There are two different but well known theories for calculating loss in the mixing process of the nonuniform flow discharged from the impeller, into the vaneless diffuser. One is by Dean and Senoo, the other by Johnston and Dean. In this paper explanations are given why these two very different theories predict similar total pressure losses. The mixing process in the vaneless diffuser based on the Dean and Senoo theory is re-examined. It is found that the reversible work exchange makes a significant contribution to the non-uniform flow becoming axisymmetric. This process is, however, greatly accelerated by wall friction and interface shear forces between the jet and wake, which lead to loss in total pressure.

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