Experimental study on flow boiling CHF in annulus channel under heaving conditions using simulant fluid R134a targeting nuclear reactor applications

Abstract

Floating nuclear power plants have attracted significant attention as a prospective means to deliver clean energy, particularly in geographically remote areas where conventional power infrastructure is inaccessible. The ocean environment, however, can affect the thermal–hydraulic phenomena in them. Notably, in the two-phase system, Critical Heat Flux (CHF) can change due to the periodic variation of additional forces. While a few experimental studies have been conducted on CHF under heaving motion, expanding the range of test conditions is necessary to elucidate its mechanism at the operating pressure of a Pressurized Water Reactor (PWR). Hence, the present study performed an experiment on the flow boiling CHF under heaving motion. To simulate the heaving motion of the ocean environment, a heaving motion platform was designed and constructed. It can simulate the heaving motion with a maximum acceleration of 0.6 g and a period ranging from 3 to 6 s. The CHF test loop utilized refrigerant R134a as the working fluid, and the tests were conducted under the thermal–hydraulic conditions corresponding to PWR operation conditions applying a fluid-to-fluid scaling. The test section consists of a single heater rod in the annulus channel. CHF was measured under both static and heaving motion conditions, and a comprehensive parametric study was conducted on the CHF variation due to the oscillating acceleration field. The results demonstrated that, in most cases, CHF decreased compared to the static condition. The reduction in CHF due to heaving motion could reach up to 9% in the present test configuration, and the reduction ratio was proportional to the magnitude of the heaving acceleration. In addition, the effect of heaving motion was pronounced in two distinct thermal–hydraulic regions: when the critical quality approached zero and when it exceeded 0.6. Based on the experimental observations, the CHF reduction mechanism in each region was suggested.