A CHF characteristic for downward flow in a narrow vertical rectangular channel heated from both sides

Abstract

A new correlation for the countercurrent flow limitation (CCFL) in vertical rectangular channels was applied to the prediction of critical heat flux (CHF) for downward flow in a vertical rectangular channel heated from both sides. The new correlation was based on CCFL experiments carried out with a two-phase flow system of air and water under 1 atm with a superficial air velocity range of 1–17 m/s. The CHF experiments were carried out with an inlet water subcooling range of 25–75 K and an inlet water mass flux range of 2–600 kg/m 2s under about 1 atm for a 750 mm long, 50 mm wide and 2.25 mm gap flow channel and a 375 mm long, 50 mm wide and 2.80 mm gap flow channel which were heated from both sides. The comparison between the prediction based on the new correlation for the CCFL and the CHF experimental results could provide a good quantitative understanding of CHF characteristics, which is required for the thermal-hydraulic design and safety analysis of nuclear research reactors in which the downward flow is adopted for core cooling with flat-plate-type fuel. The role of the aspect ratio of the rectangular channel was evident in both the CCFL and CHF characteristics for countercurrent flow and it was strongly implied that the CHF for downward flow is at a minimum under the flooding condition in the case of large inlet water subcooling and when the inlet downward water mass flux is greater than that under the flooding condition in the case of small inlet water subcooling.