ON THE INTERPRETATION OF MEASURED PROFILE LOSSES IN UNSTEADY WAKE - TURBINE BLADE INTERACTION STUDIES

The interaction of wakes shed by a moving blade row with a downstream blade row causes unsteady flow. The meaning of the free-stream stagnat ion pressure and stagnation enthalpy in these circumstances has been e xamined using simple analyses, measurements, and CFD. The unsteady flo w in question arises from the behavior of the wakes as so-called negat ive jets. The interactions of the negative jets with the downstream bl ades lead to fluctuations in static pressure, which in turn generate f luctuations in the stagnation pressure and stagnation enthalpy. It is shown that the fluctuations of the stagnation quantities created by un steady effects within the blade row are far greater than those within the incoming wake. The time-mean exit profiles of the stagnation press ure and stagnation enthalpy are affected by these large fluctuations. This phenomenon of energy separation is much more significant than the distortion of the time-mean exit profiles that is caused directly by the cross-passage transport associated with the negative jet, as descr ibed by Kerrebrock and Mikolajczak. Finally, it is shown that if only time-averaged values of loss are required across a blade row, it is ne vertheless sufficient to determine the time-mean exit stagnation press ure.