Rheological Properties Of Viscoelastic Fluids Research Articles

Improving the prediction of turbulent kinetic energy for drag reduction in turbulent viscoelastic pipe flow

Reducing turbulence in pipe flows using polymer additives is crucial for industrial applications like crude oil, water, and sewage transportation. While previous research has accurately predicted friction factor and velocity profiles, none has fully understood turbulent kinetic energy (TKE) behavior in such fluids. Authors are now focusing on exploring turbulence models to better understand the TKE behavior. In this research, we have introduced a model to improve the behavior of TKE in a modified generalized Newtonian fluid (GNF). The developed model aims to simulate the viscoelastic effects of fluids that result in drag reduction in turbulent pipe flow. The work is noteworthy as it integrates turbulence and viscoelastic components, offering a comprehensive understanding of the phenomenon. By incorporating the rheological properties of viscoelastic fluids and replacing the damping function with a non-Newtonian alternative proposed by Cruz and Pinho, the Launder–Sharma k–ε turbulence model is now suitable for simulating dilute non-Newtonian viscoelastic fluids. The viscoelastic aspect of the model employs the modified GNF model. The developed model has been subjected to simulations using the computational fluid dynamics software. The results obtained for fluid TKE demonstrate a significant improvement in comparison to our previous research and the findings of other researchers. Furthermore, the model's prediction for the Darcy friction factor has been enhanced, resulting in an average error of only 3.71% in this section. It is noteworthy that the model consistently maintains a high level of accuracy in predicting other essential flow parameters such as mean axial velocity and Reynolds stresses. The provided model advances our understanding of viscoelastic fluid behavior in turbulent pipe flow by applying the modified GNF model.

Electrokinetic ion transport of viscoelastic fluids in a pH-regulated nanochannel

The objects of nanofluidics-based biological detection are usually viscoelastic fluids, and the rheological properties of viscoelastic fluids can have complex effects on electroosmotic flow and electrokinetic ion transport in nanochannels. However, researchers have mainly focused on Newtonian fluids. In this study, numerical simulations were performed on the electrokinetic ion transport of viscoelastic fluids in a pH-regulated nanochannel. The Oldroyd-B model is used to describe the behavior of the polyacrylamide (PAM) solution, and the Poisson-Nernst-Planck model is used to describe the potential and ion concentration in the electrolyte solution. Compared with the Newtonian fluid, at pH = 8, the PAM solution of cp = 500 ppm has a larger region of low ion concentration in front of the nanochannel entrance and has a larger region of high ion concentration behind the nanochannel exit. When the pH is increased to 8, the surface charge density |σw| increases sharply, which is about 5 times of that at pH = 6. At pH = 8, cKCl = 20 mM, and cp = 500 ppm, two distinct vortices appear behind the nanochannel exit, and rotate in opposite directions.

Mixed in-situ rheology of viscoelastic surfactant solutions using a hyperbolic geometry

Microfluidic devices can be used to represent industrial and natural mixed extensional-shear flows and to measure, simultaneously, in-situ rheological properties of viscoelastic fluids. However, such measurements are challenging due to the nature of mixed flow kinematics. In this study, we present a new mathematical framework to quantify the extensional behavior of viscoelastic fluids in mixed flows through a hyperbolic contraction-expansion microfluidic device. This approach provides a better estimation of apparent extensional viscosity when the non-Newtonian fluid kinematic does not promote additional flow phenomena such as vortices, shear banding or high-velocity jet. We then apply this approach to analyze the mixed rheology of two different viscoelastic surfactant solutions. The results indicate that there is a critical flow rate where extensional properties of the studied viscoelastic fluids start losing importance possibly due to additional flow phenomena. We also found a viscoelastic transition regime from extensional tension-thickening to tension-thinning as flow rate increases.

Electro-osmotic oscillatory flow of viscoelastic fluids in a microchannel

This work presents an analytical solution for electro-osmotic flow (EOF) in small amplitude oscillatory shear (SAOS) as a measuring tool suitable to characterize the linear viscoelastic properties of non-Newtonian fluids in microchannel flow. The flow in the straight microchannel is driven by applying oscillating sinusoidal electric potentials. Fourier series are used to derive an expression for the velocity field, under an externally imposed generic potential field aimed at the practical application of SAOS in characterizing the rheological properties of viscoelastic fluids. This extensive investigation covers a wide range of parameters and considers the multi-mode upper-convected Maxwell (UCM) model, which represents the rheology of viscoelastic fluids in the limit of small and slow deformations. Particular focus is given to two cases of practical interest: equal wall zeta potentials at both channel walls and negligible zeta potential at one of the walls. The results show that in flows with thin electric double layers (EDL), Reynolds numbers below 0.001, and Deborah numbers below 100, corresponding to a viscoelastic Mach number of 0.32, the velocity field outside the EDLs is linear and has a large enough amplitude of oscillation, which may allow the quantification of the storage and loss moduli in the linear regime. This technique requires the use of significantly smaller sample sizes than the traditional SAOS in a rotational rheometer.

Effect of the Reynolds number on viscoelastic fluid flows through axisymmetric sudden contraction

Viscoelastic fluid flows through sudden contractions can be found in several industrial applications, such as polymer extrusion and processing, polymer solution flows, and food and paints processing. However, this kind of flow has not yet been studied within the whole extent of laminar flows. In the current work, a numerical study of viscoelastic fluid flow through an axisymmetric 4:1 sudden contraction is conducted. The approach employs the exponential PTT model, the Reynolds number is varied from 0.0005 to 250, the Weissenberg number from 0.01 to 100 and two values for the extensional parameter e are used, 0.02 and 0.25. The results show that for Re > 25 and low Weissenberg numbers the Couette correction and the size of the corner vortices are almost independent of the Weissenberg number and of the extensional parameter. For lower values of Re, however, the size of the corner vortices is strongly dependent of We and e for We ≥ 0.1. The recirculation zones are larger for e = 0.02 than for e = 0.25, since an increase of the extensional viscosity hinders the fluid passage through the contraction, creating larger vortices. The Couette correction varies linearly with the Reynolds number within the whole range of the Weissenberg number investigated. It can be concluded that while the rheological properties of viscoelastic fluids affect significantly creeping flow patterns, inertial forces reduce the effect of these properties as the Reynolds number increases.

An analytical solution for convective heat transfer of viscoelastic flows in rotating curved pipes

This research reports an analytical solution for flow and convective heat transfer of an Oldroyd-B viscoelastic fluid in rotating curved pipes with viscous dissipation effects. In order to solve the momentum and energy conservation equations, a perturbation method is employed. Fully-developed hydrodynamic and thermal boundary conditions are considered with a constant heat flux imposed at the pipe wall. A physical solution based on the H2 boundary condition is presented and it is shown that the results of the present study are more physically realistic than those reported in previous studies utilizing only the H1 assumption. The computations have shown that due to the large gradient of velocity field and rheological properties of viscoelastic fluids, the internal heat generated by viscous dissipation exerts a significant role on both flow and heat transfer characteristics in the present regime. Furthermore, the interesting phenomenon of dissipation of mechanical energy is also investigated. Therefore, the current study emphasizes the substantial effects of viscous dissipation on the heat transfer in non-symmetric temperature distributions. The present analysis evaluates in detail the collective effects of Coriolis force, inertia forces, elastic force and dissipation effects, via the Rossby, Reynolds, Weissenberg and Brinkman numbers, respectively. An increase in Rossby number for the co-rotation case is observed to displace the minimum temperature location toward the outer side of the pipe and additionally enhances the sensitivity of the flow to Reynolds, Weissenberg, Prandtl numbers, viscosity ratios and curvature ratios. Visualization of the main and secondary flows is also presented. The study is relevant to thermal polymeric flow processing.

Probing linear and nonlinear microrheology of viscoelastic fluids

Bulk rheological properties of viscoelastic fluids have been extensively studied in macroscopic shearing geometries. However, little is known when an active microscopic probe is used to locally perturb them far from the linear-response regime. Using a colloidal particle dragged periodically by scanning optical tweezers through a viscoelastic fluid, we investigate both, its linear and nonlinear microrheological response. With increasing particle velocity, we observe a transition from constant viscosity to a thinning regime, where the drag force on the probe becomes a nonlinear function of the particle velocity. We demonstrate that this transition is only determined by the ratio of the fluid's equilibrium relaxation time and the period of the driving.

A new instrument for dynamic helical squeeze flow which superposes oscillatory shear and oscillatory squeeze flow

Simple shear flow has been widely used as a model flow system to investigate the rheological properties of complex fluids. Although it is useful and easy to apply, the shear flow is too simple compared with the complicated flows encountered in real processes. For this reason, it is necessary to study the rheological properties of viscoelastic fluids subjected to more complicated flow field. In this paper, we report a new instrument in which dynamic helical squeeze flow is generated. It superimposes the oscillatory squeeze flow on the oscillatory shear flow, and was designed to get more information on the microstructural change of complex fluids in the flow field which is more complex than conventional simple shear but is still well-defined. This instrument has advantages: the shear and normal stresses can be obtained simultaneously under the coupled flow field; the present design can be used to investigate the structural anisotropy in the direction of both rotational and vertical motion. This study provides a new design and methodology to investigate the rheological responses of complex fluids under dynamic helical squeeze flow.

Viscoelastic medium flows in convergent-divergent channels

The problem of viscoelastic fluid flow in a convergent-divergent channel is considered. A mathematical model and the results of numerical investigations are presented. A model of the differential type is used to describe the rheological properties of viscoelastic fluids. A comparative analysis of the flow parameters of generalized Newtonian and viscoelastic fluids in the channels considered is carried out on the basis of the results of numerical calculations.

Multiparticle collision dynamics modeling of viscoelastic fluids

In order to investigate the rheological properties of viscoelastic fluids by mesoscopic hydrodynamics methods, we develop a multiparticle collision (MPC) dynamics model for a fluid of harmonic dumbbells. The algorithm consists of alternating streaming and collision steps. The advantage of the harmonic interactions is that the integration of the equations of motion in the streaming step can be performed analytically. Therefore, the algorithm is computationally as efficient as the original MPC algorithm for Newtonian fluids. The collision step is the same as in the original MPC method. All particles are confined between two solid walls moving oppositely, so that both steady and oscillatory shear flows can be investigated. Attractive wall potentials are applied to obtain a nearly uniform density everywhere in the simulation box. We find that both in steady and oscillatory shear flows, a boundary layer develops near the wall, with a higher velocity gradient than in the bulk. The thickness of this layer is proportional to the average dumbbell size. We determine the zero-shear viscosities as a function of the spring constant of the dumbbells and the mean free path. For very high shear rates, a very weak "shear thickening" behavior is observed. Moreover, storage and loss moduli are calculated in oscillatory shear, which show that the viscoelastic properties at low and moderate frequencies are consistent with a Maxwell fluid behavior. We compare our results with a kinetic theory of dumbbells in solution, and generally find good agreement.

Rheological Properties of Viscoelastic Fluids from Continuous Flow Through a Channel Approximating Infinite Parallel Plates

Important rheological properties of viscoelastic fluids can be obtained by studying continuous flow through a channel approximating infinite parallel plates. This flat plate rheometer eliminates or reduces some problems of conventional ones and extends the region of useful measurements to high shear rates. Four pressure transducers are flush mounted on the top plate to measure the normal stress T22. From these measurements the shear stress can also be obtained. The normal stress in the direction of flow (T11) is obtained from the measurement of the impact of the exit stream. A Weissenberg rheogoniometer (WRG) is used to measure the shear stress and the primary normal stress difference (N1=T11−T22). Data are presented for polyethylene oxide (M̄w=4×106) solutions of 0.25%, 0.9%, 2.0%, and 3.0% in water. Newtonian fluids were used to test and compare the apparatus. Fluid viscosities from 0.01 to 6000 poise and normal stresses from 100 to 450,000 dynes/cm2 were determined for shear rates from 0.03 to 130,000 sec−1. The rotational geometry of the WRG limits its use to shear rates under 102–103 sec−1. Further, the precision of normal stress measurement with the parallel plates is better than obtained with the WRG. The shear stress values obtained with the plates are of equivalent precision and higher accuracy. Normal stress dependence upon shear rate is fit by the molecular dumbbell model of Bird et al. over a wide range of conditions. In the upper region the model predicts a slope of 2/3 and 0.66±0.03 is obtained by regression analysis of the data. In the lower region a slope of 2 is predicted and 2.08±0.37 obtained. Extrapolation of T11 and T22 data using this model allows N1 to be obtained at shear rates far higher than with the WRG. The magnitude of the ratio T22/T11 decreases as Polyox concentration increases and the fluid becomes more elastic. The complete state of stress is determined for all Polyox solutions, and pressures at the exit surface and exit centerline are found to contribute substantially to the total stress tensor.

Velocity profiles in laminar oscillatory flow in tubes

The oscillatory laminar flow of a Newtonian fluid in a circular tube has been investigated when the fluid is subjected to a periodic pressure gradient. A technique has been developed which allows the velocity amplitude to be determined as a function of radius and the results agree closely with theoretical predictions. The work is being continued with a view to employing the technique for the determination of the rheological properties of viscoelastic fluids.

Measurement of rheological properties of viscoelastic fluids in laminar shearing flow in tubes

Measurement of rheological properties of viscoelastic fluids in laminar shearing flow in tubes