Numerical investigation of two-phase ethanol ignition in uniform droplet-laden weakly turbulent flows

The ignition of spray flames is a process of stochastic nature, especially relevant to the high-altitude relight of aeroengines. The present work aims to quantify the role of spray inhomogeneities in a realistic droplet-laden ignition set-up through Direct Numerical Simulations (DNS). Computations of the ignition process of weakly turbulent ethanol spray flows are performed using complex chemistry, a realistic energy deposition model and a polydisperse description of the spray. A good qualitative agreement of the flame radius evolution is found between the DNS and experiments, and both long-time failure and ignition modes are recovered with comparable radii, hence reproducing the experimentally-observed range of behaviour of ignition kernels. In contrast to experiments where the presence of large droplets at the spark impact both the actual energy deposited in the flow and the local fuel-air equivalence ratio, the effect of both parameters on the ignition outcome are assessed independently in this work by carefully choosing ignition locations in the flow representative of time-wise fluctuations. The minimum ignition energy (MIE) representative of the global flow condition is then evaluated using results from the DNS simulations and flow and spark characteristics. In order to understand the origin of the ignition stochasticity observed experimentally, local values of the fuel-air equivalence ratio Φt and turbulence level u′ at the spark plug are measured in the simulations using an averaging in volumes of size Δb. It is found that Φt best correlates to the MIE at three different locations in the domain for a size around 4 mm, which corresponds approximately to the kernel size at 0.1 ms, that is, the typical time scale of plasma processes and right when the flame becomes toroidal. The results give support to the experimental conjecture that stochasticity is largely due to the spatial fluctuations of Φt induced by polydispersity and in a lesser extent to those of u′.