Lab-scale research on acoustically transparent stacks for hot exhaust jets in cooler cross-flow

Abstract

From the exhaust stack of open cycle gas turbine power stations, the sound radiated is strongly refracted in the near-field by the thermal and velocity gradients of the jet. Research undertaken by the authors has identified that the presence of a cooler cross-flow leads to a bent-over plume downwind of the exhaust stack and a significant increase in sound in the far field by up to 10dB downwind when compared to spherical spreading. This paper is the first in a series that explores a unique “acoustically transparent silencer,” which prevents this by separating the sound from the flow. The concept of the approach is demonstrated in this paper, with aero-acoustic simulations conducted on a supercomputer and small-scale experiments in a wind tunnel. The numerical simulations have predicted the effects of using an acoustically transparent exhaust stack to reduce the refraction of sound in the near field, which reduced the lobes in the acoustic directivity downstream of the exhaust by up to 4.5 dB. This was further supported by experimental results in a wind tunnel with a reduction of up to 4 dB. These results were used to assist in the second iteration of designs for the Refrastack in a subsequent paper.