The interaction between internal heat gain and heat loss on compound-drop spray flames

The effects of the shell-fuel mass fraction, the compound drop radius, and the liquid loading on one-dimensional laminar premixed flames are theoretically studied using large-activation-energy asymptotics. A compound drop is composed of a water core encased by a shell of n-octane. A completely prevaporized mode is identified, in which no liquid droplets exist downstream of the flame. The shell-fuel mass fraction dominates the internal heat transfer and vaporization rate for an individual compound drop, which may induce a positive effect (overall heat gain) or a negative effect (overall heat loss) on the flame. The liquid loading represents the total quantity of the compound-drop spray. The combined effects of the shell-fuel mass fraction and the liquid loading on the premixed flame show that the flame intensity is enhanced (suppressed) by overall internal heat gain (heat loss), i.e., the flame speed increases (decreases) due to the overall internal heat gain (heat loss). As a result, the residence time required for the drops to achieve prevaporization and the temperature profile of the pre-heating zone are significantly influenced by the flame speed. The critical values of the initial drop radius and the shell-fuel mass fraction that correspond to the critical condition of the completely prevaporized mode are determined by liquid loading and the flame propagation mass flux. The correlations among these factors are investigated.