| With the rapid development of microelectronics manufacturing and packaging technology,the miniaturization and integration of electronic devices and components are becoming higher and higher,which results in the continuous increase of high heat flux and operating temperature.The stability and reliability of devices are seriously threatened.This poses a serious challenge to the cooling technology of miniaturized and high heat flux microelectronic devices and components.The pulsating heat pipe is a new type of high-efficiency heat-dissipating component,which has many advantages such as simple structure,low cost,good heat transfer performance and good work adaptability.However,with the increasing miniaturization and integration of electronic devices and components,the traditional pulsating heat pipe with capillary channel is also developed in the direction of integration and miniaturization.Accordingly,the micro pulsating heat pipes(MPHPs),which provides an effective means for achieving heat dissipation of microelectronic components with high heat flux,have been emerged.However,compared with the traditional pulsating heat pipe with capillary-scale channel,the scale effect,interface effect and wall effect in the MPHPs become more and more obvious,and more new phenomena and new laws appear during the vapor-liquid two-phase pulsating flow.Therefore,the experimental and theoretical studies on the vapor-liquid two-phase flow and heat transfer performance in MPHPs is of great academic significance for further revealing the vapor-liquid two-phase flow and heat and mass transfer mechanism in MPHPs,and provides strong theoretical support for guiding the optimization design of the pulsating heat pipe cooling device in engineering.At present,the experimental and theoretical studies on pulsating heat pipe with micro-scale channels are still very limited.The existing thermally-driven vapor-liquid two-phase pulsating flow operation mode,operation characteristics and evolution behavior of the flow pattern in MPHPs are still in the exploratory stage,and the related visualization experimental data is very scarce,especially for visualization experimental study on the generation and evolution of unsteady flow patterns in MPHPs with different channel cross-sectional shapes.In addition,the coupled mechanisms of thermodynamics and dynamics on the vapor-liquid two-phase flow in pulsating heat pipe are not clarified by the existing theoretical studies.To this end,silicon-based MPHPs with different channel cross-section shapes are developed by using MEMS processing technology in this paper.To explore the relationship of the operating state of the working fluid,the wall temperature pulsating characteristics and heat transfer performance,visualization experimental observations on the internal thermally-driven vapor-liquid two-phase pulsating flow operation mode,operation characteristics and dynamic evolution of flow patterns are realized by high-speed imaging system..Based on the volume of fluid(VOF)method,taking into account the evolution of vapor-liquid two-phase flow pattern and the coupled heat transfer relationship of evaporation/boiling,condensation and heat conduction in the MPHP,a three-dimensional theoretical model is established and numerically simulations are conducted.The periodic shrinkage-expanded cross-section channel structure is introduced to strengthen the fluid flow and heat transfer performance of the working fluid in the channel.The results and conclusions are summarized as follows:(1)A MPHP,with a rectangular cross-section channel of a hydraulic diameter of 550 ?m,is developed based on the deep reactive ion etching process.The experimental test platform of the thermally-driven vapor-liquid two-phase pulsating flow,heat transfer and two-phase flow instability in the closed loop microchannel is designed.Visualization experiment are carried out to observe the operating state,main flow pattern and dynamic evolution process of the thermally driven vapor-liquid two-phase pulsating flow in the closed-loop rectangular cross-section microchannel under different working conditions.The instability of vapor-liquid two-phase flow and heat transfer performance are discussed.And the relationship of the operating state of the working fluid,the wall temperature pulsating characteristics and heat transfer performance of the MPHP is explored.The results show that the pulsating operating state in the MPHP mainly includes three types of elements:small pulsation,bulk pulsation and stopping.These three types of elements can appear separately or intermittently.With the increase of heat load,the working state of the working fluid in the pulsating heat pipe changes from stopping to bulk pulsation,the proportion of the stopping gradually decreases,that of small pulsation increases first and then decrease,and that of bulk pulsation continuous increases.Accordingly,the heat transfer performance of the heat pipe is greatly improved,and the wall temperature pulsation tends to be stable.During the pulsating process,the change of relative position and velocity of the vapor-liquid interface with time is a quasi-sinusoidal wave with some short small pulsations.The vapor-liquid two-phase flow patterns in MPHP mainly include bubbly flow,slug flow,annular flow and transition flow patterns.Affected by the scale effect,characteristic flow patterns,jet flow and filament flow,appear in the evaporation section and the adiabatic section of MPHP.Under the experimental conditions,the optimal filling rate of MPHP is about 50%.In addition,the gravity component along the axial direction of the channel is favorable for establishing a non-equilibrium pressure state between the adjacent channels and reflux of condensate to the evaporation section.(2)Based on the wet etching process,MPHPs with a trapezoidal and triangular cross-section channel with a hydraulic diameter of 550 ?m are fabricated.The operating characteristics of working fluid pulsation,vapor-liquid two-phase flow pattern and phase change behavior and heat transfer performance under various working conditions in MPHPs with different cross-sectional shapes(rectangular,trapezoidal and triangular)are experimentally studied and compared.And then,the influence of channel cross-section on vapor-liquid two-phase flow and heat transfer performance in MPHP is investigated.The results show that the nucleate boiling,which appears in the MPHP with rectangular channels,is not observed in MPHPs with the trapezoidal and triangular channels.The generation of vapor slug mainly relies on breaking of the vapor column impacted by the structure and surface tension at the U-bend of the evaporation section.Moreover,breakage of vapor slug but not injection flow is observed in MPHPs with trapezoidal and triangular channels.This is due to the liquid bridge,which is generated by the continuous concentration of refluxing condensate and the rising of mixing flow generated at the bottom,cut off the vapor slug.The sharp corner structure in the trapezoidal and triangular cross-section microchannels enhances the capillary suction and contributes to the reflux of condensate to prevent the "dry-out" in the evaporation section.Therefore,the thermal resistance of the pulsating heat pipes with trapezoidal and triangular cross-section channel is reduced.With the increase of heat load,the thermal resistance of the MPHPs is gradually reduced,and the convective heat transfer coefficient of the evaporation section and the condensation section is gradually increased.Due to the smaller flow resistance in the trapezoidal and triangular cross-section microchannels the existence of the sharp corner structure which is favorable for the working fluid to flow back to the evaporation section,the thermal resistance of MPHPs with the triangular and trapezoidal cross-section channel is smaller than that with the rectangular cross-section channel.The convective heat transfer coefficient of the evaporation section and the condensation section is higher.(3)Based on the VOF method,by considering the evolution of vapor-liquid two-phase flow pattern and the coupled heat transfer relationship of evaporation/boiling,condensation and heat conduction,a three-dimensional theoretical model is found to numerically analyze the vapor-liquid two-phase pulsating flow behavior and heat transfer performance in MPHP under different heat loads.The results show that the theoretical model can effectively simulate the vapor-liquid two-phase behavior of the working fluid in MPHP,and predict the temperature distribution of MPHP and temperature pulsating characteristics well,which is in good agreement with the relevant experimental data.At the start-up stage,the volume fraction of vapor in MPHP gradually increases under different heat load.And as the operation of MPHP tends to stable,the evaporation rate and condensation rate are gradually balanced,and the volume fraction of vapor gets stable.With the increase of heat load,the heat accumulates in the evaporation section,and than the temperature difference between the evaporation section and the condensation section increases and results in increasing driving force between the hot and cold sides and gradually transforms local pulsation of the vapor-liquid two-phase into a sustainable bulk pulsation.Consequently,the latent heat carrying capacity and sensible heat transfer capacity of the working fluid in the pipe are improved.The equivalent thermal resistance of MPHP decreases continuously as well as the equivalent thermal conductivity increases continuously,and the overall heat transfer performance is enhanced.(4)Three kinds of channel structures with periodic shrinkage and expansion cross-section are introduced to optimize the microchannel structure design of MPHPs,and the enhanced effect and mechanism of the design on the heat transfer performance of MPHPs are studied experimentally.The results show that there are vapor-liquid two-phase flow patterns such as bubble flow,slug flow and annular flow during the operation of MPHPs with variable cross-section channel,and due to the existence of triangular cavity structure,there are also flow patterns such as dovetail slug flow/bubble flow and wave annular flow.The capillary suction force will keep a amount of liquid at the cavity,which is conducive to inhibiting the "dry-out" in the evaporation section.In the A-type MPHP with "upper narrow and lower width" variable cross section channel,the additional capillary pressure difference induced by shrinkage and expansion cross-section channel unit promotes and hinders the movement of the working fluid to the evaporation section and the condensation section,respectively.However,in the C-type MPHP with "upper width and lower width" variable cross section channel,the additional capillary pressure difference hinders and promotes working fluid moving to the evaporation section and the condensation section,respectively.In addition in the B-type MPHP with uniform contraction cross-section channel,the effect of the additional capillary pressure difference can be neglected.In general,the additional capillary pressure difference in the C-type and A-type promotes and hinders the motion of working fluid,and its influence becomes more and more obvious as heat load increases.Due to competition among the additional capillary pressure difference,destruction heat transfer boundary layer by the internal disturbance of the fluid and the augment in flow resistance,the heat transfer performance of MPHPs with the variable cross-section channel is inferior to that of MPHP with the uniform cross-section channel under the lower heat load.When the heat load is high,the C-type MPHP with variable cross-section channel has smaller thermal resistance,higher convective heat transfer coefficient,lower evaporating section temperature and better heat transfer performance.In summary,the current work systematically gains an insight into the vapor-liquid two-phase flow and heat transfer characteristics in MPHPs,clarifies the relationship between the operating state of the working fluid and the wall temperature pulsating characteristics and heat transfer performance,and provides method for enhancement of MPHP's heat transfer performance by channel structure optimization.This study is not only of important scientific significance for perfecting the basic theory of heat and mass transfer in micro-scale phase change,but also provides strong technical support for the optimization design of micro thermally-driven vapor-liquid two-phase pulsating flow heat exchange devices. |
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