Physical Mechanisms Leading to Large Unsteady Pressure Fluctuations in a Gas Turbine Testing Facility

Abstract

This paper is a detailed computational study of the flow within a scale model of a gas turbine engine testing facility. At mass flows representative of tests for large, high bypass ratio turbofans, large amplitude low-frequency pressure fluctuations have been observed experimentally at full- and model-scale. These can be so large as to have deleterious effects on downstream facility components. Improved, delayed, detached eddy simulations (IDDES) of the scale model facility are carried out two operating points using OpenFOAM: one where the high amplitude fluctuations occur, and another where they do not. By comparing detailed assessments of the unsteady flow fields for both conditions, the underlying physical mechanism responsible for the problematic pressure fluctuations is identified. The first key finding is that the shape of the chamber housing the engine being tested can result in excitation of a cut-on mode leading to high-pressure amplitudes and propagation. The second key outcome is that the shear layer shedding frequency will only lead to high amplitudes of pressure fluctuation if the excited mode causes periods of high/low pressure that are synchronized around the circular shear layer. An analytical model is derived for predicting whether tonal propagation occurs. Finally, it is found that far downstream flow behavior is mostly determined by the excitation in the test chamber, with minimal downstream dissipation.