In many combustion applications the fuel is originally present as either liquid or solid. In order to facilitate mixing and the overall burning rate, as pointed out in Section 6.4.1, the condensed fuel is frequently first atomized or pulverized, and then sprayed or dispersed in the combustion chamber. Consequently, in these devices combustion actually takes place in a two-phase medium, consisting of the dispersed fuel droplets or particles in a primarily oxidizing gas.

A description of two-phase combustion consists of three components, namely the gasification and dynamics of individual and groups of droplets (or particles); a statistical characterization of the spray; and the collective interaction of these droplets with the bulk gaseous medium through the description of the two-phase flow in terms of heat, mass, and momentum transfer. The first component was introduced in Section 6.4 through the d2-law of droplet vaporization and burning, and will be extensively studied in this chapter for both droplets and particles. Specifically, we shall first discuss the general phenomenology of droplet combustion with and without external convection, and the experimental methodologies commonly used in investigating droplet combustion. We shall then study the combustion of single-component droplets by relaxing the various assumptions associated with the d2-law, and hence examine effects of droplet heating, fuel vapor accumulation, and variable transport properties that were briefly mentioned in Section 6.4.4. We shall also relax the assumptions of gas-phase quasi-steadiness, stagnant environment, and solitary droplet by discussing effects due to gas-phase transient diffusion, external convection, and droplet interaction, respectively.