Evaluation of a Measuring System for Determining Extensional Properties

Licentiate thesis, 2011

Effective and reliable characterisations of fluids are important to gain knowledge of the behaviour and to govern the properties of a fluid. Since food products often are non- Newtonian, viscoelastic liquids, the characterisation becomes more complicated. There are various techniques available for characterising viscoelastic liquids in shear but extensional properties are at least as important and presently there are not many techniques available commercially. In this work a measuring technique based on hyperbolic contraction is evaluated both by numerical and experimental studies. The evaluation was performed with a set of model test fluids with various rheological properties, a Newtonian, Boger and a shear-thinning fluid. Numerical simulations were performed through a hybrid finite element/finite volume scheme to evaluate the behaviour of the flow through the contraction nozzle in the measuring system. Several configurations were considered, differing in contraction angle, starting from sharp going down to a smooth hyperbolical shape. The hyperbolical shape was found to give an almost constant strain rate throughout the measuring regime in contrast to the other configurations. The numerical scheme was also utilised to evaluate the influence of shear from the wall in the contraction. Four different mathematical models resembling the test fluids were exploited and the pressure drop outcome depending on the different model properties was compared. It showed the advantages of a hyperbolically shaped nozzle in minimizing shear contribution to the measured pressure drop. The numerical results were compared with experimental results, demonstration increasing pressure drops with increasing deformation rates, which is consistent with the numerical results. However, the rise in pressure drop is more pronounced for the experimental results.