Measurement of Heat Transfer During Drop-Wise Condensation of Water on Polyethylene

Heat transfer coefficients associated with drop-wise condensation are quite large. Because the ensuing driving temperature difference is small, experimental determination of heat transfer coefficient is a challenge. The statistical nature of droplet distribution in the ensemble contributes to the intricacy of analysis and interpretation. Against this background, the spatial distribution of temperature during drop-wise condensation over a polyethylene substrate was measured using liquid crystal thermography (LCT) simultaneously with actual visualization of the condensation process by videography. Experiments were conducted in such a way that pendant drops form on the underside of the liquid crystal sheet. Temperature variation at the base of the droplets, as small as 0.4 mm, were satisfactorily resolved. The signature of the drop shape was visible in the LCT images. The drop size distribution on the substrate was simultaneously visualized. Static contact angles of water on polyethylene are measured and drop shapes were estimated via a mathematical model for comparison. Using a one-dimensional heat transfer approximation, heat flux profiles through individual droplets were obtained. The temperature profiles from LCT combined with drop sizes from direct visualization provide sufficient data for understanding the heat transfer mechanism during drop-wise condensation. Results show that the measured heat flux as a function of drop diameter matches published data for large drop sizes but fails for small drops where the thermal resistance of the LCT sheet is a limiting factor. To a first approximation, the present work shows that drop size can be correlated to the local heat flux. Hence, the average heat flux over a surface can be obtained entirely from the drop size distribution.