Abstract
The critical heat flux (CHF) of downward facing heating process and its heat transfer has been a hot study topic heat flow in recent years. The application has been widely applied into a variety of industries, including nuclear, cooling of electronic devices, fuel cells, steel-making and chemical reactors. Many studies have found that the CHF of downward facing heating is lower than that of upward facing heating for the reason that of bubbles are easily accumulated at the heating surface due to buoyancy and gravity, causing premature CHF. Downward facing heating is a special heat transfer phenomenon; the feature is very different from traditional upward facing heating, especially nuclear boiling heat transfer and the process of CHF. The bubbles are forming constantly at the heating surface during downward facing heating because of buoyancy and gravity, and the surface is superheated, leading to a layer of gas film and CHF. This phenomenon may exist during many industrial production, manufacturing and application, such as reactor core boiling and severe accidents related to heat transfer of coolant and its mitigation system - In Vessel Retention External Vessel Cooling (IVR-ERVC) in nuclear industry. Under this condition, coolant and its heat transfer capability become very critical. In addition, different fluid properties and water inlet distance will also affect the CHF at the heat transfer surface. This study aims to investigate the CHF at different distances between coolant inlet and heating surfaces, different coolant inlet flow rates, and how different degas coolant conditions affect the CHF. The result shows that shorter coolant inlet distance or larger inlet flow rate leads to better heat transfer effect, and vice versa. The purpose of degas is to prevent air in the water from sub-cooled boiling and premature purge boiling bubbles on the surface, causing premature CHF. The result also indicates that the higher the temperature for degas is, the better the degas effect is, and the CHF also increases positively. Finally, the heating surface with inclining effects are also discussed and a new CHF model with downward facing inclined angles has been derived of this study.