Effects of T-junction size on bubble generation and flow instability for two-phase flows in circular microchannels

This paper investigates numerically the characteristics of gas–liquid sliding Taylor flow in a two-dimensional (2D) T-junction rectangular microchannel having both inlets perpendicular to the horizontal mixing chamber. The governing equations describing the flow are discretized and solved by employing finite volume method based computational tool ANSYS Fluent 15.0. The volume of fluid (VOF) multiphase method is used for capturing the gas–liquid interface. A dynamic mesh, based on the adaption of gradients of volume fraction, is used for grid refinement to tackle sharp gradients during the two-phase flow. A focus is laid on exploring the influence of wall contact angle on the flow physics of sliding Taylor flows inside the microchannel in which the inlet liquid velocity is varied in the range 0.05 < uL < 0.25 m/s) (or inlet Reynolds number, 4.97 < Re < 248.75), and the wall contact angle is varied in the range 0° < Θo < 170°. The effect of contact angle hysteresis on the overall pressure drop, bubble and liquid slug lengths, and the dispersed phase volume fraction is reported. The bubble length and the overall pressure drop obtained from the simulations are compared with the benchmark correlations available in the literature. It is found that the trends of the variation of bubble length with liquid velocity are in reasonable match with the correlations. In addition, the pressure drop and slug length decrease in hydrophilic channels unlike in hydrophobic channels, which signifies the importance of contact angle in two-phase sliding Taylor flows.

Hydrodynamic characteristics of gas–liquid slug flow in a downward inclined pipe

Study and characterization of gas-liquid slug flow in an annular duct, using high speed video camera, wire-mesh sensor and PIV

Recently, two-phase flow in microfluidics has attracted much attention because of its importance in generating droplets or bubbles that can be used as building blocks for material synthesis and biological applications. However, there are many unresolved issues in understanding droplet and bubble generation processes, especially when complex fluids are involved. In this study, we investigated elastic effects on bubble generation processes in a flow-focusing geometry and the shapes of the produced bubbles flowing through a microchannel. We used dilute polymer solutions with nearly constant shear viscosities so that the shear-thinning effects on bubble generation could be precluded. We observed that a very small amount of polymer (poly(ethylene oxide) at ~O(10) ppm) significantly affects bubble generation. When the polymer was added to a Newtonian fluid, the fluctuation in bubble size increased notably, which was attributed to the chaotic flow dynamics in the flow-focusing region. In addition, it was demonstrated that the bubbles were thinner along the minor axis in the viscoelastic fluid than they were in the Newtonian fluid. We expect that the current results will contribute to understanding the dynamics of two-phase flow in microchannels and the design and operation of the microfluidic devices to generate microbubbles.

This study addresses gas–liquid two-phase flows in polymer (PMMA) micro-channels with non-molecularly smooth and poorly wetting walls (typical contact angle of 65°) unlike previous studies conducted on highly wetting molecularly smooth materials (e.g., glass/silicon). Four fundamentally different topological flow regimes (Capillary Bubbly, Segmented, Annular, Dry) were identified along with two transitory ones (Segmented/Annular, Annular/Dry) and regime boundaries were identified from the two different test chips. The regime transition boundaries were influenced by the geometry of the two-phase injection, the aspect ratio of the test micro-channels, and potentially the chip material as evidenced from comparisons with the results of previous studies. Three principal Segmented flow sub-regimes (1, 2, and 3) were identified on the basis of quantified topological characteristics, each closely correlated with two-phase flow pressure drop trends. Irregularity of the Segmented regimes and related influencing factors were addressed and discussed. The average bubble length associated with the Segmented flows scaled approximately with a power law of the liquid volumetric flow ratio, which depends on aspect ratio, liquid superficial velocity, and the injection system. A simplified semi-empirical geometric model of gas bubble and liquid plug volumes provided good estimates of liquid plug length for most of the segmented regime cases and for all test-channel aspect ratios. The two-phase flow pressure drop was measured for the square test channels. Each Segmented flow sub-regime was associated with different trends in the pressure drop scaled by the viscous scale. These trends were explained in terms of the quantified flow topology (measured gas bubble and liquid plug lengths) and the number of bubble/plug pairs. Significant quantitative differences were found between the two-phase pressure drop in the polymer micro-channels of this study and those obtained from previous glass/silicon micro-channel studies, indicating that the effect of wall surface properties is important. Pressure drop trends on the capillary scale along gas bubbles extracted from the measurements in square micro-channels indicated a linear dependence on the Capillary number and did not agree with those predicted by highly idealized theory primarily because explicit and implicit assumptions in the theory were not relevant to practical conditions in this study.

The mechanism of bubble generation using a slit elastic tube and an acoustic pressure wave in the gas phase

Characterization of slug initiation for horizontal air-water two-phase flow

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.

Numerical Investigation of Heat Transfer Enhancement due to Two-Phase Flow in Microchannel

Even though numerical simulation of two-phase flows in microchannels has been attempted by many investigators, most efforts seem to have failed in correctly capturing the flow physics, especially the slug-flow regime characteristics. The presence of a thin liquid film in the order of 10 &amp;mu;m around the bubble (sometimes called gas pocket or gas slug) may be a contributing factor to the above difficulty. Typically, liquid films have a significant effect on the flow field. Thus, there is a strong motivation to employ numerical simulation methods to avoid some of the experimental difficulties. In this paper, the characteristics of two-phase slug flows in microchannels are calculated with the help of the volume-of-fluid (VOF) method. Formation of the slugs for different superficial velocities, capillary numbers, and gas volume fractions are investigated. The minimum mesh resolution required to capture the liquid film surrounding the gas bubble is reported employing a dynamic mesh adaptation methodology with interface tracking. Results are shown to be in good agreement with experimental data and empirical correlations.

Microreactors have a wide application, especially in heterogeneous reactions limited by mass and or heat transfer. In many chemical reactions, microreactors show good performance for obtaining a high yield, selectivity, and conversion up to 100%, which is not easy to realize using conventional reactors. The excellent performance of the microreactor is a result of the high stability of two phase flow, namely slug flow pattern. A comprehensive study of slug flow characteristics inside the microchannel is needed to have a good performance. For that reason, the present work focused on the study of slug flow characteristics and its stability by using a circular microtube with 0.5; 0.8, and 1 mm inside diameter, and the 2liquid phase consisted of aquades-kerosene and aquades-ethyl acetate with aquades as a dispersed phase and kerosene or ethyl acetate as a continuous phase. These liquids represent two liquid mixtures with different physical and chemical properties, which significantly influence the formation of 2-phase flow patterns in a microtube. Variables used in the experiment were temperature, channel diameter, and volumetric flow rate. Observation results show that the slug flow pattern was found at the ratio of the volumetric flow rate of disperse phase to the continuous phase (Qd/Qc) is 1; 1.4; 1.8; and 2.2. The slug flow formed at a flow rate of 70 ml/h for both the dispersed phase and the continuous phase (Qd/Qc = 1) had the most stable droplet length and the distance of consecutive droplets. Increasing Qd/Qc ratio increases the droplet length formed, and in the range of discharge used, the change in inside tube diameter from 0.5 to 1 mm does not change the flow pattern model, but it affects the slug length.

In recent years, micro-technologies have become very important to cutting edge industries such as aerospace and biotechnology. In the fluidic research area, two-phase flow study in microchannels has been an emerging topic in the past few years. Characteristics of two-phase flow in microchannels such as flow regimes, pressure drop, void fraction, and heat transfer are now being extensively studied by numerous groups. The ultimate goal of this research ranges from compact heat exchangers to small-sized refrigeration systems. One of the major drawbacks in this area to date is the lack of a universal flow regime map enabling the prediction of flow regimes in microchannels. In the present study, a new test rig was designed and constructed to extend the range of the existing data on flow regimes. Several flow regime maps, for a hydraulic diameter of less than 1.0 mm obtained from a comprehensive literature review, are tested and compared. A total of approximately 1475 experimental data points from present and previous studies performed in channels with Dh ≤ 1.0 mm were used for comparison. Two universal flow regime maps were created, one each for horizontal and vertical channels with a hydraulic diameter ranging between 0.1 mm and 1.0 mm. The resulting universal maps presented here are based on flow regimes observed in all the studies on two-phase flow in microchannels. We suggest that the use of these flow regimes could help diminish the amount of discord present among research groups pertaining to the definition of flow regimes. Based on the comparisons between the universal maps and different experimental flow regime maps, appropriate conclusions on the effects of channel orientation and geometry are suggested.

The effects of gas-liquid inlet geometry and mixing method on adiabatic gas-liquid two-phase flow in a microchannel of 100 μm diameter have been investigated using a T-junction inlet with the same internal diameter as the microchannel. Two-phase flow patterns, void fraction, and friction pressure drop data obtained with the T-junction inlet were found to be significantly different from those obtained previously with a reducer inlet. For the T-junction inlet, the two-phase flow patterns in the microchannel were predominantly intermittent flows with short gas and liquid plugs/slugs flowing with nearly equal velocities. The void fraction data then conformed nearly to that of a homogeneous flow model, and the two-phase friction multiplier data could be described by the Lockhart–Martinelli correlation applicable to larger channels. However, when a reducer inlet was used previously and the diameter of the inlet section was much larger than that of the microchannel, an intermittent flow of long gas slugs separated by long liquid slugs became prevalent and the void fraction decreased to values far below the homogeneous void fraction. The differences in the two-phase flow characteristics between a T-junction inlet and reducer inlet were attributed to the differences in the gas bubble/slug generation mechanisms.

Due to the high surface area to volume ratios leading to intensified heat and mass transfer rates, microreactors have been subject of interest for some time. Liquid-liquid two-phase flow is a very common phenomenon in microchannels. During the scale-up using a numbering-up approach, rectangular and square microchannels are preferred to circular microchannels in terms of easier integration of the former with a less volume. Therefore, liquid-liquid two-phase flow in non-circular microchannels has been investigated recently. However, there are still gaps in the fundamental understanding of liquid-liquid two-phase flow, such as the effect of inlet junctions or arrangements on flow patterns in non-circular microchannels. The present work aims to study the effect of inlet arrangements on liquid-liquid two-phase flow dynamics and flow patterns of square glass microchannels. In this paper, oil is used as the dispersed phase and de-ionized water is used as the continuous phase. The special inlet arrangement in the cross-junction is compared to these common inlet arrangements of T-junction and cross-junction square microchannels. The effect of the inlet continuous phase velocity on the slug length is studied. Then, the slug lengths with the same inlet velocities of the three inlets and equal velocities of the two phases are carried out, respectively. Meanwhile, typical liquid-liquid flow pattern transitions are achieved at specific conditions. Finally, a special phenomenon without the droplet flow pattern is introduced, due to introduction of the novel inlet arrangement.

Nitrogen/non-Newtonian fluid two-phase upward flow in non-circular microchannels