Mechanism Of The Effect Of Liquid-Solid-Surface-Free-Energy-Difference On Condensation Heat Transfer Enhancement

The paper studies the influence of surface free energy difference between condensate and solid surface on condensation heat transfer performance. Experimental and theoretical investigations have been conducted to analyze the impact of interfacial phenomena such as contact angle, departure diameter on condensation heat transfer performance, and attempted to get deep insights in the mechanism of condensation heat transfer enhancement with the concept of liquid solid surface free energy difference.Based on the experimental observation, a new physical model of the departure diameter of a drop flowing down along a vertical solid plate is proposed in terms of the equilibrium, advancing and receding contact angles. The experimental results using water and ethylene glycol as working fluids agree very well with the values predicted by the present model.It is well known that dropwise condensation heat transfer performance depends on not only the condensing conditions, but also the interfacial interaction between liquid and solid. Based on Rose's theory and taking into the effect of contact angle and departure diameter consideration, A dropwise condensation heat transfer model is established. The calculated results indicate that the heat transfer coefficient increases with the surface free energy difference increasing. The controversy among experimental results in literature can be well understood with the idea of the present paper.For condensing pressures of atmosphere 0.12MPa 0.14MPa and 0.16MPa , the steam dropwise condensation heat transfer properties are experimentally investigated on 9 kinds of coated surfaces to find the impact of the surface free energies difference. When the surface subcooling is less than 10K, the experimental heat transfer performance is higher than the calculated results of conventional Nusselt theory. The increment varies with the surface free energy of solid, and maximum fold is approximately 2.4~4.6. The experimental results also indicate that the condensation heat flux and heat transfer coefficient increase with the increasing of the magnitude of the surface free energies difference of the condensate liquid and the solid surface, at the identical operating conditions. The present model calculated results are well consistent with those of experiments.The condensation heat transfer properties of water-ethanol binary mixture vapor at atmosphere pressure are experimentally investigated on a coated surface and a smooth copperplate. Both of them demonstrate the similar relation between condensation pattern states and surface free energy difference, and the condensation pattern states changes from filmwise, transition, and finally to dropwise condensation.Both results of the present experiments and reported in literature have proven that filmwise condensation occurs when surface free energies difference is less than 13-15mJ/m". Condensation pattern state changes greatly when surface free energies difference is greater than 13-15mJ/m2, and meanwhile condensation heat transfer coefficient increase steeply. Up to surface free energies difference of 18-22 mJ/m2, condensation heat transfer coefficient approaches the peak value and dropwise condensation occurs. The concept of surface free energies difference is first introduced to explain the specific phenomena of binary mixture condensation.