Numerical Study Of Stage Effects On Performance Of Centrifugal Compressors With Large Flow Coefficient

Centrifugal compressors are widely used in various fields of the national economy because of its high pressure ratio, wide operating range and reliable structure. With large complete sets of equipment, centrifugal compressor gradually develops towards large flow coefficient and high pressure ratio. Therefore, the development of high performance large flow coefficient model stages and the study of interaction between model stages have attracted more and more attention and recognition.The design of centrifugal impeller generally assumes that the flow is uniform, regardless of the prewhirl. But for multistage centrifugal compressor, The outlet flow field of former stage and the radial inlet duct are non-uniform, which have a certain degree of circling. This is particularly true of model stages with large flow coefficient. At the inlet distortion condition, the operating condition of next stage will deviate from the design condition, which affects the aerodynamic performance of the whole machine. How inlet distortion affects the flow field and how to control the distortion is one of the key issues to improve the aerodynamic performance of multistage centrifugal compressors.Numerical simulations for two-stage centrifugal compressor with large flow coefficient are applied in order to investigate interaction between stages. By calculating with two-stage centrifugal compressor or with single stage, the distribution characteristics of distortion flow at first stage outlet and formation reasons are analyzed. The changes of flow field and stage performance under different inlet flow conditions are compared. Under the multistage conditions, the influence of distortion flow at first stage outlet to the operating characteristic of second stage is clearly expounded. It can be found that distortion distribution at outlet of the first stage makes the aerodynamic performance of the second stage deviate from design performance on the trend and value. The build-up of secondary flow and the mixing loss in the impeller increased significantly. Especially in large flow conditions, the condition of blockage is obviously ahead of time. At design point, polytropic efficiency of the second stage has fallen by 4% under two-stage environment than uniform flow.In addition, for the impeller in this article, radial distortion at first stage outlet has more significant effect on performance of the second stage than circumferential distortion. The influence of airflow with different prewhirl is mainly embodied in the entrance section of the blade. The airflow with strong negative prewhirl makes the load of blade inlet section bigger. In comparison, the influence of airflow with different prewhirl on the blade loading distribution increases from blade root to blade tip gradually. The influence of distortion flow at first stage outlet to performance of second stage is more obvious in impeller and diffuser than in bend and return channel. In addition, the optimization scheme of tandem return channel is used to improve the flow field at first stage outlet and the performance of next stage.In addition to the relevant numerical study, the experimental scheme of interaction between centrifugal compressor stages is introduced. Due to the boundedness of experiment rig, only single impeller rotation experiment can be done. In order to achieve the study of interaction between model stages, the static guide vanes are used to simulate the real diffuser outlet Angle of the first stage instead of first-stage impeller and diffuser blade. In this way, the actual 2-stage becomes 1.5-stage. Based on this idea, blade modelling software (BladeGen) is applied in order to design inlet guide vane of 1.5-stage centrifugal compressor, and the feasibility of the experiment scheme is verified by the numerical method.