Nonpremixed and premixed flamelets LES of partially premixed spray flames using a two-phase transport equation of progress variable

Partially premixed spray flames are simulated with flamelet-based tabulated chemistry parameterized by the mixture fraction and progress variable. The transport equation of the reaction progress variable C is reconsidered, and its formulation for the reacting two-phase flows is derived and employed, which allows the inclusion of spray impacts through a new spray source term that is absent in its gaseous form. Both the nonpremixed and premixed flamelets assuming single reaction regime are implemented in LES, and their validities in spray flames and dependence on the evaporation effect when considering two-phase C equation are examined. The effect of spray, reaction and turbulence interaction is then investigated in comparison with experiments of Sydney reacting acetone sprays, covering the rich, lean and stoichiometric cases. The computed results generally follow the experimental data, but a disagreement between two flamelet simulations is observed especially in rich and lean flames. The premixed flamelets tend to capture the downstream jet spreading while overestimating the peak temperature compared to the nonpremixed chemistry. Flame index analysis indicates that in the present spray flames an evaporation dominated regime exists inside the upstream core jet and it promotes the coexistence of subsequent interacting premixed and nonpremixed reaction zones, which impedes accurate flame prediction by the single regime flamelets. Furthermore, the spray source term appearing in the derived C equation is identified to act as scalar fluxes driven by sprays in flamelet structures. Including this new source term is found to be important to account for the dissipation effect induced by evaporation on the reaction zone in the flamelet simulation of turbulent spray flames. (C) 2017 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute.