Paper Number
VP83 

Session
Pre-recorded Flash Presentations (virtual)

Title
Entrance flow of unfoamed and foamed Herschel-Bulkley fluids

Presentation Date and Time
All Week (Asynchronous) Any Time

Track / Room
Pre-recorded Presentation / Virtual

Authors (Click on name to view author profile)

  1. Mishra, Kim S. (ETH Zürich, Institute of Food, Nutrition and Health)
  2. Grob, Lucas (ETH Zürich, Institute of Food, Nutrition and Health)
  3. Kohler, Lucas (ETH Zürich, Institute of Food, Nutrition and Health)
  4. Zimmermann, Simon (ETH Zürich, Institute of Food, Nutrition and Health)
  5. Gstöhl, Stefan (ETH Zürich, Institute of Food, Nutrition and Health)
  6. Fischer, Peter (ETH Zurich)
  7. Windhab, Erich J. (ETH Zürich, Laboratory of Food Process Engineering)

Author and Affiliation Lines (in printed abstract book)
Kim S. Mishra, Lucas Grob, Lucas Kohler, Simon Zimmermann, Stefan Gstöhl, Peter Fischer and Erich J. Windhab
Institute of Food, Nutrition and Health, ETH Zürich, Zürich, Zürich 8092, Switzerland

Speaker / Presenter 
Mishra, Kim S.

Keywords
computational methods; additive manufacturing; applied rheology; confined flows; foams; non-Newtonian fluids; suspensions

Text of Abstract
The present study investigates extrusion processing of unfoamed and foamed cocoa butter (CB) crystal-melt suspensions (CMS) with varying crystal volume fraction Φ SFC. The yield stress to wall shear stress τ0/τw of CB CMS was fitted with the Herschel-Bulkley Papanastasiou (HB-P) model. Foamed CB CMS behaved fluid-like for Φ SFC ≤ 11.9% and according to a brittle porous solid for ΦSFC > 11.9%. The dimensionless entrance pressure loss nen/α as a function of dimensionless shear stress τ* was higher for foamed compared to unfoamed CB CMS at ΦSFC ≤ 11.9% and lower for foamed compared to unfoamed CB CMS at ΦSFC > 11.9%. The ΦSFC dependent difference in nen/α was attributed to the crystal confinement in the die entrance flow which is increased in the case of elastic gas bubble deformation and decreased in the case of plastic gas pore collapse. The computational fluid dynamics (CFD) simulated flow of unfoamed and foamed CB CMS through an abrupt circular 20:1 contraction with the HB-P model was compared with experimental results by quantitative entrance flow visualisation (QEFV). Differences in measured and simulated τ* and critical yielding radius Rcrit were related to the ability of the HB-P model to describe the viscous flow in the die and the limitations the HB-P model to describe elastic stress contributions in the die entrance flow. Furthermore, the QEFV derived half center incidence angle θ as well as the entrance flow shear- and elongational rates γef, εef were derived and used to establish a model predicting the Bagley entrance pressure loss ΔPBag and calculate an entrance flow characteristic shear- and elongational viscosity ηef, shear and ηef, elongation.