An experimental study on the critical heat flux (CHF) has been accomplished for stable low flow of water in a vertical round tube and an annulus near atmospheric pressure (110 kPa) under forced and natural circulation flow condition. CHF data were obtained for water flow in a 0.01m-diameter vertical round tube of variable heated length ($L_h$ = 0.6 m or 0.9m), varying mass flux (0 - 258 kg/㎡s), inlet subcooling (135.8 - 344.8 kJ/kg), inlet throttling (large and no), and flow direction (upward and downdard). The CHF values varied from 21 kW/㎡ for complete bottom blockage to a maximum of 1130 kW/㎡ for a maximum inlet water mass flux of 258 kg/㎡s in the experiments.
The following parametric effects were observed during the round tube experiments :
(1) The CHF increased with the decrease of heated length at the same tube diameter and inlet condition (L/D Effect).
(2) The CHF was somewhat weakly affected by the flow instabilities for upward flow while much more susceptible in case of downward flow. The inlet throttling effect was significant in downward flow experiments.(Instability Effect).
(3) In the natural circulation experiment, the lower CHF values were observed than those of the forced circulation experiment contrary to the findings of Mishima. It seems to be related to the flow instability.(Natural Circulation Effect).
Experiments for annulus geometry were performed to assess the possibility of the following rod-bundle experiment and to visualize the flow pattern transition before and at the occurrence of CHF.
Finally, the measured CHF data were compared with predictions by available low pressure and low velocity CHF correlations.