2012 Volume 38 Issue 1 Pages 19-24
Fluid mixing is generally judged to be complete when two or more different fluid materials are uniformly distributed over the whole system. In practice, mixing time in mixing processes is often measured by detecting color change due to a chemical reaction. However, the conceptual mixing time does not necessarily coincide with the time of completion of color change. In the present study, we reconsidered the relation between the conventional concepts of fluid mixing and experimentally measured mixing time. In addition, by comparing the difference in the spatiotemporal concentration change between an irreversible decolorizing reaction and a reversible color reaction, we investigated the possibility that mixing times measured by use of these reactions may differ. Experimentally, the completion time of color change in the reversible reaction was found to be longer than that of the irreversible reaction. In a reversible reaction, an initial color change may be reversed until the concentration becomes uniform in the system. Hence, the completion time of color change agrees well with the mixing time in a reversible reaction. On the other hand, in an irreversible reaction, a region once decolorized cannot be colored again even if the concentration in the system is not homogeneous. Thus, error may arise if an irreversible reaction alone is used to conclude that mixing is complete. Moreover, the difference in mixing time measured by irreversible and reversible reactions becomes larger as Reynolds number increases.
