Filmwise Condensation From Humid Air On a Vertical Superhydrophilic Surface: Explicit Roles of the Humidity Ratio Difference and the Degree of Subcooling
Abstract The process involving heat and mass transfer during filmwise condensation (FWC) in presence of non-condensable gases (NCG) has great significance in a large variety of engineering applications. The vapor mass flux leading to condensation and the resulting condensation heat transfer coefficient (CHTC) are dependent on the gradients of temperature and vapor mass fraction established near the condenser plate. The effects of the two most influencing thermodynamic parameters, i.e., the degree of subcooling and the difference of humidity ratio (between the free stream environment and on the condenser plate), have been characterized in this work both experimentally and through a mechanistic model. The vapor mass flux during condensation on a subcooled vertical superhydrophilic surface under a free convection regime is experimentally measured in a controlled environment (temperature and humidity) chamber. A mechanistic model, based on the similarity of energy and species transports, is formulated for the thermogravitational boundary layer over the condenser plate and tuned against the experimental results. Further, the model is used to obtain comprehensive data of the condensate mass flux and CHTC as functions of the salient thermal operating conditions over a wide parametric range. Results indicate that humidity ratio difference has a more pronounced influence on the condensation mass transfer rather than the degree of subcooling. The mechanistic model lends to the development of empirical correlations of condensate mass flux and CHTC as explicit functions of these two parameters for easy use in practical FWC configurations.