Correlation for dropwise condensation heat transfer: Water, organic fluids, and inclination

Two generalized dropwise condensation heat transfer coefficient correlations (a heat flux dependent and a non-heat flux dependent) covering water and organic fluids are presented. The correlations accurately cover all fluids using two curve-fit parameters that are common to all fluids. The derived heat transfer coefficient correlations take the form of a power law expression. The generalized form of both correlations is derived by making several modifications to the Le Fevre and Rose model. Key additions to the Le Fevre and Rose model include the effect of contact angle on droplet height along with a complimentary method of estimating contact angle on well promoted surfaces. The final expression for the dropwise condensation Nusselt number re-emphasizes the view that dropwise condensation is a conduction heat transfer process limited primarily by the maximum departing droplet size, droplet height, and interfacial heat transfer coefficient. Using a data set which includes steam, propylene glycol, ethylene glycol and glycerol data, an error tolerance of ±15% is reported for 92% of the data and ±27% for all of the data using a non-heat flux dependent expression with common empirical coefficients. A heat flux dependent correlation is also presented which re-introduces a term that is neglected in the derivation of the non-heat flux dependent correlation. However, due to ambiguities between data acquired for water and especially some of the lower surface tension organic fluids, the heat flux dependent correlation is only recommended for water. Both models also match reasonably well with data acquired on inclined surfaces when a straight forward correction is made to the gravitational force acting on departing droplets. A comment is also included on the effect of the promoter thermal resistance on dropwise condensation in light of recent theoretical and experimental results regarding the thermal resistance of promoter/water interfaces.