Experimental study of downward-facing boiling heat transfer with impinging coolant flow - 15187

Boiling heat transfer has a high heat removal capability in convective cooling. However, the heat removal capability of downward-facing boiling is significantly worse than that of upward-facing cases due to the confined buoyancy effect. This study was inspired by the conception of external reactor vessel cooling (ERVC) condition relevant to the in-vessel retention (IVR) design of Westinghouse AP1000 plant. In the present study, a small-scale test facility had been established to investigate the local phenomena of boiling heat transfer under a downward-facing horizontal heated surface with impinging coolant flow. In this study, the surface temperature, heat flux information and several specific snapshots of bubbles are taken down throughout the boiling processes for detailed investigation. It is observed that bubbles are confined under the downward-facing heated surface, which causes a worse heat transfer capability and a lower critical heat flux (CHF) limit than upward-facing boiling. Nevertheless, the impinging coolant flow is found to raise the temperature gradient formed by the heated surface, hence the CHF increases with an increase of coolant flow rate. In addition, during nucleate boiling, it is discovered that the formation, condensation and motion of bubbles induce turbulent wakes and therefore enhance the heat transfer rate. (authors)