Stability Enhancement Mechanism And Multi-optimization Of Casing Treatment In A Transonic Centrifugal Compressor

With the increasingly aggravated energy shortage,the energy saving and gas emission reduction of the internal combustion engine receive more attention.Turbocharger becomes the key equipment of vehicles and boats due to the advantages of fuel economy and gas emission.As a core part of the turbocharger,the centrifugal compressor turns mechanical energy into pressure energy.The compressor speed is becoming larger and larger with the rising requirement of pressure ratio,resulting in a more complex transonic flow.The interaction between strong shock wave and transonic flow in compressor passage considerably decreases the stable range of operation.In order to guarantee the high efficiency and wide stable range of operation under high pressure ratio condition,casing treatment(CT)is the most effective way to achieve it.The research focuses on a self-recirculation casing treatment,an experimentally validated 3-D numerical simulation method was applied to a high pressure ratio transonic centrifugal compressor with or without casing treatment to explore the stability enhancement mechanism under different operating conditions.At choke condition,part fluid enters the compressor through casing treatment and the increasing of circulation area leads to the rising choke mass flow rate.Near stall condition,the effect of casing treatment downstream slot on the blade tip leakage vortex can cut off the passage vortex at upstream side of blade.The angle of attack at leading edge is decreased simultaneously under the effect of upstream slot,which inhibits the flow separation at leading edge and delays the stall inception.To extend the stable range of operation for a centrifugal compressor and without the expense of efficiency and pressure ratio,an auto-optimization of casing treatment for was conducted.Data mining techniques were firstly adopted before optimization to extract the non-linear relations and trade-off between casing treatment geometry and compressor performances.As a result,the key geometry parameters were identified and design space were refined.Analysis results show that the position and width of the downstream slot,the angle of the upstream and the tilt of bypass have a significant influence on performance,to which more attention should be paid in the optimization.Based on data mining results,a multi-objective multi-points(maximizing isentropic efficiency at design and near stall condition,not less than total pressure ratio at design and near stall condition and choke mass flow)optimization of casing treatment was established by integrating Optimal LHS,numerical simulation,kriging model and NSGA-II algorithm.The optimization results indicated a considerable improvement in the pressure ratio and efficiency of the compressor over the whole operating condition.