NOTES ON THE DIFFUSION-TYPE MODEL FOR THE LONGITUDINAL MIXING OF FLUIDS IN FLOW (REPRINTED FROM CHEM-ENGNG-SCI, VOL 6, PG 227-233, 1957)

The longitudinal mixing of fluids in flow can sometimes be characteriz ed 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 similari ty criterion for longitudinal mixing. Also, it is pointed out that the obvious and direct method of calculating the mean velocity of flow, b y injecting a tracer into the fluid stream at one point and measuring its maximum concentration at a given point downstream, may in some cas es lead to an appreciable error, even in situations where the diffusio n-type model is applicable. Methods are shown for evaluating D from ex perimental measurements, examples are worked out and conditions for ap plicability of the model are discussed.