Experimental investigation of entropy waves’ evolution for understanding of indirect combustion noise in gas turbine combustors

Abstract

Achieving clean and quiet combustion in gas turbines is essential for improving many low-carbon energy and propulsion technologies. This often requires suppression of combustion instabilities and combustion generated noise in gas turbine combustors. Entropy noise is the less explored mechanism of combustion generated sound. Central to the emission of entropic sound is the survival of entropy wave during convection by the mean flow and reaching the combustor exit nozzle. Yet, the annihilation of entropy waves in this process is still poorly understood. To address this issue, the evolution of convected entropy waves in a fully-developed, cold flow inside a circular duct is investigated experimentally. Entropy waves are produced by a well-controlled electrical heater. Fast-response, miniaturized thermocouples arranged over a moveable cross-section of the duct are employed to record the state of entropy waves at different axial locations along the duct. Hydrodynamic parameters including Reynolds number and turbulence intensity are varied to investigate their effects upon the wave decay. The results show that the decay process is strongly wavelength dependent. It is found that the wave components with wavelengths larger than the duct diameter are almost unaffected by the flow and therefore remain essentially one-dimensional. However, other spectral components of the wave are subject to varying degrees of dissipation and loss of spatial correlation. Overall, the results support the recent numerical findings about the likelihood of wave survival in adiabatic flows. They further clarify the validity range of the one-dimensional assumption commonly made in the literature.

Scattering of entropy waves into sound by isolated aerofoils

“…This result was further supported by experimental results in a circular duct (Hosseinalipour et al. 2020). The latter study also showed that the components of the entropy wave with convective wavelengths longer than the duct-diameter remain highly coherent throughout the duct, which supports the use of one-dimensional entropy noise models.…”

On the dispersion of entropy waves in turbulent flows

“…2020; Hosseinalipour et al. 2020), where shear dispersion was identified as the dominant mechanism. There are indeed several mechanisms at play that contribute to the decay of entropy waves (e.g.…”

“…Eckstein & Sattelmayer 2006;Schulz et al 2019) and with their measurements (Wassmer et al 2017;De Domenico et al 2019). Most of the modelling efforts to date have been about entropy-wave dispersion in fully developed turbulent duct flows (Sattelmayer 2002;Morgans, Goh & Dahan 2013;Morgans & Duran 2016;Giusti et al 2017;Fattahi, Hosseinalipour & Karimi 2017;Christodoulou et al 2020;Hosseinalipour et al 2020), where shear dispersion was identified as the dominant mechanism. There are indeed several mechanisms at play that contribute to the decay of entropy waves (e.g.…”

Experimental investigation of the hydrodynamic effects upon convecting entropy waves in nozzle flows

“…Importantly, however, Bake et al made no attempt to study the spatial evolution of the entropy wave. Most recently, Hosseinalipour et al [28] evaluated experimentally the spatial coherence of the convective entropy waves in a low-speed channel flow. These authors used a network of fast-response thermocouples located at different axial locations along the channel and explored the temporal and spatial decay of the wave [28].…”

“…Most recently, Hosseinalipour et al [28] evaluated experimentally the spatial coherence of the convective entropy waves in a low-speed channel flow. These authors used a network of fast-response thermocouples located at different axial locations along the channel and explored the temporal and spatial decay of the wave [28]. This study was constrained to simple channels, and thus constant mean flow velocities, and did not consider variable cross-sectional areas.…”