Liquid Two-phase Flow In Microchannels Research Articles

A Review on the Hydrodynamics of the Liquid–Liquid Two-Phase Flow in the Microchannels

The ability of small and microchannels to enhance mass transfer, reduce diffusional limitations, and create a safer process has made them the best choice for process intensification. This study provides a review of the available experimental and computational literature on the hydrodynamics of liquid–liquid two-phase flow in microscale, namely, flow regimes, pressure drop, and flow structure. When two immiscible liquids flow in small channels, various flow patterns can be observed, with the development of those patterns being influenced by several parameters, such as the geometry of the mixing junction and channel, the channel material, the fluids properties, and the orientation of the flow. As part of the flow patterns observations and measurement, the velocity fields and internal circulation reported in the literature were reviewed in the context of the microparticle velocimetry (μPIV) implementation. Also, relevant to our study is the literature outlining, pressure drop, and the available models that have been used to predict it. These models are discussed in detail with special reference to flow patterns and fluids viscosities. Moreover, the numerical studies results using computational fluid dynamics (CFD) have also been reviewed and discussed, in terms of the validity of the simulation and the different parameters affecting slug formation and flow patterns. Each section of the discussion also points out the gaps in the research and proposes future work that could further develop the technology under a review of liquid–liquid two-phase flow in microchannels.

A comprehensive review on liquid–liquid two-phase flow in microchannel: flow pattern and mass transfer

Liquid–liquid two-phase flow in microchannel is very common in micro-chemical and micro-biological system, etc. Deep understanding of the liquid–liquid two-phase flow mechanisms and mass transfer in microchannel can promote industrial applications significantly. To summarize the recent research progress on the liquid–liquid two-phase flow in microchannel, this paper collects research work about this topic, especially focusing on flow pattern and mass transfer. To begin with, flow patterns observed in various conditions are identified and factors which influence the flow patterns are analyzed. Then, mass transfer in liquid–liquid two-phase flow is discussed, especially the mass transfer during droplet flow, with both experiments and simulations. Furthermore, energy dissipation involved in liquid–liquid two-phase flow in microchannel is also briefly discussed. Finally, future needs are proposed for extending the researches on liquid–liquid two-phase flow and enlarging its application fields.

Heat transfer characteristics and field synergy analysis of gas–liquid two-phase flow in micro-channels

To meet the extreme cooling requirements in many high-tech fields, it is essential and meaningful to reveal the dominant factors of heat transfer performance of gas–liquid two-phase flow in micro-channel. In the present work, the flow and heat transfer characteristics of bubbly, slug and annular flow in circular micro-channel are numerically studied with the VOF model. Moreover, the field synergy principle is introduced to reveal the dominant mechanisms of heat transfer improvement under various flow patterns. As a result, the evolutions of local Nusselt number, volume fraction and near-wall average field synergy angle along the flow direction under various flow patterns are obtained. Moreover, the effects of gas and liquid flow rates are discussed on the average Nusselt numbers and average field synergy angle. The numerical results indicate that the heat transfer performance of gas–liquid two-phase flow is better than single-phase flow, and the slug flow owns the best heat transfer performance. It should be noted that the heat transfer enhancement of bubbly and slug flow is attributed to the decrease in field synergy angle in the boundary layer. By contrast, the liquid phase velocity near the wall plays a dominant role in heat transfer enhancement of annular flow. Furthermore, the heat transfer performance of gas–liquid two-phase flow is strongly influenced by the gas flow rate, and Reg= 24.5 is the optimal value to enhance heat transfer performance for the condition of Rel= 445. The results are expected to provide theoretical guidance for the design and optimization of the micro-channel heat exchangers.

Slug Formation Analysis of Liquid–Liquid Two-Phase Flow in T-Junction Microchannels

Slug flow is a common flow pattern in the liquid–liquid two-phase flow in microchannels. It is an ideal pattern for mass transfer enhancement. Many factors influence the slug formation such as the channel geometries (channel widths, channel depth), flow rates of the two phase, and physical properties. In this paper, in order to investigate the liquid–liquid two-phase slug formation in a T-junction microchannel quantitatively, the volume of fluid (VOF) method is adopted to simulate the whole slug formation process. With the validated model, the effects of the disperse phase channel width, channel depth, and two-phase flow rate ratio on slug formation frequency and slug size (slug volume and slug length) are analyzed with dimensionless parameters. Dimensionless parameters include the disperse-to-continuous phase channel width ratio, height-to-width ratio, and two-phase flow rate ratio. Results show that both the channel geometry and two-phase flow rate ratio have a significant influence on slug formation. Compared with the conventional slug formation stages, a new stage called the lag stage emerges when the disperse phase channel width decreases to half of the continuous phase channel width. When the channel depth decreases to one third of the continuous phase channel width, the flow patterns become unstable and vary with the two-phase flow rate ratio. Moreover, empirical correlations are proposed to predict the slug formation frequency. The correlation between slug formation frequency and slug volume is quantified.

Hysteretic heat transfer study of liquid–liquid two-phase flow in a T-junction microchannel

Liquid–liquid two-phase flow in microchannels is capable of boosting the heat removal rate in cooling processes. Formation of different two-phase flow patterns which affect the heat transfer rate is numerically investigated here in a T-junction containing water-oil flow. For this purpose, the finite element method (FEM) is applied to solve the unsteady two-phase Navier–Stokes equations along with the level set (LS) equation in order to capture the interface between phases. It is shown that the two-phase flow pattern in microchannels depends on the flow initial condition which causes hysteresis effect in two-phase flow. In this study, the hysteresis is observed in flow pattern and consequently in the heat transfer rate. The effect of wall contact angle on the hydrodynamics and heat transfer in the microchannel is investigated to gain useful insight into the hysteresis phenomenon. It is observed that the hysteresis is significant in super-hydrophilic microchannels, while it disappears at the contact angle of 75°. The effect of water to oil flow rate ratio (Qwat/Qoil) on the heat transfer is also studied. The flow rate ratio has a negligible effect on the Nusselt number (Nu) in the dripping regime, while the Nu decreases with an increase of Qwat/Qoil in the co-flow regime. The thickness of the oil film, velocity, and temperature distribution are studied in the co-flow regime. It is revealed that the normalized slip velocity reduces at higher values of Qwat/Qoil, which causes a reduction in the averaged Nu. In dripping regimes, higher flow rate ratios lead to a more frequent generation of droplet/slugs at a smaller size. The passage of the slugs or droplets increases the local Nu. Larger droplets generated at lower flow rate ratios cause a larger increase in the local Nu than smaller droplets. The temperature and velocity field around the droplets are also illustrated to investigate the heat transfer improvement. The generated vortex at the tip of the oil jet causes an increase in the velocity and Nu on the water side.

Mixing Efficiency Analysis on Droplet Formation Process in Microchannels by Numerical Methods

Liquid–liquid two-phase flow in microchannels has attracted much attention, due to the superiority of mass transfer enhancement. One of the biggest unresolved challenges is the low mixing efficiency at the microscale. Suitable mixing efficiency is important to promote the mass transfer of two-phase flow in microchannels. In this paper, the mixing efficiency in three junction configurations, including a cross-shaped junction, a cross-shaped T-junction, and a T-junction, is investigated by the volume of fluid (VOF) method coupled with user-defined scalar (UDS) model. All three junction configurations are designed with the same hydraulic diameter of 100 μm. Mixing components are distributed in the front and back parts of the droplet. The mixing efficiency in the droplet forming stage and the droplet moving stage are compared quantitatively. Results show that different junction configurations create very different mixing efficiencies, and the cross-shaped T-junction performs best, with relatively lower disperse phase fractions. However, with an increase of the disperse phase fraction, the cross-shaped junction is superior.

Investigation of Ring Waves in Gas–Liquid Two-Phase Flow in a Microchannel

Gas–liquid two-phase flow in minichannels and microchannels displays a unique flow pattern called ring film, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In this paper, the behaviors of ring waves, which correspond to ring films that appeared in ring film flow and disturbed ring film flow regions, have been investigated experimentally in gas–liquid two-phase flows of nitrogen-distilled water and nitrogen/30 wt% ethanol–water solution in a 150-μm-diameter silica tube to elucidate their generation mechanism and propagation behavior. In order to clarify the existence region and characteristics of ring waves, the flow patterns observed in a microchannel were investigated and flow pattern maps were made. Furthermore, the velocity of the ring wave was also investigated and compared with the gas slug velocity. In these velocity measurements, high-speed video images were taken at 6,000 frames per second and the formation of ring films and the relationship between the wave amplitude and velocity were determined. The results indicate an interfacial instability leading to the formation and growth of ring waves with both low and high wave amplitudes. The wave velocity is correlated to the wave amplitude, with the large amplitude waves moving much faster than the low amplitude waves. As a result, coalescence of large and low amplitude waves has been observed.

E-10 マイクロチャンネル内非ニュートン流体気液二相流の圧力損失

E-10 マイクロチャンネル内非ニュートン流体気液二相流の圧力損失

Bubble formation and breakup dynamics in microfluidic devices: A review

Bubbles are always encountered for gas–liquid two-phase flow in microchannels, and have potential applications in chemical engineering, polymer engineering, food engineering, and biological engineering. The bubble formation and breakup dynamics in microfluidic devices are reviewed in this article. The effects of the confinement of microchannels and the fluid flow on bubble formation and breakup dynamics are highlighted. The dynamical evolution of the gas–liquid interface during bubble formation and breakup in confined spaces is analyzed. The manipulation for the bubble formation and breakup in microfluidic devices is presented. The scaling laws for the bubble size in such devices are also reviewed. Finally, the key issues for the scaling-up of bubble generation in microfluidic devices are demonstrated.

Gas–liquid two-phase flow in microchannel at elevated pressure

The present study deals with pressure effects on the hydrodynamic characteristics of gas–liquid two phases within a T-junction microchannel. The operating pressure is in the range of 0.1∼5.0MPa. Nitrogen and de-ionized water are selected as the test fluids. The gas Weber numbers vary from 1.37×10−5 to 3.46 at atmospheric pressure and from 1.70×10−3 to 70.32 at elevated pressure, respectively. The liquid Weber numbers are in the range of 3.1×10−3∼4.9. The operating pressure plays an important role in gas–liquid two phases flow. Seven typical flow patterns such as bubbly flow, slug flow, unstable slug flow, parallel flow, slug-annular flow, annular flow, and churn flow are observed. Based on the force analysis of the gas and liquid phase in microchannel, the formation mechanisms of flow patterns are discussed at great length, and the flow pattern maps are divided into five regions using WeGS and WeLS as coordinates. These results are beneficial for future investigation to understand gas–liquid two-phase mass transfer and reaction characteristics in microchannel at elevated pressure.

Adiabatic air–water two-phase flow in circular microchannels

Gas–liquid two-phase flow in microchannels with hydraulic diameters of 100–500 μm exhibits drastically different flow behaviors from its counterpart in conventional macroscopic channels. Of particular interests are the two-phase flow patterns and the two-phase frictional pressure drop for given flow conditions in these microchannels. This paper presents an experimental study of the effects of channel size and superficial phasic velocity on the two-phase flow pattern and pressure drop of air–water mixture in circular microchannels with inner diameters of 100, 180 and 324 μm. Two-phase flow patterns were visualized using high-speed photographic technique. Four basic flow patterns, namely, bubbly flow, slug flow, ring flow and annular flow, were observed. The two-phase flow regime maps were constructed and the transition boundaries between different flow regimes identified. In an effort to unify the flow transition boundary in microchannels of different sizes, a new flow map was developed using the modified Weber numbers as the coordinates. The two-phase frictional pressure gradient in the microchannels was measured and the data were compared with predictions from the separated flow model, the homogeneous flow model and the flow pattern-based phenomenological models. Results show that the flow pattern-based models provide the best prediction of the two-phase pressure drop in the microchannels.

Numerical Study on Taylor Bubble Formation in a Micro-channel T-Junction Using VOF Method

A volume of fluid (VOF) method is used to study the immiscible gas–liquid two-phase flow in a microchannel T-junction, through which the accurate interface of the Taylor bubble flow inside the micro-channel is captured and compared with visualization experiment of Taylor bubbles’ generation inside a T-junction microfluidic chip. The numerical results are in good agreement with the experimental measurements, which confirms the validation of our model. Then the length of gas–liquid slugs and velocity distribution inside slugs at various conditions are investigated with the superficial velocity of gas and liquid phase ranging from 0.01 to 0.90 m/s, and capillary number ranging from 6.4 ×10 − 4 to 1.7 ×10 − 2. A comprehensive description of mechanism of bubbles’ break-off is achieved and the transition capillary number from squeezing regime to shearing regime is found around 5.8 ×10 − 3. Finally the influences of fluid viscosity, surface tension of the gas–liquid interface and the velocity of both gas and liquid phases on the characteristic of the gas–liquid two-phase flow in micro-channel are also discussed in detail.

Gas–liquid two-phase flow in micro-channels

Gas–liquid two-phase flow in micro-channels

Gas–liquid two-phase flow in microchannels: Part II: void fraction and pressure drop

Void fraction and two-phase frictional pressure drop in microchannels were experimentally investigated. Using air and water, experiments were conducted in transparent circular microchannels with 1.1 and 1.45 mm inner diameters and in microchannels with semi-triangular (triangular with one corner smoothed) cross-sections with hydraulic diameters 1.09 and 1.49 mm. Gas and liquid superficial velocities were varied in the 0.02–80 m/s and 0.02–8 m/s ranges, respectively, and void fractions were calculated by analyzing photographs taken from the test sections with circular cross-section. Measured void fractions were compared with several correlations. The homogeneous flow model provided the best prediction of the experimental void fractions in bubbly and slug flow patterns. The homogeneous flow model and all other tested empirical correlations significantly over predicted the void fractions in annular flow pattern, however. A one-dimensional model, based on the numerical solution of mass and momentum conservation equations was applied for the calculation of test section pressure drops, using various two-phase friction models. For bubbly and slug flow patterns, the two-phase friction factor based on homogeneous mixture assumption provided the best agreement with experimental data. For annular flow the homogeneous mixture model and other widely used correlations significantly over predicted the frictional pressure drop.