Experimental Investigation Of Steam Dropwise Condensation Heat Transfer At Low Pressure

The heat transfer of steam condensation has broad applications in the fields of petro-chemical,spaceflight,power generation and air-conditioning and refrigeneration system. The improvement of the heat transfer property of steam condensation also is of significance in saving energy sources and the project expenditure.Dropwise condensation has a higher heat transfer coefficient and is the best approach to enhance heat transfer by means of interfacial effects.This paper mainly focuses on the dropwise condensation heat transfer of steam at low pressure and has experimentally investigated the heat transfer properties of low-pressure steam dropwise condensation including the droplets dynamic characteristics.Finally,the dropwise condensation heat transfer model based on the interface effect is applied to analyze the variation of the heat transfer characteristics with the steam pressure.Hydrophobic coatings of n-octadecyl mercaptan self-assembled monolayers and PFA/Cr₂O₃ are prepared on copper plate.The contact angle and topography are analyzed. Both coated surfaces have promoted perfect dropwise condensation of steam.Condensation heat transfer results indicate that the condensation heat transfer coefficients decrease with the steam pressure decreasing.For n-octadecyl mercaptan self-assembled monolayers coated surface(SAM),the heat transfer coefficients at the condition of steam saturation pressures of 10kPa,40kPa and 70kPa are 55%～58%,68%～69% and 78%～84%of that at 101kPa,respectively.For PFA/Cr₂O₃ coated surface,these are 47%～49%,69%～77%and 86%～98%,respectively.High-speed camera is applied to monitor the dynamic characteristics of droplets.When the steam pressure decreases,the departure diameter of droplet increases,and the growth cycle of the droplet is getting longer.Both changes above have negative effect on the enhancement of dropwise condensation heat transfer,resulting in the decrease in dropwise condensation heat transfer coefficients.Using the established dropwise condensation heat transfer model in terms of liquid-solid interfacial effect,the variations of thermal resistance with steam pressure are analyzed.It can found that steam pressure has an insignificant effect on the thermal resistance due to the curvature of droplet.When the steam pressure is decreased,the thermal resistance due to conduction through droplet decreases slightly.The only increased one is the interfacial thermal resistance.The interfacial heat transfer coefficient of steam at 3kPa is about two orders of magnitude lower than that at atmospheric pressure.As a result,the interfacial heat transfer resistance is much higher than that at atmospheric condition.So the interfacial thermal resistance due to the lower molecular diffusion rate through the interface is the crucial to the dropwise condensation heat transfer at low pressure.