Effects of Ribbed Surfaces on Profile Losses of Low-Pressure-Turbine Blades

Abstract In this work, streamwise oriented riblets were installed on a flat plate exposed to an adverse pressure gradient simulating the diffusing part of a low-pressure turbine (LPT) blade and, successively, on the suction side of an LPT cascade operating under unsteady inflow condition. Different riblets heights, spacings and positions have been tested, and complementary measuring techniques have been used to quantify their effects on the boundary layer evolution, and on losses. Experiments carried out on the flat plate allowed the detailed description of the riblet effects on the coherent structures developing within the boundary layer affecting transition, thus providing a rationale for the identification of the optimal riblet geometry once scaled in wall units. For riblet heights equal to about 20 wall-units, a maximum loss reduction of 8% was observed with respect to the smooth case. Otherwise, for larger riblet dimensions, earlier transition occurs due to enhanced boundary layer instability and losses significantly increase. Interestingly, the streamwise extension of the ribbed surfaces with respect to the transition region was found to play a minor role compared with the riblet dimension. The riblet configurations providing the highest reductions of viscous losses on the flat plate were then tested in the LPT blade cascade for different Reynolds numbers. In this case tests have been carried out with impinging upstream wakes. An overall reduction of the profile losses comparable to that observed in the flat plate case has been confirmed also in the unsteady operation of the turbine cascade. Low sensitivity of the profile losses to the riblet streamwise extension was also observed in the cascade application. This confirms that positive effects in terms of loss reduction can be obtained even when the exact transition position is not known a priori.