Experimental and numerical investigation of evaporation from line sources of water in low porosity surfaces

Abstract

We report evaporation characteristics due to higher heating from above from surfaces having an array of line sources. The line sources are created by vertically stacked rectangular plates in a box with water. Two different types of plates were used. In one case line source or film thickness was 66 µm and open area ratio was 4%, and in the second case film thickness was 28 µm and 20% was the open area ratio. Even at the 4% open area ratio the evaporation rate was ∼85% compared to a bare water surface, at the same heat flux. Lateral conduction of heat from the impervious hotter regions to the line sources and the 2-D nature of diffusion near these tiny line sources enhances the evaporative flux, owing to increase in the concentration gradient of water vapour, explains the high evaporation rate, observed in the present work. This system bridges the gap between the understanding of evaporation from bare water surfaces (1-D vapour diffusion) and leaf surfaces (3-D vapour diffusion). Evaporation rates for the fully saturated conditions are in good agreement with the theoretical predictions of Suzuki and Maeda (1968) and Schlünder (1988). The computed surface temperatures and its width-wise variation match well with the experimental values. We also propose a simple film model for the unsaturated condition of the porous medium and show that the temperature distribution obtained using this model is in reasonably good agreement with the measured values.