Computational evaluation of diffuser performance in a turbine engine altitude test cell

Exhaust system diffusers in turbine engine altitude test cells are designed to remove hot exhaust gases from the vicinity of the test article and to provide additional pumping to further lower the pressure in the test section. For economic reasons, it is desirable to utilize the same test configuration to simulate as much of the engine operating envelope as possible. To extend the test envelope, a cut-andtry approach is usually taken using available test data, one-dimensional analyses, and past experience. In this study, a computational approach is utilized which reduces the costs and limitations associated with the current design techniques. Specifically, a compuational model is used to show how the performance of an altitude test cell at low altitude conditions can be examined with particular emphasis on potential test section heating and reduction of diffuser pumping to achieve near sea-level conditions. A computational model which utilizes the NPARC Navier-Stokes code was applied to several configurations in steady-state mode and to a single configuration in the presence of unsteady pressure fluctuations. The study identified diffuser design modifications which address both the cell heating and diffuser pumping issues and provided insight into the mechanisms of hot gas "puffing" (blow back) which can occur at low altitude operating conditions.