Loss Modeling of Turbine Tip Leakage Flows

Arthur Huang
Advisors: Prof. Greitzer, Dr. Tan

Formation of tip leakage vortex (Mischo, Behr, Abhari, 2008). DS1 is the dividing streamline between incidence-driven flow and pressure-driven flow. DS2 is the dividing streamline between flow ending up in the passage vortex and flow ending up in the leakage vortex.

A major source of inefficiency in a turbine results from pressure-driven flow leaking across the rotor tip from the pressure side to the suction side. The flow emerges from the tip gap in a jet, which rolls up into a vortex near the shroud/suction side corner of the blade passage. Entropy is generated as the leakage flow mixes with the mainstream flow. In addition to creating aerodynamic losses, tip leakage flows also transfer heat to the rotor tip so that an uncooled rotor tip may be damaged. Because of this, turbine designers introduce cooling flows, which bring with them their own mixing losses, as well as lower total work due to the cooling flow bypassing the combustor. This project aims to model the losses associated with turbine tip leakage in order to better design the rotor tip.

Schematic of tip leakage flow (Krishnababu et al., 2009)

Currently used models for aerodynamic tip leakage losses are correlations based on rotor tip lift coefficients, blade geometries, or simply an efficiency penalty proportional to the gap height. We have modeled the tip gap region as a series of 2D planes in the leakage streamline and radial directions. In each of these 2D planes, the flow is viewed as a 1D sudden expansion over a vena contracta. Mass and momentum control volume equations are solved to determine leakage mass flows and velocities, and hence entropy due to mixing, which is the efficiency loss. The next steps in this project are to conduct CFD analysis of tip leakage flow to determine whether the assumptions used in the modeling are reasonable and to develop and test models for losses from required cooling associated with the tip leakage.