Flow pattern of boiling in micro-channel by numerical simulation

Boiling flows of R-134a and R-22 fluids in a 0.50 mm circular channel have been simulated to analyze bubbly flow, bubbly/slug flow, slug flow and slug/semi-annular flow depending on bubble evolution. The vapor–liquid interface was captured using VOF method. We studied the behavior of bubble growth and coalescence related to flow pattern transitions (bubbly/slug flow to slug flow, slug flow to slug/semi-annular flow) and analyzed the effect of fluid properties on transition lines. Some parameters, including heat flux, mass velocity, ONB point, vapor velocity, bubble lifting diameter, growth rate and generation frequency, have been analyzed in detail. The results show that bubble growth and coalescence are important factors for flow pattern transitions. The flow patterns at the micro-channel outlet predicted by simulation were in agreement with phenomena observed in experiments for bubbly/slug flow, slug flow and slug/semi-annular flow. In addition, the peak bubble frequency at the outlet was predicted and the general shape of the bubble frequency distribution at the outlet from simulation was found to be consistent with the achieved experimental results.     

AN INTERFACIAL PRESSURE JUMP MODEL FOR TWO-PHASE BUBBLY FLOW

Interfacial pressure jump terms based on the physics of phasic interface and bubble dynamics are introduced into the momentum equations of the two-fluid model for the bubbly flow. The pressure discontinuity across the phasic interface due to the surface tension force is expressed as a function of the fluid bulk moduli and the bubble radius. The consequence is that we obtain from the system of equations the real eigenvalues representing the void-fraction propagation speed and pressure wave speed in terms of the bubble diameter. Inversely, we can obtain an analytic closure relation for the radius of bubbles in the bubbly flow by using the kinematic wave speed given empirically in the literature. It is remarkable to see that the present mechanistic model can indeed represent both the bubble dynamics and the two-phase wave propagation in bubbly flow. Finally, it has been shown that the numerical stability is improved significantly if the interfacial pressure jump terms are used in lieu of the virtual mass terms.     

Numerical study of condensation flow regime transition in wavy microchannels

A numerical research on flow regime transition in wavy microchannels was conducted. The model was based on the volume of fluid approach and user-defined routines including interfacial mass transfer and latent heat. The observed droplet flow, annular–wavy flow, injection flow, and slug–bubbly flow were qualitatively compared against experimental data and transition lines were established. The effects of inlet vapor velocity, wall heat flux, and microchannel geometry characteristics on the annular length, occurrence frequency of injection flow, initial slug volume, and bubble detachment frequency were investigated.     

Analysis of magnetohydrodynamics peristaltic transport of hydrogen bubble in water

This paper established the theoretical and analytical analysis of a unidirectional laminar bubbly two-phase flow in a symmetric channel with flexible wall. The two-phase model uses water as base fluid with hydrogen bubble suspended in it. Rayleigh-Plesset equation in term of volume fraction is used to model void produce due to presence of hydrogen. The flow is driven by symmetric peristaltic movement of the wall. A uniform magnetic field in the transverse direction to peristaltic motion is applied. Homotopy perturbation Method (HPM) is utilized to formulate the series solution, after simplifying the differential governing equations under the influence of long wave length and low Reynolds number. The volume of the void and radius of the bubble is analyzed graphically. The consequence demonstrates that the void fraction bubbles rapidly approaches to unity, which is due to quasi-statically unstable. Due to Lorentz force fluid velocity suppresses by increasing the transverse magnetic field while reverse performance is noted for Weber number and power law index.     

Instability,heat transfer and flow regime in a two-phase flow thermosyphon loop at different diameter evaporator channel

In this study, the influence of different channel geometries on heat transfer, flow regime and instability of a two-phase thermosyphon loop, is investigated. Instabilities in flow regime and heat transfer, at low and high heat fluxes, are observed. Bubbly flow with nucleate boiling heat transfer mechanism, confined bubbly/slug flow with backflow for small channel height (H) and finally slug/churn flow at high heat fluxes are observed. This study shows that flow and thermal instability increases as channel height (H) decreases and also heat transfer coefficient increases with increasing channel height and heat flux. Bubbly flow characterizes the flow regime at high heat transfer coefficients while confined bubbles, backflow and intermittent boiling are more significant for low channel heights with lower heat transfer coefficient and critical heat flux.     

Experimental analysis of the unit cell approach for two-phase flow dynamics in curved flow channels

Flow behavior of gas–liquid mixtures in thin channels has become increasingly important as a result of miniaturization of fluid and thermal systems. The present empirical study investigates the use of the unit cell or periodic boundary approach commonly used in two-phase flows. This work examines the flow patterns formed in small tube diameter (<3 mm) and curved geometry flow systems for air–water mixtures at standard conditions. Liquid and gas superficial velocities were varied from 0.1 to 7.0 (～±0.01) m/s and 0.03 to 14 (～±0.2) m/s for air and water respectively to determine the flow pattern formed in three geometries and dispersed bubble, plug, slug and annular flow patterns are reported using high-frame rate videography. Flow patterns formed were plotted on the generalized two-phase flow pattern map to interpret the effect of channel size and curvature on the flow regime boundaries. Relative to a straight a channel, it is shown that a ‘C shaped’ channel that causes a directional change in the flow induces chaotic advection and increases phase interaction to enhance gas bubble or liquid slug break-up thus altering the boundaries between the dispersed bubble and plug/slug flow regimes as well as between the annular and plug/slug flow regimes.     

Momentum interaction in buoyancy-driven gas–liquid vertical channel flows

Drag coefficient correlations for bubbles in buoyancy-driven two-phase flows have generally been derived from data on low-viscosity media and within the bubbly flow regime. In a number of applications, e.g. evaporative crystallizers, there is a need to extend this correlation to higher viscosity flows and slug regimes. In this paper, the momentum interaction in gas–liquid vertical channel flow has been studied experimentally over a wide range of void fractions using a circulation loop facility where the buoyancy is the only driving force for liquid circulation. A model for the drag in gas–liquid buoyant flows has been developed, and is applicable for a wide range of viscosity and void fractions.     

The effect of tube diameter on vertical two-phase flow regimes in small tubes

Flow boiling flow patterns in four circular tubes with internal diameters of 1.10, 2.01, 2.88 and 4.26 mm were investigated in the present project. The experiments were conducted in vertical upward two-phase flow using R134a as the working fluid. The observed flow patterns include dispersed bubble, bubbly, confined bubble, slug, churn, annular and mist flow. The flow characteristics in the 2.88 and 4.26 mm tubes are similar to those typically described in normal size tubes. The smaller diameter tubes, 1.10 and 2.01 mm, exhibit strong “small tube characteristics” as described in earlier studies. The sketched flow maps show that the transition boundaries of slug-churn and churn-annular depend strongly on diameter. On the contrary, the dispersed bubble to churn and bubbly to slug boundaries are less affected. The transition boundaries are compared with existing models for normal size tubes showing poor agreement.     

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2‐phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.     

Two-phase flow instability for boiling in a microchannel heat sink

The present study explores experimentally the two-phase flow instability in a microchannel heat sink with 15 parallel microchannels. The hydraulic diameter for each channel is 86.3 μm. Flow boiling in the present microchannel heat sink demonstrates significantly different two-phase flow patterns under stable or unstable conditions. For the stable cases bubble nucleation, slug flow and slug or annular flows appear sequentially in the flow direction. On the other hand, forward or reversed slug/annular flows appear alternatively in every channel. Moreover, the length of bubble slug may oscillate for unstable cases with reversed flow demonstrating the suppressing effect of pressure field for bubble growth. It is found that the magnitude of pressure drop oscillations may be used as an index for the appearance of reversed flow. A stability map on the plane of inlet subcooling number versus phase change number is established. A very narrow region for stable two-phase flow or mild two-phase flow oscillations is present near the line of zero exit quality.     

Unsteady steam condensation flow patterns inside a miniature tube

Unsteady steam condensation inside a single miniature tube has been studied. The visualization of different instantaneous and periodically two-phase flow is conducted for different experimental conditions. The two-phase flow characterization is obtained using the image processing. Annular, slug bubbly, spherical bubbly, and wavy flows are observed by varying the steam inlet pressure and cooling heat transfer. The cycle of the periodically flows are compared. It is shown that increasing the cooling heat flow rate reduces the number of the instabilities and the injected bubbles. The axial vapor velocity decreases during the waves growth. The local distribution of the condensate film thickness is analyzed. It is shown that the liquid film becomes thinner near the meniscus-like interface because of the surface tension effect. The reverse annular flow is observed at the end of each periodic flow when the bubbles leave the channel. It can be concluded from experimental results that the stratification effect is not significant during the condensation inside the miniature tube. The capillary pressure evolution is measured. The maximum values are obtained in the waves locations and near the meniscus of the annular flow.     

油水乳化液中长气泡漂移速度的研究

建立圆管内滞止液体中长气泡漂移速度动量分析模型。用高速动态分析仪测量不同含水率β下,滞止油水乳化液中弹状流流型时Taylor气泡的漂移速度。结合前人的实验数据,依据Wallis的流动分类准则,给出了油水乳化液中长气泡漂移速度的半经验性公式,揭示了流动特性不同的液体中,长气泡的运动规律。     

CFD simulations of hydrodynamic characteristics in a gas–liquid vertical upward slug flow

Computational fluid dynamics (CFD) simulations are conducted using the volume-of-fluid (VOF) method to investigate the hydrodynamic characteristics of slug flow and the mechanism of slug flow induced CO₂ corrosion. The hydrodynamic characteristics are significantly affected by the viscous, interfacial, and inertial forces. In inertia dominated flows, the velocity of fully developed falling liquid film is increased with increased Taylor bubble rising velocity. The developing falling liquid film is formed at about the length of 0.5 diameter from the Taylor bubble nose, the fully developed falling liquid film is reached at about the length of 1.5–2.1 diameter from the Taylor bubble nose. The average mass transfer coefficient in the falling liquid film is always higher than that in the Taylor bubble wake zone. The iron ion near wall mass transfer coefficient is higher than that of hydrogen ion. The wall shear stress is increased with increased Taylor bubble rising velocity in fully developed falling liquid film zone, and the wall shear stress has a large fluctuation due to the chaotic and turbulent vortexes in Taylor bubble wake zone. The formation and the damage mechanism of the corrosion product scale are proposed for the gas–liquid two-phase vertical upward slug flow induced CO₂ corrosion. It is found that the wall shear stress of upward gas–liquid slug flow is alternate with high frequency, which is the key factor resulting in the corrosion product scale fatigue cracking. The CFD simulation results are in satisfactory agreement with previous experimental data and models available in literature.     

Numerical study on the formation of Taylor bubbles in capillary tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.     

VOF modelling of gas–liquid flow in PEM water electrolysis cell micro-channels

In this study, the gas–liquid flow through an interdigitated anode flow field of a PEM water electrolysis cell (PEMEC) is analysed using a three-dimensional, transient, computational fluid dynamics (CFD) model. To account for two-phase flow, the volume of fluid (VOF) method in ANSYS Fluent 17.2 is used. The modelled geometry consists of the anode channels and the anode transport layer (ATL). To reduce the complexity of the phenomena governing PEMEC operation, the dependence upon electro-chemistry is disregarded. Instead, a fixed source of the gas is applied at the interface between the ATL and the catalyst layer. An important phenomenon that the model is able to capture is the gas–liquid contact angle on both the channel wall and ATL-channel interface. Particularly, the latter interface is crucial in capturing bubble entrainment into the channel. To validate the numerical simulation, photos taken of the gas–liquid flow in a transparent micro-channel, are qualitative compared against the simulation results. The experimental observations confirm the models prediction of long Taylor bubbles with small bubbles in between. From the simulation results, further intriguing details of the flow are revealed. From the bottom to the top of the outgoing channel, the film thickness gradually increases from zero to 200 μm. This increase in the film thickness is due to the particular superficial velocity field that develops in an interdigitated flow. Here both the superficial velocities change along the length of the channel. The model is capable of revealing effect of different bubble shapes/lengths in the outgoing channel. Shape and the sequence of the bubbles affect the water flow distribution in the ATL. The model presented in this work is the first step in the development of a comprehensive CFD model that comprises multiphase flow in porous media and micro-channel, electro-chemistry in catalyst layers, ion transport in membrane, hydrogen evolution, etc. The model can aid in the study of gas–liquid flow and its impact on the performance of a PEMEC.     

Numerical Study of Bubbly Flow Using the Second-Order Moment Turbulence Model and the Population Balance Method

A comprehensive computational fluid dynamics (CFD) turbulence model, coupled with a population balance method (PBM), is presented to simulate the two-phase gas–liquid bubbly flow. The simulation results are in good agreement with the experimental data. Furthermore, considering the bubble size distribution including the drag force and lift force, the results showed that the calculated results are in good agreement with the experimental measurements. The studies reveal the liquid recirculation and bubble uprising flow patterns, and anisotropic liquid and bubble normal Reynolds stresses. Moreover, both the liquid velocity gradient and the bubble–liquid interaction are important for the generation of liquid turbulence.     

Transient flow patterns and bubble slug lengths in parallel microchannels with oxygen gas bubbles produced by catalytic chemical reactions

Transient flow patterns and bubble slug lengths were investigated with oxygen gas (O₂) bubbles produced by catalytic chemical reactions using a high speed camera bonded with a microscope. The microreactor consists of an inlet liquid plenum, nine parallel rectangular microchannels followed by a micronozzle, using the MEMS fabrication technique. The etched surface was deposited by the thin platinum film, which is acted as the catalyst. Experiments were performed with the inlet mass concentration of the hydrogen peroxide from 50% to 90% and the pressure drop across the silicon chip from 2.5 to 20.0 kPa. The silicon chip is directly exposed in the environment thus the heat released via the catalytic chemical reactions is dissipated into the environment and the experiment was performed at the room temperature level. It is found that the two-phase flow with the catalytic chemical reactions display the cyclic behavior. A full cycle consists of a short fresh liquid refilling stage, a liquid decomposition stage followed by the bubble slug flow stage. At the beginning of the bubble slug flow stage, the liquid slug number reaches maximum, while at the end of the bubble slug flow stage the liquid slugs are quickly flushed out of the microchannels. Two or three large bubbles are observed in the inlet liquid plenum, affecting the two-phase distributions in microchannels. The bubble slug lengths, cycle periods as well as the mass flow rates are analyzed with different mass concentrations of hydrogen peroxide and pressure drops. The bubble slug length is helpful for the selection of the future microreactor length ensuring the complete hydrogen peroxide decomposition. Future studies on the temperature effect on the transient two-phase flow with chemical reactions are recommended.     

Flow Boiling in an Inclined Channel With Downward-Facing Heated Upper Wall

Wall boiling and bubble population balance equations combined with a two-fluid model are employed to predict boiling two-phase flow in an inclined channel with a downward-facing heated upper wall. In order to observe the boiling behavior on the inclined, downward-facing heated wall, a visualization experiment was carried out with a 100 mm × 100 mm of the cross section, 1.2-m-long rectangular channel, inclined by 10° from the horizontal plane. The size of the heated wall was 50 mm by 750 mm and the heat flux was provided by Joule heating using DC electrical current. The temperatures of the heater surface were measured and used in calculating heat transfer coefficients. The wall superheat for 100 kW/m² heat flux and 200 kg/m²s mass flux ranged between 9.3°C and 15.1°C. High-speed video images showed that bubbles were sliding, continuing to grow, and combining with small bubbles growing at their nucleation sites in the downstream. Then large bubbles coalesced together when the bubbles grew too large to have a space between them. Finally, an elongated slug bubble formed and it continued to slide along the heated wall. For these circumstances of wall boiling and two-phase flow in the inclined channel, the existing wall boiling model encompassing bubble growth and sliding was improved by considering the influence of large bubbles near the heated wall and liquid film evaporation under the large slug bubbles. With this improved model, the predicted wall superheat agreed well with the experimental data, while the RPI model largely overpredicted the wall superheat.     

Numerical investigation of bubble coalescence characteristics under nucleate boiling condition by a lattice-Boltzmann model

A numerical study was performed to investigate the characteristics of bubble growth, detachment and coalescence on vertical, horizontal, and inclined downward-facing surfaces. The FlowLab code, which is based on a lattice-Boltzmann model of two-phase flows, was employed. Macroscopic properties, such as surface tension (σ) and contact angle (β), were implemented through the fluid–fluid (G_σ) and fluid–solid (G_t) interaction potentials. The model predicted a linear relationship between the macroscopic properties (σ,β) and microscopic parameters (G_σ,G_t). The simulation results on bubble departure diameter appear to have the same parametric dependence as the empirical correlation. Hydrodynamic aspects of bubble coalescence are investigated by simulating the growth and detachment behavior of multiple bubbles generated on horizontal, vertical, and inclined downward-facing surfaces. For the case of horizontal surface, three distinct regimes of bubble coalescence were represented in the lattice-Boltzmann simulation: lateral coalescence of bubbles situated on the surface; vertical coalescence of bubbles detached in a sequence from a site; and lateral coalescence of bubbles, detached from the surface. Multiple coalescence was predicted on the vertical surface as the bubble detached from a lower elevation merges with the bubble forming on a higher site. The bubble behavior on the inclined downward-facing surface was represented quite similar to that in the nucleate boiling regime on a downward-facing surface.