Study on natural circulation flow stagnation and critical heat flux in narrow rectangular channel

Experiments of critical heat flux (CHF) under natural circulation condition in narrow rectangular channel were conducted in Natural Circulation Laboratory of North China Electric Power University. The phenomena of flow stagnation and heat transfer deterioration were observed during the experiments. The developing mechanism of flow stagnation/heat transfer deterioration in narrow rectangular channel under saturation condition was proposed. Flow pattern instability would appear when the flow excursion occurred, thus leading to the continuous flow oscillation and flow stagnation. Then the liquid layer near the outlet would completely evaporate under a certain heat condition, and the CHF occurred. While in narrow rectangular channel, the vapor stream could produce extrusion action to the liquid layer on the heating wall due to the limitation caused by gap size, which cause the liquid to become very thin and heat transfer deterioration could happen under a much lower heat flux condition. Based on the mechanistic analysis, a corresponding calculation model was provided. A dimensionless number of constraints and a natural circulation characteristic factor were brought in to optimize the proposed model for the purpose of considering the effects of gap size and circulation mode in the model respectively. The multiple regressions were applied to the calculation model according to the experimental results, and the accuracy of which was validated. The comparison between experimental results and model calculation values indicated that CHF would increase with the inlet velocity and system pressure increased, and would decrease with the outlet quality increased. (authors)