Self-recirculating Casing Treatment Research Articles

Influence of bleeding positions in self-recirculating casing treatments on the stability of a subsonic axial compressor

Influence of bleeding positions in self-recirculating casing treatments on the stability of a subsonic axial compressor

Redesign of a turbocharger compressor based on multi-component full-passage optimization

Self-recirculation casing treatment (SRCT) is often used to improve the flow stability of turbocharger compressors. However, it usually introduces penalties of the peak efficiency or the choke margin of compressors though assisted by auto-optimizations. For this, a multi-component optimization of SRCT and diffuser for a turbocharger compressor is implemented based on the full-passage computational model with a volute. As a result, the aerodynamic performances of the compressor are improved over the whole speed line. Specifically, the efficiency and pressure ratio at the design condition are increased by 1.23% and 1.31% respectively, those at the near-stall condition are improved by 1.84% and 3.13% respectively. The choke flow rate is improved by 1.44%, and the stable flow range is expanded by 2.7 percentage points. The flow mechanisms are as follows. Firstly, the bleed slot width is increased, then the recirculation flow rates in different impeller passages are improved and the low-energy fluids in corresponding passages are removed. The blockages and the circumferential asymmetry in impeller passages are reduced, hence, the flow stability is improved. Secondly, the flow area of diffuser passages is enlarged due to the reduced solidity of vaned diffuser, and then the swallowing capacity of the compressor is improved. Also, the mismatching effect introduced by the SRCT is compensated by the reduced solidity of vaned diffuser. Finally, the injection slot angle and height are decreased, then the distortions and mixing losses in impeller inlet are reduced. Also, the aerodynamic losses in impeller inducer are reduced by removing more stagnation fluids near inducer shroud due to the increased bleed slot width.

Effect of Self-Recirculating Casing Treatment on the Aerodynamic Performance of Ultra-High-Pressure-Ratio Centrifugal Compressors

The motivation to design a more efficient and compact aircraft engine leads to a continuous increase in overall pressure ratio and decrease in the stage number in compressors. Compared to the traditional multi-stage compressor, a single-stage ultra-high-pressure-ratio centrifugal compressor with a pressure ratio higher than 10.0 can significantly improve the engine’s power-to-weight ratio and fuel economy with a reduced structure complexity. Thus, it has great potential to be adopted in the compression system of advanced aero engines, such as turboshaft engines, in the future. However, the highly narrow Stable Flow Range (SFR) of ultra-high-pressure-ratio centrifugal compressors is a severe restriction for engineering applications. This research focuses on the aerodynamic performance of a ultra-high-pressure-ratio centrifugal compressor, and three-dimensional simulation is employed to investigate the effect of Self-Recirculating Casing Treatment (SRCT) on the performance and stability of the centrifugal compressor. Firstly, the parametric model of SRCT is established to investigate the effect of geometry parameters (rear slot distance and rear slot width) on the aerodynamic performance of the centrifugal compressor. It is concluded that SRCT improves the compressor’s SFR but deteriorates its efficiency. Also, a non-linear and non-monotone relationship exists between the SFR and rear slot distance or width. Then, the flow mechanism behind the effect of SRCT is explored in detail. By introducing the SRCT, an additional flow path is provided across the blade along the circumferential direction, and the behavior of the shock wave and tip leakage flow is significantly changed, resulting in the obviously different loading distribution along the streamwise direction. As a result, the mixing and flow separation loss are enhanced in the impeller flow passage to deteriorate the efficiency. On the other hand, the blockage effect caused by the mixing of slot recirculation and mainstream flow near the impeller inlet increases the axial velocity and reduces the incidence angle below the 90% spanwise section, which is considered to effectively stabilize the impeller flow field and enhance the stability.

Numerical investigation on the cross-stage self-recirculating casing treatment for a counter-rotating axial compressor

Although the counter-rotating axial compressor (CRAC) can promote the thrust-to-weight ratio of aero-engines, the “stall transposition” phenomenon poses a huge technology challenge in controlling the flow due to the speed ratio effect. Traditional flow control technology hardly operates under off-design conditions, posing an urgent need to develop a new flow control approach suitable for CRAC's variable speed ratio conditions. Hence, based on the conventional rear-jet self-recirculating casing treatment (SRCT), this work designs two cross-stage SRCT cases (front-jet SRCT and dual-jet SRCT) to resolve the “stall transposition” issue in a two-stage CRAC. The results demonstrate that although all SRCT cases improve the stall margin of the CRAC at the design condition, both front-jet SRCT and dual-jet SRCT cases have a significant stabilization capacity without changing the first stall stage, opposing the rear-jet SRCT case, which changes the first stall stage. The dual-jet and front-jet SRCT cases have a significant stabilization capacity under the off-design speed ratio conditions; since they can simultaneously act on the tip clearance flow of both the upstream and the downstream rotors and increase the stall margin by about 6.23%∼10.78% and 6.66%∼10.42%, respectively. Variations in speed ratio may shift the location of the first stall stage, forcing the rear-jet SRCT case to fail in expanding the stall margin at the partial speed ratio conditions. Moreover, the suppression of SRCT on the flow instability of the tip leakage flow decreases the unsteady pressure fluctuation intensity near the rotor tip.

Mechanism Affecting the Performance and Stability of a Centrifugal Impeller by Changing Bleeding Positions of Self-Recirculating Casing Treatment

This study aimed to investigate the influence of the bleeding position of a self-circulating casing on the aerodynamic performance of a transonic centrifugal compressor. Three types of self-circulating structures with the bleeding positions of 11% Ca (the axial chord length of the blade tip), 14% Ca and 20% Ca from the leading edge of the blade were studied by using the numerical simulation method, with the Krain impeller taken as the research object. It was found that all three types of self-recirculating casing treatments can expand the stable operating range of the impeller, and that at medium and small flow rates, the total pressure ratio and efficiency of the impeller increase gradually with the backward movement of the bleeding position. The self-circulating casing treatment can restrain the development of tip leakage vortex, reduce the blockage area, and improve the stability of the impeller by sucking low-energy fluid. The farther back the bleeding position is, the greater the bleeding mass flow rate of the self-circulating casing for the low-energy fluid in the blade-tip passage becomes. Additionally, a greater inhibition effect on the tip leakage vortex, and a better effect of improving the performance and stability of the impeller, can be obtained. The best air bleeding position is 20% Ca, but it is not directly above the blade-tip blockage center of the solid wall casing passage. Instead, it is downstream of the blockage area.

Experimental investigations on effects of the self-circulation casing treatment on acoustic and surge characteristics in a centrifugal compressor

Self-recirculation casing treatment is an effective technique to improve centrifugal compressor surge margin. This paper experimentally investigates the impacts of casing treatment on a centrifugal compressor using experimental methods and focuses on aerodynamic performance, surge trigger, surge cycles, and inlet acoustic characteristics. The measuring data were obtained from fast dynamic pressure transducer and microphones, and temporarily and spectrally analyzed to show surge inception and noise variation. The experimental results lead to the conclusions that (1) two separate phenomena trigger compressor to surge with different surge frequencies in the compressor with and without the casing treatment, and (2) the casing treatment suppresses the broadband noise sources but enhances the discrete frequency noise sources during surge cycles.

Flow mechanism of self-recirculating casing treatment in a low-reaction transonic compressor rotor

Casing treatment is used to ensure the stability of an aeroengine compressor when it is operated away from its design point. However, currently, there is little published data about the effect of casing treatment on highly loaded axial compressors. In this study, unsteady numerical simulations are applied to explore the flow mechanism responsible for the stability enhancement of a low-reaction (LR) transonic compressor rotor with ultra-high loading owing to self-recirculating casing treatment (SRCT). A special SRCT configuration is designed based on the suction effect, which can simultaneously improve the stall margin and aerodynamic performance. Radial skew in the direction of blade rotation is employed to enhance the control outcomes further. The results show that the best skew angle for stall margin and performance improvement is 45 degrees. Further analysis of the instantaneous flow structures reveals that for the smooth casing, the mechanism by which tip leakage vortex (TLV) breakdown, TSV, and blade loading interact induces periodic oscillations of the tip flow field. As the unsteady suction effect introduced by the SRCT dominates the evolution processes of vortical structures, the original oscillation frequency is converted into the SRCT-induced excitation frequency. Furthermore, a substantial amount of double-leakage flow is inhaled into the recirculation channel without rolling up into the adjacent TLV, which dampens the shock/TLV interaction and the formation of downstream TSVs. The blockage along the pressure side caused by the aforementioned large-scale vortices is effectively eliminated.

Effect of self-recirculating casing treatment on the unsteady flow and stability of counter-rotating axial-flow compressor

Abstract Counter-rotating axial-flow compressor (CRAC) is a promising technology to enhance the thrust-to-weight ratio of aero-engines. Self-recirculating casing treatment (SRCT) is an efficient flow control technique for increasing stall margin in conventional compressors. With the purpose of investigating the applicability and mechanism of SRCT in the CRACs, a two-stage CRAC is selected to investigate the stability enhancement mechanism of SRCT and its effect on the unsteady flow near the rotor tip, and the effect of injector location on the stability improvement capacity of SRCT is also studied. Results show that about 7.73% stall margin improvement can be achieved by configuring the SRCT on the near rotor top, and the injector location also has a significant influence on the stability expansion potential of SRCT. The SRCT delays the stall occurrence by weakening the intensity of tip leakage flow (TLF) and restraining the leading-edge spillages of TLF. The SRCT reduces the unsteady interference between the adjacent rotors by receding the disturbance of the upstream wake and inhibiting the potential flow effect of the downstream. Furthermore, the SRCT reduces the self-excited oscillation frequency of TLF and damps its fluctuation amplitude.

Stability improvement without efficiency penalty of a transonic centrifugal compressor by casing treatment and impeller/diffuser coupling optimization

Self-recirculation casing treatment (SRCT) is usually used to improve the flow stability of centrifugal compressors, while extra aerodynamic losses are often introduced. It is challenging to remove the efficiency penalty for the design of SRCT. In this study, a coupling optimization is implemented to redesign a transonic centrifugal compressor with an SRCT. Firstly, the physical design space is expanded to include the design parameters of both the SRCT and the impeller/diffuser. Secondly, data-mining techniques are used to explore and refine the expanded design space. Consequently, seven critical design parameters are quantitatively identified, and they generally present non-linear and trade-off correlations with the compressor performances. Then, the global optimization is carried out for the refined design space. As a result, significant improvements of the aerodynamic performances of the compressor are obtained. The efficiency penalty introduced by the SRCT is eliminated, and the stable flow range is maintained simultaneously. This is infeasible for the single redesign of the SRCT or the impeller/diffuser. The flow mechanisms of the coupling optimization are as follows. The recirculation flow rate of the SRCT is increased, and more low-energy fluid near the shroud is removed. The incidence angle and the leading-edge blade loading at high spanwise region of the impeller are reduced. The interaction between the tip leakage vortex and the inducer shock wave is delayed and weakened. The inlet mixing loss and the inlet radial distortion of the impeller are decreased. The friction loss is reduced and the flow separation is suppressed in the vaned diffuser.

Numerical study on the stability enhancement mechanism of self-recirculating casing treatment in a counter-rotating axial-flow compressor

Counter-rotating axial-flow compressor (CRAC) is a promising technology to increase the thrust-to-weight ratio of aero-engines. Self-recirculating casing treatment (SRCT) is selected to explore its stability enhancement capacity in the CRAC. In the present work, an effective SRCT is designed by the design of experiment method, and the stability improvement potential of the SRCT and its mechanism of stall margin enhancement are studied. Results show that the optimal SRCT scheme increases the stall margin by about 7.73% and without remarkable peak efficiency loss, and the overall performance of the compressor is enhanced at the near-stall condition. The SRCT improves the quality of the tip flow field by sucking out low-energy fluid and restrains the spillage of the tip leakage flow by the jet effect from the injection port. In addition, the SRCT also reduces the unsteady interference between the adjacent rotors. The effect of speed ratio on the effectiveness of SRCT is further investigated, and results indicate that the SRCT increases the stall margin of the CRAC by about 4.13–5.80% at some off-design speed ratio conditions. The speed ratio affects the first stall stage of the CRAC, thus affecting the effectiveness of the SRCT.

Aeroelastic Aspects of Axial Compressor Stage With Self-Recirculating Casing Treatment

Abstract For successful implementation of casing treatment designs in axial compressors, apart from the stall margin improvement benefits, aeroelasticity also plays a major role. This manuscript addresses the not often discussed aeroelastic aspects of a new discrete type of passive self-recirculating casing treatment (RCT) designed for a transonic axial compressor stage. Experiments are carefully designed for synchronized measurement of the unsteady fluidic disturbances and vibrations during rotating stall for compressor with baseline solid casing and self-RCT. The modal characteristics of the axial compressor rotor-disk assembly are studied experimentally and numerically. Experimentally it is observed that the rotating stall cells excite the blades in their fundamental mode in a compressor with baseline solid casing at the stall flow condition. In contrast, there is no excitation of the blades in the compressor with self-RCT at the same solid casing stall flow condition. Also, the self-RCT compared to the solid casing can significantly reduce the overall vibration levels of the blades that are excited at the stall flow condition. The casing treatment is able to alter the flow field near the tip region of the rotor blade, and hence influencing the forcing function of the rotating cantilever blades to have the aeroelastic benefit.

Aerodynamic behavior of a transonic axial flow compressor stage with self-recirculating casing treatment

In this manuscript, a new discrete type of passive Self-Recirculating Casing Treatment (RCT) is designed for a transonic axial compressor stage, fabricated, and tested for its performance. Parametric evaluation of the casing flush-mounted RCT for injection skew angle is studied experimentally and numerically to achieve considerable stall margin improvements. Detailed flow investigations are carried out for the RCT configurations and baseline solid casing behavior at different flow conditions using steady and unsteady measurements. The unsteadiness in the flow and the rotating stall signatures are captured through Kulite pressure sensors. Results indicate improvements in the stall margin and other overall compressor performance parameters for the investigated speeds. Improvements of stall margin in axial compressor stage with self-recirculating casing treatment are found to be because of the RCT by investing the already worked high-pressure fluid is able to overcome losses associated with the tip leakage flow. There is a delay in the onset of rotating stall instability with casing treatment which plays a role in the enhanced performance improvements.

Mechanism of affecting the performance and internal flow field of an axial flow subsonic compressor with self-recirculation casing treatment

This investigation aims to understand the mechanisms of affecting the axial flow compressor performance and internal flow field with the application of self-recirculation casing treatment. Besides, the potentiality of further enhancing the compressor performance and stability by optimizing the geometric structure of self-recirculation casing treatment is discussed in detail. The results show that self-recirculation casing treatment generates about 7.06, 7.89% stall margin improvements in the experiment and full-annulus unsteady calculation, respectively. Moreover, the compressor total pressure and isentropic efficiency are improved among most of operating points, and the experimental and calculated compressor peak efficiencies are increased by 0.7% and 0.6%, respectively. The comparisons between baseline shroud and self-recirculation casing treatment show that the flow conditions of the compressor rotor inlet upstream are improved well with self-recirculation casing treatment, and the degree of the pressure enhancement in the blade top passage for self-recirculation casing treatment is higher than that for baseline. Further, self-recirculation casing treatment can restrain the leading edge-spilled flows made by the blade tip clearance leakage flows and weaken the blade tip passage blockage. Hence, the flow loss near the rotor top passage is reduced after the application of self-recirculation casing treatment. The rotor performance and stability for self-recirculation casing treatment are greater than those for baseline. The flow-field analyses also indicate that the adverse effects caused by the clearance leakage flows of the blades tip rear are greater than those made by the clearance leakage flows of the blades leading edge. When one injecting part of self-recirculation casing treatment is aligned with the inlet of one blade tip passage, the flow-field quality in the passage is not the best among all the passages between two adjacent injecting parts of self-recirculation casing treatment. Further, the flow-field analyses also indicate that the effect of the relative position between the blade and self-recirculation casing treatment on the flows in the self-recirculation casing treatment may be ignored during the optimization of the recirculating loop configuration.

Roles and mechanisms of casing treatment on different scales of flow instability in high pressure ratio centrifugal compressors

The development of single-stage high pressure ratio centrifugal compressors for small gas turbine engines is hindered by compressor flow instabilities with various temporal and spatial scales. In this paper, the effect of a self-recirculation casing treatment device for suppressing various scales of flow instability has been discussed. Rig tests of a high pressure ratio centrifugal compressor with and without a casing treatment device are performed, and transient pressure signals are measured at endwalls by fast response pressure transducers. By analyzing measured pressure signals in both time and frequency domain, the casing treatment device is found effective in eliminating rotating instability and stall at the impeller inlet at low speeds, as well as suppressing surge of the compression system at middle and high speeds. The flow recirculation process generated inside the casing treatment device removes the vortical structures at the impeller inlet tip section, therefore eliminating the regional rotating instability and stall. Besides, the casing treatment device turns the pressure rise characteristics of the impeller more negative by enhancing the impeller work input, but turns that of the vaned diffuser more positive by increasing the incidence at the vaned diffuser inlet. Combining both effects, mild surge and the deep surge are suppressed.

Design and Experiment of Casing Treatment for a Centrifugal Compressor

AbstractCentrifugal compressors have been widely used in light-duty turboshaft engines and automotive turbochargers. One of the essential requirements for centrifugal compressors in these applications is to have adequate stable flow range. As the pressure ratio increases, the stable flow range drops dramatically, and this highlights the importance of stability improvement for centrifugal compressors. This paper first gives a brief review on the measures available for extending the stable flow range, and then focuses on the realization of self-recirculating casing treatment for a centrifugal compressor. The design principles of the casing treatment were introduced. Then the simulations were conducted to design and optimize the geometry of the casing treatment, and finally an optimum case was selected for experimental validation. The experiment results indicate that the designed casing treatment improve the stable flow range by about 5 percent at design speed, while keeping the efficiency almost identical.

Characterizing Flow Effects of Ported Shroud Casing Treatment on Centrifugal Compressor Performance

This paper presents an investigation of the effects of ported shroud (PS) self-recirculating casing treatment used in turbocharger centrifugal compressors for increasing the operable range. The investigation consists of computing three-dimensional flow in a representative centrifugal compressor with and without PS at various levels of approximations in flow physics and geometrical configuration; this provides an enabler for establishing the causal link between PS flow effects and compressor performance changes. It is shown that the main flow path perceives the PS flow as a combination of flow actuations that include injection and removal of mass flow and injection of axial momentum and tangential momentum. A computational model in which the presence of the PS is replaced by imposed boundary conditions (BCs) that reflect the individual flow actuations has thus been formulated and implemented. The removal of a fraction of the inducer mass flow has been determined to be the dominant flow actuation in setting the performance of PS compressors. Mass flow removal reduces the flow blockage associated with the impeller tip leakage flow and increases the diffusion in the main flow path. Adding swirl to the injected flow in the direction opposite to the wheel rotation results in an increase of the stagnation pressure ratio and a decrease of the efficiency. The loss generation in the flow path has been defined to rationalize efficiency changes associated with PS compressor operation.

Flow Control by Slot Position and Noise Baffle in a Self-Recirculation Casing Treatment on an Axial Fan-Rotor

To address the situations where the casing treatment needs to be used to stabilize axial compressors through strong recirculation, this paper initiated a CFD study to investigate how the flow could be suitably controlled in the casing treatment to minimize the efficiency penalty and increase the flow range. A counter-swirl self-recirculation casing treatment was first designed on a low speed axial fan rotor as a baseline case. Then three different slot positions and the influence of including the noise baffle were numerically studied. Based on the understanding of their coeffects, the shorter noise baffle was considered and it was found that the highest efficiency was achieved in the case of the upstream slot when the length of baffle was suitably adjusted to balance the incoming flow and recirculation. The largest flow range was achieved by locating the slot at the most downstream position and using a 50% length baffle since it suitably controlled the recirculating flow and relieved the separation at the low-span region. An optimization study showed that the optimum length of the baffle for efficiency was always larger than for the flow range. Both of the two optimum values reduce as the slot moves downstream.

Experimental study of self-recirculating casing treatment in a subsonic axial flow compressor

Parametric studies of recirculating casing treatment were experimentally performed in a subsonic axial flow compressor. The recirculating casing treatment was parameterized with injector throat height, injection position, and circumferential coverage percentage. Eighteen recirculating casing treatments were tested to study the effects on compressor stability and on the compressor overall performance at three blade speeds. The profiles of recirculating casing treatment were optimized to minimize the losses generated by air recirculation. In the experiment, the stalling mass flow rate, total pressure ratio, and adiabatic efficiency of the compressor were measured to study the steady-state effects on the compressor performance of recirculating casing treatments, and static pressure disturbances on the casing wall were monitored to study the influence on stall dynamics. Results indicate that both the compressor stability and overall performance can be improved through recirculating casing treatment with appropriate geometrical parameters for all the test speeds. The influence on stall margin of one geometric parameter often depends on the choice of others, i.e. the interaction effects exist. In general, the recirculating casing treatment with a moderate injector throat and a large circumferential coverage is the optimal choice to enhance compressor stability. The injector of recirculating casing treatment should be placed upstream of the blade tip leading edge and the injector throat height should be lower than four times the rotor tip gap for the benefits of compressor efficiency. At 71% speed, the blade tip loading is decreased through recirculating casing treatment at the operating condition of near peak efficiency and increased near stall. Moreover, the outlet absolute flow angle is reduced in the tip region and enhanced at lower blade spans for both operating conditions. The stall inceptions are not changed with the implementation of recirculating casing treatment for all the test speeds, but the stall patterns are altered at 33% and 53% speeds, i.e. the stall with two cells is detected in the recirculating casing treatment compared with the solid casing with only one stall cell.

Unsteady influence of Self Recirculation Casing Treatment (SRCT) on high pressure ratio centrifugal compressor

Self-Recirculation-Casing-Treatment (SRCT) is a widely employed method to enhance aerodynamic stability of a centrifugal compressor. This paper investigated unsteady effects of SRCT on the flow in a transonic centrifugal compressor via numerical method validated by experimental test. Firstly the static pressure distribution in the compressor without SRCT is measured for information of boundary conditions as well as validation. Then a 1-D unsteady model of a single passage is built and validated based on the experimental results. Next, the 1-D model of a passage with SRCT is built to investigate the unsteady influence of the SRCT on the flow in the passage. Finally 3-D unsteady CFD is employed to investigate the detailed influence of SRCT on the flow field in impeller passages. Results show that the topology of the passage with SRCT can remarkably damp the distortion propagating from downstream, hence depress the magnitude of the inlet flow distortion. Furthermore, the width of the rear slot in SRCT is the key factor for the damping effect. The 3-D simulation results further show that the fluctuations of the re-circulated flow rate via the front slot is depressed by the SRCT which is attributed to the damping effect of its configuration.

Simulation on Expanding Steady Working Range of Centrifugal Compressor by Self-Recirculation Casing Treatment

Numerical simulation method is adopted to simulate and research Self-Recirculation Casing Treatment (SRCT) on expanding the scope of steady work of high-pressure-ratio centrifugal compressor, and increase the compressor surge margin. Contrast the influences which are caused by structure sizes of the SRCT on the performance of compressor. The results showed that SRCT make the compressor stable operating range average broaden more than 13% on all kinds of speed, the surge margin increased by 11% at most. During the structure sizes, the distance between the bleed slot and the splitter blade has the largest affect on the performance of centrifugal compressor, followed by the bleed slot width, others have less affect. SRCT make the distribution of the compressor impeller inlet relative Mach number is more uniform, weaken the phenomenon of blade leading edge local high Mach number. It is also decrease the blade inlet positive incidence, inhibit separation of the blade surface boundary layer.