Polyatomic gases, mixtures

Whichever set of tensors is employed, the methodology for the explicit evaluation of the transport coefficients follows a route analogous to that described in Section 4.2.1.2 for pure gases. Thus, just as for the pure gas, all of the dynamic encounters between molecules in the gas contribute to the transport coefficients through a series of effective cross sections details of the theoretical development may be found elsewhere (McCourt et al. 1990, 1991). Here it suffices to emphasize that theory yields approximations to the transport coefficients of the gas mixture which can be evaluated to an arbitrary order if desired and, just as for pure gases, it is also possible to account for the effects of spin-polarization, as discussed in Section 4.2.2.3. However, in the case of mixtures, for most practical purposes the first nonzero approximation is adequate accordingly, this is quoted here. The results are given first in a way in which they apply to polyatomic gas mixtures, since the results include monatomic systems as a special case. 

Explicit formulas for the transport coefficients of polyatomic gas mixtures In the limit of zero density the viscosity of an iV-component gas mixture is given by the expression... 

Chapman-Enskog (Bird et al.) and Wilke and Lee [31 ] The inherent assumptions of these equations are quite restrictive (i.e., low density, spherical atoms), and the intrinsic potential function is empirical. Despite that, they provide good estimates of for many polyatomic gases and gas mixtures, up to about 1000 K and a maximum of 70 atm. The latter constraint is because observations for many gases indicate that DABP is constant up to 70 atm. 

Counting gases consists usually of one of the noble gases mixed with a small amount of polyatomic gas. The latter makes the gas multiplication factor less dependent on applied voltage, and increases the speed of electron collection. Typical counting gas mixtures are... 

The flow rate, gas mixture, and solvent used affect formation of polyatomic species and therefore the degree of interference from polyatomic species including refractory oxides. For reproducible results, the operating conditions must be rigorously controlled. Plasma conditions can be chosen that minimize the formation of interferences but may cause loss of sensitivity. 

Non-Blanc phenomena for mobility and diffusion occur for all ions in any gas mixture and thus may be viewed as standard high-field effects (2.1). Now we transition to nonstandard effects that involve inelasticity of collisions or alignment of polyatomic ions. 

Here, as earlier, we are restricting our attention to systems of pure monatomic gases. The theory as well as the comparison with experiment can be generalized to include dilute gas mixtures and dilute gases of polyatomic molecules (cf. Sections 2.4.4.1 and 2.4.4.2). 

Calculations of mass flow rate through a short channel, i. e. when the length L, width b and height a are arbitrary, would be useful in practice. In many applications, e. g. micropumps, the channels/tubes have a spiral form. Thus, calculations of gas flow through curve channels is needed. Until now, only some simple flows of polyatomic gas and gaseous mixtures were calculated. A further investigation in this direction would be interesting. 

The first term of equation (4.127) is an approximation to the translational contribution to the thermal conductivity of the mixture. It is obtained by making use of equations (4.122)-(4.125) for the thermal conductivity of a monatomic gas mixture. For this purpose approximate translational contributions to the thermal conductivity of each pure component X, tr and an interaction thermal conductivity for each unlike interaction Xqq are evaluated by the heuristic application of equation (4.125) for monatomic species to polyatomic gases. Thus, the technique requires the availability of experimental viscosity data for pure gases and the interaction viscosity for each binary system or estimates of them. As the discussion of Section 4.2 makes clear, the use of... 

Experimental observation of relaxation phenomena in binary mixtures of polyatomic gases affords much more information about vibration-vibration transfer. The nature of the vibrational relaxation process for a mixture of a relaxing gas, A, with a non-relaxing gas, B, has been discussed in Section 4.3. It involves two collision processes... 

---