Developments on Wetting Effects in Microfluidic Slug Flow

Wetting effects form a dimension of fluid dynamics that becomes predominant, precisely controllable and possibly useful at the micro-scale. Microfluidic multiphase flow patterns, including size, shape and velocity of fluidic particles, and mass and heat transfer rates are affected by wetting properties of microchannel walls and surface tensions forces between fluid phases. The novelty of this field, coupled to difficulties in experimental design and measurements, means that literature results are scarce and scientific understanding is incomplete. Numerical methods developed recently have enabled a shortcut in obtaining results that can be perceived realistic, and that offer insight otherwise not possible. In this work the effect of the contact angle on gas-liquid two-phase flow slug formation in a microchannel T-junction was studied by numerical simulation. The contact angle, varied from 0 to 140 degrees, influenced the interaction of the gas and liquid phases with the channel wall, affecting the shape, size and velocity of the slugs. The visualisation of the cross-sectional area of gas slugs allowed for insight into the existence of liquid flow along rectangular microchannel corners, which was affected by the contact angle and determined the occurrence of velocity slip. The velocity profile within the gas slugs was also found to change as a function of contact angle, with hydrophilic channels inducing greater internal circulation, compared to greater channel wall contact in the case of hydrophobic channels. These effects play a role in heat and mass transfer from channels walls and highlight the value of numeral simulation in microfluidic design

Numerical Modeling and Experimental Investigation of Gas-Liquid Slug Formation in a Microchannel T-Junction

Gas-liquid two-phase flow in a microfluidic T-junction with nearly square microchannels of 113 μm hydraulic diameter was investigated experimentally and numerically. Air and water superficial velocities were 0.018–0.791 m/s and 0.042–0.757 m/s, respectively. Three-dimensional modeling was performed with computational fluid dynamics (CFD) software FLUENT and the volume-of-fluid (VOF) model. Slug flow (snapping/breaking/jetting) and stratified flow were observed experimentally. Numerically predicted void fraction followed a linear relationship with the homogeneous void fraction, while experimental values depended on the superficial velocity ratio UG/UL. Higher experimental velocity slip caused by gas inlet pressure build-up and oscillation caused deviation from numerical predictions. Velocity slip was found to depend on the cross-sectional area coverage of the gas slug, the formation of a liquid film and the presence of liquid at the channel corners. Numerical modeling was found to require improvement to treat the contact angle and contact line slip, and could benefit from the use of a dynamic boundary condition to simulate the compressible gas phase inlet reservoir

Gas-Liquid Slug Formation at a Rectangular Microchannel T-Junction: A CFD Benchmark Case

Computational fluid dynamics (CFD) is an important tool for development of microfluidic systems based on gas-liquid two-phase flow. The formation of Taylor slugs at microchannel T-junctions has been studied both experimentally and numerically, however discrepancies still exist because of difficulties in correctly representing experimental conditions and uncertainties in the physics controlling slug flow, such as contact line and velocity slip. In this paper detailed methods and results are described for the study of Santos and Kawaji [1] on the comparison of experimental results and numerical modeling. The system studied consisted of a rectangular microchannel T-junction nominally 100μm in hydraulic diameter, used to generate Taylor slugs from air-water perpendicular flow. The effect of flow rates on parameters such as slug length, velocity slip, void fraction and two-phase frictional pressure drop were studied. Numerical simulation was performed using FLUENT volume-of-fluid (VOF) model. It is proposed in this paper that this microfluidic problem be taken up by researchers in the field as a benchmark case to test other numeric codes in comparison to FLUENT on the prediction of micro-scale multiphase flow, and also to model in more detail the experimental system described to obtain greater accuracy in prediction of microfluidic slug formation

Process Intensification Routes for Mineral Carbonation

Mineral carbonation is a realistic route for capture and storage of carbon dioxide. The principal advantages of this approach are the chemical stability and storage safety of mineral carbonates, the opportunities for process integration available, and the potential for conversion of low-value materials into useful products. In this work the valorisation of alkaline waste materials from thermal processes by mineral carbonation utilizing intensified and integrated mineral carbonation routes is explored. Process intensification is the chemical engineering of the 21st century, and aims at providing the paradigm-shifting techniques that will revolutionize the industry. The combination of process intensification and process integration strategies has the potential to produce economically feasible and industrially acceptable carbonation technologies that can soon be implemented at large-scale, several examples of which are already proven at the laboratory scale and are herein discussed

Synthesis of Pure Aragonite by Sonochemical Mineral Carbonation.

The objective of this work was to promote the formation of the aragonite polymorph of calcium carbonate, which has some valuable applications in industry, via the mineral carbonation route. The combination of ultrasound with magnesium ions promoted the formation of pure aragonite crystals at optimum conditions. It was possible to synthesize high purity aragonite precipitates at temperatures ranging from 24 oC to 70 oC, with the resulting powders possessing varying particle size distributions (from sub-micron up to 20 μm) and crystal morphologies (from acicular needles to novel hubbard squash-like particles). Several process parameters were found to influence the produced calcium carbonate polymorph ratios (aragonite over calcite). Higher values of magnesium-to-calcium ratio, intermediate ultrasound amplitude (60%), continuous ultrasound application (100% cycle), introduction of ultrasound pre-breakage, lowering of the CO2 flow rate, and increase in the relative concentration (g/L Ca(OH)2), all promoted aragonite formation. A potential route for industrial production of this material has been identified via a fed-batch process, which effectively reutilizes magnesium chloride while maintaining high aragonite yield. The results presented herein are significantly superior to aragonite formation using only single promoting techniques, typically found in literature, and go beyond by focusing on pure (\u3e99%) aragonite formation

Geotechnical evaluation of rocks and soils in Catoca kimberlitic mining complex (Angola)

Landslides and rock sliding occur very frequently in the mining area of Catoca, located in Angola. Therefore, a physical/mechanical and geotechnical characterization of the massif and the rock matrix was carried out adopting the landslide classifications as proposed by Hutchinson and Varnes. The safety factor was applied based on the structural weakness coefficient (λ); resulting in 0.70 in surface rocks, sandstones and intraformational sands; 0.58 in oversaturated eluvial gneiss; 0.50 in cracked gneiss and 0.47 in the ore compound of weathered, moist kimberlitic porphyric and weathered porphyric kimberlite. These results indicate the low strength of the massif and the need to reformulate the activities in the mine and the construction of more stable slopes. It could also be observed that deformation of rocks in the slopes and the cuts in the Catoca mine is conditioned by the movement of underground water within the rock massif itself