Study On The Application Of Unsymmetrical Impeller In Double-sided Centrifugal Compressor And The Uniformity Of Circumferential Flow Field

The turbocharger with a double-sided centrifugal compressor could improve the matching relationship between compressor and turbine,and between the turbocharger and engine.Compared with the conventional centrifugal compressor,the double-sided centrifugal compressor has been got more and more attention because of the the relatively small size,the fast transient response and the wider stable operating.Due to the limited installation space in the engine compartment,the two side impellers of double-sided centrifugal compressor employed different inlet pipes to supply air,the front and rear impellers respectively corresponded the straight pipe and the bent pipe.Due to the longer pipe of the rear impeller and the big change in cross-section area and shape,the air flow friction loss via the rear pipe is noticeably bigger than that via the front pipe,then the air mass flow between two side impellers is different,especially,the mass flow between two side impellers is seriously unbalanced in small mass flow condition,which will affect the stable operating range of compressor.Because of the asymmetric volute structure and The rear non-uniform inlet distribution,the mass flow distribution in each impeller passages and the static pressure difference between pressure surface and suction surface have difference,especially in rear impeller.In order to enhance the operating range of double-sided centrifugal compressor,the work ability of rear impeller connected with bent pipe need to be increased,and the centrifugal compressor circumferential non-uniform flow structure also need to be studied.In this paper,the asymmetric impellers'influence on the double-sided centrifugal compressor characteristic and the stable operating range was investigated through experimental and numerical methods,and the non-uninform phenomena in circumferential flow field of double-sided centrifugal compressor also has been studied.The main study contents and results as follows:On the basement of the symmetric impeller model,two asymmetric impeller models is proposed by increasing the rear impeller radius respectively corresponding:1.015R₂ model and 1.03R₂ model.The numerical simulation has been carried on the symmetric model and two asymmetric models,and the performance curves of three models has been obtained,the results shows:to increase properly the rear impeller outlet radius could enhance the pressure ratio and efficiency of double-sided centrifugal compressor.It also reduce the flow unbalance characteristics between front and rear impellers in symmetric impeller model and delay the double-sided impellers converting from the parallel working mode to single-sided impeller working mode in small mass flow condition,which broaden the stable working range and improve the circumferential non-uniform distribution of each impeller passages of the double-sided centrifugal compressor,especially for real impeller.The distribution and influence of tip leakage mass flow of each impeller and each passage of one impeller at different operating condition for three models have been studied based on numerical simulation,the results shows that due to the asymmetric impeller effect on mass flow modulation between two impellers and the different working models at small mass flow condition for three impeller models,the tip leakage mass flow of each impeller and the ratio of tip leakage mass flow on the impeller passage mass flow were also changed.The asymmetric structure of the volute leads to circumferential asymmetric distribution of the static pressure difference between blade pressure side and suction side,which results into circumferential non-uniformity of the blade tip leakage mass flow in each impeller passages,and the bigger static pressure difference between blade pressure side and suction side is,the bigger tip leakage mass flow is.Especially for the rear impeller,affected by the non-uniform inlet distortion,the circumferential non-uniform for both the static pressure difference between pressure and suction surface and the tip leakage mass flow of each rear impeller blades are bigger than that of front impeller blades.At the big and small mass flow condition,the inner flow flied circumferential distribution in real inlet pipe,impeller passage and the diffuser of three double-sided centrifugal models has been studied,the results show that increased properly the rear impeller outlet radius could improve the flow field structure at the outlet of rear inlet pipe and the inner impeller passage of double-sided centrifugal compressor and make the flow field structure more uniform at big mass flow,meanwhile,it could suppress the flow separation phenomena at inlet of two side impellers at small mass flow and lead to flow separation at same time for both two side impellers,and then delaying the conversion of operation mode.