Article

Aerodynamic effects of blade sweep and skew in low-speed axial flow rotors at the design flow rate: An overview

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Abstract

A detailed literature survey is presented herein in order to overview the aerodynamic impact of non-radial blade stacking techniques applied to axial flow fan and compressor rotors. The literature suggests a consensus that forward blade sweep and skew provides a means for the following advantages in the part load operational range of low-speed axial flow turbofan and compressor rotors: improvement of efficiency and performance, and extension of stall-free operating range. However, the published research results are rather diversified regarding the judgment of performance and loss modifying effects of sweep and skew at the design point. The current paper summarizes the major aerodynamic phenomena related to such blade stacking techniques, in order to contribute to a general reasoning of performance and efficiency modification at the design flow rate. Furthermore, it provides guidelines how to consider these phenomena in tailoring the blade geometry for potential efficiency gain and for achievement of the prescribed total pressure rise at the design point. The role of adequate computational fluid dynamics tools was considered in the paper to be essential in evaluation of aerodynamic effects of non-radial blade stacking, as well as in incorporation of sweep and skew in systematic blade design techniques.

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... An explanation for the increased efficiency is the interaction of streamwise vortices (that develop on the leading edge serrations) with the radial directed flow (Section I). A reduction in the radial flow directly affects the tip vortex formation [22][23][24][25] and hence reduces losses in the tip region as observed by Corsini et al. 17,18 Under free inflow conditions, the acoustic characteristic curves also show a great influence of the leading edge geometry. In nearly all operating points, the averaged overall sound pressure levels L p for the fans with sinusoidal leading edges are lower than those of the reference fan with straight leading edges. ...
... The vortex development is amongst others dependent on the induced radial velocity component. [22][23][24][25] Obviously, similarly to the observations by Corsini et al., 17,18 the counter rotating vortices on the leading edges interact with and weaken the tip vortex. Additionally, the induced vortices are expected to interact with the radial directed flow near the fan blade surface. ...
... A decrease in this component further suppresses the tip vortex formation. [22][23][24][25] IV.C.3. Impact on broadband sound components Leading edge serrations on airfoils have been mainly introduced to reduce turbulence ingestion noise, i.e. broadband sound (Section I). ...
... In this context the use of forward sweep applied to axialflow fan and compressor bladings is widely studied to improve performance and efficiency (see, e.g., [6] and [7]). Vad [8] presents a comprehensive review of the literature that was published until 2007 about this topic. Some of these works consider the tip leakage (see, e.g., [9]) and the radial shift of the meridional flow path within the blade passage (see, e.g., [10]). ...
... Although valid phenomenological explanations exist in the literature to support the advantages deriving from sweep of CVD blades, there is still no agreement on the real quantification of these advantages [8]. Moreover, at present neither an accepted preliminary design methodology nor specific guidelines accounting for blade sweep angle exist to improve the performance of fans featuring CVD blades. ...
... Note that higher values (see, e.g. 45°) for the maximum λ is reported in [8], because of the higher hub-totip ratios. The backward-sweep angle is also chosen equal to ...
Conference Paper
The controlled vortex design is a common criterion to distribute circulation along the blade span for high total pressure axial fan rotors when diameter and/or rotational speed limitations are imposed. In addition, industrial fans have to comply with Standards which impose high total efficiency. Researchers involved in the field of fan blading fluid-dynamics showed that forward sweep of the profiles stacking line may give beneficial effects in controlled vortex design blading. However, the published literature is not always unanimous in quantifying these effects and still lacks of clearly outlined design criteria. The paper searches for design guidelines that increase the performance of a rotor-only tube axial fan featuring a constant swirl blade design without reduction in total efficiency. The original fan is experimentally tested and considered as base design for CFD models that were build to estimate the effect of design modifications. The study of the interaction between blade sweep angle, tip clearance and radial shift of the meridional flow across the rotor suggested a design procedure which increases fan total pressure of about 10% at design point and significantly extends the stable operation range while keeping similar values of total efficiency within the whole operation range. Copyright © 2014 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
... Fanb lade skew,i .e. ac ombination of fanb lade sweep and dihedral [1,29,30], is an effective mean to reduce the sound radiation of axial fans. The impact of fanblade skew on the sound emission can be seen on several sound generation mechanisms [1,2,31,32,33,34,35,36]. The most prominent sound sources for axial fans are anticipated in the outer part of the fanb lade as the circumferential velocity increases from the hub to the tip [1,37,38]. ...
... V for fanB. Skewed fans are known to change the blade loading along the fanblade span [31,33,37] and to induce aradially inwards directed velocity in vicinity of the fanb lade (forward-skewed fans)oraradially outwards directed velocity (backward-skewed fans) [ 50,51]. This violates the radial equilibrium condition [12,44], i.e. the streamlines are no longer located on cylindrical surfaces during the blade passage. ...
... In this range, the forward-skewed fanFhas ahigher efficiencythan fans U and Bu nder all inflowc onditions. This can be explained by an increase in the total pressure peak and ar eduction of near-tip losses [31,52,53]. Accordingly,the rotational speed n of fanFc an be decreased by up to 180 rpm to match the operating point of fanBf or all inflowc onditions. ...
Article
In many applications, low-pressure axial fans are used in off-design operating conditions with a distorted flow field at the fan inlet. To assess the impact of such boundary conditions on the aerodynamic and acoustic fan characteristics, the sound emission of skewed and unskewed fans is compared under different inflow conditions and at matching operating points, including the design point and the off-design range. The results show that the forward-skewed fan has the highest efficiency and the least sound emission in the typical operating range under all considered inflow conditions. However, in the stall range and in the moderately loaded operating range, the backward-skewed fan has a lower sound emission. The unskewed fan has the least favorable characteristics under all operating conditions. For each fan, operating range and inflow condition, specific features in the acoustic spectrum are identified that lead to these findings.
... Thickness and Gutin noise are reduced by a dephasing effect of sound sources from hub to tip because each part of a skewed fan blade interacts with the inflow at a different instant [14,15]. Owing to the different types of fan blade skew (i.e., backward or forward), the tip vortex and the leakage flow are reduced (forwardskewed fan blade) or increased (backward-skewed fan blade) [16][17][18][19]. A weaker tip vortex leads to a reduction of narrowband tip noise, which is generated by coherent flow structures that interact with the fan blades in the tip region [10,12,20]. ...
... Therefore, the chord length for each skewed blade section was increased compared with the chord length of an unskewed blade section in the design procedure [38,54]. Additionally, the total-to-static pressure difference Δp ts for the fan design was slightly adapted for each fan individually, accounting for expected additional losses in the tip region for backward-skewed fans [19,22,24], with the aim of each fan providing the same pressure rise at the design flow coefficient. Hence, the design total pressure coefficient ψ t;d in Table 2 is different for each fan. ...
... In the range φ ∈ 0.15; 0.24, fans F and BF show an extended operating range with increased ψ ts values. This effect has been observed in previous studies and was associated with a stabilizing effect of forward-skewed fan blades, preventing flow separations in the hub region [17,19,26]. The same effect, albeit slightly less pronounced, is visible even for the combined-skewed fan BF, with backward skew in the inner part of the fan blade and forward skew just in the outer part. ...
Article
Several blade-stacking strategies exist for low-pressure axial fans with potential benefits with respect to the aeroacoustic sound emission. In this study, the impact of such different blade-stacking strategies (including singletype and combined-skewed fan blades) on the fan characteristic curves, aerodynamic properties, and aeroacoustic properties was investigated. It was found that the fan aerodynamic and acoustic characteristic curves are strongly governed by the type of fan blade skew in the outer part of the fan blade. This was also observed for the flow-field on the suction side and on the pressure side. Sound pressure spectra and beamforming sound maps of the fans were further studied for one representative point in the typical fan operating range. The spectra showed similar tendencies as the fan characteristic curves, with spectra progressions being mainly dependentonthe typeof fan blade skewinthe outer part of the fan blade. The sound maps revealed that, for the most part, sound sources are located in the outer fan region but that source characteristics may still be different for single-type and combined-skewed fans. These findings can be further used for reducing the sound emission of low-pressure axial fans with a focus on the fan blade shape in the outer part of the fan blade.
... As a rule of thumb, confirmed also by our recent CFD calculations [4], forward sweep of rotor blades without other design modifications shifts the stall towards lower flow rates without other relevant changes in the performance and efficiency curves at the only cost of a limited decrease of fan total pressure at the design point. On the other hand, in a literature survey [5] dated 2008, Vad stated that there was no general consensus on the actual advantages of forward sweep until some years ago, also because of the difficulties in isolating the effect of blade sweep alone from the global aerodynamic blading behaviour. In fact, the incorporation of sweep angle on a blade originally designed as envelope of radially stacked airfoil sections is usually coupled in the literature to the modification of other geometrical and operation parameters of the annular airfoil cascade. ...
... To avoid ambiguities, most of the papers appeared in the literature dealing with swept blades explicitly reports the definition of sweep angle being used (see, e.g., [12,5,2,22]). The sweep angle is commonly defined in the turbomachinery field as a rigid translation of each airfoil section along the chord of the spanwise adjacent airfoil section (see, Fig. 2-b), whereas the aeronautic practice defines sweep as the tilt of the whole wing around an axis perpendicular to the plane containing both the airplane and the wing axes (see, Fig. 2-a). ...
... A first estimate of drag could be conceptually done by simplified considerations similarly to lift 1 . However, it is likely that actual cascade drag deviates appreciably from the prediction of very simplified approaches which do not account for loss mechanisms, blade surface boundary layer dynamics, secondary flows, tip leakage vortices and other contributions (see the overview [5] for detailed references on these topics). Thus, any conclusive statement about advantantages/disantvantages deriving from sweep on the aerodynamic efficiency of the cascade is out of the capability of the theoretical tools employed in this paper. ...
Conference Paper
Many modern axial flow compressor and fan rotors feature forward swept blades. However, there was no general consensus on advantages due to blade forward sweep until some years ago, also because of the difficulties in isolating the effect of blade sweep alone from the global aerodynamic blading behaviour. Thus, while some researchers try to collect in a “sweep package” all modifications connected to the introduction of blade sweep, there is no attempt in the recent literature to formalise a theory on the aerodynamic effect of sweep angle on axial flow fan blades. This paper first recalls the swept wing concept suggested by Adolf Busemann, then the original swept wing meaning is reinterpreted to present a simplified theory of blade forward sweep. This theory allows to explain the not yet completely understood changes in total pressure and efficiency due to the inclusion of sweep in the design of high circulation gradient controlled vortex design rotors. A methodology to select the degree of forward sweep to be chosen in the preliminary design of constant-swirl bladings for rotor-only axial flow fan featuring low hub-to-tip ratio is suggested as well. A specifically designed unswept fan and its modified configurations according to the suggested theory has been verified by using CFD. The forward swept design increases the unswept fan total pressure in the whole operation range from about 1% to 4% and expands the stall margin at throttling by more than 10% of the design flow rate leaving efficiency and duty unchanged. Copyright © 2015 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
... Subsequent investigations of the effects of sweep, dihedral, and skew reported beneficial effects, such as improvement of efficiency (g ts or g tt ), reduction of end-wall (hub and shroud) and tip gap losses, and control of secondary flow (radial flow). Most of these advantages are found for forward types [17]. ...
... Therefore, FSW and FCS are often investigated together, as in Refs. [13] and [17]. ...
... This radial migration induced by forward swept blades is illustrated in Refs. [17], [22], and [23], showing that for forward swept blades the flow path of radially outward flow near the blade surface is shortened. This is confirmed by the measured pressure distribution in Refs. ...
Article
Full-text available
Axial fans with low hub-to-tip diameter ratio are used in many branches of industry. Optimization of their aerodynamic performance is important, for which using sweep, dihedral and skew of the blades' stacking line form an important method. Investigations on axial fans with medium to high hub-to-tip diameter ratio have shown that forward sweep of blades can give improved aerodynamic performance, especially the total-to-total efficiency. However, only few studies for fans with small hub-to-tip diameter ratio have been reported. For such fans, extensive regions of backflow are present behind the fan near the hub. Based on a validated Computational Fluid Dynamics simulation method, effects of sweep, dihedral and skew in axial and circumferential directions (in forward and backward direction) on the aerodynamic performance of small hub-to-tip ratio fans are investigated, with a linear stacking line. Current results show that forward sweep and circumferential skew are beneficial for higher total-to-total efficiency and that higher total-to-static efficiency can be obtained by forward dihedral and axial skew. The backward shape variety generally gives negative aerodynamic effects. Forward sweep and circumferential skew shorten the radial migration path, but more flow separation is present near the hub. With forward dihedral and axial skew the backflow region is reduced in size and axial extent, but a more significant hub corner stall region is found. The pressure reduction due to sweep and dihedral is more limited than what could be expected from wing aerodynamics.
... 17,18 Owing to different types of fan blade skew, i.e. backward or forward, the tip vortex is reduced (forward-skewed fan blade) or increased (backward-skewed fan blade). [19][20][21][22] A weaker tip vortex, in turn, leads to a reduction of narrowband tip noise. Similarly to the dephasing effect, peak load is reduced for skewed fans, which leads to a reduction of instationary blade forces, and consequently to decreased unsteady loading noise. ...
... Additionally, the total-to-static pressure difference ∆p ts for the fan design was slightly adapted for each fan individually, accounting for additional losses in backward-skewed fans. 22,24,26 Hence, the design total pressure coefficient ψ t,d in Table 2 is different for each fan. ...
... This effect has been observed in previous studies and was associated with a stabilizing effect of forward-skewed fan blades, preventing flow separations in the hub region. 20,22,28 The same effect, albeit slightly less pronounced, is visible even for combined-skewed fan BF, with backward-skew in the inner part of the fan blade and forward-skew just in the outer part. In a similar way, characteristics of fan B are also visible for fan FB. ...
... The curvature is given by orienting the blade either forward (in the direction of rotation) or backward. It is therefore a combination of sweep and lean changes, which are in theory a stacking change either in the direction of the chord or perpendicular to the chord [9,10]. ...
... It should also be noted that the backward-curved fan produces an overall less tangential velocity and it is compensated by a largely positive radial velocity, whereas the forward curvature tends to create negative radial velocities. Some authors have already presented some effects of sweep or lean modifications [6][7][8][9]11,18], and it explains, for instance, that the blade curve creates a force acting on the flow that has a direction perpendicular to the surface of the pressure side. In the case of a forward fan, this force would be centripetal, and centrifugal in the case of a backward curvature. ...
Article
Full-text available
Two blade curvatures representative of those found in automotive fans are compared. Measured performances are analyzed for forward and backward curved blades, either with or without heat-exchangers placed in front of them. The backward fan demonstrated good efficiency but poor acoustics, whereas it is the contrary for the forward fan. Investigations are completed by a numerical analysis of the flow in the cooling module. Different integration effects are highlighted depending on the blade curvature, showing variation in pressure, torque and efficiency. Analyses of blade loadings show that the flow is more homogeneous with a forward curved fan and it produces less unsteadiness at the blade tip. Post-processing of detached eddy simulations (DES) shows density fluctuations on the blade wall and confirms the correlation between the large vortical structures and the acoustic sources for both fans. In addition, with the forward fan, the sound propagation is less directed towards the axis of rotation and it yields up to −3.6 dB of sound pressure level (SPL) measured in front of the cooling module. As a conclusion, any choice for a fan must result from a compromise between aerodynamics and aeroacoustics, and the final performances must be carefully checked on the module.
... Its application has been widely spread from compressors to turbines and from rotors to stators [10], especially applied to blade ends to improve the endwall flowfields [11]. For compressor rotors, a detailed survey [12] suggested that forward swept blades improved the efficiency and extended the stall-free operating range by reducing the near-wall and tip clearance losses, as well as controlling the secondary flows and radial migration of high-loss fluid. More recently, Okui et al. [13] optimized the sweep feature, the camber line and the chord length of a transonic axial compressor rotor. ...
... It is illustrated that the front loading near the tip is decreased with the enlarged sweep angle, while that near the hub is increased. Because the front of main blade locates at the axial part of the impeller, the loading redistribution effect is similar with that in the axial case [12]. Such effect is also reported in a former centrifugal case [25], but the extent in the current case is much stronger due to the significant change of sweep angle. ...
Article
Full-text available
Transonic centrifugal compressors with high performance are required in the oil and gas industries, modern gas turbine engines, and turbochargers. The sweep of the blades is one of the crucial features that have a significant influence on their performance. This paper numerically investigates mechanisms by which sweep affects the performance of a transonic impeller with twin splitters. Sweep is defined as scaling up or down the shroud chord, and the variation range of the sweep angle has been chosen from −25 to +25 •. In the current case, results show that the variation of choke mass flow rate, pressure ratio, and efficiency value is around 1%. If the centrifugal compressor has a higher pressure ratio or a higher front loading, the sweep effect on compressor performance will be even stronger. The essential aerodynamic effect of sweep is the spanwise redistribution of the front loading, resulting in effects on the shock structure, the tip leakage vortex, and the flow separation. On the shroud section, forward sweep restricts the front loading, the shock strength, and the tip leakage vortex, which reduces the loss near the casing. On the hub section, aft sweep suppresses the front loading and the flow separation, which reduces the loss near the hub. It is the delicate balance between controlling the loss near the hub and the loss near the casing that determines the optimal sweep angle design.
... On the other hands, the second type stall inception, "modal", which is the small amplitude, long scale length disturbance propagating with usually less than 50% of rotor speed. Modal-type stall inception occurs at a lower point than the peak point of the total-to-static pressure characteristics [5]. ...
... It grew into the stall cell and slowed down. However, it seemed to be difficult to identify the other stall inception, which is the long-length-scale (modal) type fluctuation clearly in this pressure trace data because the amplitude of modal oscillation usually remains small and it is thought to be not associated with the separation near the tip region [5]. Therefore, further analysis for the pressure data was performed to clarify whether modal oscillation existed. ...
Article
In this study, we aim to elucidate the effect of a forward-swept rotor on the stall margin and flow field at the distorted inflow conditions. The rig in this research is a low-speed, single-stage axial compressor, which has two types of rotor blades: the radially stacked blade (Radial) and the forward-swept blade (Sweep). The distortion screen that circumferentially generates distorted inflow is located upstream of the rotor. The stall margin of Sweep was found to be larger than that of Radial. Sweep was considered to improve the flow fields at the distorted inflow conditions. From the results of the study, it was observed that Sweep suppressed the circumferential expansion of the high-load regions and the spike-type disturbances generated at the distorted sector. Therefore, Sweep enlarged the stall margin more than to Radial.
... The efficiency for the fans with serrated leading edges is expected to be improved, due to streamwise vortices that develop on the serrations and interact with the radial directed flow [11,29,30,28,17,17]. This also affects the tip vortex formation [31,32,33,34] and hence reduces losses in the tip region, as observed by Corsini et al. [35,36]. ...
... It can be expected, that similarly to the observations by Corsini et al. [35,36], counter rotating vortices that are induced by leading edge serrations interact with and weaken the tip vortex. Additionally, the vortices are expected to interact with the radial directed flow near the fan blade surface, which further suppresses the tip vortex formation [31,32,33,34]. ...
... Comprehensive research of the blade sweep mechanism was also described in Refs. [41,43,44]. ...
... A large number of researchers [39][40][41][42][43][44][45][46][47] have focused on the effect of sweep on the compressor performance. Denton et al. [26] gave a novel way to look at the effects of sweep which is shown in Fig. 32. ...
... non-radial stacking line) methods, such as sweep, dihedral, bow, and lean, have been conducted. 1,2,5,6 However, the results are much diversified in the literatures, and no consensus of quantitative 7,8 and/or even qualitative 9,10 rules could be found for a certain blade 3D design type, resulting in an absence of widely accepted design criteria for blade 3D designs. Hence, further studies on the working mechanics of blade 3D designs are still needed. ...
... Therefore, most of the related researches were conducted to concern the effects of one or several parameters, for further simplicity, and were mainly conducted in a single-blade row or cascade configurations. 5,7,[10][11][12][13][14][15][16][17] Although some related works have been conducted in single-stage or multi-stage compressors, the influences of the blade row interaction flow phenomena on the effects of blade 3D designs were often not under serious consideration. 1,18,19 It is well known that in a multi-stage configuration, particularly for the subsonic compressors in the middle and rear stages of a high-pressure compressor, the design of one blade row might have significant effects on the flows in both the upstream and downstream blade rows. ...
Article
Full-text available
To realize whether the rules for controlling the blade three-dimensional stacking line in a compressor with conventional loading level could be used for the design of a highly loaded compressor, the effects of three-dimensional bladings in an ultra-highly loaded compressor stage were studied numerically. A low-speed compressor stage (Stage-C) with ultra-high loading coefficient (=0.52) was designed at first. Due to the well-chosen through-flow design parameters accompanied using controlled diffusion airfoil with spikeless leading edge, Stage-C achieved the design goal of loading level with high peak efficiency of about 0.89. However, all the blades in Stage-C were designed with radial stacking lines. And then, Stage-C-three-dimensional was re-designed with non-radial stacking blades based on Stage-C, after which 1-point compressor efficiency profit was achieved. Based on the numerical simulations, the performance change in the two compressors and also the effects of blade three-dimensional stacking were discussed in depth. It was found that the endwall corner separation and secondary flows could be suppressed effectively using endwall bending; however, the blade forward sweep design at the rotor tip failed due to strong rotor–stator coupling effects in the highly loaded compressor stage.
... Comprehensive research of the blade sweep mechanism was also described in Refs. [41,43,44]. ...
... A large number of researchers [39][40][41][42][43][44][45][46][47] have focused on the effect of sweep on the compressor performance. Denton et al. [26] gave a novel way to look at the effects of sweep which is shown in Fig. 32. ...
Article
This paper reviews the literature published over the past 30 years on the blade-end treatment in axial compressors. The blade-end treatment reduces the endwall losses and extends the stable margin by modifying the blade shape near the endwall region with end-bend, end-dihedral and end-sweep flow control measures. The end-bend improves the overall performance by aligning the blade inlet/outlet to the flow stream direction. The end-dihedral reduces the blade force on the endwalls, while the end-sweep not only reduces the shock losses, but also controls the spanwise migration of the blade surface boundary layer. All these effects strongly influence the compressor performance by modifying the blading loading distribution in the streamwise or spanwise directions. However, the benefit of the endwall flow control comes with increased losses in the mainstream so there is a trade-off between the improved endwall region flows and the degraded mid-span flows. Thus, how to combine end-bend, end-dihedral and end-sweep to achieve the correct balance of loss distribution, appears to be the key to a successful three-dimensional compressor design.
... In order to improve the performance of compressor, flow control measures are necessary, especially near the end wall region. Thus far, many kinds of flow control researches [14][15][16][17][18][19][20][21] have been done to improve the compressor performance. Re-cambered design (also called end-bend) is one of the effective methods to achieve improvement in the end wall region. ...
Article
The development pattern of the end wall boundary layer (BL) in whole conditions and its effect on the matching of multistage compressor have been studied in detail in this paper. Moreover, one method of end wall zone blade modification is carried out, using computational fluid dynamics, to improve the stage matching by re-camber. It is found that the pitch-averaged thickness of end wall BL gradually increases along streamline direction and the BL is highly skewed in the pitchwise direction. In addition, the value of BL thickness, mainly depends on the stage pressure rise coefficient . For a fixed rotating speed, the axial BL displacement thickness is changed dramatically from choked condition to near surge operating point. However, this thickness shows insensitive to the change of rotating speeds. On the other hand, BL degrades the pressure-rise characteristics of stages with low efficiency, and it results in mismatching of the compressor stage. Furthermore, the deviation angle of the blades is strongly affected by the end wall BL. Finally, two rows of stators are re-cambered on the leading edge to improve the matching near the hub region where it is influenced by end wall BL. The results show that the total pressure loss coefficient of the stators is reduced by 0.6 points and 1.73 points each. Moreover, the overall isentropic efficiency of multistage compressor is enhanced by 0.39 points without compromising pressure-rise capability and surge margin.
... During the last fifteen years, the advancements in the knowledge of the effects due to non-radial stacking line of the blade [7] have allowed to improve performance [8] and to reduce noise [9] of tube-axial industrial fans. In a recent work [10] we collected within a sequence of two procedures (which can be performed without using CFD) some design guidelines that allow to increase total pressure of a rotor-only tube axial fan featuring a constant swirl blade design without reduction in total efficiency. ...
Article
Common blade design techniques are based on the assumption of the airflow laying on cylindrical surfaces. This behaviour is proper only for free-vortex flow, whereas radial fluid migration along the span is always present in case of controlled vortex design blades. The paper presents a design procedure to increase aeraulic efficiency of fan rotors originally designed using a controlled vortex criterion, based on the assumption that a blade section positioning taking into account the actual airflow direction could be beneficial for rotor aeraulic performance. The proposed procedure employs a three-dimensional aerofoil positioning and blade forward sweep. The procedure is applied to a rotor-only tube-axial fan featuring a 0.44 hub-to-tip ratio, an almost constant swirl velocity distribution at the rotor outlet and a quite low blade Reynolds number. Rotor prototypes deriving from step-by-step blade modifications are experimentally tested on an ISO 5801 standard test rig. Results show the importance of considering radial fluid migration for highly loaded rotors.
... This suggests the presence of a separation zone that is probably due to local blade overloading caused by the negative dihedral. 30 Increasing the clearance, the suction effect and the adverse pressure gradient, manifesting the blade load, tend to decrease, and tend to confine to lower radii. As seen in the acoustic power level plot for zero clearance (lower row, first plot), the separation zone near the tip LE appears as a source of peak noise. ...
Article
A radial flow rotor with radially aligned straight blades, used in electric motor cooling, has been considered as datum fan. The aerodynamic performance and acoustic behaviour of the datum fan have been measured, in order to establish a basis for redesign. As replacement for the datum fan, an axial flow shrouded rotor with skewed blades has iteratively been designed, involving computational fluid dynamics and computational aero-acoustics tools. The aim of redesign was reducing fan noise and moderating motor shaft power absorbed by the fan, while retaining the original cooling performance. Special flow features have been taken into consideration in three-dimensional axial rotor design, such as leakage flow in the axial clearance between the rotor shroud inlet and the perforated cover, and strong radial flow as well as deviation due to the motor shield located close downstream. The axial rotor has been manufactured via rapid prototyping. Measurements on the prototype confirmed the achievement of the redesign goal. The effect of axial clearance size on the operation of the axial rotor has been investigated by computational fluid dynamics, computational aero-acoustics and experimental means. It has been pointed out that the axial clearance size is a sensitive parameter in influencing fan aerodynamics and aero-acoustics, for which the major mechanisms, associated with the leakage flow, were qualitatively identified. Both the computational and experimental studies revealed the existence of an acoustically unfavourable clearance size, for which maximum noise emission can be expected. A semi-empirical model was outlined as starting point in prediction of cooling flow rate as a function of axial clearance size as well as other parameters. © IMechE 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.
... The opposite is regarded as aft-sweep. For axial cases, swept blade design has been extensively studied in compressor rotors [9] and fans [10]. It is generally found that the stall margin was improved by forward sweep, and most forward swept cases showed better efficiency than that of the unswept or the aft-swept ones. ...
Preprint
Full-text available
Transonic centrifugal compressors with high efficiency and wide stable flow range are required in modern gas turbine engines. Blade design with complex three-dimensional features is one of the promising methods to further improve the performance of such cases. Aiming to increase the efficiency while maintaining similar level of the stable flow range, this paper investigates aerodynamic potentials of complex three-dimensional features in a transonic centrifugal compressor by multi-point and multi-objective optimizations, in which the camber curves, the sweep feature and the lean feature have been optimized. During the first round of optimizations, the aforementioned three groups of variables are optimized individually, and their sensitivities to the performance have been analyzed. When optimizing the camber curves, the best result shows an end-bend feature at the front of the hub section, and the efficiency is improved by 1.0% due to the lowered shock strength. When optimizing the sweep feature, the best result presents an S-shape leading edge and a forward sweep feature. The efficiency is increased by 0.5% because of the reduced wake region. The optimized lean feature only improves the efficiency by 0.2%, which shows its relatively low potential. The final round of optimizations couples both the camber curves and the sweep feature, and the best geometry combines both the end-bend and S-shape leading edge patterns. The peak efficiency and the choke mass flow rate has been increased by 2.2% and 8.1% respectively, which is owing to the combination of the lowered shock strength by optimized camber curves and the reduced wake region by optimized sweep feature. The result indicates significant potential of complex three-dimensional features to improve the performance of transonic centrifugal compressors.
... Hence, with an increasing demand for MFFs with high efficiency and performance (Gunner and Hultmark et al., 2014), optimization study has been a focus. The most recent advances in aerodynamic performance focused on the effects of tip clearance size (Liu et al., 2008;Jin et al., 2010) and skewed and swept blades (Vad, 2008;Mohammed, 2012); but diffuser optimization has been researched relatively rarely. ...
Article
Mixing fans (MFFs) are widely used in ventilation of agricultural buildings to improve the uniformity of the air supply, thereby improving the ventilation efficiency. In order to improve the ventilation performance of MFFs, a new visor-shaped diffuser was designed and installed on a MFF. The angle and the length of the diffuser were crucial parameters that affected the performance of the MFFs with the diffusers. Thereby, in this study numerical simulation with 42 diffusers of different angles (ranged from 90-270°) and different lengths (ranged from 150-650 mm) with the MFF were studied with Computational Fluid Dynamics (CFD) simulation to acquire the optimal design of diffusers. The numerical simulation results show that the diffusers of 90°/450 mm, 120°/350 mm, and 150°/250 mm with jet lengths of up to 5.85, 5.90, and 5.85 m, respectively, had better performances among all the diffusers. The optimal prototype diffusers of 90°/450 mm, 120°/350 mm, and 150°/250 mm of MFFs were tested by laboratory study and field test. The test was conducted in wind speed distributions at distances of 0.5 to 1.0 m from the axial of MFFs. During the test, we evaluated the MFFs performance such as maximum flow flux, maximum energy efficiency, and non-uniformity coefficient. The diffuser of 150°/250 mm showed the best performance, increasing the flow flux and energy efficiency by 3.8% and 11%, respectively, and obtain higher axial wind speeds and larger non-uniformity coefficients. Finally, the diffusers of 150°/250 mm were tested in a free-stall dairy barn. The field test result shows that the diffusers of 150°/250 mm increased overall average wind speeds by 7.4% and local average wind speeds at bedding 1 and bedding 2 by 31.0% and 27.7%, respectively, which agreed with our numerical simulation and laboratory test. This optimal design of mixing fans could be applied to improve the air mixing in agricultural buildings. © 2018 American Society of Agricultural and Biological Engineers.
... In general, the pressure coefficient for the backward-skewed fans is lower than for the corresponding forward-skewed fan. This is the result of additional losses that occur during the blade passage of backwardskewed fans, as described in [25,26,12]. Furthermore it was observed that there are differences for the blade loading distribution as the fans do not exactly share a common operating point. ...
Article
An experimental investigation on the noise emission of axial fans under distorted inflow conditions was conducted. Three fans with forward-skewed fan blades and three fans with backward-skewed fan blades and a common operating point were designed with a 2D element blade method. Two approaches were adopted to modify the inflow conditions: first, the inflow turbulence intensity was increased by two different rectangular grids and second, the inflow velocity profile was changed to an asymmetric characteristic by two grids with a distinct bar stacking.
... Focusing on the sweep applied to axial-flow fan rotors, the review [5] collects the findings of the main researchers on the topic. The introduction of forward sweep in the radial stacking line of rotor blades for low-speed axial fans allows an appreciable gain in the stall margin at the expense of a reduction of the fan pressure at the design point. ...
Conference Paper
Full-text available
Forward swept blades in low-speed axial fan rotors allow for appreciable gain in the stall margin and a small percentage gain in the maximum fan efficiency if the rotor blade circulation increases from the hub to tip. However, a reduction of the fan pressure at the design point counteracts these advantages. The paper investigates the effectiveness for small tube-axial fans of a design method suggested to increase the performance of an existing arbitrary vortex design by introducing the span-wise uniform distribution of blade forward sweep. The following three rotors for a 315-mm tube-axial fan have been tested: unswept, forward swept, and forward swept with additional sweep at the blade tip. Experimental data prove the effectiveness of the design method for these small fans.
... During the last fifteen years, the advancements in the knowledge of the effects due to non-radial stacking line of the blade [7] have allowed to improve performance [8] and to reduce noise [9] of tube-axial industrial fans. In a recent work [10] we collected within a sequence of two procedures (which can be performed without using CFD) some design guidelines that allow to increase total pressure of a rotor-only tube axial fan featuring a constant swirl blade design without reduction in total efficiency. ...
... However, the adoption of dynamic control devices has an inherent difficulty in identifying the characteristic frequencies of the vortical structures for the positional adjustment and intensity of such devices [4]. The second approach makes use of static means such as changes in blade configuration [5] or tip modification techniques [6][7][8] to achieve the same desired outcome. More recently, Corsini et al. [9][10][11][12] investigated the effect of locally thickening the blade tip based on a safe chord-wise Rossby number distribution on a family of commercially available axial flow fans. ...
Conference Paper
This paper reports on numerical investigations of passive control techniques used for the performance enhancement of a large diameter axial flow cooling fan through modification of the blade tip geometry. Using open-source software, a novel meshing strategy is developed to carry out both steady and unsteady numerical computations of a periodic section of an axial flow fan. Analysing the flow near the blade tip with a reduction in tip clearance, two predominant flow phenomena are identified. These two flow phenomena are further investigated with the aim of controlling them through implementation of a tip appendage design. Both introduced end-plate designs indicate effective control of each relevant flow phenomena. The constant thickness (CT) end-plate design is found to increase all fan performance characteristics at lower than design point (DP) flow rates while increasing the fan’s peak efficiency plateau towards the rotor stall margin. However, none of the CT end-plate designs are able to improve the fan’s performance characteristics at its DP. The introduction of a novel trailing edge (TE) end-plate design is found to increase all fan performance characteristics across the entire evaluated stable operating range, with an indicated increase of 37.3 percent in total-to-static pressure rise and a 2.9 percentage point increase in total-to-static efficiency at the fan’s DP flow rate. The aerodynamic performance results attest to the associated benefits of the investigated passive control techniques.
... For such fans, stators (or diffusers) are often not employed [1]. Their rotors have frequently been investigated [2][3][4][5][6], with the improvement of operational efficiency as an important design objective. An important characteristic of axial fans with small HTR, as indicated by Eck [1] and shown in Refs. ...
Article
Full-text available
Axial fans with small hub-to-tip diameter ratio (HTR) are widely used in industry, especially for cooling and ventilation purposes. Optimization of their aerodynamic performance is important, for which the vortex distribution method is well-established for axial fans with medium to high HTR. However, only few studies have focused on small HTR fans. For such fans, downstream backflow regions are often present near the hub. The vortex distribution (polynomial in spanwise coordinate) and the HTR have been determined by maximizing the total-to-static efficiency of a baseline axial fan with small HTR. For free vortex designs, analytical expressions for the maximum total-to-static efficiency and the optimal HTR have been formulated. By combining the vortex distributions thus obtained with a suitable choice for the spanwise lift coefficient distribution, fan blade designs have been established. The effects of different vortex distributions on the aerodynamic performance have been investigated, employing a Computational Fluids Dynamics (CFD) simulation strategy that has been validated for the baseline axial fan. The current CFD results show that the free and the polynomial vortex distribution designs satisfy the desired pressure rise, with significantly improved total-to-static and total-to-total efficiency (maximum improvement by 3.9% and 4.6%, respectively). For the free vortex design with larger HTR, neither flow separation nor backflow is observed. For the other designs at the design flowrate, only flow separation near the hub is found. Backflow is observed only for the designs with smaller HTR.
... This sound generation mechanism is linked to a backflow from the pressure side to the suction side in the tip-gap region (Section 2). Forward-skewed fan blades tend to weaken the tip-vortex formation [28][29][30]64,65], which leads to a lower magnitude of this sound generation mechanism. Tonal noise is most likely connected to inflow distortions, as no turbulence control screen was used [66,67]. ...
Article
The type of fan-blade skew used in axial fans has a strong impact on the sound field, the flow field and their interrelations. In this study, the sound emission and the velocity distributions of three low-pressure axial fans with a similar design point and forward-, backward- and unskewed fan blades are investigated. The forward skewed fan is found to have best aerodynamic performance as well as the least sound emission over a broad operating range. More detailed investigations at the fan design point, based on sound pressure spectra and beamforming evaluations, reveal very dominant subharmonic narrowband components and increased turbulence-ingestion noise for the backward- and the unskewed fan. Moreover, an increase in the meridional velocity in the tip region is observed for the forward-skewed fan, which weakens the tip-vortex formation. For the backward- and the unskewed fan however, a reduced meridional velocity in the tip region is found, which is an indicator of an intensified tip gap flow. Additionally, values of the turbulent kinetic energy are substantially increased for the backward- and the unskewed fan, which also hints at unsteady flow phenomena in the tip region. These flow processes are linked to the subharmonic narrowband sound emission. Finally, unsteady flow features in the tip region lead to an increase in turbulence-ingestion noise.
... However, skewed and swept blades still do not have uniform definitions. The definitions adopted in this paper are based on those of Seo et al. (2008) and Vad et al. (2008) but are not used by all researchers. The shapes of skewed and swept blades are determined by the shape of the stacking line, but relatively few studies have been devoted to stacking line design, and there is yet no consensus on the shape of the stacking line; the most commonly used stacking lines are straight lines, standard arcs and multiple curves (Jin et al., 2011;Rohkamm et al., 2003). ...
Article
With the rapid development of modern agriculture facilities, agricultural fans have been widely used due to their low pressure and large airflow characteristics. However, existing agricultural fans have large flow losses and low energy efficiencies. To increase the airflow and energy efficiency of these fans, optimization designs based on skewed and swept blades were carried out. First, a “DDZ” agricultural fan (a leaf model agricultural fan commonly used in China) was chosen as the archetype fan. Its performance curves and flow field distribution were obtained by performance testing and numerical simulation. Second, the stack lines of the skewed blade and swept blade were designed based on the original blade, 3 skewed blade parameters (skewed angle a, x direction control parameter kx, and y direction control parameter ky), and 3 swept blade design parameters (swept angle ß, z direction control parameter kz, and r direction control parameter kr). Finally, the optimal skewed blade design parameters (a = 16.8°, kx = 1.65, and ky = 0.5) and optimal swept blade design parameters (ß = 10.6°, kz = -0.33, and kr =0.6) were obtained using numerical simulations and orthogonal testing, which is a response surface method. The numerical simulation results showed that the airflow and energy efficiency ratios of the optimal skewed blade fan were increased by 4.3% and 20.5%, and those of the optimal swept blade fan were increased by 4.5% and 15.4%, respectively, in comparison with those of the original fan. The flow fields showed that the optimal skewed blade mainly reduced the radial flow at the blade root and the leakage flow. The optimal swept blade mainly reduced the leakage flow by changing the distribution of the static pressure on the blade surfaces. Keywords: Agricultural fan, Skewed-Swept blade, Numerical simulation, Optimization.
... The application of forward-skew (with otherwise identical design parameters) leads to an increase in the maximum efficiency and a shift of the maximum to V = 0.9 m 3 /s. As observed in other studies [6,9,25,53], forward-skewed fan blades lead to a better aerodynamic performance, even for this simplified test setup. ...
... Although the improved lift-drag ratio for aeroplane wings was well known in advance, in early turbomachines the sweep was an accidental bi-product of a manufacturing process. According to recent studies, blade sweep is particularly important for the low speed machines because of its ability to improve stall margin and efficiency [2]. Incorporating sweep in design also modifies 1. meridional flow, 2. blade to blade flow and 3. boundary layer growth [3]. ...
... Although the improved lift-drag ratio for aeroplane wings was well known in advance, in early turbomachines the sweep was an accidental bi-product of a manufacturing process. According to recent studies, blade sweep is particularly important for the low speed machines because of its ability to improve stall margin and efficiency [2]. Incorporating sweep in design also modifies 1. meridional flow, 2. blade to blade flow and 3. boundary layer growth [3]. ...
Conference Paper
Full-text available
Three fans designed with forward, backward and unswept blades are analysed in the present work. The simulations are performed using Reynolds-Averaged Navier Stokes equations using k − ω SST turbulence closure model. The y + ∼1 is maintained on blade surfaces and computational domain is discretized using hybrid mesh. The predicted overall performances agree reasonably well with experiments and reproduce the observed hysteresis. The analysis of flow field and noise predictions are assessed at design point i.e at 5083CMH and 1400RPM. The mean flow field before and after the fans also agree well with laser Doppler anemometry measurements. The farfield noise is predicted using Amiet's model for leading edge and trailing edge noise sources separately. The former is found dominant and the latter negligible. Secondary flow such as tip vortices are significantly contributing to the noise spectrum confirming that tip noise is a dominant noise source in such machines.
... The application of forward skew (with otherwise identical design parameters) leads to an increase in the maximum efficiency and a shift of the maximum to _ V 0.9 m 3 ∕s. As observed in other studies [6,9,25,54], forward-skewed fan blades lead to a better aerodynamic performance, even for this simplified test setup. ...
Article
Axial fans represent a major source of noise in technical systems. Based on investigations on airfoils, a promising measure for reducing the sound emission of such systems with axial fans is the application of leading-edge serrations to the fan blade. Hence in this study, the joint impact of fan-blade skew—as a commonly used noise-reduction approach—and leading-edge serrations on the sound emission of low-speed axial fans was investigated. Forward-, backward-, and unskewed fans with five different leading-edge modifications were used. For each fan, a reference configuration with straight leading edges and four configurations with sinusoidal leading-edge serrations (with two different amplitude values and two different wavelength values) were examined. The results show that, among the reference fans with unmodified leading edges, the forward-skewed fan had the lowest sound emission. The serrations applied to the unskewed fan lead to an increase in the efficiency and a decrease in the sound emission. When applied in combination with fan-blade skew, the serrated fans showed an even lower sound emission and a higher efficiency—even the forward-skewed fan, which already has a low sound emission without any leading-edge modifications. The highest sound reduction was achieved by applying the serrations with the smallest wavelength and the highest amplitude. The findings prove that even for low-noise fans with forward-skewed fan blades, a further sound reduction is possible with the use of leading-edge serrations.
... Another strategy of sound reduction involves the installation of endplates at the blade tip (see Figure 8) to modify the behavior of the leakage flow and the related vortical structure [52]. Apart from the acoustic interests, the effects of forward-swept designs on the performance and efficiency of fans with non-free-vortex blades have then been investigated by Corsini and Rispoli [53] and Vad [54,55]. These authors respectively highlighted the advantages of swept blades into providing an extension of the stall margin and to obtain an efficiency gain in case of rotors designed with NFV techniques featuring positive gradients of circulation along the radius. ...
Article
Full-text available
The paper presents a historical overview of the developments of aerodynamic design methods for low-speed axial-flow fans. This historical overview starts from the first fan applications, dating back to the 16th century, and arrives to the modern times of computer-based design techniques, passing through the pioneering times of aerodynamic theories and the times of designing before computers. The overview shows that the major achievements in the axial fan design discipline have actually been related to other technological fields, such as marine and aeronautical propulsion, as well as to the development of wind tunnels. At the end of the paper, the reader will have acquired a complete panorama of how the historical developments of the discipline have brought us to the current state of the art.
... However, the adoption of dynamic control devices has an inherent difficulty in identifying the characteristic frequencies of the vortical structures for the positional adjustment and intensity of such devices [4]. The second approach makes use of static means such as changes in blade configuration [5] or tip modification techniques [6][7][8] to achieve the same desired outcome. More recently, Corsini et al. [9][10][11][12] investigated the effect of locally thickening the blade tip based on a safe chord-wise Rossby number distribution on a family of commercially available axial flow fans. ...
Article
This paper reports on numerical investigations of passive control techniques used for the performance enhancement of a large diameter axial flow cooling fan through modification of the blade tip geometry. Using open-source software, a novel meshing strategy is developed to carry out both steady and unsteady numerical computations of a periodic section of an axial flow fan. Analysing the flow near the blade tip with a reduction in tip clearance, two predominant flow phenomena are identified. These two flow phenomena are further investigated with the aim of controlling them through implementation of a tip appendage design. Both introduced end-plate designs indicate effective control of each relevant flow phenomena. The constant thickness (CT) end-plate design is found to increase all fan performance characteristics at lower than design point (DP) flow rates while increasing the fan's peak efficiency plateau towards the rotor stall margin. However, none of the CT end-plate designs are able to improve the fan's performance characteristics at its DP. The introduction of a novel trailing edge (TE) end-plate design is found to increase all fan performance characteristics across the entire evaluated stable operating range, with an indicated increase of 37.3 percent in total-to-static pressure rise and a 2.9 percentage point increase in total-to-static efficiency at the fan's DP flow rate. The aerodynamic performance results attest to the associated benefits of the investigated passive control techniques.
... Reasonable blade shape can reduce the blade tip vortex and the wake vortex shedding caused by flow separation to reduce the noise of such fan systems. The most striking features of this low-speed axial fan are its swept and skewed blade shape [13]. Previous studies have demonstrated that the forward-tilted blade reduces the total pressure loss near the rotor hub and expands the stable working range of the fan [14][15][16]. ...
Article
Full-text available
Outer edge bending is already used on the axial fan blades of air conditioners, reducing the leakage flow loss at the blade tip and suppressing the tip vortex development, thereby improving fan aerodynamic and acoustic performance. However, there are few studies on the multi-parameter design and optimization of this complicated structure, and most studies only focus on the overall sound pressure level rather than the sound quality when evaluating the fan noise. This study investigated the effects of outer edge bending structure on the aerodynamic performance and sound quality of air conditioners’ axial fans by experiments and numerical methods. Based on the orthogonal design method, the effects of three bending parameters, the circumferential starting angle, radial relative position, and the bending degree effects on the performance of the axial flow fan blade were analyzed, and the best efficiency scheme was selected. A comparative analysis of the preferred and the original bending schemes shows that the bending towards the blade suction surface successfully inhibits the development of tip leakage vortex at the blade tip, thereby achieving efficiency enhancement and noise reduction. The experimental results show that the preferred bending scheme with a 10° circumferential starting angle, 90% radial relative position, and 8% bending degree can effectively reduce the fan’s broadband noise within 200~1000 Hz by 0.54~2.68 dB (A) at different operating conditions. Additionally, the preferred bending blade with reasonably designed bending effectively reduced the loudness and roughness of the fan noise in the rated conditions, and the sound quality of the studied fan was correspondingly improved.
... Therefore, there is interest within both the academic and industrial community in improving the aerodynamic performance of an axial fan and reducing its noise. 1 Many researches have been done to investigate the impact of the shape and dimensions of the fan on its aerodynamic performance and aeroacoustic performance. First, the influences of a forward, a radial and a backward sweep on the downstream turbulent flow of the axial flow fan are studied. ...
Article
Experimental and numerical investigations on the effect of deflecting rings featuring different axial lengths on aerodynamic performance of the small axial flow fan were conducted under the condition of maximum flow rate. Two deflecting rings, the semi-open type and closed type, were investigated. Aerodynamic and aeroacoustic performances have been measured in experiment, and key analysis of flow was based on computational fluid dynamics results. The numerical and experimental results show that the deflecting ring has great influence on the performance of an axial flow fan. For the semi-open-type deflecting ring, the fan has better P-Q performance and higher efficiency; pressure, vorticity, and vorticity gradient distributions on the blade surface are more uniform; and noise level of the fan is lower at wider frequency bands. For the closed-type deflecting ring, the performance curve has a convex feature; blade pressure difference between the pressure surface and the suction surface is much bigger. The result shows that better aerodynamic and aeroacoustic performances of the axial flow fan can be acquired when the semi-open-type deflecting ring is adopted.
Article
Full-text available
This paper shows methodology that has been used to further enhance performance of a backward-forward sweep blade in an axial fan system. A DES simulation has shown that blade profiles and stagger angles for a swept blade are no longer adapted to the inlet velocity triangle which differs from the theoretical one. Local conditions at the radius where most of the structures are located are extracted from a blade simulation and used as inlet conditions in a profile study aimed to improve the aerodynamics. Velocity, chord length, camber and stagger angle effects were investigated in a parametric study through a URANS numerical design of experiments. An optimization was ultimately performed to identify a new profile which minimizes drag and performs the same lift. Two additional simulations with a DES technique have confirmed improvement of the new profile. An additional blade simulation and post-processing shows a decreased level of unsteadiness for the optimized profile.
Article
In this paper, the three-dimensional flow field at downstream of lower pressure axial fans with circumferential forward-skewed, backward-skewed blades are successfully measured using a hot-wire CTA system at off-design conditions. An X hot-wire probe is used to obtain three-dimensional velocity. Based on measurement results, the evolution of three-dimensional flow in these fans according to flow rate, and the effect of circumferential skewed blades on transporting of boundary layer at off-design conditions are discussed. The measurement results show that as flow rate decreases, three-dimensional flow structure is changed by circumferential skewed blades; the obvious suppression of passage flow is found near shroud and hub region, which is benefitial to improve stable operation range of circumferential skewed rotors; as centrifugal force increases, blade radial force Fr of circumferential forward-skewed blade has a positive effect on pressure gratitude, and seems to be efficient to control the velocity and direction of boundary layer movement.
Article
The impact of leading edge sweep in an attempt to reduce shock losses and extend the stall margin on axial compressors has been extensively studied, however only a few studies have looked at understanding the impact of leading edge contouring on the performance of centrifugal compressors. The present work studies the impact of forward and aft sweep on the main and splitter blade leading edge of a generic high flow coefficient and high pressure ratio centrifugal compressor design and the impact on its overall peak efficiency, pressure ratio and operating range. The usage of aft sweep on the main blade led to an increase of the pressure ratio and efficiency, however it also led to a reduction of the stable operating range of the impeller analyzed. The forward sweep cases analyzed where the tip leading edge was displaced axially forward showed a slight increase in pressure ratio, and a significant increase on operating range. The impact of leading edge sweep on the sensitivity of the impeller performance to tip clearance was also studied. The impeller efficiency was found to be less sensitive to an increase of tip clearance for both aft and forward sweep cases studied. The forward sweep cases studied also showed a reduced sensitivity from operating range to tip clearance. The studies conducted on the splitter leading edge profile indicate that aft sweep may help to increase the operating range of the impeller analyzed by up to 16% while maintaining similar pressure ratio and efficiency characteristics of the impeller. The improvement of operating range obtained with the leading edge forward sweep and splitter aft sweep was caused by a reduction of the interaction of the tip vortex of the main blade with the splitter tip, and a reduction of the blockage caused by this interaction.
Article
In recent years, the different methods of stacking of the blade profiles from hub to the tip are studied frequently for axial fans and compressors. The blade profiles can be stacked on a radial line where the centers of gravity of the profiles coincide or the blade shape can be obtained by shifting the profiles in axial direction or in other directions (parallel or perpendicular to the chord line of the profile). In this manner, it is possible to make a modification on the 3D flow structure without making any change on the angle of the profile on 2D plane. In this study, the effect of the leading edge shape (related to the stacking process) on the performance and on the 3D flow structure of an axial fan rotor is handled. First, a reference fan where the centers of the gravity of the profiles situate on the radial stacking line is designed. This fan is optimized by means of CFD tools to give the maximum efficiency. The results of the CFD calculations are validated by experiments. Subsequently, new fans are obtained by shifting the profiles in axial direction to give the desired leading edge shape. The performances and the flow structures of the new fans are investigated and compared by means of CFD calculations. The CFD results show that, neither convex nor concave leading edge has positive effects on the aerodynamic performance of the axial fan studied. However, some potential advantages of the new geometries are observed especially at the hub and the tip region of the fans which are aerodynamically the most problematic regions of an axial turbomachine.
Thesis
Full-text available
Centrifugal fans are machines for moving a gas, such as air, by accelerating it radially outward in an impeller towards a surrounding casing. They are often used in heating and ventilation and air conditioning (HVAC) systems. But, the efficiency of the fan can be further improved by studying their detailed performance characteristics and modifying their geometry. Beyond its design point, the fan's performance begins to drop. In this context, a numerical simulation with the aim of predicting the flow field which passes through two axial fans in a tunnel or cage which contains electronic components has been carried out. The flow is three-dimensional and turbulent, such as can be encountered in the suction and bulge of air. The CFD Fluent code based on the finite volume method was used for turbulent flow using the k-ε model and the model. Our numerical simulations were presented in two different cases: with and without heat transfer. The results obtained our allowed a better understanding of the phenomenology of the flow in question, in particular the improvement of heat transfer by air discharge in chambers or tunnels contain components. Keywords: centrifuge fans, turbulent flow, CFD, performanc
Thesis
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The sound emission of low-pressure axial fans is substantially influenced by the fan blade geometry and the inflow conditions, induced by the fan installation system. However, the combined impact of these parameters has not yet been comprehensively investigated or understood. Hence the motivation for this thesis was to undertake a compact, systematic experimental study on the sound emission of axial fans with different blade geometry parameters under distorted inflow conditions. The impact of the fan blade design was investigated on the basis of nine fans with different blade loading distributions and different fan blade skew, but otherwise identical geometric parameters. Additionally, the effectiveness of leading edge serrations in reducing axial fan noise was examined with a parametric study of a generic flat-plate fan and eleven sets of fan blades with leading edge modifications that included single-sine (sinusoidal), double-sine and random amplitude leading edge serrations. The inflow conditions were altered to incorporate either an increased inflow turbulence intensity or an inhomogeneous inlet velocity profile. These conditions were realised by grids -- three turbulence grids and two velocity gradient grids -- that were mounted upstream of the fans. A large increase in the sound radiation with elevated tonal and broadband components was observed with the modified inflow conditions. Thereby, the sound emission of the forward-skewed fans showed a greater susceptibility for distorted inflow conditions than that of the backward-skewed fans. It was found that owing to the fan blade shape, the forward-skewed fans are prone to higher unsteady blade forces and increased pressure fluctuations on the fan blade leading edges. These factors determine the tonal and broadband sound radiation. For the backward-skewed and unskewed fans, dominant subharmonic components, originating from flow phenomena in the tip region, occurred under both free and distorted inflow conditions. All types of leading edge serrations achieved a sound reduction compared with the reference fan with straight leading edges -- for both free and distorted inflow conditions. The greatest reduction was observed for the sinusoidal leading edges, followed by the double-sine and random amplitude leading edges. The reduction was mainly dependent on the serration wavelength rather than the serration amplitude: the smaller the leading edge wavelength, the greater was the overall sound reduction. Overall, the investigations showed that the sound emission of low-pressure axial fans is highly dependent on the combined impact of the fan blade geometry and the inflow conditions. The findings contribute to a better understanding of the sound generation mechanisms in axial fans and show means for designing low-noise fans in complex cooling or ventilation systems.
Article
Purpose High speed axial flow pumps are widely used in aircraft fuel systems. Conventional axial flow pumps often generate radial secondary flows at partial-load conditions which influence the flow structure and form a “saddle-shaped” region in the Q-H curve that can destabilize the operation. Thus, the “saddle-shaped” Q-H region must be eliminated. The paper aims to discuss these issues. Design/methodology/approach The swept stacking method is often used for radial flow control in turbo-machinery impeller blade design. Hence, this study uses the swept stacking method to design a high speed axial flow pump. The detached eddy simulation method and experiments are used to compare the performance of a swept blade impeller in a high speed axial fuel pump with the original straight blade impeller. Both the pump performance and internal flow characteristics are studied. Findings The results show separation vortices in the impeller with the straight blade design at partial-load conditions that are driven by the rotating centrifugal force to gather near the shroud. The swept geometry provides an extra force which is opposite to the rotating centrifugal force that creates a new radial equilibrium which turns the flow back towards the middle of the blade which eliminates the vortices and the “saddle-shaped” Q-H region. The swept blade impeller also improves the critical cavitation performance. Analysis of the pressure pulsations shows that the swept blade design does not affect the stability. Originality/value This study is the initial application of swept blades for axial flow liquid pumps. The results show how the swept stacking changes the radial equilibrium of the high density, high viscosity flow and the effects on the mass transfer and pressure pulsations. The swept blade effectively improves the operating stability of high speed fuel pumps.
Article
The purpose of this study is to optimize the 1st stage of the transonic high pressure turbine (HPT) for enhancement of aerodynamic performance. Isentropic total-to-total efficiency is designated as the objective function. Since the isentropic efficiency can be improved through modifying the geometry of vane and rotor blade, lean angle and sweep angle are chosen as design variables, which can effectively alter the blade geometry. The sensitivities of each design variable are investigated by applying lean and sweep angles to the base nozzle and rotor, respectively. The design space is also determined based on the results of the parametric study. For the design of experiment (DoE), Optimal Latin Hypercube sampling is adopted, so that 25 evenly distributed samples are selected on the design space. Sequentially, based on the values from the CFD calculation, Kriging surrogate model is constructed and refined using Expected Improvement (EI). With the converged surrogate model, optimum solution is sought by using the Genetic Algorithm. As a result, the efficiency of optimum turbine 1st stage is increased by 1.07 % point compared to that of the base turbine 1st stage. Also, the blade loading, pressure distribution, static entropy, shock structure, and secondary flow are thoroughly discussed.
Article
This paper concentrates on the application of blade-end treatment to axial compressors by means of the optimization algorithm. The blade-end treatment reduces the end wall losses and extends the stable margin by modifying blade shape near the end wall region. It contains three types of passive flow control measures, i.e., the end-bend, end-dihedral and end-sweep treatment. Firstly, the effects of blade-end treatment were reviewed based on the open literatures published over the past 30 years. All of these effects essentially influence the compressor performance by changing the blading loading distributions in the streamwise or spanwise directions. There is a trade-off between the improved end wall flows and the deteriorated mid-span flows. It’s difficult to quantitatively apply these measures to achieve an optimal balance according to the traditional engineering experience. Optimization algorithm provides an efficient access to resolve this issue by automatically obtaining the utmost benefit. Secondly, an optimization example of NASA Stage 35 was conducted to validate against the summarized flow mechanisms. The optimal geometry parameters of cantilever stator vane near the end wall region were obtained by employing a surrogate model in conjunction with a genetic algorithm for optimization. Finally, optimization results indicated that the optimal vane blade featured an obvious combination of forward end-sweep, positive end-dihedral and end-bend. The stator total pressure losses were reduced with the blade-end treatment based on optimization method. A significant reduction of loss occurred near the shroud region, from the 80% span to the casing, while the performance was degraded within the mid-span region, approximately 50% to 80% span. The resulting mechanisms are consistent with the knowledge obtained from the literature review and this will provide meaningful guidance on the further compressor design process.
Article
A swept blade is one of the efficient ways to control unsteady flow and passively attenuate the noise radiated from an axial flow fan. Herein, an experimental study was conducted to investigate the effect of the blade sweep on the acoustic characteristics of an axial fan system operating under system resistances at an anechoic chamber facility. The noise level and fan performance of the axial fans with forward-swept, backward-swept, and straight blades were systematically measured using an acoustic fan tester, which can simultaneously measure them at various system resistance conditions. The variations in the overall A-weighted sound pressure levels (OASPLs) as a function of the static pressure and air flow rate, and acoustic spectra at the specific operating conditions were examined. The results showed that the forward-swept blade has superior aerodynamic and acoustic performance in a wide range of operating conditions to the other blades, except for the high system resistance corresponding to rotating stall conditions. In conclusion, the use of a forward-swept blade can achieve noise level reductions of 2–4 dBA under system resistance conditions since the amplitude of tonal noise associated with blade passing frequency is decreased.
Article
Centrifugal compressors have been widely used in light-duty turboshaft engines and automotive turbochargers. One of the essential requirements for centrifugal compressors in these applications is to have adequate stable flow range. As the pressure ratio increases, the stable flow range drops dramatically, and this highlights the importance of stability improvement for centrifugal compressors. This paper first gives a brief review on the measures available for extending the stable flow range, and then focuses on the realization of self-recirculating casing treatment for a centrifugal compressor. The design principles of the casing treatment were introduced. Then the simulations were conducted to design and optimize the geometry of the casing treatment, and finally an optimum case was selected for experimental validation. The experiment results indicate that the designed casing treatment improve the stable flow range by about 5 percent at design speed, while keeping the efficiency almost identical.
Article
This paper presents an overview of the characteristics potentially influencing the profile vortex shedding (PVS) phenomenon being relevant in noise and vibration of low-speed axial fan rotor blades. Dimensional analysis has been applied to explore the essential dimensionless quantities in a systematic and comprehensive manner. On this basis, limitations have been established, and simplifying assumptions have been set up in terms of PVS investigation. Groups of dimensionless characteristics playing a role in the semi-empirical model for predicting the PVS frequency were identified. The available semi-empirical model and its unique features related to the measurement evaluation methodology and Reynolds number dependence have been outlined. The presented comprehensive analysis provides guidelines from the perspective of transferability of the literature data on PVS from steady, isolated blade profile models to low-speed axial fan rotors. It also results in the formulation of objectives of future research related to PVS.
Article
The swept blade is one of the most efficient and practical techniques to alleviate the noise radiated from rotating fans. In this study, the swept effects of an automotive cooling fan on its acoustic characteristics were investigated using numerical and experimental approaches. Straight, forward, and backward-swept blades with a serrated shroud configuration were fabricated for the automotive cooling systems, and their performance and noise levels were measured. Moreover, measurements on the acoustic spectra were used to validate the numerical predictions. The pressure fluctuation on the cooling fan system, including the fan and shroud, was calculated from unsteady computational fluid dynamics simulation, and the acoustic signal at observer positions was predicted using an acoustic analogy. The swept effects in terms of noise reduction were compared for all three swept blades. Results indicate that the sweep angle in the forward direction can reduce the noise level of the cooling fan with a serrated shroud. Hence, the characteristics of the noise source caused by swept blades surrounding the shroud were identified in automotive cooling fans. Our findings in the present study provided significant insight into the acoustic characteristics depending on the direction of the swept blade, they can be used as a guide for designing a low-noise fan blade.
Article
Limitations still exist in the research on the effects of leading edge sweep on transonic centrifugal compressors. The lack of understanding of this mechanism also leads to confusion regarding the magnitudes of the performance changes that occur due to the use of leading edge sweep. According to a detailed numerical simulation verified by experiments on two different compressors, the effect and flow mechanism of leading edge sweep on transonic centrifugal compressors were investigated in detail to alleviate the confusion as clearly as possible. The conclusions show that leading edge sweep has different impacts on shock waves at different positions; a forward sweep leading edge can weaken the strength of the shock wave in front of the leading edge, while a backward sweep leading edge can weaken the strength of the passage shock wave that is swallowed in the blade passage. The improved compressor performance attributed to leading edge sweep is due to the weakening of the shock waves and the interaction between the shock waves and the secondary flow of the inducer tip. Furthermore, the intensity of the inducer tip secondary flow influences the potential for improved performance induced by using sweep leading edges in transonic centrifugal compressors. This study provides a valuable reference for applying leading edge sweep to existing transonic centrifugal compressors or future advanced centrifugal compressor designs.
Article
A series of numerical simulations were conducted in both a 1.5 stage axial compressor model and a simplified planar cascade model. The cases with different blade sweep schemes in the cascade model were simulated with free-slip endwalls to minimize the influence of endwall 3-D (three-dimensional) flows. The results obtained in the cascade model were discussed at first for getting a clear insight into the effects of blade sweep without other influences. And then the simulation results of the 1.5 stage compressor model were discussed accordingly. The discussions focused on the influences of different meridional flowpath designs, i.e. constant mid radius (CMR) design, constant outer radius (COR) design and constant inner radius (CIR) design, on the performance of the swept blade. The results showed that the most critical effect produced by blade sweep was attributed to the redistribution of local mass-flow-rate in blade spanwise direction. The change of meridional flowpath did not change the mass-flow-rate redistribution behaviors. However, the trends for local mass-flow-rate redistribution in compressor stage model showed some discrepancies compared with the results shown in the planar cascade model.
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Experimental studies were carried out in order to survey the performance and efficiency aspects of spanwise constant forward and backward sweep in axial flow rotors of low aspect ratio (AR) blading for incompressible flow, for part-load, near-design, and overload operational ranges. The experiments involved overall performance tests, laser Doppler velocimetry, and stationary total pressure probe measurements. The overall performance data and pitchwise averaged and resolved flow characteristics were evaluated in detail. For moderate and high flow rates, it was pointed out that positive or negative sweep tends to reduce or increase the blade load in the vicinity of the endwalls, respectively. It has been concluded that the loss-modifying effect of sweep can be judged by considering the three-dimensional viscous phenomena, and the influence of sweep on local blade efficiency depends on the balance of changes in blade load and losses. For low flow rates, forward sweep was found beneficial over the entire span from the aspect of improved stall margin and efficiency. The influence of AR on the performance reducing effect of sweep was studied on the basis of literature data.
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Sweep, when the stacking axis of the blade is not perpendicular to the axi-symmetric streamsurface in the meridional view, is often an unavoidable feature of turbine design. Although a high aspect ratio swept blade can be designed to achieve the same pressure distribution as an unswept design, this paper shows that the swept blade will inevitably have a higher profile loss. A modified Zweifel loading parameter, taking sweep into account, is first derived. If this loading coefficient is held constant, it is shown that sweep reduces the required pitch-to-chord ratio and so increases the wetted area of the blades. Assuming fully turbulent boundary layers, and a constant dissipation coefficient, the effect of sweep on profile loss is then estimated. A combination of increased blade area and a raised pressure surface velocity means that the profile loss rises with increasing sweep. The theory is then validated using experimental results from two linear cascade tests of highly loaded blade profiles of the type found in low pressure aeroengine turbines: one cascade is unswept, the other has 45 degrees of sweep. The swept cascade is designed to perform the same duty with the same loading coefficient and pressure distribution as the unswept case. The measurements show that the simple method used to estimate the change in profile loss due to sweep is sufficiently accurate to be a useful aid in turbine design.
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In the recent past, experimental studies have shown some advantages of blade lean and sweep in axial compressors. As most of the experimental results are combined with other features, it is difficult to determine the effect of individual parameters on the performance of the compressor. The present numerical studies are aimed at understanding the performance and three-dimensional flow pattern within and at the exit of swept and unswept rotors. Three rotors, namely, unswept, 20° forward swept, and 20° backward swept rotors, are analysed with a specific intention of understanding the three-dimensional flow pattern within the rotors and also the pattern of the blade boundary layer flow. The analysis was done using a fully three-dimensional viscous CFD code CFX-5. Results indicated a reduction in pressure rise with sweep. Backward sweep adversely affects the stall margin. Forward sweep changes the streamline pattern in such a way that the suction surface streamlines are deflected towards the hub and the pressure surface streamlines are deflected towards the casing. An opposite behaviour is observed in the backward swept rotors. High axial velocities reduce the secondary losses near the hub, resulting in a high pressure rise in forward swept rotor.
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The recent trend in using aerodynamic sweep to improve the performance of transonic blading has been one of the more significant technological evolutions for compression components in turbomachinery. This paper reports on the experimental and analytical assessment of the pay-off derived from both aft and forward sweep technology with respect to aerodynamic performance and stability. The single-stage experimental investigation includes two aft-swept rotors with varying degree and type of aerodynamic sweep and one swept forward rotor. On a back-to-back test basis, the results are compared with an unswept rotor with excellent performance and adequate stall margin. Although designed to satisfy identical design speed requirements as the unswept rotor, the experimental results reveal significant variations in efficiency and stall margin with the swept rotors. At design speed, all the swept rotors demonstrated a peak stage efficiency level that was equal to that of the unswept rotor. However, the forward-swept rotor achieved the highest rotor-alone peak efficiency. At the same time, the forward-swept rotor demonstrated a significant improvement in stall margin relative to the already satisfactory level achieved by the unswept rotor. Increasing the level of aft sweep adversely affected the stall margin. A three-dimensional viscous flow analysis was used to assist in the interpretation of the data. The reduced shock/boundary layer interaction, resulting from reduced axial flow diffusion and less accumulation of centrifuged blade surface boundary layer at the tip, was identified as the prime contributor to the enhanced performance with forward sweep. The impact of tip clearance on the performance and stability for one of the aft-swept rotors was also assessed.
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Comparative studies have been carried out on two axial flow fan rotors of controlled vortex design (CVD), at their design flowrate, in order to investigate the effects of circumferential forward skew on blade aerodynamics. The studies were based on computational fluid dynamics (CFD), validated on the basis of global performance and hot wire flow field measurements. The computations indicated that the forward-skewed blade tip modifies the rotor inlet condition along the entire span, due to its protrusion to the relative inlet flow field. This leads to the rearrangement of spanwise blade load distribution, increase of losses along the dominant part of span, and converts the prescribed spanwise blade circulation distribution towards a free vortex flow pattern. Due to the above, reduction in both total pressure rise and efficiency was established. By moderation of the radial outward flow on the suction side, being especially significant for non-free vortex blading, forward sweep was found to be particularly useful for potential reduction of near-tip loss in CVD rotors. Application of reliable CFD-based design systems was recommended for systematic consideration and control of both load-and loss-modifying effects due to non-radial blade stacking.
Chapter
The goals in performance that are presently put forward for advanced turbomachines can only be achieved by a careful consideration of the three-dimensional (3D) geometric effects and secondary flows. The two-dimensional (2D) cascade concept for blade design is nowadays useful only as a first guess of the optimum geometry.
Article
The data outlined in this paper show that swept-back blades are still fairly sparingly used in steam turbines, even though an initial rough estimate can now be made for each turbine of the gain in efficiency through the use of nozzle blades with build-up varying over the radius. Even taking into account the more complex technology of manufacture, it can prove to be technically and economically viable for several, and in some cases, many stages. The use of swept-back blades examined, does not mean that other ways of improving the efficiency of turbines (both of the flow section and of all channels, from the valves to the condenser inlet), or of different types of heat exchangers in turbine plants, have been exhausted. In some cases, taking physical investigations into account, it is desirable to do away with the usual ways of improving conventional solutions and designs. Particular attention must be paid to the need for aerodynamic improvement of turbine blades and the development of unusual, nonconventional profiles with a characteristic high level of efficiency within a given wide range of variation in operating parameters.
Article
The forward swept blades are expected to reduce the accumulation of boundary layer fluid resulting from the effect of centrifugal forces, near the tip re gion of an axial flow impeller blade. An experimental study was carried out on three sets of impeller blades with different forward sweep, keeping the blade element profile geometry the same. It is seen from the comparison of the overall performance of the impellers with swept and unswept blades that the swept blades operate more efficiently than the unswept blades, especially at low volume flows. Moreover, the blade element stall is delayed, if the blades are swept forward. An analysis of the experimental results shows that forward sweep effectively reduces the deteriorating effects of the radially outward boundary layer flow in the tip region. (A)
Article
An approximate method for including the effects of sweep and dihedral when designing axial-flow turbomachinery blading is presented. Blades are said to have sweep when the flow direction is not perpendicular to the spanwise direction, and dihedral when the blade surface is not normal to the surface of an end wall. It is shown that blade cross sections should be cut by sectioning surfaces that are tangent to the axisymmetric stream surfaces of the meridional flow, but that these cross sections should be viewed by looking parallel to the axis (stacking line) of the blade. When this is done the observed blade shapes and flow angle distributions are most nearly comparable to those obtained from two-dimensional cascade experiments and analyses. This approach is found to be inadequate at the blade ends, however, and an analytical method is presented which yields a wall correction for blade rows of semi-infinite span. For all practical variations of the parameters involved in the design of axial-flow compressors and turbines, the wall correction can be conveniently calculated from a set of approximate formulas presented in this paper. The importance of an adequate axisymmetric solution (method not presented herein) as the first step in the analysis is pointed out; many of the effects of sweep and dihedral are traceable to the skewness of the force and thickness-blockage fields of the axisymmetric model. Finally, the paper summarizes the blading design procedure and applies the present work within the framework of the overall design. As an example, the method is used to design a swept cascade; previously reported test results for a similar cascade tend to substantiate the validity of the design procedure, but experimental results for a direct comparison with the theory are not available.
Article
Isolated stationary airfoils of simple geometry were tested in incompressible flow in order to study the combined aerodynamic effects of sweep, spanwise changing circulation, and their combination. Endwall effects were excluded from the studies. The tool of study was computational fluid dynamics, supplemented with wind tunnel experiments involving laser Doppler anemometry and flow visualisation. The computational results suggested unloading effects due to leading and trailing edge sweep. A model has been proposed for the description of such effects. It has been rendered probable that harmonization of the sweep with the spanwise circulation gradient results in reduction of the fluid pathline length on the suction side, giving a potential for reduction of profile losses.
Article
This paper presents a new methodology for quantifying compressor endwall blockage and an approach, using this quantification, for defining the links between design parameters, flow conditions, and the growth of blockage due to tip clearance flow. Numerical simulations, measurements in a low-speed compressor, and measurements in a wind tunnel designed to simulate a compressor clearance flow are used to assess the approach. The analysis thus developed allows predictions of endwall blockage associated with variations in tip clearance, blade stagger angle, inlet boundary layer thickness, loading level, loading profile, solidity, and clearance jet total pressure. The estimates provided by this simplified method capture the trends in blockage with changes in design parameters to within 10 percent. More importantly, however, the method provides physical insight into, and thus guidance for control of, the flow features and phenomena responsible for compressor endwall blockage generation.
Article
The influence of two stacking lines, namely sweep and dihedral, has been investigated in a linear compressor cascade. Both types of blade considered are symmetric about midspan and consist of a straight central section with either swept or dihedral sections toward the endwalls. Two types of experiment have been carried out. First, a parametric study was performed by changing both the magnitude and the extent of the sweep or dihedral. In the case of swept blades, those with forward sweep (SWF), for which the stacking line is swept in the upstream direction toward the endwall, were found to have better performance than backward-swept blades. Subsequently, four sets of SWFs were compared. In the case of dihedral blades, it is well known that the dihedral is advantageous when the angle between the suction surface and the endwall is obtuse, i.e., positive dihedral. Thus, four sets of positive dihedral blades (DHP) were compared. In both SWF and DHP blades, those configurations that have better efficiency than straight blades were determined. Second, detailed three-dimensional measurements inside the blade passage were performed in the cases that showed the best performance in the parametric study. Both SWF and DHP showed significant effects on the flowfield. In the SWF case, a vortex, which has the opposite sense to the passage vortex, was observed in the forward portion inside the blade passage. This vortex supplies high-energy fluid to the endwall region and reduces the corner stall. The secondary flow is greatly reduced. In the DHP, the blade loading was reduced at the endwall and increased at the midspan. Reduction of the corner stall and the secondary flow was also observed.
Article
A qualitative discussion of the geometrical properties of dihedral and sweep in turbo-machine blade rows is presented with a discussion of the consequent fluid dynamic effects upon blade to blade and meridional flows. Results of blade to blade calculations for four heavily loaded turbine cascades of infinite span and various angles of sweep are presented. Annulus end wall interference effects on blade to blade flow are briefly discussed with reference to previous work. The principal analytical contribution is a development of actuator disc theory to deal with meridional flows for swept cascades and for blade rows in cylindrical annuli.
Article
Tip clearance flows have a major impact on both performance and stability of high-pressure compressors (HPC). The purpose of this paper is to underline how tip clearance variations affect the matching of compressor stages, hence modifying the efficiency and stall margin of the compressor. The practical application covered by the scope of this article is a modern highly loaded HPC dedicated to civil aircraft propulsion. The first part of the paper gives a very simple overview of stagewise matching in multistage compressors. Also, this part introduces the subject of the impact of tip clearance on stage matching. The second part of the paper illustrates the effect of increased rear block clearances on performance and stability, using some available experimental data. Finally, it is shown that three-dimensional multi-stage calculations can predict the effect of tip clearance variations on stage pressure-rise characteristics and on stage matching. This validated numerical tool therefore allows the aerodynamic design to be adjusted to the range of in-service clearances, thus providing a more robust compressor design.
Article
The paper discusses the use of sweep as a remedial strategy to control the aerodynamic limits in low-speed axial fan rotors. In this respect, the present work contributes to the understanding of the potential effect of blade lean on the shifting of the rotor stall margin. Numerical investigations have been undertaken on highly loaded fans of non-free vortex design, with the ideal total head rise coefficient typical of the industrial application range. Two rotors with identical nominal design parameters and, respectively, with 35° forward swept blades and unswept blades have been studied. The investigation has been carried out using an accurate in-house developed multilevel parallel finite element RANS solver, with the adoption of a non-isotropic two-equation turbulence closure. The pay-off derived from the sweep technology has been assessed with respect to the operating range improvement. To this end, the flow structure developing through the blade passages and downstream of the rotors, as well as loss distributions, have been analysed at design and near-peak pressure operating conditions. The analyses of three-dimensional flow structures showed that, sweeping forward the blade, the non-free vortex spanwise secondary flows are attenuated, and a control on the onset of stall is recovered. Moreover, the swept rotor features a reduced sensitivity to leakage flow effects. Consequently, it operates more efficiently approaching the throttling limit.
Article
The application of improved blade tip geometries is studied with the aim of identifying an effective design concept for industrial fan passive noise control. The concept developed optimizes a datum blade by means of profiled endplates at the tip, reducing fan noise by changing the tip leakage flow behaviour. Experimental and computational investigations have been carried out on a family of axial fans, in fully ducted configuration, to establish the aerodynamic merits of the proposed blade tip design concept. The flow mechanisms in the fan tip region are correlated to specific blade design features that promote a reduction of the fan aero-acoustic signature in both tonal and broadband noise components. The tip vortical flow structures are characterized, and their role in creation of overall stage acoustic emissions clarified. The reported research identifies modification of tip geometry as markedly affecting the multiple vortex behaviour of blade tip leakage flow by altering the near-wall fluid flow paths on both blade surfaces. Blade tip endplates were also demonstrated to influence the rotor loss behaviour in the blade tip region. Improvement of rotor efficiency was correlated to the control of tip leakage flows.
Article
In the early development of gas turbines, many empirical design rules were used; for example in obtaining fluid deflection using the deviation from blading angles, in the assumption of zero radial velocities (so-called radial equilibrium) and in expressions for clearance loss (the Lakshminarayana formulas). The validity of some of these rules, and the basic fluid mechanics behind them, is examined by use of modern ideas and computational fluid dynamics (CFD) codes. A current perspective of CFD in design is given, together with a view on future developments.
Article
The paper describes an experimental investigation of the stator hub and blade flow in two different stators of a highly loaded single-stage axial-flow low-speed compressor. The first stator (A) is a conventional design with blades of rectangular planform. The second stator (K) is an unconventional, more advanced design with blades of a special planform, characterized by an aft-swept leading edge with increasing sweep angle towards hub and casing. The experimental results show that stator K exhibits a much better hub performance than stator A, finally leading to a better overall performance of stage K compared to stage A. The better hub performance of stator K is, primarily, the result of a planform effect of the newly introduced blades with an aft-swept leading edge and the aerodynamics of an aft-swept wing.
Article
This paper describes the introduction of 3D blade designs into the core compressors for the Rolls-Royce Trent engine with particular emphasis on the use of sweep and dihedral in the rotor designs. It follows the development of the basic ideas in a university research project, through multistage low-speed model testing, to the application to high pressure engine compressors. An essential element of the project was the use of multistage CFD and some of the development of the method to allow the designs to take place is also discussed. The first part of the paper concentrates on the university-based research and the methods development. The second part describes additional low-speed multistage design and testing and the high-speed engine compressor design and test.
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Article
The available literature on aerodynamic and acoustic properties of axial fans with swept blades is presented and discussed with particular emphasis on noise mechanisms and the influence of high-intensity inlet turbulence on 'excess' noise. The acoustic theory of Kerschen and Envia for swept cascades is applied to the problem of axial fan design. These results are compared to available data and a provisional model for specifying sweep angles is presented. The aerodynamic performance theory for swept-bladed rotors of L.H. Smith and H. Yeh is adapted for use in designing low-speed axial fans. Three prototype fans were designed using the resultant computer codes. One is a baseline fan with blade stocking lines radially oriented, and two are fans having swept blades of increasingly greater forward sweep.
Article
A numerical analysis of the flow in axial flow fans with skewed blades has been conducted to study the three-dimensional flow phenomena pertaining to this type of blade shape. The particular fans have a low pressure rise and are designed without stator. Initial studies focused on blades skewed in the circumferential direction, followed by investigations of blades swept in the direction of the blade chord. A Navier-Stokes code was used to investigate the flow. The simulation results of several fans were validated experimentally. The three-dimensional velocity field was measured in the fixed frame of reference with a triple sensor hot-film probe. Total pressure distribution measurements were performed with a fast response total pressure probe. The results were analyzed, leading to a design method for fans with swept blades. Forward swept fans designed accordingly exhibited good aerodynamic performance. The sound power level, measured on an acoustic fan test facility, improved.
Article
An experimental analysis using three-dimensional laser Dopplervelocimetery (LDV) measurements and computational analysis usingthe Reynolds stress model of the commercial code, FLUENT, wereconducted to give a clear understanding on the structure of thetip leakage flow in a forward-swept axial-flow fan operating atthe peak efficiency condition, and to emphasize the necessity ofusing an anisotropic turbulence model for the accurate predictionof the tip leakage vortex. The rolling-up of the tip leakage flowwas initiated near the position of the maximum static pressuredifference, which was located at approximately 12% axial tipchord downstream from the leading edge of the blade, and developedalong the centerline of the pressure trough on the casing. Areverse flow between the blade tip and the casing due to the tipleakage vortex acted as a blockage on the through-flow. As aresult, high momentum flux was observed below the tip leakagevortex. As the tip leakage vortex proceeded to the aft part of theblade passage, the strength of the tip leakage vortex decreaseddue to the strong interaction with the through-flow and the casingboundary layer, and the diffusion of the tip leakage vortex byhigh turbulence. Through the comparative study of turbulencemodels, it was clearly shown that an anisotropic turbulence model,e.g., Reynolds stress model, should be used to predict reasonablyan anisotropic nature of the turbulent flow fields inside the tipleakage vortex. In comparison with LDV measurement data, thecomputed results predicted the complex viscous flow patternsinside the tip region in a reliable level.
Article
To study blade sweep effects on loss production and energy transfer a numerical analysis has been undertaken on three 10-bladed fan rotors with low-pressure ratio and different sweep. Pressure ratio and sweep are in the typical range of propfans. The results of two rotors with 30-deg forward- and backward-swept blades are compared to those of an unswept propfan rotor. The geometry of the profiles of the swept rotors on each cylindrical position is identical with that of the unswept rotor. The numerical analysis shows that the total pressure ratio of the swept rotors is lower than that of the unswept rotor over the whole span. Only in the vicinity of the hub is the pressure ratio higher in the case of the forward-swept rotor. The decrease of the total pressure ratio at midspan, by sweeping the rotor, is due to an oblique shock with lower preshock Mach number and an increase of the boundary layer at the suction side. Sweeping the rotor has the effect to induce strong radial pressure gradients at the leading edge. The deflection of the flow towards and off the end-walls due to this pressure gradient has the effect of a changed diffusion and expansion of the flow in the vicinity of the end-walls. The emphasis of this article is not to directly provide rotors with better pressure ratios and efficiencies, but to explain the basic effects by sweeping fan-rotors as references for improving the rotor-aerodynamic.
Article
Three-dimensional laser Doppler anemometer (LDA) measurements were carried out downstream of isolated axial fan rotors of non-free vortex design in order to investigate the role of radial velocity components in design. The structure of secondary flows due to non-free vortex operation was studied in detail. It is pointed out that the tangential gradient of radial velocity at midspan is nearly in direct proportion with the spanwise gradient of ideal total head rise prescribed in design. Design criteria have been established for the neglect of torsion of stream surface segments inside the blading. A linear relationship was proposed in order to estimate the pitch-averaged radial velocities at the rotor exit. Using this relationship, a proposal has been put forward for taking the radial velocity components into account in non-free vortex design with the assumption of conical stream surfaces through the blading.