Large-Eddy Simulation of a Turbulent Diffusion Flame Laden With Evaporating Droplets

To investigate the effect of water dilution on turbulent flames, large eddy simulations (LES) of a turbulent reactive jet laden with evaporating droplets are performed under various conditions. A hybrid Eulerian-Lagrangian formulation is adopted for the liquid-gas reactive system described by finite-rate chemistry. Fully dynamic subgrid-scale (SGS) models are developed for the filtered momentum, energy and chemical species equations while a scale similarity SGS model is used for the filtered reaction rate. The developed LES methodology is capable of capturing highly unsteady and localized features and is applied to studying the interaction among turbulence, combustion and evaporating droplets at various droplet size and mass loading ratio (MLR). It is found that smaller droplets can seamlessly follow the flow structures into the reaction zones while larger droplets are insensitive to flow fields, leading to different outcome of combustion suppression. With the same MLR, smaller droplets have a larger total surface area, which increases the evaporation rate and leads to more effective separation of the reaction zones from the oxidizer. For the same droplet size, increasing the MLR initially leads to stronger suppression of combustion. However, further increase in the MLR beyond a critical value would inhibit evaporation and thus limit its effect on combustion.