Mechanical degradation of dilute polymer solutions under turbulent flow

Effect of Shear Flow on Drag Reducer Performance and Its Microscopic Working Mechanism

As the development of unconventional oil and gas resources goes deeper, the stimulation of reservoirs goes deeper year by year. Flow in longer wellbores poses a challenge to the stability of drag-reduction performance of fracturing fluid. However, at present we have limited understanding of the mechanism of drag-reduction damage caused by shear flow, especially the microscopic mechanism. Therefore, in this work, the variation pattern of drag reducer solution performance with shear rate has been analyzed by using a high precision loop flow drag test system. The test results show that there is a critical shear rate for the performance damage of the drag reducer solution, and high strength shear flow and cumulative shear flow time are the main factors leading to the performance degeneration of the drag reducer. Based on the nanometer granularity distributions, rheological properties and microscopic structures observed with a transmission electron microscope of drag reducer solutions subjected to shear flows of different velocities, it is confirmed that the damage to the microscopic structure of the solution is the main reason leading to its performance degeneration. The destruction of the microscopic structure causes the drag reducer solution to degrade in non-Newtonian characteristics, so it becomes poorer in its capability of reducing turbulent dissipation and drops in drag-reduction capability. This research can provide a reference for improving and optimizing drag-reduction capability of fracturing fluid.

Prediction and New Insight for the Drag Reduction of Turbulent Flow with Polymers and Its Degradation Mechanism

A physicochemical understanding of the mechanism of turbulent flow drag reduction with polymer and its degradation is of great interest from both science and industry perspectives. Although the correlation based on the Fourier series has been proposed to predict the drag reduction and its degradation, its physical meaning was not clear until now. This letter aims to clarify this issue. We develop a comprehensive model to predict the drag reduction and degradation of polymers in turbulent flow from a chemical thermodynamics and kinetics viewpoint. We demonstrate that the Fourier series employed to predict the drag reduction and its degradation is due to the viscoelastic property of drag-reducing polymer solution, and the phase angle in the model, in physical nature, represents the hysteresis of the polymer in turbulent flow. Besides, our new insight of drag reduction with flexible polymers can also explain why a maximum drag reduction in rotational flow appears before degradation happens.

Epidermal biopolysaccharides from plant seeds enable biodegradable turbulent drag reduction

The high cost of synthetic polymers has been a key impediment limiting the widespread adoption of polymer drag reduction techniques in large-scale engineering applications, such as marine drag reduction. To address consumable cost constraints, we investigate the use of high molar mass biopolysaccharides, present in the mucilaginous epidermis of plant seeds, as inexpensive drag reducers in large Reynolds number turbulent flows. Specifically, we study the aqueous mucilage extracted from flax seeds (Linum usitatissimum) and compare its drag reduction efficacy to that of poly(ethylene oxide) or PEO, a common synthetic polymer widely used as a drag reducing agent in aqueous flows. Macromolecular and rheological characterisation confirm the presence of high molar mass (≥2 MDa) polysaccharides in the extracted mucilage, with an acidic fraction comprising negatively charged chains. Frictional drag measurements, performed inside a bespoke Taylor-Couette apparatus, show that the as-extracted mucilage has comparable drag reduction performance under turbulent flow conditions as aqueous PEO solutions, while concurrently offering advantages in terms of raw material cost, availability, and bio-compatibility. Our results indicate that plant-sourced mucilage can potentially serve as a cost-effective and eco-friendly substitute for synthetic drag reducing polymers in large scale turbulent flow applications.

Effect of guar gum and salt concentrations on drag reduction and shear degradation properties of turbulent flow of water in a pipe

Concentrated solutions of guar gum in water (1000-3000ppm) with and without KCl salt (1000-4000ppm) were injected near the wall for a short period (2.5min) to investigate their effect on drag reduction in turbulent flow of water through a pipe (Re≈17000-45000). Relative to bulk solution, the concentrations of polymer and salt were 50-150ppm and 50-200ppm, respectively. A drag reduction of 71.45% was observed for 3000ppm of biopolymer without salt. Guar gum experienced mechanical degradation under high shear conditions and addition of KCl improved shear stability up to 47% (for Re≈45000). A polymer concentration of 3000ppm and salt concentration of 2000ppm in the injection fluid were found to be optimum for achieving the highest drag reduction with better shear stability. Results indicated that boundary layer injection shows better drag reduction ability than pre-mixed solutions.

Polymer mechanochemistry: techniques to generate molecular force via elongational flows

Long chain polymers have a unique ability to become highly extended in elongational flow fields. The forces developed along the backbone give rise to scission of the chains near their center. Recently, this unique property of polymers has been adopted to explore new chemical transformations by embedding structural elements into the backbone designed to undergo site-specific bond cleavage, termed mechanophores. Experimental techniques to generate elongational flow fields exist in a variety of different arrangements and have been used to study polymer mechanochemistry in solution. This tutorial review will discuss progress in the field of polymer mechanochemistry as well as survey the techniques used to generate elongational flow fields. Ultrasonication will be highlighted as the technique that has been widely adopted to screen mechanophore reactivity in solution.

Photoreversible micellar solution as a smart drag-reducing fluid for use in district heating/cooling systems

A photoresponsive micellar solution is developed as a promising working fluid for district heating/cooling systems (DHCs). It can be reversibly switched between a drag reduction (DR) mode and an efficient heat transfer (EHT) mode by light irradiation. The DR mode is advantageous during fluid transport, and the EHT mode is favored when the fluid passes through heat exchangers. This smart fluid is an aqueous solution of cationic surfactant oleyl bis(2-hydroxyethyl)methyl ammonium chloride (OHAC, 3.4 mM) and the sodium salt of 4-phenylazo benzoic acid (ACA, 2 mM). Initially, ACA is in a trans configuration and the OHAC/ACA solution is viscoelastic and exhibits DR (of up to 80% relative to pure water). At the same time, this solution is not effective for heat transfer. Upon UV irradiation, trans-ACA is converted to cis-ACA, and in turn, the solution is converted to its EHT mode (i.e., it loses its viscoelasticity and DR) but it now has a heat-transfer capability comparable to that of water. Subsequent irradiation with visible light reverts the fluid to its viscoelastic DR mode. The above property changes are connected to photoinduced changes in the nanostructure of the fluid. In the DR mode, the OHAC/trans-ACA molecules assemble into long threadlike micelles that impart viscoelasticity and DR capability to the fluid. Conversely, in the EHT mode the mixture of OHAC and cis-ACA forms much shorter cylindrical micelles that contribute to negligible viscoelasticity and effective heat transfer. These nanostructural changes are confirmed by cryo-transmission electron microscopy (cryo-TEM), and the photoisomerization of trans-ACA and cis-ACA is verified by (1)H NMR.

Universal scaling for polymer chain scission in turbulence

We report that previous polymer chain scission experiments in strong flows, long analyzed according to accepted laminar flow scission theories, were in fact affected by turbulence. We reconcile existing anomalies between theory and experiment with the hypothesis that the local stress at the Kolmogorov scale generates the molecular tension leading to polymer covalent bond breakage. The hypothesis yields a universal scaling for polymer scission in turbulent flows. This surprising reassessment of over 40 years of experimental data simplifies the theoretical picture of polymer dynamics leading to scission and allows control of scission in commercial polymers and genomic DNA.

Measurements and CFD Modeling of Drag-Reduction Effects

Summary

Computational-fluid-dynamics (CFD) modeling of drag reduction (DR) by polymer additives dissolved in hydrocarbon was carried out in pipe flows and in a rotating shear viscometer. A two-layer turbulence model was applied that described turbulence damping by the molecules of drag-reducing additives (DRAs) in the near-wall regions. The von Karman constant in the near-wall region was used as the model parameter. Extensive measurements of the DR effect for a rotating viscometer were performed at different Reynolds numbers, apparent molar masses, and DRA concentrations. The model with only one fit parameter was able to reproduce the experimental results in both pipe flows and the rotating viscometer. Experimental results were used in relating the model parameter to the relevant physical properties.

Introduction

The phenomenon of DR by low concentrations of long-chain polymers has been widely studied since Toms discovered the effect more than 50 years ago. In long oil pipelines, the pumping capacity can be significantly increased and pumping costs decreased by applying small amounts of DRA. A reduction of up to 70% in pressure loss in pipe flow has been achieved.

The DR effect has mostly been studied in pipe flow because it is the most important application.1–3 With pipe diameters larger than 40 mm, DR effects can also be measured accurately at high Reynolds numbers for high-viscosity (3·10–5 m2/s) oil. How- ever, pipeline construction may be rather expensive and the measurements time-consuming. The advantage of pipe-flow measurements is the large amount of general knowledge about turbulent flow in pipes.

In a spinning wheel consisting of a ring-shaped pipe and equipped with torque-measurement devices, pressure loss can be measured accurately with a small amount of fluid and without degradation losses. One possible drawback of this method is the short liquid-volume length that provides only partial DR effects.

Measurements in a rotating-viscometer-type apparatus have been carried out by, for example, Tong et al.,4 Cadot et al.,5 and Choi et al.6 It is easy to control the turbulent flow conditions in this type of experiment by varying the diameter of the rotating cylinder and the rotational speed.

We have carried out extensive measurements of DR effects in a viscometer equipped with a rotating cylinder in a baffled vessel. DR efficiency was measured with varying polymer molar masses and concentrations as well as rotational speed. The experimental setup is relatively easy to model with CFD.

Although a vast amount of research on DR has been done, the mechanism of the phenomenon is still poorly understood. Lumley,7 Thirumalai and Bhattacharjee,8 and Ryskin9 have presented possible explanations and theoretical analysis.

The commonly approved theory of DR is based on polymer molecules stretching outside the viscous sublayer because of shear forces and the subsequent enhanced damping of the transverse turbulent fluctuations close to the pipe wall.

The polymer chain elongates in an extensional flow if the strain rate exceeds a critical value. The DR effects are related to the polymers' elongation and relaxation properties and depend on polymer characteristics, such as apparent molar mass, molar mass distribution, and chain structure, as well as polymer concentration and shear stresses. Furthermore, fully stretched polymer chains can be degraded by high shear stresses, leading to losses in DR effects.

CFD modeling of the DR effect in turbulent flow is challenging. Attempts to model the DR effect have been made by Hassid and Poreh10 with a one-equation turbulence model, Poreh and Hassid 11 and Patterson et al.12 with a modified k-e model, and Sureshkumar et al.13 with direct numerical simulation.

Turbulence damping should be based on anisotropic turbulence models in which the damping effects are described, in each direction, by polymer DR properties. These models, however, are computationally expensive and, therefore, not suitable for engineering applications.

In this paper, the effect of drag reducers on turbulence has been modeled with an isotropic two-layer turbulence model that employs the one-equation model of Hassid and Poreh10 in the near-wall region and the standard k -e model in the fully turbulent regions. The results of measurements with the rotating viscometer were used to fit the model parameters, and the validity of the model was tested in pipe flows. The developed DR model was incorporated into the commercial CFD code STAR-CD14 for simulating local DR effects in arbitrary flow conditions.

Measurements of DR and Degradation Effects in Hydrocarbon Solvent

Description of Measuring Device.

In this study, the flow conditions necessary to elongate the polymer, the DR efficiency of polymers of various apparent molar masses, and their degradation kinetics have been measured with the rotating shear viscometer shown in Fig. 1.

The vessel is equipped with a rotating cylinder 99 mm in diameter and baffles around the vessel and near the bottom. The rotation speed of the cylinder can be adjusted within the 0- to 3,200-rpm range. The vessel stands on two bottom discs with an annular ball bearing in between, allowing frictionless movement of the upper disc. DR effects can be exactly measured under turbulent conditions by measuring the torque force of the bottom plate against the balance sensor.

Influence of chain architecture on the mechanochemical degradation of macromolecules

We detail here studies into the nature of mechanochemical degradation of macromolecules, effected by means of ultrasonic (US) irradiation. Specifically, we have investigated the effect of long-chain branching (LCB), in the star configuration, on the degradation mechanism of polystyrene dissolved in DMAc/LiCl. The information obtained from size-exclusion chromatography with triple detection (refractometry, viscometry, multi-angle light scattering) shows that the degradation mechanism of stars is radically different from that of linear polymers. Whereas in the latter, from a macromolecular standpoint, it is merely necessary for the molar mass to be greater than some limiting value (M(lim)), in the former both molar mass and structural factors affect ultrasonic degradation. We have examined the effects of arm number and of arm molar mass on star degradation, and propose the concept of a spanning molar mass (M(span) approximately 2M(arm)) such that, even in the event that M(arm)<M(lim) for the stars, mechanochemical degradation may nonetheless proceed if M(span)>M(lim). This mechanism has been extended to other types of architecture (e.g., H-branched, dendritic), where it is proposed that a continuous path must exist with M(path)>M(lim) for degradation to occur. Examination of the different radii afforded by viscometric and light-scattering detection gives insight into the solution thermodynamics and conformation of the stars with differing arm number and molar mass and of the effects of insonation on macromolecular structure.