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Gas dispersion in non-Newtonian fluids is a challenging task due to the formation of large cavities behind the impeller blades, which leads to the generation of very large bubbles. In this study, the effects of impeller speed, impeller type, pumping direction, and CMC concentration on the local and overall gas holdup inside a coaxial mixing tank comprised of two central impellers and an anchor were investigated through tomography, computational fluid dynamics (CFD), and response surface methodology (RSM). The results showed that an increase in the fluid apparent viscosity resulted in decreasing the gas holdup except for the pitched blade impeller in upward-pumping mode. Although the highest overall gas holdup was accomplished for the downward pumping and co-rotating mode, the local gas holdup data revealed a non-uniform distribution of gas by this configuration. The lowest gas dispersion efficiency was achieved by a system comprised of two Scaba impellers and an anchor.

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Volumetric mass transfer coefficient, k L a, bubble size, and interfacial area were investigated in an aerated coaxial mixing system with an aspect ratio of 1.25 furnished with two central impellers and an anchor. The simplified dynamic pressure method and a combination of the dynamic gas disengagement and electrical resistance tomography methods were utilized for measuring k L a and bubble sizes, respectively. Carboxymethyl cellulose (CMC) solutions were used as the power-law non-Newtonian fluids. New correlations for the prediction of k L a in aerated coaxial mixing systems containing non-Newtonian solutions were developed. The pitched blade impeller in downward pumping generated the highest mass transfer and interfacial area and smaller bubble sizes at all anchor speeds and concentrations of the CMC solution. The anchor speed had a positive impact on the oxygen mass transfer up to 20 rpm. Bubble stability increased while the interfacial area decreased with an increase in the fluid apparent viscosity.

a b s t r a c tIn this paper, Computational fluid dynamics (CFD) modeling of turbulent heat transfer behavior of Magnesium Oxide-water nanofluid in a circular tube was studied. The modeling was two dimensional under keε turbulence model. The base fluid was pure water and the volume fraction of nanoparticles in the base fluid was 0.0625%, 0.125%, 0.25%, 0.5% and 1%. The applied Reynolds number range was 3000 e19000. Three individual models including single phase, Volume of Fluid (VOF) and mixture were used. The results showed that the simulated data were in good agreement with the experimental ones available in the literature. According to the experimental work (literature) and simulation (this research), Nusselt number (Nu) increased with increasing the volume fraction of nanofluid. However friction factor of nanofluid increased but its effect was ignorable compared with the Nu on heat transfer increment. It was concluded that two phase models were more accurate than the others for heat transfer prediction particularly in the higher volume fractions of nanoparticle. The average deviation from experimental data for single phase model was about 11% whereas it was around 2% for two phase models.