Accommodation coefficient definition

Uptake across the interface into solution (a). By definition, this is described by the mass accommodation coefficient, a, and l/a is the interfacial resistance. ... 

A more basic difficulty and one not yet adequately resolved is that encountered in the use of artificial models to represent molecules. From a rigorous point of view the entire behavior of a molecular encounter is determined by the force field surrounding each molecule. By representing molecular force fields by artificial models we avoid the impossible mathematical problem involved in the rigorous approach. The result, however, is to introduce an entirely new set of molecular parameters which remain as yet unpredictable from simpler molecular properties. In the case of the hard sphere model we have introduced the molecular diameter additional parameters which were somewhat concealed in the discussion, namely, the two accommodation coefficients, one for velocity transfers between molecules in collision and the other for collision between molecules and surfaces. 

For the more complicated molecular models such as, for example, those that assume central forces, we replace the above set of parameters by a new set involved in defining the force field. If we add to this the problem of complex molecules (i.c., those with internal structure), then there is the additional set of parameters needed to define the interactions between the internal molecular motions and the external force fields. From the point of view of the hard sphere model this would involve the definition of still more accommodation coefficients to describe the efficiency of transfer of internal energy between colliding molecules. 

From the definition of the accommodation coefficient, we can write the following... 

Theoretical knowledge on pressure-driven gas microflows is currently well advanced, especially in the slip flow regime. However, there is yet a need for accurate experimental data, both for steady and unsteady gas microflows, with or without heat transfer. For example, in order to definitely validate the choice of the best boundary conditions in the slip flow regime, we need to isolate the influence of the accommodation coefficients, as it remains an open issue. Relationships between their values, the nature of the substrate, and the microfabrication processes involved are currently not available. There are also few data about flows of gas mixtures in microchannels. 

However, if mass exchange takes place, this definition is only valid for molecules which, after falling on the surface, are not condensed and are reflected or re-emitted from it. Obviously their accommodation coefficient cannot be determined directly by bulk measurements. 

For this experiment, the treatment of N as a constant results in the best approximation of the series coefficients as virial coefficients. The most accurate as a true virial coefficient is, of course, the second (linear) coefficient since the experiment was optimized for the determination of the linear behavior and not for the nonlinear behavior of the sample gas. However, the role of the nonlinear terms is more than that of virial remainders terms to accommodate only the analytical fit to the data. The nonlinear coefficients are characteristic of the true virial coefficients to the extent suggested to us by the variations of their values and by the definitiveness of their temperature behavior. Our listed values of the third virial coefficient agree within 2% with those determined by Douslin and Harrison (2). In Table II, we present what we consider to be our best values for the virial coefficients for N constant. Where several groups of... 

Actually, the wall shear stresses found in a cylindrical tube are not really material parameters at all, because their dependence on geometry limits their applicability to cylindrical-tube flow. For cylindrical tubes of similar geometry and a constant length/diameter ratio, the wall shear stresses determined according to equation 2 are identical. Other shapes, however, such as cones (die) or auger chaimels demand a different, more suitable approach, with equation 2 modified to accommodate the corresponding geometric boundary conditions. Nonetheless, the waU shear stress remains superior to the coefficient of friction as a useful material parameter, because it yields a correct, practical, near-system definition of a compound s material properties, as explained above. 

The diffusion coefficient is now concentration dependent, reflecting dimensional changes in the fiber. Such changes can be accommodated by appropriate definitions of length coordinates to avoid shrinkage effects within the diffusion coefficients [31]. 

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