Heat Transfer Analysis for Dropwise-Filmwise Hybrid Surface of Steam on Vertical Plate

Abstract

A model is proposed to predict and evaluate the heat transfer characteristics of dropwise-filmwise hybrid surface. This is approached by modifying the original drop-size distribution, which is defined for fully dropwise condensation (DWC) and making it applicable for dropwise-filmwise condensation (DFC). The modification is achieved by simulation work to determine two parameters: the area fraction occupied by large drops f and the ratio of the maximum radius of newly formed drops to that of larger drops γ. Simulation results show a good agreement with the literature for fully DWC and provide a correlation for each parameter with respect to DWC region width in the DFC hybrid surface. The present model evaluates the heat transfer performance of DFC by utilizing these correlations. A comparison is made between the proposed model with experimental work from the literature and results show a good agreement. While changing filmwise condensation (FWC) region width significantly affects the overall heat transfer performance, utilizing smaller width to that of DWC region has the advantage over fully DWC. Furthermore, surface renewal within the hydrophobic region is controlled by adjusting the DWC region width. When the ratio of drop maximum diameter to DWC region width is unity, surface renewal is achieved by drops merging to adjacent FWC regions only. When the ratio is less than unity, surface renewal is achieved by sweeping of departing drops within the hydrophobic region and by merging. For each case, an optimum DWC region width which corresponds to the maximum DFC heat flux is defined.