Abstract
The rate of drainage of a viscous liquid from initially full cylindrical tubes inclined at various angles to the vertical (0°, 30°, 45° and 60°) was studied in glass and polymethylmethacrylate (Perspex™) tubes of various lengths and diameters using three food materials: honey (Newtonian) and two variants of Marmite™ spread (both exhibiting complex rheological behaviour, including shear-thinning and thixotropy). The behaviour was marked by an initially steady rate of drainage in which an air slug descended the tube, followed by slower drainage from an annular film remaining on the wall. Eventually the liquid stopped draining as a filament and entered a dripping regime. Drainage was insensitive to the tube material, whereas the stages of drainage were influenced by the geometry and angle of inclination. Quantitative models are presented for the rate and extent of the initial drainage stage, the rate in a second linear stage (where it existed), and the rate of drainage in the third, falling rate stage. The fourth and final stage, characterised by drop formation, was not modelled. The initial rate can be predicted with reasonable accuracy, allowing the time to remove approximately 50% of the material in a short waiting phase to be calculated, e.g. t=8νL/R2g for a Newtonian liquid with kinematic viscosity ν in a vertical pipe of radius R and length L. The agreement with the other models is less exact but they capture the general trends reasonably.
Highlights
Viscous liquids are widely used in food processing
We present a model, derived for Newtonian liquids such as those studied by Taylor and by Cox, which predicts the effect of experimental parameters on m* and compare the model predictions with the results obtained with the Newtonian fluid and the non-Newtonian ones
The self-drainage of three food-related viscous liquids from circular pipes was investigated in experiments featuring different pipe diameters, lengths and angles of inclination to the vertical
Summary
Products such as sauces and spreads (e.g. MarmiteTM, White et al, 2008) are manufactured and sold as viscous, non-Newtonian liquids Others such as ice cream and chocolate Food processing operations regularly require the lines carrying these viscous liquids to be cleared, either as part of shutdown for maintenance, changeover to a different batch, or for cleaning and disinfection. This is often achieved by recirculating water as part of a cleaning-in-place cycle. Water flushing can be fast but causes product loss and generation of large volumes of contaminated water, which must be treated
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