Experimental Investigation of the Effects of Intermediate Gravity Waves on the Water Evaporation Rate

A perfectly still water surface is an ideal hypothesis that is rarely encountered in real-world applications. It is impossible to find an open surface of water without any disturbance or waves, as well as an occupied closed water surface, such as a swimming pool. This experiment was done to predict the evaporation rate from the wavy water surface under the different convection regimes (free, forced, and mixed) at turbulent airflow conditions over a wide range of the ratio (Gr/Re-2). The evaporation rate from the wavy water surface is strongly affected by combinations between wave steepness and main airflow velocity above the wavy water surface. Experimental results show that no pattern can be followed for which combinations of evaporation rate will increase. Thus, only two facts can be noticed: the evaporation rate is larger than that measured under the same airflow velocity conditions with no waves existing on evaporated water surface because the airflow is smooth and attached along the still water surface and when increasing the wave steepness (H/L, H/T), airflow will separate at the lee side of the wave crest near the bottom of the wave trough. Thus, the vortex will be generated in the airflow separation region. These vortexes are unstable and cause an increase in turbulence, reducing the water surface's resistance to vertical transport water vapor and increasing the evaporation rate. Also, experimental results show that the evaporation rates are somewhat less than that measured under the same airflow velocity with smaller wave steepness due to the air trapped region observed at the leeside of the wave crest near the bottom of the wave trough. Also, the result shows the evaporation rate increases with increased airflow velocity under the same convection regime. The current study implemented the particle image velocimetry (PIV) technique to analyze the airflow structure above the evaporated wavy water surface.