Analytical and numerical study of direct and indirect combustion noise through a subsonic nozzle
Résumé
Two mechanisms control sound generated by combustion in a gas turbines: direct combustion noise, in which acoustic waves generated by the flame propagate from the combustion chamber to the outlet through the turbine stages, and indirect combustion noise (or entropy noise), in which entropy waves created by unsteady combustion generate noise as they are accelerated through the turbine stages. These mechanisms can be studied in laboratories by sending acoustic and entropy waves through a nozzle as done in the EWG experiment of DLR [1]. Previous studies have addressed the case where the EWG nozzle is choked and have demonstrated that indirect noise was large compared to direct noise, suggesting that indirect noise only could be retained for gas turbine studies. In the present study, subsonic cases (where the nozzle is unchoked) are analysed using first a full numerical resolution of the unsteady Euler equations, second an analytical method based on the work of Marble and Candel [2] in the low frequency limit and finally the one-dimensional linearized Euler equations in the frequency domain. Results show that direct noise cannot be neglected in these situations and will have to be included for real gas turbines where the flow remains mostly subsonic.
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