Investigation On Enhancement Of Flow Boiling Heat Transfer In Minichannel Under Pin Electrode

With the rapid development of modern high-new technology,heat dissipation challenges with high heat flux have occurred in many fields,and the traditional single-phase heat transfer enhancement methods have been unable to meet heat dissipation needs.With the advantages of high heat transfer capability,simple structure and small size,the minichannel phase change heat transfer technique is considered to be one of the most promising methods to solve the future thermal management problem of high heat flux devices.However,there is still a gap between the current minichannel phase change heat transfer capability and the future cooling demand,therefore the development of minichannel heat transfer enhancement method becomes more urgent.In this paper,electric field heat transfer enhancement technique,nanofluid heat transfer enhancement technique and minichannel structure heat transfer enhancement technique are applied into minichannel to carry out the study of enhanced flow boiling heat transfer,especially to study the enhancement effect of their combined action.According to this objective,the main contents of this thesis are as follows:（1）Study on flow boiling of pure working fluid in minichannel under pin electrodes.Test section with pin electrodes is manufactured,and an intermittent inhomogeneous electric field is formed using pin electrodes arranged on the top of the minichannel.R141b is used as working fluid.The flow boiling characteristics of pure working fluid in minichannel under pin electrodes is studied experimentally.Visualization studies are carried out using high-speed camera,and bubble image processing techniques are used to quantify the behavior of boiling bubbles in minichannel.In addition,the boiling bubble detachment behavior under the action of pin electrode is studied with numerical simulation software,and the electric stress on the bubble surface is calculated theoretically,and the mechanisms for EHD enhancement of flow boiling heat transfer is discussed from the perspective of bubble force.The results show that electric field facilitates the bubble detachment.The bubble in visualization window becomes more and the size decreases,so that the bubble merging speed becomes slower and the length of confined bubble becomes shorter,so as to strengthen flow boiling heat transfer.The pin electrode significantly enhances flow boiling heat transfer,and enhancement effect is greatly influenced by the flow pattern in minichannel,and the electric field enhancement effect is between 1.0 and 1.64 under the experimental conditions.（2）Study on effect of pin electrode arrangement parameters on flow boiling heat transfer in minichannel.Combined with numerical simulation and theoretical derivation,the effective action range of pin electrode along the flow direction of minichannel is theoretically calculated,and the effect of different pin electrode arrangement parameters（electrode distance and arrangement area）on flow boiling heat transfer is investigated,so as to explore the mechanism for flow boiling heat transfer enhanced by pin electrode.The results show that the intermittent inhomogeneous electric field formed by pin electrodes can improve flow boiling heat transfer,and the effective action range of pin electrode is 1.6 mm at U=800 V.The action range of all pin electrodes accounts for 29.5%of the whole minichannel.By adjusting electrode distance and arrangement area,the control of heat transfer intensification in different areas can be realized,which reveals pin electrode have certain advantages in controlling the surface heat flow than wire electrode.（3）Study on nanofluid flow boiling heat transfer in minichannel under the action of pin electrodes.Al₂O₃/R141b nanofluids with different nanoparticle concentrations（0.05%,0.1%,0.2%）,different surfactant types（SDBS,CTAB,Span80）and different surfactant concentrations（0.05%,0.1%,0.5%）are prepared,and the zeta potential of nanofluids is measured to study their stability.The effects of parameters such as particle concentration,surfactant type and concentration on flow boiling heat transfer and pressure drop characteristics of minichannel under the action of pin electrodes are investigated,and nanofluid flow boiling process of minichannel under pin electrodes is visualized.In addition,the enhanced flow boiling heat transfer mechanism of nanofluid under electric field is discussed from the perspective of kinetic properties of nanoparticles under electric field.The results show that the addition of nanoparticles makes bubbles easier to detach and more discrete,and delays flow pattern transition from bubbly to slug flow.The heat transfer coefficient of nanofluid is higher than that of pure R141b,and it can still obtain better boiling heat transfer performance under high heat flux.The heat transfer effect of nanofluid is further enhanced after the application of electric field,and the heat transfer coefficient is increased by80%.Among the nanofluids in this study,Al₂O₃/R141b nanofluid with 0.1%concentration has the best heat transfer effect;Al₂O₃/R141b nanofluid modified by anionic surfactant SDBS has better boiling heat transfer effect than CTAB and Span80;nanofluid with 0.1%surfactant concentration has the best enhanced heat transfer effect under electric field.（4）Effect of inlet restrictor and counter-flow structure on flow boiling heat transfer in minichannel under electric field.Test section which can place inlet restrictor and realize counter-flow structure is designed.The flow boiling characteristics of minichannel with inlet restrictor under pin electrode are experimentally studied.The visualization research is carried out,the bubble velocity in the boiling process is calculated,and the mechanism of improving heat transfer and reducing instability of inlet restrictor is analyzed.The results show that the introduction of inlet restrictor improves boiling heat transfer and electric field enhancement,especially at high heat flux,the average increase is 21.3%.Electric field reduces the wall temperature,and inlet restrictor can reduce pressure drop fluctuation and surface temperature fluctuation.On this basis,experimental study on flow boiling characteristics of new counter-flow minichannel with inlet restrictor is carried out,focusing on effect of counter-flow structure on temperature uniformity along minichannel.The temperature uniformity of counter-flow minichannel is better than that of parallel-flow minichannel.Under the same experimental conditions,the minichannel with counter-flow structure shows higher heat transfer coefficient,especially at low mass flow rate.（5）Comprehensive performance evaluation of electric field,nanofluid and channel structures and their composite effects.The comprehensive performance evaluation results show that most of the enhancement methods have good overall performance at low and medium heat flow density conditions,and the overall performance starts to decrease as the heat flow density increases.The comprehensive performance of the three groups of composite enhancement methods is significantly higher than that of the single enhancement method.Within the range of experimental heat flux,most of the comprehensive performance of the three composite enhancement methods are above 1.25,and with the increase of heat flux,the reduction range of comprehensive performance is not as large as that of single enhancement method.