Experimental Investigation Into the Generation Mechanisms of Additional Loss due to Incoming Wakes in Low-Pressure Turbine Cascades

Abstract For low-pressure (LP) turbines, there exists the loss due to incoming wakes, whose magnitude is sometimes comparable to the profile loss. For a substantial improvement in LP turbine performance, it is necessary to reduce not only the profile loss but also the additional loss. This paper aims at clarifying the generation mechanisms of additional loss so that a design guide for reducing the additional loss is introduced. A parametric set of boundary layer measurements was conducted using a low-speed linear turbine cascade test facility with a moving-bar mechanism for blades with no separation bubble on the suction surface. Four different wake-passing frequencies and three different Reynolds numbers were examined. From the boundary layer measurements, the streamwise variations of the boundary layer integral parameters were obtained along the rear part of the blade surfaces. Also, the wall-normal profile of root-mean-square of streamwise velocity fluctuation was obtained at each boundary layer measurement location. The sources of additional loss were derived by using the measured boundary layer integral parameters together with the estimated profile loss under each flow condition. It was found that the wake-boundary layer interaction loss was attributed to the breakdown of Klebanoff streaks generated in the suction surface boundary layer and the magnitude of the loss was determined by the wake-passing frequency.