A Study Of Rarefied Gas Flow Mechanism Based On Molecular Dynamics Simulation

In recent years,with the rapid developments of micro-nano electromechanical systems,aerospace and vacuum technology,some basic questions about rarefied gas dynamics are getting more and more attention.Among them,the understanding of gas-surface interactions and flow characteristics in the near-wall region is still unclear.In this paper,molecular dynamics simulation method is used to study the flow mechanism of rarefied gas in micro and nanochannels,and the gas-surface interaction mechanism is also well investigated in the microscopic aspect.The influence of surface effects,temperature effects and rarefied gas effects on flow were analyzed.The accommodation coefficients of gas molecules to surfaces under different conditions were calculated,and the momentum and energy exchange mechanism between gas molecules and surface was quantitatively analyzed.The study of shear driven gas flows in nanochannels shows that the flow field in the channel is clearly divided into two areas: the near wall area affected by the surface force and the bulk flow region not affected by the surface force.In the near wall region(approximately 1nm),the distribution of gas density,normal stress and shear stress only show a dependence on the gas-surface interaction properties.In the bulk flow region which is not affected by the wall force,the gas flow characteristics conforms to the law of aerodynamics.The density,normal stress and shear stress are at constant values,and the gas velocity distribution conforms to the linear response relationship of stress-strain.Through the correlation between shear stress and Knudsen number as well as shear rate,the tangential momentum accommodation coefficient(TMAC)and viscosity coefficient of gas molecules under different conditions are predicted.The results show that TMAC shows an independence from the Knudsen number for the same gas-surface interaction property,while the viscosity coefficient of the gas decreases with the decrease of the Knudsen number.The value of TMAC changes with the change of surface properties,and the shear stress changes,which further affects the velocity distribution in the flow field.The wall elastic modulus has little effect on TMAC,while the increase of wall potential energy coefficient and the surface roughness significantly promoted the accommodation of tangential momentum and kinetic energy between surface and gas molecules.As the wall potential energy coefficient increases,the sticking capacity of the wall increases,and the density and velocity of the gas in the near wall region also increases.When the wall coefficient increases to a certain extent,the gas molecules adsorb on the surface form an adsorption layer.While the surface roughness only causes fluctuations in velocity,density and stress distribution in the near wall region,and the influence on the bulk flow region is not obvious.For the same surface situation,the values of TMAC,NMAC and EAC decrease with the increase of the tangential velocity of the flow.It indicates that the increase of tangential velocity has a negative effect on the momentum and energy accommodation of gas molecules on the surface.However,the increase of the normal velocity promotes the momentum and energy accommodation.The surface roughness has a very significant effect on the momentum and energy accommodation of gas molecules and surfaces.When the roughness is large enough,the TMAC is close to 1.0,NMAC and EAC are close to a fixed value,and they also show an independence of the velocity flow and the wall temperature.The limitations of the traditional scattering kernel are pointed out by comparing the velocity distribution of reflect gas and the contour of velocity correlation obtained from the MD and the traditional scattering kernel model.The velocity distribution of gas molecules scattered on a smooth surface exhibits a typical ”head and shoulder” distribution,and the velocity distribution after scattering on a rough surface does not appear,which scattering characteristics are close to diffuse reflection.Gas molecules of different energies are incident perpendicularly to different rough surfaces.For different roughness surfaces,the tangential and normal energy distribution of the reflected gas molecules exhibit the same behavior.When gas molecules are incident on a smooth surface at a nearly horizontal angle,the tangential and normal momentum and energy changes are almost independent of each other.The average tangential momentum and energy after molecular reflection remain essentially the same,and the scattering is close to specular reflection.However,the roughness changes the interaction mode between the tangential gas molecules and the wall surface,and promotes the momentum and energy adaptation of the gas molecules and the surface.