Coupled Level Set Research Articles

Numerical Investigation of Liquid Water Transport Dynamics in Novel Hybrid Sinusoidal Flow Channel Designs for PEMFC

This study numerically investigates liquid water dynamics in a novel hybrid sinusoidal flow channel of a proton exchange membrane fuel cell (PEMFC). The two-phase flow is examined using a three-dimensional, transient computational fluid dynamics (CFD) simulation employing the coupled level set and volume of fluid (VOF) method. Simulations for hybrid and non-hybrid sinusoidal flow channels, including a straight flow channel, are compared based on their water exhaust capacities and pressure drops. Additionally, the effects of inlet gas velocity, wall wettability, and droplet interaction in the flow channel on the dynamic behaviour of liquid water are investigated. Results reveal that the novel hybrid sinusoidal channel designs are consistent in terms of quicker water removal under varying hydrophilic wall conditions. Also, it is found that the liquid surface coverage, detachment, and removal rate depends on droplet proximity to the walls, inlet gas velocity, and wall contact angle. Also, the time a droplet makes contact with the side walls affect the discharge time. Additionally, there is an improvement in the gas velocity magnitude and vertical component velocity across the hybrid sinusoidal channel designs. Therefore, the unique geometric configuration of the proposed hybrid design makes it a viable substitute for water management in PEMFC applications.

Evolution and heat transfer after droplet impact on heated liquid film with vapor bubbles inside

To better understand the droplet impact on the liquid film with vapor bubbles in spray cooling, a two-dimensional symmetric numerical model is set up using the Coupled Level Set and Volume of Fluid method (CLSVOF). Three simulative cases are taken, considering the effects of film thickness and the presence of vapor bubbles or not. The main purposes of this paper are to investigate the evolution of vapor bubbles during droplet impact and to identify the effect of vapor bubbles on convection heat transfer. The results indicate that vapor bubbles will detach from the wall and break up at the surface of the liquid film during droplet impact, for a thinner film, later a “sawtooth” liquid film appears at the non-impact region. However, for a thicker film, no bubbles rupture and the detached bubbles will flow inside the liquid film and then some will merge into larger bubbles. In the presence of vapor bubbles, the crater radius is larger for a thicker liquid film. The presence of vapor bubbles will facilitate the subcooled droplet to spread to the heated wall, leading to a substantial increase in surface heat flux.

Computational and experimental analysis of droplet transportation/jetting behaviours driven by thin film surface acoustic waves

A Coupled Level Set Volume of Fluid (CLSVOF) approach has been applied to investigate severe deformation/transportation/jetting behaviours of sessile droplet driven by thin-film surface acoustic waves (SAW) devices. For validation of this computational method, a series of experimental studies of droplet transportation/jetting were performed using ZnO/Si thin film based SAW devices with resonant frequencies ranging from 64.49 MHz to 271.36 MHz. Good agreements between the computational and experimental results showed the capability of the developed CLSVOF method in modelling complex acoustofluidics phenomena such as significant internal streaming, pumping and jetting of the droplet driven by the propagating SAW.Results obtained from the computational model are used to clarify the fluidic mechanisms of droplet oscillation and wobbling behaviours during transportation. Numerical results reveal the liquid streaming patterns and airflow velocity field around the droplet at different stages of transportation/jetting process. Effects of droplet volume, the resonant frequency of SAW devices and applied SAW power on droplet transportation/jetting were investigated both theoretically and experimentally. In particular, comparisons between experimental and computational results showed that the model predicted well the minimum RF power to start droplet pumping and jetting at various resonant frequencies.

Detailed nonlinear dynamics of the liquid spike development in gaseous medium caused by a three-dimensional Rayleigh-Taylor instability

Previous researches on the single-mode Rayleigh-Taylor (RT) instability were mainly focused on the advancing speed of gaseous bubbles, while few of them shed light on the nonlinear dynamics of the liquid spikes (jets) where the droplets are constantly generated in the liquid atomization case. In this paper, a three-dimensional (3D) numerical simulation on a single-mode RT instability (Atwood number is close to unity) with the focus on the development of the liquid spike was conducted by the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method. Nonlinear dynamic characteristics of the RT instability were then analyzed thoroughly based on the detailed information of pressure and velocity fields obtained from the numerical simulation. The numerical results revealed that a local maximum pressure point, where an equilibrium was achieved between the inertial force and pressure gradient, was formed at the root of liquid spike region caused by the horizontal impinging flow. Combined with the theoretical analysis, the relationships among the steady-state characteristic parameters, the initial perturbation wavelength and the inertial acceleration were established. It is found that the velocity and width of the liquid entering the freed liquid spike at the maximum pressure point exhibit the same characteristics as those of a vertical jet emanating downward from an orifice injector under gravity. Thus, theories concerning the low-speed jet can be used to predict the disintegration behavior of a liquid spike caused by RT instability.

Free convection film boiling over a discrete flat heater with different surface orientations

In the present numerical study, a two-dimensional, laminar, free convection saturated film boiling of water at near critical conditions over a discrete heater surface flush mounted over a flat plate has been investigated. For the numerical simulations, a multi-directional advection algorithm based Coupled Level Set and Volume of Fluid (CLSVOF) interface capturing method is used. In this study, both the flow and heat transfer characteristics are evaluated for various heater sizes, wall superheats, and the angle of orientations of the flat plate. The numerical modeling of the three phase moving contact lines is evaluated with the experimental data of a single droplet impact and spreading over a horizontal flat solid surface. From the numerical study, it is observed that the heat transfer rate from a small size discrete heater surface is higher compared to that of a large size heater surface. It is also observed that the variation of the plate orientation angle, α of the heater surface from upward facing horizontal position (α = 0°) to vertical position (α = 90°) shows increase in the magnitude of upward rising vapor velocity due to increase in the buoyancy force. It leads to higher rate of heat transfer at vertical position compared to that of horizontal position.

Saturated film boiling over a circular cylinder subjected to horizontal cross-flow in the mixed regime

Film boiling over a circular cylinder in a horizontal cross-flow of saturated liquid is studied in the mixed regime that is characterized by a combined influence of buoyancy and flow inertia at low magnitudes of the Froude number (Fr). Liquid-vapor interface evolution and the ensuing vapor wake dynamics together with heat transfer have been determined through a computational framework developed for phase change problems on two-dimensional unstructured grids using a coupled level set and volume of fluid interface capturing method. While the quasisteady nature of ebullition cycle is gradually lost as Fr increases, the effect of cross-flow orientation with respect to gravity is shown to be nontrivial in the mixed regime. A direct consequence of the orthogonal gravity and flow fields is an anomalous impairment of heat transfer with an increase in cross-flow velocity under certain conditions, which is discussed in detail. Simultaneously, the film boiling behavior as influenced by several other hydrodynamic and thermal parameters is also ascertained. The interplay between buoyancy and inertia is further highlighted while discussing the interdependent liquid and vapor wake characteristics in the mixed regime with horizontal cross-flow. The liquid wake behavior is shown to result not only from the bluff body geometry but also the instantaneous vapor wake profiles, with the wall superheat affecting the time scale of wake interaction. Finally, a reduction factor (ξ) is proposed and determined as a function of the Froude number, which is used in conjunction with a correlation for upward cross-flow film boiling to predict the heat transfer.

Numerical simulation of dam-break flow impacting a stationary obstacle by a CLSVOF/IB method

In this paper we develop a Coupled Level Set and Volume Of Fluid and Immersed Boundary (CLSVOF/IB) method, and apply it to simulating the interaction between dam-break flow and stationary obstacles. The projection method which adopts the second-order Adams–Bashforth algorithm is performed for solving the Navier-Stokes equations on a fixed staggered Cartesian grid. The Point Successive Over-Relaxation (PSOR) method is then employed to solve the discretized linear system of the Poisson pressure equation. For fluid-fluid interface treatment, the CLSVOF model is implemented using the Tangent of Hyperbola for INterface Capturing (THINC) scheme with the Weighted Linear Interface Calculation (WLIC), and the level set function which is a distance function is used to calculate interface normal vectors. For solid-fluid interface treatment, we adopt the direct forcing IB method which can be easily extended to three-dimensional simulations. The artificial momentum forcing term is applied at certain points in cells containing both fluid and solid, which imposes a velocity condition to drive the solid body motion. The proposed CLSVOF/IB model is used to simulate dam-break flow and its interaction with stationary structures, and the results agree well with numerical and experimental data in the literature.

Effect of heaving motion on two-phase flow in horizontal channel: Flow and heat transfer characteristics

Experiments were carried out to investigate the effects of vibration frequency (0Hz~8Hz) and amplitude (0 mm~8 mm) on gas-liquid two-phase flow characteristics in a channel under heaving motion. The magnitude of instantaneous frictional pressure drop under heaving motion increases with the increasing vibration parameters. A 3-D numerical simulation on heat transfer characteristics at various working conditions with a constant applied wall heat flux was performed using the CLSVOF (Coupled Level Set and Volume of Fluid) model combined with dynamic mesh and UDF (User Defined Function) in Fluent. The results show that the heaving motion has a favorable effect on heat transfer coefficient. The fluid temperature reaches its maximum value is about 0.1s–0.2s later than the wall temperature. According to the field synergy analysis, there exists a most appropriate combination of vibration parameters. It was also found that, similar to the temperature variation, the maximum field synergy number occurs approximately when the channel moves from the highest point of heaving motion to the equilibrium position.

Numerical approach for generic three‐phase flow based on cut‐cell and ghost fluid methods

SummaryIn this paper, we introduce numerical methods that can simulate complex multiphase flows. The finite volume method, applying Cartesian cut‐cell is used in the computational domain, containing fluid and solid, to conserve mass and momentum. With this method, flows in and around any geometry can be simulated without complex and time consuming meshing. For the fluid region, which involves liquid and gas, the ghost fluid method is employed to handle the stiffness of the interface discontinuity problem. The interaction between each phase is treated simply by wall function models or jump conditions of pressure, velocity and shear stress at the interface. The sharp interface method “coupled level set (LS) and volume of fluid (VOF)” is used to represent the interface between the two fluid phases. This approach will combine some advantages of both interface tracking/capturing methods, such as the excellent mass conservation from the VOF method and good accuracy of interface normal computation from the LS function. The first coupled LS and VOF will be generated to reconstruct the interface between solid and the other materials. The second will represent the interface between liquid and gas.

The Importance of Viscous and Interfacial Forces in the Hydrodynamics of the Top-Submerged-Lance Furnace

The purpose of this work is to focus on the hydrodynamics of a Top-Submerged-Lance (TSL) smelting furnace, understanding how liquid properties and operational parameters act on key factors of a TSL process, such as splashing, mixing, mass transfer area, and bubble development. A deep knowledge of all those aspects is needed since they all influence the smelting reaction rates; hence the efficiency of the reactor. The characterization and scaling of the TSL gas injection are commonly based on the modified Froude number, the ratio of dynamic and gravitational forces. Detailed literature research reveals a potential weakness of this approach, since it does not consider the effects of viscosity and surface tension. To investigate this question an extensive parametric study was performed applying computational fluid dynamics to cold and non-reactive flows, which provided a broad overview of the physics of the flow. The analysis was performed on fluid dynamic properties (liquid density, liquid viscosity, surface tension) and operational variables (gas volume flow, lance immersion depth). The coupled Level Set—Volume of Fluid model, available in the commercial solver ANSYS Fluent®, was used to resolve the gas–liquid interface in the multiphase flow. The results of the work underscore the significance of the viscous and interfacial forces for gas injection in smelting slags, confirming the incompleteness of applying only the Froude number to describe such flows.

Numerical simulation of hydrodynamic and heat transfer characteristics of slug flow in serpentine microchannel with various curvature ratio

A numerical investigation is carried out to study hydrodynamic behavior and thermal performance of slug flow in serpentine microchannel with different curvature ratio and wall properties. By using coupled Level Set and Volume of Fluid (CLSVOF) method implemented in Fluent, gas-liquid interface shows good consistency with experimental results especially for slug flow. Distribution of sectional velocity profile in curved microchannel with various curvature ratios was investigated. On the basis of fluid flow in microchannel with non-slip boundary condition, by means of user defined function (UDF), influence of slip velocity on hydrodynamic characteristics was performed numerically. For a better prediction on thermal performance, further analysis of combined effect of wall properties and curvature ratio on fluid flow characteristics was also conducted. As a consequence, the increasing slip velocity on channel wall can reduce hydraulic resistance. Both the contact angle and the curvature ratio have an appreciable influence on thermal performance of gas-liquid two-phase flow in serpentine microchannel.

Deformation and coalescence of water droplets in viscous fluid under a direct current electric field

Deformation and coalescence behaviors of water droplets in viscous fluid under a direct current electric field are investigated with experiments and numerical simulations. An electro-hydrodynamic model is developed based on fluid dynamics equations and electrostatic equations. A coupled level set and volume of fluid method is proposed to capture the dynamic interface. The numerical results are validated to be in good accordance with classic results in the literature. The effects of electric capillary number Ca and viscosity ratio λ on droplet deformation and coalescence are systematically analyzed. Two different droplet response modes are obtained, namely overdamping response and underdamping response. The conversion of response modes is discussed with the (Ca, λ) phase diagram. Moreover, the distributions of electric field and flow field before and after the droplets contact are compared and analyzed. Numerical results show the time evolution of liquid bridge is dominated by the interfacial tension and viscosity of continuous phase. The electric stress is extremely weak at the liquid bridge and has a slight influence on evolution of liquid bridge. The correlation formula between dimensionless liquid bridge radius and conical angle during the growth of liquid bridge is regressed. These results are helpful for the optimization of electro-coalescence process.

Numerical study on dynamic characteristics of double droplets impacting a super-hydrophobic tube with different impact velocities

ABSTRACTThe coupled level set and volume of fluid method is applied to the numerical study on the successive impact of double droplets on a super-hydrophobic tube. The impact velocity varies from 0.25 to 2 m/s. These impact processes present spread, retract, rebound, breakup and splash. The out-of-phase impact takes place with the impact velocity from 0.25 to 1.25 m/s, while the in-phase impact takes place with the impact velocity from 1.44 to 2 m/s. With the impact velocity larger than 1.25 m/s, the liquid crown presents and deforms after the trailing droplet impact, then it would gather at the film edge, rebound or break up. When impact velocities range from 1.44 to 1.5 m/s, the finger liquid film presents before the liquid crown appearing. The finger head breaks with the impact velocity of 1.5 m/s during the leading droplet spreading. The zigzag liquid film becomes more obvious for larger velocities.

Research on dam-break induced tsunami bore acting on the triangular breakwater based on high order 3D CLSVOF-THINC/WLIC-IBM approaching

In present study, high order three dimensional coupled level set and volume of fluid (CLSVOF) based simulations about dam-break induced tsunami bore interacting with the stationary triangular breakwater was numerically investigated. Under our CLSVOF approaching, VOF advection equation is solved by tangent of hyperbola for interface capturing (THINC) with weighed linear interface calculation (WLIC) scheme, and level set advection equation is solved directly by the high-resolution optimized compact reconstruction weighted essentially non-oscillatory (WENO) scheme. Navier-Stokes equations is solved by a projection method on a fixed staggered Cartesian finite difference mesh. The pressure Poisson equation is solved by the point successive over-relaxation (PSOR) method. The direct forcing immersed boundary method (IBM) was implemented for breakwater-tsunami bore interface treatment. Numerical experiments based on our high order 3D CLSVOF-THINC/WLIC-IBM method were carried out in a rectangular water channel with a stationary typical triangular breakwater. The numerical results were compared with corresponding laboratory experimental results and reasonable agreements were obtained with satisfied accuracy. Numerical cases with different shape of triangular breakwater are carried out then in order to investigate the free surface evolution and energy dissipation due to the breakwater and useful conclusion is obtained.

Numerical modeling of the dynamics of bubble oscillations subjected to fast variations in the ambient pressure with a coupled level set and volume of fluid method.

A numerical method for modeling and understanding the dynamics of bubble oscillations subjected to fast variations in the ambient pressure is proposed under low Mach number conditions. In the present work, the method uses a single-fluid continuum formalism of weakly compressible axisymmetric Navier-Stokes equations for the numerical simulation of liquid-gas flows with surface tension and adopts the interface capturing approach based on a coupled level set and volume of fluid (CLSVOF) method for describing the moving and deformed interfaces. To demonstrate the efficacy of the proposed method, first, the numerical results of the radial oscillations of a spherical gas bubble are tested with the numerical solutions of Rayleigh-Plesset equation. Then, the numerical method is applied to reproduce the growth and subsequent collapse of a bubble in an infinite liquid medium observed in experiments. Finally, the numerical simulation of the interaction of two oscillating bubbles at small separation distance is evaluated in response to a moderate step change in the ambient pressure. It is shown that two deformable bubbles undergo coupled radial and oscillatory translational motions which eventually results in the bubbles' attraction and coalescence caused by the secondary Bjerknes forces. The numerical predictions show very good accuracy with the experimental and numerical results reported in the literature.

Numerical study of a laser generated cavitation bubble based on FVM and CLSVOF method

Based on the open source software OpenFOAM, the finite volume method (FVM) is applied for discretization of the Navier-Stokes equation. A coupled level set and volume of fluid (CLSVOF) method is established to track the movement of the gas-liquid interface, for improving the accuracy of the simulation of interface curvature and surface tension force. The spherical collapse of a laser generated bubble in water is modeled with the consideration of viscosity, compressibility of both the gas and the liquid, and the surface tension of the liquid. The variations of the bubble radius during collapse predicted by the CLSVOF method are compared with the Gilmore model, the VOF method, which are more consistent with the experimental results. The velocity and the pressure around the bubble are also investigated. And the shock waves emitted by the bubble during the process are illustrated.

LES analyses of the air-core vortex in intake flow field of pumping station

Air-core vortex is a universal and attention-getting phenomenon in intake flow field of pumping station. Due to the difficulties and complexities of two-phase flow of the air-core vortex, the Coupled Level-Set and Volume-of-Fluid (CLSVOF) method is adopted in the simulation, and a horizontal intake pipe in pump sump was calculated. Results of the simulation were consistent with that of experiment. The air-core vortex is successfully reproduced in the simulation, and four stages during its formation is observed. At the early stage of the simulation, vortices generated in the vicinity of the free surface are gathered in the stagnation region above the pipe intake. The intensity of the gathering vortices increases due to the stretching effect of ωy (vertical vorticity component), and move toward to the pipe intake due to the downward flow motion of the stagnation region. Furthermore, air-core vortex meandering is observed from the time series of instantaneous vortex motion, and quasi-periodicity is founded in its meandering, which is caused by the quasi-periodic characteristic of the velocity filed. The dominant frequency (fD/vD) of coordinate x of the air-core vortex center is around 0.0096, and coordinate z is around 0.0191.

Understanding the Influence of Rheological Properties of Shear‐Thinning Liquids on Segmented Flow in Microchannel using CLSVOF Based CFD Model

AbstractIn this study, two phase gas‐shear‐thinning liquid flow in a square microchannel is numerically investigated using the coupled level set and volume of fluid (CLSVOF) methods. A systematic investigation is carried out to explore the influence of polyacrylamide (PAM) concentration, surface tension, velocity ratios, and contact angle on the gas slug length, volume, unit cell length, and pressure drop. Three different concentrations of PAM solutions, which exhibit shear‐thinning behaviour are considered as the continuous phase. Gas slug length, volume, and unit cell length decreased with increasing the PAM concentration. Velocity and non‐homogeneous viscosity distributions in the liquid slug for three different PAM concentration solutions are reported. Gas slug length decreases with an increase of the contact angle and the bubble shape change from convex to concave. This numerical work provides the fundamental insights in segmented flow formation and two‐phase flow characteristics comprising shear‐thinning liquids.

Three-dimensional VOF simulation of droplet impacting on a superhydrophobic surface

It is very important to analyze and study the motion process of droplets impacting superhydrophobic surface, which is of great significance to understand the mechanism of superhydrophobic surface and guide the design and manufacture of superhydrophobic surface. Taking by three-dimensional volume of fluid (VOF) simulation coupling coupled level set (CLS) algorithm, on the one hand, we simulate the morphological changes in the process of droplet impingement, as well as the internal velocity and the pressure distribution; on the other hand, we focus on the effects of droplet impact velocity, surface wettability, surface tension on the dynamics of the droplets. The CLSVOF model inherits the advantages of the VOF model for accurately constructing the phase interface and inherits the advantage that the level set can accurately calculate the surface tension, which improves the accuracy of the calculation of the droplet impact on the superhydrophobic surface. The computed results distinctly demonstrated there were four stages: falling, spreading, shrinking and rebounding. The time history of each stage agreed well with the pictures captured by high-speed camera, which indicated the computational fluid dynamics scheme was effective. Moreover, the motion mechanism of the droplets impacting on the solid surface is elaborated, which was helpful to control the solid–liquid interface to achieve a variety of solid interface characteristics.

Numerical simulation of bubble changes in an oil film impacted by oil droplets via a coupled level set and volume-of-fraction method

A coupled level set and volume-of-fraction method is applied to investigate the impact of an oil droplet on an oil film containing a bubble. Subsequently, the deformation of the bubble is analyzed. Simulation results show two different movement forms of bubbles after an oil droplet impacts an oil film under different impacting velocities: stable deformation and bubble breaks. Bubble breaks originate from the combined action of surface tension and viscous shear force. Oil film thickness also affects bubble deformation. As the film thickness decreases, the deformation quantity of bubbles increases. Thus, bubbles are easily broken. This study provides a basic guide for bubble dynamics when a droplet impacts a film.