Dilipkumar Bhanudasji Alone

Improvement of Moderately Loaded Transonic Axial Compressor Performance Using Low Porosity Bend Skewed Casing Treatment

This paper presents experimental results of a single stage transonic axial flow compressor coupled with low porosity bend skewed casing treatment. The casing treatment has a plenum chamber above the bend slots. The depth of the plenum chamber is varied to understand its impact on the performance of compressor stage. The performance of the compressor stage is evaluated for casing treatment and plenum chamber configurations at two axial locations of 20% and 40%. Experimental results reveal that the stall margin of the compressor stage increases with increase in the plenum chamber volume. Hot-wire measurements show significant reduction in the turbulence intensity with increase in the plenum chamber volume compared to that with the solid casing at the stall condition. At higher operating speeds of 80% and at 20% axial coverage, the stall margin of the compressor increases by 20% with half and full plenum depth. The improvement in the peak stage efficiency observed is 4.6% with half plenum configuration and 3.34% with the full plenum configuration. The maximum improvement in the stall margin of 29.16% is obtained at 50% operating speed with full plenum configurations at 40% axial coverage.

Performance Characterization of the Effect of Axial Positioning of Bend Skewed Casing Treatment Retrofitted to a Transonic Axial Flow Compressor

The performance of an aero-engines to a large extend depends on the performance behavior of axial flow compressors and is restricted by the compressor instabilities like rotating stall and surge. In the present study, attempts have been made to design and develop the bend skewed casing treatment geometries with lower porosities to improve the stable operating range of single stage axial flow compressor. Experimental investigations were undertaken to study the impact of axial position of one of the casing treatment geometry on the single stage transonic axial flow compressor. The transonic compressor used for the current experimental studies has a stage total to total pressure ratio of 1.35, corrected mass flow rate of 22 kg/s at an operating speed of 12930 rpm. The compressor stage steady and unsteady state response for 20%, 40%, 60% and 100% axial chord coverage relative to the rotor tip chord of the bend skewed casing treatment with a porosity of 33% was studied experimentally. The objective was to identify the optimum axial location; which will give maximum improvement in the stall margin with minimal loss of compressor stage efficiency. Through an experimental study it was observed that the axial location of bend skewed casing treatment plays a very crucial role in governing the performance of the transonic compressor. For all the investigated axial coverages, compressor stall margin increases but the optimum performance in terms of stall margin improvement and efficiency gains were observed at 20% and 40% of the rotor chord. This trend shows good agreement with existing published literature. An improvement of 31.7% in the stall margin with an increase in the stage efficiency was obtained at one of the axial coverage. Maximum improvement of 37% in the stall margin above the solid casing was noticed at 60% axial coverage. The stalling characteristics of the compressor stage also changes with the axial positions. In the presence of solid casing the nature of stall was abrupt and stalls cells travels at half the rotor speed. The blade element performance also studied at the rotor exit using pre-calibrated aerodynamic probe.Copyright

On Understanding the Effect of Plenum Chamber of a Bend Skewed Casing Treatment on the Performance of a Transonic Axial Flow Compressor

Bend skewed casing treatment was designed to improve the stable operating range of single stage transonic axial flow compressor and also to understand the effects of its plenum chamber volume on the performance. This paper presents the original experimental research work undertaken to study the effect of plenum chamber depth and thus its volume on the performance of single stage transonic axial flow compressor coupled with the bend skewed casing treatment. The bend skewed casing treatment with porosity of 33% was selected for the present experimental study. The bend skewed casing treatment has slot width equal to the maximum thickness of the rotor blade. The casing treatment geometry has an axial front segment and a 45° staggered rear segment following the blade tip stagger. Both the segments were skewed by 45° in the radial plane, in such a way that the flow emerging from the casing slots would do so with swirl contrary to the direction of rotor rotation. The plenum chamber which can also be called as stagnation zone exists above the skewed slots. The plenum chamber has an axial length equal to the axial length of the casing treatment slots. The maximum depth of the plenum chamber was 11 mm and which was equal to the depth of bend skewed casing slots. The depth of plenum chamber was varied from zero, half the slot depth, and equal to slot depth in order to get variable volume. The porosity and axial location of the casing treatment relative to the rotor tip chord were chosen from the earlier experimental programs on effect of bend skewed casing treatment porosities and axial coverages for the present compressor stage. Optimum performance of the transonic compressor stage was obtained at 20% and 40% axial coverages and for 33% porosity configurations. The axial coverages of 20% and 40% were chosen for the present study to understand the effects of plenum chamber volume on the performance of single stage transonic axial flow compressor.The performance of the compressor stage with solid casing and casing treatment with different plenum volume was obtained and compared at different operating speeds. The compressor performance was derived for the fixed casing treatment porosity of 33% and for three different configurations of plenum chamber volumes at two different axial coverages. Experimental investigations reveal that the plenum chamber volume does have an impact on the stable operating range of the compressor. The compressor stall margin improves with increase in the plenum chamber volume. Bend skewed casing treatment coupled with plenum chamber of depths equal to the slots depth results in maximum stall margin improvement of 37.62% as compared to 26.40% without plenum chamber over the solid casing at 40% axial coverage. For this combination 0.8% improvement in the peak stage efficiency above the solid casing was noticed at 60% design speed.Copyright

Experimental Investigation on the Effect of Porosity of Bend Skewed Casing Treatment on a Single Stage Transonic Axial Flow Compressor

A bend skewed casing treatment was designed, to study the influence of one of its geometrical parameter porosity on the stable performance of single stage transonic axial flow compressor. The compressor was designed for the stage total-to-total pressure ratio of 1.35, corrected mass flow rate of 22 kg/s at corrected design speed of 12930 RPM. Bend skewed casing treatment has an axial inlet segment till 50% of the total length and rear segment that is skewed by 45° in the direction of the rotor tip section stagger. Both the sections were oriented at a skew angle of 45° to the radial plane such that the flow exiting the slot is in counter-clockwise direction to that of the rotor direction. The casing treatment slot width was equal to the maximum thickness of the rotor blades. Three casing treatment configurations were identified for the current experimental investigation. All the treatment geometries considered for the experimental research have lower porosities than reported in the open literatures. The effect of the porosity parameter on the performance of transonic compressor stage was evaluated at two axial coverages of 20% and 40% relative to the rotor tip axial chord. Performance maps were obtained for the solid casing and casing treatment with three different porosities. Comparative studies were carried out and experimental results showed a maximum of 65% improvement in the stable operating range of the compressor for one of the treatment configurations. It was also observed that the stable operating range of the compressor increases with an increase in the casing treatment porosity. All the casing treatment configurations showed that the compressor stall occurs at lower mass flows as compared to the solid casing. Compressor stage peak efficiency shows significant degradations with increase in the porosity as compared to solid casing. Detailed blade element performances were also obtained using calibrated multi-hole aerodynamic probe. Comparative variations of flow parameters like absolute flow angle, Mach number were studied at full flow and near stall conditions for the solid casing and casing treatment configurations. Hot wire measurements show very high fluctuation in the inlet axial velocity in the presence of solid casing as compared to casing treatments. Experimental investigation revealed that the porosity of the casing treatments has strong influence on the transonic compressor stage performance.Copyright

Understanding the Steady and Transient Behavior of the Moderately Loaded High Speed Axial Flow Compressor Stage at Off-Design Conditions

High performance high speed transonic axial flow compressors and fans are the major components of efficient aero-engines. Engine operating envelope and performance normally depends on the compressor performance and its distortion tolerance. Even though the high speed compressors are favorite amongst modern aero-engines, they are more prone to instabilities like surge and stall, which further restricts the engine operations. The present paper deal with experimental study carried on a moderately loaded high speed single stage axial compressor and is focused on the steady state as well as transient behavior at off-design conditions. The single stage transonic compressor is designed for stage total pressure ratio of 1.35 and delivers 23 kg/s mass flow rate at corrected speed of 12930 rpm. The stall inception phenomenon is studied using high frequency miniature Kulite pressure transducers and hot wire probe. The fluctuations in the wall static pressure were also studied by placing three Kulite transducers, one at rotor inlet, second at rotor exit and the third at stator exit. The unsteady pressure signals from these Kulite transducers are processed and analysed to understand the stall inception process. The FFT was performed to identify the stall frequency and pressure oscillation amplitude. Unsteady pressure signals clearly shows the instabilities occurred at the rotor inlet and then gradually moves towards the rotor exit, while the flow at the stator exit remains undisturbed. The intensity of the instabilities along the blade span was also studied using three point Kulite rakes placed at the rotor inlet. Stable operating margin of the compressor was evaluated by deriving the compressor stage performance map. The compressor stage performance map was derived using conventional instrumentation like total pressure rakes and thermocouples. The compressor stage experienced abrupt stall at higher operating speeds. The turbulence intensity was evaluated using single component hot wire probe. Maximum stable operating range of 22% is obtained at 50% design speed. The flow parameters were also evaluated at the rotor inlet and rotor exit using aerodynamic probe at full flow and near stall conditions.Copyright

Numerical Investigation on Effect of Aspect Ratio of Axisymmetric Circumferential Groove Casing Treatment Coupled to a Transonic Axial Flow Compressor Stage

While the effects of axisymmetric casing treatment on performance of an axial compressor stage have been extensively studied numerically as well as experimentally, the major geometrical parameters which govern these effects have been identified. Studies are now focused on understanding how each of these parameters individually impacts the performance of a casing treatment. The present work aims to study the impact on performance of casing treatment geometry when aspect ratio of the grooves is varied in a circumferential groove casing treatment. The compressor geometry chosen for this study has design characteristics of a transonic compressor stage. Flow field solutions were derived for baseline model by solving steady state 3-D Reynolds-Averaged Navier-Stokes (RANS) equations for three grid densities and the grid independence was proved. The basic casing treatment geometry has 10 circumferential grooves of width 4mm and axial spacing of 2mm between each groove. The aspect ratio was varied by changing the depth of the grooves in each case. These casing treatment geometries were superimposed over the rotor domain with the grooves extending over the entire blade tip chord and flow field solutions were again obtained for various aspect ratios of grooves. These results depict improvement in the range of operation in terms of mass flow rate. Results also show that the aspect ratio of the grooves significantly influences the overall effectiveness of casing treatment on the performance of compressor stage. Improvement in overall compressor efficiency is noted with lower aspect ratio casing treatments when compared to those with higher aspect ratios, however, the range improvement is higher with higher aspect ratios. It is also observed that, after a certain depth of grooves is reached, there is no significant improvement in performance on further increasing the depth and hence the aspect ratio. Post processing results of the flow solutions are presented which confirm the trends and show that the flow behavior near rotor tip governs this effect.

Stall margin improvement of a single stage transonic axial flow compressor using naturally aspirated slots

This paper describes the method to improve the stall margin of transonic axial flow compressor by controlling the boundary layer on the suction surface of the rotor blade tip through natural aspiration. Aspiration slots in the compressor blade are intended to energize the flow by increasing its momentum on the suction surface. This phenomenon of boundary layer control can delay the flow separation and hence results in enhancement of the stall margin of the compressor stage. Flow behavior with aspiration slots and its performance are evaluated using commercially available numerical software. Steady state RANS simulations with three dimensional implicit pressure-based coupled solver and turbulence model SST k-ω are used.The effect of natural aspiration slot on the rotor blade performance is computed numerically. The main objective of the study was to identify the optimum location of the aspiration slot along the chord of the compressor on the rotor blade. The axial location chosen for the performance evaluations were 20%,40%,50%,60% and 70% of the rotor blade axial tip chord. By comparing the numerical simulation results with the steady state behavior in the absence of the aspirated slots, the optimized location of the aspiration slot that results in maximum stall improvement is identified. At the optimized location, natural aspiration slots on the rotor blade tip improved the stall margin with the minimum reduction in efficiency and stage pressure ratio when compared to base model. After critically understanding the performance with straight aspiration slots the compressor stage performance has enhanced further by orienting the aspiration slots. The numerical three dimensional results conclude an optimal improvement in the stall margin for the slots near the trailing edge of the rotor. The prediction shows that with the inclined aspiration slots at proper location it is possible to improve the stall margin of the compressor stage and also to restore the stage efficiency.Copyright

Computational Analysis of Effect of Circumferential Groove Casing Treatment With Different Axial Coverage Over Rotor Blade Tip Chord on the Performance of a Transonic Axial Compressor Stage

={Previous studies on circumferential groove casing treatments have shown that the effectiveness of casing Grooves highly depends on their axial location over blade tip. The present work aims to study the flow behavior and its impact on the performance of the compressor stage when the casing treatment grooves are placed to provide different axial coverage over rotor chord in each case. Geometry of a transonic compressor stage was modeled for this study. Flow field solutions for this model with smooth casing wall were obtained by solving steady state 3-D Reynolds-Averaged Navier-Stokes equations for three different grids to prove the grid independence of the solutions. Results obtained with the intermediate grid density were used as the baseline results to compare with results of casing treatment geometries. The basic casing treatment geometry has 10 circumferential groves of width 4mm, depth 16mm and axial spacing of 2mm between each groove. This casing treatment geometry was superimposed over the rotor domain with the grooves extending axially over the entire axial chord (58mm) of rotor blade tip and flow field solutions were again obtained. After that, for each case the grooves are removed from the rear side and axial coverage is shortened. Flow solutions for various axial coverage and hence for various number of grooves are thus obtained and compared. These results depict improvement in the operating range when compared to the Base-line results. Results also exhibit that as the grooves from the rear end are removed gradually, recovery in the overall efficiency is seen in compressor performance. Post processing of the flow solutions confirms the trend and shows that the grooves in the rear of the chord are almost idle not providing sufficient flow to pass over from pressure surface to suction surface of the blade and hence contributing very less towards performance enhancement.