Two-phase Flow In Microchannels Research Articles

Distribution of gas-liquid two-phase flow in parallel microchannels with the splitting of the liquid feed

The instability of gas-liquid two-phase flow caused by bubble collision downstream in parallel microbranching T-junctions is discovered, and a strategy of splitting liquid feed stream in two for the prevention of bubble collision is proposed. The two main flow regimes of breakup and repartition of bubbles are observed and characterized under the slug flow, and the phase distribution among five branches are studied. It is proved that, for the breakup regime, splitting liquid feed stream with appropriate proportion enhances the uniformity of the bubble size at the branches, eliminating the instability caused by bubble collision, and the phase distribution is improved and more stable. The influence of the flow rate and viscosity of fluids on the gas-liquid two-phase distribution is studied. A proportional relationship is given to predict bubble volumes and to reveal the gas distribution among branches with liquid feed stream split.

Linear and Nonlinear Stability Analysis in Microfluidic Systems

In this article we use analytical and numerical modeling to describe parallel viscous two-phase flows in microchannels. The focus is on idealized two-dimensional geometries, with a view to validating the various methodologi... | Find, read and cite all the research you need on Tech Science Press

Miniaturization as One of the Paths to Process Intensification in Chemical Engineering

Miniaturization of chemical equipment is one of process intensification methods. The main advantage of microstructured chemical equipment originates from the small cross sizes of microchannels (2–3 mm or less, usually less than 1 mm). Thus, the capillary and viscous forces dominate over the inertial and gravitational forces, leading to the formation of structured flows. The microsized reactors, mixers, heat exchangers, extractors, etc., have the following significant advantages: (1) lower mass and heat losses during processing; (2) better temperature control of processes; (3) shorter diffusional paths for molecules and ions; (4) ultimate safety due to a small reaction volume; (5) much higher heat and mass transfer rates as compared to macrolevel devices; and (6) narrow residence time, consequently, better selectivity and yield. This special issue includes several papers in the field of hydrodynamics and heat and mass transfer in two-phase flows in microchannels.

Catalytic exchange of hydrogen isotopes intensified by two-phase stratified flow in wettability designable microchannels.

This work develops a reliable method to achieve stratified flow in squared cross-section microchannels by endowing the four channel walls with very different wettabilities. The gas-liquid flows are studied experimentally in six kinds of microchannels with different wetting partitions on the channel walls, where occurrence of the stratified flow is found to strongly depend on the wetting partition and the individual flow rate of gas or liquid. Among them, the microchannel possessing three hydrophobic and one hydrophilic walls (3S1H) shows great potential in facilitating liquid phase catalytic exchange (LPCE) of hydrogen isotopes. Subsequently, a 3S1H microchannel reactor coated with hydrophobic Pt/activated carbon/polydimethylsiloxane (Pt/AC/PDMS) is fabricated to perform the LPCE at 30-80 °C and in a wide range of flow rates of hydrogen and water. The experimental outcomes not only unveil the relationship of the isotopic exchange performance with the temperature and flow pattern but also exhibit an outstanding isotopic exchange performance under the stratified flow.

Experimental investigation on the heat transfer characteristics of novel rectangle radial microchannel heat exchangers in two-phase flow cooling system for data centers

As electronic component technology becomes increasingly miniaturized, how to keep them from overheating is challenging. Advanced microchannel two-phase flow cooling technology attracts more and more attentions due to its much better performance when compared with conventional forced air convection cooling technology. To fundamentally alleviate the Ledinegg instability of two-phase flow, three novel rectangle radial expansion multiple microchannel heat exchangers are proposed and manufactured, with two types of cold plates and covers. Experimental investigation on the flow boiling heat transfer performance of three heat exchangers was carried out. The rectangular radial design effectively suppresses the reverse two-phase flow, without dry-out phenomenon occurred. The heat exchanger, with cutting in its cold plate and without gap in its cover, was finally clarified having the best performance. Under the operation conditions of heating load of 400 W and flow rate of 8 L h−1, the maximum measured wall temperature, the calculated averaged heat transfer coefficient, and the pressure drop are 104 °C, 0.009 kPa, and 65.444 kW m−2 K, respectively. This type of heat exchanger has the best dynamic performance on pressure drop, and its cover could be further investigated by enhancing the bubble departure. Contributions of this work are expected to directly optimize the pumped two-phase flow cooling system for data centers.

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.

Dynamics and modelling of bubble formation in asymmetric parallel microchannels

Dynamics of gas-liquid two-phase flow in parallel microchannels is the bottleneck to be solved during numbering-up. This paper focuses on the stability and distribution of gas-liquid two-phase flow in asymmetric parallel microchannels. Nitrogen and glycerol-water solutions are used as the gas and liquid phases respectively. The stability of bubble formation is better under higher flow rates of gas and liquid phases. When the flow rate ratio of gas and liquid phases is smaller, the uniformity of bubbles is better. The design of cavities in the microchannel degrades stability, while results in better uniformity. The uniformity of the bubble size is the best in the microchannel configuration with both cavities in the front and the rear. The predictive models of the relative deviation E(L), representing the uniformity of bubbles, are proposed according to the principle of conservation of pressure drop and the resistance relationship in the parallel microchannels.

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.

Development of an online measurement system using an alpha liquid scintillation counter and a glass-based microfluidic solvent extraction device for plutonium analysis

An online measurement system using an alpha liquid scintillation counter (α-LSC) coupled to microchip solvent extraction has been developed. A flow-through cell of α-LSC has been prepared by packing PTFE tube into glass tube to combine microchip. Two-phase flow in microchannel has been stabilized by using coiled tube. The Pu in organic phase has been mixed with scintillation cocktail by T-junction connectors. The system separates and detects Pu by online with detection limit of 6.5 Bq/mL, generating only μL-level wastes.

Numerical simulation of hydrodynamic and heat transfer characteristics of slug flow in serpentine microchannel with various curvature ratio

A numerical investigation is carried out to study hydrodynamic behavior and thermal performance of slug flow in serpentine microchannel with different curvature ratio and wall properties. By using coupled Level Set and Volume of Fluid (CLSVOF) method implemented in Fluent, gas-liquid interface shows good consistency with experimental results especially for slug flow. Distribution of sectional velocity profile in curved microchannel with various curvature ratios was investigated. On the basis of fluid flow in microchannel with non-slip boundary condition, by means of user defined function (UDF), influence of slip velocity on hydrodynamic characteristics was performed numerically. For a better prediction on thermal performance, further analysis of combined effect of wall properties and curvature ratio on fluid flow characteristics was also conducted. As a consequence, the increasing slip velocity on channel wall can reduce hydraulic resistance. Both the contact angle and the curvature ratio have an appreciable influence on thermal performance of gas-liquid two-phase flow in serpentine microchannel.

Hydrodynamics and mass transfer of gas-liquid flow in a tree-shaped parallel microchannel with T-type bifurcations

The numbering-up of microchannel could cause negative effects on the hydrodynamics and mass transfer of gas-liquid two-phase flow under inappropriate operating conditions. In this study, the hydrodynamics and mass transfer of gas-liquid two-phase flow in a tree-shaped parallel microchannel constructed by T-type bifurcations are investigated. Four flow regimes are obtained: bubbly flow, slug-foam flow, slug flow and compact slug flow. The results show that, there exists a critical ratio of gas flow rate to liquid flow rate to attain ideal flow distribution and optimal mass transfer performance. Beyond the critical point, the flow regime would become much different with before, the change of hydrodynamics could cause the remarkable deteriorations in flow distribution and mass transfer efficiency. The influencing mechanism of the operating conditions on hydrodynamics and mass transfer performance are analyzed. Furthermore, the correlations for predicting the volumetric mass transfer coefficient are proposed.

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.

Effects of geometry on simulation of two-phase flow in microchannel with density and viscosity contrast

With respect to the importance of a multiphase flow behaviour in a microchannel, this study is focused on developing a numerical modelling of two-phase flows using lattice Boltzmann method (LBM). This method utilizes a different vision for surveying the behaviour of fluids. The LBM be able to capture fluid motion via a microscopic vision in contrary to classical CFD methods (Navier–Stokes approach). Based on this capability, the LBM is a powerful method for solving complex problems in multiphase and multicomponent fluid dynamics problems. This research attempts to employ the chromo-dynamic model, which is compatible for high-density ratio flow in a microchannel and introduces different approaches to achieve this objective. This study examined both Huang and Lishchuk methods and the obtained results show a sharp interface between two phases by using Haung model. However, utilization of this method can impose some difficulties on stability of calculation. The research tries to eliminate or at least weaken numerical errors caused by density contrast in the interface of different phases from the exertion of an artificial force. A numerical modelling approach comprises an accurate treatment of the surface tension between phases and a passive-scalar heat transfer method. This research used a high-performance simulation to obtain real-time flow visualizations. The proposed method is applied to a new geometry with variable cross section. A correlation introduced between this geometry and dimensionless numbers (Bejan number, Laplace number, Reynolds number). This new correlation defines an operational criterion for a step-contracted microchannel and enables us to capture the delay time between fluctuations. The results are compared to available exact solutions for high-density ratio flow in a microchannel.

Experimental and theoretical study of two-phase flow in wide microchannels

Experimental and theoretical studies of the two-phase flow regimes in a wide microchannel with the height of 164 μm were performed. The ranges of parameters of the formation of the main flow regimes are determined: jet, bubble, churn, stratified, and annular. The pressure drop in single-phase and two-phase flows was measured. Using a flat flow model, we performed a theoretical study of the pressure drop and compared it with experimental data. Regular waves were experimentally detected in film flow regimes. The formation of waves on a liquid film was investigated, and characteristic wavelengths were measured. As for the theory, the study of linear stability of the stratified two-phase flow regime in a wide microchannel was performed. The comparison of numerical and experimental results indicated the validity of the proposed approach for modeling waves in film regime in wide microchannels.

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.

COMPUTATIONAL FLUID DYNAMICS SIMULATION OF WATER-OIL TWO-PHASE SLUG FLOW IN MICROCHANNELS

Oil-water slug flow in microchannels is one of the basic problems of pore-scale seepage. In recent years, more computational fluid dynamics (CFD) simulations concerning two-phase flow in microchannels have been developed. However, few simulations have studied the flow resistance of oil slug in microchannels. In this study, a CFD method was used to simulate the movement of slug in oil-water two-phase flow in circular microchannels with diameters of 20 and 200 μm. The flow resistances of oil slugs were investigated at different contact angles. The effects of the length and speed of the movement of oil slugs on their flow resistances were analyzed. The simulation results indicate that the shape of the oil slug as well as the flow pattern are influenced by the oil contact angle, defined as the angle between the oil-water and oil-wall interfaces. A certain number of vortices appear in the front, back, and inside of an oil slug when the oil contact angle is 60°. However, few vortices arise at the back and front of an oil slug when the oil contact angle is 140°. If the oil contact angle is less than 30°, the microchannel wall is strongly lipophilic and the oil slug beomes an annular film.

Experimental study on gas and Newtonian and non-Newtonian liquid two-phase flow in rectangular micro-channel

Experimental study on gas and Newtonian and non-Newtonian liquid two-phase flow in rectangular micro-channel

Distribution of gas-liquid two-phase slug flow in parallel micro-channels with different branch spacing

An experimental study was conducted in order to investigate the phase distribution of gas-liquid slug flow in six parallel micro-channels in presence of different branch spacing of 0.8 mm(2d), 4 mm(10d) and 12 mm(30d), respectively. The parallel micro-channels was composed of a header with hydraulic diameter of 0.48 mm and six branch channels with hydraulic diameter of 0.40 mm, all with rectangle cross sections. The entire test section was machined by PMMA to facilitate flow visualization. Nitrogen and 0.03 wt% sodium dodecyl sulfate (SDS) solution at ambient pressure and room temperature were used as the test fluids. A flow-regime map of bubbly, slug, slug-annular and annular was generated, covering the range of gas and liquid inlet superficial velocities of 0.28 ≤ JG ≤ 33.3 m/s and 0.008 ≤ JL ≤ 2.52 m/s, respectively. The phase distribution experiments of slug flow were conducted. The fluid dynamics of two-phase flow splitting in parallel micro-channels was captured by high speed recording technique. It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depend on the inlet flow conditions and the distance between channels. Besides, the effect of branch spacing took on distinct characteristics under different inlet flow conditions. At low mass flux and quality, the increase of branch spacing can facilitate the liquid phase to flow into channels at the rear part of the header, while the first three channels are more supplied with liquid as the branch spacing increasing at high mass flux and quality. Specially, an improvement of gas distribution was also observed with the increase of the branch spacing. Finally, a correlation capable of predicting the liquid phase distribution of slug flow in parallel micro-channels was developed.

Liquid-liquid extraction in laminar two-phase stratified flows in capillary microchannels

In this work, we study the mass transfer of a solute in laminar, two-phase, stratified flow in a circular microchannel. The model developed in our earlier work (Picardo et al., 2015) has been used. We adopt a semi-analytical approach to model liquid-liquid extraction in the circular channel with stratified flow. A bipolar cylindrical coordinate system is employed for an elegant description of the boundaries in the form of iso-coordinate surfaces.The model developed for the circular channel is computationally intensive. We present a methodology to estimate the mass transfer predictions for the 2D circular channel from a 2D channel of rectangular cross-section and a 1D channel i.e. flow between infinite parallel plates. This is analyzed for different carrier phase holdups and fluid properties. It is found that the 1D model can accurately predict the performance of the capillary. Further, we conclude that a more viscous carrier phase results in enhanced mass transfer.