The Mechanism And Passive Control Of Endwall Flow In High-load Axial Flow Compressors

The design of high-load compressors makes the endwall flow a big challenge.Induced by the secondary flow,the corner separation becomes a major loss source in the passage.Moreover,the corresponding blockage changes the through flow and leads to mismatching between stages and thus the stability problems.To ensure the operating of high-load compressor in the engine,the current paper focused on a type of passive flow control technique,i.e.endwall contouring.The research was based on the open questions about endwall flow control,aiming at promoting the theory and application of endwall contouring to improve the endwall flow and thus the performance of compressors.The overall research includes five parts.Based on a first review of the endwall contouring technique,a new empirical method was built for the design of endwall contouring in compressors.A datamining study coupled with a numerical optimization were carried out in the second part to obtain the effective profiling method.An experiment of endwall contouring was then conducted to validate the above empirical method and profiling method.After that,an inverse method of designing endwall contouring was put forward,with its effectiveness confirmed by the numerical study.The final part investigated the flow control method and interaction of endwall contouring in the high load stages with tandem stator by using the numerical optimization.The details were provided as following.1.To enable the control of local endwall flow,a new empirical method has been developed for better performance in the application on compressors.In this method,the concept of profiling units has been defined for different functions in controlling the local endwall flow.A superposition was made for the profiling units with their weight to form a complex endwall surface with all the functions of profiling units.A software was developed based on the empirical method.According to the numerical result on a high-load compressor cascade,the new method slightly reduced the overall loss,but still perform better than the conventional empirical method.The software is suitable for the profiling in compressor.An adjustment of the weight factor of different units could further improve the performance.2.Based on the simulation on a high-load compressor cascade,a data mining study has been carried out using the relative analysis and the self-organizing neuron network to conclude the effective flow control method for compressors.The conclusion provided a guideline for the design of endwall contouring.It is found that the overall vortex intensity,loss and secondary kinetic energy will be closely related to each other only when the separation developed to a considerable level,meaning that the control of loss can be achieved by suppressing the secondary flow.To effectively control the separation,the endwall contouring should slow down the endwall flow along stream wise and accelerate the cross flow at the origin point of separation.Meanwhile,at the region of the accumulated low energy fluid of separation,the cross and stream wise endwall flow both should be accelerated to reduce the separation loss.The cross flow in the mid-pitch should also be generally reduced to relief the corner separation.3.Experimental study was carried out on a subsonic axial compressor stage.The endwall contouring was designed using the new developed empirical method,guided by profiling rules obtained during the data mining study.During the experiment,the overall performance was tested using the three-hole probes and five-hole probes.The endwall flow field was measured with pressure taps and visualized using the oil-flow test.The result showed that the generation and development of corner separation under the influence of real unsteady inhomogeneous inflow was almost the same with in the compressor cascade.The endwall contouring effectively relief the corner separation and improved the efficiency at the point of minimum stator loss by 0.45%.The endwall loss was reduced across the operation line of compressor.The effectiveness of the empirical endwall contouring method and the profiling rules were confirmed.4.A new reverse method for endwall contouring has been built based on the character of endwall flow in compressors.The complex real endwall flow field was simplified to a twolayer flow structure.A simplified equation group with the ability to calculate the profiled endwall according to a given target end wall flow was then derived.The numerical result proved the new developed reverse method superior to the empirical method,because the geometry of the end wall surface was decided by calculation,meaning a more accurate control of endwall flow avoiding steep rising and dipping of surface which could deteriorate the flow.The endwall designed by the new revers method spend much less time than the optimization but reduced loss to a similar level at the design point.Near stall point,the loss was reduced by about 50% of the optimization profiling result.5.A numerical study was carried out on a high-load compressor stage with tandem stator.After the validation of the numerical method,the rotor and stator endwall were respectively parameterized by 30 and 28 free control points.Endwall contouring were then designed using numerical optimization for the rotor,the stator and the full stage of compressor.The CFD result found a significant blockage near pressure corner of the hub endwall of rotor which almost remained the same regardless of the throttling in compressor.The optimum flow control method should suppress the separation started in the pressure corner region in the main blade and accelerate clearance flow to blow away the blockage.In the rear blade passage,the endwall contouring should suppress separation flow at the start point of corner separation and accelerate the cross and stream wise flow near the accumulation region of corner separation to reduce loss.The simulation also identified the strong interaction of endwall contouring between stages.The optimum rotor endwall contouring was found to reduce the loss in the stator by changing its inflow condition.The coupled profiling in both rotor and stator was found to improve the overall performance of the stage to the best level rather than to maximize the benefit of endwall contouring in the rotor and stator respectively.