This paper presents a simple method for predicting tip leakage losses in transonic axial-flow turbines. The method is based upon experimental work conducted on a flat plate at 5° incidence and with isentropic exit Mach number of 1.26. The tip gap height was varied from zero up to 15% of chord. Measurements were made (using Laser-2-Focus) of velocity vectors around the tip gap region. These revealed a strong shear layer emerging from the gap onto the suction side of the plate. The relative angle between the leakage flow and the freestream was identified as a key parameter determining the subsequent mixing and overall loss generation. The proposed model applies two-dimensional potential flow analysis to estimate the flow angle as a function of tip gap height and the angle of incidence.
Subsequently, comparisons were made with experimental results obtained in an annular cascade on the outer profile of the last-stage blade of a steam turbine. The predicted tip leakage losses compare favourably with the measured values.