| The use of passive safety systems whose operation takes advantage of natural forces such as convection and gravity are being considered for numerous reactor concepts after the Fukushima nuclear accident. During a passively driven flows under the natural circulation conditions, an interesting phenomenon known as “intermittent discharge and refilling“ may occur in the coolant channels due to the weak driving forces. This Geysering Boiling Phenomenon(GBP) will lead to a large driving head between the reactor core and the heat sink which located at a higher position and an increased flow rate in the coolant channels. Thus this flow instability can be acted as an efficient method for cooling the reactor core under the conditions when there are no forced circulations or the natural circulations are too weak, and would remarkably impact the process of nuclear accidents and the safe operations of reactors. This intermittent flow and heat transfer process is essentially a kind of two-phase flow instability, and has various distinctive characteristics such as periodic vapor liquid transitions, dynamic two-phase flows, and various flow pattern transitions and so on. However, because of the special constructional features and operating conditions of the nuclear reactor, the relevant experimental study and theoretical analysis on this field are very rare, and many features of this instability such as the basic characteristics of flow and heat transfer, the key influence factors, the potential mechanism for its occurrence and the mathematical models have not been well defined and clearly clarified. Based on the above problem, the chief research content is presented as follows:(1) A visualized experimental apparatus for the geysering flow and heat transfer characteristic in a vertical heated channel are presented and visualization experiments have been carried out to investigate the change rules of hydraulic parameters such as- IV-temperature, system pressure, differential pressure, mass flow rate, etc. It turns out that four flow patterns of bubbly flow, slug flow, churn flow and mist packet annular flow are observed and new definitions are proposed for the observed intermittent flow process, i.e., thermal storage and violent boiling, vapor eruption and liquid refilling. The established geysering flow through channels presents a pulsating periodic characteristic. In addition, a dimensionless number Ge* is defined as the ratio of normalized pressure difference to the relative temperature difference in the vertical riser channel in order to describe the information of the geysering process.(2) The effects of five system parameters such as heat flux, system pressure, subcooled temperature, coolant inventory and geometry ratio on the period and intensity of geyser boiling are investigated. It is revealed that the geyser period decreases from approximately 267 s to 36.1s by increasing the heat flux from 2.0×104 W·m-2 to 7.2×104 W·m-2. This result clearly indicates that the geyser boiling occurs more frequently at a higher heat load, and the effect of heat flux on the period of geysering boiling and intensity of temperature oscillation is so important that by increasing the heat input, the period and intensity of temperature oscillation decreases until it becomes very low and then completely disappears. Meanwhile, it seems that much of the geysering time is occupied by the thermal storage stage which is the most important and one of the key requirements for the occurrence of the geysering phenomenon. The geyser boiling occurs obviously below 0.1MPa, but this intermittent flow and boiling disappears when the system pressure is raised beyond 0.3MPa, and this fact reveals that the system pressure is an important factor that decided its occurring degree. The geysering period increases with an increase in upper subcooling at low heat flux, and its influence decrease gradually as the heat flux increases. The geyser period changed slightly as the coolant inventory ratio ranged from 12.5% to 87.5%, which indicates that the coolant inventory ration has a little effect on the geysering flow. The geysering period decreases from 33 s to 29 s when the channel ratio increases from 55 to 83, and this fact shows that a larger geometry ratio was a favorable factor for geysering which illustrated that geysering occurred more easily at a larger geometry ratio.(3) Based on the calculation of power spectral density(PSD) and probability density function(PDF) of pressure, differential pressure and temperature in the vertical heated channels under various experimental conditions, the results show a typical multi-peak distribution of signal analyzed by the power spectral density method. These results indicate that there are many different flow patterns which have been observed successfully in the experiments such as bubble flow, slug flow, churn flow and mixed flow regimes during the geysering flows. There are mainly churn flows in the heated section with higher gas voids and slug flows with lower gas voids in the riser section. Besides, the change of flow patterns and pressure variations in the channels under different conditions are analyzed and the influence of five system parameters on the geysering flow characteristics are presented based on the above mentioned methods.(4) Based on the experimental results, the geysering phenomena in a vertical closed system without circulation are divided into six basic patterns which were determined by the severity extent during the eruption and refilling process, i.e., violent geysering, geysering, weak geysering, slug flow, bubbly flow and single phase flow. The flow pattern map for the two-phase flow conditions in vertical channels based on geometry ratio L/D and different input powers Qin is presented. Eventually, the mechanism of geysering flow and boiling is proposed according to the experimental results.(5) A mathematical model of geysering flow in the vertical channels is developed based on the conservation principle of mass and energy, temperature variations and geysering period during geysering flow are analyzed with this mathematical model.The numerical results are verified by comparison with the experimental results, which shows the computation is reliable and effective.In this paper, the flow and heat transfer characteristics, key influence factors and mechanism during geysering process are presented based on the experimental study and theoretical analysis, and the conclusions and method of this paper can be helpful to design the passive safety systems theoretically and technologically. |
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