A systematic numerical study will be described considering a coupled arrangement of turbine and diffuser flow fields. For this purpose a multi-stage solver commonly used for industrial applications has been applied. The interaction with the turbine stage was investigated assuming two annular axis-symmetric diffuser configurations. One of these has been designed according to the performance charts by Sovran and Klomp [1] at optimum performance and the other close to stall conditions. In addition to a classical mixing plane approach, a frozen rotor solution has been considered as a first-order approach to the unsteady flow to study the effect of rotor wakes on the diffuser flow field. The predicted diffuser performance was strongly influenced by the rotor wakes, and their effect could only be obtained by an approximate frozen-rotor calculation. The results for the computed specific work and pressure recovery for both configurations showed an optimum distance of a/cR = 0.431 between rotor exit and diffuser inlet. Increase of radial rotor clearance gap has a beneficial effect on the diffuser characteristic for the built near the stall point. However, the larger tip leakage loss of the turbine rotor cannot be compensated by improved diffuser performance.

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