Article

An Approximate Analysis and Prediction Method for Tip Clearance Loss in Axial Compressors

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Abstract

A simple model for loss created by the rip clearance flow in axial compressors is presented, based on an experimental program performed in conjunction with the Dawes three-dimensional Navier-Stokes calculation method. The principal mechanism of loss (entropy creation) caused by tip leakage flow has been established to be the mixing of flows of similar speeds but different direction. Calculations show that relative motion of the endwall relative to the rip has a small effect on clearance flow. The simple model correctly predicts the magnitude of tip clearance loss and the trend with changes of tip clearance for the cascade tested. For a given geometry the loss is almost exactly proportional to the ratio of tip clearance to blade span; the loss directly associated with the clearance is smaller than often assumed. The simple model for tip clearance loss has been expressed in terms of conventional nondimensional design variables (for example: solidify, aspect ratio, flow coefficient, loading coefficient) and from these the contribution to the overall loss of efficiency caused by rip leakage flow is conveniently represented. The trends are illustrated far a number of possible compressor design choices. Blade row loss increases more slowly than blade loading (for example, diffusion factor). As a result the decrement in stage efficiency associated with clearance flow decreases as the stage loading is raised in the practical range of flow and loading coefficients.

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... The model was tested on several inlet guide vanes with different geometric parameters and different profiles. The influence of radial clearance parameters on the level of losses in the stage of the axial compressor was considered in works [10,11]. In [10], losses in the end clearance of a single-stage compressor are analyzed for different parameters at the inlet duct. ...
... In [10], losses in the end clearance of a single-stage compressor are analyzed for different parameters at the inlet duct. The authors of [11] proposed a technique for calculating the total pressure loss in the end clearance of the rotor wheel of an axial compressor. ...
... An analysis of the result of works [3][4][5][6][7][8][9][10][11] has shown that studies of the mutual influence of the rotor and stator elements in the compressor is an actual problem. At the same time, the tasks associated with detailed investigation of the losses in individual blade rows of the compressor stage and their interaction remains unresolved. ...
Article
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Purpose: the purpose of this work is to investigate the total pressure losses in the input guide vanes, in the rotor wheel and the guide vanes of the axial compressor stage taking into account their gas-dynamic interference. Methods: the study was carried out by numerical simulation of three-dimensional flow in the stage of an axial compressor. An unstructured adaptive computational grid is constructed. The gas dynamic calculation of the flow in the stage of the axial compressor is performed using the Navier-Stokes system of equations, which was closed by the SST turbulent model. Results: a series of gas-dynamic flow calculations was performed at different values of the axial velocity at the inlet to the compressor stage. The circuit velocity at the peripheral radius in the calculated mode was u= 238.64 m/s. The coefficient of velocity at the entrance to the stage varied in the range =0.35…0.7. Based on the calculation results, the dependences of the total pressure loss coefficients from the input velocity coefficient for various elements of the stage of the axial compressor were constructed. Analysis of the results of the study shows that the greatest contribution to the overall balance of total pressure losses is made by losses in the guide vanes. Discussion: losses in the guide vanes increase due to the gas-dynamic interference of the rotor wheel and the guide vanes. The mutual influence of the blade rows of the rotor wheel and the guide vanes leads to a significant transformation of the velocities and pressures in the interblade channels. As a consequence, there is a redistribution of losses caused by circumferential and radial flow irregularity, end-flow and centrifugal forces. It can be expected that a reduction in the level of losses in the step of the axial compressor can be achieved by influencing the boundary layer in the end clearance of the rotor wheel.
... The tip clearance flow and the tip leakage vortex make up the 'tip leakage flow'. Tip leakage flows are generally detrimental to the stability and efficiency of compressors 1 (Peacock, 1982;Freeman, 1985;McDougall et al., 1990;Storer and Cumpsty, 1994;Graf et al., 1998). This work considers the tip leakage flow in low-speed compressors, but many of the concepts are also relevant to high-speed machines Suder and Celestina, 1996;Chima, 1998;Gerolymos and Vallet, 1999). ...
... Compressor blades are normally too thin for the flow to re-attach, so the flow emerges from the clearance as a wall-bounded jet with separated flow near the blade. This emerging flow is not aligned with the core flow so there is a shear layer between the two ('5'), which generates large losses (Storer and Cumpsty, 1994). The tip leakage flow then meets the inflow and separates from the casing ('6'), before rolling up to form the tip leakage vortex ('7'). ...
... Both of these effects further increase the loss and blockage associated with the vortex as stall approaches. Storer and Cumpsty (1994), Khalid et al. (1999) and Suder (1998) found that tip leakage flow causes most of the loss and blockage in the tip region, and Howard et al. (1994) show that the tip leakage flow can also cause additional losses in downstream blade rows. Hoeger et al. (2000) show that the blockage caused by the tip leakage flow reduces through-flow velocities near the casing. ...
Thesis
Aircraft jet engines must operate in a stable manner at all times. One source of instability is compressor stall. Stall problems can be reduced by machining cavities into the compressor casing adjacent to the rotor blades. This ‘casing treatment’ is the focus of the present work. Two treatment configurations are tested: circumferential grooves cut into the casing above the rotor blades, and axial slots cut into the casing adjacent to the rotor blade leading edges. The performance of a single casing groove is measured at different axial locations over the blade tips. For the first time, it is shown that there are two locations where compressor stability is maximised; near the leading edge and near mid-chord. The interaction between the groove and the compressor flow field is then studied. It is found that when located near the leading edge, the groove has a strong interaction with the near-casing flow and tip leakage vortex, but when located near mid-chord, the interaction is more subtle and less damaging to efficiency. Since the groove works well in both locations, it is concluded that manipulating the tip leakage vortex is not critical for improving compressor stability. Different groove numbers and cross-sections are then tested. For multiple grooves, the effi- ciency reduction is the sum of the constituent grooves, while the stall margin improvement is less than this sum. A simple square-section groove is found difficult to improve on, although in certain circumstances, a new ‘intermittent groove’ geometry is beneficial. The performance of axial slots is then investigated. Different slot shapes are tested and the results added to previous work to suggest an optimum slot geometry. A computational flow study shows that large variations in blade loading occur as the blades pass the slots, which could cause noise and vibration. It is found that while the flow inside the slot is principally a quasi-steady recirculation, the interaction between the slots and blades is highly unsteady, and this unsteadiness should not be neglected in design. In general, it is found that casing treatments that generate large stability improvements cause large efficiency losses. It is shown for the first time that the performance of casing grooves can be seriously reduced by changes in the stall inception mechanism. Maximum performance is achieved when the treated compressor stalls with a spike inception. Models from the literature are tested, but do not predict the stall inception mechanism well, which makes predicting the performance of casing grooves in a given compressor hard. Finally, it is shown that designing the blades and casing treatment as a unit may improve compressor performance. Access though DOI number, or: https://www.repository.cam.ac.uk/handle/1810/252194
... To maximise the fan blade performance and reduce engine fuel con-sumption, tip clearances are designed as small as possible. This reduces leakage flow through the gap between the tip of the blade and the stationary casing which, upon entering the blade passage, generates a significant amount of losses and reduces blade performance [1][2][3]. To allow minimum clearances while also preventing damage to the blade due to contact with the casing, a 'tip rubbing' solution is used, where a sacrificial abradable liner is added to the casing. ...
... Realistic casing profiles, with steps up and downstream of the blade, and the implications, are not generally investigated. The trend of increasing overall clearance reducing blade performance and operability is well known [1][2][3], however work in this area focuses on varying clearance with simple crops of the blade tip. This paper aims to develop a detailed understanding of a modern fan blade's tip aerodynamics for datum clearance, before investigating the impact that variations to the casing profile caused by rubbing have on its performance. ...
... The importance of the chordwise distribution of tip leakage has not been discussed in detail in the literature. Previous researchers have acknowledged that chordwise variations in leakage flow occur and that it is the tip pressure field that controls this [2]. However the impact that this can have on the tip leakage aerodynamics has not been highlighted. ...
Article
During engine operation, fan casing abradable liners are worn by the blade tip, resulting in the formation of trenches. This paper describes the influence of these trenches on the fan blade tip aerodynamics. A detailed understanding of the fan tip flow features for cropped and trenched clearances is first developed. A parametric model is then used to model trenches in the casing above the blade tip and varying blade tip positions. It is shown that increasing clearance via a trench reduces performance by less than increasing clearance through cropping the blade tip. A response surface method is then used to generate a model that can predict fan efficiency for a given set of clearance and trench parameters. This model can be used to influence fan blade design and understand engine performance degradation in service. It is shown that an efficiency benefit can be achieved by increasing the amount of tip rubbing, leading to a greater portion of the tip clearance sat within the trench. It is shown that the efficiency sensitivity to clearance is biased toward the leading edge (LE) for cropped tips and the trailing edge (TE) for trenches.
... Many studies show that a larger gap height increases the tip-leakage flow losses [1,4,5]. A bigger gap height leads to a smaller pressure rise, efficiency and operating range of the compressor [6]. ...
... The effect of the casing boundary layer thickness on the tip-leakage flow has been rarely studied. Storer and Cumpsty [5] have shown that a thicker boundary layer on the casing increased the total pressure loss coefficient without changing its evolution with the gap height. ...
... In the latter case, for small gaps, Denton [8] and then Storer and Cumpsty [5] adopted an alternative approach which consists in modelling the tip-leakage flow as a jet in crossflow in order to investigate the total pressure losses. ...
Conference Paper
The tip-leakage flow has detrimental effects on the performance of compressors. In this paper the effects of gap height and incoming casing boundary layer thickness are analyzed. Velocity and total pressure measurements are carried out in a plane behind the trailing edge of an isolated fixed blade. The total pressure loss is decomposed in a vortex loss and a wake loss. It appears that the increase of total pressure losses with the gap height comes essentially from the vortex part. This observation motivated the development of a model based on an analogy with a jet in crossflow to estimate the tip-leakage vortex circulation. The predictions of this model are consistent with the experimental data for gaps smaller than 4 % of chord.
... To maximise the fan blade performance and reduce engine fuel consumption, tip clearances are designed as small as possible. This reduces leakage flow through the gap between the tip of the blade and the stationary casing which, upon entering the blade passage, generates a significant amount of losses and reduces blade performance [1][2][3]. To allow minimum clearances while also preventing damage to the blade due to contact with the casing, a 'tip rubbing' solution is used, where a sacrificial abradable liner is added to the casing. ...
... Realistic casing profiles, with steps up and downstream of the blade, and the implications, are not generally investigated. The trend of increasing overall clearance reducing blade performance and operability is well known [1][2][3], however work in this area focuses on varying clearance with simple crops of the blade tip. This paper aims to develop a detailed understanding of a modern fan blade's tip aerodynamics for datum clearance, before investigating the impact that variations to the casing profile caused by rubbing have on its performance. ...
... The importance of the chordwise distribution of tip leakage has not been discussed in detail in the literature. Previous researchers have acknowledged that chordwise variations in leakage flow occur and that it is the tip pressure field that controls this [2]. However the impact that this can have on the tip leakage aerodynamics has not been highlighted. ...
Conference Paper
Full-text available
During engine operation fan casing abradable liners are worn by the blade tip, resulting in the formation of trenches. This paper investigates the influence of these trenches on the fan blade tip aerodynamics. A detailed understanding of the tip flow features for the fan blade under investigation is developed. A parametric model is then used to model trenches in the casing above the blade tip. It is shown that increasing clearance via a trench reduces performance by less than increasing clearance through cropping the blade tip. A response surface method is then used to generate a model that can predict fan efficiency for a given set of clearance and trench parameters. It is shown that the efficiency sensitivity to clearance is greater for cropped tips than trenches, and is biased towards the leading edge for cropped tips, and the trailing edge for trenches.
... To maximise the fan blade performance and reduce engine fuel con-sumption, tip clearances are designed as small as possible. This reduces leakage flow through the gap between the tip of the blade and the stationary casing which, upon entering the blade passage, generates a significant amount of losses and reduces blade performance [1][2][3]. To allow minimum clearances while also preventing damage to the blade due to contact with the casing, a 'tip rubbing' solution is used, where a sacrificial abradable liner is added to the casing. ...
... Realistic casing profiles, with steps up and downstream of the blade, and the implications, are not generally investigated. The trend of increasing overall clearance reducing blade performance and operability is well known [1][2][3], however work in this area focuses on varying clearance with simple crops of the blade tip. This paper aims to develop a detailed understanding of a modern fan blade's tip aerodynamics for datum clearance, before investigating the impact that variations to the casing profile caused by rubbing have on its performance. ...
... The importance of the chordwise distribution of tip leakage has not been discussed in detail in the literature. Previous researchers have acknowledged that chordwise variations in leakage flow occur and that it is the tip pressure field that controls this [2]. However the impact that this can have on the tip leakage aerodynamics has not been highlighted. ...
Conference Paper
Tip leakage flow has a significant impact on fan blade performance. It is therefore critical to understand the effect of both the uniform and non-uniform tip clearance variations that occur in engines. Firstly, a detailed understanding of the flow physics present in the tip region of a modern axial flow fan blade is developed using RANS CFD. Analysis is undertaken of the tip leakage flow structure, tip vortex characteristics and tip leakage mass flow distribution. It is shown how flow acceleration upstream of the pressure surface passage shock causes an unusual chord-wise tip leakage distribution with ’reverse’ tip leakage near the leading edge. The importance of the leakage distribution on the tip flow behaviour is explained. The sensitivities of blade efficiency to both uniform and non-uniform tip clearances are studied, and it is shown how increasing tip clearances towards the leading edge has the greatest impact on efficiency.
... That is why knowing its influence on the tip-leakage flow is essential. Previous works have shown that larger gaps lead to higher total pressure losses Lakshminarayana and Horlock, 1963;Storer and Cumpsty, 1994;Doukelis et al., 1998a) and reduce the pressure rise capabilities and the surge margin of compressors (Saathoff, 2001;Kameier and Neise, 1997;Smith and Cumpsty, 1982). However most of these studies focus on a global analysis of total pressure losses and give little information on the underlying mechanisms of loss generation. ...
... A computational analysis of its influence on the tip-leakage flow in a compressor rotor and a cascade was performed by Brandt et al. (2002), yielding that a thicker inlet boundary layer leads to higher total pressure losses. Similar results were obtained experimentally by Storer and Cumpsty (1994). In addition, when the boundary layer thickness is increased, not only the tip-leakage vortex detaches from the blade closer to its leading edge but its trajectory also yields more inclination about the blade passage. ...
... To sum up, entropy production is not only found in the jet mixing layer, as previously observed by Storer and Cumpsty (1994), but also in the wake and in a mixing layer coming from the separation of the boundary layer at the casing wall induced by the tip-leakage vortex. ...
Thesis
Full-text available
To reduce the environmental footprint of aviation, it is essential to develop lighter and more fuel efficient turbojet engines. This can be achieved by increasing the efficiency of axial compressors and by extending their range of stable operation. The main limitations of axial compressors are caused by the flow going through the gap between the tip of rotor blades and the casing, also known as tip-leakage or tip-gap flow. This study aims at providing a detailed description of the tip-leakage flow, in order to analyze its sensitivity to design parameters and to control devices based on air injection from the casing. The setup is composed of a single blade, set orthogonal to a flat plate that plays the role of the casing wall. This simplification makes optical measurements easier and allows a fine tuning of the gap height, the incoming boundary layer thickness at the casing wall and the blade loading. The analysis is based on experiments conducted in a low-speed wind tunnel that are complemented by CFD calculations. The tip-leakage flow is primarily driven by the gap height. The incoming boundary layer thickness and the blade loading also have a notable effect on the lateral postion of the tip-leakage vortex. The distribution of local entropy creation rate has been used to identify areas of losses in the flow. Moreover, the total pressure losses have been decomposed in two terms identified as a vortex loss and a wake loss. This vortex loss drives the increase of total pressure losses with the gap height. An empirical model has been developped to predict the evolution of the tip-leakage vortex circulation and of the total pressure losses with respect to the gap height. A steady injection of air from the casing has been evaluated, using normal jets in the gap. With this control strategy, the tip-leakage vortex tends to be closer to the blade, which could lead to an extension of the range of stable operation for axial compressors. In addition to that, the control device smoothes out the axial vorticity field in the tip-leakage vortex, which could be interesting to reduce rotor-stator interactions. However, this control strategy leads to higher total pressure losses and is less effective with larger gaps.
... That is why knowing its influence on the tip-leakage flow is essential. Previous works have shown that larger gaps lead to higher total pressure losses (Flachsbart 1931;Lakshminarayana and Horlock 1963;Storer and Cumpsty 1994;Doukelis et al. 1998a) and reduce the pressure rise capabilities and the surge margin of compressors (Saathoff 2001;Kameier and Neise 1997;Smith and Cumpsty 1982). However, most of these studies focus on a global analysis of total pressure losses and give little information on the underlying mechanisms of loss generation. ...
... A computational analysis of its influence on the tip-leakage flow in a compressor rotor and a cascade was performed by Brandt et al. (2002), yielding that a thicker inlet boundary layer leads to higher total pressure losses. Similar results were obtained experimentally by Storer and Cumpsty (1994). In addition, when the boundary layer thickness is increased, not only the tip-leakage vortex detaches from the blade closer to its leading edge but its trajectory also yields more inclination about the blade passage. ...
... The second approach, initiated by Rains (1954), assimilates the tip-leakage flow as a jet in cross-flow. For instance, Denton (1993) with Storer and Cumpsty (1994) built a simple model based on the assumption that the jet flow exiting the gap completely mixes-out with the main flow. Their model gives a good estimation of the increment of total pressure losses with gap height. ...
Article
Full-text available
Improving pressure rise capabilities of axial compressors requires an in-depth understanding of the losses produced in the tip leakage region. Here, a generic setup that magnifies the tip region of an isolated, non-rotating blade is used with the objectives of describing the main flow components and evaluating the related sources of loss. The flow at the tip is structured by the jet flow out of the gap which, under the effect of the main stream, rolls-up into a tip-leakage vortex. The current setup is characterized by the tip gap height and the thickness of the incoming boundary layer at the casing, here a flat plate, for a given incidence of the blade. Measurements are performed using LDV and a multi-port pressure probe. Variations in the tip-leakage flow are found to be mainly driven by gap height. A small, intermediate and large gap regimes are more specifically found, with threshold around 4% and 8% of gap to chord ratio for the present setting. The incoming boundary layer thickness is shown to provoke a notable effect on the vortex lateral position and total pressure losses. The local entropy creation rate is computed from LDV data and used to identify the sources of loss in the flow. A decomposition into wake and vortex losses is further proposed, allowing to relate the contributions of the various flow components to the overall losses. An empirical model of the formation of the tip vortex is developed to account for the increased losses as a function of gap height. The model provides a useful mean for the practical approximation of the gap sensitivity of pressure losses. Graphic abstract
... That is why knowing its influence on the tip-leakage flow is essential. Previous works have shown that larger gaps lead to higher total pressure losses Lakshminarayana and Horlock, 1963;Storer and Cumpsty, 1994;Doukelis et al., 1998a) and reduce the pressure rise capabilities and the surge margin of compressors (Saathoff, 2001;Kameier and Neise, 1997;Smith and Cumpsty, 1982). However most of these studies focus on a global analysis of total pressure losses and give little information on the underlying mechanisms of loss generation. ...
... A computational analysis of its influence on the tip-leakage flow in a compressor rotor and a cascade was performed by Brandt et al. (2002), yielding that a thicker inlet boundary layer leads to higher total pressure losses. Similar results were obtained experimentally by Storer and Cumpsty (1994). In addition, when the boundary layer thickness is increased, not only the tip-leakage vortex detaches from the blade closer to its leading edge but its trajectory also yields more inclination about the blade passage. ...
... To sum up, entropy production is not only found in the jet mixing layer, as previously observed by Storer and Cumpsty (1994), but also in the wake and in a mixing layer coming from the separation of the boundary layer at the casing wall induced by the tip-leakage vortex. ...
Thesis
To reduce the environmental footprint of aviation, it is essential to develop lighter and more fuel efficient turbojet engines. This can be achieved by increasing the efficiency of axial compressors and by extending their range of stable operation. The main limitations of axial compressors are caused by the flow going through the gap between the tip of rotor blades and the casing, also known as tip-leakage or tip-gap flow. This study aims at providing a detailed description of the tip-leakage flow, in order to analyze its sensitivity to design parameters and to control devices based on air injection from the casing. The setup is composed of a single blade, set orthogonal to a flat plate that plays the role of the casing wall. This simplification makes optical measurements easier and allows a fine tuning of the gap height, the incoming boundary layer thickness at the casing wall and the blade loading. The analysis is based on experiments conducted in a low-speed wind tunnel that are complemented by CFD calculations.The tip-leakage flow is primarily driven by the gap height. The incoming boundary layer thickness and the blade loading also have a notable effect on the lateral postion of the tip-leakage vortex. The distribution of local entropy creation rate has been used to identify areas of losses in the flow. Moreover, the total pressure losses have been decomposed in two terms identified as a vortex loss and a wake loss. This vortex loss drives the increase of total pressure losses with the gap height. An empirical model has been developped to predict the evolution of the tip-leakage vortex circulation and of the total pressure losses with respect to the gap height. A steady injection of air from the casing has been evaluated, using normal jets in the gap. With this control strategy, the tip-leakage vortex tends to be closer to the blade, which could lead to an extension of the range of stable operation for axial compressors. In addition to that, the control device smoothes out the axial vorticity field in the tip-leakage vortex, which could be interesting to reduce rotor-stator interactions. However, this control strategy leads to higher total pressure losses and is less effective with larger gaps.
... That is why knowing its influence on the tip-leakage flow is essential. Previous works have shown that larger gaps lead to higher total pressure losses Lakshminarayana and Horlock, 1963;Storer and Cumpsty, 1994;Doukelis et al., 1998a) and reduce the pressure rise capabilities and the surge margin of compressors (Saathoff, 2001;Kameier and Neise, 1997;Smith and Cumpsty, 1982). However most of these studies focus on a global analysis of total pressure losses and give little information on the underlying mechanisms of loss generation. ...
... A computational analysis of its influence on the tip-leakage flow in a compressor rotor and a cascade was performed by Brandt et al. (2002), yielding that a thicker inlet boundary layer leads to higher total pressure losses. Similar results were obtained experimentally by Storer and Cumpsty (1994). In addition, when the boundary layer thickness is increased, not only the tip-leakage vortex detaches from the blade closer to its leading edge but its trajectory also yields more inclination about the blade passage. ...
... To sum up, entropy production is not only found in the jet mixing layer, as previously observed by Storer and Cumpsty (1994), but also in the wake and in a mixing layer coming from the separation of the boundary layer at the casing wall induced by the tip-leakage vortex. ...
Thesis
L’objectif de cette ´étude est de d´écrire en détail l´écoulement de jeu afin d’en ´évaluer la sensibilité´e auxparamètres de conception ainsi que la r´réceptivité´e `a des dispositifs de contrôle par injection d’air au carter. Pourcela, on considère une géométrie simplifiée qui est constituée d’une aube fixe isolée, placée perpendiculairement `aune plaque plane représentant le carter. L’analyse de cette configuration simplifiée s’appuie sur la complémentarité´edes mesures en soufflerie et simulations numériques. La taille du jeu est le paramètre principal qui affectel’écoulement de jeu. L’épaisseur de la couche limite incidente au carter et le chargement de l’aube ont ´égalementun effet visible sur la position latérale du tourbillon de jeu. Le calcul du taux de création d’entropie local a permisd’identifier plusieurs zones de pertes dans l’écoulement. Ensuite, les pertes de pression totale ont été décomposéesen la somme d’un terme li´e au tourbillon et d’un autre li´e au déficit de vitesse longitudinale. Ce terme li´e autourbillon est responsable de l’augmentation des pertes de pression totale avec la taille du jeu. Enfin, un modèleempirique a été développé pour estimer la circulation du tourbillon de jeu ainsi que les pertes de pression totaleen fonction de la taille du jeu. Un système d’injection continue d’air dans le jeu a été évalué, les jets étant orientésperpendiculairement au carter. D’une part, cette stratégie de contrôle permet de rapprocher le tourbillon de jeude l’aube, ce qui pourrait permettre d’augmenter le domaine de fonctionnement stable d’un compresseur. D’autrepart, le champ de vorticité axiale dans le tourbillon de jeu devient plus homogène, ce qui serait intéressant pourlimiter les interactions rotor-stator. Cependant cette approche tend `a augmenter les pertes de pression totale etperd en efficacité avec un élargissement du jeu.
... The first thirty percent chord of the blade are modified accordingly. More interestingly, Chen's works [20] showed that the clearance flow was driven by the blade loading away from the clearance region, which pushes into considering midspan blade loadings in TLF models, as done for example by Storer and Cumpsty [134]. Depending on the size of the gap, the topology of the flow can change. ...
... This is summarized in Figure 1.21. This is consistent with what Storer and Cumpsty [134] observed in their experiment, in which they calculated losses due to TLF for two boundary layer thicknesses. For the thin boundary layer, the existence of an optimum for a non-zero clearance was also observed but for the thicker boundary layer, the evolution was linear as obtained by Chen. ...
... Storer and Cumpsty [134] developed a model regarding losses due to TLF. It follows on from the topology described by Rains where the encounter of the leakage jet and the mainstream flow generates the losses due to their mixing. ...
Thesis
Full-text available
The present thesis aims at investigating tip-leakage flow (TLF) in axial compressors with the help of two resolution levels. The study particularly focuses on three characteristics of the tip-leakage vortex (TLV) found in the tip region of compressor rotors: the location of where the TLV detaches from the blade, its trajectory in the blade passage and its circulation, quantifying its intensity. Their prediction is analyzed with both resolutions. Zonal Detached Eddy Simulations (ZDES) are used on a rotor of an experimental high-speed compressor. The results are compared to a steady Reynolds-Averaged Navier-Stokes (RANS) simulation, less costly but also generally poorly adapted to this kind of flows. The results show the benefits of the ZDES approach compared to RANS, especially in the treatment of the TLV. With ZDES, the TLV diffuses less and is found to be a more persistent structure along the chord. A parametric study on the tip gap size is then led with the ZDES. Three values of the tip gap are tested. The time-averaged analysis confirms results from the literature on the position of the detachment point. It also indicates that the mean trajectory of the TLV could be predicted by a model, based on the knowledge of the midspan loading. Finally, it puts forward the increase of circulation with the tip gap size, with a larger TLV core and higher levels of axial vorticity. The unsteady analysis focuses on the occurrence of a vortex breakdown, only observed with the largest tip gap size through a Q-criterion and contours of axial vorticity. A frequency analysis on the axial velocity in the TLV core enables to identify a low-frequency phenomenon on the largest gap, could be associated to a vortex wandering.The second approach investigates the capabilities of a potential method, a low-resolution method, applied to tip-leakage flow. The Vortex Lattice Method (VLM) relies on the use of vortex singularities to replicate the presence of an aerodynamic body, which induces a perturbation on the velocity field. This approach is computationally cheap and is dedicated to preliminary design, when a large quantity of geometries have to be evaluated. After a validation on external aerodynamics cases, the method has been tested on a TLF configuration, consisting of a single blade interacting with a casing endwall, where the tip gap size can be varied. A new model for the prediction of the TLV circulation is constructed, using the results of the VLM as inputs. By considering the mixing of pairs of contra-rotative viscous analytical vortices, the experimental increase of the circulation with the gap can be well captured by the VLM model over a large range of tip gaps. However, further work is needed to improve the module in order to be used on a compressor geometry.
... A lot of experiments and numerical simulation show that tip clearance plays an important role [1][2][3] in the performance and stability of axial compressors. However, a large number of published literature studies are based on axial compressors with uniform tip clearance, which may not fully reveal the true physical laws. ...
... where λ is an inertia parameter related to compressor geometry: λ = Axialchord/radius 1 2 cos 2 (stagger) (2) Then Song [11] combined the continuity equation, axial and circumferential momentum equations with two shock disk models to analyze the effect of non-uniform tip clearance on compressor blade load. After 2000, with the introduction of active control and stall warning, more and more researchers have conducted experiments to investigate the relationship between non-uniform tip clearances and stall signals. ...
Article
Full-text available
The tip clearance has an important effect on the performance of an engine compressor. While the impact of tip clearance on a concentric compressor has been widely explored in previous research, the flow field distribution of an eccentric compressor has only been minimally explored. Both the steady and unsteady computational fluid dynamics (CFD) methods have been widely used in the studies of concentric axial-compressors, and they have similar simulation results in terms of flow field. However, they have been rarely applied to axial-compressors with non-uniform tip clearance to investigate their flow field. In this paper, ANSYS CFX is used as CFD software, and both steady and unsteady CFD methods are applied to study a single rotor of ROTOR67 to investigate the compressor characteristic line and flow field under different eccentricity conditions. The results show that non-uniform tip clearance creates a non-uniform flow field at the inlet and tip regions over the whole operating range. The circumferential position where the flow coefficient and the axial velocity are the smallest occurs at a position close to the maximum tip clearance and is located on the side deviating toward the direction of rotation of the rotor. Compared with steady CFD, unsteady CFD has better predictive capability for the flow field distribution in axial compressors with non-uniform tip clearance.
... Based on Rains' work, 8 Storer proposed a simple model for tip clearance flow. 9 By assuming inviscid, incompressible flow, infinitely thin blades, the velocity of the leakage jet where it leaves the tip gap is described as 1 School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, China 2 Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China a function of its total pressure and the local static pressure on the suction side of the tip. Moreover, an empirical discharge coefficient is used to account for the vena-contracta effect. ...
... Control volume method has been applied in previous studies to model tip clearance flow/main flow interaction 15 and to estimate tip clearance loss. 9 The benefit of this method is that the averaged outflow properties can be easily solved by conservation constraints without concerning about the detailed mixing process, as soon as the inflow and mixing flow properties are appropriately given. As for application in transonic compressors, there are several issues to be dealt with. ...
Article
Full-text available
An improved compressible model for estimating tip clearance loss in transonic compressors is presented with the emphasis on the effects of blade tip loading distribution and double leakage flow. Tip clearance flow is treated as three parts along the chord and the progressive relations from upstream to downstream part is revealed to be responsible for the formation of tip clearance flow. Control volume method is applied to simplify the mixing process and calculate the mixed-out loss for the three parts, separately. Computational study shows that mass flow of the incoming flow entering the control volume is consistent with that passing through an equivalent area of about half of tip leakage vortex region. The new model reveals that the second part of tip clearance flow has a larger mixed-out loss capacity than the two other parts. This difference is attributed to two factors: larger injection flow angle and more enrolled incoming flow, and both factors tend to increase the mixed-out loss. The success of the model implies that blade design or flow control strategies turning the tip clearance/main flow interface’s arrival onto blade tip pressure side downstream and the shock’s impingement point onto blade tip suction side upstream may be beneficial in desensitizing compressor performance to tip clearance size, without trading off pressure rise.
... In den letzten Jahren ist man verstärkt auch dazu übergangen, automatisierte Optimierungsalgorithmen mit 2D-Profildesignprogrammen zu koppeln [9]. [10,11,12] und insbesondere für die Profilsaugseite. Abbildung 2-5 (rechts) zeigt den Einfluss des Spaltwirbels auf die Machzahlverteilung in einem S1-Schnitt nahe der Blattspitze. ...
... Es ist sinnvoller, sich der bekannten Fachsprache zu bedienen. Der Literatur[10,11] und Kapital 3.3 folgend, sind die Verluste im Blattspitzenbereich hauptsächlich abhängig von:Die geometrischen Profileigenschaften, von denen ein merklicher Einfluss auf die Spaltströmung erwartet wird, sind in Tabelle 9 zusammengefasst. Die Gesamtbelastung der Schaufel, der Gehäuseverlauf und die Spalthöhe blieben nach Vorgabe unverändert. ...
... The model attempts to study the growth of perturbations in space. Losses are assessed exploiting the following correlations: Tip clearance effects are taken into account following correlation by Storer and Cumpsty [28]; secondary flows are predicted by means of approach suggested by Wright and Miller [29]; blade deviation losses are assessed according to the correlation of K€ onig et al. [30]. ...
Article
The main role of the intake is to provide a sufficient mass flow to the engine face and sufficient flow homogeneity to the fan. Intake-fan interaction off design represents a critical issue in the design process because intake lines are set very early during the aircraft optimization. The off-design operation of an aero-engine, strictly related to the intake flow field, can be mainly related to two different conditions. When the plane is in near ground position, vorticity can be ingested by the fan due to crosswind incidence. During the flight, distortions occur due to incidence. In these conditions, the windward lip is subjected to high acceleration followed by strong adverse pressure gradients, high streamline curvature and cohabitation of incompressible and transonic flow around the lip. All these features increase the risk of lip stall in flight at incidence or in crosswind near ground operation and increase the level of forcing seen by the fan blades because of the interaction with non-uniform flow from the intake. This work deals with the study of two sources of distortions: ground vortex ingestion and flight at high incidence conditions. A test case representative of a current installation clearance from the ground has been investigated and the experimental data available in open literature validated the CFD approach. An intake, representative of a realistic civil aeroengine configuration flying at high incidence, has been investigated in powered and aspirated configurations. Distortion distributions have been characterized in terms of total loss distributions in space and in time. The beneficial effect of the presence of the fan in terms of distortion control has been demonstrated. The mutual effect between fan and incoming distortion from the intake has been assessed in terms of modal force and distortion control. CFD has been validated by means of comparisons between numerical results and experimental data have been provided. Waves predicted by CFD have been compared with an actuator disk approach prediction. The linear behavior of the lower disturbance frequency coming from distortion and the waves reflected by the fan has been demonstrated.
... The model attempts to study the growth of perturbations in space. Losses are assessed exploiting the following correlations: Tip clearance effects are taken into account following correlation by Storer and Cumpsty [28]; secondary flows are predicted by means of approach suggested by Wright and Miller [29]; blade deviation losses are assessed according to the correlation of K€ onig et al. [30]. ...
Article
The main role of the intake is to provide a sufficient mass flow to the engine face and a sufficient flow homogeneity to the fan. Intake-fan interaction off design represents a critical issue in the design process because intake lines are set very early during the aircraft optimization. The offdesign operation of an aero-engine, strictly related to the intake flow field, can be mainly related to two different conditions. When the plane is in near ground position, vorticity can be ingested by the fan due to crosswind incidence. During the flight, distortions occur due to incidence. In these conditions, the windward lip is subjected to high acceleration followed by strong adverse pressure gradients, high streamline curvature, and cohabitation of incompressible and transonic flow around the lip. All these features increase the risk of lip stall in flight at incidence or in crosswind near ground operation and increase the level of forcing seen by the fan blades because of the interaction with nonuniform flow from the intake. This work deals with the study of two sources of distortions: ground vortex ingestion and flight at high incidence conditions. A test case representative of a current installation clearance from the ground has been investigated and the experimental data available in open literature validated the computational fluid dynamics (CFD) calculations. An intake, representative of a realistic civil aero-engine configuration flying at high incidence, has been investigated in powered and aspirated configurations. Distortion distributions have been characterized in terms of total loss distributions in space and in time. The beneficial effect of the presence of fan in terms of distortion control has been demonstrated. The mutual effect between fan and incoming distortion from the intake has been assessed in terms of modal force and distortion control. CFD has been validated by means of comparisons between numerical results and experimental data which have been provided. Waves predicted by CFD have been compared with an actuator disk approach prediction. The linear behavior of the lower disturbance frequency coming from distortion and the waves reflected by the fan has been demonstrated.
... Chen et al. (1991) studied the structure of tip leakage flow in axial compressor through similarity analysis. Storer et al. (1991Storer et al. ( , 1994 the proposed model and the result obtained from the numerical simulation at that time were not sufficient to provide accurate guidance for the tip leakage flow in the fan bearing tips with complex geometry. Jang et al. (2001a,b ) used laser Doppler velocimetry (LDV) measurement and large eddy simulation (LES) to investigate the leakage vortices and the authors confirmed that the periodical movement of the tip leakage vortex is related to the blade passing frequency (BPF) noise. ...
Article
This study is concerned with the improvement in efficiency of axial flow fans that are being used in numerous fields including outdoor units of air conditioners. The tip leakage flow occurring between a blade tip and shroud is one of the major losses in the axial flow fan. A well-known method used to control such tip leakage flow is locating winglet on the suction side of blade tips. Only a few articles have studied the impact of tip clearance on the flow structure of tip leakage flow of axial flow fans with winglet. In this study, the flow structure occurring on the blade tip due to the location of a winglet was analyzed. We confirm the existence of an optimal tip clearance which results in the maximum efficiency for an axial flow fan with a shroud height measuring 30% of the axial chord length.
... Further studies on the turbulence model for predicting complex flows in compressors should be conducted to minimize the design risk. The major difficulty in developing an advanced highperformance compressor is the occurrence of complex flows inside it, such as corner separation [3, 4] , bladerow interaction [5], and tip leakage flow (TLF), which significantly influences the pressure ratio, efficiency, and stall margin [6][7][8]. Thus, casing treatment has been widely investigated and used to enhance the stall margin related to TLF [9][10][11]. Numerous efforts have been exerted to investigate the TLF on the basis of theoretical models [12][13][14], experimental investigations [15][16][17][18][19][20][21], and numerical simulations [22][23][24][25][26][27]. ...
... Since the pioneering study of Rains, 3 several theoretical models have been derived to simplify the flow mechanisms and mixing process of TLF. [4][5][6] Meanwhile, many empirical models have been proposed to estimate the tip blockage, loss production, or vortex trajectory caused by TLF in the tip region. [7][8][9][10] The influence of tip clearance size (TCS) on the flow structure of TLF is also a focus of many research efforts. ...
Article
Full-text available
Tip leakage flow is usually responsible for the deterioration of compressor performance and stability. The current paper conducts numerical simulations on the impact of casing aspiration on the axial compressor cascade performance. Three aspiration schemes with different chordwise coverage are studied and analyzed. It is found that the cascade performance can be effectively improved by the appropriate casing aspiration, and the optimum aspiration scheme should cover the area including the onset point of tip leakage vortex and its vicinity. The control mechanisms are different for the aspiration schemes located at different blade chord ranges. For the aspiration scheme covering the onset point of tip leakage vortex, the improvement of the cascade performance is mainly due to that the starting point of the tip leakage vortex is shifted downstream. The original tip leakage vortex structure is divided into two parts if the aspiration scheme is located behind the onset point of tip leakage vortex and the final control effect is the combination of the influence from the two different parts of tip leakage vortex. Additionally, the casing aspiration redistributes the blade loading along the chord near blade tip. The results of these investigations may offer guidance for the appropriate design of aspiration scheme in the future updated compressors and the overall total pressure loss coefficient caused by aspiration slot should be considered in the design process.
... The tip leakage flow has a detrimental impact on the efficiency, pressure rise and stability of axial compressors. 1 There are two aspects of the tip leakage flow; one is blockage and the other one is aerodynamic loss. [2][3][4] Complicated tip flow structures are generated by the tip leakage flow and its interactions with the passage shock wave, the casing boundary layer and the blade wake in transonic compressor rotors, which are often considered as the reasons of rotating instability. The investigation of tip leakage flow in axial transonic compressor rotors has received an increasing interest in the published reference, and some basic understanding has been revealed. ...
Article
Full-text available
The tip leakage flow has an important influence on the performance of transonic compressor. Blade tip winglet has been proved to be an effective method to control the tip leakage flow in compressor, while the physical mechanisms of blade tip winglet have been poorly understood. A numerical study for a highly loaded transonic compressor rotor has been conducted to understand the effect of varying the location of blade tip winglet on the performance of the rotor. Two kinds of tip winglet were designed and investigated. The effects of blade tip winglet on the compressor overall performance, stability and tip flow structure were presented and discussed. It is found that the interaction of the tip winglet with the flow in the tip region is different when the winglet is located at suction-side or pressure-side of the blade tip. Results indicate that the suction-side winglet (SW) is ineffective to improve the performance of compressor rotor. In addition, a significant stall range extension equivalent to 33.74% with a very small penalty in efficiency can be obtained by the pressure-side winglet (PW). An attempt has been made to explain the fundamental mechanisms of blade tip winglet in detail.
... Tip clearance flow can be characterized in two ways. First is the flow blockage [4] and the second effect is the thermal loss as mentioned by Storer and Cumpsty [5]. Turbomachines often operate with a tip clearance larger than the design value due to the change in operating conditions as well as manufacturing limitations. ...
... The inspection of blade tip clearance in transonic compressor reveals that the tip gap chokes the flow near the wall and it increases downstream, and because of the interaction between vortices resulted from the tip gap and the main flow, the flow rate decreases (Ren et al. 2016). Researches done by Cumpsty showed that for each 1% increase in tip clearance over 6% increase in the blade chord, the pressure and the flow coefficient increase by, respectively, 23% and 15% (Storer et al. 1994). Denton has cited the tip clearance loss as the main loss source in compressors (Denton 1993). ...
Article
Full-text available
In the current study, the influence of blade tip clearance in different stages of a three-stage compressor is investigated. Performance diagrams of compressor were verified against experiment when there is no change in the tip clearance, after which the effect of tip clearance for the cases, 1, 1.5 and 2 mm, in the first, second and third stages of rotor was studied. The results indicated that the impact of tip clearance increase did not have any effect on choked flow rate value in the first and second stages, and only the change in the third stage tip clearance reduced the choked flow rate. For the same tip clearance value, the highest compressor performance loss occurs in the case of applying the tip clearance in the third stage, which is also the final stage and is highly sensitive to tip clearance changes. Moreover, modifying the tip clearance is effective on the flow angle in the trailing edge and as the tip clearance increases, the same thing happens about the flow angle. The maximum value of flow angle changes by modifying the tip clearance belongs to the third stage of the compressor.
... Flow through tip clearance of an axial compressor is an important element affecting the inception of stall and surge and thus is a key concern in improving the stability of the compressor. One of the main causes of the decrease in the stable operation of a single-stage transonic axial compressor is the flow through tip clearance, the leakage jet flow and leakage vortex at the tip of the blades are the main source of losses, according to Storer and Cumpsty (Storer and Cumpsty 1994). To solve this phenomenon, many researchers have used methods such as: Casing grooves, casing slots, air bleeding, and flow recirculation. ...
Article
Full-text available
This paper presents the effect of an inclined casing groove on the aerodynamic performance of a single-stage transonic axial compressor, NASA Stage 38, using three-dimensional Reynold-averaged-Navier-Stokes equations with the k-ε turbulence model. The research was carried out to examine the effects of four casing groove parameters: angle, width, depth, and location. Validation of a numerical model for a single-stage transonic axial compressor was conducted to evaluate the computational fluid dynamics method. Most of the simulations showed positive results with an increase in stall margin, adiabatic efficiency, and total pressure ratio, in which the maximum stall margin, adiabatic efficiency, and total pressure ratio can be raised by 87.09%, 0.13%, and 1.57%, respectively, as compared to the smooth casing case. ARTICLE HISTORY
... This is because, once the pressure difference has been determined by the main flow in the blade passage, the in-gap TLFs become relatively independent 37 . The stationary casing-wall assumption is a compromise; fortunately, many critical features of compressor TLFs are unaffected by a moving casing wall 38,39 . This is valid in the case of no strong inlet distortions 2 . ...
Preprint
Full-text available
Time-resolved schlieren visualization is used to investigate the unsteady flow structures of tip leakage flows in the clearance region. A common generic blade tip model is created and tested in a wind tunnel under operating conditions ranging from low-subsonic to transonic. A multi-cutoff superposition technique is developed to achieve better flow visualization. Quantitative image processing is performed to extract the flow structures and the instability modes. Additional numerical simulations are performed to help classify the observed flow structures. Unsteady flow structures such as over-tip shock oscillation, shear-layer flapping, and vortex shedding are revealed by Fourier analysis and dynamic mode decomposition. The results show that, under subsonic conditions, the trigger position of the shear layer instability is monotonically delayed as the blade loading increases; however, this pattern is reversed under transonic conditions. This implies that flow compressibility, flow acceleration, and the oscillation of over-tip shock waves are critical factors related to tip flow instabilities. The over-tip shock waves are observed to be locked-in by frequency and position with the shear-layer flapping mode. An intermittent flow mode, termed the escaping vortex-shedding mode, is also observed. These flow structures are key factors in the control of tip leakage flows. Based on the observed flow dynamics, a schematic drawing of tip leakage flow structures and related motions is proposed. Finally, an experimental dataset is obtained for the validation of future numerical simulations.
... Storer and Cumpsty (1991) showed that the mechanisms of tip-gap flow is primarily inviscid and is mostly controlled by the near region static pressure field. Storer and Cumpsty (1994) also showed that much of the loss from tip-gap flows was associated with the down-stream mixing of the tip vortices with the main flow field. ...
Thesis
Full-text available
The prediction of an axial compressor’s loss early on in the design phase is a valuable and important part of the design process. The work presented here focuses on assessing the accuracy of current prediction methods, Reynolds Averaged Navier Stokes (RANS), compared with highly accurate Large Eddy Simulations (LES). The simulations were performed at the challenging running conditions of engine relevant Mach (0.67) and Reynolds (300,000) numbers. The work looks at the effects of off-design incidence and the influence of different free-stream disturbances on loss prediction. From the highly accurate datasets produced by the LES the work is able to show how loss attribution varies under different conditions, and goes on to compare how well RANS captures these changes. It was found that overall loss trends are captured well by RANS but substantial differences exist when comparing individual loss sources, which are shown to vary significantly under different running conditions. The investigation into loss attribution is performed using the Denton (1993) loss breakdown as well as a novel application of the Miller (2013) mechanical work potential. In addition to the discovery of the variation in the sources of loss, the comparison between the loss analyses highlighted some of the limitations of the Denton loss breakdown, which was shown to have increasing error under large off-design incidence or in the presence of discrete disturbances. From the comparison of the loss breakdown analyses and LES and RANS flow field results, new insight into the characteristics, limitations and short comings of current modeling techniques have been found. The variation in the sources of loss under different running conditions was also discovered.
... Results from experiment of Inoue and Kuroumaru [1] showed the structure of the tip clearance flow and the detrimental interaction of reverse flow due to vortex with through flow. Storer and Cumpsty [2] carried out experimental and numerical investigation to explore the characteristics of tip leakage flow. The results revealed tip leakage flow and vortex are major source of loss in compressor. ...
Article
Full-text available
This paper investigates a novel casing treatment method, called recirculation-bleeding channels, which combine a recirculation channel with additional bleeding channels. The system consists of 36 channels distributed around the blades and located on rotor shroud surface of a single-stage transonic axial compressor. This study focuses on its effects on aerodynamic performance of a single-stage transonic axial compressor, NASA Stage 37. Validation of numerical model of NASA Stage 37 was performed using experimental data for the single-stage transonic axial compressor. A common drawback of flow recirculation and air bleeding is the reduction in efficiency; however, numerical results showed that with the presence of recirculation-bleeding channels, both stall margin and adiabatic efficiency of the single-stage transonic axial compressor were increased as compared to the smooth casing with small penalty in pressure ratio. A parametric study of the recirculation-bleeding channels was performed for six geometric parameters. With recirculation-bleeding channels, the compressor could reach the stall margin of 13.85% at maximum while still retaining an increase in peak adiabatic efficiency. It is also showed that proper adjustments of the channels design can eliminate the deficiency in pressure ratio at peak efficiency condition.
... The inspection of blade tip clearance in transonic compressor reveals that the tip gap chokes the flow near the wall and it increases downstream, and because of the interaction between vortices resulted from the tip gap and the main flow, the flow rate decreases (Ren et al. 2016). Researches done by Cumpsty showed that for each 1% increase in tip clearance over 6% increase in the blade chord, the pressure and the flow coefficient increase by, respectively, 23% and 15% (Storer et al. 1994). Denton has cited the tip clearance loss as the main loss source in compressors (Denton 1993). ...
... The efficiency, the pressure rise and the stable operating range are three prominent parameters for energy and power machinery, such as axial fans and compressors. The tip leakage flow (TLF), driven by the pressure difference between the pressure side and suction side of the blade in the tip region, plays a significant role in all three parameters mentioned above [1][2][3]. In recent years, the TLF, especially the tip leakage vortex (TLV), has been found to be an inherently unsteady flow phenomenon, with several distinct unsteady behaviors, including vortex wandering [4], vortex splitting [5,6] and vortex breakdown [7,8]. ...
Article
Full-text available
Though the proper orthogonal decomposition (POD) method has been widely adopted in flow analysis, few publications have systematically studied the influence of different POD processing methods on the POD results. This paper investigates the effects of different decomposition regions and decomposition dimensionalities on POD decomposition and reconstruction concerning the tip flow in the compressor cascade. Stereoscopic particle image velocimetry (SPIV) measurements in the blade channel are addressed to obtain the original flow field. Through vortex core identification, development of the tip leakage vortex along the chord is described. Afterwards, each plane is energetically decomposed by POD. Using the identified vortex core center as the geometric center, the effects of different decomposition regions with respect to the vortex core are analyzed. Furthermore, the effects of different single velocity-components as well as their combination are compared. The effect of different decomposition regions on the mode 1 energy fraction mainly impacts the streamwise velocity component. Though the addition of W velocity component in the decomposition does change the spatial structures of high-order modes, it does not change the dynamic results of reconstruction using a finite number of POD modes. UV global analysis is better for capturing the kinetic physics of the tip leakage vortex (TLV) wandering.
... In addition to the experimental measurements, numerical simulation has proved to be a reliable approach to study the tip leakage flow. Storer and Cumpsty [16] conducted RANS calculations and showed that the principal mechanism of loss creation due to tip leakage flow is the mixing of passage and leakage flows. Shin [17] and Khorrami et al. [18] performed RANS computations to predict the flow field in a low-speed linear compressor cascade with stationary end-wall. ...
Article
Full-text available
The tip leakage vortex (TLV) has aroused great concern for turbomachine performance, stability and noise generation as well as cavitation erosion. To better understand structures and dynamics of the TLV, large-eddy simulation (LES) is coupled with a homogeneous cavitation model to simulate the cavitation flow around a NACA0009 hydrofoil with a given clearance. The numerical results are validated by comparisons with experimental measurements. The results demonstrate that the present LES can well predict the mean behavior of the TLV. By visualizing the mean streamlines and mean streamwise vorticity, it shows that the TLV dominates the end-wall vortex structures, and that the generation and evolution of the other vortices are found to be closely related to the development of the TLV. In addition, as the TLV moves downstream, it undergoes an interesting progression, i.e., the vortex core radius keeps increasing and the axial velocity of vortex center experiences a conversion from jet-like profile to wake-like profile.
... According to (Storer & Cumpsty, 1994) clearance loss in incompressible flow can be approximated by ...
... Therefore, numerous studies have been carried out to understand these effects. For instance in (Smith & Cumpsty, 1984) as well as in (Wisler, 1985) the impact of the tip flow is quantified while in (Chen, et al., 1991), (Denton, 1993) and (Storer & Cumpsty, 1993) the tip flow is described analytically. Since these models are limited to a few basic geometric input parameters, complex structures and their associated losses cannot be mapped with common methods. ...
... The performance and stability of any type of compressor can be strongly influenced by the tip leakage flow. For axial compressors the tip clearance effects are mainly determined by the clearance-to-bladeheight ratio according to Storer & Cumpsty [12]. Due to the small blade heights, the magnitude of this effect is relatively high for mixed-flow compressors. ...
Article
Full-text available
Compact, electrically-driven compressors are a core component of a novel active high-lift system for future commercial aircraft. A newly-developed aeromechanical optimization process was used to design the compressor stage. The optimization resulted in an unusual mixed-flow compressor design with very low aspect ratio blades and a high rotational speed of up to 60,000 rpm. Due to the unusual design, experimental validation of the performance predictions by means of CFD is necessary. This paper presents the first experimental results obtained using a preliminary prototype at part-speed, i.e. rotational speeds from 20,000 to 30,000 rpm. The experimentally-determined pressure ratios deviate up to 1.5 %, the polytropic efficiencies up to 4 percentage points from the CFD predictions. Besides the deficiencies of available turbulence models, the underestimation of overall losses is presumably due to the omission of the volute in the CFD model. An experimental validation of the CFD predictions at full-speed is under way.
... Experimental results from Inoue and Kuroumaru [1] revealed the structure of the tip clearance flow and the deleterious interaction between the vortex and through flow. Experimental and numerical investigations by Storer and Cumpsty [2] showed that the tip leakage vortex is a major source of loss in a compressor. ...
Article
This paper investigates a novel casing treatment for an axial compressor that combines a circumferential casing groove with flow recirculation channels. This casing treatment is located in the rotor's tip region and comprises a single shallow circumferential groove with 36 recirculation channels that are distributed equally around the annulus. The effects of the casing treatment on the stability and aerodynamic performance were evaluated numerically on a NASA Stage 37 single-stage transonic axial compressor. The results show that the addition of the recirculation channels improved the gain in stall margin while mitigating the penalty in the adiabatic efficiency and pressure rise compared to a casing groove without recirculation channels. Six geometric parameters were examined in a parametric study. The maximum increase in stall margin was 42.5% with small reductions in the efficiency and pressure ratio. To maximize the stall margin and minimize efficiency loss, two design variables were selected to perform a multi-objective optimization using a multi-objective genetic algorithm coupled with surrogate models using a radial basis neural network. The optimization successfully improved both the objective functions for all representative designs compared to the reference design.
... Results from experiment of Inoue and Kuroumaru [1] showed the structure of the tip clearance flow and the detrimental interaction of reverse flow due to vortex with through flow. Storer and Cumpsty [2] carried out experimental and numerical investigation to explore the characteristics of tip leakage flow. The results revealed tip leakage flow and vortex are major source of loss in compressor. ...
Article
Full-text available
This paper investigates a novel casing treatment method, called recirculation-bleeding channels, which combine a recirculation channel with additional bleeding channels. The system consists of 36 channels distributed around the blades and located on the rotor shroud surface of a single-stage transonic axial compressor. This study focuses on its effects on the aerodynamic performance of a single-stage transonic axial compressor, NASA Stage 37. Validation of the numerical model of NASA Stage 37 was performed using experimental data for the single-stage transonic axial compressor. A common drawback of flow recirculation and air bleeding is the reduction in efficiency; however, numerical results showed that with the presence of recirculation-bleeding channels, both stall margin and adiabatic efficiency of the single-stage transonic axial compressor were increased as compared to the smooth casing with small penalty in pressure ratio. A parametric study of the recirculation-bleeding channels was performed for six geometric parameters. With recirculation-bleeding channels, the compressor could reach the stall margin of 13.85% at maximum while still retaining an increase in peak adiabatic efficiency. It is also showed that proper adjustments of the channels design can eliminate the deficiency in pressure ratio at peak efficiency condition.
Conference Paper
This paper presents experimental investigation conducted on a 2-stage low speed axial research compressor with cantilevered stator vanes. Flow fields at four different axial locations in the radial stacking and bowed stator passage were measured at maximum flow point and near stall point using 4 five-hole pressure probes. The aim of the work is to study the effects of the bowed stators with hub clearance on the flow field of compressor blade passage. The investigations were conducted with the first stage of the compressor. The hub clearances of both original and bowed stators are 1.1% of span. The results show that the scale of the separation vortex, the hub leakage vortex and the lower passage vortex in the bowed blade passage becomes much smaller and the hub leakage vortex is closer to the suction side at near stall point, which causes a much smaller mixing loss in the blade passage.
Article
Fast-response pressure measurements collected in a three-stage axial compressor highlight the development of the rotor tip leakage flow. Data collected from an array of high-frequency-response pressure transducers measure timeresolved static pressure over the rotors at several loading conditions for three tip clearance heights. The influence of surrounding vane rows on the rotor tip leakage flow is investigated by adjusting the position of the vanes with respect to fixed sensor positions. One key result is that the wake from the upstream vane creates a modulation of the leakage flow. However, the upstream propagating potential field from the downstream vane row has no measureable impact on the leakage flow trajectory. In some cases, variations of the leakage flow trajectory angle due to these blade row interactions are greater than the differences due to a doubling of the rotor tip clearance height, an important finding not previously reported in the literature. Differences of the leakage flow trajectory trends with tip clearance are identified for a high loading condition compared to other operating points. Based on these observations, alternate methods are introduced for achieving collapsed leakage flow trajectories with changing tip clearance in two different flow ranges along the speed line.
Article
The turbulent low Mach number flow through an axial fan at a Reynolds number of 9.36 × 105 is investigated by large-eddy simulation (LES). Computations are performed for a fixed flow rate coefficient and two tip-gap widths, i.e., and . A finite-volume flow solver in an unstructured hierarchical Cartesian setup for the compressible Navier–Stokes equations is used. To account for sharp edges, a fully conservative cut-cell approach is applied. A periodic boundary condition is used in the azimuthal direction such that only one out of five blades is resolved. The focus of this numerical analysis is on the impact of the tip-gap size on the overall flow field. Special attention is paid to the vortical structures around the tip-gap region and the turbulent transition on the suction side of the blade. It is shown that the reduction of the tip-gap size totally changes the flow field. The blade-wake interaction, which was evident for vanishes for . Furthermore, depending on the tip-gap width different mechanisms are responsible for the turbulent transition on the suction side of the blade, i.e., for a cyclic transition is observed which is triggered by the blade-wake interaction, where for a separation bubble leads to a permanent transition. A reduction of the tip-gap width decreases the amplitude of the tip-gap vortex wandering and as such the region of influence of the turbulent wake and increases the frequencies of the dominant modes. Since this is confirmed by the experimental results of the sound power level, it also shows the dominant role of the tip-gap vortex in the noise emission.
Chapter
Um die gesteckten Ziele eines umweltfreundlichen Flugtriebwerkes, einen möglichst hohen Vortriebswirkungsgrad bei möglichst niedriger Lärmemission zu erreichen, werden immer höhere Bypassverhältnisse realisiert. Dieser Trend wird auch in naher Zukunft anhalten, um die politischen Vorgaben zu erfüllen. So strebt die EU-Kommission „European Aeronautics: Visio 2020“ in Kooperation mit dem „Advisory Council for Aeronautics Research in Europe" (ACARE) eine Halbierung der Lärm- und CO2-Emission, sowie eine Absenkung der NOx-Werte um 80 % an.
Article
An Improved Delayed Detached Eddy Simulation (IDDES) model as a kind of hybrid RANS/LES approach has been developed and applied to calculate tip leakage flow in a linear compressor cascade with moving casing, and the result was compared with the experimental results and LES results. The hybrid approach combines delayed detached eddy simulation (DDES) with an improved RANS–LES hybrid model aimed at wall modeling in LES (WMLES). In this code, the convective flux was solved using fourth-order skew-symmetric scheme and the Spalart–Allmaras (S–A) model was applied as a subgrid scale (SGS) model. This hybrid RANS/LES model code was successfully validated by simulation of homogeneous isotropic turbulence, turbulent channel flow, and backward facing step flow. After validation, a 3D compressor cascade tip leakage flow with different specific blade tip clearances was simulated via this code. Result shows that the blade surface pressure distribution agrees well with the results of experiment and LES. The hybrid RANS/LES model captures intensive vortices clearly, including tip leakage vortex, tip separation vortices, induced vortices, and abundant vortex structures generated by the interaction between tip separation vortices and tip leakage vortex. The end-wall vortical structures were observed in detail being described by Q vortex identification method and meanstream lines. The effect of tip-gap size on total pressure loss was investigated and it was found that there is a best tip gap with most stable tip leakage vortex and least pressure loss.
Article
Based on a validation of the numerical methods with an experiment, numerical simulations are carried out to study the effect of tip clearance size on the performance and tip leakage flow in a dual-stage counter-rotating axial compressor. The predicted results showed that the variation of the tip clearance size in rotor2 has a more significant impact on the overall performance and stall margin of the compressor. In addition, the impact of the tip clearance size effect is mainly on the rotor with the tip clearance size variation. The variation of the tip clearance size in rotor2 almost has no influence on the performance of rotor1, while the performance of rotor2 is increased about 1.37% at near-stall point when the tip clearance size of rotor1 is increased to 1.0 mm from 0.5 mm. At peak efficiency condition, the tip clearance size variation in rotor1 has remarkable influence on the tip leakage vortex intensity, onset point and trajectory in rotor1, but has little influence on those in rotor2. However, the tip clearance size variation in rotor2 has remarkable effect on those in both rotors. Different tip clearance size combination schemes can impact the stall-free characteristic in the counter-rotating axial compressor.
Chapter
Beim Entwurf und der Auslegung von Gasturbinen-Luftstrahltriebwerken stehen in den ersten Auslegungsphasen das stationäre Betriebs- und Leistungsverhalten der Antriebe im Mittelpunkt. Mit den fortschreitenden Entwicklungsstufen hinsichtlich der Flugerprobung und der Serienreifmachung von Antriebssystemen treten das Regelungs- und das Übertragungsverhalten und damit die instationären Betriebszustände in den Vordergrund, die letztlich nicht selten für den Erfolg oder Mißerfolg einer Triebwerksentwicklung entscheidend sein können [Sel75, Ric82, Fio93, Hol97, Bau99B, Wal00B, RTO07B, GasTurb].
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In this paper, experiments and numerical modeling are used to quantify the effects of clearance and eccentricity on compressor performance and to examine the influence of each on flow distribution and stall margin. A change in the size of the tip-clearance gap influences the pressure rise and the stall margin of a compressor. Eccentricity of the tipclearance gap then further exacerbates the negative effects of increasing tip-clearance. There are few studies in the literature dealing with the combined effect of clearance and eccentricity. There is also little guidance for engine designers, who have traditionally used rules of thumb to quantify these effects. One such rule states that the stall margin of an eccentric machine will be equal to that of a concentric machine with uniform clearance equal to the maximum eccentric clearance. In this paper, this rule of thumb is checked using experimental data and found to be overly pessimistic. In addition, eccentric clearance causes a variation in axial velocity around the circumference of the compressor. The current study uses a three-dimensional model which demonstrates the importance of radial flow gradients in capturing this redistribution. Flow redistribution has been treated analytically in the past, and for this reason, previous modeling has been restricted to two dimensions. The circumferential variation in axial velocity is also examined in terms of the local stability of the flow by considering the stalling flow coefficient of an equivalent axisymmetric compressor with the same local tip-clearance. The large clearance sector of the annulus is found to operate beyond its equivalent axisymmetric stall limit, which means that the small clearance sector of the annulus must be stabilizing the large clearance sector. An improved rule of thumb dealing with the effects of eccentricity is presented.
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In this paper, the effects of inlet flow distortion on performance maps and details of the flow field are investigated using the Streamline Curvature Method. The results are presented for both design and off-design conditions and compared with experimental data, which shows good agreement. The effects of inlet flow distortion are investigated by inlet total pressure variation in three different cases in the way that the average total pressure remains constant and equal to the design value. The results show that a relative increase in the total pressure at tip causes an increase in the pressure ratio and efficiency as well as a better performance in the choking region. Alternatively, a relative increase in the total pressure in hub causes opposite behavior and a better performance at the surging region. © 2018 Polish Society of Theoretical and Allied Mechanics. All rights reserved.
Article
Flow structures in an axial compressor with a non-uniform tip clearance were predicted by solving a simple prediction method. For more reliable prediction at the off-design condition, off-design flow characteristics such as loss and flow blockage were incorporated in the model. The predicted results showed that flow field near the design condition is largely dependent on the local tip clearance effect. However overall flow field characteristics are totally reversed at off-design condition, especially at the high flow coefficient. The tip clearance effect decreases, while the local loss and flow blockage make a complicated effect on the compressor flow field. The resultant fluid induced Alford`s force has a negative value near the design condition and it reverses its sign as the flow coefficient increases and shows a very steep increase as the flow coefficient increases.
Chapter
Luftbeaufschlagte Turboverdichter und gasbeaufschlagte Turbinen stellen dynamisch arbeitende Fluidenergiemaschinen dar, bei denen im Wesentlichen durch die Änderung von Strömungsgeschwindigkeiten die Energie umgesetzt wird. Mit diesem Arbeitsprinzip bewirken sie eine Änderung des Druckniveaus im Fluid. Bei Energiezufuhr im Verdichter wird der statische Druck erhöht, bei Energieabgabe in der Turbine wird der statische Druck abgesenkt. Beide Maschinen Verdichter und Turbinen haben also geometrische und kinematische Gemeinsamkeiten (Abb 4.1-1 und 4.1-2).
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Tip clearance flow significantly influences the aerodynamic and aeroacoustic performances of a fan. Its mechanism and control have remained a hotspot in the field of turbomachinery. In this paper, blended blade tip and winglet (BBTW) technology is proposed based on the combination of blended blade and end wall and winglet technologies, while the flow field and sound field of an original blade and the BBTW-blade of a diagonal fan are numerically simulated. The calculation results show that the BBTW design reduces the pressure difference between the suction surface and the pressure surface in the blade tip area and enhances the necking effect of tip clearance, thus inhibiting tip clearance leakage flow and improving the aerodynamic performance of the diagonal fan. In addition, as BBTW weakens the tip leakage vortex intensity, thereby reducing the sound pressure level of the near-field and the far-field noise.
Article
A numerical study on the influence of compressor blade aspect ratio on profile and secondary loss has been conducted. In order to more accurately estimate the change in secondary loss, a new analytical model has been developed. The aspect ratio has been increased by reducing blade chord while maintaining blade height and solidity. A simplified compressor cascade geometry and an engine-like HPC stage geometry (rotor blade and stator vane) have been analysed with 3D CFD simulations. For these simulations, the solver TRACE has been used together with the k-ω turbulence model and a Low-Reynolds approach. A negative effect of increased aspect ratio on profile loss due to the lower Reynolds number has been observed as expected from literature. Moreover a decrease of secondary loss at increased aspect ratio due to smaller endwall regions has been noticed. While this effect is also well known, a significant influence of the assumptions regarding the incoming boundary layer thickness has been observed based on the cascade simulations. This leads to the conclusion that changing the aspect ratio of all blades and vanes of a multistage compressor causes a much stronger decrease in secondary loss per blade row than changing the aspect ratio of a single rotor or stator within the compressor. In literature so far only the first case is considered in common loss correlations. However considering the latter would increase the accuracy of secondary loss estimation for a non-uniform change in aspect ratio within a compressor.
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Tip leakage significantly affects the axial-compressor performance (i.e., its pressure ratio, peak efficiency, and surge margin). The size of the clearance is one of the most important parameters in compressor design. Generally, the tip clearance is uniform from the leading edge to the trailing edge of the compressor-blade section at the tip. In this paper, the effect of chordwise nonuniform tip clearance on the axial-compressor performance is investigated. In addition, the optimum distribution of clearance size is obtained by employing a surrogate model in conjunction with a multi-objective genetic algorithm for optimization. It is shown that the nonuniform tip clearance can effectively reduce the leakage mass-flow rate and eliminate the flow separation, thus enhancing the overall performance of the compressor. Based on the results of the sensitivity analysis, it is found that the changes in clearance size near the trailing edge of the blade tip contribute more toward the improvement of the compressor efficiency than those obtained by changing the clearance near the leading edge. The Pareto optimal front obtained by employing the multi-objective genetic algorithm shows that the transonic axial compressor with well-designed distributions of tip-clearance size gives 0.64% improvement in peak efficiency and 48.18% enhancement in surge margin.
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Robust design is a multi-objective optimization framework for obtaining designs that perform favorably under uncertainty. In this paper robust design is used to redesign a highly loaded, transonic rotor blade with a desensitized tip clearance. The tip gap is initially assumed to be uncertain from 0.5 to 0.85% span, and characterized by a beta distribution. This uncertainty is then fed to a multi-objective optimizer and iterated upon. For each iteration of the optimizer, 3D-RANS computations for two different tip gaps are carried out. Once the simulations are complete, stochastic collocation is used to generate mean and variance in efficiency values, which form the two optimization objectives. Two such robust design studies are carried out: one using 3D blade engineering design parameters (axial sweep, tangential lean, re-cambering and skew) and the other utilizing suction and pressure side surface perturbations (with bumps). A design is selected from each Pareto front. These designs are robust: they exhibit a greater mean efficiency and lower variance in efficiency compared to the datum blade. Both robust designs were also observed to have significantly higher aft and reduced fore tip loading. This resulted in a weaker clearance vortex, wall jet and double leakage flow, all of which lead to reduced mixed-out losses. Interestingly, the robust designs did not show an increase in total pressure at the tip. It is believed that this is due to a trade-off between fore-loading the tip and obtaining a favorable total pressure rise and higher mixed-out losses, or aft-loading the tip, obtaining a lower pressure rise and lower mixed-out losses.
Article
Detailed measurements were made of the casing wall boundary layer development across a large-scale, low-speed axial compressor rotor blade row. An important feature of the work was the use of blading which allowed the tip clearance to be varied. A conventional pressure probe was used to obtain time-averaged measurements of the outer-wall boundary layer downstream of the rotor whilst a hot-wire anemometry technique yielded the three-dimensional, blade to blade structure of the flow. The downstream boundary layer was found to thicken as the rotor loading and blade-end clearance were increased, with fluid tending to accumulate towards the pressure side of the passage. By its pronounced effects upon wall boundary layer development, tip clearance had a deleterious effect upon the performance of the compressor. (A)