| Evaporation of water from a shallow rectangular pan to a turbulent forced convective airflow is examined experimentally and analytically.; In the experiments, the pan, partially filed with water, is located in the floor of a rectangular duct. The exposed surface of the water forms the base of a cavity over which a fully developed airflow passes, resulting in evaporative mass transfer. Duct Reynolds numbers between 7200 and 21050 and two different cavity lengths were investigated. For the longer of the two cavities, an aspect ratio (cavity length/cavity depth) range between 15 and 55 was examined, while for the shorter, the corresponding range was between 8 and 25. The mass transfer coefficient was found to increase with increasing duct Reynolds number but decreased with increasing cavity length and depth. The transfer coefficient for the short cavity was more sensitive to changes in the cavity depth than was the long cavity.; The analytical study involved the numerical solution of two model cases, each of which represents a limiting case of mass transfer from a shallow cavity. The first model involved turbulent mass transfer in a duct with a cavity of zero depth. The second dealt with mass transfer in a duct with a sudden one-sided expansion. The results from each of these models were used in defining and clarifying trends.; Two correlation equations for the experimentally-determined mass transfer coefficients were formulated. The first of these correlations, which serves as an interpolation formula between the operating conditions for the two cavities, uses the cavity depth as the main characteristic length along with a weak cavity length correction. The mass transfer coefficients predicted by this equation were typically within (+OR-)5.2 percent of the experimental results.; The second correlation, which incorporates a semi-analytical approach, was based on the data for the longer of the two cavities and serves as an extrapolation to still longer cavities. The experimentally determined mass transfer coefficients were typically within (+OR-)2.6 percent of the correlation. |
