Direct measurement of skin friction in a turbine cascade

Abstract

This research was carried out to design and utilize an instrument to directly measure the skin friction on the suction surface of an advanced turbine blade. A floating head gauge configuration was developed to detect the small shear force created by the flow passing over the nonintrusive sensing element. The floating head was mounted to a cantilever beam arrangement with minute deflection due to the shear force, which allows the balance to be a nonnulling type without introducing misalignment errors. This results in a simpler, more reliable design. Measurements were conducted in cold flow in the Cascade Tunnel at Virginia Tech for exit Mach numbers from 0.84 to 1.37. The cascade was representative of a modern turbine stage. The output from the gauge was found to be repeatable for the same nominal input conditions. The measured values of skin friction coefficient ranged from 0.002 to 0.004 over the conditions tested. No other values can be found in the literature for comparison. Finally, a full Navier-Stokes computational fluid dynamics code with a modern turbulence model was used to predict the flow and skin friction coefficient for an exit Mach number of 1.2. The observations compared reasonably well with the numerically predicted value for Cf.