Mechanism Analysis And Experimental Investigation Of Nucleate Pool Boiling Heat Transfer In A Large-scale Confined Space

Although boiling is ubiquitous,this phenomenon is not completely clear,mainly due to the complex phase transition process involved phase change process,bubble dynamics,and convective flow.The nucleate boiling heat transfer mechanism and the cause of the efficient heat transfer capacity are the focus of long-term research.Current empirical or semi-empirical correlations are commonly used to calculate boiling heat transfer.Due to the complexity of boiling,existing empirical correlations and research results have their own application limitations,and most of the correlations are not widely applicable.In addition,the differences in experimental methods and working conditions make the correlations related to boiling heat transfer often have large deviations.Boiling heat transfer outside the vertical heat exchange tubes has been widely used in new generation nuclear power systems,such as reactors,passive safety systems,and spent fuel storage cooling.The characteristics of boiling heat transfer on a large-scale vertical tube in a confined space are different from the conventional pool boiling,and more different from the flow boiling in the tube.The current research focuses on the microscopic mechanism of wall superheat,core number of vaporization,formation of bubbles,and evaporation.Therefore,the study of the pool boiling on a large-scale vertical heat transfer tubes,as well as the dynamic process of vapor bubble and the combination between the mesoscopic and macroscopic heat transfer mechanism,has scientific significance for revealing the fluid and heat transfer.At the same time,it also has important guiding significance for the design of phase-change equipment.Taking the pool boiling on a large-scale vertical tube as the research object,this paper adopts numerical simulation and experimental research methods to analyze of the heat transfer,flow,local heat transfer mechanism and the overall heat transfer characteristics,bubble behavior and dynamics.The main research work and results obtained are as follows:Adopting the VOF model and the Lee model,the CFD simulation was performed for the pool boiling on the vertical tube in a confined space.A numerical model for the multi-scale boiling coupled conduction in a confined space was established.The bubble behavior was analyzed as the key factors affecting the boiling heat transfer characteristics.The local heat transfer mechanism was discussed under the bubble impact.Combining the bubble morphology and behavior observed in numerical simulation and visualization experiments,the growth and merging process of the bubble are analyzed in the lower and middle part in confined space.The large bubble behaviors in the upper area is discussed including the rupture and wake characteristics.According to the morphology,distribution and interaction of the vapor bubbles,four regions can be found along the height of the vertical tube:isolated bubble regions with uniform dimensions;the bubble coalescence and growth regions among the different dimensions;Bubble bubble coalescence and collapse region;bubble behavior chaotic region,bubble behavior and morphology become the diversification and complexity.The local heat transfer characteristics are closely related to the evolution of bubble behavior and mechanism.The reliability and accuracy of the numerical simulation method are also established.This paper constructed the experimental platform of pool boiling heat transfer on a vertical tube.The experimental study of pool boiling heat transfer coupling conduction in a confined space was carried out under atmospheric pressure.The results indicate that form natural convection to subcooled boiling,and finally to saturation boiling,there are obvious turning points of the heat transfer mechanism in the different heat exchange stages.Moreover,natural convection and supercooled boiling have different development times at different altitudes.The duration of natural convection in the lower part of the heat transfer tubes is the longest,and there is a steep rise in the heat transfer coefficient in the subcooled boiling stage.The whole heat transfer tubes reach saturation boiling at the same time.Compared with the boiling heat transfer in the open space,the heat transfer in the confined space can accelerate the temperature of the lower part of the tank to reach saturation,and effectively weaken the thermal stratification in the tank.Based on the PRHR HX of a new generation nuclear power system,we simulated the boiling heat transfer on the original length of the vertical tube in the channel of the tube bundle.The accuracy of the numerical model was validated by the empirical correlations and experimental data obtained the Westinghouse Company.Numerical results show significant differences in nucleate boiling along the vertical tube heights in industrial scales.This also shows that the application of traditional integral heat transfer design will create deviations.In addition,although the heat transfer performance of the wall surface corresponding to the small spacing have a large fluctuation,the small space of heat transfer can form relatively high convection velocity.Then the effective combination of large spacing and small spacing is a very effective way to strengthen heat exchange.The difference of heat transfer coefficient on the same hight of heat transfer tube is not obvious by comparing 4m and 5.486m tube length,and the short tube is slightly lower than that of the long tube.The short tube is better than the long tube for the whole heat transfer coefficient.Therefore,it is of great significance to guide the design of passive cooling systems and other applications of nuclear power engineering design.