Research On Capillary Flow And Capillary Evaporation In High Aspect Ratio Microchannels

With the rapid development of information technology in the 5G era,the miniaturization,ultra-thin and micro-scale heat sinks of electronic components have attracted widespread attention.At the same time,the heat dissipation of internal chips in electronic devices has also become a key topic in chip development.When the working temperature of electronic chips is too high,its processing performance will be greatly limited,which not only makes it difficult for electronic components to operate efficiently,but also brings potential safety hazard.Silicon-based materials can be compatible with chips,and the integrated structure of chips and heat sink can greatly reduce the contact thermal resistance.Loop Heat Pipe(LHP)has the advantages of fixed-point heat dissipation and gas-liquid pipeline separation,and high aspect ratio array microchannels have high specific surface area and strong pumping performance,which can effectively supplies the liquid working fluid inside the evaporators.Therefore,this paper aims to deeply explore the mechanism of flow evaporation in micro-scale radiator,and proposes a silicon-based ultra-thin loop heat pipe(S-UTLHP)with high aspect ratio array microchannels as evaporator.Firstly,a series of high aspect ratio array microchannels with different characteristic parameters were designed,and the pressure drop of single-phase flow was simulated by simulation,and the theoretical formula of pressure drop in rectangular channels at micro-scale was fitted.Secondly,the closed-loop horizontal parallel microchannels were pumped,and the Computational Fluid Dynamics(CFD)method was used to simulate the micro-scale capillary pumping.The results showed that the suction speed gradually slowed down with time and finally reached its suction limit.At the same time,the variation law of suction distance with time accords with Lucas-Washburn(L-W)theoretical formula,and it has different suction performance for different feature sizes.The array microchannels corresponding to the pumping model were established,and their evaporation characteristics were studied by inputting heat flux in a steady state.The heat transfer coefficient,gas phase ratio and pressure drop were analyzed respectively.The results showed that the array microchannels with widths of 12μm,20μm and 30μm had high heat transfer coefficient(Nearly 9000W/(m~2·K)),reasonable gas phase ratio(20%-40%)and low pressure drop resistance(200Pa-2000Pa).Three kinds of microchannels(depth of 180μm,width of 12μm,20μm and 30μm,respectively)were selected to build a visual pumping experimental platform,and the pumping process inside the microchannels was observed.The results showed that the pumping law was consistent with the L-W theoretical formula.At the same time,the constant power and variable power of microchannels were studied,and the internal macroscopic visualization and microscopic bubble dynamics were observed to explore its internal evaporation mechanism.The results showed that a stable gas-liquid meniscus was generated below the limit heat flux,and its internal evaporation mechanism was the thin liquid film evaporation,and the limit heat flux was 12.72 W/cm~2.The surface temperature was monitored by infrared temperature measurement technology to study the heat dissipation characteristics.The data showed that all microchannels could achieve variable load adaptation characteristics,but the 12μm array microchannel was the best in the research range.Based on the experimental study of visual pumping and heat transfer,the 12μm width microchannel with the best comprehensive heat transfer performance was selected to manufacture the evaporator of S-UTLHP,and its overall size was 40 mm× 20 mm× 0.4 mm.The heat transfer experiments of the heat dissipation device were carried out,including rising power,rising and falling power and sudden power.The results showed that the ultra-high heat load parameters were obtained under the 12μm channel size,and the ultimate heat load reached 4.6W.The start-up operation state and circulation flow of S-UTLHP were analyzed,and the evaporation phenomena of S-UTLHP in macro and micro images were monitored.It was concluded that the evaporation circulation mechanism in micro-scale radiator was film boiling.Overall,this work is expected to provide theoretical guidance for future chip cooling technology.