Direct numerical simulation of spray evaporation and autoignition in a temporally-evolving jet

This work investigates spray evaporation and autoignition in a temporally-evolving jet by means of Direct Numerical Simulation (DNS). The impact of shear on evaporation and spray autoignition mechanisms is quantified by comparing droplets evolving in a high-speed jet flow or in a nearly quiescent environment. Comparisons are based on the topology of the reaction fronts, as characterized by mixture fraction, scalar dissipation rate, flame index, as well as fields of temperature and heat release. The impact of local equivalence ratio, droplet diameter and jet velocity have been investigated by varying these parameters. The results reveal that the temporally-evolving jet is a promising numerical configuration to study spray-turbulence interaction, evaporation, mixing, and auto-ignition mechanisms. It is observed that the autoignition delay time strongly depends on the droplet diameter and jet velocity, while it is far less sensitive onto the equivalence ratio. With high shear, ignition occurs on the lean side, peak heat release being found near stoichiometry in a premixed combustion mode. In the absence of shear, ignition occurs simultaneously over a broad range of mixture fractions, and peak heat release is found for a rich mixture involving both premixed and nonpremixed flames. It is also noticed that the conditional mean of the scalar dissipation rate shows non-monotonic behavior when varying jet velocity.