Modeling of spray jet flame under MILD condition with non-adiabatic FGM and a new conditional droplet injection model

This paper reports a numerical study on the Delft Spray-in-Hot-Coflow (DSHC) flame with a new Open FOAM solver developed by the authors, in which the Flamelet Generated Manifolds (FGM) model has been implemented, and used to account for the Turbulence-Chemistry Interaction (TCI). The enthalpy loss effect due to droplet vaporization is considered by employing an additional controlling parameter in the FGM library. Different configurations for generating three dimensional non-adiabatic FGM library are tested. Analysis of the DSHC experimental data suggested that flash boiling influences the atomization of liquid fuel in the DSHC burner. This introduces new challenges for specifying precise spray boundary conditions. A conditional injection model is proposed to provide precise spray information at the injector exit plane (Z = 0 mm). In this conditional injection model, the droplets have an asymmetric distribution around the spray half angle, in agreement with experimental observations. Also, the possible range of droplet injection angle is conditioned upon the droplet size (mass). Droplet initial velocity magnitude is scaled such that it peaks at the spray half angle direction and reaches a minimum at the edge of the spray cloud. The results obtained show that this model significantly improves the prediction of all the properties examined compared to standard injection model. Comparison between Unsteady Reynolds Averaged Navier Stokes (URANS) and Large Eddy Simulation (LES) is made, and it is found that the LES predicted similar gas phase velocity and better temperature profiles compared to experimental data than URANS, mainly due to the better performance of the dynamic model for mixture fraction variance used in LES. The improvement of the temperature prediction by the non-adiabatic FGM was clearly demonstrated by comparing the results obtained with an adiabatic FGM table. Strong droplet-flame interaction was shown to exist in the DSHC flame. Large droplets have ballistic trajectories at the early stage of their lifetime, and can penetrate the flame and survive until far downstream outside the main reaction region. Movement of these large droplets strongly deform the shape of the flame. Rapid evaporation of droplets in the reaction region may also cause local quenching. Small droplets are quickly convected towards the central region and vanish soon, forming a fuel rich and high temperature condition in the center. These phenomena have been captured with the LES/non-adiabatic FGM modeling approach developed in this paper. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.