Analytical model for CHF in narrow gaps with surface orientation effects

The CHF in rectangular narrow gaps has been investigated to develop a reasonable predictive model for CHF, accounting for the surface orientation effects. The model was based on Wallis formulation of the counter current flow limitation (CCFL) for flooding of the flow entrance gap. The results by the predictive model were compared with the experimental data by Kim and Suh (2003), and a good agreement was obtained for gap sizes of 1 mm and 2 mm with surface inclination angles ranging from 15° to 90°. However, when the surface inclination angle was less than 15° (nearly downward-facing position), the predictive model underestimated the CHF. A modified Katto-Kosho correlation based on Kutateladze approach was further proposed and it could predict the CHF well for inclination angles less than 15°, due to the surface tension effects. For the gap sizes of 5 mm and 10 mm, the predictive results were far larger than experimental data by Kim and Suh. And the large differences between the predictive results and experimental data were attributed to the fact that the mechanism of CHF in large gap is quite different from a narrow gap. It indicated that the validity of the model based on CCFL was limited to gaps of less than 5 mm. The present work is instructive for the safety analysis of the lower head of PWR in case of core meltdown during severe accident.