Evaluation of a Measuring System for Determining Extensional Properties
Licentiatavhandling, 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.
LPTT-model
Viscoelastic fluid
Boger fluid
numerical simulation
FENE-CR model
extensional rheology
White-Metzner model
hyperbolic contraction flow
Hörsalen, Frans Perssons väg 6, SIK, Göteborg
Opponent: Prof. Bengt Hagström, Swerea-IVF and Chalmers, Sweden