Mixing by rotary jet heads: Indications of the benefits of head rotation under turbulent and transitional flow conditions

Mixing times were obtained by the iodine-thiosulphate decolorization technique using rotary jet heads (RJH) for mixing in a Perspex tank with an inner diameter of 0.75 m and an aspect ratio of 2.5 using both water (turbulent flow) and shear-thinning, carboxymethyl cellulose (CMC) solutions (turbulent and transitional flow). The RJH system is based on mixing by liquid jets, but due to the rotation of the head the position of the jets is continually changing. Experiments were undertaken with RJHs rotating as normal, or held stationary, in order to ascertain whether and to what extent the 'randomness' imposed by the head motion improves mixing. It was found that the head rotation leads to a decrease in mixing time compared to stationary jets, even under turbulent conditions. However, if the head is rotating too fast, the jets do not reach the top and bottom of the tank whereby the mixing time increases. Experiments in 0.75 wt% and 1.25 wt% CMC showed that the transition from transitional to turbulent flow occurs at a lower Reynolds number with head rotation than without. Furthermore, the decrease in mixing time caused by the rotation is much more pronounced when the flow is transitional than when turbulent.