Experimentally investigating boiling characteristics in the transition boiling for the downward facing heating

• Heat transfer in transition boiling for a downward facing heated surface is investigated.
• Boiling characteristics are studied by measurements and observation.
• Effects of injection flow rate and gap distance are considered.
• CHF increases with increasing injection flow rate or decreasing gap distance.
• Leidenfrost temperature would increase with decreasing gap distance.

The external reactor vessel cooling (ERVC) can retain the molten corium to ensure the vessel remains intact via boiling heat transfer on the outer wall of the vessel. This downward facing heating layout research attracts much attention for nuclear safety. The heat removal capability for the boiling heat transfer is limited to the CHF value and the rewet of overheated vessel wall is determined by the Leidenfrost temperature. This paper investigates the heat transfer characteristics and bubble dynamics of transition boiling regime for the downward facing heating by way of experimental measurements and observation. Effects of injection flow rate and gap distance are considered. The CHF can be elevated as the injection flow rate increases or the gap distance is reduced. The Leidenfrost temperature would increase with decreasing the gap distance. The measured data reveal that nucleate boiling/transition boiling/nucleate boiling cycles exist in the transition boiling region of the boiling curve. These cycling and data fluctuation characteristics increase with the higher injection flow rate or the smaller gap distance. In addition, based on the experimental observations, the bubble size stuck on the heating surface during the transition boiling regime is smaller for the higher injection flow rate due to condensation and flow disturbance.