Comparison With the Matrix Methods

If the nonzero fluxes have the same sign (i.e., they are all in the same direction), then effective diffusivity methods are more likely to give reasonable results. This is nearly always the case in condensation and absorption processes and this goes some way at least to explaining why effective diffusivity methods usually give good estimates of the total amount condensed and the total heat load even if the individual condensation rates are not so well predicted. Webb et al. (1981) discussed in detail the conditions that must apply for an effective diffusivity method to be a useful model in multicomponent condensation. 

The effective diffusivity formula of Stewart (Eq. 6.1.8) is by far the best of this class of methods. This should not come as a surprise since this method is capable of correctly identifying the various interaction phenomena possible in multicomponent systems. Indeed, for equimolar countertransfer, this effective diffusivity method is equivalent to the linearized theory and to both explicit methods discussed above. In fact, for some systems Stewart s effective diffusivity method is superior to Krishna s explicit method (Smith and Taylor, 1983). However, since the explicit methods are actually simpler to use than Stewart s effective diffusivity method (all methods require the same basic data) and, in general 

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