Viscoelastic Polymer Solutions Research Articles

Cavitation in viscoelastic dilute polymer solutions through a Venturi nozzle

This research experimentally examines the influence of viscoelastic dilute water solutions of polyethylene oxide on Venturi cavitation. Variations in solutions are engineered to manipulate the viscoelastic properties that in turn affect cavitation patterns and attributes. The consequences of viscoelasticity and flow conditions on cavitation are quantified using dimensionless numbers, including the elasticity number (El), the Reynolds number (Re), and the pressure ratio (κ). The experiment identifies three distinct cavitation patterns in the solutions, with their transitions being impacted by alterations in El and κ. As El amplifies, the cavitation bubbles expand and get smoother, and the reentrant jet thickens and amplifies. The behavior of cavitation aligns with the model proposed by Zhang et al. [Phys. Fluids 31, 097107 (2019)], suggesting the critical role of the reentrant jet in the shedding of the cavity cluster. The study also substantiates that the reentrant jet intensifies with ascending El or Re. The collective influence of El, Re, and κ is discovered to shape the cavitation length and shedding frequency of cavity clusters. An increased El or a decreased Re reinforces the vorticity and the reentrant jet, which inevitably leads to a reduction in cavitation lengths and an uptick in the shedding frequency. Conversely, a larger El results in a more gradual response of the bubble to pressure alterations and pronounced rebounds, extending the cavitation length.

Effect of fluid elasticity and shear thinning on heat transfer from a heated sphere at moderate Reynolds numbers

The drag and heat transfer characteristics of an isothermal sphere in a FENE-P-type viscoelastic fluid have been studied in the steady axisymmetric flow regime. The governing equations, namely, mass, momentum, and energy equations, together with FENE-P type viscoelastic constitutive equations, have been solved using the finite volume method based opensource CFD code OpenFOAM over the following ranges of conditions: Reynolds number, 1 ≤ Re ≤ 100 ; Prandtl number, 1 ≤ Pr ≤ 50 ; Weissenberg number, 0 ​ ≤ Wi ≤ 10 and polymer extensibility parameter, 10 ≤ L 2 ≤ 500 for a fixed value of the polymer viscosity ratio of β = 0.5 . At low Reynolds numbers, fluid viscoelasticity causes an upward and downward shifting in the axial velocity profiles with reference to that in a Newtonian fluid along the upstream and downstream axis of the sphere, respectively. However, such a shift progressively diminishes with the increasing Reynolds number. Furthermore, a “velocity overshoot” and/or a “negative wake” is also observed in a viscoelastic polymer solution at low Reynolds numbers, and this is accentuated with the increasing Weissenberg number. The separation of boundary layers is seen to be suppressed with the increasing Weissenberg number. The drag coefficient decreases with the Reynolds number irrespective of the fluid type and the corresponding dependence on the Weissenberg number is found to be non-monotonic depending upon the values of Re and L 2. The average Nusselt number always increases with the increasing values of Re, Pr, and Wi and with the decreasing value of L 2. A limited number of simulations have also been carried out to show the effect of the temperature-dependent viscosity of the fluid on the flow dynamics and heat transfer characteristics. Finally, simple correlations for the drag coefficient and average Nusselt number are presented which not only capture the functional dependence but also can be used for the interpolation of the present results for the intermediate values of the governing parameters in a new application.

NONLINEAR EFFECTS IN VISCOELASTIC DROP SHAPE OSCILLATIONS

A study of axisymmetric shape oscillations of viscoelastic drops in a vacuum is conducted, using the method of weakly nonlinear analysis. The motivation is the relevance of the shape oscillations for transport processes across the drop surface, as well as fundamental interest. The study is performed for, but not limited to, the two-lobed mode of initial drop deformation. The Oldroyd-B model is used for characterizing the liquid rheological behavior. The method applied yields a set of governing equations, as well as boundary and initial conditions, for different orders of approximation. In the present paper, the equations and solutions up to second order are presented, together with the characteristic equation for the viscoelastic drop. The characteristic equation has an infinite number of roots, which determine the time dependency of the oscillations. Solutions of the characteristic equation are validated against experiments on acoustically levitated individual viscoelastic aqueous polymer solution drops. Experimental data consist of decay rate and oscillation frequency of free damped drop shape oscillations. With these data, solutions of the characteristic equation dominating the oscillations are identified. The theoretical analysis reveals nonlinear effects, such as the excess time in the prolate shape and frequency change for varying initial deformation amplitude. The influences of elasticity, measured by the stress relaxation and deformation retardation time scales, are quantified, and the effects are compared to the Newtonian case in the moderate-amplitude regime.

Surface-tension change of graphene-based water nanofluid and its effects on heat-transfer process

Nanofluids with high thermal conductivities are gaining attention for heat-exchange applications, particularly in boiling processes. Nanofluid heat-transfer efficiency is significantly influenced by the surface tension (ST) and wettability patterns of the nanofluid. This study investigated graphene-based water nanofluids prepared from three different viscoelastic polymer solutions. The effects of the dispersant, graphene concentration, and temperature on the ST were investigated, and the wetting behavior of nanofluids on stainless steel and polypropylene random surfaces was determined. The results indicated that the ST of the nanofluids exhibited a negative correlation with the dispersant concentration and temperature. Additionally, the inclusion of graphene reduced the ST in xanthan-gum systems but increased it in sodium carboxymethyl cellulose and polyethylene-oxide systems. Overall, the nanofluids exhibited a lower ST than water. The decrease in the nanofluid ST is ascribed to the development of a rigid network resulting from the co-adsorption of graphene nanoparticles and the dispersant at the gas–liquid interface. Furthermore, graphene has been demonstrated to enhance the wettability of the base fluids of various materials. A decrease in ST led to increased frictional resistance; however, this was counteracted by an increase in the contact angle, resulting in an overall reduction in frictional resistance. Moreover, reducing the ST can improve the fluid boiling heat transfer. Therefore, these nanofluids exhibited the dual effect of reduced flow resistance and enhanced heat-transfer efficiency. This study offers valuable insights into the behavior of nanofluids and their potential applications in areas such as energy storage and spacecraft thermal management.

The Micro-Flow Mechanism of Polymer Flooding in Dual Heterogeneous Reservoirs Considering the Wettability.

There have been some studies conducted about the single factor viscoelasticity of polymer solution or wettability effect on the micro-flow mechanism of polymer flooding. In this paper, the flow mechanism of polymer solution in dual heterogeneous reservoir considering the wettability and gravity was studied. The influences of wettability and rock particle shape on flow characteristics were studied based on the characteristics of saturation and pressure distribution. Compared with the simulation results of polymer flooding in three different rock particle shapes porous media, the oil displacement efficiency of the circular particle model is the highest at 91.57%, which is 3.34% and 11.48% higher than that in the hexagonal and diamond models, respectively. The influence of wettability was studied by the circular particle model. The oil displacement efficiency under water-wet conditions was higher than that under oil-wet conditions. The displacement process considering gravity was affected by the crossflow caused by gravity and viscous force, and the micro-oil displacement efficiency was 9.87% lower than that of non-gravity. Considering the wettability, vertical crossflow will be formed. The oil displacement efficiency under water-wet conditions was 3.9% higher than in oil-wet conditions. The research results can not only expand and enrich the micro-flow mechanism of viscoelastic polymer solution, but also provide reference and guidance for polymer flooding scheme design.

Efekty lepkosprężyste w warunkach nawadniania kolektora szczelinowo-porowego cieczą wodno-polimerową

The work is devoted to the numerical simulation of the flow of simple and viscoelastic (polymer solution) fluids through a fracture by using polymer solutions for enhanced oil recovery from a reservoir. Polymer solutions have viscoelastic properties. Therefore, when polymer solution flows through the slot, we use the well-known Maxwell’s fluid model with the Jaumann derivative to evaluate deformation characteristics of the flow (stream functions, distributions of the longitudinal velocity gradient and normal stress) resulting in the manifestation of abnormal (compared with the behaviour of the ordinary fluid) effects. The case of slow flow is considered. In this case, the inertial terms can be neglected, the velocities, stresses, and stream functions can be written as the decomposition by Weisenberg number, and we can assume that the Weissenberg number is less than one. The determined regularities of viscoelastic (polymer solution) liquid behaviour with longitudinal velocity gradient and the elastic deformations effects manifested in this case have a decisive meaning in understanding the mechanism of anomalously high oil recovery capacity of a reservoir by using water-polymer flooding of the fractured-porous reservoir. Understanding the nature of the effects of elastic deformations under the conditions of water-polymer flooding of the fractured-porous reservoir enables hydrodynamic calculations of the optimal flow of the polymer solution. One of the main issues that need to be solved when developing the technology for increasing oil recovery from formations using polymer solutions is to determine the optimal flow regime in the fractured-porous reservoir. The calculation results verify the ideas obtained from the experimental solution of this problem concerning the strain-stress state of polymer macromolecules (liquid elements) during polymer flow in the inlet area of the fracture in the oil reservoir and confirm the possibility of using numerical analysis of convergent polymer flow for calculating longitudinal velocity gradients in the inlet area of the fracture and can also serve as additional substantiation of the proposed earlier mechanism for increasing oil recovery from reservoirs by using polymer solutions.

Dilute viscoelastic polymer solutions for dielectric heat transfer applications: A molecular dynamics study

Dilute viscoelastic polymer solutions for dielectric heat transfer applications: A molecular dynamics study

Entrapment and mobilization dynamics during the flow of viscoelastic fluids in natural porous media: A micro-scale experimental investigation

Capillary desaturation process was investigated as a function of wetting phase rheological signatures during the injection of Newtonian and non-Newtonian fluids. Two sets of two-phase imbibition flow experiments were conducted on a water-wet sandstone core sample using brine and viscoelastic polymer solutions. During the experiments, a high-resolution micro-computed tomography scanner was employed to directly map pore-level fluid occupancies within the pore space. The results of the experiments revealed that at a given capillary number, the viscoelastic polymer was more efficient than the brine in recovering the non-wetting oil phase. At low capillary numbers, this is attributed to the improved accessibility of the viscoelastic polymer solution to the entrance of pore elements, which suppressed snap-off events and allowed more piston-like and cooperative pore-body filling events to contribute to oil displacement. For intermediate capillary numbers, the onset of elastic turbulence caused substantial desaturation, while at high capillary numbers, the superimposed effects of higher viscous and elastic forces further improved the mobilization of the trapped oil ganglia by the viscoelastic polymer. In the waterflood, however, the mobilization of oil globules was the governing recovery mechanism, and the desaturation process commenced only when the capillary number reached a threshold value. These observations were corroborated with the pore-level fluid occupancy maps produced for the brine and viscoelastic polymer solutions during the experiments. Furthermore, at the intermediate and high capillary numbers, the force balance and pore-fluid occupancies suggested different flow regimes for the non-Newtonian viscoelastic polymer. These regions are categorized in this study as elastic-capillary- and viscoelastic-dominated flow regimes, different from viscous-capillary flow conditions that are dominant during the flow of Newtonian fluids. Moreover, we have identified novel previously unreported pore-scale displacement events that take place during the flow of viscoelastic fluids in a natural heterogeneous porous medium. These events, including coalescence, fragmentation, and re-entrapment of oil ganglia, occurred before the threshold of oil mobilization was reached under the elastic-capillary-dominated flow regime. In addition, we present evidence for lubrication effects at the pore level due to the elastic properties of the polymer solution. Furthermore, a comparison of capillary desaturation curves generated for the Newtonian brine and non-Newtonian viscoelastic polymer revealed that the desaturation process was more significant for the viscoelastic polymer than for the brine. Finally, the analysis of trapped oil clusters showed that the ganglion size distribution depends on both the capillary number and the rheological properties of fluids.

Pressure drop in a converging channel with viscoelastic polymer solutions having power-law viscous behaviour

Pressure drop in a converging channel with viscoelastic polymer solutions having power-law viscous behaviour

Effect of viscoelasticity on displacement processes in porous media

Improving the displacement efficiency of capillary entrapments in porous media by adding high molecular weight polymers to the invading phase has various industrial applications, from enhanced oil recovery to soil remediation. Apart from an increased viscosity contrast compared to regular water flooding, the flow of viscoelastic polymer solutions exhibits unstable flow behavior even at small Reynolds numbers, which can lead to an additional displacement mechanism of the capillary entrapments. In this work, we employ a microfluidic approach to unravel the underlying physics and mechanism of this enhanced pore scale displacement. To this end, we show that the major complex topological flow features in a typical porous medium can be mimicked by a flow geometry consisting of a single capillary entrapment connected to two symmetric serpentine channels. This design excludes the effect of viscous stresses and allows direct focus on displacement processes driven solely by elastic stresses. We show that the unique viscoelastic fluid features, such as the significant storage and release of elastic stresses and first normal stress difference, combined with the flow geometry, lead to purely elastic instability and secondary flow, which in turn provide the stresses necessary to overcome the capillary threshold and displace the capillary entrapment.

Lubrication with Non-Newtonian Fluids

Hydrodynamic lubrication is studied for both shear thinning and viscoelastic polymer solutions. We find that elasticity, notably strong normal stresses, does not change the friction significantly for the range of parameters tested in this manuscript. Shear-thinning properties, on the other hand, do change the formation of the lubricating layer thickness and the dependence of friction on velocity relative to Newtonian fluids. A hydrodynamic model that includes shear thinning is developed and compared to experimental data. The model describes the dependence on lubrication parameters well, but underestimates the lubricating layer thickness by a constant factor of roughly 1.5. The theory allows us to define a Hersey-like number for shear-thinning fluids that describes the lubricating layer thickness as a result of the balance between normal load and viscous force. For each tested liquid it succeeds in collapsing friction measurements onto the same curve. The friction analysis for both lubrication theory and experiments then reveals that shear thinning mainly changes the layer thickness, which is the main determinant of the friction coefficient.

Pinch-off of bubbles in a polymer solution

The formation of gas bubbles in a liquid occurs in various engineering processes, such as during foam generation or agitation and mixing in bubbly flows. A challenge in describing the initial formation of a gas bubble is due to the singular behavior at pinch-off. Past experiments in Newtonian fluids have shown that the minimum neck radius follows a power-law evolution shortly before the break-up. The exponent of the power-law depends on the viscosity of the surrounding Newtonian liquid, and ranges from 0.5 for low viscosity to 1 for large viscosity. However, bubble formation in a viscoelastic polymer solution remains unclear, and in particular, if the evolution is still captured by a power-law and how the exponent varies with the polymer concentration. In this study, we use high-speed imaging to analyze the bubble pinch-off in solutions of polymers. We characterize the time evolution of the neck radius when varying the polymer concentration and thus the characteristic relaxation time of the polymer chains, and describe the influence of viscoelasticity on the bubble pinch-off. Our results reveal that the presence of polymers does not influence the thinning until the latter stages, when their presence in sufficient concentration delays the pinch-off.

The role of essential oils in maintaining the postharvest quality and preservation of peach and other fruits.

Fruits are highly susceptible to postharvest losses induced majorly by postharvest diseases. Peach are favored by consumers because of their high nutritional value and delicious taste. However, it was easy to be affected by fungal infection. The current effective method to control postharvest diseases of fruits is to use chemical fungicides, but these chemicals may cause adverse effects on human health and the residual was potentially harmful to nature and the environment. So, it is especially important to develop safe, non-toxic, and highly effective strategies for the preservation of the fruits. Essential oil, as a class of the natural bacterial inhibitor, has been proven to exhibit strong antibacterial activity, low toxicity, environmental friendliness, and induce fruit resistance to microorganism, which could be recognized as one of the alternatives to chemical fungicides. This paper reviews the research progress of essential oils (Eos) in the storage and preservation of fruits, especially the application in peach, as well as the application in active packaging such as edible coatings, microcapsules, and electrospinning loading. Electrospinning can prepare a variety of nanofibers from different viscoelastic polymer solutions, and has broad application prospects. The paper especially summarizes the application of the new Eos technology on peach. The essential oil with thymol, eugenol, and carvacrol as the main components has a better inhibitory effect on the postharvest disease of peaches, and can be further applied. PRACTICAL APPLICATIONS: As an environmentally friendly natural antibacterial agent, essential oil can be used as a substitute for chemical preservatives to keep fruits fresh. This paper summarizes the different preservation methods of essential oils for fruits, and especially summarizes the different preservation methods of essential oils for peaches after harvesting, as well as their inhibitory effects on pathogenic fungi. It could provide ideas for preservation of fruits and vegetables by essential oils.

Multistage transitions in viscoelastic turbulent flow: Vortex structures and self-sustaining mechanisms

Viscoelastic dilute polymer solutions are known to display drag reduction in turbulent flows. At constant Re and as the fluid becomes more elastic (increasing Wi), the flow undergoes transitions between several stages of different behaviors. The first is the onset of drag reduction, before which the turbulent friction factor remains indistinguishable from its Newtonian limit. The second is the transition from low-extent (LDR) to high-extent drag reduction (HDR), which increasing evidences have shown to be a qualitative transition between different types of turbulent dynamics. The third is the convergence to the maximum drag reduction (MDR) asymptote, after which the friction factor becomes constant. Although increasing drag reduction with increasing Wi is intuitively expected, transitions between these stages are accompanied by drastic changes in flow statistics and structural patterns, indicating fundamental differences in the underlying turbulent dynamics. Mechanistic understanding into these transitions is far from complete. Our latest work has offered fresh insights into both the LDR-HDR and HDR-MDR transitions. For the former, we developed a new vortex analysis method, called VATIP, and applied it to analyze the differences between the conformation statistics in LDR and HDR regimes. The result links the occurrence of HDR to the dominance of a new turbulence self-sustaining mechanism. For the latter, although a full answer to the MDR problem remains elusive, our analysis of the so-called elastoinertial turbulence (EIT) challenges the long-held presumption of MDR being an ultimate flow state that is no longer affected by polymers. The finding calls for a new direction of thought in constructing a theoretical explanation for MDR.

Extensional flow affecting shear viscosity: Experimental evidence and comparison to models

The effect of extensional flow on apparent shear viscosity has never previously been directly measured nor is it often considered. Here, for the first time, through using a novel flow configuration (two-phase shear response under extensional flow), we have directly measured the effect extensional flow has on the apparent shear viscosity of a viscoelastic polymer solution in a controlled and kinematically mixed manner. We show, via a control transient shear experiment, that the apparent shear viscosity of the solution under mixed deformation depends not only on the shear rate but also on the extension rate and their relative direction: shear thinning being enhanced by parallel and reduced by perpendicular extensional flow, respectively. A 62% reduction in apparent viscosity with parallel extension was seen in this work. We then test the ability of the commonly used Giesekus and Carreau–Yasuda (incorporating generalized shear rate) models to predict the effect of extension rate on apparent shear viscosity against our data. The Giesekus model was found to predict the correct qualitative behavior under both parallel and perpendicular extensional flow, and depending on the fitting parameters, also provided a loosely quantitative agreement. Conversely, the generalized shear rate description does not capture the qualitative behavior, with the most significant errors occurring for perpendicular extension (i.e., expansion) flows. This work emphasizes the rarely noted shortcomings of the latter approach when used for experimental analysis and engineering design when extensional flows are additionally present.

Non-Newtonian flows and instabilities in 3D glass microfluidic devices

<h2>Abstract</h2> In this virtual seminar we will discuss the use of selective laser-induced etching (SLE) for the fabrication of glass microfluidic devices, in particular for the study of non-Newtonian fluid dynamics problems [1]. SLE enables the precise monolithic fabrication of three-dimensional (3D) channels and structures embedded in a rigid and transparent fused-silica substrate, and thus provides opportunities for a wealth of novel microscale flow experiments. We will highlight various recent and ongoing microfluidic projects from our laboratory that have been made possible by SLE fabrication. Our focus will be on flows of viscoelastic wormlike micellar and polymer solutions in microscale geometries featuring, for instance, slender rigid or cantilevered posts [2,3]. 3D intersections [4,5], and axisymmetric contraction/expansions [6]. We make use of state-of-the-art quantitative techniques such as high-speed flow-induced birefringence imaging and volumetric 3-component micro-particle image velocimetry to characterize and understand purely-elastic and inertioelastic flow instabilities arising in these complex systems.

Scaling purely elastic instability of strongly shear thinning polymer solutions.

Flow of viscoelastic polymer solutions in curved channels exhibits instability caused by the elastic nature of polymers even at low Reynolds numbers. However, scaling of the onset of this purely elastic instability in semidilute polymer solutions has not been previously reported. Here we experimentally investigate the flow of highly elastic polymer solutions above their overlap concentrations using pressure measurements and particle image velocimetry. We demonstrate that the onset of instability can be scaled by including shear dependent rheological properties of the polymer solutions in the nonlinear stability analysis. As a result, a universal criterion as function of normalized polymer concentration is provided for scaling the onset of purely elastic instability in the semidilute regime regardless of the type and molecular weight of the polymer.

On the reduction of the residual oil saturation through the injection of polymer and nanoparticle solutions

Polymer flooding has been traditionally applied to achieve favorable mobility ratios compared to water flooding, and thus increase formation sweep efficiency in oil displacement processes. Based on laboratory studies, several authors have reported lower residual oil saturations, Sorc, to the waterflood residual oil saturation, Sorw, when viscoelastic polymers were used during polymer flooding. The reported mechanisms contributing to the observed reduced residual oil saturation include mobility control, improved sweep efficiency, and improved microscopic displacement efficiency. The limitations of polymer flooding are obvious: high oil viscosity requires high concentration of polymer which increases the process cost, presence of clays which increases polymer adsorption, formation brine and water make-up salinity and type of ions which will affect the injected polymer viscosity, and extensively fractured formations which will cause sweep challenges. This review paper summarizes the most recent research findings, observations, conclusions, and reported physical mechanisms contributing to more efficient oil displacement processes and lower than Sorw residual oil saturation. The discussion focuses on viscoelastic polymer oil displacement processes and includes formation water and polymer solution salinity, resistance factor (RF), and polymer molecular weight effects. The use of nanoparticles (NPs) during polymer flooding is examined and results obtained are also reported in this review. More specifically, the polymer-NP system rheological behavior for several different types of NPs and surface modified silica NP are examined to find the most efficient system for oil displacement. Mechanisms contributing to more effective oil displacement processes when carefully screened NPs are added into the viscoelastic polymer solutions include interfacial tension reduction, formation wettability alteration, increased injected polymer solution viscosity, and prevention of asphaltene precipitation.

Effect of elastic instability on mobilization of capillary entrapments

The flow of viscoelastic polymer solutions and their use as displacing agents in porous media are important for industrial applications, such as enhanced oil recovery and soil remediation. The complexity of flow and high elasticity of conventionally used viscoelastic polymer solutions can lead to purely elastic instability in porous media. In this study, we investigate the impact of this instability on displacing capillary entrapments at low Reynolds numbers using a microfluidic approach. Our unique design consists of a single-capillary entrapment connected to two symmetric serpentine channels. This design excludes the effect of viscous forces and enables a direct focus on displacement processes driven solely by elastic forces. After the onset of purely elastic instability, an unstable base flow is observed in the serpentine channels. We discuss that the pressure fluctuations caused by this unstable flow create an instantaneous non-equilibrium state between the two ends of the capillary entrapment. This provides the driving pressure to overcome the capillary threshold pressure and eventually displace the entrapped oil. In our geometry, we observe that the displacement coincides with the emergence of a fully developed elastic turbulent state.

Liquid Transfer for Viscoelastic Solutions.

Viscoelastic liquid transfer from one surface to another is a process that finds applications in many technologies, primarily in printing. Here, cylindrical-shaped capillary bridges pinned between two parallel disks are considered. Specifically, the effects of polymer mass fraction, solution viscosity, disk diameter, initial aspect ratio, final aspect ratio, stretching velocity, and filling fraction (alike contact angle) are experimentally investigated in uniaxial extensional flow. Both Newtonian and viscoelastic polymer solutions are prepared using polyethylene glycol and polyethylene oxide, with a wide variety of mass fractions. The results show that the increase in polymer mass fraction and solvent viscosity reduces the liquid transfer to the top surface. Moreover, the increase in the initial and final stretching heights of the capillary bridge also decreases the liquid transfer for both Newtonian and viscoelastic solutions. Finally, the shape of the capillary bridge is varied by changing the liquid volume. Now, Newtonian and viscoelastic solutions exhibit opposite behaviors for the liquid transfer. These findings are discussed in terms of interfacial shape instability and gravitational drainage.