Axial mixing in a reciprocating plate column with very small density gradients

Abstract Axial dispersion coefficients have been measured in single‐phase flow conditions in a 5.08‐cm‐dia. reciprocating plate extraction column. The measurements were done under steady‐state conditions by analyzing temperature profiles (for hot and cold water mixing) and concentration profiles (for mixing of water and salt solutions). The results confirm previous published data of Holmes et al. (1991) in showing that axial dispersion is increased strongly in the unstable situation, where liquid density increases with height. The earlier work is extended to the condition where buoyant energy dissipation is exceeded by mechanical energy dissipation by four orders of magnitude. Even in this case, axial mixing is increased significantly by density gradient‐induced instability. Results have been correlated by a simple equation based on a mixing length model.