Slug flow in microchannels: Numerical simulation and applications

Numerical Modelling of Mixing in a Microfluidic Droplet Using a Two-Phase Moving Frame of Reference Approach

Droplets generated in microfluidic channels are effective self-contained micromixers and micro-reactors for use in a multiplicity of chemical synthesis and bioanalytical applications. Droplet microfluidic systems have the ability to generate multitudes of droplets with well-defined reagent volumes and narrow size distributions, providing a means for the replication of mixing within each droplet and thus the scaling of processes. Numerical modelling using computational fluid dynamics (CFD) is a useful technique for analysing and understanding the internal mixing in microfluidic droplets. We present and demonstrate a CFD method for modelling and simulating mixing between two species within a droplet travelling in straight microchannel, using a two-phase moving frame of reference approach. Finite element and level set methods were utilised to solve the equations governing the coupled physics between two-phase flow and mass transport of the chemical species. This approach had not been previously demonstrated for the problem of mixing in droplet microfluidics and requires less computational resources compared to the conventional fixed frame of reference approach. The key conclusions of this work are: (1) a limitation of this method exists for flow conditions where the droplet mobility approaches unity, due to the moving wall boundary condition, which results in an untenable solution under those conditions; (2) the efficiency of the mixing declines as the length of the droplet or plug increases; (3) the initial orientation of the droplet influences the mixing and the transverse orientation provides better mixing performance than the axial orientation and; (4) the recirculation inside the droplet depends on the superficial velocity and the viscosity ratio.

Numerical study on gas–liquid two-phase flow and mass transfer in a microchannel

A numerical study of the gas–liquid two-phase flow and mass transfer in a square microchannel with a T-junction is carried out in this work. Through numerical simulation methods, the flow patterns of bubble flow, slug flow and annular flow are determined. By proposing a new flow pattern conversion relationship with different media and different speeds, 100 sets of CO2-water flow patterns and 100 sets of CO2-ethanol flow patterns are obtained. The effects of surface tension on flow pattern, bubble length and liquid plug length are studied. The pressure distribution and pressure drop are analyzed, and mass transfer is obtained through slug flow simulation, and the influencing factors of gas–liquid mass transfer are studied. The results show that the effect of surface tension on the length of the bubble and the length of the liquid plug is completely opposite, the pressure distribution is stepped, and the pressure drop increases with the increase of the gas–liquid velocity. In addition, it was found that the volumetric mass transfer coefficients of the bubble cap and the liquid film gradually decreased with time, and eventually stabilized. The increase in bubble velocity accelerates the mass transfer rate, while the increase in unit cell length slows the mass transfer rate. However, the influence of film thickness and liquid film length on mass transfer varies with time.

Investigation of Hydrodynamic and Heat Transfer Characteristics of Gas-liquid Taylor flow in Square Microchannel

Heat transfer and flow characteristics under air-water Taylor flow in a square microchannel with T-junction were investigated in this work. Different hydraulic diameters of models were discussed numerically by VOF method. Flow patterns such as bubbly flow, slug flow, annular flow and churn flow were identified by both numerical simulation and experimental methods. Simulation results including bubble formation process, bubble length, bubble velocity, void fraction and heat transfer fit well with literature data. The pressure differential of two sides in gas phase played an important role in bubble development. The gas and liquid superficial velocities were found to have a significant impact on bubble behavior. And the higher liquid viscosity would promote higher bubble velocity, also enhance heat transfer, but weaken the void fraction. The results showed a tiny but not ignorable effect of geometric dimensioning on bubble and liquid slug lengths. An appropriate correlation was proposed to estimate bubble length, and the deviation was −10 ~ + 15 %. By using moving frame of reference technique, the internal circulations inside the moving slugs were displayed more clearly.