Experimental investigation of re-wetting of hot horizontal tubes

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Re-wetting of a hot dry surface is the establishment of direct contact between the surface and a liquid at a lower temperature. Re-wetting heat transfer is characterized by a large increase in heat transfer from the surface and occurs when a vapor film existing between the dry surface and the liquid is destabilized. Study of re-wetting heat transfer is very important in nuclear reactor safety for limiting the extent of core damage during the early stages of severe accidents after loss of coolant accidents LOCA and is essential for predicting the rate at which the coolant cools an overheated core. Surface re-wetting is established by the formation of a wet patch on the hot dry surface which then grows in size to cover the entire surface. The leading edge of the wet patch is called the re-wetting front and consists of transition and nucleate boiling heat transfer regions. The aim of this study is to introduce and study two important variables related to the re-wetting front as it moves on hot horizontal tubes cooled by water jet impingement. These variables are: the rebound phenomenon of the re-wetting front and the width of the re-wetting front. Experimental observations of this study showed that the re-wetting front could rebound a small distance just after the formation of the wet patch due to rapid heat conduction in the solid towards the wet patch. The rebound distance was found to increase by decreasing water temperature and the velocity of the jet. The width of the re-wetting front was found to increase by increasing water temperature and decreasing the initial surface temperature. As the solid thermal conductivity increases, this width was found to increase.mp201