CFD Validation of Incompressible Cross-Flow Discharge Coefficients

Abstract

Validation against air systems problems is required to enable Computational Fluid Dynamics (CFD) codes to be confidently used in the design of turbine cooling air systems. CFD calculations of orifice cross-flow discharge coefficients (Cd) have been compared with measurements by Rohde et al [1]. Simulations have been carried out for cases with a low main duct Mach number (Md ~ 0.25) using incompressible flow modelling. Comparisons have been made of cross-flow discharge coefficients for a range of pressure-head ratios and Mach numbers. Results at a main duct Mach number of 0.07 were obtained using the standard k-ε.: turbulence model which gave agreement to better than 5% for absolute values of pressure-head ratios and discharge coefficients. The trends in the data for pressure-head ratios and Mach numbers were also reproduced. At a higher main duct Mach number of 0.25, the Mach number in the vicinity of the orifice reached 0.8. As expected this rendered incompressible flow modelling unsuitable, resulting in inaccurate determinations of orifice pressure-drops. Work is already in progress to simulate high Mach number cases using a more suitable compressible flow model. The results obtained so far give confidence that CFD will become a valuable tool for evaluating air system losses in novel configurations.