Study On Heat And Mass Transfer Characteristics Of Carbon Nanotube Capillary Wicks

Ultra-thin vapor chamber heat sinks have been widely used in recent years with the thinning of electronic equipment.However,there are still great challenges in achieving controllable thermal conductivity of the capillary wick to reduce the startup time and overall thermal resistance.After filling with water,the heat transfer performance of carbon nanotubes will change,and a reasonable arrangement of carbon nanotube capillary wicks can effectively improve the heat dissipation performance of the radiator.In this paper,molecular models of capillary wicks of single-walled carbon nanotubes with different water-filled positions and different lengths are established.The effects of filling ratio,water filling position,length size effect,wall defect and other factors on its heat and mass transfer performance were explored.Using quartz flakes,high-purity iron flakes,and 304 stainless steel as substrates,multi-walled carbon nanotube capillary wicks were prepared and hydrophilically modified,and the in-plane thermal conductivity before and after modification and water filling were measured.Combined with the simulation results,the actual properties of carbon nanotube capillary wicks are analyzed and predicted.A molecular model of water-filled single-walled carbon nanotubes was established,and the changes in its heat and mass transfer performance under different working conditions were predicted by changing the filling ratio.Through molecular dynamics simulation results,it was found that the thermal conductivity of water-filled single-walled carbon nanotubes decreased by 17.8%-23.8%.This is beneficial to reduce the start-up power and time,and reduce the temperature of the heat sink surface of the evaporator under overheating conditions.When the ambient temperature increased from 300 K to390 K,the thermal conductivity and heat flow decreased by 18.6%,8%.When the filling ratio is 0.25,the thermophoretic velocity of water molecules can reach 74 nm/ns,and the mass exchange between the radially dense layers occurs driven by the temperature gradient.By fixing the water molecule and calculating the phonon density of states and spectral heat flow integrals,it is found that both the van der Waals force and the thermal motion of the water molecule lead to a decrease in thermal conductivity.Both induce low-frequency phonon scattering,but the latter also provides an additional thermal pathway,making it less suppressive for heat transfer performance.By calculating the thermal conductivity and heat flow of external water-filled single-walled carbon nanotubes,it is found that the proportion of its decrease with the increase of temperature is smaller than that of internal water-filled.Molecular models of single-walled carbon nanotubes with different lengths were established.By fitting the linear relationship between the reciprocal thermal conductivity and the reciprocal tube length,it is found that the thermal conductivity of infinite-length single-walled carbon nanotubes is 1066.35 W m^-1 K^-1,which is 13.1%-13.6%lower than that after water filling.After the introduction of hole defects,the temperature gradient around the single-walled carbon nanotube defects increase significantly,resulting in a decrease in thermal conductivity.There is a slight increase in thermal conductivity and heat flow after water filling.Driven by the same temperature gradient,the peak value of thermophoretic motion of the externally water-filled single-walled carbon nanotubes is much lower than that of the inner water-filled SWCNTs,and the inner high-density layer has circumferential density fluctuations.This indicates that the flow pressure drop is larger when externally filled with water.Multi-walled carbon nanotube capillary wicks were prepared on different substrates by chemical vapor deposition and were hydrophilically modified.Among them,the iron element catalyst particles produced by 304 stainless steel after high-temperature redox have the highest activity,and the grown carbon nanotube layer is the thickest and the shape is better.Using the HotDisk thermal constant analyzer,it was found that both modification and water filling increased the in-plane thermal conductivity of the capillary wick by 3.26 times.