Investigation Of Operability Margin Improvement In A Centrifugal Compressor With Inlet Recirculation Casing Treatment

Compressor is one of the important parts of aeroengines and gas turbines.With the tackle of key technology of“two machines”during the 13th Five-Year Plan period,the indepth study of compressor and improvement of the performance has become the research topic of researchers.Centrifugal compressors often need to work under the non-design conditions,so the development of a wide range of work with high efficiency and stability of the centrifugal compressor is the key to the development of high-performance engines.Both experimental and numerical simulation approachs are adopted to research the operability margin improvement mechanism of a centrifugal compressor with inlet recirculation casing treatment.The experimental results verify the reliability of the numerical simulation.Two kinds of guide vanes,which are installed in the casing tretment,are designed:the pre-swirl vanes and the negative pre-swirl vanes are calculated and compared with the prototype compressor.The conclusions are as follows:The choke may occur at the guide vane,the splitter or the diffuser.the shock at the guide vane is weaked due to the diverging action of the upstream slot and the centripetal flow of the discharge slot,which improves the flow conditions here.And for the splitter and diffuser,there is little impact,therefore,the compressor choke flow is not widened.Near the surge conditions,the tip leakage flow seriously,low-energy fluid accumulation and extended to most of the passage,impact greatly on the performance of the compressor.The casing treatment can pump down low-energy fluid,reduce the angle of attack in the leading edge of the blade,and increase the load of the latter part of the blade,improve the flow characteristics,increase the pressure ratio,delay the occurrence of surge and broaden the stability of the working range about 12%.After adding the casing treatment,the flow in the compressor is changed.The main change contains the increase of the axial velocity near the tip of the impeller and the increase of the relative flow angle,so that the countercurrent condition upstream the impeller and the upstream vortex flow can be obviously weakened,the low-energy fluid upstream the tip of blade is eliminated,gas flow speed up.The downstream vortex flow has not improved,and the is no significant change in velocity field,indicating that although the use of casing treatment can make the downstream flow deterioration,but the improvement of the upstream is more obvious,so the casing treatment has played a significant role in improving the flow in the compressor.Negative pre-swirl guide vanes have almost no effect on the performance of centrifugal compressor under high flow conditions,but the maximum pressure ratio of the compressor increases,the work capacity is strengthened and the operability margin is improved significantly.The pre-swirl guide vanes also did not change the performance of the high flow conditions,and its improvement of the pressure ratio is not as obvious as the negative pre-swirl,the surge boundary widening only at high rotation speed.The introduction of the negative pre-swirl guide vane increases the work of the first half of the blade and increases the maximum pressure ratio of the compressor.The relative Mach number of the ₁S flow surface can be effectively increased to eliminate the low-energy fluid at the top of the impeller.