Investigation On The Heat Transfer Characteristics Of Gravity-capillary Evaporation In An Axially Grooved Heat Pipe

In recent years,with the miniaturization,modularization and high power of electronic components,their heat flux increases rapidly.Thermal dissipation and temperature equalization problems have become the main obstacles that affect the stability of electronic systems and restrict the performance development of electronic devices.Axially grooved heat pipe is an efficient heat transfer element based on the principle of evaporation-condensation coupled phase change heat transfer.It has the characteristics of low thermal resistance,high heat transfer coefficient and good homogeneity,which is an optimum scheme to solve the heat dissipation problem of high heat flux electronic devices.It has been widely used in the field of heat dissipation of electronic devices.Therefore,it is of great significance to study the gas-liquid two-phase flow behavior and phase change heat transfer characteristics in micro-channel heat pipes for the promotion and utilization of micro-channel heat pipes.However,due to the limitation of application scenario and working space,the axial groove heat pipe is always in an inclined working state.Previous studies have shown that gravity has a significant effect on the evaporation behavior during the gas-liquid phase transition in the channel.However,the mechanism of gravity-capillary evaporation in microgrooves is not well known.In addition,the research on the maximum heat transfer capacity of the axially grooved heat pipe in the inclined state is less,and further research is needed.In this paper,the visualization experiment of gas-liquid two-phase flow in an axially grooved heat pipe is carried out.The gas-liquid interface distribution characteristics of the axially grooved heat pipe during gravity-capillary evaporation under small inclination angle are visually observed.The thermal response characteristics of gravity-capillary evaporation under inclined state are experimentally tested,the effects of heating power and inclination angle on heat transfer characteristics of gravity-capillary evaporation are studied.At the same time,the maximum heat transfer capacity model of gravity-capillary evaporation axially grooved heat pipe under small angle inclination is established.The flow and heat transfer characteristics of gravity-capillary evaporation axially grooved heat pipe under small angle inclination are obtained.The influence of working inclination and channel structure on its maximum heat transfer capacity is also studied.Overall,the main contents and conclusions of this paper are as follows:Firstly,the visualization experiment of gas-liquid two-phase flow and heat transfer in gravity-capillary evaporation axially grooved heat pipe under small inclined angle was carried out.The results indicate that:(1)corner-film evaporation and fin-evaporation may coexist in channel evaporation section when heating power is low,and only corneal evaporation occurs in evaporation section when heat load is large enough;(2)The imbalance of shear force,gravity and capillary force caused by the reflux condensate leads to the instability of two-phase flow,which makes the interface of the projectile oscillate randomly;(3)The wall temperature of the evaporation section of the axially grooved heat pipe increases gradually in the horizontal and inclined states,and there is no temperature overshoot during the start-up process,however,when the heat load is high,the phenomenon of overshoot start and temperature oscillation appears in the inclined axially grooved heat pipe.In addition,the theoretical study on critical heat flow of gravity-capillary evaporation in axially grooved heat pipe is also carried out.The results imply that:(1)in the fin-film evaporation mode,the meniscus radius remains basically unchanged under the action of surface tension.At this time,the liquid evaporation mainly depends on reducing the liquid flow area and increasing the liquid flow velocity.With the increase of heating power,the liquid level decreases continuously,and finally the bottom of meniscus contacts with the bottom of channel,which makes the liquid meniscus unable to keep its original state,thus entering the corneal evaporation mode from the fin-film evaporation mode;(2)When the heat pipe approaches the heat transfer limit,the capillary radius of the evaporation section and the adiabatic section is much lower than the critical radius,but the capillary radius of the condensation section is higher than the critical radius.The flow area of the evaporation section decreases sharply with radius,and the maximum liquid flow rate appears in the evaporation section;(3)The maximum heat transfer capacity of the heat pipe with axial groove increases with the increase of angle and aspect ratio.The gas-liquid two-phase flow and heat transfer characteristics in micro-channel heat pipe is systematically studied in this paper,and the fluctuating state of gas-liquid two-phase refrigerant under gravity-capillary evaporation is explored,the relationship between the operating state of refrigerant and heat transfer performance in micro-channel heat pipe is revealed at the same time,and the influence of heat load and inclination angle on heat transfer characteristics and maximum heat transfer capacity is obtained.The research results in this paper can provide strong theoretical support for the design and optimization of micro-channel heat pipe in the field of electronic equipment heat dissipation,and further improve the mechanism of micro-scale gas-liquid two-phase flow and phase change heat transfer.