  The present work investigates forced ignition and oscillating propagation of
spray flame in a mixture of fine ethanol droplets and air. Eulerian-Eulerian
method with two-way coupling is used and detailed chemical mechanism is
considered. Different droplet diameters and liquid fuel equivalence ratios (ER)
are studied. The evaporation completion front (ECF) is defined to study the
interactions between the evaporation zone and flame front. The gas composition
at the flame front is quantified through an effective ER. The results show that
the kernel trajectory is considerably affected by droplet size and liquid ER.
Generally, the flame ER reaches the maximum when the ECF start to move from the
spherical center. It gradually decreases and reaches a constant value when the
flame freely propagates. Quasi-stationary spherical flame is observed when the
liquid ER is low, whilst kernel extinction/re-ignition appears when liquid ER
is high. These flame behaviors are essentially affected by the heat conduction
and species diffusion timescales between the droplet evaporation zone and flame
front. The dependence of the minimum ignition energy (MIE) on liquid ER is
U-shaped and there is an optimal liquid equivalence ratio range (ERo) with the
smallest MIE. Long and short ignition failure modes are observed, respectively
for small and large liquid ERs. When liquid ER is less than ERo (long failure
mode), larger energy is required to initiate the kernel due to very lean
composition near the spark. For liquid ER larger than ERo (short mode),
ignition failure is caused by strong evaporative heat loss and rich gas
composition due to heavy droplet loading.
