| The flow speed of centrifugal impeller exit from root to tip have great differences, which make the flow structure of diffuser at downstream is complex. Because of it, the performance of diffuser is weaker and the working range is more narrow, and it becomes the important factor of restricting the centrifugal compressor performance. In order to grasp the influence of the impeller outlet flow on the internal flow mechanism of diffuser and put forward the corresponding solutions, in this paper, the performance and flow field structure of the diffuser with the different inflow conditions is studied, including the fllowing contents :1. The study of the flow angle development in the import extension and its influence on diffuser flow, established the method for determining its length.Through the numerical calculation and analysis of flow development in the import extension, the flow angle in boundary layer decreased first and then increased caused by the favorable pressure gradient because of the given inflow condition and then the adverse pressure gradient because of channel expansion; the flow angle within the mainstream increased first and then decreased because of the development of boundary layer which makes the effictive flow width narrow. It was analyzed in detail about reason of flow angle development from two aspects of the radial velocity and circumferential velocity. It was found that the large flow angle within the boundary layer of import extension caused the strong secondary flow and then generated separation in the passage of diffuser downstream. Combining with the influence of the working status and structure size and downstream blade on the flow angle development in import extension, it has established the general extension length determination method.2. Research on the mechanism of the radial diffuser performance deterioration and changes of flow structure caused by the inflow with linear distribution velocity at spanwise directionThe performance and internal flow structure of vaned diffuser with uniform and non-uniform inflow conditions were calculated and analyzed, and the research that the influcence of non-uniform speed inflow at spanwise direction on the performace and flow structure of diffuser was done. Design the unilateral expansion channel test system, and verify the correctness of the flow mechanism analysis. It is known that the mechanism of flow structure changing through the analysis of pressure gradient distribution. While the inflow is non-uniform, it has mutual influcence between flow layers at spanwise direction, and pressure distribution tends to the flow layer with the average inlet Mach number. The flow at low Mach number inflow layer goes through relatively higher pressure resistance with low kinetic energy, and generates a new separation characteristic at back of vane. The separation causes severe blocking phenomenon and makes the performance of diffuser weaker, which made expanding pressure level fall by nearly 4%, losses increase by about 0.7%.3. Explore skewed vane which can adapt to the non-uniform speed inflow at spanwise direction, and analyze the principle of performance improvementThe design method of the sweepback type vaned diffuser is presented, and the performance of sweepback vaned diffuser performance with different sweep angles is caculated. Through the contrast and analysis of performance and flow field structure of straight vande and sweepback vaned diffuser, it is concluded that skewed shape can make accelerate zone of blade leading edge change and decrease the initial reversed pressure gradient value, which can make the flow easier.4. The design of the whole circular radial-diffuser experimental systerm for the subsequent experimental workThe whole circular radial-diffuser test systerm is designed. The impeller is insteaded by the pre-swirl blade and it is proved the feasibility. At last, the structure connection of different parts and performance neasurement methods are introduced. |
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