Understanding interactions between gas molecules and solid surface is key to the aerodynamic design of high-speed,high-altitude aerospace vehicles,but there is a large dispersion of gas-surface interaction parameters ...Understanding interactions between gas molecules and solid surface is key to the aerodynamic design of high-speed,high-altitude aerospace vehicles,but there is a large dispersion of gas-surface interaction parameters or namely accommodation coefficients.The uncertainty results partly from different considerations of the interaction between gas molecules in various experimental and numerical methods.In this study,effects of gas-gas molecules interaction are systematically discussed by comparing two different approaches of molecular dynamics simulation of high-speed argon molecules scattering on a graphite surface.The popularly-used“single scattering”approach repeats the scattering process of a single gas molecule without considering the gas-gas molecules interaction.The newly-developed“continual scattering”approach continually shoots gas molecules at the surface,considering collisions between gas molecules in addition to gas molecules’collisions with surface.Gas-surface interaction features in the two approaches are compared and discussed under various affecting factors including rarefaction degree,gas-surface interaction strength,surface temperature and incident velocity.It is shown that these two approaches usually produce different accommodation coefficients,and the corresponding mechanisms are explained.This study could help clarify some doubts about the selection of accommodation coefficients in engineering practice,and also provide an instruction on design of an appropriate molecular dynamics simulation approach.展开更多
The molecular dynamics(MD)simulations were used to understand the heat transfer process between the gas phase and the solid skeleton in the nanoporous silica aerogels.The amorphous silica nanoparticles were generated ...The molecular dynamics(MD)simulations were used to understand the heat transfer process between the gas phase and the solid skeleton in the nanoporous silica aerogels.The amorphous silica nanoparticles were generated by the MD simulations and the energy accommodation coefficient(EAC)between the gases and the nanoparticles was calculated based on the results of the nonequilibrium molecular dynamics(NEMD)simulations.The apparent thermal conductivity(ATC)of the gases between the heat source and heat sink was also obtained.The effects of the temperature,the particle diameter and the molecule type on the EAC and the ATC were investigated.The results indicate that the EAC decreases with the increase of temperature within the calculating range.When the preset temperature is constant,the EAC increases with the increasing of the particle diameter and eventually approaches a specific value.When the preset temperature is 300 K and the particle size is 4 nm,the obtained EAC for the N2 gas and the O2 gas is close to each other and both are less than that of the Ar gas.The results also indicate that the heat transferred through the gas-nanoparticle interface is far less than that through the neighbouring nanoparticles in silica aerogels.展开更多
Highly rarefied gas flows through a rough channel of finite length with small bumps appended to its surfaces are investigated,by varying the accommodation coefficientin Maxwell’s diffuse-specular boundary condition,t...Highly rarefied gas flows through a rough channel of finite length with small bumps appended to its surfaces are investigated,by varying the accommodation coefficientin Maxwell’s diffuse-specular boundary condition,the characteristic size and position of the bumps,and the channel length.First,we study the influence of the surface bumps and consider the rarefied gas flow in a unit channel with periodic boundary conditions to remove the end effect.It is found that the surface bumps have a significant impact on the flow permeability.Whenis very small(i.e.,nearly specular reflection of gas molecules at the channel surface),the apparent gas permeability is dramatically reduced,even in the presence of small bumps,to a value that is almost comparable to the one when fully diffuse gas-surface scattering is assumed.This impact can be taken into account through an effective accommodation coefficient,i.e.,the permeability of the rough channel is taken equivalently as that of a smooth channel without bumps but having gas-surface scattering under the effective accommodation coefficient.Second,we study the end effect by connecting a smooth channel of length L_(0) to two huge gas reservoirs.It is found that(i)the end correction length is large at small.Consequently,the mass flow rate barely reduces with increasing L_(0) rather than scales down by a factor of 1/L_(0) as predicted by the classical Knudsen diffusion theory;and(ii)the end correction is related to the channel’s aspect ratio.Finally,based on the effective accommodation coefficient and end correction,we explain the exotic flow enhancement in graphene angstrom-scale channels observed by Geim’s research group(Keerthi et al,Nature 558:420-424,2018).展开更多
基金This work was supported by the National Natural Science Foundation of China(No.12072343).
文摘Understanding interactions between gas molecules and solid surface is key to the aerodynamic design of high-speed,high-altitude aerospace vehicles,but there is a large dispersion of gas-surface interaction parameters or namely accommodation coefficients.The uncertainty results partly from different considerations of the interaction between gas molecules in various experimental and numerical methods.In this study,effects of gas-gas molecules interaction are systematically discussed by comparing two different approaches of molecular dynamics simulation of high-speed argon molecules scattering on a graphite surface.The popularly-used“single scattering”approach repeats the scattering process of a single gas molecule without considering the gas-gas molecules interaction.The newly-developed“continual scattering”approach continually shoots gas molecules at the surface,considering collisions between gas molecules in addition to gas molecules’collisions with surface.Gas-surface interaction features in the two approaches are compared and discussed under various affecting factors including rarefaction degree,gas-surface interaction strength,surface temperature and incident velocity.It is shown that these two approaches usually produce different accommodation coefficients,and the corresponding mechanisms are explained.This study could help clarify some doubts about the selection of accommodation coefficients in engineering practice,and also provide an instruction on design of an appropriate molecular dynamics simulation approach.
文摘The molecular dynamics(MD)simulations were used to understand the heat transfer process between the gas phase and the solid skeleton in the nanoporous silica aerogels.The amorphous silica nanoparticles were generated by the MD simulations and the energy accommodation coefficient(EAC)between the gases and the nanoparticles was calculated based on the results of the nonequilibrium molecular dynamics(NEMD)simulations.The apparent thermal conductivity(ATC)of the gases between the heat source and heat sink was also obtained.The effects of the temperature,the particle diameter and the molecule type on the EAC and the ATC were investigated.The results indicate that the EAC decreases with the increase of temperature within the calculating range.When the preset temperature is constant,the EAC increases with the increasing of the particle diameter and eventually approaches a specific value.When the preset temperature is 300 K and the particle size is 4 nm,the obtained EAC for the N2 gas and the O2 gas is close to each other and both are less than that of the Ar gas.The results also indicate that the heat transferred through the gas-nanoparticle interface is far less than that through the neighbouring nanoparticles in silica aerogels.
基金the National Natural Science Foundation of China(No.52222402,No.52074235).
文摘Highly rarefied gas flows through a rough channel of finite length with small bumps appended to its surfaces are investigated,by varying the accommodation coefficientin Maxwell’s diffuse-specular boundary condition,the characteristic size and position of the bumps,and the channel length.First,we study the influence of the surface bumps and consider the rarefied gas flow in a unit channel with periodic boundary conditions to remove the end effect.It is found that the surface bumps have a significant impact on the flow permeability.Whenis very small(i.e.,nearly specular reflection of gas molecules at the channel surface),the apparent gas permeability is dramatically reduced,even in the presence of small bumps,to a value that is almost comparable to the one when fully diffuse gas-surface scattering is assumed.This impact can be taken into account through an effective accommodation coefficient,i.e.,the permeability of the rough channel is taken equivalently as that of a smooth channel without bumps but having gas-surface scattering under the effective accommodation coefficient.Second,we study the end effect by connecting a smooth channel of length L_(0) to two huge gas reservoirs.It is found that(i)the end correction length is large at small.Consequently,the mass flow rate barely reduces with increasing L_(0) rather than scales down by a factor of 1/L_(0) as predicted by the classical Knudsen diffusion theory;and(ii)the end correction is related to the channel’s aspect ratio.Finally,based on the effective accommodation coefficient and end correction,we explain the exotic flow enhancement in graphene angstrom-scale channels observed by Geim’s research group(Keerthi et al,Nature 558:420-424,2018).
