An experimental study on critical heat flux in deformed channel

Abstract

During accident case in nuclear power plant, cladding can be deformed by rapid environmental change in the core. Deformed cladding can block coolant channel partially and it leads to change of heat transfer characteristics. In this research, critical heat flux on deformed channel, which were not researched a lot, was measured. In order to simulate partially blocked flow path, tubes with different diameters were connected by very low temperature welding. Manufactured test sections had clear inner surface and heated directly by electrical rectifier. Uniform heat flux along the test section was enabled by adjusting different thickness for different diameter. 3 blockage ratio, 2 blockage length, 4 mass flux and 2 inlet subcooling conditions were used in this research. When blockage ratio was small, CHF was occurred at the outlet of test section. On the other hand, the cases for large blockage ratio with high mass flux, CHF were occurred at the sudden expansion part. CHF was evaluated by using Macbeth’s correlation, which was developed for CHF under low mass flux and low pressure conditions. When blockage ratio was small, CHF was slightly enhanced compared with CHF without blockage. There was about 10% of CHF enhancement due to lower quality in same distance from inlet. Increased mass flux in partially blocked part affected quality increment along the test section. In case of high blockage ratio with high mass flux, CHF was occurred in churn flow and in rather low heat flux, between 100 and $300kW/m^{2}$. Bubble accumulation induced by backflow at sudden expansion part takes a significant role in early CHF for these cases.