Experimental study on the onset of wall temperature fluctuation near critical heat flux under rolling motion in a flow boiling annulus channel using R134a

Abstract

This study examines rolling motion's impact on heat transfer in an annulus flow channel using R134a fluid, crucial for offshore nuclear reactors facing unique two-phase flow phenomena due to platform motion. Unlike previous studies on rolling’s effect on Critical Heat Flux (CHF), this research focuses on the pre-CHF regime, defining Periodic Dryout Heat Flux (PDHF) as the heat flux where temperature fluctuations indicate dry patches. PDHF was lower than vertical and inclined CHF, highlighting increased dryout susceptibility. It was influenced by superficial liquid and gas velocities and roll period: higher liquid bulk velocities in Departure from Nucleate Boiling (DNB) delayed drypatch formation, while higher vapor velocities in dryout accelerated it. Shorter roll periods reduced PDHF due to intensified perpendicular acceleration. Two kinds of modified Froude number (Fr) were introduced to characterize rolling effects. In DNB, PDHF approached rolling CHF as Fr increased, while in dryout, PDHF decreased with increasing Fr. When Fr exceeded 0.5 in dryout region, PDHF remained below rolling CHF, marking a critical dryout risk region. These findings emphasize the need to consider PDHF in heat transfer predictions for offshore reactors and offer essential data for improved thermal-hydraulic models.