Experimental study of boiling heat transfer of inclined down-ward facing heated curved wall under low flow and pressure conditions

Abstract

This study aims to investigate the behavior of bubbles and the mechanisms of heat transfer in a downward-facing curved channel under boiling flow conditions. A curved wall, representing a portion of the external surface geometry of a nuclear light water reactor vessel, is used to examine the influence of channel curvature on boiling phenomena. Experiments are conducted across a range of heat fluxes to understand the behavior of bubbles and slugs at different thermal loads on the curved surface. High-speed videography is employed to visualize bubble dynamics and their impact on heat transfer. The study quantifies parameters such as slug bubble velocity, length, width, and frequency, and examines their dependence on heat flux. The findings reveal that under a heat flux range of 30 to 100 kW/m2, the bubble motion and dynamics exhibit varying patterns, with the generation of smaller and faster bubbles. At approximately 150–200 kW/m2, a transition phase occurs, alternating between isolated nucleates and the formation of large slugs. Beyond 200 kW/m2, in the slug boiling regime, bubbles undergo substantial transformations, characterized by the emergence of larger vapor structures and more intricate boiling patterns. The resulting heat transfer rate is, thus, affected by the dynamics and behavior of these vapor structures, as well as the streamwise evolving thermal boundary layer and the surface curvature of the heater.