Variability of Mass Transfer Coefficient with Roughness Elements and Approach Flow Velocity

Water bodies are often surrounded by plants and trees. Presence of these around the water bodies may influence the flow field and thus the evaporation from water body. To understand the role of these obstructions or roughness elements, an experimental investigation was carried out for evaporation rates of water under forced convective air flow in a wind tunnel. The effect of obstruction in the form of creation of additional turbulence caused by artificial roughness of small height on the evaporation rates and hence on the convective mass transfer coefficient has been investigated under varying conditions of air velocities and vapor pressure differential for circular pan of 0.93 m diameter fitted at the bottom of the test section. The respective ranges of flow velocity and pan water temperature for this study were 4 m/s to 18 m/s and 30° C to 50° C, respectively, at relative roughness height of 0.011. It has been found that there is a significant effect of the presence of roughness elements and this effect is a strong function of flow velocity and the vapor pressure differential.