Non-newtonian fluids discharge through circular and square orifices from a tank

Abstract

Orifices are commonly used to control and measure the discharge of Newtonian fluids from pipes, channels, reservoirs and tanks and are available in different types, shapes and sizes. The accuracy of non-Newtonian fluid flow rate measurements through orifices from tanks is not well understood with little research available on discharge of non-Newtonian fluids from circular orifices and not at all on square orifices. In this work,square and round orifices were used. Circular and square cross-section sharp crested orifices with similar hydraulic radii were fitted at the bottom of the tank. A rectangular tank suspended from a weighbridge fitted with a load cell was used to measure the rate of discharge. A rotary viscometer was used to establish the rheological parameters of the t est fluids. Water was used for calibration purpose. Different concentrations of glycerine and model non-Newtonian test fluids were used. From the experimental results, the coefficients of discharge (Cd) and orifice Reynolds numbers for each orifice shape were determined. Cd vs Re relationships for the laminar and turbulent regions for each shape orifice were plotted. The average Cd values for both orifices was found to be 0.62 which is close to the oft-quoted value of 0.61 for Newtonian fluids, and there was no difference in Re in the fully turbulent region. The Re values ranged between 10 and 80 000. There is no effect of orifice shape for non-Newtonian fluids but there is an effect of the fluid viscosity and yield stress. This must be incorporated in the process engineering predictions.