Indirect combustion noise: Experimental investigation of the vortex sound generation in nozzle flows

Abstract

In recent years combustion noise of aero-engines has gained in importance. Combustion noise consists of direct noise, related to the unsteady combustion process itself, and indirect noise. As known, entropy noise, contributing to indirect noise, is produced when entropy fluctuations originating from the combustor are accelerated through the turbine. According to the characterisation of a flow by pressure, entropy and vorticity perturbations, accelerated vorticity fluctuations are likewise expected to generate an indirect noise component. Within this work, the vortex sound generation mechanism was studied in a model experiment simplifying the combustor-turbine combination of aero-engines. Vorticity fluctuations were generated artificially by air-injection into swirl free as well as swirling tube flows and the proof of the emission of vortex sound during their acceleration in a convergent-divergent nozzle could be provided. The spatial and temporal change of the velocity field was determined with Hot-Wire Anemometry measurements upstream of the nozzle and the produced acoustic waves were detected downstream of it. For various flow fields imposed with perturbations the identification and separation of direct and vortex sound was achieved. Beside the dependency on the vorticity fluctuation amplitude, increasing the air-injection into the mean flow augments the vortex sound in case of a choked nozzle, the swirl intensity of the mean flow was figured out as a further parameter for this indirect sound generation process. In addition to the sound generation caused by the acceleration of a predominant, artificially produced vortex structure, the broadband noise emission of an accelerated tube flow was studied. With a linear theory approach, the alterations of the fluctuating velocity components through the acceleration were estimated. The coupled influence of the flow field upstream of the nozzle and of its occurring changes due to the imposed mean flow gradient revealed a significant effect on the vortex sound and therewith on the indirect noise generation mechanism.