Effect of Geometry and Heat Flux on Turbine Over-Tip Flow Power Extraction

The tip leakage flow is known to be a substantial loss contributor. A precise local loss decomposition of the relevant laminar and turbulent terms for viscous losses and thermal losses is required to predict the impact of tip design modification and revise current design guidelines. While losses due to viscous shear stress cause an irreversible reduction from the theoretical maximum power output, tip leakage vortices also penalize the flow turning. This paper combines a novel method for calculating volume-based loss terms and power extraction with an approach to track the tip leakage flow and to analyze the effect of heat transfer and varying tip gap height. Three different flow topologies were categorized for relative tip gap sizes between 0.23% and 1.10% passage height. The contribution of the tip leakage flow to the overall losses could be quantified at each streamwise location. The prime driver for efficiency improvements with tight clearance is achieved by a reduction in turbulent losses in the tip leakage vortices. Of secondary importance, but ubiquitous for all the tip gaps is the laminar loss, occurring in the overtip region and the main passage. The wall heat flux causes a massive effect on the aerothermal efficiency, the internal heat transfer increases exponentially with increasing wall heat flux.