Optimum bottom-stirring in a horizontal cylindrical vessel

Mixing times have been measured in 470 L of water in an 87 cm diameter horizontal cylindrical tank, using air blown in from submerged plugs. It was found that the mixing time depended strongly upon the arrangement of the plugs in the tank. For example, an equally-spaced row of plugs along the tank axis gave a relatively slow mixing process which could be interpreted in terms of a dispersion model. Much faster mixing was achieved with an uneven spacing between the plugs and in this case the mixing depended strongly on lateral circulation currents and did not fit the dispersion model well. In most of the cases, the mixing time decreased as the gas flow rate to the plugs was increased. However with certain plug arrangements, the mixing time increased as the gas flow increased. This unexpected result was explained in terms of the changes in circulation pattern with respect to the gas flow rate.