Effects of Tip Clearance on Aerodynamic Damping in a Linear Turbine Cascade

Abstract

This paper documents an investigation into unsteady flow in a three-dimensional oscillating turbine cascade with emphasis on the influence of tip clearance. Systematic experimental measurements were acquired on a low-speed turbine cascade rig. The cascade consists of seven prismatic turbine blades, with the middle blade being driven to oscillate in a three-dimensional bending/flapping mode. Blades were instrumented with pressure tappings at six spanwise sections to facilitate three-dimensional steady and unsteady pressure measurements on the blade surface. The steady pressure measurements are complemented by computational fluid dynamics simulations. Both are in good agreement and indicate a marked local pressure drop at 70-90 % C on the suction surface resulting from the tip-clearance vortex. The measured unsteady pressure shows that at a small tip clearance (1.25-2.5%C), the tip-clearance flow generates a stabilizing contribution around the midchord on the suction surface near the tip. This behavior is in line with a quasi-steady analysis. Whereas at a large tip clearance (5%C), a dominant destabilizing effect is observed around 80 % C on the suction surface, which is associated with a well developed tip-clearance vortex.