The passive safety features of nuclear power plant against station blackout (SBO) and intact containment integrity are the main key issues after Fukushima accident. As a corresponding safety system, passive containment cooling system (PCCS) received attention as one of the candidate systems applying to advanced light water reactors. Next generation of light water reactor, AP-1000 and ESBWR have suggested their own PCCS design. However, PCCS have difficulty in its heat exchanger volume due to low heat transfer coefficient of condensation under the presence of non-condensable gas condition. Several attempts had been studied worldwide to enhance the heat transfer coefficient of PCCS and this paper focused on dropwise condensation, which has much higher heat transfer coefficient than those found with filmwise condensation. Historically, surface coating or applying organic promoter on the cooling surface were typically used to induce dropwise condensation, but those method had disadvantage of their duration time. In general, surface coating and organic promoter were considered to have few years and few days of their duration, respectively. Therefore, an aim of our experiment was to determine whether SUS316 without any surface treatment is able to utilize dropwise condensation for PCCS heat exchanger. Following studies were compared to the filmwise condensation on SUS316 with same condition and also the experiment results reported by other researchers. The overall results determined how much the heat transfer coefficient was enhanced. To analyze the behavior of dropwise condensation, visualized images of cooling surface with droplets were graphically processed. All experiments were conducted on 13mm diameter of vertical-oriented flat surface with 6mm thickness. Air concentration (non-condensable concentration) was estimated by the partial pressure ratio of steam and air. Subcooled temperature, the difference between steam saturated temperature and surface temperature, was tested from 1 to 30 degree Celsius. High-speed camera visualized the condensate on the cooling surface with several magnification and frame speed. All measurement was measured after the whole system reached to equilibrium state and sustained it more than 30 minutes. Each data was recorded for 60 seconds and time-averaged its measurement. Experiment results indicated that SUS316 surface without any surface treatment could sustain dropwise condensation phase over 12 hours of experiment under low subcooled temperature below approximately 20 degree Celsius. Also, dropwise condensation had around 3 to 4 times enhanced heat transfer coefficients than those of filmwise condensation. The result of dropwise condensation with pure steam condition was well proportional to the power of subcooled temperature. Otherwise, the result with non-condensable gas showed heat transfer coefficient, shaded in high uncertainty of systematic error, seems to be decreased in low subcooled temperature under 5 degree Celsius. Higher sensitivity of non-condensable concentration upon heat transfer coefficient was observed on dropwise condensation more than filmwise condensation. The variation of drop-size distribution with the periodic time passage after surface sweeping was observed and patterns of drop-size distribution were repeated after the surface sweeping regardless of experimental conditions. The study provides feasibility and benefit of the utilization of dropwise condensation to PCCS heat exchanger if the system has designed to be operated within low subcooled temperature.
