Loss reduction by means of two-dimensional roughness elements on the suction surface of a linear turbine rotor cascade

This paper reports the results of experimental investigations carried out t o reduce pressure losses by means of two-dimensional roughness elements (in the form of stainless steel tubes of different diameters). The roughness e lements are fixed at various axial stations on the suction surface of 120 d egrees turning, 175 mm chord, impulse turbine rotor blades. Flow measuremen ts are carried out at the exit of the cascade at five axial stations, using a five-hole probe operating in non-nulling mode. In addition, the blade su rface static pressure distribution is measured. The data from the five-hole probe measurements are used to calculate pressure, velocity and flow angle distributions at the cascade exit and these results are used to calculate mass averaged values and integral parameters such as wake half-width, loss coefficient, etc. The static pressure distribution is altered very little e xcept near the roughness element. The lift coefficient remains almost const ant for all configurations and the drag coefficient is reduced for some con figurations. The non-dimensional total pressure defects in the wake for all configurations followed Gaussian distribution. A two-dimensional roughness element of 0.6 mm diameter placed at 0.65 chord on the suction surface sho wed an appreciable reduction in pressure losses.