Entropy noise: experimental and numerical investigation in turbomachinery

Entropy noise is a type of combustion noise produced by the acceleration of entropy inhomogeneities in a fluid, and is typically produced in aeroengines when flow at the outlet from a combustor is accelerated in high pressure turbines. The entropy inhomogeneities arise from unsteady burning of the combustor flame and cooling introduced through the endwalls. To date there have only been limited attempts to experimentally study entropy noise generation. Some analytical and numerical contributions were made to its effect in a subsonic nozzle and turbine stage configuration. The ever increasing noise emissions regulations produced a growing interest in optimising combustion noise and entropy noise in particular because the combustor exit temperature profile generates entropy noise due to its large temporal and spatial temperature variations. Therefore it became imperative to investigate and potentially quantify the sound pressure level of entropy noise emitted by a simple nozzle geometry with its subsequent application to a full turbine configuration. The present study utilised numerical and experimental methods to extend the understanding of entropy noise and its generation to a simple nozzle configuration and half a turbine stage. Numerical simulation largely employed Large Eddy Simulation as a turbulence model to achieve a better accuracy and resolve energy transformation from kinetic energy contained between the shear layers of a temperature profile into acoustic perturbations. The accurate modelling of temporal and spatial temperature variations of a non-uniform temperature profile was performed, thus allowing the possibility to evaluate sound pressure levels of entropy noise generated in the Oxford Turbine Research Facility. Experimental investigation was concentrated on two simple nozzle geometries with different acceleration rates in the nozzle guide vane region. The non-uniform and uniform temperature profiles were generated in a combustor simulator and noise measurements were performed at the outer wall of the duct extension. The noise evaluation showed that the presence of a temperature difference between the âhot spotâ and its environment produced an additional acoustic response of approximately 10dB in the crucial lower frequencies. Moreover, when the second nozzle was operated the sound pressure level of the uniform temperature profile was the same for two nozzle configurations, while that of the non-uniform temperature profile gave a noise difference in the frequency range from 10 to 3000Hz. This demonstrates that different acceleration rates produce different acoustic responses only when operated with non-uniform temperature profile. Following the acoustic analogy developed by James Lighthill it can be concluded that entropy noise was successfully measured. The method developed for the numerical modelling of temperature variations of the non-uniform temperature profile was validated using the acquired experimental data. A good agreement was achieved between the predicted and measured results. This allowed an evaluation of sound pressure levels produced by typical rich-burn and lean-burn combustors. The numerical results confirmed that the lean-burn combustor is likely to produce a noise level larger than that of rich-burn by approximately 5-10dB in the lower frequencies for an equivalent mean temperature. A numerical parametric study of lean-burn combustor temperature profiles was performed and some recommendations on noise optimisation were explored.