The Manufacturing Of A Large-scale Pulsating Heat Pipe And Its Heat Transfer Performance

With current social appeals for energy conservation and emission reduction,and the increasing demand for heat exchange in large space environments such as communication base stations,heat pipes have been widely used as an indirect surface heat exchange device.It provides a heat transfer channel for heat while isolating hot and cold fluids,so that the indoor heat is taken out of the room.With the use of natural cold fluid,it reduces the indoor air temperature and the time of base station air conditioning utilization for the purpose of energy saving.This paper presents a large-scale pulsating heat pipe with a special cross-sectional structure,and an experimental bench was designed and built to study the start-up and thermal performance of the new pulsating heat pipe.At this stage,the application of pulsating heat pipes is mainly miniaturization.In order to solve the problems of pulsating heat pipes in the size and heating power enlargement,in this paper,the flow channels of the working fluid formed by two coaxially arranged copper tubes and copper wires between the tubes are connected by U-shaped channels on both sides to form a novel pulsating heat pipe.The heat pipe has the characteristics of compact structure,low cost,flexible installation and good heat transfer effect,and because of its heat transfer power several orders of magnitude higher than that of the conventional pulsating heat pipe,it has the application prospect in high power scenes.By testing the start-up process of the pulsating heat pipe,it was found that two different start-up modes due to different heat flux and inclination angle.Firstly,the start-up temperature of the heat pipe increases continuously with the increase of the inclination angle.However,with the increase of the heating power,it shows a trend of decreasing first and then increasing,and the smaller the inclination angle,more obvious the trend became.In terms of start-up time,both a smaller inclination angle or greater heat flux can make the novel pulsating heat pipe have a shorter start-up time.It has the best starting performance when the filling ratio is between 40%-55%,and can start quickly at low temperatures.Secondly,the thermal resistance and wall temperature fluctuation signals of the novel pulsating heat pipe under different working conditions are analyzed,and it is found that the pulsating heat pipe exhibits two kinds of flow heat transfer mechanisms under different filling ratios.It has excellent heat transfer performance when the filling ratio is small,but the limit of heating power is small.When the filling ratio is large,the heat transfer of the heat pipe is limited.Heat transfer performance is optimal when the filling ratio is 55%.The minimum thermal resistance that can be reached at this working condition is 0.131℃/W and the maximum heating power is 700W.The effect of gravity on the novel pulsating heat pipe is more obvious when the inclination angle is between 0°-45°,and it gradually decreases with the increase of the inclination angle,and good anti-gravity characteristics appears in the inclination angle range of 45°-90°.According to the wall temperature oscillating characteristics,its operating state is sequentially divided into five stages,the bubble formation and expansion,local pendulum oscillating with plug flow,the intermittent flow of the gas-liquid plug,the unidirectional circulating flow with plug flow and annular flow and dry-out mode.The deterioration of heat transfer during operation was found,and its causes were analyzed.In the unidirectional circulation flow of the heat pipe,as the power increases,the main frequency of the temperature oscillation gradually increases,while the amplitude gradually decreases.The overall performance is a transition from low-frequency oscillation with high amplitude to high-frequency oscillation with low amplitude.Finally,a neural network model is established to predict the thermal resistance of the novel pulsating heat pipe.According to the 237 sets of data obtained in the experiment,three main parameters that affect the operating characteristics of the heat pipe:heating power,filling ratio,and inclination angle are used as the input layer of the neural network.The thermal resistance of the pulsating heat pipe is used as the output of the neural network.A two-layer neural network model with 7 nodes has been established.It has good robustness,and its predicted value agrees well with the experimental value.The maximum relative error of the model is 44%,the average relative error is 11%,and the data of 85.7%can fall within the range of relative error 20%.The prediction of thermal resistance under high heating power has high accuracy.