Clocking Effects on Surface-Pressure Fluctuations for Variable Axial-Gaps

Abstract

The present paper addresses the influence of clocking on the unsteady pressure field and corresponding forces on the second-stator vane of a 1 1 2 -stage subsonic low-pressure turbine rig while simultaneously varying the upstream and downstream axial-gaps. Three different axial gaps are investigated. The sources of unsteady pressure in terms of unsteady flowstructures are identified, distinguished, and the influence of the axial gap on the intensity of pressure fluctuations is discussed. It is shown that the distribution of the intensity of the pressure fluctuation is strongly dependent on the clocking of the vane, i.e. its relative circumferential position. The amplitude of the unsteady pressure-disturbances on the stator-vane surface is highest when the volume of the blade passage on the suction side or pressure side is subjected to the wake of the upstream blade-row only. The more the inter-blade-passage of the second-stator vanes is additionally contaminated with the wake of the first-stator vanes, the weaker the pressure fluctuations are. The mechanisms leading to the observed pressure fluctuations are either spatially limited in their extent or interfere with each other in a way which leads to a cancellation of the disturbances. This leads to spots on the surface with pressure fluctuations of very low amplitude. An evaluation of the unsteady forces of one stator-vane shows that, at certain circumferential positions, the pressure fluctuations on pressure side and suction side oscillate in phase or in opposite phase. The corresponding unsteady blade forces have been found to be highest when the wake of the first-stator vane impinges on the vane’s leading edge with a slight shift towards the suction side.