There are two different, but well-known, theories for calculating the mixing process of the distributed flow discharged from the impeller into the vaneless diffuser. One is by Dean and Senoo, the other by Johnston and Dean. It is intended in this paper to make clear the reason for these two theories predicting 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 mixing of the flow is greatly accelerated by wall friction and shear force acting between wake and jet, and it causes the large loss at the diffuser inlet. Nevertheless, the reversible work exchange makes a significant contribution to the mixing out of the flow compared with the stationary jet and wake pattern.

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