Measurement and analysis of the re-wetting front velocity during quench cooling of hot horizontal tubes

- Two phase flow & re-wetting front velocity were studied for quench of hot tubes.
- The velocity decreased as temperature difference between tube and coolant decreased.
- Increasing surface curvature was found to decrease the re-wetting front velocity.
- Increasing tube thermal conductivity decreased the velocity.
- Correlations were developed to predict the front velocity.

When a liquid is put into contact with a hot dry surface, there exists a maximum temperature called the re-wetting temperature below which the liquid is in actual contact with the surface. Re-wetting occurs after destabilization of a vapor film that exists between the hot surface and the liquid. If re-wetting is established at a location on the hot surface, a wet patch appears at that location and starts to spread to cover and cool the entire surface. The outer edge of the wet patch is called the re-wetting front and can proceed only if the surface ahead of it cools down to the re-wetting temperature. 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. One of the important parameters in re-wetting cooling is the velocity at which the re-wetting front moves on the surface. In this study, experimental tests were carried out to investigate the re-wetting front velocity on hot horizontal cylindrical tubes being cooled by a vertical rectangular water multi-jet system. Effects of initial surface temperature in the range 400-740 °C, water subcooling in the range 15-80 °C and jet velocity in the range 0.17-1.43 m/s on the re-wetting front velocity were investigated. The two-phase flow behavior was observed by using a high-speed camera. The re-wetting front velocity was found to increase by increasing water subcooling, decreasing initial surface temperature and decreasing jet velocity. Effects of surface curvature, solid material, tube wall thickness, jet orientation, number of jets and number of tubes were also investigated. Empirical correlations for the re-wetting front velocity have been developed and provided good prediction of experimental data.