Browsing by Subject "Taylor-Couette flow"

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    Dynamics of polyelectrolyte solutions and oil-in-water emulsions in Taylor-Couette flows
    (2022-07) Panwar, Vishal
    The increasing water scarcity and environmental pollution concerns continue to raise the need for enhanced understanding of water treatment methods. Removal of small, stabilized insoluble contaminates, such as suspended particulate and emulsified droplets, are a key part of drinking and waste water treatment needs. In this thesis, two main areas are presented which examine the treatment of stabilized particulate and droplets from the multiphase systems, using the well-controlled hydrodynamic conditions of Taylor-Couette (TC) cell. (1) First, flocculation of particulate contaminants with polyelectrolytes in Taylor-Couette flows is explored. Based on our recent work in (Metaxas et al. 2021), the effects of transient physicochemical and hydrodynamic conditions on floc growth and floc microstructure are determined with ionic strength and polyelectrolyte molecular weight and reviewed in Chapter 1. Following on to this work, Chapter 2 will focus on systematic exploration of Taylor-Couette flow states and conditions caused by the non-Newtonian nature of the polyelectrolytes used in the flocculation process. As a part of these studies, the ionic strength is also varied to modify the elasticity and degree of shear thinning of the solution and the conformation (shape) of the polymer chains. (2) Second, shear-induced coalescence of emulsified oil droplets is studied, also using TC flows. Chapter 3 and 4 explores the stability and dispersion dynamics, respectively, of complex oily bilgewater emulsions using direct radial injection of the emulsion into the flow field. The effect of salinity is also explored in this focus area, as the salt screening of the ionic surfactants used to stabilize the emulsified droplets has a key influence on the droplet coalescence during flow. In the first area, the focus is specifically related to the flocculation of charged polyelectrolytes (cationic polyacrylamide) with bentonite clay in Taylor-Couette flows. Metaxas and Panwar et al. 2021 explored the radial injection of polymeric fluid into the particle laden flow in the custom-built TC cell to investigate in-situ floc nucleation and growth. In this thesis, Chapter 2 extends the work to include a study of changing rheological polyelectrolyte solution properties to non-Newtonian, giving rise to new TC flow vortex states and structure. This can impact the inter-vortex mass transfer of the flocculant in the modified flow and change the flocculation results. The fundamental work is explored here to understand the polyelectrolyte solution behavior in TC flows with changing rheological properties. The effects of changing polymer chain conformation of polyelectrolyte due to changing ionic strength on flow states and transitions have been investigated in TC cell. It is found and discussed in Chapter 2 that at lower ionic strength, expanded polymer conformation with higher elasticity and greater degree of shear thinning, the flow states and critical transition points are affected more by the elasto-inertial instability and ability of the solution to shear thin readily. New flow states are discovered like elongated jellyfish wavy vortex (EJWV) flow, distorted and turbulent EJWV flow and helical wavy vortex flow. At higher ionic strength, the charge around the polymer is screened, and the flow behavior is impacted by the more coiled, flexible nature of the polymer chains. New flow states, such as alternating standing wavy vortex flow, longitudinal wavy vortex flow, expanded wavy vortex flow, and over-expanded wavy vortex flow are found. Additionally, flow maps are constructed from the ramping experiments to mark the types of flow states and the critical transition points relative to the stationary, co-, and counter-rotation of the outer cylinder for each of the four different ionic strength solutions. Finally, in the future outlooks presented in Chapter 5, the coagulation and removal of microplastics is explored using a sustainable bio-flocculant like Moringa oleifera where surface properties and charge have a significant impact on the results. In the second area, oily bilgewater emulsion stability and dispersion are studied in Taylor-Couette flows. Chapter 3 discusses the effects of shearing flow and salinity on the stability of complex bilgewater emulsions. Mixing intensity can influence the droplet size distribution and indirectly impact emulsion stability as larger droplet are more susceptible to creaming out effect. It is found that shearing at lower order TC flow states like laminar wavy vortex and turbulent wavy vortex resulted in increase in coalescence but higher order flow states like turbulent Taylor vortex lead to droplet breakup. This non-monotonic effect of shear on the droplet size distribution is consistent with other previous studies. The presence of salt resulted in destabilizing of the emulsion. The effects of shearing are enhanced for both lower order flow states with more droplet coalescence and higher order flow states with more droplet breakup in the presence of salt. Chapter 4 focuses on the dispersion and mixing dynamics of bilgewater emulsion in a TC cell with radial injection of a concentrated emulsion into the well-defined flow field. The dispersion coefficient is found to be increasing with Reynolds number. The impact of dispersion coefficient and observation time is also explored on the droplet size distribution of the bilgewater emulsion. It is observed that in the initial mixing stage the effect of dispersion is more dominant on the droplet size distribution, where higher dispersion rates results in more uniform distribution of oil into the solution. While at the final stages of mixing, shearing effects like droplet coalescence/breakup based on shear rate also observed in Chapter 3 becomes dominant, and the same non-monotonic shear behavior is observed. Impact of salinity are also determined, where higher dispersion coefficients are found compared to the no salt case. The higher dispersion coefficients lead to more uniform oil distribution among the droplet sizes, and destabilizing effect of salt are visible in the later mixing stage. Overall, this thesis seeks to provide additional understanding of treatment process of complex multiphase systems, under a variety of physicochemical and flow conditions. The stability and properties of the solutions are modified using changes in ionic strength, to screen charges of the polymer and surfactants and to modify the stability of the particulate and droplets. The flow conditions are modified and controlled using Taylor-Couette flow, to access a wide variety of flow states by changing the rotation rate and direction of the inner and outer cylinders. Understanding the changes in rheological properties of polyelectrolyte used in flocculation on the Taylor-Couette flows can help make the flocculation process more efficient by the optimal selection of flow states for mixing and mass transfer of polymer. Likewise, the improved understanding of bilgewater stability parameters like shearing, observation time, salinity and dispersion rate mass transfer can improve the prediction and control of droplet size distribution, which ultimately can improve the treatment process of bilgewater.