On the role of droplet clusters in spray combustion

Spray combustion is central, e.g., in internal combustion engines and gas turbines for aviation or power production. To improve the efficiency and lower emissions, we need to better understand the complex interactions of the turbulent carrier gas and the inertial fuel droplets. Since droplet Stokes numbers are typically around one or greater, droplets are centrifuged away from vortex centers and cluster in high-strain regions. While the combustion of fuel droplets has been extensively studied, the impact of clustering has received comparably little attention. In this contribution, we present a systematic direct numerical simulation study of decane droplets that evaporate and burn in turbulent air. To account for the shear flow in real systems, we opted for a planar mixing layer configuration involving a cold central spray jet that is surrounded by two hot stagnant air layers. To arrive at a canonical setup, the boundaries were chosen to be periodic in all three spatial directions. By varying the central jet velocity, the carrier gas Reynolds number could be changed. Moreover, we examined the impact of the droplet loading and Stokes number, where the later controls the clustering of the mono-disperse droplets. Depending on the selected parameters, a distinct diffusion flame separates from the spray, which in turn is composed of droplet clusters that are each surrounded by a premixed flame. Large droplets, however, move individually and also tend to burn individually with diffusion flames in their vicinity.