On the evolution of fuel droplet evaporation zone and its interaction with the flame front in ignition of spray flames

TL;DR

This paper develops a general theory to describe the evolution of fuel droplet evaporation zones and their interaction with flame fronts during spray flame ignition, analyzing effects of various parameters on flame behavior.

Contribution

It introduces a comprehensive model for spherical spray flame ignition, considering heterogeneous and homogeneous regimes, and examines the influence of droplet mass loading, heat exchange, and Lewis number.

Findings

Flame trajectories are significantly affected by ignition energy.

Successful ignition requires the coincidence of inner and outer flame balls.

Evaporative heat loss varies with flame regime and affects ignition success.

Abstract

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.