Research On The Heat Transfer And Capillary Flow In Carbon-Nanotube-Based Heat Pipe Wick

Heat pipes are efficient heat transfer devices that utilize the transportation of latent heat of the working substance.They can reach effective thermal conductivities that are over 100 folds higher than that of metals such as copper.Wick is the essential component of heat pipes,which offers the capillary pressure generated from the micro pores to drive the circulation of the liquid.In the last decade,due to the fast progress of technology,especially in the electronics industry,the heating components are being diminished in size and enhanced in power,while the allowed heat transfer channels are continuously being restricted.All those developments pose great challenges on the heat transfer capability of conventional heat pipes.Base on such a background,this paper studied the possibility of adopting carbon nanotube(CNT)array as wicking structure of the next-generation heat pipes,and conducted investigations on the fundamental aspects including the conduction,wetting,liquid transfer and phase transition of carbon nanotube wicks.The theoretical approaches presented in this paper may offer references for solving the practical problem of high-density heat transfer within small sizes.The main research content and conclusions are summarized as follows:Firstly,the heat conduction characteristics of carbon nanotube array has been discussed in general.The axial heat conduction in CNT is dominated by ballistic phonon transportation,thus the intrinsic thermal conductivity is one magnitude higher than diamond.Theoretical and experimental results on the thermal conductivity of CNT has been collected and organized from literature,with which the general characteristics and influence factors of CNT conductivity have been discussed.The previously reported conductivity values tend to have great dependence on samples,with no consistent evaluation criteria or equations.Similar to the corresponding states principle,a non-dimensionalization method has been proposed to evaluate the conductivity of CNT,which makes the vastly diverged measurement and theoretical results collapse into a single curve.A thermal conductivity equation with clear physical interpretation is proposed.Verification over the equation shows that it can accurately predict the thermal conductivity of single CNT,aligned and unaligned CNT array,and even graphite material over the temperature range of absolute zero to over room temperature.The thermal conductivity results provide necessary and accurate data support for the theoretical study of carbon nanotube wicks.Secondly,the wetting characteristics of CNT array has been studied theoretically from both the microscale and macroscale.The micro-meniscii are the sources of the capillary pressure which drives the liquid flow inside the wick.With the numerical method based on the principle of minimizing surface energy,the influences of the configuration parameters of CNT array and wetting parameters upon the capillary pressure and evaporation characteristics have been comparatively analyzed.The effect of random distribution of CNT positions on the wetting characteristics has been quantitatively calculated,which showed that minor offset from ideal positions slightly enhances the evaporation,while larger offsets lead to worsened wetting effects.For the problem in the simulation of macroscale wetting in large number of CNT or micro-pillar array,the idea of “equivalent surface tension” has been proposed,based on which an operational approach and corresponding numerical model have been developed.The macroscale wetting characteristics are then discussed with the calculation via the conventional numerical tool.Quantitative results on the extension of the micro-meniscus and the expanding tendency of the bulk liquid confirm the advantage of carbon nanotube wick in static wetting perspective.Thirdly,for the heat and mass transfer processes of CNT wick under sub-micrometer scale,the scale effects and the correction methods have been investigated.From the perspective of flow,a representative CNT unit from the array has been identified as the physical model,and lattice Boltzmann method has been adopted to simulate the liquid flow to evaluate the effect of velocity slip at the CNT wall.Results show that even though the main flow resistance comes from the blocking of the array,velocity slip still leads to significant enhancement over the flow.Further calculations show that capillary pressure and the capillary length are both affected by the characteristic length scale and the velocity slip.From the perspective of heat transfer,the thermal resistance network in a CNT unit and the dominating resistance components have been analyzed.The dependence of the conductive resistance on the density of the array has been calculated.Analysis over the effects of the nanoscale evaporation shows that the overall evaporation rate is enhanced due to the combined effects of the non-evaporating and thin-film regions,in which the highest enhancement is over two magnitudes.The liquid flow and heat transfer in carbon nanotube wick are enhanced by different levels due to the scale effects,which help further elevate the capability of wicking liquid and transfering heat.Further,the dynamic capillary spreading characteristics of liquid in the CNT array has been studied.Visualization approach has been adopted using a high-speed camera to record the capillary spreading process with ethanol and acetone as the working substances,showing that there are two difference spreading modes in arrays with different heights.For the steady and uniform spreading mode,a corrected three-phase spreading assumption is proposed based upon the classic Washburn spreading model.Combined with the previously obtained capillary model and permeability model corrected with slip effect,the temporal variation law in the Washburn regime has been accurately described,which also validates the correctness of the theoretical model.The inclusion of the evaporation effects help quantitatively explain the deviation from the Washburn curve during the third regime of the spreading.The effects of the self-assembly behavior after the evaporative dry-out has also been discussed.Finally,the characteristics of CNT wick has been studied from the perspective of heat pipes.A highly optimized theoretical model based on lattice Boltzmann method is developed,which can efficiently simulate the transient performance of heat pipes.The accuracy and correctness of the model have been validated through a typical working condition.Using the heat pipe model,a parametric study on the low-permeability wick is conducted,which is an essential characteristic of the CNT wick.Results show that when the capillary pressure is sufficient to overcome the flow resistance,low permeability causes redistribution of the flow profiles,but has relatively insignificant effect on the heat transfer.However,since the increase of capillary pressure is unable to balance the increase of the flow resistance,ease of dry out become the main performance bottleneck for CNT wick.Therefore,improved structure of bi-porous carbon nanotube wick has been proposed,so that the advantage of high capillary pressure can be retained while the liquid flow resistance can be largely reduced,which helps enhance the capability of heat pipes to transfer heat.Numerical investigation on a heat pipe with bi-porous CNT wick proved its ability to transfer high-density heat flux at sub-millimeter channels.