Gas-phase transient diffusion in droplet vaporization and combustion

By suppressing the relaxation process of fuel vapor accumulation through the use of d²-law results as the initial conditions, the present study isolates gas-phase transient diffusion as the only transient process during droplet vaporization and combustion, and thereby successfully identifies its influence on the bulk droplet gasification characteristics. The case of pure vaporization and flame sheet combustion are analytically solved using perturbation methods and the matched asymptotic expasion technique in the limit of the small gas-to-liquid density ratio. Results demonstrate that transient diffusion enhances the vaporization and burning rates, reduces the flame front standoff ratio, and elevates the flame temperature. However, contrary to predictions of previous studies which have inadvertently included fuel vapor accumulation, these transient diffusion effects are very small so that gas-phase quasi steadiness is indeed an adequate and useful assumption for the modeling of subcritical droplet combustion.