Numerical Simulation and Experimental Visualization of the Isothermal Flow of Liquid Containing a Headspace Bubble Inside a Closed Cylinder During off-Axis Rotation

Abstract

Numerical simulations and visualization of the isothermal flow of Newtonian and non-Newtonian liquids inside a rotating cylindrical Perspex ‘can’, which includes a headspace air bubble, are presented. The can has a diameter of 76 mm and is 66 mm long and, in the experiments, rotational speeds up to 40 rpm are considered. The Newtonian liquid is Corena Oil 27, which has a viscosity of 0.09 P as at 20 °C, and the non-Newtonian liquid is Keltrol, which is a shear-thinning liquid with a zero shear viscosity of 349 P as at 20 °C and a viscosity of 0.07 P as at a shear rate of 120s–1, which is typical of the largest shear rates that occur in this study. The work forms part of a broader study of the in-container convective processing of foods. The centroid of the can rotates around a circle in a vertical plane and the axis of symmetry of the can may be normal to the plane of rotation (termed ’axial’ rotation) or lie in the plane of rotation (termed ‘end-over-end’ rotation). The numerical simulations are (spatially) two- or three-dimensional and time-dependent, use a rotating co-ordinate system and determine the air–liquid interface (i.e. the headspace bubble position), for Newtonian and non-Newtonian liquids, using a homogeneous two-phase flow model. These complex simulations are carried out with the aid of the CFX software. A specially constructed rig is used for the experiments. Numerically predicted headspace bubble positions are in very good agreement with the experimental flow visualizations. It is shown that for a certain combination of off-axis position of the can and rotational speed the headspace bubble will move through the liquid. Such behaviour significantly enhances mixing and so is of great interest to companies who use this method of food processing. The circumferentially averaged median shear rate is used to quantify the effect of varying the off-axis position and rotational speed and, for both Newtonian and non-Newtonian liquids, conditions are identified under which this parameter is maximized. Two examples illustrating how the validated numerical simulations can be used to carry out parametric studies are then given.