Flow regimes in the mixing of municipal sludge simulant using submerged, recirculating jets

• Mixing of a sludge simulant with submerged jets studied using flow visualization techniques.
• Different flow regimes discovered and analysed using imaging tools.
• Phenomena characterising individual regimes and transitions between them discussed.

We study the flow regimes that develop when a non-Newtonian fluid that simulates municipal wastewater sludge is agitated in a mixing vessel using a submerged, downwards-pointing, recirculating jet and report a series of empirical scaling laws that emerge. A stream extracted from the vessel itself feeds the nozzle that discharges the jet via a pump. We observe that the volume that remains unperturbed (inactive volume, Vi) decreases steadily over time and when plotted against a normalised time-scale Nt = (Q̇/VT)t   where Q̇ is the recirculation rate, VT the total volume of the vessel and t the elapsed time, the quantity (Vi/VT) follows distinct trends that can be related to the apparent flow patterns within the mixing vessel. We study these domains experimentally using analytical video images to establish the differences between the flow behaviours that develop in these domains. Our experiments demonstrate that (Vi/VT) decreases slowly at first in the initial regime (Regime 1 and 2) and then rapidly in the terminal regime (Regime 3) with a rate almost an order of magnitude larger than the initial decay. While the rate of decay in the initial regime depends on the rheological properties of the fluid, the rate becomes independent of the same as the terminal regime is reached. Our experiments show that the transition between the initial and the terminal regimes can be a complex one and that the initial regime can persist for a remarkably long period of time. We provide empirical evidence to prove that that these regimes are affected by the rheology of the fluid and the principal operating condition, the injection velocity, when the geometrical factors are held constant.