Turbine Blade Aerodynamic Wall Shear Stress Measurements and Predictions

Abstract

The correct prediction of the aerodynamic wall shear stress is a good test of a numerical codes ability to predict profile loss. Its measurement with heated thin film gauges is significantly easier than attempting a complete measurement of a turbine blade boundary layer. A modified form of previously published heated thin film gauge calibrations allow wall shear stress measurement in laminar incompressible flow with favourable pressure gradients and turbulent incompressible flows with small pressure gradients. In this paper, measurements are presented of the distribution of aerodynamic wall shear stress over the suction surface of a turbine blade in a linear cascade. Gauge voltage signal analyses show a laminar separation bubble between about 53% and 63% of suction surface length that is confirmed by surface flow visualisation. By-pass transition is detected by downstream gauges. Wall shear stress measurements are presented at two cascade incidence angles and for tripped and natural transition. The commercial code FLUENT is used to predict the surface pressure distribution, the aerodynamic wall shear stress distribution in the laminar region and the turbulent surface shear distribution for the tripped boundary layer. Comparisons are made between measurements and predictions.