Impact of unsteady secondary air flow interaction with main flow on loss generation in axial turbines

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 121-122).Secondary air, often called purge air, is injected through the endwall gap between stationary vanes and rotating rotors in axial turbines to prevent ingestion of the hot working gas into the endwall cavities. Three-dimensional flow computations, unsteady as well as steady, have been carried out to assess the role of vane-rotor stage induced flow unsteadiness on loss generation from purge flow interacting with main flow. Computational models of varying physical and geometry complexity that range from simple bladeless annulus configurations to vane-rotor stage configurations were used for identifying the specific flow features responsible for loss generation; these were also complemented by control volume analyses of simple two streams flow model. Computed results showed that the vane induced circumferentially non-uniform static pressure field, and to a much lesser extent rotor blade induced bow waves, significantly increases loss generation upon introduction of injected flow with and without swirl; any flow approximations that renders the pressure field imposed on the purge flow as circumferentially uniform would underestimate the loss due to purge flow-main flow interaction. Flow unsteadiness induced by vane-rotor interaction has marginal, if any, impact on loss generation from introducing purge flow into the main flow path. Judicious flow path modification such as endwall contouring that reduces or eliminates vane-induced static pressure circumferential non-uniformity imposed upon the purge flow has been shown to significantly reduce the purge flow driven loss. Computed results also show that loss generation increases linearly with injected mass flow rate ( 7.5% more loss per 1% purge flow) and decreases quadratically with purge flow swirl until it matches the main flow circumferential velocity ( 85% reduction in purge flow induced based loss at that point). Purge flow induced loss has a weak dependence on the width of the purge duct and angle of injection.by David Clifton.S.M