Abstract
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. 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. Furthermore, the heat transfer rate evaluation, bubble dynamics analysis and pictures regarding the heat transfer mechanisms are performed under the present condition. It is observed that bubbles are confined under the downward-facing heated surface, which causes a worse heat transfer capability than upward-facing boiling. Nevertheless, the impinging coolant flow is found to raise the temperature gradient formed by the heated surface, hence the heat transfer rate 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.