Volume Of Fluid Model Research Articles

CFD simulations of gas-liquid-liquid three-phase co-current flow in horizontal pipe by tracking volume fractions using VOF model

The gas-liquid-liquid three-phase co-current flow is frequently observed in upstream petroleum pipelines. The identification of three-phase flow patterns is important to characterize three-phase flow dynamics. The three-phase flow patterns change with the pipe diameter, superficial velocities and physical properties of the fluids. Computational fluid dynamics (CFD) analysis of three-phase flow through 0.1524 m diameter and 3.5 m long horizontal pipe has been conducted. The superficial velocity ranges of gas-oil-water were 0.5-4, 0.08-0.32 and 0.08-0.32 m/s respectively. The volume of fluid (VOF) model was used to simulate the three-phase interfacial structures. According to the results, the three-phase flow patterns have been successfully simulated. Three-phase flow regime map has been constructed to analyze the flow pattern transition with the superficial velocities. The three-phase flow pressure drop increased with the superficial velocities of oil and water. However, the drop in pressure first increased and then decreased with the increase in superficial velocity of the gas.

Fluid dynamics in a full-scale flat sheet MBR, an experimental and numerical study.

Fluid dynamics is used for fouling mitigation in membrane bioreactors (MBRs), whereby a proper understanding of the fluid dynamics is of great interest. The influence of fluid dynamics has led to the use of computational fluid dynamics for optimizing MBR systems. In this work, a model has been validated for flat sheet membranes, with use of the Eulerian multiphase method. The model is validated against a comparable setup where the liquid velocities are measured with a laser Doppler anemometer (LDA). Furthermore, the Eulerian multiphase approach is validated against the more numerical direct volume of fluid (VOF) approach with sludge properties for the liquid, resulting in an error between the models of less than 2% for the wall shear stresses. The VOF model further showed that the horizontal components contribute significantly to the total wall shear stresses. The model has been applied to a full-scale setup for studying the effect of deflecting membranes as deflections have been seen in production. Minimizing the deflection of the membrane sheets was crucial to achieve a good operating condition as a deflection of 2 mm in a setup with a gap of 7 mm decreased the wall shear stresses with as much as 40% on average on the specific membrane surface.

Effect of Immersion Depth of a Swirling Flow Tundish SEN on Multiphase Flow and Heat Transfer in Mold

The effect of the immersion depth of a new swirling flow tundish SEN (Submerged Entry Nozzle) on the multiphase flow and heat transfer in a mold was studied using numerical simulation. The RSM (Reynolds Stress Model) and the VOF (Volume of Fluid) model were used to solve the steel and slag flow phenomena. The results show that the SEN immersion depth can significantly influence the steel flow near the meniscus. Specifically, an increase of the SEN immersion depth decreases the interfacial velocity, and this reduces the risk for the slag entrainment. The calculated Weber Number decreases from 0.8 to 0.2 when the SEN immersion depth increases from 15 cm to 25 cm. With a large SEN immersion depth, the steel flow velocity near the solidification front, which is below the mold level of SEN outlet, was increased. The temperature distribution has a similar distribution characteristic for different SEN immersion depths. The high temperature region is located near the solidification front. Temperature near the meniscus was slightly decreased when the SEN immersion depth was increased, due to an increased steel moving distance from the SEN outlet to the meniscus.

A numerical study on flow patterns inside an electrical submersible pump (ESP) and comparison with visualization experiments

This paper presents a numerical study of flow pattern recognition inside the rotating impeller of an electrical submersible pump (ESP) using the transient multiphase Computational Fluid Dynamics (CFD) simulations. The calculation domain is constructed based on the previous experimental facility for visualizing flow patterns in an ESP (Barrios (2007)). The high-quality structured mesh comprising hexahedral grids is generated using multi-block technique in ANSYS ICEM. For CFD simulations, the realizable k-ε turbulence model with volume of fluid (VOF) and Eulerian-Eulerian multiphase models is successfully implemented in ANSYS Fluent solver. The sliding mesh technique is applied to interfaces where rotating and stationary parts interact. By incorporating the same boundary conditions as Barrios experimental study, three flow cases with constant gas flow rates and varying liquid flow rates are selected to conduct numerical simulations. The comparison of simulation results with Barrios’ observations shows that the Eulerian-Eulerian multiphase model is superior to VOF model for simulating gas-liquid two-phase flow in a rotating ESP. The single-phase simulation results match catalog curves, which validates the numerical methodology. For two-phase simulations, the simulated flow patterns using Eulerian-Eulerian model agree well with visualization experiments. Different flow patterns prevailing inside the rotating ESP impeller are captured. At low gas flow rate, bubbles are dispersed in liquid phase, and the flow pattern is categorized as dispersed bubble flow. As gas flow rate increases, bubbles can accumulate and coalesce, causing large gas-pocket formation leading to intermittent/slug flow. Transient multiphase CFD simulation is an efficient and reliable tool to predict flow patterns inside ESPs.

Two-phase flow in the mixed-wettability gas diffusion layer of proton exchange membrane fuel cells

Polytetrafluoroethylene (PTFE) is widely employed to improve the hydrophobicity of gas diffusion layer (GDL) in proton exchange membrane (PEM) fuel cells. In this study, the effects of different PTFE loadings on the relationship of the capillary pressure Pc and water saturation s in the mixed-wettability GDL, i.e. Pc-s, are investigated using a three-dimensional (3D) volume of fluid (VOF) model. The simulated Pc-s curves are presented and compared with results obtained from the lattice Boltzmann model (LBM) and experiments. The good agreement between the VOF predictions and experiment data is achieved, indicating that the mixed wettability in the PTFE treated GDL is an important feature to understand two-phase behaviors in fuel cells. The homogeneous and heterogeneous PTFE distributions resulted from two PTFE drying methods (i.e. the vacuum and air dryings, respectively) are studied. It was found that the air drying GDL yields a high PTFE concentration near the water inlet and reduces water imbibition near the inlet. The simulated Pc-s correlation from VOF model was compared with standard Leverett correlation.

Experimental and numerical studies on molten salt migration in porous system with phase change

Molten salt is promising high temperature heat transfer fluid, and its transport in porous media is an important problem for molten salt application. In this paper, molten salt migration and phase change in cold porous system packed with sand particles is experimentally and numerically studied. Experimental results show that high temperature molten salt continuously migrates and a transparent liquid molten salt layer appears during discharge stage with molten salt pouring into porous bed, and then it solidifies as white opaque solid block during post-discharge stage. A transient axial-symmetrical flow and heat transfer model is developed using volume of fluid model and linear approximation in mushy zone, and the simulated results very well fit with experiment. After molten salt discharges and contacts with the cold surface, a thin solid layer quickly forms for solidification, and then it gradually expands and finally becomes a solid block. Since the earlier solid layer hinders molten salt vertical flow, liquid molten salt will flow across the outer boundary of solid layer, and then molten salt layer below the surface becomes thicker. After molten salt totally solidifies, an inner region with little molten salt will probably exist inside molten salt solid block. The maximum migration region for molten salt in cold porous system is affected by structural and operating parameters. For larger porosity and particle diameter or higher molten salt temperature, molten salt flow in porous system has less flow resistance, and then the maximum migration height can be increased, while the migration diameter is reduced.

Study of heat and fluid flow during melting of PCM inside vertical cylindrical tube

This paper presents numerical investigation to study the melting of Phase Change Material (PCM) partially filled in a vertical cylindrical tube. The top space of tube was filled with air to take into account the volumetric expansion of PCM. The natural convection inside PCM liquid phase was considered via the temperature dependency of density and gravitational force. The finite-volume method was adopted to discretize the conservation equations of mass, momentum, and energy. The enthalpy porosity formulation was employed to solve the energy equation in both regions of PCM, liquid and solid. In this multiphase system, the Air-PCM interactions have been treated using the Volume of Fluid model (VOF). The mathematical model is based on conjugate heat transfer in PCM subjected to a constant temperature at the external surface of cylindrical shell. The obtained results have been analyzed and compared with literature, and a good agreement was showed. Then, a parametric study was carried out to establish correlations for the liquid fraction and time of complete melting as a function of all dimensionless parameters that governing this problem, such as Fourier number, Grashof number, Stefan number, wall to PCM thermal diffusivity ratio, tube aspect ratio, shell-to-tube diameter ratio and dimensionless initial temperature. The results show that all parameters of the problem can really affect the phase change phenomena and consequently, affect the melting time.

3D simulation of interaction and drag coefficient of bubbles continuously rising with equilateral triangle arrangement in shear-thinning fluids

Numerical simulations by the volume of fluid model (3D-VOF) were performed for the dynamical behavior of bubbles with equilateral triangle arrangement continuously rising in shear-thinning fluids. The influences of bubble diameter, bubble formation frequency, initial bubble distance and liquid rheological property on the interaction between bubbles and drag coefficient were studied systematically. With the increase of bubble diameter, bubble formation frequency and the decrease of initial bubble distance, both the horizontal attractive effect and vertical wake effect of bubbles strengthened, resulting in the decrease of drag coefficient. The intensified liquid shear-thinning property for the lower flow index could also enhance these two effects, and accordingly lead to a decrease of bubble drag coefficient. To characterize the interaction within multi-bubble systems, the bubble interaction coefficient Ψ was introduced, and an empirical correlation of Ψ was proposed. Based on the Magnaudet model, a new bubble drag coefficient relationship for the multi-bubble system was proposed by Ψ with an average deviation of 5%.

A correction method of hole-to-hole variation mass flow of diesel injector equipped on a common-rail DI diesel engine

In the present paper, the inner flow characteristic and cavitation phenomena for different injector shapes (characterized by angle α and length–diameter ratio) are analyzed experimentally and numerically. Mathematical models including multi-phases model, volume of fluid model and the k-epsilon turbulence model are validated by experiment. The numerical results show that with the increase of injection angle α, the inception time of cavitation is earlier, the extension velocity of cavitation to nozzle exit is higher and the nozzle fuel mass is less for each cycle. With the increase of length–diameter ratios, the time consumed from the inception to stable state of cavitation is longer and the single nozzle fuel mass increases. Furthermore, a correction method is proposed based on inconsistent length–diameter ratios. It could amend the difference of single nozzle fuel mass and guarantee the uniform fuel mass in axis symmetry direction of engine cylinder.

Operational Time and Melt Fraction Based Optimization of a Phase Change Material Longitudinal Fin Heat Sink

Abstract In the present study, the numerical investigation has been performed for a phase change material (PCM)-based longitudinal fin heat sink. The fins are taken as an integral part of the heat sink and are made up of aluminum. The PCM considered in the study is RT44HC. Heat is transferred to the heat sink through its horizontal base. In order to simulate the melting behavior of the PCM, volume of fluid model has been used. To attain the best configuration with optimum operational time, Taguchi method has been used followed by analysis of melt fraction and maximum base temperature. The optimized heat sink configuration with maximum operational time has been obtained at the critical temperatures of 54.8 °C, 63 °C, and 72.6 °C.

Control of Two-Phase Flows During Startup Operations of Casting Sequences in a Billet Tundish

The startup operations of continuous casting sequences of steel in a 4-strand tundish, using three different turbulence inhibitors (TI), were investigated using a 1/3-scale water model in combination with numerical simulations through the volume of fluid model. The two-phase flow water–air is used to model the system, and the liquid steel–air and the liquid–gas interfaces are tracked by a donor–acceptor principle applied in the computational mesh. In the actual caster, one of the inhibitors releases the liquid steel with a sensible heat high enough to avoid freezing in the regions near the outer strands during the startup of the casting sequence. A second inhibitor improves the fluid flow control by yielding higher plug flow volume fractions. However, it promotes steel freezing in the outer strands. The analysis of the results lead to the design of the third TI with intermediate capability to control the steel flow, preventing steel freezing in the regions near the outer strands.

Nearshore wave processes numerical study for a better prediction of hydrodynamic loads on coastal structures

Taking into account the sensible approach in the coastal zones, in these types of areas were found different ways of understanding and description of the phenomena occurring during the water wave propagation. The waves hit the zone with a force that can affect the shoreline. Coastal protection measures were implemented, which include the hydrodynamic structures. The hydrodynamic process plays an important role in coastal area which involves the need for knowledge on the prediction of hydrodynamic loading required up to a very detailed level, such as maximum or minimum pressures. In this paper, we will present a study of wave fields that propagates in a simplified, constant-slope coastal model and relations between waves and hydrodynamics structures. The processes of wave fields include dissipative mechanisms such as depth-induced wave breaking, bottom friction and the interaction between wave field and coastal dike. The numerical models were constructed using ANSYS-Fluent program for modelling fluid flow and to solve the unsteady, two dimensional Navier-Stokes equations of incompressible fluid. Water waves are generated in two-phase fluid domain, where Volume of Fluid Model is used for generation of the two-phase flow model - the water-air interaction.

Effect of saturation temperature on the condensation of R134a inside an inclined smooth tube

In this study, numerical simulations of the condensation of R134a inside an inclined smooth tube are conducted. The effects of different parameters such as saturation temperature, tube inclination angle, vapour quality, and mass flux, on variations of the condensation heat transfer coefficient and pressure drop along the tube were investigated. The simulations were performed at a uniform heat flux of 5050 W/m2, mass fluxes of 100–400 kg/m2s, saturation temperatures of 30–50 °C, vapour qualities of 0.1–0.9, and inclination angles of −90 to + 90° It was assumed that the flow field was three-dimensional, transient, and turbulent. The volume of fluid model was used to solve the governing equations. The simulated results for the condensation heat transfer coefficient and pressure drop showed good correlation with the available experimental data. The results also demonstrated that the condensation heat transfer coefficient and pressure drop along the tube increased with decreasing saturation temperature, and increasing mass flux and vapour qualities. It was also determined that there was an optimum inclination-angle-range, −30 to −15°, in which the condensation heat transfer coefficient attained a maximum value.

Coupling of Lattice Boltzmann and Volume of Fluid Approaches to Study the Droplet Behavior at the Gas Diffusion Layer/Gas Channel Interface

With the increased concern about energy security, global warming, and air pollution, the possibility of using polymer electrolyte fuel cells (PEFCs) in approaching renewable and sustainable energy systems has reached significant momentum [1,2]. A typical PEFC flow field consists of a series of micro/minichannels. The continuous removal of liquid water from the cathode channels is a critical issue, as water droplets forming in the channels can block the transport of gaseous oxygen to the active sites. This phenomenon gives not only an uneven current distribution, substantial loss of performance, but also, unstable operation and increased degradation rates [1]. Water generated by the electrochemical reactions often condenses into liquid form, potentially flooding the GC (gas channel), the GDL (gas diffusion layer), the micro porous layer (MPL) and the catalyst layer. Insight into the fundamental processes of liquid water transport and evolution is still not complete, preventing further FC development [1]. Computational fluid dynamics (CFD) models make it possible to reduce the number of experiments needed for cell design and development [1,3]. The aim of this work is to obtain an increased understanding of the droplet behavior at the GDL/GC interface by coupling of Lattice Boltzmann and Volume of Fluid (VOF) approaches, for gas channels relevant for PEFCs. The VOF model is an interface-resolving method and has become a widespread methodology for solving two-phase flow phenomena inside PEFCs. The volume fraction of liquid water at the computational cell volume schemes, are used to track the interface between the phases [1,3]. Input parameters in the VOF model are extracted from our in-house Lattice Boltzmann calculations [4], i.e., establishing a multiscale environment. It is clear that the contact angle as well as the size of the liquid droplet vary with positions at the interface, depending on the stochastic GDL geometry. A model describing one straight channel with one gas inlet, one liquid inlet (at the GDL surface) and one two-phase outlet is developed. The operating parameters belong to the laminar flow regime. Notice that a developing flow region covers the entire channel. The uniqueness of this work includes a calculation of contact angles at the GDL interface as well as the size of the liquid inlet, by the authors, with Lattice Boltzmann calculations (3D nineteen-velocity (D3Q19) pseudopotential multiphase multicomponent isothermal LBM) [4].

Coupling of Lattice Boltzmann and Volume of Fluid Approaches to Study the Droplet Behavior at the Gas Diffusion Layer/Gas Channel Interface

A typical polymer electrolyte fuel cell (PEFC) flow field consists of micro/minichannels. The continues removal of liquid water from the cathode channels is a critical topic, as water droplets forming in the channels may block the transport of gaseous oxygen to the active sites, which not only gives an uneven current distribution and substantial loss of performance, but also, increases degradation rates and unstable operation. Water generated by the electrochemical reactions condenses, depending on temperature mainly, into liquid form, potentially flooding various part of the PEFC. The aim of this work is to obtain an increased understanding of the droplet behavior, especially at the gas diffusion layer (GDL) interface, by the coupling of Lattice Boltzmann (LB) and Volume of Fluid (VOF) approaches, for gas channels in PEFCs. A multiscale environment is established with input parameters in the VOF model being extracted from in-house LB calculations. It is clear that the contact angle as well as the size of the liquid droplet vary with positions at the GDL surface, depending on the stochastic GDL geometry. A VOF model describing one straight channel with one gas inlet, one liquid inlet (at the GDL surface) and one two-phase outlet is employed.

Multi-scale simulation of droplet–droplet interaction and coalescence

In this work, droplet–droplet interaction is modelled using a multi-scale approach which couples multiphase flow simulation using a volume of fluid method to a surface thin film model operating on the sub-grid scale. The volume of fluid model is based on a multiple marker method with a smoothed surface tension calculation, and a thin film model is derived to simulate film drainage using a Reynolds equation approach. A novel method of coupling the two allows for the prediction of coalescence or rebound of colliding droplets essentially from first principles, relying only on a critical film thickness parameter. The model is implemented using the open source tool set OpenFOAM and tested against experimental results of colliding hydrocarbon droplets from the literature. It is found to produce accurate interface deformation results for the duration of the collision, and to consistently predict the outcome of the collision process.

Coupled 3D Numerical Model of Droplet Evolution Behaviors during the Magnetically Controlled Electroslag Remelting Process

A transient three-dimensional coupled numerical model has been established to clarify the influence of the external transverse static magnetic field (TSMF) on droplet evolution behaviors during the electroslag remelting (ESR) process with an alternating current (50 Hz). The development of the electromagnetic field code and the calculation procedure showed that the periodic inverse Lorentz force (50 Hz) could be fully coupled with the volume of fluid model in each iteration when an external TSMF of 0.05 T is superimposed. The droplet evolution behaviors with and without the external TSMF were demonstrated and analyzed. The numerical results showed that the huge periodic inverse Lorentz force could smash the droplet neck into numerous smaller droplets during the ESR process with the external TSMF; simultaneously, the interfacial area and falling time could be increased, indicating that the purification efficiency could be expected to improve.

Numerical Analysis of Initial CO2 Bubbles Leaked in Seawater from Ocean CO2 Storage using Volume of Fluid Method

The aim of this article is to propose a model to predict initial size and shape of CO2 bubbles leaked in shallow seawater from ocean CO2 storage. The volume of fluid (VOF) model in FLUENT was employed. The initial bubble sizes were predicted to increase with the increases in leakage velocity and leakage orifice diameter. The leaked bubbles were predicted to form into various shapes. A comparison of bubble shape predicted by the VOF model and observed from the recently published experimental data showed a reasonable agreement. It was found that the VOF method can be a reliable approach for predicting the initial size and the shape which are used to calculate a rising velocity and a dissolution rate of the bubble during the occurrence of the CO2 leakage in the ocean.

Dynamic Simulation and Control of an Active Roll Reduction System Using Free-Flooding Tanks With Vacuum Pumps

Ship roll motion is critical for offshore operations due to its lack of damping mechanism. This paper demonstrates a dynamic simulation scheme of an active roll reduction system using free-flooding tanks controlled by vacuum pumps. A tank is installed on each side of a catamaran. Both the tank hatches are opened to the sea and the air chambers of both tanks are connected by an air duct. Vacuum pumps and air valve stabilized the wave-induced roll motion by controlling the water levels in the tanks through a feedback controller. The catamaran is a dynamic model with single degree-of-freedom (DOF) in roll, and its hydrodynamic behavior is calculated using potential theory by SHIPX. The air chambers are modeled as isothermal processes of ideal gas. The behavior of the liquid flow in a tank is simulated by incompressible Reynolds-averaged Navier–Stokes solver with the volume of fluid model, then summarized as a response function for the vessel model. A simplified control plant model for the vacuum pumps is proposed where higher order behaviors are neglected and the external wave-induced load is unknown. A parameter-dependent observer and a backstepping controller are adopted to estimate the external load and to reduce the roll motion. The system stability is proved by Lyapunov's direct method. The performance of the entire system is evaluated in terms of roll reduction capability and power cost. The system is more suitable for roll reduction in low-speed or resting conditions.

Drawdown mechanism of light particles in baffled stirred tank for the KR desulphurization process

Abstract To improve the efficiency of the desulfurization process, the drawdown mechanism of light particles in stirred tank is studied in this paper. For both up and down pumping modes, the just drawdown speeds (Njd) of floating particles in transformative Kanbara Reactor (KR) are measured in one and four baffled stirred tanks experimentally. Then numerical simulations with standard k-e model coupled with volume of fluid model (VOF) and discrete phase model (DPM) are conducted to analyze the flow field at the just drawdown speed Njd. The torques on the impeller obtained from experiments and simulations agree well with each other, which indicates the validity of our numerical simulations. Based on the simulations, three main drawdown mechanisms for floating particles, the axial circulation, turbulent fluctuation and largescale eddies, are analyzed. It's found that the axial circulation dominates the drawdown process at small submergence (S = 1/4T and 1/3T) and the large-scale eddies play a major role at large submergence (S = 2/3T and 3/4T). Besides, the turbulent fluctuation affects the drawdown process significantly for up pumping mode at small submergence (S = 1/4T and 1/3T) and for down pumping mode at large submergence (S = 2/3T and 3/4T). This paper helps to provide a more comprehensive understanding of the KR desulphurizer drawdown process in the baffled stirred tank.