Turbulence, evaporation and combustion interactions in n-heptane droplets under high pressure conditions using DNS

Abstract

The evaporation and combustion of n-heptane droplets under high pressure (20 atm) conditions were investigated using three-dimensional direct numerical simulations (DNS) in the present work. Two ambient temperatures were considered, i.e. 860 K and 1180 K. Low-temperature combustion, which is controlled by low-temperature chemical pathways, occurs in the 860 K cases while single-stage combustion occurs in the 1180 K cases. Droplet evaporation and combustion in isotropic turbulence were considered. The interactions of turbulence, evaporation and combustion in a cloud of droplets were explored. It was found that turbulence promotes the evaporation process. The temperature and mixture-fraction are negatively correlated after evaporation, which impacts the subsequent ignition process. For both low-temperature and single-stage combustion, ignition occurs in regions with low scalar dissipation rate. Low-temperature ignition first appears in lean mixtures while single-stage ignition in rich mixtures. Ignition kernels evolve into reaction fronts, which propagate towards thermally and compositionally stratified mixture and consume the remaining fuel. Spontaneous ignition front is dominant in low-temperature combustion while deflagrative front is playing an important role in single-stage combustion. The interactions of turbulence and the reaction front structures were examined. Cylindrical elements are the most probable shape of the reaction front for turbulent droplet combustion. The reaction front normal is misaligned with the vorticity vector for both low-temperature and single-stage combustion, indicating that the vortical structures preferentially locate along the tangential plane of the reaction front, which promotes the observed cylindrical reaction front structures in turbulent combustion of droplets.