NUMERICAL STUDIES OF DIFFUSER INLET CONFIGURATIONS FOR SMALL FLOW RATE RADIAL COMPRESSORS

摘要

A computational fluid dynamics (CFD) study is presented to predict the impact of a number of inlet configuration features to the performance of centrifugal compressors diffusers. These features include impeller-diffuser alignment, diffuser inlet edge radius, impeller tip gap width, flow angle, and tip leakage/injection.In small flow rate centrifugal compressors the distance between hub and shroud at impeller discharge could be so small that due to thermal expansion and component mechanical tolerance stacking accurate alignment between the impeller and the diffuser is difficult to achieve. The impeller tip gap is relatively big and the leakage percentage is high. These are some of the major factors that contribute to the high loss and low efficiency of small flow rate compressors. In order to reduce the sensitivity of diffuser performance to misalignment, the designers often make a round, i.e., a radius of curvature in the meridional plane, on the diffuser entrance edges. The designers also need to properly determine the impeller tip gap width and estimate the impact of tip leakage. In this study the influences of these design features are investigated. Axisymmetric models are used for the flow inside a vaneless diffuser. Groups of cases are calculated with focus on the impact of each individualfeature in a multiple configuration variable environment. In the cases calculated the impeller discharge is positioned in incremental axial misalignment relative to the diffuser. Different sizes of impeller tip gap, diffuser inlet rounds, and quantity of leakage or injection through the gap are calculated. The influences of the skewed impeller discharge velocity field on each of these configuration variations are investigated. The results are compared and the optimized configurations with balanced misalignment tolerance and peak performance are discussed.The study found that diffuser inlet round radius Φ has a significant impact on the diffuser performance. At the optimum value of Φ the diffuser has the highest tolerance to misalignment and the minimum penalty in pressure recovery. For a given impeller/diffuser misalignment there exists an optimum impeller tip gap width. Small quantities of impeller tip gap leakage or injection always has an adverse effect on diffuser performance. When the meridional flow angle offsets from radial direction the optimum impeller/diffuser alignment shifted as well.