An experimental investigation of combined turbulent free and forced evaporation

The wide variation in correlations available in the literature for predicting water evaporation rates in a moving air stream necessitated a new investigation to determine which correlations can be considered reliable. Water evaporation measurements were made from a heated pool (a class-A pan) into a low speed wind tunnel. The evaporation regime examined combined turbulent free and turbulent forced convection over the range 0.1 < Gr_m/Re² < 10.0. The data includes the range in which combined convection modes are important, as well as the limits where either free or forced convection effects may dominate. The data are compared to several evaporation correlations based on laboratory wind tunnel data. These historical correlations do not produce consistent estimates in predicting evaporation rates. It is believed that the apparent inconsistencies arise because many correlations do not adequately describe the appropriate evaporation regimes for which they are valid. A new correlation using the combined free/forced convection Sherwood number has been developed to predict evaporation rates for a moving air stream. This correlation allows the results of this study to be extended to other evaporating conditions (i.e. variation in surface geometry and air turbulence levels) than those described here. For a 95% confidence limit, the Sherwood number correlation matches the data within ±7.9%.