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Simulation And Experimental Study Of Flow And Boiling Heat Transfer Characteristics In Capillary Microchannels

Posted on:2024-05-06Degree:MasterType:Thesis
Country:ChinaCandidate:S L LiangFull Text:PDF
GTID:2542306920485684Subject:Power Engineering and Engineering Thermophysics
Abstract/Summary:PDF Full Text Request
With the highly integrated and miniaturized electronic devices,the heat flow density during their operation rises rapidly,and the heat dissipation capability of electronic devices is greatly tested.Among the electronic device heat dissipation technologies,the passive heat dissipation technology based on capillary effect shows great potential in the field of electronic device heat dissipation due to its advantages of spontaneity,low energy consumption,light weight and high stability.However,its application in the field of electronic devices is restricted due to its disadvantages such as low heat dissipation limit and large capillary core size,and promoting its performance improvement and miniaturization has become the key to promote its development.Research on the flow heat transfer characteristics in capillary flow-based passive two-phase heat sinks is urgently required.In this paper,an in-depth study of the flow and phase change heat transfer characteristics in capillary microchannels have been carried out by a combination of simulation and experimental research methods,with the following results:1.The simulation study of capillary flow and heat transfer in capillary microchannels with different cross-sectional shapes is carried out based on the Phase-Field method.By comparing the Level-Set model,the Phase-Field model shows a clear superiority in phase interface capture.The results show that the advancing concave liquid surface reconstruction leads to an oscillatory mechanism at the phase interface during the initial stage of capillary flow.The non-circular channels present better capillary and heat transfer properties compare to the circular channels with equal hydraulic diameters due to the change in liquid surface curvature at the acute corners and the expansion of the adsorption layer of the walls.On this basis,the capillary microchannels with different cross-sectional shapes are designed in parallel dense rows.The parallel tube bundle capillary microchannels exhibit excellent capillary and heat transfer performance despite their simple geometry.Meanwhile,the capillary dense row structure with non-circular cross-section obtains a smaller thermal resistance and higher heat transfer efficiency compared to other structures.2.The VOF model and evaporation-condensation model are used to simulate the mechanism related to the two-phase boiling phase change in the microchannel under capillary drive.Under the action of capillary force,the fluid in the capillary microchannel flows spontaneously in the axial direction,while the fluid in the microchannel undergoes nucleation boiling,and the flow pattern transitions from bubbly to slug flow.The fluid flow in the capillary microchannel is controlled by both the phase change heat transfer process and capillary action.In the direction of flow,the fluid boiling disturbance gradually increases,and the temperature field is more uniformly distributed.The near-wall heat transfer coefficient is influenced by the near-wall vapor rate,and the liquid film occupancy is lower in regions with higher vapor rates,resulting in poorer heat transfer capacity.The best heat transfer efficiency is obtained in the transition region between high vapor rate and high liquid filling rate.3.A novel capillary microchannel heat sink with a dendritic bifurcated infusion structure and an outlet chamber structure is designed,which can inhibit the generation of bubbles inside the channel during the liquid injection process and the backflow of vapor.Visualized microchannel chips of the same structure are fabricated and the capillary flow behavior of the fluid inside the chip is observed and analyzed using the visualization experimental bench.It is found that under the influence of the dendritic bifurcation infusion structure,the fluid in the microchannel form a continuous wave-like liquid surface,and the steepness of the wave gradually decrease with the increase of the microchannel width,and the flow velocity of the fluid decrease.4.Visualization of flow boiling in capillary microchannels shows that bubbles are preferentially generated from the exit chamber and the gas-liquid interface gradually retreats to the capillary microchannel exit while saturation boiling occurs in the channel.The entrainment effect of steam is obvious,the periodic formation and breakage of liquid bridges appear at the channel exit,and the liquid level in the near-inlet section of the closed bubbles produce a continuous oscillation phenomenon after reaching the equilibrium position.As the power increases,the faster steam flow rate can make the liquid bridge unable to form,and eruptions occur at the outlet.As the heating power is further increased,an explosive jet phenomenon occurs inside the microchannel with violent expansion triggered by nucleated bubbles.5.The experimental study of capillary microchannel heat dissipation system is carried out to verify the feasibility of the capillary microchannel-based heat dissipation system for electronic devices,and the start-up characteristics and failure characteristics of the heat dissipation system were analyzed by means of variable power testing.The minimum start-up power is 21 W.The cooling system achieves stable operation in the range of 21-109 W.The system fails when the heating power is greater than 134 W.The start-up of the cooling system is accompanied by an evaporative phase change and a boiling phase change.After the system starts,the temperature of the heating center decreases rapidly and becomes dynamically stable,compensating for the condensate reflux in the chamber and thus forming a stable cycle.Fluctuations in temperature and pressure during operation confirm the existence of boiling instability.As the heating power continues to increase,the system fails and is accompanied by a large amount of phase change steam reflux,and reflux bubbles appear in the compensation chamber.At the same time,the temperature of the heating area rises rapidly,and the phenomenon of alternating fluid and steam oscillation occurs in the heat sink inlet.
Keywords/Search Tags:capillary microchannels, capillary flow, boiling phase change, visualization, phase-field model, VOF model
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