Three-Dimensional Flowfield Calculation of High-Loaded Centrifugal Compressor Diffusers

Abstract

In this paper a method for calculating inviscid three-dimensional flowfields in vaned diffusers of high-loaded centrifugal compressors will be considered. Following the classical theory of Wu different kinds of streamsurfaces have been introduced. The complete three-dimensional result is approximated by a combination of blade-to-blade streamsurfaces (S1 surface) and S2 surfaces between the side walls of the diffuser. The geometry of each of the stream sheets depends on the others. A special curvilinear coordinate system has been introduced to take into account the twisted shape of the surfaces. Because of the expected transonic flow pattern due to the high loading of the considered units, a time-marching procedure is applied for solving the conservative form of the governing equations in each kind of streamsurface. To demonstrate the capability of the described calculation procedure, the flow pattern in a radial diffuser with twisted vanes has been considered. The impeller speed is assumed to be so large that the flow may become transonic behind the impeller exit. Shock waves may therefore occur in front of the diffuser vanes. Pressure distributions have been calculated in several stream sheets and have been compared with available experimental data. Also integral results predicted by this theory have been compared with measured performance maps. A simple method to estimate a diffuser blockage factor will be given. As a conclusion it will be indicated that a good agreement between theory and experiment justifies the application of an inviscid three-dimensional method for calculating essential details of the pressure field in radial diffusers. However, a coupling of inviscid theory and boundary layer theory does not provide a sufficient prediction of the losses.