Generalized Newtonian fluid constitutive equation for polymer liquids considering chain stretch and monomeric friction reduction for very fast flows modeling

In this work, novel rectangular and circular orifice (zero-length) dies were used to measure planar and uniaxial extensional viscosities as a function of strain rate for various linear isotactic polypropylene melts by using Cogswell methodology. The obtained experimental data were combined with shear and uniaxial extensional viscosity data determined at very high strain rates. The ability of the molecularized Generalized Newtonian Fluid (mGNF) [M. Zatloukal and J. Drabek, "Generalized Newtonian fluid constitutive equation for polymer liquids considering chain stretch and monomeric friction reduction for very fast flows modeling," Physics of Fluids 33(8), 083106 (2021)], Giesekus and explicit Yao constitutive equations to describe the measured data was tested. It has been shown that including the effect of the chemical environment (i.e., the role of the oligomeric solvent) using a simplified version of the mGNF constitutive equation (instead of the commonly used Newton's law) can significantly improve the ability of the Giesekus and Yao viscoelastic constitutive equations to describe the measured experimental data, especially at very high strain rates with using adjustable parameters with a clear physical meaning.

Measurement and modeling of uniaxial and planar extensional viscosities for linear isotactic polypropylenes

Drábek

Zatloukal

2023

Three-dimensional numerical simulations and experimental studies on the viscoelastic rheological and deformation behavior of polymer catheter in gas-assisted extrusion processing

Zhang,

Huang,

Chen

2024

Gas-assisted extrusion is an effective method for improving the deformation behavior of polymer catheters during extrusion. However, the underlying mechanisms that dictate how geometrical and constitutive models influence the complex rheological behavior of the melt are not yet fully understood, which hinders further utilization and optimization. In this study, the three-dimensional (3D) gas–liquid–gas model for catheter gas-assisted extrusion was constructed. Subsequently, the Bird–Carreau model and the Phan–Thien–Tanner (PTT) model were employed in finite element numerical simulations to analyze the complex behavior. For comparative analysis, simplified two-dimensional (2D) model numerical simulations were also conducted. Additionally, experiments on catheter gas-assisted extrusion and parameterization studies of key constitutive model parameters were performed. The findings indicate that the 3D model, when integrated with the PTT constitutive model, demonstrates superior predictability and aligns more closely with experimental results. Furthermore, as the flow rate increases, discrepancies among different models diminish, and the distance required for the melt and gas to achieve motion equilibrium decreases. The internal mechanisms behind these phenomena are elucidated through the analysis of velocity and stress field distributions. This research enhances our understanding of the complex rheological behavior in polymer catheter gas-assisted extrusion, providing valuable insights for both academic research and industrial production in this field.

Polymer scission and molecular weight prediction in continuous abrupt contraction–expansion microchannel

Yin,

Nakamura,

Suzuki

et al. 2024

This study conducted a two-dimensional numerical simulation of flow in continuous abrupt contraction–expansion microchannel, using experimental measurements as input parameters to validate the reliability of the simulation by comparing the experimental and simulated flow regimes. Memory effects and polymer scission altered flow regime and decrease the elasticity of the solution. Consequently, the relaxation time and maximum extensibility parameter were continuously adjusted in the simulation to match the flow regime in each cavity, successfully predicting the polymer solution's molecular weight after each instance of scission for the first time. The simulation also provided distributions of velocity, extensional rate, and the first normal stress difference, clarifying the mechanism of continuous polymer chain scission during flow. The results suggest that scission likely occurs near the contraction region and the corners between the throat and expansion sections. Along the channel, velocity gradients and extensional rates near the contraction region increase, while first normal stress difference decreases, indicating that chain scission reduces solution elasticity, lowers flow resistance, and increases extensional rates, promoting further chain scission.