Large-Eddy-simulation prediction of indirect combustion noise in the entropy wave generator experiment

Abstract

Compact and non-compact analytical solutions of the subsonic operating point of the entropy wave generator experiment are compared with detailed numerical results obtained by large Eddy simulations. Two energy deposition methods are presented to account for the experimental ignition sequence and geometry: a single-block deposition as previously used and a delayed deposition that reproduces the experimental protocle closely. The unknown inlet acoustic reflection coefficient is assumed to be fully reflective to be more physically consistent with the actual experimental setup. The time delay between the activation of the heating modules must be considered to retrieve the temperature signal measured at the vibrometer and pressure signals at the microphones. Moreover, pressure signals extracted from the large Eddy simulations in the outlet duct using the delayed ignition model clearly reproduce the experimental signals better than the analytical models. An additional simulation with actual temperature fluctuations directly injected at the inlet of the computational domain clearly shows that the pressure fluctuations produced by the acceleration of the hot slug yields indirect noise almost entirely. Finally, the entropy spot is shown to be distorted when convecting through the turbulent flow in the entropy wave generator nozzle. Its amplitude decreases and its shape is dispersed, but hardly any dissipation occurs. The distortion appears to be negligible through the nozzle and become important only when convected over a long distance in the downstream duct. As the dominant frequencies of the entropy wave generator entropy forcing are very low, the effects of dispersion by the mean flow are however weak.

Influence of viscosity on entropy noise generation through a nozzle

“…When larger frequencies are considered, Morgans et al [32] and Giusti et al [33] numerically demonstrated that turbulent dissipation of entropy is negligible for constant section duct flows. This finding was also observed through a nozzle flow by Becerril [34] and Moreau et al [35]. Common to these studies is the importance of shear dispersion of entropy perturbations by the mean flow and possible contribution of turbulent mixing in entropy dispersion, the latter being restricted to large frequencies because of the limited diameter of the duct where only small-scale turbulence can develop.…”

“…[25] but a potential noise generator when viscosity is considered [34,35,38]. In addition, viscosity influences the mean flow development, in particular through the presence of boundary layers, and may affect entropy dispersion and noise generation in comparison to inviscid configurations.…”

A comparative analysis of the evolution of compositional and entropy waves in turbulent channel flows

“…Gant et al [37,38] developed an analytical model to predict the response of a flame to entropic waves, [38] and emphasized the role of turbulent mixing in practical configurations [37]. By conducting LES, Moreau et al [39] showed that the amplitude of entropy wave subsided and its shape dispersed during convection in a long path. These authors cast doubt on the importance of shear dispersion due to the low frequency of the investigated entropy waves and reported little wave dispersion.…”

Dynamics of entropy wave generation in a simplified model of gas turbine combustor: A theoretical investigation

“…Recently, large eddy simulations in simplified geometries [34,35], convergent-divergent nozzles [36] and real combustors [37] provided further information on the propagation of entropy waves. For example, several investigations indicated that entropy waves are annihilated during downstream advection [19,38,39,40].…”