Numerical Study On The Effect Of Transmission Distance On Thermal Characteristics Of Separate Heat Pipe

With the development of information technology,the number of communication base stations is increasing.In order to reduce energy consumption of communication base station,heat pipe technology is used to save energy and reduce energy consumption of communication base station.As an enhanced heat transfer device,heat pipe has become a research hotspot,because of its high efficiency in energy saving.Separate heat pipe is a new type of heat pipe,because its evaporation section and condensation section can be arranged separately and achieve long-distance heat transfer.It has the advantages of low maintenance cost and strong heat transfer capacity,which can greatly improve the energy utilization rate and significantly reduce energy waste and achieve sustainable development.Based on CFD method,the physical model of separate heat pipe is established.VOF model and CSF model are selected to calculate the flow of gas-liquid two-phase flow and the surface tension of gas-liquid interface.The gas phase,liquid phase and energy source phase are defined,and the numerical simulation method of gas-liquid two-phase flow and phase change of separate heat pipe is established.The flow pattern of gas-liquid two-phase flow in the evaporation section of the separate heat pipe and the heat transfer performance of that with different filling rates and heating power were analyzed by numerical simulation.And the distribution of the condensate film thickness of the condensation section and condensation heat transfer coefficient and working medium temperature at different transmission distance(vertical height and horizontal length)were also analyzed.The main contents and conclusions of this paper are as follows:(1)When the separate heat pipe reaches the nucleate boiling state,there will be a variety of flow patterns in the tube,including bubble formation,flow,coalescence,rupture and so on.(2)The effects of different filling rates of 40%,60%and 90%on the heat transfer performance of separate heat pipe were numerically analyzed.The results show that when the filling rate is 40%and 60%,the wall temperature fluctuates greatly and the wall temperature becomes overheated,however the filling rate is 90%,the fluctuation of wall temperature is small,the average temperature difference of each point is about7K,and there is no overheating in the upper part of the evaporation section,and the effective mass cycle and the actual mass cycle of the heat pipe system are closer,and the heat transfer performance is better.The liquid filling rate is larger,the more bubbles and the more obvious the change of section void fraction.(3)The effect of different heating power on heat transfer performance of separate heat pipe were numerically analyzed.The results show that the higher the heating power is,the smaller the overall thermal resistance of the heat pipe system is,and the higher the heat transfer efficiency of the separate heat pipe is.With the increase of heating power,the number of bubbles on the wall increases.Bubbles coalesce with each other during the flow process,and the position of slug flow moved forward.The wall heat transfer coefficient and the steam volume circulation flow increase with the increase of heating power.(4)The vertical distance of H=1200mm is more remarkble than H=800mm and1000mm between evaporation section and condensation section for condensation effect.With the increase of the vertical height,the thickness of condensate film increases,and the thickness of condensate film presents an unstable distribution along the length of condensation section.(5)With the increase of the vertical height,the steam flow speed increases.And the steam velocity of the condensation section is relatively high and there is a steam recirculation vortex phenomenon at the entrance.Because of the loss of local pressure,the steam velocity decreases gradually along the length of the condensation section tube.When the vertical height increases from 800 m to 1200 mm,the condensation heat transfer coefficient decreases along the condensation section,the condensation heat transfer coefficient at the outlet of the condensation section is basically kept at about 1800 W/(m~2.K),and the heat transfer capacity of the heat pipe system increases by 6.7%.(6)Horizontal distance 400 mm,350 mm compared with 300 mm,working medium temperature at the entrance of the condensation section decreases slightly,but it has little effect on the overall thermal characteristics of the heat pipe system.The steam velocity decreases gradually along the horizontal pipe length at different horizontal length.The longer the horizontal length is,the larger the steam velocity decreases.