Molecular Dynamics Study on the Effect of Adsorbed Layer on Accommodation Coefficients in Micro Gas Flow

2008 ◽  
Author(s):  
Jun Sun ◽  
Zhixin Li
Keyword(s):  
Gas Flow ◽  
Rarefied Gas ◽  
Driving Forces ◽  
Adsorbed Layer ◽  
Accommodation Coefficient ◽  
Micro Channels ◽  
Adsorbed Layers ◽  
Micro Gas Flow ◽  
Accommodation Coefficients ◽  
Wall Interactions

Tangential momentum accommodation coefficient (TMAC) is reported to be less than unity in rarefied gas flow and greatly influenced by many factors such as temperature and adsorbed layers. According to the definition, a proper statistical algorithm in NEMD method is described and verified. Adsorbed layers occur on walls due to strong gas-wall interactions and the effect on TMAC are studied in two dimensional isothermal Poiseuille flow in smooth micro-channels under the conditions of various temperatures, driving forces, and Kn. The simulation results indicate that when gas-wall interactions become stronger, TMAC increases to maximum firstly and decreases a little. Besides, the effects of temperature and Kn on TMAC are not monotonous with the existence of adsorbed layer. In addition, normal momentum accommodation coefficient (NMAC) is almost unity for isothermal flow in smooth microchannels.

A Molecular Dynamics Study on Energy Accommodation Coefficients in Microchannels

2008 ◽  
Author(s):  
Jun Sun ◽  
Zhixin Li
Keyword(s):  
Temperature Difference ◽  
Wall Temperature ◽  
Gas Flow ◽  
Accommodation Coefficient ◽  
Energy Accommodation ◽  
Micro Gas Flow ◽  
Different Temperatures ◽  
Accommodation Coefficients ◽  
Lower Temperature ◽  
Non Equilibrium

Energy accommodation coefficient (EAC), used in thermal boundary condition in micro gas flow and heat transfer, is reported to be always less than unity and greatly influenced by the wall characters. According to EAC’s definition, the statistical algorithm was described and EAC for argon gas was studied by two dimensional NEMD simulations with heat conduction between two smooth platinum plates at different temperatures. With one wall’s temperature fixed, the non-equilibrium EACs were calculated by changing the other wall’s temperature. Meanwhile, the equilibrium EAC at one temperature can be extrapolated from a series of non-equilibrium EACs as the temperature difference approaches to zero. The effects of wall temperature, wall temperature difference, and Kn on EAC were investigated. Non-equilibrium EAC increases with wall temperature difference decreased, and becomes larger with increased Kn. And equilibrium EAC is larger for lower temperature and larger Kn.

Heat Transfer Predictions for Micro-/Nanochannels at the Atomistic Level Using Combined Molecular Dynamics and Monte Carlo Techniques

2009 ◽  
Vol 131 (3) ◽  
Author(s):  
S. V. Nedea ◽  
A. J. Markvoort ◽  
A. A. van Steenhoven ◽  
P. A. J. Hilbers
Keyword(s):  
Molecular Dynamics ◽  
Heat Flux ◽  
Monte Carlo ◽  
Boundary Conditions ◽  
Accommodation Coefficient ◽  
Monte Carlo Techniques ◽  
Effective Coefficients ◽  
Accommodation Coefficients ◽  
Dynamics Simulations ◽  
Wall Interactions

The thermal behavior of a gas confined between two parallel walls is investigated. Wall effects such as hydrophobic or hydrophilic wall interactions are studied, and the effect on the heat flux and other characteristic parameters such as density and temperature is shown. For a dilute gas, the dependence on gas-wall interactions of the temperature profile between the walls for the incident and reflected molecules is obtained using molecular dynamics (MD). From these profiles, the effective accommodation coefficients for different interactions and different mass fluid/wall ratio are derived. We show that Monte Carlo (MC) with Maxwell boundary conditions based on the accommodation coefficient gives good results for heat flux predictions when compared with pure molecular dynamics simulations. We use these effective coefficients to compute the heat flux predictions for a dense gas using MD and MC with Maxwell-like boundary conditions.

scholarly journals A rarefied gas flow around a rotating sphere: diverging profiles of gradients of macroscopic quantities

2019 ◽  
Vol 862 ◽  
pp. 5-33 ◽  
Author(s):  
Satoshi Taguchi ◽  
Kazuyuki Saito ◽  
Shigeru Takata
Keyword(s):  
Gas Flow ◽  
Rarefied Gas ◽  
Velocity Distribution Function ◽  
Normal Derivative ◽  
Accommodation Coefficient ◽  
Jump Discontinuity ◽  
Rotating Sphere ◽  
Wide Range ◽  
Normal Distance ◽  
The Moment

The steady behaviour of a rarefied gas around a rotating sphere is studied numerically on the basis of the linearised ellipsoidal statistical model of the Boltzmann equation, also known as the ES model, and the Maxwell diffuse–specular boundary condition. It is demonstrated numerically that the normal derivative of the circumferential component of the flow velocity and that of the heat flux diverge on the boundary with a rate $s^{-1/2}$, where $s$ is the normal distance from the boundary. Further, it is demonstrated that the diverging term is proportional to the magnitude of the jump discontinuity of the velocity distribution function on the boundary, which originates from the mismatch of the incoming and outgoing data on the boundary. The moment of force exerted on the sphere is also obtained for a wide range of the Knudsen number and for various values of the accommodation coefficient.

Simulation of Rarefied Gas Flow in Microchannel Based on Vorticity-Stream Function Method

2007 ◽  
Author(s):  
Xueyong Wei ◽  
Mike C. L. Ward ◽  
Dejiang Lu ◽  
Zhuangde Jiang
Keyword(s):  
Stream Function ◽  
Function Method ◽  
Gas Flow ◽  
Rarefied Gas ◽  
Taylor Series Expansion ◽  
Accommodation Coefficient ◽  
Parallel Plates ◽  
Slip Boundary ◽  
Tangential Momentum ◽  
New Formula

Vorticity-stream function method is successfully used to solve an incompressible gas flow in the parallel-plates micro-channel. A new formula in finite difference scheme is developed to describe the boundary vorticity based on the slip boundary theory and Taylor series expansion. Results show that the boundary vorticity are not only influenced by the Knudsen number (Kn) but also influenced by the tangential momentum accommodation coefficient (TMAC).

A High Order Theory for an Isothermal Rarefied Gas Flow in Micro Channels

2003 ◽  
Author(s):  
Nevena D. Stevanovic
Keyword(s):  
Small Parameter ◽  
Gas Flow ◽  
Rarefied Gas ◽  
Reynolds Numbers ◽  
Order Theory ◽  
Gas Flows ◽  
Regular Perturbation ◽  
Micro Electro Mechanical Systems ◽  
Slip Boundary ◽  
Micro Channels

Gas flows take place in a number of micro-electro-mechanical systems (MEMS). Since the dimensions of the MEMS are within μm range, it is necessary to take into account the gas rarefaction effects in investigations of these flows. This paper presents the solution and analysis of isothermal compressible gas flow through micro channels with slow varying cross section under low Mach number conditions. The problem is solved by the introduction of the small parameter ε that presents the square of the Mach and Reynolds numbers ratio. Small parameter ε is used in a regular perturbation analysis of the problem. The exact dependence among Mach, Reynolds and Knudsen number is utilized, which leads to accurate prediction of the influence of the inertia forces and the slip boundary conditions.

scholarly journals Experiments on the exchange of energy between gas, solid, and adsorbed layer in vacuo. —I. A method of detecting variations in the thermal efficiency of molecular collisions

1930 ◽  
Vol 128 (808) ◽  
pp. 432-444 ◽  
Keyword(s):  
Solid Surface ◽  
Thermal Efficiency ◽  
Rarefied Gas ◽  
Adsorbed Layer ◽  
Accommodation Coefficient ◽  
Heterogeneous Chemical Reaction ◽  
Thermal Exchange ◽  
Gas Molecule ◽  
Gas Molecules ◽  
Clean Metal

1. Introduction . —Following Knudsen and Smoluchowski, the thermal efficiency of encounters between the molecules of a rarefied gas and of a solid surface may be characterised by the “accommodation coefficient,” α = T 1 – T 2 /T 1 – T' 2 , where T 1 is the temperature corresponding to the energy of a gas molecule approaching the solid surface, T 2 the temperature on leaving the surface, and T 2 ' the temperature of the molecules which constitute the surface layer. The value of α has been measured in certain cases, and is generally considered to depend on the physical condition of any given surface; for instance, Langmuir has given reasons for expecting α at a clean metal wire not to be the same as when the metal carries adsorbed vapour, while Hughes and Bevan have shown that α for H 2 on nickel can be increased from 0.25 to 0.48 by oxidation. Since Langmuir has shown that a connection exists between the time taken for gas molecules to reach thermal equilibrium with a surface and the probability of their condensing thereon, variations in α , for different surface conditions of any given solid in any given gas, are of importance to the physics of adsorption. Since differences in thermal exchange at a bare lattice and at an adsorbed layer will share largely in determining whether the latter is monomolecular or multimolecular in structure, these differences are among the data preliminary to a physics of heterogeneous chemical reaction, in so far as this depends on the structure of the interfacial layer.

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