Endwall Cavity Flow Effects on Gaspath Aerodynamics in an Axial Flow Turbine: Part I — Experimental and Numerical Investigation

Abstract

Axial flow turbine designers are currently using Navier-Stokes flow solvers to reveal the details of the three dimensional flowfield inside individual bladerow passages. This new capability has allowed designers to focus on secondary flow reduction to improve turbine efficiency. These steady bladerow solvers include viscous and film cooling effects and show good agreement with test measurements in the midspan region. However, the difference between computational results and data at the endwalls is significant due to the exclusion of endwall cavity effects. A clear understanding of how the flow entering and exiting the cavity interacts with the gaspath aerodynamics, in conjunction with an accurate computational model, are needed to predict accurately the secondary flow patterns and endwall losses. This investigation confirms that endwall cavity flows have a significant influence on gaspath aerodynamics and that they need to be included in bladerow computations for accurate results. Part I presents the experimental and computational results from an investigation of the endwall cavity and gaspath flow interaction in a low pressure turbine. Detailed test measurements were obtained in a low speed research turbine using state of the art geometry and served as a benchmark for the computational model. Hot wire and total pressure measurements were taken at multiple planes between bladerows to establish the interaction between the hub cavity and gaspath flows. Ethylene tracer gas was also applied to evaluate secondary flow characteristics of the stator and the migration of the cavity flow through the downstream rotor. Steady and unsteady computational analyses were utilized to model different combinations of the cavity and bladerow geometries. This building block approached allowed for separation of the flow physics involved in the interaction and identified the geometry and flow features that were critical to producing the best agreement with test data. In Part II, the development of a source term model for a steady bladerow solver that simulates endwall cavity flows in a low pressure turbine is reviewed. The source term model adequately captured endwall cavity effects and accurately predicted secondary flow in the adjacent bladerow. This source term model gives designers the capability to investigate new ideas of reducing secondary flow in a timely manner, leading to improvements in overall turbine efficiency.