Analysis Of Unsteady Feature In High Speed Centrifugal Compressor With Vaneless Diffuser

The unsteady flow phenomena represented by stall and surge will affect the operation range of centrifugal compressor and can even cause serious accidents.In the high-speed centrifugal compressor,these phonomena are more complicated.In order to improve the safety and reliability of centrifugal compressor,it is necessary to study the characteristics of unsteady phenomena.Meanwhile,mastering the technique to identify the spatiotemporal information of unsteady flow is of great significance.Research is carried out on a high speed centrifugal compressor with vaneless diffuser.The unsteady characteristics at small flow rates are studied by dynamic test and numerical simulation.Following are the main contents and conclusions:1.Paticipating in the construction of high speed centrifugal compressor test bench.By deploying Kulite dynamic pressure sensor and using NI-9215 acquisition card,dynamic pressure measurement is realized.2.The aerodynamic performance of the high speed centrifugal compressor at four rotating speeds is obtained.The dynamic pressure signals from steady to unsteady flow conditions are acquired by the dynamic test technology,including the entire process from stall to surge.3.Based on the performance curve,the similar fracal dimension of dynamic pressure signals is analyzed.It is found that the signals have fractal dimension and multifractal characteristics.By calculating the correlation integral,the results show that the correlation integral at the impeller outlet decreases with the reduction of the mass flow rate,and the extent of variation is the most drastic when approaching the unsteady condition.4.The methods of empirical mode decomposition(EMD)and spatiotemporal intrinsic mode decomposition(STIMD)are used to analyze the dynamic pressure signals at the impeller outlet during the process of compressor instability,and several intrinsic mode functions are obtained.The comparison shows that the STIMD method can ameliorate the false component and modal aliasing problems in EMD.The rotating stall and surge precursor in the centrifugal compressor are captured by STIMD method during the process of instability.5.Based on the unsteady numerical simulation conducted in the twodimensional vaneless diffuser model at different inlet flow angles,the unsteady flow fields in the process of convergence and divergence are analyzed by dynamic mode decomposition(DMD)and proper orthogonal decomposition(POD).The methods visually reveal the development and suppression process of disturbance in the diffuser.6.The steady calculation of the centrifugal compressor with vaneless diffuser is carried out by using a single channel model.The performance curves at different rotating speeds are obtained and compared with the experimental results.7.The unsteady calculation of the centrifugal compressor with vaneless diffuser is carried out by using the full channel model.The performance curve at design speed is obtained,which is in good agreement with the experimental result.The evolution process of low-velocity fluid at low mass flow rate can be observed from the flow fields,and the frequency is close to the experimental result.DMD analysis of the unsteady flow field is performed to obtain the information of flow modals at different frequencies.