Experimental Study on the Natural Circulation Subcooled Boiling of Square Channel in Fusion Reactor Blanket

Blanket is implemented in fusion reactor for removing heat, shielding neutrons, and tritium breeding. Under accident conditions, natural circulation flow will emerge due to the buoyancy forces caused by density gradient in the different parts of the loop. Without the function of moving machinery, the natural circulation will occur to remove the heat of the nuclear reactor core, which denotes natural circulation is one of the significant passive cooling mechanisms of a new reactor. The appearance of subcooled boiling will greatly affect density gradients in the closed loop and lead to the flow instability of coolant. Experimental study has been performed to investigate natural circulation subcooled boiling characteristics in square channels of fusion blanket. Experimental results show that the natural circulation mass flow ascends with the increase of the heat flux and the decline of pressure, and subcooled boiling is easier to appear in natural circulation because natural circulation flow is usually small and heat transfer intensity is lower compared to forced circulation. The location of onset of nucleate boiling (ONB) moves to inlet of test section and the wall superheat increases with the ascend of heat flux. A new empirical experimental correlation is developed on the foundation of experimental data, which correlates heat flux at ONB in natural circulation with the maximum relative error of 35%. Furthermore, the correlations of wall superheat, location of ONB, and subcooled boiling heat transfer coefficient are also acquired to predict the occurrences of natural circulation subcooled boiling of square channel.