Carbomer microgels as model yield-stress fluids

The review presents current research results for Carbopol-based microgels as yield-stress materials, covering three aspects: chemical, physical and rheological. Such a joint three-aspect study has no analog in the literature. The chemical aspects of Carbopol polymers are presented in terms of a cross-linking polymerization of acrylic acid, their molecular structure, microgel formulation, polyacid dissociation and neutralization, osmotic pressure and associated immense microgel swelling. The physical characterization is focused on models of the shear-induced solid-to-liquid transition of microgels, which are formed of mesoscopic particles typical for soft matter materials. Models that describe interparticle effects are presented to explain the energy states of microgel particles at the mesoscale of scrutiny. Typical representatives of the models utilize attributes of jamming dispersions, micromechanical and polyelectrolyte reactions. Selected relationships that result from the models, such as scaling rules and nondimensional flow characteristics are also presented. The rheological part presents the discussion of problems of yield stress in 2D and 3D deformations, appearance and magnitude of the wall slip. The theory and characteristics of Carbopol microgel deformation in rotational rheometers are presented with graphs for the steady-state measurements, stress-controlled oscillation and two types of transient shear deformation. The review is concluded with suggestions for future research.

Numerical investigation of hydrodynamics and heat transfer for Bingham-pseudoplastic fluids in an industrial coiled stirred tank

Computational fluid dynamics (CFD) was used to investigate the influence of complex rheological properties for Bingham-pseudoplastic fluids on hydrodynamics and heat transfer performance in an industrial polymerization coiled agitated reactor. The fluid rheology was described by the Herschel–Bulkley rheological model. The power consumption obtained by CFD simulation was in good agreement with the plant data. The relationship between the Metzner–Otto constant and power-law index for dual axial flow impellers was investigated. The hydrodynamics strongly depends on the rheological parameters and rotational speed. The flow domain is composed of two parts: the cavern around the impellers, and the stagnant zone adjacent to helical coils and tank wall which resulting in poor mixing and heat transfer performance. With the increase of rotational speed, the stagnant zone could be effectively eliminated, while the heat transfer performance could be significantly improved.

A new model of cavern diameter based on a validated CFD study on stirring of a highly shear-thinning fluid

Results of numerical simulations of momentum transfer for a highly shear-thinning fluid (0.2% Carbopol) in a stirred tank equipped with a Prochem Maxflo T type impeller are presented. The simulation results were validated using LDA data and both tangential and axial force measurements in the laminar and early transitional flow range. A good agreement between the predicted and experimental results of the local fluid velocity components was found. From the predicted and experimental values of both tangential and axial forces, the power number, Po, and thrust number, Th, were also calculated. Values of the absolute relative deviations were below 4.0 and 10.5%, respectively, for Po and Th, which confirms a satisfactory agreement with experiments. An intensive mixing zone, known as cavern, was observed near the impeller. In this zone, the local values of fluid velocity, strain rate, Metzner–Otto coefficient, shear stress and intensity of energy dissipation were all characterized by strong variability. Based on the results of experimental study a new model using non-dimensional impeller force number was proposed to predict the cavern diameter. Comparative numerical simulations were also carried out for a Newtonian fluid (water) and their results were similarly well verified using LDA measurements, as well as experimental power number values.

Numerical study of fluid flow and power consumption in a stirred vessel with a Scaba 6SRGT impeller

The present work deals with agitation of non-Newtonian fluids in a stirred vessel by Scaba impellers. A commercial CFD package (CFX 12.0) was used to solve the 3D hydrodynamics and to characterise at every point flow patterns especially in the region swept by the impeller. A shear thinning fluid with yield stress was modelled. The influence of agitator speed, impeller location and blade size on the fluid flow and power consumption was investigated. The results obtained are compared with available experimental data and a good agreement is observed. It was found that an increase in blade size is beneficial to enlargement of the well stirred region, but that results in an increased power consumption. A short distance between the impeller and the tank walls limits the flow around the agitator and yields higher power consumption. Thus, the precise middle of the tank is the most appropriate position for this kind of impeller.