Analytical model for CHF in narrow gaps concerning surface orientation

The CHF in rectangular narrow gaps have been investigated to develop a reasonable predictive model on CHF concerning surface orientation effects. The model is based on the counter current flow limitation (CCFL) by flooding of the flow entering gap. The results by the predictive model were compared with the experimental data by Kim and Suh (2003), and a good agreement was obtained in case of 1 mm and 2 mm gap size with surface inclination angle ranging from 10°-90°. However, when the surface inclination angle is less than 10° (nearly downward-facing position), the predictive model underestimated the CHF. The reason may be that the heating surface is covered by a stable vapor film and the vapor film is almost stationary, and that the effects of CCFL on CHF became smaller. A modified Katto-Kosho correlation was further proposed and it could predict the CHF well for inclination angle less than 10°. For gap size equaling to 5 mm and 10 mm, the predictive results were far larger than experimental data by Kim and Suh, which indicated that the validity of model based on CCFL controlled CHF was limited to gaps of less than 2 mm.