Numerical modeling of a vaporizing multicomponent droplet

Abstract

A numerical investigation of the fundamental processes governing the energy, mass and momentum exchange between the liquid and gas phases of vaporizing, multicomponent liquid droplets is presented. The configuration studied is unsteady, axisymmetric and consists of an isolated, multicomponent droplet vaporizing in a high-temperature, convective, oxidative environment. the model considers the different volatilities of the liquid components, and accounts for variable liquid properties due to variation of the species concentrations as well as for non-Fickian multicomponent gaseous diffusion. Additional features include variable thermophysical properties in the gas phase, variable liquid viscosity, conductivity and heat of vaporization, consideration of the surface blowing effect and droplet surface regression due to vaporization, internal liquid circulation with transient droplet heating, and droplet deceleration with respect to the free flow due to drag. The numerical calculation employs finitedifference techniques and an iterative solution procedure. The time-varying spatially-resolved data for a bi-component (n-octane/benzene) liquid droplet show depletion of the more volatile liquid substance on the droplet surface due to preferential vaporization. The surface depletion is subsequently transferred to the droplet interior through convective mechanisms that develop as a result of the liquid-surface motion under the influence of the fast-moving surrounding gas. The preferential vaporization results in a gradual reduction of the more volatile species concentrations throughout the droplet interior with maximum concentrations occuring at regions around the center of the vortex formed within the droplet. An examination of the potential for microexplosion of the n-octane/benzene droplet, indicated no possibility of a catastrophic fragmentation under the conditions investigated. The bi-component droplet model was employed to examine the commonly used assumptions of unity Lewis number in the liquid phase and Fickian gaseous diffusion. We found that even though the individual surface quantities involved may change substantially, the droplet drag coefficients, the vaporization rates and the related transfer numbers are not influenced by the above assumptions in a significant way.