| After the Fukushima nuclear accident, the safety of the reactor began to attract attention.For cooling and depression of containment accident condition, a new generation of reactors introduced the passive containment cooling system (PCCS), the system did not need to use external power, only relied on the laws of physics such as natural circulation . However, the natural circulation capability and the system safety have been obviously influenced by the two-phase flow characteristics .Therefore, it is necessary to study bubbles' polymerization behavior characteristics under the natural circulation condition and the effect of bubble behavior on natural circulation. This research provided the basis for thorough analysis of the mechanism of gas bubble enhancing the natural circulation capabilities.In this paper,a set of visualization experiments were carried out under natural circulation conditions. By adjusting the gas injection rate, the impact of flow pattern , interaction behavior between the rising gas bubbles and bubbles polymerization mechanism on natural circulation was studied. Experimental research was conducted on the flow characteristics in an open natural circulation loop. The gas mass flow rate ranged from 0-4.0 kg/h and the water level in tank ranged from 150 mm?950 mm. The system pressure was under the atmosphere pressure.The visualization experimental studies showed that flow patterns could be divided into:bubbly flow, slug flow and churn flow. Taking liquid Reynolds and gas superficial velocity as the characteristic parameters, the effects of liquid turbulence intensity and gas flow on character and range of flow pattern were analyzed. The main research of this article was bubbles' polymerization mechanism and impact of bubbles on natural circulation. This paper first analysed the individual behavior characteristic of the bubble, including the relationship among bubble velocity, bubble diameter, rising height and gas flow, and probed into the periodic law of bubble spiral movement. Then, the aggregation behavior of bubbles was divided into two categories including two parallel bubbles and two vertical bubbles and analysed the relationship between individual characteristic and bubble aggregation. Finally,effects of bubble individual behavior of and bubbles polymerization on natural circulation ability were discussed. The results showed that when the gas mass flow rate was less than 0.500 kg/h, the gas flow and bubbles' average diameter existed a good linear relationship.With the increase of gas flow, the diameter increased; When the gas mass flow rate was greater than 0.500 kg/h, the change of bubbles' average diameter was no longer linear but fluctuated in the range of 3.500mm; Natural circulation flow increased with the increase of gas flow rate. When gas flow was less than 1.300 kg/h, augmenting gas flow is better for the natural circulation ability promotion, but the effect under the condition of greater than 1.300kg/h is falling.By adjusting the injection rate and tank level, the enhancing performance of bubble pump for the natural cycle capabilities was studied. For the macroscopic flow characteristics of the natural circulation under bubble pump action, the calculation model of natural circulation average flow rate changing with the system gas injection rate was proposed. The results showed the existence of critical gas injection rate for the natural circulation system under the effect of bubble pump. When the gas injection rate was less than the corresponding critical value, natural circulation would not occur. The gas injection rate was related with the tank water level in the system. The injection rate decreased with the increase of water level. For the same tank water level, when the gas injection rate reached a critical value,average flow rate of the natural circulation system increased rapidly with the increase of gas injection rate. However, with further increasing of gas injection rate, the increasing rate of system average flow rate slowed down and the curve turned to be flatten. Through the established theoretical calculation model, it was found that there was an optimum injection rate in the natural circulation system with the action of bubble pump, where could achieve the maximum natural circulation capacity under the same working conditions. |
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