Accommodation coefficient energy

The energy of an incident molecule will not normally be the same as that of the molecule when it is scattered from the surface, i.e., ZsP Ef There will be an accommodation to the surface and an exchange of energy with the surface. Complete accommodation or equilibration with the surface would imply that the scattered molecules have the same temperature as the surface. The energy accommodation coefficient, ac, is defined for each surface involved in the problem by the expression... 

For convenience in this derivation, we assume that the energy accommodation coefficient is the same for collisions at surfaces 1 and 2. Thus, we may write... 

Table 4 Experimental Data for Energy Accommodation Coefficient...

As the pressure increases from low values, the pressure-dependent term in the denominator of Eq. (101) becomes significant, and the heat transfer is reduced from what is predicted from the free molecular flow heat transfer equation. Physically, this reduction in heat flow is a result of gas-gas collisions interfering with direct energy transfer between the gas molecules and the surfaces. If we use the heat conductivity parameters for water vapor and assume that the energy accommodation coefficient is unity, (aA0/X)dP — 150 I d cm- Thus, at a typical pressure for freeze drying of 0.1 torr, this term is unity at d 0.7 mm. Thus, gas-gas collisions reduce free molecular flow heat transfer by at least a factor of 2 for surfaces separated by less than 1 mm. Most heat transfer processes in freeze drying involve separation distances of at least a few tenths of a millimeter, so transition flow heat transfer is the most important mode of heat transfer through the gas. 

Fig. 5. The dependence of the recombination coefficient, the energy accommodation coefficient and the stationary concentration of adatoms on temperature for the hydrogen—tungsten system when P2 — 1 torr and the other parameters have the values specified in Fig. 4.

The parameter used to characterise the energy transfer to the surface is the energy accommodation coefficient (]3) which is defined as the fraction of the total energy of recombination adsorbed by the catalyst. 

Fig. 22. Temperature dependence of the energy accommodation coefficient and of the recombination coefficient for the nitrogen—tungsten system. P2 = 1 torr, a = 100. ° P> > 7 (Courtesy Halpem and Rosner [112 ].)...

The values of the energy accommodation coefficient for the recombination of H and O atoms by metals measured by Wood et al. [89] and by Melin and Madix [77] are given in Table 6. The /3(H) values of Wood et al. for nickel, platinum and tungsten are substantially higher than those reported by Melin and Madix, but no explanation appears to be forthcoming for this difference. From Table 6, it is seen that a given metal has... 

The amount of incident energy of the molecule ( incident) that is transferred to the surface can be represented by an energy accommodation coefficient y ... 

For an incident atom, = 0 if the kinetic energies of the incident and scattered atoms are the same incident = scauered- Conversely, = 1 if the kinetic energy of the scattered species is equal to the kinetic energy expected for desorbing from a surface of thermal energy kgT urface> since, in this circumstance, scattered = surface-For a scattered atom, the energy accommodation coefficient becomes the translational-energy accommodation coefficient. 

Figure 4.8. Translational, vibrational, and rotational energy accommodation coefficients for NO scattered from Pt(l 11) as a function of crystal temperature [93J.

Gp for the CL scattering kernel obtained in [5] are given in Table 2. In all regimes the influence of the energy accommodation coefficient on the flow rate is weak, while the momentum accommodation coefficient ott affects significantly the flow rate in the transition (8 = 1) and near the free molecular (5 = 0.01) regimes. 

A much larger set of basic parameters is required for description of the thermal force than is necessary for the drag force. Transfer of heat to a particle by the host gas is central to the phenomenon. In the case of a polyatomic host gas, one must work from the more difficult kinetic theory of polyatomic gases [2.123]. Heat transfer at the particle-gas interface presents the problem of specifying the energy accommodation coefficients which often differ substantially from perfect accommodation. Additional complexity will appear in the discussion which follows. 

Starting from the entropy produced at the liquid surface the transport matrix for the heat and mass transfer is derived. Onsager symmetry and the role of the evaporation coefficient, the condensation coefficient and the energy accommodation coefficient are discussed. 

Calculate the angle-averaged energy accommodation coefficient (a) using the Baule formula (1.27). 

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