Effect of round orifice aspect ratio on nonnewtonian fluid discharge from tanks

Abstract

Non-Newtonian fluids have complex viscous characteristics, which make it difficult to measure the flow rate during their processing and handling. Several researchers have conducted studies on the flow rate measurement of non-Newtonian fluids using orifices installed in horizontal pipes. However, the same cannot be said for the gravitational flow of non-Newtonian fluids through circular orifices from the bottom of tanks. The aim of this work was to determine the coefficient of discharge (Cd) values for different non-Newtonian fluids from a tank through circular orifices of varying aspect ratios (the relationship between the length and the diameter of an orifice denoted by L/d) fitted at the bottom of the tank, as a function of fluid properties. The Flow Process Research Centre at Cape Peninsula University of Technology built a rig that was used for the experiments. Two circular orifices of aspect ratios 0 and 5 were fitted one at a time in the center of the bottom of the tank and flush with the inside surface. Water was used for calibration and various concentrations of glycerine solutions, carboxymethyl cellulose (CMC) solutions, and bentonite and kaolin suspensions were used as test fluids. The rheology of the test fluids was established using a rotary viscometer. The analysis is presented in the form of the coefficient of discharge (Cd) against the orifice Reynolds number (Re). While the existing literature showed that in turbulent flow, Newtonian and non-Newtonian fluids have an average Cd value of 0.62 and 0.67 respectively, the turbulent flow region of this study, shows that Cd values of both Newtonian and non-Newtonian fluids varied with orifice thickness. The flow trends of L/d of 0 are separate from the flow trends of L/d of 5 in the laminar and turbulent regions.