Direct numerical simulations of autoignition in turbulent two-phase flows

Three-dimensional direct numerical simulations (DNS) were carried out to investigate the impact of evaporation of droplets oil the autoignition process under decaying turbulence. The droplets were taken as point sources and were tracked in a Lagrangian manner. Three cases with the same initial equivalence ratio but different initial droplet size were simulated and the focus was to examine the influence of the droplet evaporation process on the location of autoignition. It was found that all increase in the initial droplet size results in an increase in the autoignition time, that highest reaction rates always occur at a specific mixture fraction zeta(MR), as in purely gaseous flows, and that changes in the initial droplet size did not affect the value of zeta(MR). The conditional correlation coefficient between scalar dissipation rate and reaction rates was only mildly negative, contrary to the strongly negative values for purely gaseous autoigniting flows, possibly due to the continuous generation of mixture fraction by the droplet evaporation process that randomizes both the mixture fraction and the scalar dissipation fields. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.