Influences of liquid fuel atomization and flow rate fluctuations on spray combustion instabilities in a backward-facing step combustor

Combustion instabilities occurring in spray combustion fields inside a backward facing step combustor have been investigated by performing large-eddy simulations (LES). In this study, the influence of fluctuations in the incoming oxidizer air velocity (caused by drastic pressure oscillations in the combustor during combustion instability) on the droplet diameter distribution (due to atomization) of the injected liquid fuel spray, as well as the influence of pressure oscillations on the fuel flow rate have been taken into consideration using appropriate models. For the temporal fluctuations in fuel droplet diameter distribution, a model for the Sauter Mean Diamter (SMD) of atomized droplets, obtained as a function of spray injection parameters and gas/liquid properties, is incorporated in the LES. Additionally, to consider the temporal fluctuations in fuel flow rate along with its phase difference with the pressure oscillations, a model derived from Bernoullis principle is proposed and employed in the LES. The objective is to examine in detail, the impacts of the fluctuations in fuel droplet diameter distribution and the fluctuations in fuel injection rate individually, as well as the impact of the mutual interaction of these two fluctuations, on the spray combustion instability characteristics. Results of the LES reveal that the temporal fluctuations in fuel droplet diameter distribution resulting from combustion instability, lead to a reduction in the intensity of pressure oscillations and hence the combustion instability's strength. Additionally, the temporal fluctuations in liquid fuel flow rate strongly influence the intensity of spray combustion instability, and it is observed that the combustion instability intensity increases with the increase in phase difference between the fuel flow rate fluctuations and pressure oscillations. Furthermore, the effect of the temporal fluctuations in fuel droplet diameter distribution resulting in the reduction of combustion instability intensity, becomes more pronounced as the phase shift between the fuel flow rate fluctuations and pressure oscillations becomes larger. It is clarified that the above-mentioned behavior of spray combustion instability, results from the change in the correlation between heat release rate fluctuations and pressure oscillations near the combustor's dump plane, which is caused by the change in the local residence time of fuel droplets and the local fuel droplet evaporation rate. (C) 2020 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute.