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2^mk/pk

For ideal gases, it is possible to write component density, pk in terms of the molecular weight of component k, temperature, operating pressure and universal gas constant. For non-ideal fluids, one can use empirical correlations to represent variations of fluid density with temperature. The values of fluid viscosity, thermal conductivity and molecular diffusivity can be estimated using kinetic theory (Hirschfelder et al., 1964). Kinetic theory allows one to estimate variations of these properties with temperature and pressure. Sometimes it is necessary to use empirical or semi-empirical formulae to estimate these properties while solving the basic conservation equations. For multicomponent flows, it is customary to use mixture properties as a mass fraction weighted average of the individual component properties, such as p

It is possible to use alternative formulations considering mole fractions rather than mass fractions. For most cases, mass fraction formulations will be adequate. An estimation of the diffusion coefficient (of component k) in a multicomponent mixture (Dkm ), however, is not straightforward. For mixtures of ideal gases, the diffusion coefficient in a mixture can be estimated as (Hines and Maddox, 1985)

j,j=k where Xk is the mole fraction of component k and Dkj is a binary diffusion coefficient for species k in species j. Pure component properties may be estimated by following standard practices (Reid et al., 1987). Whenever possible, experimental values of transport properties and equations of state should be used. Typical values of the physical properties of different fluids are listed in Table 2.3. Data for pure compounds can be found in various handbooks (for example, Vargaftik, 1983; Yaws, 1995; Perry's Handbook, 1997). Data for mixtures (especially diffusion coefficients) are difficult to find. In the absence of experimental data, some help can be obtained from estimation methods. For dilute gases and gas mixtures, the kinetic theory of gases can be used to make reasonable estimates of transport properties (see, for example, Hirschfelder et al., 1964). For liquids, theory is much less well developed. A recent review on theory and experiment may provide some assistance in estimating or measuring the required transport properties (Millat et al., 1996).

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