Simulation of Two-phase Natural Circulation Cooling using Air Water System

Abstract

Abstract A closed loop natural circulation system employs thermally induced density gradients in single phase or two-phase liquid form to induce circulation of the working fluid thereby obviating the need for any mechanical moving parts such as pumps and pump controls. Due to its passive nature the reliability of these systems is very high. That is the reason natural circulation cooling has been considered in advanced reactor core cooling and in engineered safety systems. Natural circulation cooling has been proposed to remove reactor decay heat by external vessel cooling for in-vessel core retention and for external core catcher cooling during sever accident scenario. First a scaling analysis was carried out for natural circulation flow in a closed loop. Based on the scaling analyses simulation of steam-water natural circulation is carried out with air-water flow in an inclined rectangular channel. The heat flux corresponding to the decay heat is simulated with air flux to the test section to produce equivalent flow quality. Design calculations were carried out for typical core catcher design to estimate the expected natural circulation rates. The natural circulation flow rate of the water and two-phase pressure drop were obtained for different air injection rate expressed as void fraction for a select downcomer pipe size. These results can be scaled to steam water system using scaling consideration presented.