(402k) CFD Simulation and Analysis of Gas-Liquid Segmented Flow with Mass Transfer in Microfluidic Devices: Case Study of CO2 Gas Solubility and Diffusivity Measurement in Aqueous NaCl Solution

The microfluidic gas-liquid segmented flow has the potential for measurement of thermodynamic properties such as solubility, diffusivity, and reaction rates. In multiphase systems, mass transfer resistance across the phase boundaries controls the mass transfer rates and hinders determination of thermodynamic properties. Microfluidic gas-liquid segmented flow minimizes this problem by increasing the specific interfacial area and improving mass transfer efficiency across phase boundaries [1]. Moreover, microfluidic devices significantly reduce measurement equipment size, experimental resource requirement, and measurement time. Using both experiment and numerical simulation, we study the solubility and diffusivity of CO₂ in the aqueous NaCl solution. In the experimental work, we developed long serpentine channel microfluidic devices that may be used at various CO₂ pressures and liquid solution flow rates to achieve segmented flow conditions. CO₂ bubble size is measured (at 293K) along the length of the channel to determine CO₂ solubility and diffusivity. To accurately measure these properties, we require pressure profile along the length of the channel which can be accurately calculated through CFD simulations. In simulation work, we use CFD to simulate and analyze gas-liquid segmented flow in the same microfluidic designed device. CFD analysis allows precise examination of the pressure profile along the channel length, flow behavior and localized concentration field. These simulations use Volume of Fluid method to address gas-liquid phases paired with momentum balance and continuity equations to capture the flow behavior [2]. Mass transfer between vapor and liquid phases is addressed by the diffusion equation, and application of Henry’s law at the phase boundaries is used to analyze temporal and spatial variations in CO₂ concentration. We show CFD simulation results for pressure profile and use it to calculate the CO₂ solubility and diffusivity. We also show that CFD can simulate and predict the solubility of CO₂ in different salt solutions and different geometry devices which will help in reducing the experimental effort.

References

[1] Günther, A. and Jensen, K.F. “Multiphase microfluidics: from flow characteristics to chemical and materials synthesis.” Lab on a Chip, 6(12), 1487-1503, 2006.

[2] Hirt, C.W. and Nichols, B.D. “Volume of fluid (VOF) method for the dynamics of free boundaries.” Journal of Computational Physics, 39(1), 201-225, 1981.