The Effects And Flow Mechanisms Of Tip Injection And Recirculating Casing Treatment In Axial Flow Compressors

Improving the stability of compression systems has always been a key issue for the design of modern aero engines.Passive endwall technologies such as casing treatment are the primary methods to extend the stable operating range of compressors.Compared with traditional casing treatments,tip injection and recirculating casing treatment(RCT),which have the potential to improve or not decrease the compressor efficiency as well as to enhance compressor’s stall margin,have been popular research topics in recent years all over the world.The present study experimentally and numerically implemented parametric studies of tip injection and RCT on a subsonic rotor,a transonic rotor and a transonic multistage compressor.The main purpose is to explore the design rules and underlying flow mechanisms of tip injection and RCT in a vast scope ranging from a subsonic flow condition to a transonic flow condition and from a single-stage environment to a multistage environment,and thus to design high-efficiency and engineering-applicable mechanisms for enhancing compressor stabilityThe test injectors and RCTs were first optimized and designed with a modular design procedure to generate high-efficiency and compact injectors and RCTs.Parametric studies of tip injection were then performed experimentally on the subsonic rotor.The measurements of pressure fluctuations at the rotor tip and full-annulus unsteady simulations were employed to study the mechanisms of stability enhancement.Tip injections were also implemented in the transonic rotor with multi-passage unsteady simulations to find out the similarities and differences between the subsonic rotor and the transonic rotor for the application of tip injection.Second,the design rules and flow mechanisms of RCT were studied experimentally on the subsonic rotor,and the mechanisms of the improvements in both compressor stability and efficiency were analyzed in detail.Finally,a new-type RCT was developed for the transonic multistage compressor whose stall margin could not be improved with the traditional casing treatment and tip injection.The feasibility of improving simultaneously the rotor’s stability and the stator’s stability was then discussed by re-designing the stator.The main content and conclusions are summarized as follows:1.Design of injector and RCT.High-efficiency and compact actuators are the necessary prerequisite to implement experimental studies of tip injection and RCT.The injector and RCT were optimized to make injected jets attached to the casing wall as close as possible,and the total pressure recovery coefficient of the optimized RCT reached up to 0.98.The various structures of injector and RCT were achieved with a modular design procedure,resulting in 18 injection configurations and 18 RCT configurations.2.Effects and mechanisms of tip injection.Tip injections were tested on the subsonic rotor tostudy the effects of injection parameters such as injector throat height and injector spacing on compressor stability.The combinations of tip injection with traditional casing treatments were also experimentally studied.The results show that the compressor’s stall margin can be improved by 15% with an injection mass flow rate of 0.64%(normalized by the main flow at the near stall condition).The stall margin of the rotor with circumferential grooves is improved with tip injection from 9% to 19%;whereas the stall margin of the rotor with axial slots cannot be improved with tip injection.The parametric studies show that the stall margin is maximized as soon as the injectors are choked.The optimal injector throat height and circumferential coverage percentage are 2 times the rotor tip clearance and 8.3%,respectively.The injector spacing and inlet distortion have no influence on the effects of tip injection on compressor stability.Tip injections were also investigated on the transonic rotor with multi-passage unsteady simulations.The quantitative analysis and correlation analysis of blade tip blockage show that injection efficiency(the averaged decrement of tip blockage in the injection domain)determines the improvements of stall margin.The depression on the tip blockage is found to be primarily responsible for the stability enhancement for both the subsonic and transonic rotors.The unsteady effect of tip injection,i.e.,the recovery of blade tip blockage lags behind the recovery of tip leakage vortex,maintains the accomplishment of blockage diminishment in the circumferential direction and thus makes the stability improvement possible with only a few injectors around the annulus.3.Effects and mechanisms of RCT.A total of 18 RCT configurations were tested on the subsonic rotor to study the effects on the compressor performance.The measurement of pressure fluctuations over the rotor tip and full-annulus numerical simulation were also implemented to understand the underlying flow mechanisms of the improvements in both the compressor stability and efficiency.The results show that the stall margin and efficiency are improved at most by 10% and 2%,respectively.The depression on the blade tip blockage related with the tip leakage vortex is the main reason for the stability enhancement,and the change in the unsteadiness of double-leakage flow and the reduction in the leaking mass flow are responsible for the efficiency improvement.4.Exploration on the design of RCT in the transonic multistage compressor.The experiment demonstrated the failure of traditional casing treatment in the transonic multistage compressor because the stall inception was blade tip stall at 90% design speed although the stall is spike-initiated.The proposed new-type RCT,which improves the rotor tip flow condition by bleeding tip blockage,improves the compressor’s stall margin and efficiency at the design point by 10.96% and 0.54%,respectively.The compressor’s stall is triggered by the stator rather than the rotor under the effect of RCT.The stall margin of the compressor with RCT isfurther improved by optimizing the stator.