Numerical Simulation of Tip Leakage Flows in Axial Flow Turbines, With Emphasis on Flow Physics: Part II—Effect of Outer Casing Relative Motion

A pressure-correction based, 3D Navier-Stokes CFD code was used to simulate the effects of turbine parameters on the tip leakage flow and vortex in a linear turbine cascade to understand the detailed flow physics. A baseline case simulation of a cascade was first conducted in order to validate the numerical procedure with experimental measurements. The effects of realistic tip clearance spacing, inlet conditions, and relative endwall motion were then sequentially simulated, while maintaining previously modified parameters. With each additional simulation, a detailed comparison of the leakage flow’s direction, pressure gradient, and mass flow, as well as the leakage vortex and its roll-up, size, losses, location, and interaction with other flow features, was conducted. Part II of this two-part paper series focuses on the effect of relative motion of the outer casing on the leakage flow and vortex development. Casing relative motion results in less mass flow through the gap and a smaller leakage vortex. The structure of the aerothermal losses in the passage change dramatically when the outer casing motion was incorporated, but the total losses in the passage remained very similar. Additional secondary flows that are seen near the casing are also discussed.