Hue, Seng Hoe; (2024) The investigation of pure viscoelastic fluids in a liquid-liquid displacement system. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

In this thesis the displacement flow of an organic Newtonian fluid by pure viscoelastic aqueous solutions is investigated in small channels. Displacement is commonly encountered in many industrial applications, from cleaning and decontamination to enhanced oil recovery. Polymeric fluids of polyethylene oxide (PEO) that exhibited viscoelastic properties but constant shear viscosity, also known as Boger fluids, were used to displace an immiscible organic liquid (mixture of silicone oils) that was initially present in the channels. The viscosity of the displaced phase was higher than the viscosity of the displacing one, making the displacement front unstable and leaving behind a film of the displaced phase along the channel wall. Different concentrations of PEO, between 500 to 1000 ppm were added to a Newtonian aqueous solution of polyethylene glycol and zinc chloride to vary the elastic relaxation time. This also increased the viscosity of the displacing phase from 19.68 cP up to 29.31 cP. To keep the viscosity ratio between the two immiscible liquids constant at 0.82, silicone oils of different viscosities were blended together. Other fluid properties such as the interfacial tension and density ratios were kept constant. The elastic relaxation time of the Boger fluid ranged from 0.063 s to 0.134 s for the lowest and highest concentration of PEO, respectively. Using optical imaging techniques such as brightfield shadowgraphy and particle image velocimetry (PIV), the displacement properties were studied in a circular 200 µm internal diameter microchannel with bends, and a straight capillary with an internal diameter of 500 µm. Flow rates of the displacing phase were varied over an order of magnitude, from 0.03 ml/min to 0.35 ml/min. The displacement front was first investigated. Between the Newtonian and the Boger displacing phase, the thickness of the film left behind was up to 7 % lower for the Boger displacing phase compared to the Newtonian one at high displacing flow rates. An empirical correlation for the film thickness which included the capillary and Weissenberg numbers was developed. It was also found that the interface after the displacement front was unstable with waves developing continuously along it. A stability analysis for the Newtonian phase confirmed this behaviour. For the Newtonian case, the top and bottom interfacial instabilities had both axisymmetric and asymmetric shapes with no periodic repetitions, while for the Boger displacing phase the instability was asymmetric and periodic. The periodicity of the interface was attributed to the elastic wave of the Boger displacing phase. When the displacement front reached a bend in the microchannel, additional instabilities from the backwaves were detected upstream. Using a straight capillary to eliminate bends in the configuration, the properties of the interfacial instability were further investigated. Wave properties such as amplitude, frequency, speed and phase difference were measured for three Boger fluids in the displacing phase with different elastic relaxation times across different flow rates, between 0.025 ml/min and 0.3 ml/min. The elastic Mach number (Ma), a dimensionless parameter defined as the velocity scale ratio between the flow and the elastic wave was introduced and successfully correlated all interfacial wave properties. Both the frequency and speed of the waves increased monotonically with the Ma. However, the amplitude of the instability showed an increase up to Ma=0.5, before decreasing again. This was attributed to the transition of flow towards elastic turbulence. The proposed empirical correlations were used to build a simple wave model, which was then extended into three-dimensions. A displacing core which resembled a helical screw was produced, and this aligned with observations from literature. To verify the transition to elastic turbulence when Ma>0.5, particle image velocimetry experiments were conducted on the displacing phase at two flow rates, 0.1 ml/min and 0.2 ml/min. These corresponded to the Ma of 0.43 and 0.86 respectively, which were below and above the critical value of 0.5. Velocity profile measurements showed slight deformations when they were close to the unstable interface. Meanwhile, velocity fluctuations revealed an increase in the turbulent intensity in the case of Ma >0.5. When the displacing phase flow rate was below Ma=0.5, the turbulent intensity was close to zero. Above this critical value however, the turbulent intensity was significantly higher. The profiles of the turbulent intensities also showed larger values when they were close to the deformed interface. Reynolds stresses were close to zero for the low Ma case, while they had large values for the high Ma case. By subtracting the interfacial wave velocity from the velocities in the flow field, a circulation pattern was observed below the interfacial instability. The circulation pattern spanned the entire thickness of the displacing core and travelled at the same velocity as the interfacial wave. The results indicate that the use of viscoelastic fluids in displacement can enhance displacement efficiencies. They demonstrate however, that the flow behaviour is complex and is affected by the channel geometry, the fluid properties and the flow properties when elastic turbulence is present. Further studies are therefore needed to include different viscoelastic fluids, channel geometries and flow regimes to fully characterise and to be able to control displacement in channels.

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