Capillary Pressure Characterization And Heat Transfer Performance Analysis Of Axial Micro-grooved Heat Pipe

Micro heat pipe is a high-efficiency heat transfer device,which transfers the heat via the phase change and circular flow of its working fluid.Because of its compact structure,light quality,and the characteristics of thermal homogeneity and start-up performance,micro heat pipe is getting wider application.Axial micro-grooved heat pipe has arisen more and more attention of many scholars due to its special internal structure,simple machining technology and low cost.The capillary pressure produced by wick structure of the microgrooves provides the essential driven force to accelerate fluid flow and cyclic phase transformation and determines the heat transfer performance of micro heat pipe.Therefore,increasing the capillary pressure of wick structure by changing inner structure is a critical way to improve heat transfer performance of micro heat pipe.On the basis of theoretical study about operating principle of micro heat pipes,axial micro-grooved heat pipes with three different sections including 0.5 mm×1.5 mm deep rectangular,0.5 mm×1.0 mm shallow rectangular,and 0.5 mm×1.0 mm V-type were fabricated as the testing elements in this paper.Next,the capillary pressure of above three axial micro-grooved heat pipes with water and acetone was tested by adopting dynamic height method,which elucidated the relationship between the capillary pressure and internal construction as well as working fluid.In order to analyze the effect of factors covering inner structure,working fluid,gravity and cooling mode on the heat transfer performance,the intact micro-grooved heat pipes was tested by experimental research.In the end,the mechanism of low temperature plasma modification for improving capillary pressure and heat transfer performance was explored.The test results of capillary pressure of axial micro-grooved heat pipes showed that the capillary pressure of water which had bigger surface tension was 1.71 times as large as acetone.And the micro-channel with small equivalent diameter could generate larger capillary pressure.The results of both capillary force test and heat transfer experiments found that capillary force played a decisive role in transferring heat.When the angular region area of various micro-grooved heat pipe was identical,heat-transfer capability increased with the increase of capillary force.Furthermore,experiments had been conducted to study the heat transfer property of 3 kinds of axial micro-grooved heat pipes with different inner structure.The assessment criterias were start-up performance,axial temperature difference,extreme heat flux density,and equivalent thermal conductivity.It turned out that all of micro-grooved heat pipes in this paper could start-up and work steadily in 70 seconds.Heat transfer ability ofmicro heat pipe with water was 1.35 times of acetone,which stated that the transmission factor could be seen as standards to choose working medium.When the direction of gravity was as same as the direction of fluid return,gravity could accelerate the velocity of circulation and heat exchange so that the heat transfer performance heightened.The heat transfer ability of micro heat pipes with 3 different inner construction followed the sequence:Rectangular deep groove>Rectangular shallow groove>V-shaped groove.Heat pipe with V-shaped grooves had strongest capillary force,but its heat transfer ability was inferior than rectangular grooves,since the angular region areas of V-shaped grooves were only half of the rectangular.Meanwhile,the increase of depth of the grooves was more effective in facilitating heat transmission.After modified by low temperature plasma,the surface hydrophilicity of micro heat pipe strengthened.Therefore,compared with unmodified surface,all of capillary force,equivalent thermal conductivity and extreme heat flux density increased 12.9%,9.2% and 14.3% respectively.All in all,low temperature plasma modification could be able to improve heat transfer ability effectively.