An experimental study has been performed on boiling phenomena and critical heat flux (CHF) in narrow gaps submerged in a pool of saturated water under atmospheric pressure. Two types of test geometries were investigated: (a) internally heated annular gaps with the opened bottom and (b) one-side heated rectangular gaps with the closed bottom.
The annular test sections were heated by a stainless steel inner tube with outside diameter of 19 mm and heated length of 200 mm. The annular gap was changed in the range of 0.5 to 3.5 mm and the orientation of the test section was changed from horizontal (θ=0°) to vertical positions (θ=90°). Natural circulation flow was established in the gap where single-phase liquid entered from the bottom and two-phase mixture flowed out through the top of the test section. The experimental results indicate that the CHF occurs due to the dryout of the liquid film on the heated wall and increases with the increases in the gap size and the inclination angle (θ). A one-dimensional two-phase flow model with a modified friction factor correlation is suggested to predict the CHF condition. The developed model reasonably predicts available CHF data.
The rectangular test section was heated by a 200 mm (length) × 60 mm (width) plated heater. Two gap sizes (0.5 and 1.0 mm) were investigated for vertical orientation with the closed bottom condition. In this case, the CHF was observed to occur due to flooding as in previous works. To understand the mechanism of flooding in narrow channels, data analysis and theoretical modeling were conducted. A new flooding correlation is suggested using Kutateladze number which shows better prediction results than other flooding correlations in narrow channels. A theoretical model is also derived for flooding in thin rectangular channels. It shows similar tendency with the flooding data for large span width, but slightly under-predict them.