Effect of surface saturation on evaporation rate.

Abstract

The effect of surface liquid saturation (the fraction of voids occupied by the liquid) on evaporation rate is studied. Evaporation is caused by convective heating of the surfaces (with and without chemical reaction). The burning rate of ethanol supplied to the horizontal surface of a ceramic wick is measured. The steady-state, one-dimensional (parallel to gravity vector) transport equations of heat, liquid- and gaseous-phase in the porous medium were solved to find the surface saturation. The evaporation of ethanol from a surface, made of solid cylinders, to a forced-convection air flow (without chemical reaction) is studied next. This two-dimensional surface model allows for an estimate of the surface area of the liquid and the surface saturation, using the static Young-Laplace formulation. Besides the measured evaporation rate, heat and mass transfer coefficients are also estimated by varying the surface saturation, Bond number, and ambient velocity. The results show that except the limiting cases where $Bo\\ \o$ 0 or the ratio of boundary layer thickness to the mean particle size is very large, the evaporation rate (with and without combustion) decreases with the surface saturation. The mechanism of this reduction of evaporation rate results from the additional diffusion in the space caused by the protuberance of the solid matrix and the alteration of the concentration profile. Under the same ambient conditions, the dependence of evaporation rate on saturation is more pronounced for combustion due to the large mass transfer number. For Bond numbers larger than 0.02 the analogy between heat and mass transfer no longer holds and deteriorates as the Bond number increases and the velocity decreases for a given surface saturation. The result also indicates that for the same boundary and geometry conditions, using a wick with the larger thermal conductivity results in the higher burning rate.