Flow In Circular Microchannel Research Articles

An experimental investigation on characteristics of liquid film thickness of gas-liquid Taylor flow in square/rectangular microchannel applied in microreactor

Microchannel reactors employing Taylor flow is 1 – 3 orders of magnitude higher than traditional reactor in handling gas-liquid reaction processes, which has found important applications in oxidation, hydrogenation, chlorination, fluorination and absorption involved in chemical industry. Liquid film thickness of gas-liquid Taylor flow in circular microchannel has been widely investigated, whereas the variation and distribution of liquid film thickness in square/rectangular microchannel are lack of comprehension. In the present work, experiments were conducted to investigate the variation of liquid film thickness in square channels with hydraulic diameter Dh = 0.3, 0.5, 0.7, 1.0 mm, and rectangular channels with a depth of 0.5 mm and widths of 0.3, 0.5, 0.7, 1.0 mm under the condition of air-water and the addition of surfactant Tween-20. The liquid film thicknesses were measured using a laser focus displacement meter (LFDM), the bubble shapes and velocities were compared using high speed photography. The experimental results illustrate that the dimensionless bubble diameter in square/rectangular microchannels can be divided into initial stage, reduction stage and stable stage with the increasing of capillary number Ca. The initial, reduction and stable stage of the dimensionless bubble diameter are all influenced by the channel hydraulic diameter, aspect ratio, asymmetry effects and surface tension coefficient of working fluid. In square channels with Dh = 0.3, 0.5, 0.7, and 1.0 mm, the dimensionless bubble diameter enters the reduction stage at Ca = 0.019, 0.015, 0.011, and 0.009, respectively. Besides, an empirical formula of dimensionless bubble diameter was developed using the dimensionless groups Ca, Re, Bo, and Li/Dh to describe the characteristics of liquid film thickness in square/rectangular microchannel.

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.

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.

Numerical investigation of bubble shape and flow field of gas–liquid slug flow in circular microchannels

Numerical simulations of gas–liquid slug flows (also known as Taylor flows) in circular microchannels are carried out using an interface tracking method based on the volume of fluid method to investigate effects of relevant dimensionless numbers on the bubble shape and the flow structure inside and outside a bubble. Uniform slug flows are dealt with, and therefore, the motion of a single unit cell consisting of a Taylor bubble and a liquid slug is predicted. Adaptive computational cells are used to capture the thin liquid film between a bubble and the channel wall. The numerical conditions are the same as those in experiments in our previous study, i.e. two channel diameters and three liquids of different viscosities are used. The conclusions obtained are as follows: (1) the interface tracking simulation can give good predictions of the bubble shape and the relationship between the void fraction and the gas volumetric flow ratio, (2) the radius of curvature at the bubble nose can be well correlated in terms of the capillary number, whereas that at the bubble tail depends not only on the capillary number but also on the Weber number, and (3) the minimum liquid film thickness appearing in the tail region of a bubble, which is thinner than the uniform liquid film in the cylindrical bubble body region in all the numerical conditions, can be correlated in terms of the capillary number as well as the uniform liquid film thickness in the cylindrical bubble body region.

Figure of merit for optimization of nanofluid flow in circular microchannel by adapting nanoparticle migration

In this paper, the laminar fully developed flow of alumina/water nanofluid inside circular microchannels subjected to a constant wall temperature (CWT) is theoretically investigated. The effect of nanoparticles migration originating from thermophoretic diffusion (temperature-gradient driven force) and Brownian diffusion (concentration-gradient driven force) on the thermophysical characteristics of nanofluids has been considered. A Navier's slip condition is considered at the wall to model the non-equilibrium region at the fluid-solid interface. In order to assume a hydrodynamically and thermally fully developed flow, the governing equations are reduced to a system of ordinary differential equation and solved using the appropriate reciprocal algorithm. The effects of pertinent parameters including the ratio of Brownian motion to thermophoresis (NBT), slip parameter (λ) and bulk mean nanoparticle volume fraction (ϕB) on the flow and thermal fields are investigated. In addition, the results are compared with the case of constant heat flux (CHF) at the wall. The figure of merit (FoM) is used to measure the thermal performance of equipment and finding the optimum thermal condition. It is shown that the anomalous heat transfer enhancement depends on the thermal boundary condition as well as the nanoparticles diameter. Furthermore, the optimum value of NBT for the case of constant wall temperature (approximately 1) is found to be greater than that of the constant wall heat flux (approximately 0.5). Thus, it can be concluded that the optimum diameter of nanoparticles for the case of constant wall temperature should be smaller than that of constant wall heat flux.

Three Dimensional Effects in Taylor Flow in Circular Microchannels

The gas-liquid Taylor flow regime in microchannels is of great interest to a range of industries (e.g. electronics cooling, automotive, biomedical, aerospace and chemical processing) as the rates of heat and mass transport are significantly enhanced in Taylor flow when compared with laminar, fully-developed single phase liquid-only flow. The bubble shape and flow are generally assumed to be axisymmetric and steady (in a frame of reference moving with the bubble) in Taylor flow in millimetre-size circular channels. Our experiments performed using a combination of highspeed imaging and micro-Particle Image Velocimetry (micro-PIV) techniques have investigated the case of Taylor flow for an ethylene glycol-nitrogen system in a horizontal channel of 2 mm diameter, where the bubble shape was observed to show significant non-axisymmetric behaviour. To further understand the non-axisymmetric flow behaviour a periodic, three-dimensional computational fluid dynamics (CFD) model employing the Volume of Fluid (VOF) method (using ANSYS Fluent) with geometric reconstruction to capture the gas-liquid interface was developed. A combination of experimental and computational techniques gives insight into the flow physics for the case of horizontal Taylor flow where three dimensional effects are important.

Prediction of Flow Regime for Air-water Flow in Circular Micro Channels Using ANN

Abstract Artificial Neural Network is used for the classifications of flow regimes in air-water flow through micro channels are presented. 561 data points from various experimental results from the published literature for air-water two-phase flow in circular micro channels have been used. Three different well known artificial neural network models have been used to predict the flow regime. The ANN model based on Levenberg-Marquardt training algorithm gives the slightly better predictability over the other training algorithms.

J053044 マイクロチャンネル内の気液混合と二相流の特性

Characteristics of the liquid-gas two-phase flow in circular microchannels (d = 486 μim) were experimentally investigated from the aspects of the bubble generation at T-junctions and two-phase pressure drop. The test fluids were chosen as water and air as continuous and dispersed phases, respectively. The T-junctions were fabricated by connecting two glass capillaries with various spacing (136-252 μm) inside a connecting chamber. Both fluids, which were pumped at constant flow rates, were mixed at T-junctions and the mixing was visualized by means of a high-speed camera. As a consequence, we observed stable slug flow in most cases, while bubbly flow or unstable slug flow, which was characterized by non-constant slug and/or bubble lengths in single flow, were appeared when the flow rates between two phases were significantly different. Meanwhile, the pressure drop measurement revealed that the two-phase pressure drop increases in cases of the use of narrower T-junctions even when the flows seem to be stable. This contradiction was explained by a bubble overpressure and scaling analysis based on the Bretherton law.

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.

Gas–liquid flow in circular microchannel. Part I: Influence of liquid physical properties and channel diameter on flow patterns

This paper presents an experimental investigation on influence of liquid physical properties and channel diameter on gas–liquid flow patterns in horizontal circular microchannels with inner diameters of 302, 496 and 916 μm. Several liquids with different physical properties, i.e. water, ethanol, three sodium carboxymethyl cellulose (CMC) solutions (0.0464%, 0.1262%, 0.2446% CMC) and two sodium dodecyl sulfate (SDS) solutions (0.0608%, 0.2610% SDS) are chosen as working fluid and nitrogen as working gas. By using a high-speed photography system, flow patterns such as bubbly flow, slug and unstable slug flow, churn flow, slug-annular and annular flow are observed and identified on the flow regime maps. The results show that the liquid physical properties (viscosity and surface tension) and channel diameter affect the flow pattern transitions remarkably. Comparison with existing models in literature implies that these transitions cannot be well predicted. As a result, an effort is put into the proposition of a new empirical model taking the effects of channel size and liquid physical properties into account.

The effects of channel diameter on flow pattern, void fraction and pressure drop of two-phase air–water flow in circular micro-channels

Two-phase air–water flow characteristics are experimentally investigated in horizontal circular micro-channels. Test sections are made of fused silica. The experiments are conducted based on three different inner diameters of 0.53, 0.22 and 0.15 mm with the corresponding lengths of 320, 120 and 104 mm, respectively. The test runs are done at superficial velocities of gas and liquid ranging between 0.37–42.36 and 0.005–3.04 m/s, respectively. The flow visualisation is facilitated by systems mainly including stereozoom microscope and high-speed camera. The flow regime maps developed from the observed flow patterns are presented. The void fractions are determined based on image analysis. New correlation for two-phase frictional multiplier is also proposed for practical applications.

Viscous dissipation effect on entropy generation for non-Newtonian fluids in microchannels

An analytical study on the viscous dissipation effect on entropy generation in fully developed forced convection for single phase non-Newtonian fluid flow in circular microchannels is reported. In the first-law analysis, closed form solutions of the temperature distributions in the radial direction for the models with and without viscous dissipation term in the energy equation are obtained. In the second-law analysis, the two models are compared by analyzing their relative deviations in dimensionless entropy generation and Bejan number for different Brinkman number and power-law index. The findings show that under certain conditions the viscous dissipation effect on entropy generation in microchannels is significant and should not be neglected.

Forced convection gaseous slip flow in circular porous micro-channels

Laminar forced convection of gaseous slip flow in a circular micro-channel filled with porous media under local thermal equilibrium condition is studied numer- ically using the finite difference technique. Hydrodynamically fully developed flow is considered and the Darcy-Brinkman-Forchheimer model is used to model the flow inside the porous domain. The present study reports the effect of several operating parameters (Knudsen number (Kn), Darcy number (Da), Forchhiemer number (�) , and modified Reynolds number (Re ∗ )) on the velocity slip and temperature jump at the wall. Results are given in terms of the velocity distribution, temperature distri- bution, skin friction (CfRe ∗ ), and the Nusselt number (Nu). It is found that the skin friction is increased by (1) decreasing Knudsen number, (2) increasing Darcy number, and (3) decreasing Forchheimer number. Heat transfer is found to (1) decrease as the Knudsen number, or Forchheimer number increase, (2) increase as the Peclet number or Darcy number increase. Nomenclature C Coefficient in the Forchheimer term Cf Skin friction coefficient Cp Constant pressure specific heat Cv Constant volume specific heat D Diameter of the circular channel Da Darcy number (K/er 2 ) h Local heat transfer coefficient K Intrinsic permeability of the porous medium Kn Knudsen number (λ/r0)

Numerical Analysis of Isothermal Gaseous Flows in Microchannel

AbstractTwo‐dimensional compressible momentum equations were solved by a perturbation analysis and the PISO algorithm to investigate the effects of compressibility and rarefaction on the local flow resistance of isothermal gas flow in circular microchannels. The computations were performed for a wide range of Reynolds numbers and inlet Mach numbers. The explicit expression of the normalized local Fanning friction factor along the microchannel was derived in the present paper. The results reveal that the local Fanning friction factor is a function of the inlet Mach number, the Reynolds number and the length‐diameter ratio of the channel. For larger Reynolds and inlet Mach numbers, the friction coefficient in the microchannel is higher than the value in a macrotube, and the gas flow in the microchannel is dominated only by compressibility. For smaller Reynolds and inlet Mach numbers, the Fanning friction factor of gas flow in the microchannel is lower than that in a circular tube of conventional size due to slip flow at the wall and thus, rarefaction has a significant effect on the fluid flow characteristics in a microchannel.

Characterization of fluid dynamic behaviour and channel wall effects in microtube

Sometimes contradictory results available for fluid flow in micropipes show that much is yet to be verified in microfluid dynamics. In this study the influence of channel wall roughness and of channel wall hydrophobicity on adiabatic flow in circular microchannels is investigated, varying in diameter from 70 μm to 326 μm. The hydrodynamic behaviour of water in smooth tubes down to 30 μm inner diameter (ID) is also ascertained. Within the current experimental accuracy it is found that the classical Hagen–Poiseuille law for friction factor vs. Reynolds number is respected for all diameters measured and Re > 300. With degassed water, no effect of slip flow due to hydrophobic channel walls was noted even at 70 μm ID, which might suggest that the liquid slip flow phenomenon is associated with local desorption of dissolved gases on the hydrophobic surface, as reported elsewhere in the literature. For roughened glass channels, an increase in Darcy friction factor above 64/ Re was observed only at the smallest diameter measured, 126 μm. Although the roughness levels of these channels were up to 10 times coarser than the untreated, smooth glass tubes, probably the higher factor was caused by actual deformation of channel circularity, rather than increased friction at the rougher wall, as similar behaviour was observed in a Teflon tube, also of imperfect circularity of cross-section. For all experiments, no anticipated transition to turbulent flow was observed, which means that the transitional Reynolds number was always found between Re ≈ 2000 and Re ≈ 3000. Finally, an introduction to the importance of viscous dissipation in microchannels is added, with quantitative indications of its influence on hydrodynamic properties. It is put forward as being an alternative to pressure measurements in the characterization of the behaviour of microscopic flow.

Microfluid Flow in Circular Microchannel with Electrokinetic Effect and Navier's Slip Condition

Electrokinetic transport phenomena and slippage of fluid at the surface are often considered independently for description of microchannel flow. Anomalous cases of experimental results disagreeing with theoretical prediction have frequently been reported. By combining the electrokinetic effect and Navier's slip condition, we illustrate a possible explanation and present an analytical solution for oscillating flow in a circular microchannel. The derived analytical solution is useful for more general time-dependent problems through superposition of time-harmonic solutions weighted by appropriate Fourier coefficients. Our parametric results suggest that these two surface phenomena can be important for a better understanding of microfluid flow with hydrophobic channel walls.

Analytical treatment of flow in infinitely extended circular microchannels and the effect of slippage to increase flow efficiency

Slippage of liquids at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in circular microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. For pressure-driven flow, our results suggest that slippage of a typical electrolyte solution at a channel wall with a 10% slip length can improve fluid flow rate by about 20%. With respect to electro-osmotic pumping at a given flow rate, we showed that the voltage requirement for a typical electrolyte solution can be greatly reduced, by as much as 90% for a 1% slip length and 99% for a 10% slip length. Our results are useful to precisely control time-dependent microflow in microfluidic microelectromechanical system devices. They also provide design guidelines to improve the efficiency of lab-on-a-chip devices and miniature mechanical pumping/cooling systems by inducing slippage of liquids at the channel wall.