Hyperbolic contraction measuring systems for extensional flow

Abstract

In this paper an experimental method for extensional measurements on medium viscosity fluids in contraction flow is evaluated through numerical simulations and experimental measurements. This measuring technique measures the pressure drop over a hyperbolic contraction, caused by fluid extension and fluid shear, where the extensional component is assumed to dominate. The present evaluative work advances our previous studies on this experimental method by introducing several contraction ratios and addressing different constitutive models of varying shear and extensional response. The constitutive models included are those of the constant viscosity Oldroyd-B and FENE-CR models, and the shear-thinning LPTT model. Examining the results, the impact of shear and first normal stress difference on the measured pressure drop are studied through numerical pressure drop predictions. In addition, stream function patterns are investigated to detect vortex development and influence of contraction ratio. The numerical predictions are further related to experimental measurements for the flow through a 15:1 contraction ratio with three different test fluids. The measured pressure drops are observed to exhibit the same trends as predicted in the numerical simulations, offering close correlation and tight predictive windows for experimental data capture. This result has demonstrated that the hyperbolic contraction flow is well able to detect such elastic fluid properties, and that this is matched by numerical predictions in evaluation of their flow response. The hyperbolical contraction flow technique is commended for its distinct benefits-it is straightforward and simple to perform, the Hencky strain can be set by changing contraction ratio, non-homogenous fluids can be tested, and one can directly determine the degree of elastic fluid behaviour. Based on matching of viscometric extensional viscosity response for FENE-CR and LPTT models, a decline is predicted in pressure drop for the shear-thinning LPTT model. This would indicate a modest impact of shear in the flow, since such a pressure drop decline is relatively small. It is particularly noteworthy, that the increase in pressure drop gathered from the experimental measurements is relatively high despite the low Deborah number range explored.

Bolus rheology and ease of swallowing of particulated semi-solid foods as evaluated by an elderly panel

“…Extensional viscosity was measured at room temperature using a hyperbolic contraction flow geometry 12 mounted on an Instron testing instrument (5542 Instron Corporation, Canton, MA, USA), where the samples were forced through a hyperbolic nozzle and the pressure drop over the nozzle was measured.…”

Predictions for circular contraction-expansion flows with viscoelastoplastic & thixotropic fluids

“…Such corrections have been implemented successfully in a number of applications, as exemplified through the simulation of -wormlike micellar fluids in complex flow in non-banding [5,8] and banding conditions [38], the flow of viscoelastoplastic fluids in contraction-expansions (rounded-corner, =4; [6][7]), and the flow of Boger fluids in contraction-type flows. The work on Boger fluids has resulted in the close match of some well-founded experimental pressure-drop data (also flow-structure), encompassing contraction-expansion and contraction forms, planar/circular configurations [39], rounded/abrupt corners [13,[39][40][41], hyperbolic shape [42][43][44], and change in contraction-ratio [13].…”

UQ eSpace

“…Testing under low-shear conditions in a hyperbolic contraction flow rheometer (HCF) can yield a viscosity profile that can be separated into shear and extensional viscosity by methods such as the Cogswell, Bagley or Binding analyses (Stading and Bohlin, n.d.;Nyström et al, 2017). For example, using the Cogswell analysis the extensional viscosity, ηε [Paᐧs], and extension rate, ε̇ [s -1 ], can be described as a function of pressure drop at the entrance, ΔPe [Pa] of an axisymmetric contraction according to the equations (Larson, 1994):…”

“…where n is a constant. In the HCF rheometer the hyperbolic geometry of the nozzle is designed to minimise the contribution of shear effects, with a constant rate of displacement being applied to the sample being moved through the nozzle, or contraction (Nyström et al, 2017). The normal forces as the fluid moves through the contraction are measured by a load cell mounted above the nozzle.…”

“…Schematics of a HCF extensional rheometer (left) (Reproduced from:Nyström et al, 2017) and capillary breakup extensional rheometer (Reproduced from: ThermoScientific, 2015).…”