VORTEX UNSTEADINESS IN THE ENDWALL REGION OF A HIGH-LIFT LOW-PRESSURE TURBINE PASSAGE

Three-dimensional vortical structures within the endwall region of turbine passages directly affect the aerodynamic efficiency and heat transfer characteristics of the turbine. Interactions between the vortical endwall structures and the suction surface flow have been shown to be a significant source of loss generation through passages. One dominant vortex extends from the leading edge junction region of the blade across the passage, where it interacts with the flow along the suction surface of the adjacent blade. In high-lift low-pressure turbine cascade passages, the vortical structure intermittently loses coherence and exhibits unsteady variations of strength and position as it extends across the passage. The present paper details the temporal behavior through high-speed measurements in a low speed linear cascade of high-lift low-pressure turbine blades. Stereoscopic particle image velocimetry measurements in the passage are used to evaluate the unsteady behavior of the vortex. Space-time iso-surface plots of Q-criterion clearly show the evolution of the vortex over time. Analysis of the data reveals the various time scales of fluctuations in strength and position. Comparisons of the temporal fluctuations in the high-lift turbine passage are made with similar phenomena found in canonical junction flow papers in the literature. Key findings support the hypothesis that in-passage vortex unsteady characteristics near the endwall are influenced by leading edge junction flow dynamics, and provide additional insight into the unsteady endwall flow physics that are necessary to further the development of endwall loss reduction techniques.