Large-eddy simulation of turbulent spray flames: Effects of scalar correlation and enthalpy reduction in flamelet modeling

Abstract

Numerical modeling of turbulent spray combustion provides a promising tool for advanced engine design. In spray flames, the droplet evaporation not only reduces the ambient gas temperature, but also influences flame structure by generating substantial local fluctuations of the mixture fraction $\widetilde{Z}$ and progress variable $\widetilde{c}$. These two scalars, conventionally assumed independent in flamelet models, exhibit significant correlations arising from the coupling among evaporation, turbulent mixing and chemical reactions. This study proposes a six-dimensional flamelet-generated manifolds (FGM) method, considering the evaporation-induced specific enthalpy reduction and scalar correlation. A novel joint presumed probability density function (PDF) method is derived using the copula theory, achieving rapid grid convergence and good feasibility. Large-eddy simulation (LES) is performed on the Sydney ethanol turbulent spray flames (EtF1, EtF4 and EtF7), which feature different ethanol mass flow rates and jet Reynolds numbers. Both gas and liquid phase statistics show good agreement with experimental data across the three flames. The incorporation of specific enthalpy reduction and scalar correlation in FGM modeling improves gas temperature predictions, along with enhanced liquid-phase prediction through refined gas-field resolution. The correlation coefficient of $\widetilde{Z}$ and $\widetilde{c}$ is found to be a competing result of local evaporation and combustion, since evaporation elevates $\widetilde{Z}$ and dilutes reaction products, whereas chemical reactions enhance $\widetilde{c}$ fluctuations.