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The understanding of the tip clearance flow in axial compressors is a key issue for developing new compressors with enhanced efficiency and reduced noise for instance. However, necessary flow measurements in the blade tip region and within the tip clearance are challenging due to the small gap width. The application of a novel optical measurement t...
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... flow field in the tip region of cantilevered stator vane rows and rotor blade rows is dominated by the tip clearance vortex (TCV), which is illustrated in Fig. 1. The pressure difference of the blade drives a balancing out clearance flow through this gap from the pressure side (PS) toward the suction side (SS). When the jetlike tip leakage flow enters the next blade passage, it inter- acts with the main flow and other secondary flow phenomena such as the passage vortex and rolls up into a ...
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... Steady State Profile Pressure Distribution. The conven- tional measuring techniques data is completed by the profile pres- sure distributions shown in Fig. 10. Here the vanes surface pressure is referred to the inlet plane measurements. The typical characteristic for highly loaded profiles due to high incidences was found at midspan. The highest loaded blade area is the first 10% of chord. From there, the decreasing pressure gradient along the SS indicates separated flow caused by the large ...
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... toward midspan at 0.94h does not show this clearly altered distribution compared to that at mid- span. From these data, the estimated radial dimension of the TCV interaction up to 0.9h from the wake measurements behind the cascade cannot be verified inside the passage. The merged velocity mean values after a coordinate transform are shown in Fig. 11. The velocity z-component c z is color coded. It clearly indicates that c z is directed upward into the gap at the PS and downward to the passage at the SS. Due to the uprolling TCV, the flow is also turned upward at the rear part of the SS. De- spite the high incidence of the inflow, the cascade flow continues along the passage formed ...
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... This applies notably at the PS while the forming TCV deviates the SS flow. The measured outflow angle a; 2 is approximately 75 deg at the PS and decreases to minimum 35 deg at the SS. This is in very good agreement with the results of the 5HP measurements presented in Sec. 3.1 (see Fig. 6). With assistance of the FM-DGV measurements (see Fig. 11) the dimensions of the TCV inside the clearance can be estimated by negative values of c z . The spanwise massflow at SS is caused by the uprolling leakage jet. In axial directions the origin seems to be at 0.05l-0.1l and the end of the vortex core at 0.9l. Despite of the high incidence, the pitchwise position is near the SS with the ...
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... to Eq. (3), the measurement uncertainty is estimated to 0.6 m/s. Comparing with the results in Fig. 11, the estimated value is smaller than the measured velocity standard deviation, which is due to the flow ...
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... an example, the PSD of the velocity from two positions of the first velocity component are presented in Fig. 13. The first position at x ¼ 0.56l x , y ¼ 0.526t is located in the center of the tip clearance, whereas the second position at x ¼ 0.56l x , y ¼ 0.738t is at the tail of the TCV. From Eq. (3), the noise power spectral Fig. 13(b) faster flow oscillations up to about 500 Hz occur. Generally, a low-pass char- acteristic is apparent in the ...
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... an example, the PSD of the velocity from two positions of the first velocity component are presented in Fig. 13. The first position at x ¼ 0.56l x , y ¼ 0.526t is located in the center of the tip clearance, whereas the second position at x ¼ 0.56l x , y ¼ 0.738t is at the tail of the TCV. From Eq. (3), the noise power spectral Fig. 13(b) faster flow oscillations up to about 500 Hz occur. Generally, a low-pass char- acteristic is apparent in the spectra near the TCV, which means the oscillation amplitude is decreasing with increasing frequency. A dominating mode occurring permanently in subsequent spectra could not be ...
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Citations
... Regarding planar methods, in particular particle image velocimetry (PIV) was successfully applied in numerous studies, ranging from the characterization of stator passages and wakes (Bryanston-Cross et al. 1992;Woisetschläger et al. 2003) to applications in rotating machinery (Wernet 2000;Voges et al. 2012). As an alternative approach to PIV, Doppler global velocimetry with frequency modulation (FM-DGV) was applied on a transonic cascade (Fischer et al. 2012) as well as a rotating turbine stage (Fischer et al. 2013). ...
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... Regarding planar laser-optical techniques, in particular PIV and, to a lesser extent, DGV were successfully applied to turbomachinery flows [4][5][6][7][8][9]. Despite their general ability to deliver timeresolved three-component velocity fields in turbomachinery environments, both PIV and DGV suffer from their susceptibility to laser scattering from surfaces, so that affected image areas have to be masked and thus cannot provide data in these areas [6,7,9]. ...
The aero-thermal characterization of turbomachinery flows typically relies on well-proven conventional measurement technology such as pneumatic probes, hot-wire sensors or thermocouples. As these devices have to be introduced into the flow, they disturb the flow field at the actual measuring position. This is especially critical in the context of measurements in narrow flow channels typically found in turbomachinery applications. In this regard, both five-hole probe (equipped with an additional temperature sensor) as well as laser-optical filtered Rayleigh scattering measurements are carried out in order to characterize pressure, temperature and flow velocities downstream of a nozzle guide vane cascade with lean-burn combustion representative inflow distortions. By comparing the optical Doppler frequency-shifts measured with filtered Rayleigh scattering and calculated from five-hole probe velocity data, the study reveals a significant bias of the probe’s velocity measurement caused by strong pressure gradients present in the airfoils’ wake regions. In addition, passage flows are notably altered by the probe’s intrusive impact on the flow. The study concludes that laser-optical filtered Rayleigh scattering diagnostics overcomes these shortcomings and is well suited to characterize aero-thermal flow properties in turbomachinery environments.
... The measurement principle is based on the optical Doppler effect, that is, measuring the frequency shift of laser light scattered on seeding particles in the flow. Recently, the FM-DGV technique was applied to a linear cascade for the first time ( Fischer et al. 2012), and the tip clearance vortex at a blade with a cylindrical standard profile from a center stage of a high pressure compressor was successfully resolved. Using scanning techniques, a velocity field in the center of the 8 mm wide tip clearance was acquired and all three velocity components were measured. ...
... Thus, the random error is dominating here, which is characterized by the resulting velocity standard deviation. The standard deviation r t i of the ith velocity component can be estimated from the corresponding scattered light power P s;i and the averaging time T by ( Fischer et al. 2012): ...
... At the suction side corner of the squealer tip, the TLF is accelerated again, followed by deceleration inside the passage. Starting from mid-chord of the blade, the tip clearance vortex occurs shedding from the blade suction side ( Fischer et al. 2012). The development of the tip clearance vortex is shown in very good agreement between measurement and numerical results. ...
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generator downstream. The computational results obtained agree well with the available experimental
data on the baseline configuration. The detailed numerical analysis shows that the tip clearance
improves the blade loading near the tip through the promotion of favorable incidence by the tip
leakage flow. The tip clearance shifts the centre of pressure on the blade surface towards the tip.
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On the other hand, with a shear inflow, the effects of tip leakage flow are observed to be partly
suppressed. The shear flow reduces the tip leakage losses substantially in terms
of kinetic energy associated with it.
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Boundary layer suction (BLS), or aspiration, is an effective methodology to eliminate flow separation and improve blade loading in axial compressors. The objective of this study is to reveal the flow physics and loss mechanism of tip leakage vortex in aspirated compressor cascades with multiple loading levels under diffusion factor ranging from 0.42 to 0.71. The effect of blade pressure distribution on tip leakage flows was investigated numerically based on three aspirated blades, including the one designed with conventional method and the other two designed with curvature induced pressure recovery. The influences of tip clearance dimension and BLS schemes (BLS location, BLS height and BLS mass flow rate) on tip leakage vortex as well as its breakdown mechanism were investigated. Results show that a pseudo-shock was found in aspirated compressor cascades designed with conventional method and curvature induce pressure recovery concept. Compared with conventional compressor blades, the main flow phenomenon of tip leakage flow of aspirated compressor blades is the tip leakage vortex breakdown (TLVB) caused by strong adverse pressure gradient introduced by BLS. The TLVB phenomenon occurred both in conventionally loaded compressor cascade and highly loaded compressor cascade. For conventionally loaded blade, TLVB phenomenon occurred under low tip clearance condition but disappeared under high tip clearance condition due to increased strength of tip leakage vortex. Lower BLS mass flow resulted in weaker pseudo-shock, which reduced the swirl number of tip leakage vortex and improved the vortex breakdown phenomenon as a consequence. The vortex breakdown of highly loaded cascades was resulted from the joint action of high blade loading and pseudo-shock. Pseudo-shock designed near blade trailing edge resulted in higher leakage mass flow but promised superior tip leakage flow field. For all the aspirated cascades studied, the tip leakage loss decreased as tip clearance dimension increased, which was opposite to the results of conventional compressor cascade.
The onset of spike stall induced by the interaction of hub corner separation flow with the tip leakage flow is investigated in detail by numerical method in this paper. The time resolved results indicate that the remarkable radial secondary flow from hub to tip near the trailing edge is formed when the compressor approaching rotating stall. The radial secondary flow is unstable and cross-passages propagates, which flows in and away out of the tip region periodically. The disturbance caused by radial secondary flow will influence the tip leakage flow directly by reforming the vortexes in blade tip region. A secondary vortex which comes from the radial migration of corner separation and is induced by the tip leakage vortex appears in the tip region. The simulation result demonstrates that the generation of the secondary vortex is an important symbol of blockage growth in the tip region at the stall inception phase. The disturbance produced by secondary vortex is an incentive of the leading edge overflow and the intensity of secondary vortex could be used as a criterion of rotating stall before leading edge spillage.
The tip leakage flow in turbines is considered to be responsible/or significant machine losses. An efficient reduction of these losses by e. g. squealer cavities at rotor blade tips requires a detailed physical and quantitative understanding of the tip leakage flow. For this purpose, numerical flow simulations are a valuable tool, but they have to be validated by measurements. However, non-intrusive, optical flow measurements in a rotating machine are challenging due to the small tip gap dimensions.
Using an optimized optical setup, all three velocity components of the tip gap flow field were resolved while the turbine (1.5 stage low Mach number turbine test rig) was running with 930Hz blade passing frequency at the design point. The measurement results are in good qualitative agreement with numerical flow simulations. The gap flow above the squealer cavity is not homogeneous, but has several flow gradients, which mainly result from the blade tip geometry and the continuity of the flow. Furthermore, the flow structure between two successive rotor blades was resolved yielding the size and shape of the tip leakage vortex downstream at the suction side of the rotor blade in the measurement plane. Consequently, the capabilities of the applied measurement approach opens promising perspectives toward the development of optimum blade tip designs with minimized tip leakage.
The article describes investigations on the 3D flowfield development near the endwall of a linear compressor cascade which is caused by specific part gap and endwall concepts of adjustable stator vanes. Their beneficial or harmful characteristics, previously measured with outlet loss and flow turning distributions, are investigated inside the passage at different stagger angles to analyse the origin and the interaction of the geometry induced vortex system. Qualitative blade to blade measurements were conducted with particle image velocimetry in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Improved 3D numerical RANS calculations with Reynolds-Stress turbulence models were carried out and enhance the experimental findings. Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vortex development and its impact to the blade boundary layers downstream the passage is visualized and enhances the understanding of the geometry effect. Also the impact of the endwall concepts without radial clearances to the endwall boundary layer is shown and explains their beneficial characteristics compared to a reference geometry.
The article describes investigations on the three-dimensional (3D) flowfield development near the endwall of a linear compressor cascade which is caused by specific part gap and endwall concepts of adjustable stator vanes. Their beneficial or harmful characteristics, previously measured with outlet loss and flow turning distributions, are investigated inside the passage at different stagger angles to analyse the origin and the interaction of the geometry-induced vortex system. Qualitative blade-to-blade measurements were conducted with particle image velocimetry in several spanwise positions as well as oil flow visualisation on the blade surfaces and the endwall. Improved three-dimensional (3D) numerical Reynolds-averaged Navier–Stokes (RANS) calculations with Reynolds-stress turbulence models were carried out and enhance the experimental findings. Results indicate extensive interactions between secondary flow and leakage flow through a penny gap depending on the aerodynamic loading. The part clearance vortex development and its impact to the blade boundary layers downstream the passage is visualised and enhances the understanding of the geometry effect. Also, the impact of the endwall concepts without radial clearances to the endwall boundary layer is shown and explains their beneficial characteristics compared to a reference geometry.
