Study On Two-phase Flow And Heat Transfer In A Header-type Separate Gravity Heat Pipe

Based on the application potential in the spent fuel pool,this paper studied the flow and heat transfer performance in a header-type separated gravity heat pipe under the pre-experimental scale,selecting R134a as the working fluid.Firstly,the liquid-gas evaporation model was established in CFD platform,to get the gas-liquid two-phase flow pattern,flow distribution characteristics and heat transfer coefficient in parallel branches of evaporation section.The results show that in the vertical flow boiling of8mm-diameter pipe,the bubble flow,slug flow and agitated flow patterns appear along the height of branches;in the calculation results of65mm-diameter pipe however,there is mainly bubble flow.When the branches in the evaporation section are inclined at a certain angle,bubbles will accumulate to the upper wall under the effect of buoyancy,forming a"dry zone"where heat transfer deteriorates.The heat transfer coefficient of the two-phase region in the 8mm-diameter pipe fluctuates between 1600W（m2K）^-1.The heat transfer coefficient in the branch has a corresponding relationship with the gas volume fraction near the wall and the average gas volume fraction in the cross section.Secondly,a flow distribution model of parallel branches in the evaporation section of separated gravity heat pipe was established,and the influence of inlet header mass flow,number of parallel branches,pool temperature,sub-cooling at inlet,and header/branch diameter ratio on uniformity of flow distribution among parallel branches was discussed.The results show that the uniformity of flow distribution in parallel branches will get improved with the increase of header/branch diameter ratio,sub-cooling at inlet and inlet mass flow;while the increase of pool temperature and the number of parallel branches will cause uneven flow distribution.Thirdly,the theoretical analysis of each element of the separated gravity heat pipe was performed,and then the modules were sequentially coupled to obtain the mathematical model of the loop.With the mathematical model of the loop,the correlation among the saturation temperature,heat absorption,heat source temperature and filling rate in the system was obtained.Under the condition of liquid filling rate being 42.3%,the pool temperature being 60℃ and the temperature of condensation section being 20℃,the heat transfer capacity of the separated heat pipe loop is 2241W.Finally,a separate gravity heat pipe loop experiment with branch pipe diameter of 8mm was established.The effects of liquid filling rate,heating power,and condensation section temperature on the performance of the separate heat pipe loop were studied and the flow and heat transfer characteristic differences of the evaporation branches and condensation branches were compared.The maximum deviation of heat transfer between the experimental and MATLAB theoretical loop model under the same working conditions,is about 15%,which shows that the separated gravity heat pipe loop model established in this paper is reasonable.