Two-phase sparse-Lagrangian MMC-LES of dilute ethanol spray flames

Abstract

A sparse Lagrangian particle method (MMC) is coupled with large eddy simulations (LES) and applied to a series of ethanol spray flames that cover a wide range of jet Reynolds numbers and droplet loadings. Varying degrees of pre-evaporation of the liquid favour mixed combustion regimes and pose severe challenges to any combustion model. Three flames with gas-phase equivalence ratios of 1.75, 0.53 and 0.05 (with overall equivalence ratios of the fuel jet of 4.5, 1.4 and 1.8) are investigated and the simulations are overall in good agreement with the measurements. The dependencies of droplet velocities, evaporation rates and mean droplet diameters on flow and flame conditions are captured well, with a possible exception of a persistent underprediction of the average diameter at far downstream locations. This could be improved by modified inflow conditions where droplets are injected through a nozzle pipe far upstream of the nozzle exit plane. This ensures a correlation between the carrier phase and droplet velocities at the burner injection plane. The predictions of temperature are good to reasonable with some qualitative differences within the central jet for flames with high droplet loadings. Further analysis of the MMC model demonstrates that particle mixing preserves localness in composition space throughout the domain despite the presence of pre-evaporation and the related deviations from a pure non-premixed flame regime. MMC-LES predictions are very good for fully turbulent spray flames where evaporation is relatively fast and mixing is dominated by turbulence.