Experimental study of spacer effects on post-dryout heat transfer in a tight-lattice rod bundle

Abstract

Tight-lattice fuel assembly is an advanced design for a compact water-cooled reactor core, but it presents challenges during loss of flow accidents, potentially suffering from critical heat flux (CHF). Downstream of the CHF point, dispersed flow with steam-entrained droplets becomes critical, significantly deteriorating heat transfer abilities. This paper experimentally studies the heat transfer capabilities downstream of spacer grids under post-dryout dispersed flow conditions. Experiments were conducted based on the high-temperature, high-pressure open-loop system at the Reactor Thermal-Hydraulics Laboratory of Shanghai Jiao Tong University. The working medium is water and the conditions tested included pressures ranging from 6 to 10 MPa, mass fluxes between 65 and 200 kg⸱m⁻²⸱s, heat fluxes from 75 to 200 kW⸱m⁻², and inlet qualities between 0.543 and 0.887. The heat transfer enhancement ratio is defined to assess the impact of spacer grids. It is shown that the heat transfer enhancement ratio is influenced by quality, mass flux, and system pressure. Specifically, spacer grids exhibit better heat transfer enhancement capabilities under conditions of low qualities, low mass flow rates, and high pressures. The circumferential wall temperature distribution is quite uniform before dryout, while local hot spots appear near the 30°, 90°, and 180° directions after dryout. Spacer grids exhibit optimal heat transfer enhancement under low qualities, low mass flow rates, and high pressure conditions, with the maximum effect observed at the spacer grid outlet, decreasing exponentially thereafter. Available correlations for spacer grid heat transfer characteristics are evaluated, with a novel correlation according to experimental data proposed. This novel correlation closely aligns with experimental results, maintaining an error range within ±15%.