Simulation Of Boiling Heat Transfer In Evaporator Of Separate Type Heat Pipe

The evaporation section and the condensation section of the separate heat pipe heat exchanger are arranged separately,which not only can achieve the heat of the long-distance transmission,but also conducive to the realization of large-scale heat exchanger.In addition,the proportion of the evaporation section and the condensation section could be adjusted according to the requirements,so the separation heat pipe has been widely used in the engineering.In this paper,the flow and boiling heat transfer in the evaporator of separate type heat pipe were investigated based on the passive containment cooling system.At present,the most common method for engineering design problems with flow boiling is to use empirical or semi empirical formulas,which have some limitations on geometric dimensions and flow parameters.So the designer needs to extend the relationship beyond its given valid range,but this leads to higher design errors.Therefore,the designer needs to extend the relationship to a given range of validity,but this may lead to higher design errors.The simulation of the phase change heat and mass transfer model in the evaporating section of the separated heat pipe based on the whole CFD is an effective method to overcome these difficulties,and the research method can reduce the experimental workload,saving the cost.At present,the numerical simulation of flow boiling in the tube is not very mature,and the research on the mass and heat transfer inside the heat pipe is also rare.Therefore,in this paper,we hope to present a more comprehensive and integrated CFD model,which includes all the details of the two phase convection,phase transition and heat transfer process in the process of the separate heat pipe.Therefore,this paper proposes a more comprehensive and integrated CFD model based on the previous studies,which includes the characteristics of two-phase convection,phase transition and heat transfer in the process of the separate heat pipe.The main contents and conclusions are as follows:(1)It is introduced that the numerical simulation method of boiling heat transfer in the heat pipe evaporator of the passive cooling system has been established.Through the simplification of the practical problems,the geometrical model is created,the grid is divided and the boundary conditions and working conditions are determined.The VOF method was used to calculate the free surface,the CSF model was used to calculate the surface tension and the Lee model was used to realize the mass transfer between gas and liquid phases to establish the two-phase flow boiling numerical simulation method.The independence of the model is tested and the feasibility of the model is verified by comparing the simulation results with the empirical formulas of the previous studies.(2)Based on the above numerical method,the process of bubble formation,growth,detachment and aggregation in the vertical heating tube were simulated.The results show the same when using the experimental results in the literature for comparative verification.The influence of different factors on the flow pattern of two-phase flow in the evaporation tube was analyzed.(3)According to the actual operating condition of the passive containment cooling system,the parameter sensitivity analysis of the heat transfer in the evaporation section of the cooling heat pipe is carried out.The relationship between the heat transfer and the flow pattern is clarified,and the following conclusions are drawn:the increase of the diameter leads to the increase of boiling heat transfer coefficient.The effect of the initial liquid filling rate on the boiling heat transfer is mainly reflected on the ratio of the gas-liquid two-phase area to the length of the tube in the state of equilibrium.The increase of temperature and vapor condensation coefficient outside the tube will enhance boiling heat transfer.