Impeller-diffuser interaction in a centrifugal compressor

A study has been conducted, using an unsteady three-dimensional Reynolds-av eraged Navier-Stokes simulation, to define the effect of impeller-diffuser interaction on the performance of a centrifugal compressor stage. The princ ipal finding from the study was that the most influential aspect of this un steady interaction was the effect on impeller tip leakage flow. In particul ar, the unsteadiness due to the upstream potential effect of the diffuser v anes led to larger viscous losses associated with the impeller tip leakage flow. The consequent changes at the impeller exit with increasing interacti on were identified as reduced slip, reduced blockage, and increased loss. T he first two were beneficial to pressure rise, while the third was detrimen tal. The magnitudes of the effects were examined using different impeller-d iffuser spacings and it was shown that there was an optimal radial gap size for maximum impeller pressure rise. The physical mechanism was also determ ined: When the diffuser was placed closer to the impeller than the optimum, increased loss overcame the benefits of reduced slip and blockage. The fin dings provide a rigorous explanation for experimental observations made on centrifugal compressors. The success of a simple flow model in capturing th e pressure rise trend indicated that although the changes in loss, blockage , and slip were due largely to unsteadiness, the consequent impacts on perf ormance were mainly one-dimensional. The influence of flow unsteadiness on diffuser performance was found to be less important than the upstream effec t, by a factor of seven in terms of stage pressure rise in the present geom etry. It is thus concluded that the beneficial effects of impeller-diffuser interaction on overall stage performance come mainly from the reduced bloc kage and reduced slip associated with the unsteady tip leakage flow in the impeller. [S0889-504X(00)01704-9].