Experimental Research On Vapor-liquid Separation And Heat Transfer Performance In Heat Pipe

For the rapid development of new energy,5G and IoT(Internet of Things),the electronic devices gradually tend to be miniaturized and integrated.At the same time,power and heat flow density continue to increase.The development of electronic industry is restrained by heat dissipation.Heat pipe is often used to solve the problem of electronic heat dissipation.It is an ideal passive heat exchanger.However,the traditional heat pipe has some shortcomings,the heat transfer ability can not meet the heat dissipation requirements of integrated electronic components.Therefore,new methods and new concepts should be used to study the heat transfer of the heat pipe,and the purpose is to further improve the heat transfer performance.To solve the contradiction between vapor and liquid replenishment in the evaporation section and reduce the thermal resistance of condensation,the principle of phase separation is borrowed.Pool boiling and steam condensation experiments are carried out.Multi scale porous wick,hydrophilic and hydrophobic stripe surface and super hydrophilic mastoid are made.The importance of phase separation in the process of phase change is examined.The loop heat pipe heat transfer performance with multi-scale porous wick is studied,and the influence of phase separation or phase distribution in the evaporator on the heat transfer of the loop heat pipe is analyzed.Then,the porous wicks are sintered on the inner surface of the evaporation section,and mastoide are set to the condensation section of the gravity heat pipe,the vapor and liquid phases in the gravity heat pipe are separated.Finally,the influence of working fluid distribution in the gravity heat pipe on the operation characteristics is explored by visualization technology.The main research work includes the following aspects:1.Study on the mechanism of phase separation effect on heat transfer:The boiling and condensation phase change both included in the heat pipe.The vapor-liquid separation has an important effect on boiling and condensation.To explore the effect mechanism of phase separation on boiling and condensation.The basic research of pool boiling and vapor condensation are carried out.In order to solve the contradiction between vapor overflow and liquid suction in pool boiling,a multi-scale porous wick was proposed to adjust the flow path of vapor-liquid two-phase.It is found that the multi-scale porous wick can realize the separation of vapor and liquid phases in pool boiling:the large pores provide channels for the overflow of vapor,and the small pores ensure the supply of liquid.As a result,the heat transfer coefficient of the boiling surface is improved.In order to separate the liquid film from the condensation surface,the super hydrophilic mastoid is used to absorb the liquid film and separate it from the condensation surface,so the effect of condensation heat transfer is improved.It is found that the heat transfer performance of the condensing wall surface is significantly improved.Compared with smooth copper surface,condensation heat transfer coefficient increased by 83%when the wall supercooling degree is 5.3 K.The increase of boiling and condensation heat transfer coefficient fully demonstrates the importance of the phase separation principle on phase change heat transfer.2.The effect of phase separation on heat transfer characteristics of loop heat pipe:The heat transfer performance of the loop heat pipe is studied in this chapter.A multi-scale porous wick is constructed by sintering.The effect of phase separation on the heat transfer characteristics of loop heat pipes is studied.There are two type of phase separation processes in the loop heat pipe.The one is the separation of the vapor-liquid two-phase flow path in the porous wick,and the other is the separation of the vapor-liquid two-phase working fluid in the evaporator and the compensation chamber.The former affects the heat transfer of the working fluid in the porous wick,and the latter determines the gas-liquid two-phase circulation mode in the loop heat pipe.The results show that in the multi-scale porous wick:vapor can escape through the large pores,and liquid can wet the porous wick through the small pores.The vapor-liquid phase separation in this multi-scale wick improves the loop heat pipe heat transfer performance and reduces the operating temperature.At ?=90° and Q=220 W,the operating temperature is reduced by 4.6? when multi-scale porous wick is used.The vapor-liquid distribution pattern in the evaporator and compensation chamber will affect the circulation of the working fluid.When the compensation chamber is occupied by vapor the loop heat pipe operates in a "double cycle mode".When the compensation chamber is occupied by liquid,the loop heat pipe will operationg on the normal circulation of the working fluid.In the experiment,an infrared test device is used to explore the diffusion process of the working fluid in the porous wick.It is found that the liquid distribution in the porous wick has an important influence on the temperature uniformity of the bottom surface of the evaporator.The temperature of the evaporator with multi-scale porous wick is more uniform.At ?=90° and Q=160 W,the multi-scale porous wick can increase the temperature uniformity of the evaporator bottom plate by 42%.3.Study of the flow and heat transfer performance of a phase-separated gravity heat pipe:The heat transfer performance of the gravity heat pipe can be improved by optimizing the vapor-liquid two-phase distribution in the evaporation section and reducing the liquid film thickness in the condensation section.Therefore,a phase-separated gravity heat pipe is constructed by using multi-scale porous wicks and super-hydrophilic mastoide in the paper.The effect of vapor-liquid separation in the gravity heat pipe on the heat transfer characteristics is studied.The experimental results show that the operation stability of the gravity heat pipe is mainly affected by the vapor-liquid two-phase flow and the diameter of the bubbles.The liquid is absorbed by the porous wick for the capillary force.while the vapor will escape from the porous to the central area of evaporation section.The separation of vapor and liquid is realized.The flow instability in the evaporation section is suppressed for the separation of vapor and liquid,and it also improves the uniformity of the distribution of the liquid working fluid in the evaporation section.Finally,the heat transfer performance of the evaporation section is enhanced.At ?=90°,Q=420 W,the operating temperature drops by 10.7?,and the critical heat flux is also increased by 110%and 53.3%at ?=60° and 90°,respectively.In the condensation section,the liquid film is separate from the condensation wall surface for the super hydrophilic mastoide,which is helpful to reduce the thickness of the film and improve the heat transfer performance.At Q=760 W,the condensation heat transfer coefficient increases by 48.4%.The physical properties of the working fluid also affect its distribution in the gravity heat pipe.The application of the self-rewetting fluid in the gravity heat pipe is also studied in this paper.It is found that when water is used as the working fluid,the liquid distribution in the heated area is uneven.As a results the temperature uniformity is not ideal.When the self-rewetting fluid is used as the working fluid,the content of the liquid in the heated area is increased significantly,the self-re wetting effect is obvious.This is the reason for the enhancement of gravity heat pipe heat transfer performance when the self-wetting fluid is used as the working fluid.