Investigation on Development of New Mechanistic Model for Post Dryout Heat Transfer

Abstract During some transient scenarios such as loss of coolant accident (LOCA) in nuclear power plants, dryout could occur in fuel assemblies. The heat transfer after dryout will determine the maximum cladding temperature when dryout occurs. Therefore, identifying the heat transfer mechanism and predicting heat transfer in Post-dryout (PDO) region will be particularly important for facility safety operation. In the post dryout region, the behaviour of droplets may significantly affect the heat transfer and subsequently, the wall temperature. When droplets move toward the wall due to turbulence dispersion effect, most of the droplets will be completely evaporated inside the vapor layer or bounced back to the mainstream. Therefore, in this paper, the flow area is divided into the central region and the near-wall region considering the large difference in vapor superheating between wall and tube center. The interfacial heat transfer is calculated separately in these two regions with a correlation of vapor temperature profile obtained from CFD results. There will be droplet mass exchange between the two regions due to the concentration difference caused by the fluctuation velocity profile. The calculation results were compared with Becker’s water PDO experimental results, and it was found that the predicted wall temperature were in good agreement with the experimental data. It shows that the new model can improve the prediction accuracy of heat transfer in the post dryout region.