Abstract
A passive residual heat removal heat exchanger is used in several nuclear reactors to remove reactor core decay heat after severe accidents. In the late stages of a severe accident, the coolant level in a water storage tank may decrease and heat transfer tubes cannot be fully submerged by coolant. To study the shell-side heat transfer performance at different low coolant levels, an experimental setup is built and experimental cases with four different coolant water levels are designed (i.e., full, 4/5, 2/3, and 1/2 coolant levels). During the experiments, saturated steam flows through the inner side of heat transfer tubes while purified water works as a coolant on the outside of tubes. According to the experimental phenomena, the shell-side heat transfer area can be divided into three regions, i.e., nucleate pool boiling region, liquid film evaporation region, and steam convection region. A new calculation method is proposed to calculate the shell-side heat transfer coefficients in different regions at low coolant levels. Experimental results show that the shell-side heat transfer performance is enhanced at 4/5 coolant level compared to the full coolant level case, similar to the full coolant level case at 2/3 coolant level, and lower at 1/2 coolant level. The heat transfer performance does not decrease proportionally to the coolant level reduction at low coolant levels because liquid film and small droplets impact the tube surface and phase-change heat transfer happens in the liquid film evaporation region.
