Study On Heat Transfer Characteristics Of Fuel Rod On Reflooding Quenching

Quenching refers to the rapid cooling phenomenon that occurs when a high-temperature object contacts a low-temperature medium,commonly used in industrial cooling and nuclear reactor safety.The reflooding quenching boiling process of fuel rods is a complex transient two-phase flow and heat transfer process,which is affected by many factors.The study focused on visualization and measurement experiments around the phenomenon of reflooding quenching boiling of rods,based on image processing and inverse heat conduction method to obtain data of film thickness,quenching front propagation rate,minimum film boiling temperature and film boiling heat transfer coefficient,to analyze the flow and heat transfer characteristics of reflooding quenching boiling of rods under the influence of subcooling,flow rate,internal heat source power and surface morphology and thermal properties.The main contents and conclusions of the research are as follows:Firstly,according to the experimental conditions,the experimental platform was designed and built to carry out the visualization experiment of the reflooding quenching boiling of the rod.Combined with image processing technology,the data of film evolution process,film thickness and quenching front propagation rate were obtained to explore the influence of different influencing factors on the film evolution of the reflooding quenching boiling of the rod.The results show that the subcooling has the most significant effect on the quenching time and the evolution of the film.As the subcooling increases from 5°C to 25°C,the film boiling time of the stainless steel rod shortens by84.1%,and the overall quenching boiling time shortens by 75%.The thickness of the film boiling film corresponding to T_w-T_sat=500°C decreases from 0.966mm to 0.15 mm,and the fluctuation range of the film also decreases significantly.The propagation rate of the transition nucleate boiling quenching front increases from 0.611 mm/s to 3.03 mm/s.The increase of the coolant flow rate will also lead to the slight decrease of the film boiling time,the overall quenching time and the film thickness of the rod,but the fluctuation of the film does not change significantly,and the propagation speed of the quenching front increases slightly.The increase of internal heat source will greatly increase the duration of film boiling stage,shorten the duration of the transition and nucleate boiling,and accelerate the propagation speed of the quenching front,but has little effect on the film thickness.Secondly,based on the temperature data of the rod reflooding quenching experiment,the surface temperature and heat flux of the rod were inverted and calculated by the inverse heat conduction method,and the influence of different influencing factors on the boiling heat transfer characteristics of the rod reflooding quenching was explored.The results show that the increase of coolant subcooling,coolant flow rate,thread form height and the decrease of heat source power in the rod can enhance the boiling heat transfer performance,increase the wall cooling rate,and accelerate the reflooding quenching boiling process,among which the effect of subcooling was the most significant.The minimum film boiling temperature T_min and critical heat flux q _CHF increase with the increase of coolant subcooling,coolant flow rate and thread tooth height.Finally,the empirical relationship between the minimum film boiling temperature T_min and the film boiling Nu number of the reflooding quenching boiling was established.Considering that the coolant subcooling has the most significant effect on the minimum film boiling temperature T_min and the film boiling Nu number,followed by the coolant flow rate and the physical parameters,and the internal heat source power has little effect on the film boiling Nu number.On this basis,the prediction relationship between the minimum film boiling temperature and the film boiling heat transfer Nu number was established.The error range between the experimental value and the experimental value was within the acceptable range,which proves its reliability.In summary,the flow and heat transfer characteristics of reflooding quenching of fuel rods under different influencing factors were experimentally investigated in this paper.The influence law and mechanism of influencing factors were analyzed and summarized,which provides ideas for strengthening the heat transfer performance of reflooding quenching and improving the safety of reactor core.