Flow Pattern Diagram Research Articles

High-throughput generation of monodisperse double emulsions via controllable symmetric splitting in Y-shaped microchannels

To achieve high-throughput generation of monodisperse double emulsions via controllable symmetric splitting, the splitting dynamics of double emulsions in Y-shaped microchannels are systematically investigated by the combination of experimental study and numerical simulation. Four distinct breakup flow patterns of double emulsions splitting are initially identified, namely non-breakup, once uneven breakup, twice uneven breakup and twice even breakup, and the underlying hydrodynamic mechanisms of each flow pattern are further elucidated through the flow field structures and pressure distributions. The results show that the upstream pressure created by the flow obstruction of continuous phase governs the stable and symmetric splitting of double emulsions in Y-shaped microchannels. Effects of the flowrates of continuous phase, the outer and inner diameters of double emulsions, the physical properties of all the inner, middle and outer fluids as well as the angle of Y-shaped microchannel on the flow pattern transition laws are systematically clarified, and the corresponding universal flow pattern diagrams for predicting the critical boundary for the transition of flow patterns are established. Based on controllable symmetric splitting of double emulsions, three-level splitting microfluidic networks are rationally designed to achieve high-throughput generation of monodisperse double emulsions as well as monodisperse microcapsules via template synthesis. The results in this study not only advance the deep understanding of the breakup dynamics of double emulsions in Y-shaped microchannels, but also offer valuable guidance for the rational design of microfluidic devices for mass production of monodisperse double emulsions, thus providing a significant contribution to the fields of micro-chemical engineering and droplet microfluidics.

Flow pattern diagram of compressible non-equilibrium gas flow around a circular cylinder

An investigation into the non-equilibrium gas flow around a circular cylinder within the Knudsen number (Kn) range of 0.001–1 and the free-stream Mach number (Ma) range of 0.01–6 is presented using the unstructured grid unified gas kinetic scheme. The primary objective is to examine the impact of Kn and Ma on flow patterns. The flow pattern diagram illustrating seven flow patterns in the Ma-Kn space is provided, including the transition boundary between bow shock-wave with laminar flow (BS+L) and bow shock-wave with vortex flow (BS+V). The relationships between Re-Kn and Ma-Re both follow the power function: y=eβxα, where α and β are constants. The study also provides a more precise critical curve of vortex shedding in subsonic inflow, the boundary of tailing shock-wave, and the boundary of vortex shedding in a transonic inflow. The flow pattern diagram indicates that the variation of flow separation with Kn is non-monotonic across the entire Ma range but is monotonic at Ma>1. In the subsonic inflow, the critical Re of flow separation (Rec) increases with Ma, while Rec initially increases and then decreases with Kn. The critical Ma at the turning point is about 0.72. In supersonic inflow, the critical Re associated with the onset of flow separation either increases or decreases with the increase in Ma or Kn. The critical Re of vortex shedding is non-monotonic with Kn. The critical Re of the trailing shock-wave decreases with both Kn and Ma. In the transonic inflow, the critical Re and critical Ma of vortex shedding decrease with Kn. As rarefaction increases, the type of flow patterns decreases. The flow pattern diagram provides a visual representation of the impact of rarefaction and compressibility effects on flow pattern transitions and assists in determining the applicable range of the drag coefficient model.

Experimental study and prediction of two-phase flow pattern distribution diagrams in multi-channel cylinder dryer

The multi-channel cylinder dryer (MCD) is designed to improve heat transfer. Although there are numerous research studies on the pressure drop, heat transfer characteristics, and flow pattern in static state of MCD, there is little research on the flow pattern in the rotating state. In this paper, the distribution of flow pattern in MCD under different rotating speeds and steam mass flow rates is studied. Furthermore, the logistic regression method (LR) is used to predict the flow pattern diagrams. The results show that in the front section of the flow channel, the flow pattern is basically annular flow, which is not affected by mass flow rate and rotating speed. On the other hand, wavy flow, vortex flow, slug flow, and bubble flow can be observed when the fluid enters the middle and the end section. The higher the rotating speed and the steam mass flow rate, the more the flow pattern tends to be an annular and wavy flow. At the end of the passage, the flow pattern is mainly slug flow. The predicted flow pattern diagrams are in good agreement with the experimental result, and to obtain an effective flow pattern in the middle and the end section of the flow channel, the influence of increasing rotating speed is greater than that of increasing steam mass flow rate. However, the specific rotating speed, steam mass flow rate, and other parameters should still be set by combining with the actual situation. This work can provide some references for the further study of MCD flow characteristics.

Hydrodynamics of gas–liquid flow in a capillary at elevated pressure: Implications for the fixation of CO2 into propylene carbonate in microreaction systems

It is an effective method to capture and utilize carbon dioxide (CO2) by using a microreactor to fix CO2 into propylene carbonate (PC). It is important to investigate the hydrodynamics of gas–liquid flow in order to ensure that the CO2 fixation in microreactor is operated under suitable conditions to obtain efficient mass transfer performance. However, few studies have been concerned with PC-CO2 two-phase flow in microreactors, especially at elevated pressures. Therefore, a high-pressure microreactor observation platform is established in this work to explore the hydrodynamics of PC-CO2 two-phase flow at a pressure range of 0.5–3.0 MPa. Three typical patterns such as bubbly flow, slug flow, and slug-annular flow were observed, and the flow pattern diagram at different pressures was plotted using WeG and WeL. The influence of the pressure on flow pattern transition lines was further discussed. Moreover, the effects of pressure on bubble size, liquid film thickness, bubble frequency, and gas volume fraction were investigated, as well as the corresponding correlation models. This work may provide guidance for the operation of gas–liquid microreactor at elevated pressure, especially for the fixation of CO2 into PC.

Spontaneous Formation Mechanisms of Droplets in Step Emulsification

Droplet step emulsification has been proven to possess the unique advantage of decoupling the flow parameters, which obviously contributes to the realization of the mass production of monodisperse droplets. However, a complete understanding of the dynamic characteristics underlying droplet spontaneous formation in step emulsification has not been fully revealed and remains a challenge because the channel confinement effect always results in the complexity in interface spatiotemporal evolution under various conditions. In this work, the spontaneous formation mechanisms of droplets in step emulsification are numerically investigated via a VOF–CSF model. The physics behind the two distinct flow patterns regarding dripping and jetting are deeply revealed based on the local flow field structures and pressure distribution, and it was found that in dripping, before final pinch-off of the neck, a finite time singularity usually exists, thus leading to infinite velocity and pressure inside the neck. However, in jetting, as the droplet steadily expands, the velocity and pressure inside the neck finally reach an equilibrium state. Besides, by taking multiple variables with a wide range into account, the flow pattern diagram and the prediction correlation of droplet size are established with several dimensionless numbers, exhibiting excellent universality. In particular, the force field characteristics previously undocumented for droplet step emulsification are also quantitatively clarified from a new perspective of momentum conservation. The results obtained in this study reveal the spontaneous formation mechanisms of droplets in step emulsification, thus providing theoretical guidance for precisely regulating the emulsification process.

Flow Pattern Study and Pressure Drop Prediction of Two-Phase Boiling Process in Different Surface Wettability Microchannel.

An experimental study of two-phase flow pressure drop using R-134a is conducted on three types of different surface wettability microchannels with superhydrophilic (contact angle of 0°), hydrophilic (contact angle of 43°) and common (contact angle of 70°, unmodified) surfaces, all with a hydraulic diameter of 0.805 mm. Experiments were conducted using a mass flux of 713-1629 kg/m2s and a heat flux of 7.0-35.1 kW/m2. Firstly, the bubble behavior during the two-phase boiling process in the superhydrophilic and common surface microchannel is studied. Through a large number of flow pattern diagrams under different working conditions, it is found that the bubble behavior shows different degrees of order in microchannels with different surface wettability. The experimental results show that the hydrophilic surface modification of microchannel is an effective method to enhance heat transfer and reduce friction pressure drop. Through the data analysis of friction pressure drop and C parameter, it is found that the three most important parameters affecting the two-phase friction pressure drop are mass flux, vapor quality, and surface wettability. Based on flow patterns and pressure drop characteristics obtained from the experiments, a new parameter, named flow order degree, is proposed to account for the overall effects of mass flux, vapor quality, and surface wettability on two-phase frictional pressure drop in microchannels, and a newly developed correlation based on the separated flow model is presented. In the superhydrophilic microchannel, the mean absolute error of the new correlation is 19.8%, which is considerably less than the error of the previous models.

Controllable fabrication of millimeter-scale double droplets in co-flowing devices

The fabrication of millimeter-scale water-in-oil-in-water (W1/O/W2) double droplets in five self-manufacturing co-flowing devices is investigated. Double droplets with a diameter range of 1000–4000 µm are successfully fabricated. Three typical flow patterns are observed in a wide range of flow rates, namely squeezing, dripping-like, and jetting-like. A flow pattern diagram is obtained to form double droplets without a single oil droplet and rupture. Furthermore, during the transition from the squeezing to jetting-like patterns, the frequency, filament length and satellite droplet size increase, while the double droplet size and formation period decrease. In addition, due to the increase in middle O and outer W2 phase tube sizes, the double droplet size and its distribution increase, but the frequency decreases. The effect of the flow rate of different phases and the sizes of the tubes on the formation process of millimeter-scale double droplets is also systematically investigated. These results provide more in-depth insight into the formation mechanism of millimeter-scale double droplets and enhance the fabrication controllability of double droplets.

Condensation Flow and Heat Transfer Characteristics of R410A in Micro-Fin Tubes and Three-Dimensional Surface Enhanced Tubes

Condensation heat transfer characteristics (using R410A as the working fluid) were studied experimentally to evaluate the heat transfer performance in copper and stainless-steel heat transfer tubes (smooth and enhanced). Experiments were carried out for a mass flux that varied from 250 to 450 kg m−2 s−1, at a saturation temperature of 318 K. Single-phase heat balance verification found that the heat loss is less than 6%, and the deviation between single-phase experimental results and various prediction correlations is less than 15%. Additionally, tube side condensation flow patterns were observed and recorded. Experimental results found that the enhancement ratio of the condensation heat transfer coefficient (enhanced tube/smooth tube) of the three-dimensional surface (1EHT) tube is in the range of 1.15~1.90, while the ratio of the micro-fin (HX) tube is in the range of 1.18~1.80. Heat transfer performance is affected by material conductivity, with the thermal conductivity of the smooth tube slightly affecting the heat transfer performance; larger heat transfer enhancements are produced in the enhanced tubes. At a low mass flow rates and vapor qualities, the flow pattern is a stratified wavy flow, while at higher mass flow rates and vapor qualities, the flow pattern is an annular flow (with the area in the enhanced tube being larger than the area of a smooth tube). Flow patterns in the smooth tube are consistent with the predicted values shown in previously reported flow pattern maps. A flow pattern diagram for condensation heat transfer in enhanced tubes is presented as part of this study. The condensation heat transfer coefficient increases with an increase in mass flow. When the mass flow rate increases, the turbulence of the liquid flow increases and the liquid film becomes thinner; thermal resistance is reduced and the heat transfer coefficient increases. Heat transfer values at lower mass velocities increase slightly with increasing mass flux values; however, at higher mass flux rates the heat transfer increase is larger than that at low mass flux values. Finally, tubes produced from high thermal conductivity materials produce larger heat transfer performance gains than the gains found in smooth tubes; small diameter tubes produce larger gains than larger diameter tubes.

Application of Flow Pattern Map for Solving Liquid Loading Problems in Well AA

About two-thirds of fields that exist in the world right now are classified as mature fields. Mature fields usually have a lot of problems that occurred during production. Mature field is classified as field which has produced as much as fifty percent of their established proved plus probable resources estimation or has produced for more than twenty-five years. Empowerment of mature well can be increasing the efficiency of well productivity. Productivity decline can be caused by low maintenance of flow pattern in a well. The other problems which commonly happen is decreasing of reservoir pressure of the well, liquid loading, slugging, and high water cut percentage. Several methods can be used to solve a lot of problems in mature well. The methods can be applied to get an optimum well productivity result. This study shows that Well AA has a slug flow pattern using matching method on a flow pattern diagram. Gas injection and velocity string are applied to solve the problems for Well AA. Gas rate of about 12 MMSCFD is obtained with gas injection method to get annular flow pattern and three different sizes of velocity string are used which are 1.25 inch, 1.5 inch, and 2.0625 inch with churn flow pattern. It is identified that the use of velocity string of 1.25 inch is the optimum method for Well AA.

Experimental Study of Gas–Liquid Pressurization Performance and Critical Gas Volume Fractions of a Multiphase Pump

Abstract Gas entrainment may cause pressurization deterioration and even failure of pumps under conditions of high inlet gas volume fraction (GVF). When the inlet GVF increases to a critical value, an obvious deterioration performance of pump occurs. Air–water pressurization performance and inlet critical GVFs of a centrifugal multiphase pump are investigated experimentally under different inlet pressures and gas–liquid flow rates. To determine the first and second critical GVFs, a new method is proposed by computing the local extreme points of the second derivative of performance curves. New prediction correlations for two critical GVFs are established with relative errors lower than ±10% and ±8%. Boundaries of three different flow patterns and the transition flow rates are determined and presented by critical GVFs on the flow pattern diagram. Moreover, boundaries of maximum pressurization are determined by performance curve clusters and a power function correlation of gas–liquid flow rates when reaching the maximum pressurization is established. With the increase of inlet pressure from 1 MPa to 5 MPa, two-phase pressurization performance is significantly increased; occurrences of pressurization deterioration are obviously delayed with the first and second critical GVFs increasing by maximums of 8.2% and 7.1%.

Evaporation Heat Transfer and Pressure Drop of Low-Global Warming Potential Refrigerant HFO-1234yf in 6.95-mm Horizontal Smooth Tube

This study investigated the evaporative heat transfer coefficient and pressure drop characteristics of R-1234yf in a horizontal tube with an inner diameter of 6.95 mm under various experimental conditions. The heat transfer coefficient increased with an increase in quality but showed a sharp decrease in the high-quality area. In addition, the heat transfer coefficient increased as the mass flux, heat flux, and saturation temperature increased. Although R-1234yf and R-134a presented similar heat transfer coefficients, that of R-134a was higher. The pressure drop increased with an increase in the quality and mass flux but decreased with an increase in the saturation temperature. The pressure drop of R-134a was larger than that of R-1234yf. In light of the flow pattern diagram by Taitel and Dukler, most of the experiments were included in the annular flow region, and some regions showed intermittent and stratified corrugated flow regions. Kandlikar’s heat transfer coefficient correlation provided the best prediction for the experimental database, with approximately 84% of the predicted data within ±30%. Moreno Quibén and Thome’s equation for pressure drop predicted approximately 88.71% of the data within ±30%.

Heat transfer investigation of a flat-plate oscillating heat pipe with tandem dual channels under nonuniform heating

To provide cooling of electronics under nonuniform heating from multiple heat sources, a novel tandem dual-channel flat-plate oscillating heat pipe (OHP) is tailored and experimentally studied with a “center-multiple-heat-source nonuniform heating and two-end air cooling” layout under different inclination angles, heat loads and nonuniform heating levels. Flow pattern diagrams together with thermal resistance contour maps are organized for two tandem mirror-symmetric halves of the flat-plate OHP, and the thermal performance differences under nonuniform and uniform heating are compared. It is indicated that, compared with uniform heating, nonuniform heating produces better thermohydrodynamic performance at a low heat load with quasi-steady “stop-oscillation” fluid motion in the flat-plate OHP, while it results in poorer thermohydrodynamic performance when the heat load is large enough to maintain sustainable fluid flow in the flat-plate OHP. Under nonuniform heating conditions, concentrating more heat to overcome the negative effects rather than to amplify the positive effects on the fluid flow and heat transfer in OHPs is more beneficial to the overall thermal performance. For the 540 tested conditions, the flat-plate OHP has a much smaller overall thermal resistance than a pure 6063 aluminum alloy plate with the same configuration (approximately 15.3% on average) along a heat transfer distance of 101.8 mm. In addition, it possesses good adaptability to gravity, with coefficients of variation of 27.6% in the overall thermal resistance for inclination angles from 0° to 90°. Accordingly, this flat-plate OHP has good potential for high-efficiency and good-adaptability cooling of electronics under nonuniform heating by multiple heat sources.

Flow regimes of polymeric fluid droplet formation in a co-flowing microfluidic device

The generation of polymeric fluid droplets via a microfluidic device with co-flowing structure is investigated by experimental, and further the influences of outer phase flow rate as well as polystyrene (PS) mass concentration on the droplet formation are systematically clarified. The experimental results indicate that the polymeric fluid droplet formation can be quantitatively distinguished by three typical flow regimes, namely breakup with satellite droplet, breakup without satellite droplet, and breakup with liquid filament. It is also found that, as the PS mass concentration increases, the satellite droplet diameter correspondingly increases. However, specially, the influence of continuous fluid flow rate on the satellite droplet diameter exhibits various, and it always depends on the PS mass concentration. Finally, a flow pattern diagram based on the dimensionless numbers is also obtained. The results obtained above could provide a guidance to prepare highly monodisperse polymeric fluid droplet without satellite droplet formation via co-flowing stream.

Visual Experimental Study on Two Phase Flow Patterns of the Evaporative Cooling System

The flow pattern is of great significance for calculating boiling heat transfer and two phase flow pressure drop, and there is no uniform flow pattern diagram for the evaporative cooling system. This paper has carried out experimental research on the flow pattern in the liquid box of the surface-mounted evaporative cooling system and establish a mathematical prediction model of flow pattern transition. The experimental results show that the flow patterns are mainly bubble flow, churn flow, and misty flow in the liquid box of the surface-mounted evaporative cooling system under different thermal loads. By comparing observed flow patterns with existing flow pattern diagrams, it can be found that the existing flow pattern diagrams are not suitable for the flow pattern division of surface-mounted evaporative cooling system. A new flow pattern diagram is proposed based on the change of interfacial area concentration in the liquid box. The prediction results of the new flow pattern diagram are in good agreement with the experimental data, which provides valuable insights into the flow pattern and paves the way for further boiling heat transfer calculation of surface-mounted evaporative cooling system.

Thermal performance of a novel dual-serpentine-channel flat-plate oscillating heat pipe used for multiple heat sources and sinks

A novel dual-serpentine-channel flat-plate oscillating heat pipe (FPOHP) is designed and fabricated for electronic cooling with multiple heat sources and sinks. The FPOHP has two mirror symmetric tandem branches of capillary serpentine square channel with a hydraulic diameter of 2 mm (i.e., Bond number Bo = 1.137~1.265 here), which is made of 6063 aluminum alloy and 55% partially filled with acetone. A corresponding experimental study is conducted to investigate the start-up characteristics and quasi-steady thermal performance of the FPOHP with arrangement of “uniform heating by multiple heat sources at center and air cooling at both ends” under different inclination angles. The combined flow pattern diagrams as well as thermal resistance and equivalent thermal conductivity contour maps are provided for two mirror symmetric tandem sections of the FPOHP, based on which the thermo-hydrodynamic performance deviations between these two sections are compared and analyzed. The results show that the quasi-steady fluid motions in the FPOHP can be characterized by intermittent or independent occurrence of three elements, namely, stop, oscillation, and circulation, depending on the heat load and inclination angle. Significantly, the thermo-hydrodynamic behaviors in two tandem sections of the FPOHP both cooperate and compete with each other, as a result of the heat, mass and momentum exchanges via the tandem structure. The FPOHP can start up successfully under all inclination angles from 0° to 90° with no obvious difference, and its start-up temperature increases smoothly and approximately reaches a plateau of 41~46 °C as the heat load goes up. For the 175 tested conditions, the coefficients of variation for the thermal resistance of two tandem sections are smaller than 21.5% with the average value of 6.8%, showing a fine heat transfer uniformity of the FPOHP over a wide range of operating conditions. Significantly, in general, the FPOHP has much larger overall equivalent thermal conductivity (about 5.8 times on average) and possesses only a weight 83.6% of a pure 6063 aluminum alloy plate with the same geometry. In addition, the FPOHP exhibits a well gravity adaptability with the relative deviation ranging from 7.1% to 25.2% in the total thermal resistances for all inclination angles. Accordingly, the FPOHP shows a good potential for the high-heat-flux electronic cooling applications with multiple heat sources and sinks.

Research on pressure drop characteristics of CO2 flow boiling based on flow pattern in horizontal Minichannel

The experimental study of the frictional pressure drop characteristics of flow boiling heat transfer of CO2 in a horizontal minichannel with an inner diameter of Φ1.5 mm was carried out. Experimental conditions: heat flux (7.5 ~ 30 kW•m−2), mass flow rate (50 ~ 600 kg•m−2•s−1), saturation temperature (−40 ~ 0 °C), vapor quality (0 ~ 1). The comparison between the experimental results and theoretical flow pattern diagrams shows that the change of heat flux has little effect on the frictional pressure drop at high vapor quality area, but it has a decisive effect on the dryout and mist flow areas. The mass flow rate is the most important factor affecting the frictional pressure drop which has a decisive effect on the flow pattern experienced during the heat transfer process. The saturation temperature has an important influence on the flow pattern transition characteristics and the friction pressure drop decreases with the increase of saturation temperature. The influence of vapor quality on the frictional pressure drop is mainly caused by the flow pattern change in the minichannel. The flow visualization study shows that the theoretical flow pattern diagram can better reflect the flow pattern of CO2 in the phase-change heat transfer process and the transition trend of flow pattern under different working conditions.

Formation of magnetic ionic liquid-water Janus droplet in assembled 3D-printed microchannel

Droplet-based microfluidics offers an effective approach for overcoming the drawbacks of high viscosity and costs of ionic liquids (ILs) in practical applications. However, studies on the hydrodynamic behavior of the formation of IL-based Janus microdroplets which can facilitate the multifunctional applications of ILs through unique structures, are still limited due to their complex interfacial properties and high viscosities. Here, magnetic ionic liquid (MIL)-water Janus droplets were generated in an assembled co-flowing 3D-printed microchannel. Seven typical flow patterns of MIL-water Janus droplet generation were presented in a mountain-shaped flow pattern diagram, which was compared to the flow regime demarcations of MIL single-phase droplet formation. The scaling law of MIL-water Janus droplet size was analyzed to further reveal the breakup mechanism of biphasic dispersed phases with large differences in viscosities and interfacial tensions. Besides, the morphology control of surfactant-free MIL-water Janus droplet in the microchannel was investigated qualitatively and quantitatively. And, by changing the continuous phase, the morphology evolution of MIL-water Janus droplet to core-shell structure was also observed. The present study would be useful for providing a deep and comprehensive understanding on the formation and structure control of IL-based Janus microdroplets which are promising candidates for the use in applications of catalysis and extraction.

Experimental and CFD-DEM numerical evaluation of flow and heat transfer characteristics in mixed pulsed fluidized beds

Pulsed fluidized beds can make gas-solid mix and contact more uniform, therefore obviously improving heat transfer efficiency. The mixed pulsed fluidized bed, whose total gas flow is composed of stable gas flow and pulsed gas flow, is proposed in this research. Firstly, the experimental device for drying particles in a mixed pulsed fluidized bed is established. Pressure signals with different frequencies and gas flow ratios are collected, and flow pattern diagrams are obtained through a high-speed camera. Secondly, the CFD-DEM parallel numerical simulation method is constructed to research the mixed pulsed fluidized bed performance. Particle mixing, motion and heat transfer characteristics under different pulse frequencies and flow ratios are studied. Results show that particles in the mixed pulsed fluidized bed exhibit regular periodic motion, thereby promoting the mixing effect of particles. Moreover the bubble nucleation point moves to the bottom of the bed with the increasing pulse frequency. When the total gas velocity is relatively low, particle mixing effect can be enhanced by increasing the proportion of pulsed gas. However, when the velocity is relatively high,particle mixing effect will be enhanced by decreasing the proportion.

Three-dimensional lattice Boltzmann simulation of Janus droplet formation in Y-shaped co-flowing microchannel

A three-dimensional ternary color-gradient lattice Boltzmann (LB) model was successfully developed to numerically investigate the hydrodynamic behavior of Janus droplet formation in Y-shaped co-flowing microchannel. The model’s ability for simulating the contact conditions of two immiscible droplet pairs in the third host fluid is validated. Janus droplet formation was categorized into five generation modes presented in a mountain-shaped flow pattern diagram, which agreed well with experimental reports. Then the size law of Janus droplet, dynamic collapsing process and the critical break-up length of biphasic dispersed thread were systematically investigated in comparison to the results of single-phase droplet formation. The simulation results showed that both single-phase and Janus droplet sizes obeyed a power-law relationship with the Capillary number of the continuous phase (Ca) in squeezing regime, where the breakup of dispersed thread showed a quasi-static character, and a critical Ca number around 0.121 for transitional regime was observed. Besides, Janus dispersed thread which is subjected to asymmetric interfacial tensions, was more inclined to collapse than the single-phase thread at the same flow rates, indicating that the operating range of Janus droplet formation might be larger than that of single-phase droplet generation. The present study expands the understanding on Janus droplet formation process, which provides a new insight on complex droplet generation in multiphase flow.

Investigation on millimeter-scale W1/O/W2 compound droplets generation in a co-flowing device with one-step structure

Millimeter-scale W1/O/W2 compound droplets generation in a co-flowing microfluidic device with one-step structure was symmetrically observed through experiments, and the formation process is distinguished according to the variation law of thread tip velocity. Specially, under a wide range of operation conditions, W1/O/W2 compound droplets form accompanying with the generation of oil phase droplet. The formation ratio of compound droplets to oil phase droplet always depends on the outer phase Capillary number, Caw2, and the flow rate ratio, R, of middle to inner phases, and thus the flow pattern diagram is obtained. Further the influences of all phases flow rates, outer phase viscosity as well as the interfacial tension on the formation period and geometric sizes are extensively studied. Finally, a prediction correlation for dimensionless inner and outer diameters with several dimensionless numbers is also obtained. The results could provide an experimental basis for controllably preparing monodisperse compound droplets with millimeter-scale.