Two-phase coupling for MMC-LES of spray combustion

Abstract

A sparse-Lagrangian particle implementation of the multiple mapping conditioning / large-eddy simulation (MMC-LES) model for two-phase flows is developed with the aid of carrier-phase direct numerical simulation (CP-DNS) of a droplet-laden temporally evolving reacting double shear layer. In sparse-Lagrangian MMC for spray combustion, the liquid fuel droplets are represented by a first set of Lagrangian particles, while the reacting gas phase is described by a second set of (stochastic) particles. The two phases exchange heat and mass and a one-to-one coupling technique of liquid and gas particles with particle pair selection conditional on mixture fraction and/or temperature is introduced. The simulations show that unconditional mean and rms of mixture fraction are accurately reproduced by MMC and are largely independent of the particle coupling. Temperature, however, is significantly underpredicted when using the Eulerian mixture fraction for particle selection as is conventional for particle-particle selection in MMC. This is due to a lack of correlation between temperature and mixture fraction caused by excessive cooling of selected fluid elements due to evaporation and subsequent local flame extinction and reignition. Conditioning on temperature leads to mild improvements but does not prevent a decorrelation of the temperatures “seen” by the droplets. Alternatively, a minimization based on stochastic particle temperature is suggested. This leads to good agreement between our MMC-LES model and the CP-DNS. The comparison between the CP-DNS data and MMC-LES also confirms that the anisotropic mixing time scale derived for gaseous flows ensures an accurate degree of micro-mixing also for two-phase flows and provides very good predictions of conditional fluctuations of reacting scalars around the conditional means.