Numerical Investigation of a Non-Confined Spray Flame Exposed to Acoustic Forcing

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. The test-case which is the focus of this work is a non-confined flame^1,2 burning at atmospheric pressure and therefore the velocity fluctuations play a key role. Acoustic waves will eventually affect the rate of combustion, and the oscillating fluctuation of the heat released by the flame might be increased by the evaporation process. The dynamic interaction between the evaporating fuel spray and the velocity fluctuations induced by an acoustic perturbation is investigated to understand the impact of the acoustic waves on the droplet dispersion and on the evaporation rate. The influence of the initial droplet diameter has been observed to be irrelevant, when two monodispersed sprays of 20 μm and 80 μm were numerically simulated. In this work the main question to address is how the interphase heat and mass transfer, and the momentum exchange are influenced, at low amplitude velocity fluctuations, by the forcing frequency, under two different imposed velocity profiles of the liquid fuel. A fast decay of the slip velocity is predicted under both steady and perturbed conditions. Thus, slip velocity fluctuations do not have a significant influence on the solved spray field. Finally, the impact of the forcing frequency and of the pilot are the main effects acting on the forced flame response. At low frequency, the entrainment of hot gases into the spray results in a clearly visible stretching of the flame which causes a high level of temperature fluctuation. At high frequency, despite the weak response of the gas velocity field, the dynamics of the combustion show a faster evaporation rate than the acoustic–free case.