Evaluation of Various Correlations for Direct-Contact Heat Transfer in Dispersed Flow Film Boiling Using DRACCAR Code

Abstract

Following the core uncovery after a hypothetical loss of primary coolant accident (LOCA), a flow of steam with dispersed droplets forms downstream of the quench front. This flow is crucial for cooling nuclear fuel rods that are not in direct contact with water, representing a typical Dispersed Flow Film Boiling (DFFB) scenario. Depending on accident severity, fuel rod deformation may partially block subchannels, diverting steam towards less deformed areas and potentially impacting cooling. As a result, characterizing heat trans-fer under such conditions is essential to enhance the nuclear reactor safety during accidents like LOCA. The COLIBRI experimental loop was designed to assess the heat flux released by steam/droplet flow in intact and deformed fuel rod subchannels. Experimental results obtained with this facility helped evaluate various correlations for direct-contact heat transfer calculations in DFFB within the DRACCAR code. In this article, the DRACCAR code is used to compare different correlations for heat transfer due to droplet impacts onto the wall calculation. This comparison was made on four COLIBRI experiments, involving two different droplet mass flow rates (5 kg/h and 11.5 kg/h) and two levels of subchannel deformation (intact and 90% blocked).