Study Of Flow And Heat Transfer Mechanism And Performance Enhancement Of Micro-oscillating Heat Pipe

At present,the continuous improvement of the miniaturization and integration of electronic components leads to the increasing heat flux.Therefore,it is necessary to carry out efficient cooling and heat dissipation to ensure its safe and effective work within a reasonable temperature range.Among many cooling technologies,the micro oscillating heat pipe is a solution with good development prospect.It's worth noting that,compared to traditional capillary channel of oscillating heat pipe with mm scale,micro oscillation heat pipes within 100 microns level micro-channel highlights surface/interface effect.In this way,the gas-liquid oscillating behavior and the evolution characteristics of the flow pattern show new rules,which has an important influence on the heat transfer performance and the effective working range.However,the current researches on these key issues are not enough.Especially,the enhancement methods of heat transfer performance of micro oscillating heat pipe need to be further developed.Therefore,in this paper,through Deep Reactive Ion Etching(DRIE)process,visual experimental chips with a uniform straight channel and a hydraulic diameter of 550?m is designed and fabricated.And carried out the flow and heat transfer characteristics of visualization experimental observation,to study auto-excitation gas-liquid oscillations flow behaviors and evolution characteristics of flow patterns.The internal relationships between heat transfer performance and effective working range of heat pipe is analyzed.On this basis,from the perspectives of enhancing the gas-liquid oscillation sensible heat transfer and enhancing the driving force of the working fluid in the heat pipe,the Sin-type periodic redirection channel structure and the variable cross-section expansion and contraction channel structure design are introduced to explore the heat transfer performance of the micro oscillating heat pipe.The enhancement effect of the two kinds of channel structure design on the heat transfer performance of the micro oscillating heat pipe is verified by experiments,and the strengthening mechanism is analyzed.In summary,the main conclusions of this study are as follows:(1)The research on uniform straight channel oscillating heat pipe shows that when the oscillating heat pipes work in a quasi-steady state,the oscillation of the working medium in the channel consists of three operating elements:stopping,small oscillation and bulk oscillation.The heat transport efficiency in the channel is the best under bulk oscillations.In addition to the capillary forces(assisted by gravity)drive the liquid back to the evaporation section,the bulk oscillation can effectively transport the liquid working medium(including liquid plug and gas-liquid mixture)to the evaporation section,so that the evaporation section can still be well wetting at high power and delay the "dry-out".The annular/semi-annular flow.plug flow,bubble flow and evolution of flow patterns,and the evolution of nucleated boiling in the evaporation section are observed in the micron scale channels.The flow and heat transfer in the oscillating heat pipe are coupled to each other.Thermodynamics drive the switching of the flow state of the working medium,and the evolution of the flow state in turn increases or weakens the heat transfer.For the experimental conditions,the optimal filling ratio is 70%.(2)The flow and heat transfer characteristics of the sine-type micro-oscillating heat pipes with 4,6 and 8 period numbers along the length of the channel and the uniform straight channel oscillating heat pipes are compared and analyzed.The results show that the distortion occurs at the gas-liquid interface of the working medium in the channel,and the distortion becomes more obvious at the higher flow rate.The obvious mixing flow is observed in the evaporation section and extends to the adiabatic section.In the condensing section,the gas plug/bubble is broken and scattered at the U-bend at the top of the condensing section to generate fine bubbles with smaller diameter.The flow resistance introduced by the Sin-type structure makes the velocity and frequency of the gas-liquid interface oscillation in the channel weaker than that in the uniform straight channel.The average flow velocity and frequency of oscillating are the best,and the filling ratio and heat load range of the effective operation are the widest in the oscillating heat pipe channel with a period of 4.And the flow and heat transfer performance at the filling ratio of 60%are better than other conditions.Under the conditions that the volume and heat transfer area of the channel are almost constant,the heat transfer capacity of unit working medium can be improved effectively by the sine-type micro-oscillating heat pipe with periodic variable structure(3)The flow and heat transfer of the oscillating heat pipes with variable cross-section expansion and contraction structures of "gradual expansion and sudden contraction","sudden expansion and gradual contraction" and "symmetric expansion and contraction"(along the direction of gravity)as well as uniform straight channel oscillation heat pipe are compared and analyzed.The results show that under the influence of the wall effect of the expansion and contraction structure channel,the main flow patterns are "wavy annular flow",dovetail-shaped bubble/plug flow,etc.At the same time,it is observed the generation of mixing flow by working fluid impacting the wall surface can effectively destroy the boundary layer,nucleate boiling in triangular cavity structure,the liquid working fluid accumulated in the cavity structure and the capillary suction liquid can effectively delay the "dry-out" under high power.Based on the observation and analysis of experimental flow patterns,among the three expansion and contraction structure channel designs,the "gradual expansion and sudden contraction" type can promote the downward flow of the working fluid,the "sudden expansion and gradual reduction" type can promote the upward flow of the working fluid,and the"symmetrical expansion and contraction" type as a control group,there is no obvious tendency for working fluid directional movement.Based on the analysis of visual data extraction,it is found that the capillary driving force of superimposed channel structure mainly affects the proportion of bulk oscillating.Under medium and high power,the bulk oscillation proportion of the "gradual expansion and sudden contraction" type and the "symmetric expansion and contraction" type oscillating heat pipe is higher,and the oscillation operation status is better than the "sudden expansion and gradual contraction" type.Judging from the equivalent thermal conductivity and thermal resistance,the strengthening design of the expansion and contraction structure is also successful,and the "gradual expansion and sudden contraction"type oscillating heat pipe shows the best performance at 70%filling ratio.The variable cross-section expansion and contraction structure design effectively improves the heat load range for the reliable operation of oscillating heat pipe.