Accommodation coefficient is a physical quantity characterizing the behavior of gas or vapor particles in their collisions with a solid or liquid body surface. The value of the accommodation coefficient depends on the surface nature and state as well as on the composition and pressure of the gas mixture in the environment and on other parameters.
One of the most commonly used models of the interaction of gas molecules with the surface is the Maxwell model. It implies that the fraction of the molecules, which after their impingement on the surface are diffuse-reflected, is equal to α, and the fraction of the molecules which are specularly reflected is 1-α. It appears here that α can also be interpreted as a loss coefficient of the tangential component of the momentum brought by the molecules incident on the wall.
The energy accommodation coefficient is introduced analogously as:
where Ei is the energy of the incident molecules, Er is the energy carried away by reflected molecules, E is the energy which would be carried away by all reflected molecules if the gas has had time to come to thermal equilibrium with the wall. Assuming that the diffuse-reflected molecules reach the wall temperature, we have α = αE according to the Maxwell model. However, in reality the momentum and energy accommodation processes proceed differently. When multiatomic molecules collide with the surface, account must be taken of the possibility of a change in their internal energy.
If evaporation or vapor condensation occurs on the body surface, then evaporation and condensation coefficients are introduced to describe the transfer process. The condensation coefficient is defined as a share of the flow of molecules, which condensed on the surface, in the total flow of the particles which have impinged on it. The evaporation coefficient (which is smaller than unity) is a correction factor in the relation for maximum evaporation rate in the vacuum that accounts for various factors lowering the evaporation rate. In applied calculations, the values of these coefficients are assumed to be equal to each other, although this assumption holds strictly only under equilibrium conditions.
Accommodation coefficient values for some materials are presented in Table 1. It should be noted, however, that these data are related to pure substances. The presence of admixtures on the evaporation surface or of chemical reactions between the vapor and the components of the external gas flow, proceeding simultaneously with the evaporation, can significantly affect the accommodation coefficient.
Table 1. Accommodation coefficients α
|Temperature range, K
It has been found experimentally that in almost every case when α is much less than 1, the vapor molecules differ from the condensate molecules due to association, dissociation or polymerization. A temperature dependence of α has not been observed with experimental accuracy. The majority of experimental studies measure α in the vacuum, using the Knudsen-Langmuir equation (see Sublimation).