Yajun Lu

Flow Control of Annular Compressor Cascade by Synthetic Jets

An experimental investigation conducted in a stationary annular cascade wind tunneldemonstrated that unsteady flow control using synthetic jets (zero mass flux) could effec-tively reduce flow separation in the axial compressor cascade. The synthetic jets drivenby speaker were introduced through the casing radially into the flow-field just adjacent tothe leading edge of the compressor cascade. The experimental results revealed that theaerodynamic performance of the compressor cascade could be improved amazingly bysynthetic jets and the maximum relative reduction of loss coefficient was up to 27.5%. Theoptimal analysis of the excitation frequency, excitation location was investigated at dif-ferent incidences. In order to obtain detailed information on flow-field structure, thedigital particle image velocimetry (DPIV) technique was adopted. The experimental re-sults indicated that the intensity of wake vortices became much weaker and streamlinesbecame smoother and more uniform with synthetic jets.

Experimental Investigation on the Effects of Unsteady Excitation Frequency of Casing Treatment on Transonic Compressor Performance

The importance of the unsteady effects between the casing treatment and rotor is still not clear. Experiments are conducted in a transonic compressor with arc skewed slot casing treatment configurations. The experimental results indicate that the unsteady excitation frequency due to the slot number of casing treatment is one of the most important factors influencing compressor performance. Also, compressor performance can be overall enhanced through optimizing this unsteady excitation frequency. For the transonic compressor herein, peak efficiency, stall margin, and maximum flow mass can be improved by 0.17%, 19.86%, and 0.81%, respectively, at near design rotating speed, which can reach up to 1.13% . 57.84%, and 1.57%, respectively, at part design speed.

Research on Sound-Vortex Resonance in Enhancing Performance of an Annular Cascade

This paper presents an experimental investigation on the effects of internal acoustic excitation on enhancing performance of an annular cascade. Based on the research on the shedding frequency characteristics of the annular cascade, the mechanism of interaction between the unsteady fluctuation and the separated flow in the annular cascade was systematically investigated by means of unsteady acoustic excitation. The experimental results reveal that the aerodynamic performance of turbomachinery would be improved amazingly if the unsteady interferences in the flow field could be reasonably organized. In experiment, the loss in one cascade passage could be reduced by nearly 30 perents. The optimal effect of the acoustic excitation was studied at different frequencies, intensities and positions of the excitation as well as at different flow incidence angles.Copyright

Experimental Investigation of the Influence of Inlet Distortion on the Stall Inception in a Low Speed Axial Compressor

Inlet distortion is one of the major concerns for high maneuverability airplanes. An experiment is performed to investigate the influence of inlet distortion on the stall inception in a low speed axial compressor, where the distorted inlet flow field is simulated with a flat baffle placed upstream of the compressor. The flow field around a rotor blade is measured using 2D Digital Particle Image Velocimetry (DPIV) under both uniform and distorted inlet flow conditions. A comparison of flow fields reveals that the distorted inlet flow condition makes the compressor flow fields asymmetric. Flow separation and blockage within Distorted Sector A and Transition Sector B are more severe compared to Sector C. The distorted Sector A and Transition Sector B are the key regions that degrade compressor performance and stability. The large axial velocity makes the flow field within the Undistorted Sector C vigorous, which helps to suppress flow separation and blockage. Compressor rotor blades experience loading and unloading in a revolution period and the compressor exhibits strong dynamic response when it operates under distorted inlet flow conditions. Time-related pressure signals acquired at the rotor exit are utilized to analyze the development of the stall disturbance and the stall characteristic of the compressor. The development period of the stall disturbance is prolonged by the dynamic response of the compressor flow field under the distorted inlet flow condition. Dynamic development of the stall disturbance induced by inlet distortion reduces the compressor stall intensity. The frequency associated with the rotating stall cell is related to the rotating velocity of stall cells, which keeps the same value for uniform and distorted inlet flow conditions. Consequently, the stall inception of the compressor is influenced by the distorted inlet flow condition. The disturbance initiated in Distorted Sector A will experience development and damping when it propagates circumferentially, and might fail to survive the damping within Undistorted Sector C. Stall inception occurs only when the damping within Undistorted Sector C is insufficient to prevent its growth. The dynamic development of the disturbance can reasonably explain the influence of inlet distortion on compressor performance.Copyright

An Experimental Investigation on the Unsteady Excitation Effect of Casing Treatment on a Low Speed Axial Compressor

The performances of compressor should be remarkably improved if the interaction between the flow within treatment slots and the separated flow field around the rotor blades could be properly organized. An experiment was performed on a single stage low speed axial compressor to investigate the unsteady excitation effect of three arc curve skewed slot casing treatments which differ only in the number of treatment slots under uniform and distorted inlet flow conditions. The separated flow field around the rotor blade was investigated with a dynamic pressure probe to obtain the information on unsteady vortex shedding, and the frequency spectrum of the dynamic information was analyzed and utilized to guide the design of the casing treatment in confirming the number of slots. The performances of the compressor were measured for smooth casing and arc curve skewed slot casing treatments under uniform and distorted inlet flow conditions respectively. The stable operating range of the compressor can be improved with all these arc curve skewed slot casing treatments compared to smooth casing. However, only the casing treatment representing the same frequency as that of separated flow around the rotor blades could achieve prominent improvement in stability and total pressure rise coefficient of the compressor without efficiency penalty, where the unsteady separated flow around the rotor blades could couple with that injected from the treatment slots. Consequently, the flow structure was remarkably improved and the time-averaged aerodynamic performances especially the stability of the compressor were significantly enhanced. The number of treatment slots would effectively influence the unsteady interaction, and hence influence the effectiveness of the casing treatment. The frequency of unsteady interaction should be tuned close to the vortex shedding frequency or within the range of lock-in frequency to couple with the separated flow field around the rotor blades, which might be utilized to guide the design of casing treatment in confirming the number of treatment slots.Copyright

An Experimental Investigation About the Effect of the Partial Casing Treatment on a Transonic Axial-Flow Compressor Performance Under Inlet Distortion

The traditional annulus casing treatment often pays the price of lowered efficiency for improving the stall margin of a compressor under inlet distortion. In view of the unsymmetry of the inlet flow-field of compressors, partial casing treatment was used to control the flow in a transonic axial-flow compressor with arc-skewed-slots deployed at different circumferential positions under inlet distortion. The experimental results indicate that when the partial casing treatment is arranged on the undistorted and distorted sectors, the stall margin is enhanced by 8.02%, with the relative peak efficiency improved simultaneously by 2.143%, compared with the case of solid casing at 98% rotating speed. By contrast, the traditional casing treatment increases the stall-margin by 23.13%, but decreases the relative peak efficiency by 0.752%. By analyzing dynamic and static experimental data, the mechanism underlying the partial casing treatment was also studied in detail here. The disturbances of inlet flow were restrained by annulus casing treatment, nevertheless the total pressure ratio was decreased obviously in the distorted sector. As a result, the stall-margin is improved, but the relative peak efficiency is decreased too. When the partial casing treatment was arranged on the undistortded and distorted sectors, the stall disturbances was thereby restrained. So the stall margin was enhanced. In addition, the total pressure ratio was improved by the partial casing treatment in the distorted and transition sectors, and thus the relative peak efficiency was also increased markedly.© 2012 ASME

Experimental Investigation on the Stall Inception of a Counter-Rotating Compressor

For investigating the flow phenomena in the stall process of a counter-rotating compressor (CRC), static and dynamic experiment have been carried out on a low speed counter-rotating compressor. Seven high response pressure sensors were mounted at the leading edge of rotor 1(R1) and rotor 2(R2) as well as at the outlet of R2 blade tip and root. Wavelet analysis and FFT transform was introduced to the post processing progress. According to the experimental results, the stall inception was found in the tip region of the second rotor. The rotating speed of stall cell in the compressor is about 35% of the rotor speed.Copyright

An Unsteady Numerical Investigation on the Hysteresis Stall Loop of a Counter-Rotating Compressor

For investigating the flow phenomena in the stall process, a full annular unsteady numerical simulation has been carried out on a low speed counter-rotating compressor. The numerical results are in good agreement with experimental results. According to the CFD results, the stall inception was found in the tip region of the front rotor. The rotating speed of stall cells in the front rotor are about 41% of the rotor speed and the direction is the same with the rotor rotating direction. The stall cells occupies about 20% of the blade span away from the casing wall when the compressor is in deep stall. The flow phenomena is well captured which explained why the compressor characteristic line appears as a hysteresis loop in the stall inception-recovery process.© 2011 ASME

Experimental Investigation of Non-Axisymmetrical Flow Control in a Low Speed Axial Compressor

The non-axisymmetric feature of the compressor separated flow field should be considered when flow control technology is utilized to improve compressor performance. An experiment is performed to investigate the effectiveness of non-axisymmetric flow control using arc curve skewed slot casing treatment in the paper. A simplified non-axisymmetric excitation model is presented with variable circumferential excitation extent and location. FFT analysis results indicate that the frequency spectrum of the non-axisymmetric excitation is similar with that of the whole circumferential excitation. The non-axisymmetric excitation possesses the same dominate frequency, smaller amplitude and wider frequency bandwidth compared to the whole circumferential excitation. A simplified circumferential non-axisymmetric arc curve skewed slot casing treatment is utilized to perform non-axisymmetric excitation on the separated flow field of a low speed single stage axial compressor under both uniform and distorted inlet conditions. Experimental results indicate that the non-axisymmetric slotted casing treatment presents strong flow control capability, which could improve compressor efficiency, total pressure rise coefficient and stall margin. For the distorted inlet condition, the stall margin, total pressure rise and efficiency of the compressor are respectively improved by 47.4%, 12.7% and 0.7% compared to the solid casing, and the compressor efficiency is improved by 1.4% compared to the whole circumferential excitation. For uniform inlet condition, the non-axisymmetric excitation can improve compressor efficiency by 1.0% and 1.5% respectively compared to the solid casing and the whole circumferential excitation. The whole circumferential excitation can also improve the compressor total pressure rise coefficient and stall margin, on the contrary, it decreases compressor efficiency. As a result, the non-axisymmetric slotted casing treatment can achieve more excellent compressor performance than the whole circumferential excitation does. Experimental results also indicate that the circumferential extent and location of the non-axisymmetric excitation can influence the effectiveness of the non-axisymmetric excitation. The best compressor performance can be achieved only when the non-axisymmetric excitation is tuned to match the asymmetric compressor separated flow field. Analysis on the experimental results indicates that compressor efficiency improvement achieved with the non-axisymmetric excitation can not simply attribute to the flow loss reduction induced by fewer casing slots. The flow loss reduction within undistorted sector, the circumferential flow exchange and the dynamic response induced by the non-axisymmetric excitation, the unsteady coupling between the non-axisymmetric excitation and the separated flow field might be the key flow factors to influence the compressor flow field structure, and hence influence the compressor performance.Copyright