  Evolution of fuel droplet evaporation zone and its interaction with the
propagating flame front are studied in this work. A general theory is developed
to describe the evolutions of flame propagation speed, flame temperature,
droplet evaporation onset and completion locations in ignition and propagation
of spherical flames. The influences of liquid droplet mass loading, heat
exchange coefficient (or evaporation rate) and Lewis number on spherical spray
flame ignition are studied. Two flame regimes are considered, i.e.,
heterogeneous and homogeneous flames, based on the mixture condition near the
flame front. The results indicate that the spray flame trajectories are
considerably affected by the ignition energy addition. The critical condition
for successful ignition for the fuel-rich mixture is coincidence of inner and
outer flame balls from igniting kernel and propagating flame. The flame balls
always exist in homogeneous mixtures, indicating that ignition failure and
critical successful events occur only in purely gaseous mixture. The fuel
droplets have limited effects on minimum ignition energy, which however
increases monotonically with the Lewis number. Moreover, flame kernel
originates from heterogeneous mixtures due to the initially dispersed droplets
near the spark. The evaporative heat loss in the burned and unburned zones of
homogeneous and heterogeneous spray flames is also evaluated, and the results
show that for the failed flame kernels, evaporative heat loss behind and before
the flame front first increases and then decreases. The evaporative heat loss
before the flame front generally increases, although non-monotonicity exists,
when the flame is successfully ignited and propagate outwardly. For
heterogeneous flames, the ratio of the heat loss from the burned zone to the
total one decreases as the flame expands.
