Investigation On The Flow Mechanisms And Design Method Of Tandem Impeller In A Highly-loaded Centrifugal Compressor

The centrifugal compressor is widly used in small and medium aero-engines due to its high pressure-ratio,compact structure and high reliability.However,the performance levels of centrifugal compressors are far from satisfaction until to now.With the intense turning of flow path and Coriolis force,there are always complex secondary flows and highly-distorted discharge flows within impellers,which finally lead to the performance deterioration of diffuser downstream and the whole compressor stage.Additionally,the increasing demand for higher loading and higher pressure ratio of centrifugal compressors triggers supersonic impeller flows and sets new challenge for designers.Since the flow quality within centrifugal impellers plays the fundamental role in the compressor performance,it is of vital importance to improve the impeller flow condition.The tandem impeller method developed by the flow control view point has the potential to achieve the compressor flow control with no additional complex configurations or control costs.In this paper,a series of tandem impellers were designed based on a single stage,high pressure ratio centrifugal compressor used on a small aero-engine,for the purpose of investigate the complex flow mechanisms and design criteria in them.The main body of this research consisted of four parts:Firstly,the aerodynamic performance of tandem impeller coupled with the inducer/exducer circumferential arrangement was investigated.The tandem impellers were initially designed as the ruled surface blades.A parameter investigation of the effects of tandem design on the compressor performance under transonic and subsonic inflow conditions was carried and a maximum stage efficiency gain of 1.4%at 100%design speed was achieved.Particular emphasis was placed on the impeller and diffuser performance gain mechanisms.According to the investigation,the critical role of circumferential arrangement of tandem impeller was identified and the influences of this parameter on the flow structure were analyzed.The optimum value of the circumferential arrangement parameter was obtained,eventually.Secondly,the inducer/exducer stream-wise arrangement was detailed investigated.At 100%design speed where the inflow transonic,two main design parameters of axial displacement space and meridional split location were analyzed in detail to figure out its effect and mechanism on the impeller flow pattern.The best value selections of the two parameters in tandem impeller design were obtained.Thirdly,the optimum range of tandem impeller application was detailed analyzed.Based on a couple of tandem impeller designs of centrifugal compressors with different impeller inlet/outlet radius ratio D_1t/D₂,the parameter D_1t/D₂ was proven to be critical to tandem impeller application superiority.The optimum D_1t/D₂ range was consistent to the geometry characteristic of future advanced high load centrifugal compressor,which contributed a lot to the establishment of the design method and criteria of tandem impeller.Finally,the free-formed transonic inducer blades of tandem impellers were evaluated in detail aiming at further enhancing the performance gain of tandem impeller.Contributed by the much more degrees of design freedom of tandem impeller,the inducer blade could be designed using3-Dimensional method with compound blade sweep and lean,which had the ability to better control or adopt the supersonic flow and secondary flow within impellers.Nevertheless,the exducer blade still used the ruled surfaces to maintain the manufacture costs.As a result,the tandem impeller coupled with free-form inducer finally achieved the stage efficiency gain of2.11%,which further proved the potential of tandem impeller method.According to the systematic in-depth investigation in this paper,the tandem impeller flow mechanisms and design methods for high pressure ratio centrifugal compressor were identified,which provided the basic theoretical support to tandem impeller technology applications and future advanced centrifugal compressor designs.