Development of dropwise condensation model considering constriction resistance and droplet distribution

Abstract

Most condensation study, including PCCX designs, is focused on filmwise condensation(FWC). However, operating condition of PCCS is likely to be dropwise condensation(DWC) and related studies also reported DWC. Also, low-conductivity surface, such as stainless steel, is profoundly affected by constriction resistance. Consequently, this study compared DWC and FWC for SUS316 surface. Feasibility of using DWC for PCCS was also assessed. Drop-size distribution data under subcooled temperature and non-condensable gas (NCG) concentration was obtained by image processing to develop constriction resistance model. Moreover, an empirical model of drop-size density for large droplet was suggested. At last, the constriction model and overall heat transfer coefficient(HTC) considering constriction model were developed. The experiment was conducted on both vertical flat and tube surface. Experimental condition was subcooled range 1~30K and NCG range 0~40%. Tube experiment was also conducted on 0.3m and 1.0m tube to check length effect. In the flat surface experiment, FWC showed 2~3 times higher HTC than DWC and maintained at least 12 hours. The advantage of FWC was disappeared subcooled over 20K. In the tube surface experiment, DWC showed 1.5 times higher HTC than FWC and maintained at least 4 hours. None of the significant length effect was observed. In addition, HTC is dramatically decreased in the presence of NCG for both experiments. A behavior of initial droplet growth is affected by subcooled temperature and critical radius. Maximum drop departure radius is 1.37mm for SUS316. All drop-size distribution function, including N(r), f(r) and A(r), was obtained, and they were independent of experimental conditions. Moreover, empirical drop-size distribution model for large droplet was developed. In conclusion, the constriction model for single drop and multiple drops were developed. The results from the validation showed that constriction resistance took account of 70~85% of total resistance.