Subcooled flow boiling heat transfer of water in circular tubes with twisted-tape inserts under high heat fluxes

This paper presents the heat transfer of subcooled water in swirl flow under high heat fluxes with respect to the heat removal technologies for the divertors in the International Thermonuclear Experimental Reactor (ITER). The experiments were conducted in a vertical circular tube that was heated uniformly with a large AC power supply. A twisted tape with a twist ratio of 2 or 4 was inserted into the circular tube to induce the swirl flow. The test parameters were as follows: heat flux q = 5-19.5 MW/m2; system pressure P = 3, 4.2, 5 MPa; mass flux G = 6000, 8000, 10,000 kg/(m2 s); and inlet bulk temperature Tb,i = 50-200 °C. The heat transfer coefficients and boiling curves are obtained from single-phase forced convection to fully developed nucleate boiling. The influences of mass flux, heat flux, system pressure, thermodynamic quality, and inlet subcooling on heat transfer are discussed in detail. Special attention is paid to the effect of the twisted tape on subcooled heat transfer. More importantly, a wide set of heat transfer correlations are evaluated on the basis of our experimental data, and relevant recommendations for engineering applications are made according to their prediction accuracies.