Investigating the Heat Transfer Characteristics of Impinging Flow on a Downward-Facing Surface

Abstract

Critical heat flux (CHF) is an important heat transfer deterioration phenomenon during boiling heat transfer. It has been extensively studied, especially in the field of nuclear energy. Previous research has found that downward-facing heat transfer is worse than upward-facing and should be paid more attention. In this paper, the boiling heat transfer process under different flow rates and inlet distances is investigated. Seven experimental cases were made including a pool boiling case. The experiment studied the effect of inlet distances under small flow rates, which is not covered by previous research. Analysis of the CHF mechanism included surface temperature curves, boiling curves, bubble behaviors, and heat transfer coefficient. The fluctuation of the surface temperature of forced convention cases was observed due to the bubbles sliding along the heating surface. The phenomenon of vapor film fragmentation could also be found. The results show that even at small flow rates, CHF occurring time is postponed and the CHF value increases. Reducing inlet distance or increasing flow rate can both promote boiling heat transfer, thereby enhancing CHF.

Keywords:

• Downward-facing heating
• forced convention
• critical heat flux
• bubble behaviors

Acknowledgments

The authors appreciate the financial support from the Development Foundation of College of Energy, Xiamen University (no. 2018NYFZ04).