Mixing viscoelastic fluid flows at the micro scale
Lead Research Organisation:
University of Strathclyde
Department Name: Mechanical and Aerospace Engineering
Abstract
Applications such as enhanced oil/gas recovery, coatings and encapsulation involve multiphase flows where rheological behaviour and interfacial dynamics plays a major part on the outcomes. Many of these processes require the addition of polymers and surfactants, to reduce friction, promote emulsification or improve wetting, among others. These additives, even when added in extremely small quantities, have a significant impact on the fluid properties, particularly in terms of its viscoelasticity and surface tension, with consequent impact on the flow behaviour.
For Newtonian fluids, flows at low Reynolds numbers (Re) are dominated by diffusion and tend to remain laminar. As a consequence mixing of two immiscible fluids, e.g. to create an emulsion or to displace one of the phases such as in enhanced oil recovery, is inherently difficult to achieve efficiently for very viscous systems or flows at small scales. However, when a small amount of high molecular-weight polymer is added to water (or any Newtonian solvent), making the solution viscoelastic, then fluid flows at arbitrarily small values of Re have been shown to exhibit instabilities and "turbulent-like" characteristics and turbulent flows mix things much more efficiently. It has been shown that small length-scales, in which surface effects are enhanced, accentuate the role of elasticity to levels far beyond those typical at conventional scales.
The purpose of this project is to fundamentally investigate the mixing effectiveness of this "elastic" turbulence for single and multiphase systems and, in particular, for immiscible two liquid systems.
We plan to make use of canonical flows and simple microfluidic flows, which provide a combination of shear and extension, to understand under well-controlled conditions a range of phenomena arising in much more complicated flows and as such address a number of engineering applications: flow through porous media, removal of ganglia in oil/recovery applications, creation of emulsions under adverse conditions.
For Newtonian fluids, flows at low Reynolds numbers (Re) are dominated by diffusion and tend to remain laminar. As a consequence mixing of two immiscible fluids, e.g. to create an emulsion or to displace one of the phases such as in enhanced oil recovery, is inherently difficult to achieve efficiently for very viscous systems or flows at small scales. However, when a small amount of high molecular-weight polymer is added to water (or any Newtonian solvent), making the solution viscoelastic, then fluid flows at arbitrarily small values of Re have been shown to exhibit instabilities and "turbulent-like" characteristics and turbulent flows mix things much more efficiently. It has been shown that small length-scales, in which surface effects are enhanced, accentuate the role of elasticity to levels far beyond those typical at conventional scales.
The purpose of this project is to fundamentally investigate the mixing effectiveness of this "elastic" turbulence for single and multiphase systems and, in particular, for immiscible two liquid systems.
We plan to make use of canonical flows and simple microfluidic flows, which provide a combination of shear and extension, to understand under well-controlled conditions a range of phenomena arising in much more complicated flows and as such address a number of engineering applications: flow through porous media, removal of ganglia in oil/recovery applications, creation of emulsions under adverse conditions.
Organisations
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509760/1 | 01/10/2016 | 30/09/2021 | |||
| 1960606 | Studentship | EP/N509760/1 | 01/10/2017 | 31/07/2021 | Gemma Houston |
| Description | The main purpose of the work is to gain a fundamental insight into the impact of elastic instabilities that arise in viscoelastic fluid flows at the microscale and assess the mixing effectiveness of elastic turbulence in multi-fluid flow systems (where two miscible or immiscible liquids are present) and exploit this understanding for enhancing potential engineering applications. So far in this PhD project we have been using experimental techniques to characterise relevant flows in different microfluidic devices (flow focusing and cross-slot devices) under a range of varying flow conditions, for which the main results are summarised below. A. The extensional flow-focusing device is commonly used in microfluidics to promote mixing and operates by having two opposing lateral streams that shape a third inlet, or dispersed, stream. • Single phase viscoelastic experimental results are in qualitative agreement with those of Oliveira et al (2009) demonstrating that at low velocities the flow is symmetrical relative to the centreline. • Increasing the velocities results in two elastic instability transitions: a first transition to steady asymmetric flow and second transition to time-dependent flow. • For all Newtonian cases the fluid flow remained symmetric. This work will be continued introducing other fluids, alongside the viscoelastic fluid, to determine the effects on the flow patterns and the onset of the elastic instabilities. B. The cross-slot geometry consists of perpendicular, bisecting rectangular channels with two sets of opposing inlets and outlets. A number of experiments were conducted to study the interface between two different Newtonian fluids. • As the viscosity of one fluid is increased, the area that the fluid occupies increases changing the interface location. In addition, as the viscosity ratio is increased a well-defined 'dimple' appears in the centre of the channel. The location of the interface, for various viscosity ratios and aspect ratios, was studied and compared to both analytical solutions and numerical simulations. The results are in quantitative agreement. The size of the 'dimple' in the centre of the geometry is also a function of the aspect ratio parameter. • The experiments were repeated using immiscible fluids, enhancing interfacial tension, which was shown to have a large impact on the size of the 'dimple' produced for a constant Capillary Number (representing the relative effect of viscous drag forces versus surface tension forces acting across an interface between the two immiscible liquids). • Further experiments were conducted to visualise the effect of changing the Capillary number. This was altered by changing the velocity of the inlet fluids. Decreasing the capillary number, for a constant viscosity ratio and interfacial tension, reduces the size of the dimple until it reaches a point where the interface becomes straight. From simulations we again find qualitative agreement. • Finally, when a viscoelastic fluid is used in this configuration and an elastic instability has been triggered, by reducing the capillary number (i.e. increasing the interfacial tension) the interface of the two fluids becomes flatter and eventually regains symmetry. Conferences presented at: - BSR Mid Winter meeting 2018 - BSR Mid Winter meeting 2019 |
| Exploitation Route | • The flow focussing experimental work discussed in the above section was previously presented in 2018. From this a collaboration with colleagues at the University of Liverpool to produce the results discussed in the cross-slot configuration. This work is a combination of experimental, analytical and numerical simulations. • This work is part of an ongoing PhD, which can lead to be the base for future undergraduate student or postgraduate projects. Furthermore, the experimental and post-processing methodologies developed will be available to other students in the department and likely to be made available more widely at a later stage. • Related industrial applications include enhanced oil/gas recovery. Further research into this area is vital to improve the efficiency of existing oil extraction methods and procedures. The work will give an insight into how viscoelasticity or complex fluids could aid separation or displacement of a particular phase in multiphase flows. In future, this research could result in the elongation of existing oil wells reducing the creation of new oil wells. |
| Sectors | Education,Environment,Manufacturing, including Industrial Biotechology |