An investigation into the mechanics of sound propagation through turbulent non-isothermal exhaust jets in cross-flow

Abstract

Open cycle gas turbines (OCGT) are increasingly being used for power generation as they are able to provide base loads in peaking or uncertain conditions and can respond quickly to power grid demands. However, it has been recorded in literature that OCGT (also known as single cycle gas turbines) have exhibited higher levels of low-frequency noise levels in communities near these plants than orthodoxy predicts. High levels of low-frequency noise within a community can cause the following effects: audible low frequency ‘rumble’, ‘beating’ sensations in the chest, nausea and acoustic excitation in structures with low resonant frequencies, such as glazing. It is hypothesised that the low-frequency noise affecting neighbouring communities is due to the sound emitted from the stack being refracted by hot turbulent exhaust gases and cross-winds. It is known that the shear layer, counter-rotating vortex pairs and horseshoe vortices that are generated from this type of flow scenario can affect the propagation of sound with effects such as refraction, diffraction and scattering. This thesis investigates the effects of this fluid flow mechanism on the propagation of low-frequency sound from a plane-wave through a heated mean flow. The investigation is undertaken numerically with a commercial computational fluid dynamics (CFD) package using ANSYS FLUENT, and with an acoustic finite element model (FEM) using ANSYS Mechanical. Subsequent investigations were completed experimentally using a reduced scale exhaust rig within a wind tunnel at the University of Adelaide. The numerical and experimental results have shown that downwind of the exhaust stack the plume causes an increase in the sound pressure level (SPL) of up to 11 dB when compared to acoustic radiation from an exhaust stack in a homogeneous medium. Furthermore, the experimental and numerical results have shown qualitative similarities, that the sound propagation path is drastically altered by the non-isothermal, turbulent exhaust jet that is deflected by a cooler cross-flow. The results of the research have shown that the measured sound radiation from exhaust stacks in cross-flow is highly dependent on the following: acoustic frequency, distance from the exhaust stack, temperature of the exhaust jet, thermal and velocity gradient in the plume, and the jet to cross-flow momentum flux ratio. The change in the sound propagation path, which impacts the downwind SPL, can also be significantly altered by changing the outlet of the exhaust stack. A prototype exhaust nozzle was developed to reduce SPLs downwind. An investigation of a straight duct section with walls that are acoustically transparent, but relatively impervious to flow, has been completed both numerically and experimentally. The investigation has shown that the SPL downwind of the stack is reduced in the presence of the nozzle. Two other prototype nozzles of different geometries have also been designed and experimentally tested. The two additional prototype nozzle geometries have walls that are both acoustically transparent and impervious to flow but are implemented to change the fluid dynamics of the plume. All three nozzles show a reduction in the acoustic refraction downwind of the exhaust stack. It is expected that the results of the research will be used to develop new noise control measures for the electrical power generation industry.