Aerodynamic Measurements on a Low Pressure Turbine Cascade With Different Levels of Distributed Roughness

The effect of surface roughness on the aerodynamics of a highly loaded low-pressure turbine airfoil was investigated in a series of cascade tests conducted in a high speed facility. Profile loss and aerodynamic loading of three different surface roughnesses with a ratio of the centerline average roughness to the profile chord of 1.1 · 10⁻⁵, 7.1 · 10⁻⁵ and 29 · 10⁻⁵ were analysed. Tests were carried out under design outlet Mach number, outlet Reynolds number ranging from 5 · 10⁴ to 7 · 10⁵ and inlet turbulence level of 2.5% and 5%. The mid span flow field downstream of the cascade and the loading distribution on the profile were measured for each investigated operating point using a five hole probe and surface static pressure taps. Additional measurements with a hot wire probe in the profile boundary layer under reference conditions (Re_2th = 2 · 10⁵) were also conducted. Experimental results show a loss reduction for the highest roughness under reference conditions, due to the partial suppression of the separation bubble on the suction side of the profile. At high Reynolds numbers a massive boundary layer separation on the suction side is observed for the highest roughness, along with a large increase in total pressure loss. The middle roughness tested has no effect on the loading distribution as well as on the loss behaviour of the airfoil under all investigated flow conditions.