The effect of acoustic reflections on combustor noise measurements

Abstract

Pressure fluctuations measured in turbine engine combustors at low engine speed show good agreement with theory. Above idle speed, the turbine chokes and a significant change in the shape of the measured combustor pressure spectrum is observed. A theoretical model of the acoustic pressure generated in the combustor due to the turbulence/flame front interaction did not account for acoustic waves reflected from the turbine. By retaining this combustion noise source model and adding a reflecting plane at the turbine and combustor inlet, a theoretical model has been developed that reproduces the undulations in the combustor fluctuating pressure spectra. Plots of the theoretical combustor fluctuating pressure spectra are compared to the measured pressure spectra obtained from the CF6-50 turbofan engine over a range of engine operating speeds. Reasonable agreement exists. It is thus concluded that the simplified combustion noise theory, when modified by a simple turbine reflecting plane, adequately accounts for the changes in measured combustor pressure spectra. The shape of the pressure spectra downstream of the turbine, neglecting noise generated by the turbine itself, will be the combustion noise spectra essentially unchanged, except for the level reduction due to the energy blocked by the turbine.