Notes on the diffusion-type model for the longitudinal mixing of fluids in flow

The longitudinal mixing of fluids in flow can sometimes be characterized by a single parameter D, the “longitudinal dispersion coefficient.” which is analogous to and has the same units as the coefficient of molecular diffusion. The results of a study of this model show that a dimensionless parameter, the Peclet number can be used as the similarity criterion for longitudinal mixing. Also, it is pointed out that the obvious and direct method of calculating the mean velocity of flow, by injecting a tracer into the fluid stream at one point and measuring its maximum concentration at a given point downstream, may in some cases lead to an appreciable error, even in situations where the diffusion-type model is applicable. Methods are shown for evaluating D from experimental measurements, examples are worked out and conditions for applicability of the model are discussed.