Effective parameter interpretation and extrapolation of dispersion simulations by means of a simple two-velocity model

The investigation of dispersion by microscopic simulations yields a lot of detailed information. To identify characteristic behaviours, it is useful t o condense this information into a few effective parameters, which describe the transport process in the model geometry on a larger scale. For this pu rpose, a very simple two-velocity model has been developed, which models th e transition from reversible to irreversible spreading of a tracer volume. It is shown that this model is very similar to Taylor-Aris dispersion and t hat it is quite suitable to approximate the time dependence of dispersion. The model is applied to characterize the effect of dead end pores on disper sion with a single correlation parameter. Up to Peclet numbers of about 500 , 'hold-up'-dispersion similar to Taylor-Aris-dispersion is found. The simu lations have been performed by the lattice Bhatnagar-Gross-Krook (BGK) meth od, which is a particular type of cellular automata and therefore allows an easy implementation of complicated geometries. The fully irreversible asym ptotic dispersion is reached in an exponential process, the parameters of w hich can be identified by the two-velocity model after the mixing has notic eably begun. These are used to extrapolate the process which reduces the co mputational effort by about one order of magnitude.