Study On Capillary Flow And Heat Transfer Characteristics In Nanochannel Based On Molecular Dynamics Simulation

With the rapid development of microelectronics technology in the fields of integrated circuits,energy utilization,and aerospace,the high heat flux generated by electronic devices during operation has become a key factor affecting the reliability and service life of equipment.In the context of the current "dual carbon" plan,passive heat dissipation technology based on capillary drive has become an important technology for cooling high heat flux electronic devices due to its advantages of spontaneity,simple structure,good antigravity performance,and strong heat dissipation ability.However,due to the small characteristic length of the capillary microchannel,it is significantly influenced by the scale effect,and is more significantly affected by the channel surface characteristics.Therefore,exploring the capillary flow heat transfer characteristics in the capillary microchannel from a nanoscale perspective is of great significance to guide the design of passive heat dissipation devices based on capillary flow.In this paper,based on method molecular dynamics simulation,in-depth research has been conducted on the mechanism of capillary flow and heat transfer in nanochannels and the mechanisms affected by surface characteristics.The main research contents and achievements are as follows:Firstly,a molecular dynamics model of contact angle was established,and static contact angles under different solid-liquid interaction intensities εwl were simulated.Walls with different εwl were divided into completely nonwetted walls,partially wetted walls,and completely wetted walls.It was found that the wall wettability increased with the increase ofεwl.On this basis,molecular dynamics simulation of capillary flow in capillary channels was carried out,and it was found that the dynamic contact angle was much larger than its corresponding static contact angle at the initial stage of capillary flow.Later,as the capillary flow progressed,the dynamic contact angle gradually decreased and approached the static contact angle.In the study of the influence of wall wettability,it was found that when the wall is completely nonwetted,fluid cannot enter the channel.When the wall is incompletely wetted,the capillary flow velocity of the fluid gradually increases with the enhancement of the strength of the solid-liquid interaction.When the wall is completely wetted and the solid-liquid interaction intensity continues to increase,the capillary flow velocity gradually slows down.In addition,the applicability of the macro scale Lucas-Washburn equation at the nanoscale was studied and analyzed,and it was found that the predicted results of the L-W equation had a significant error from the actual simulation results.By introducing slip length,the L-W equation was modified,and the modified L-W equation achieved a significant improvement in prediction accuracy.Secondly,the effects of wall wettability and wall temperature on capillary flow and heat transfer characteristics in smooth nanochannels were studied.The results show that with the continuous improvement of wall wettability,the fluid potential energy near the wall gradually decreases,and the solid-liquid coupling effect increases.The existence of"quasi solid"enhances the efficiency of solid-liquid heat transfer.However,as the strength of solid-liquid interaction increases after the wall is fully wetted,the capillary flow rate also decreases.At this time,the heat transfer rate increases first and then decreases with the increase of the strength of solid-liquid interaction.In addition,studies have shown that an increase in wall temperature can improve capillary flow velocity and heat transfer capacity to a certain extent.However,when the wall temperature is too high,the fluid evaporation speed is too fast,and the capillary flow precursor is constantly destroyed,while the flow speed becomes slower.When the wall temperature is 130 K,even the phenomenon of capillary flow distance stagnation occurs.Finally,the capillary flow and heat transfer characteristics in rough nanochannels were studied.Firstly,several different triangular nanostructured channels were established,and it was found that the heat transfer speed and heat transfer capacity of the spaced triangular channels were both optimal.In order to simulate a more realistic rough wall,chord function nanostructured channels with different periods and amplitudes have been established.Studies have shown that capillary flow velocity is negatively correlated with the period and amplitude of the chord function,and heat transfer at the same time is also negatively correlated with the period and amplitude.Due to the well-known influence of heat transfer area on heat transfer,in order to eliminate the influence of channel heat transfer area on the heat transfer characteristics in nanochannels,based on the premise that the heat transfer area is the same,a model of nanochannels with different wall roughness is established based on a chord function.The results show that wall roughness can slow down capillary flow,and as the roughness increases,the capillary flow speed gradually slows down.Unlike smooth channels,uneven capillary flow velocity also occurs in rough channels.Comparing the heat transfer characteristics of different channels,it is found that the Nu of rough channels is significantly larger than that of smooth channels,and the Nu number gradually decreases as the roughness increases.The total heat transfer in smooth channels is greater than that in rough channels,and the greater the roughness,the smaller the heat transfer.That is,although the presence of wall roughness can accelerate the heat transfer efficiency under capillary flow,it also reduces the total heat transfer.