•Different stages of drop formation under horizontal tubes are numerically simulated.•Hydrodynamic of in-line jet mode between tubes are captured numerically.•Transient nature of drop mode makes ...different film thickness around the tubes.•Average local absorbed mass flux in drop mode is ten times larger than the jet mode.
The rate of absorbed water vapor over a horizontal tube bank in a water-cooled falling film absorber depends on several parameters and working conditions. The flow regime between the tubes in falling film absorbers is one of the most influential parameters in the amount of the absorbed vapor. Among different falling film regimes, in this study, the drop and jet modes are simulated numerically. The full Navier-Stokes equations are solved and the well-known volume of fluid (VOF) method is used to capture the gas/liquid interface. In addition, the energy and diffusion equations are solved in this 3D-simulation with aid of an in-house CFD code. Adaptive mesh refinement, in accordance with the magnitude of volume fraction gradient, has strongly improved the interface capturing and therefore increased the accuracy of simulation. The simulation results reveal that by changing just the regime from drops to jets, the rate of average vapor mass flux decreases one order of magnitude, from 6.3×10-3 to 4.76×10-4kg s-1m-2.
•We perform direct numerical simulations of abrupt reconfigurations in angular pores.•During spontaneous meniscus reconfiguration, inertial effects dominate.•Oscillations cannot be controlled by ...injection rate but possess their own time scale.•An energy-based analysis explains the oscillations induced by surface energy release.•In a network of angular pores the local dynamics influence macroscopic quantities.
In porous media, the dynamics of the invading front between two immiscible fluids is often characterized by abrupt reconfigurations caused by local instabilities of the interface. As a prototype of these phenomena we consider the dynamics of a meniscus in a corner as it can be encountered in angular pores. We investigate this process in detail by means of direct numerical simulations that solve the Navier–Stokes equations in the pore space and employ the Volume of Fluid method (VOF) to track the evolution of the interface. We show that for a quasi-static displacement, the numerically calculated surface energy agrees well with the analytical solutions that we have derived for pores with circular and square cross sections. However, the spontaneous reconfigurations are irreversible and cannot be controlled by the injection rate: they are characterized by the amount of surface energy that is spontaneously released and transformed into kinetic energy. The resulting local velocities can be orders of magnitude larger than the injection velocity and they induce damped oscillations of the interface that possess their own time scales and depend only on fluid properties and pore geometry. In complex media (we consider a network of cubic pores) reconfigurations are so frequent and oscillations last long enough that increasing inertial effects leads to a different fluid distribution by influencing the selection of the next pore to be invaded. This calls into question simple pore-filling rules based only on capillary forces. Also, we demonstrate that inertial effects during irreversible reconfigurations can influence the work done by the external forces that is related to the pressure drop in Darcy’s law. This suggests that these phenomena have to be considered when upscaling multiphase flow because local oscillations of the menisci affect macroscopic quantities and modify the constitutive relationships to be used in macro-scale models. These results can be extrapolated to other interface instabilities that are at the origin of fast pore-scale events, such as Haines jumps, snap-off and coalescence.
•The condensation flow in a rectangular microchannel is numerically studied.•The simulated flow patterns agree well with the experimental observations.•Waves along the interface form necks decreasing ...the local vapor pressure.•The initial bubble size increases as the flow mass flux increases.•The initial bubble size increases as the heat flux decreases.
The condensation flow of the refrigerant FC-72 in a rectangular microchannel with a 1-mm hydraulic diameter is numerically studied using the volume of fluid (VOF) model. The heat transfer related to the condensation is taken into account by a thermal equilibrium model assuming the interface temperature is at saturation. The numerical method is validated against experiments from the literature and well predicts the flow patterns along the microchannel. The vapor phase in the microchannel forms a continuous column with a decreasing diameter from upstream to downstream. Slugs are periodically generated at the head of the column. Decreasing the wall cooling heat flux or increasing the flow mass flux increases the vapor column length. Waves along the interface cause necks in the column and locally increase the vapor velocity and decrease the pressure, facilitating breakage of the vapor column into slugs. The liquid temperature is close to saturation near the interface and lower downstream and in the thin liquid layer close to the cooling surface. The initial bubble size increases with increasing flow mass flux or decreasing cooling heat flux.
This study is devoted to the development of a VoF/GFM solver using the open source CFD library OpenFOAM. An industrial point of view is adopted in this paper, meaning that simulations are run with ...large CFL conditions. Weaknesses of the standard OpenFOAM VoF solver interFoam are identified (free surface wiggles and light phase accelerations). Using available literature, MULES algorithm has been abandoned to the profits of algebraic schemes (HRIC, MHRIC, MCICSAM) and the Ghost Fluid Method (GFM, Fedkiw et al. (1999)) has been adopted for dealing with free surface discontinuity. The resulting solver called marineFoam is tested and compared to interFoam on the following test cases : 3D wave impact on a tall structure (dam break) (Gomez-Gesteira, 2013), periodic wave propagation simulation (Larsen et al., 2019), model and full-scale ship resistance simulations (Larsson et al., 2014; Tokyo Workshop, 2015; Lloyd’s Register, 2016). marineFoam can allow large CFL numbers while avoiding free surface wiggles and most of spurious currents. interFoam achieves reliable results but requires additional VoF sub-cycle iterations or a shorter time step.
Double emulsion droplets generation is convenient for drug delivery applications since the core-shell template has the ability to increase the success of the target and release of the drug. In this ...study, the modified flow-focusing microfluidics device is proposed to generate double emulsion droplets with high monodispersity and high throughput, satisfying industrial needs. The W/O/W (water-in-oil-in-water) encapsulation template is a common combination in drug delivery. The research aims to analyze double emulsion droplets generation using the 2D Volume of Fluid (VoF) VoF approach, which is able to visualize flow regime, droplets average diameter, Coefficient of Variation (CoV), and droplets generation rate. Combination of water-in-olive oil-in-water was used as the working fluids. The diameter of the droplets, CoV, and generation rate was obtained using image processing. The simulation results showed that the injection model and the sudden expansion in the modified flow-focusing device successfully produced double emulsion droplets with two dripping instabilities. The narrowing jetting flow regime is obtained, including its droplets evolution. The average diameter for both outer and inner droplets were achieved with their CoV, including the generation rate. The outer and inner droplet's diameter generated can potentially be implemented for drug delivery, although the inner droplet's monodispersity must be further investigated. Nevertheless, the proposed device and the flow control were able to generate high-throughput double emulsion droplets.
A well tested mathematical model is used to bring out important counter intuitive results on the aspect of reducing tsunami impact forces on on-shore buildings using sea dikes. The mathematical model ...is based on numerical solution of the general Reynolds Averaged Navier Stokes equations, with SST–k–ω model for turbulence closure. Tsunami waves are allowed to propagate on a 1 in 40 sloped beach and over-top a sea dike before impacting on buildings. The combined effect of various parameters such as position of the dike and shape and size of the dike has been studied, for sloping beaches. A sea dike placed far from the building may increase the tsunami impact force, if its size is not appropriate. It is found that a sea dike positioned close to the building offers best protection only if the height of the wave is small. If the wave height is large, there is an optimal distance at which the dike should be located for minimizing the tsunami impact force. Placing a second sea dike in addition to an existing sea dike may not reduce the tsunami impact force, if the location and size of the sea dikes are not appropriate.
•CFD model used for evaluating effectiveness of sea dikes to mitigate tsunami impact forces.•Location plays insignificant role if wave height is much larger than the dike height.•There exists an optimal location for a sea dike to be most effective if dike height is larger compared to wave height.•Addition of an extra sea dike of equal height may not reduce the maximum impact force significantly.
In this paper, we are interested in an interface reconstruction method for 3D arbitrary convex cells that could be used in multi-material flow simulations for instance. We assume that the interface ...is represented by a plane whose normal vector is known and we focus on the volume-matching step that consists in finding the plane constant so that it splits the cell according to a given volume fraction. We follow the same approach as in the recent authors' publication for 2D arbitrary convex cells in planar and axisymmetrical geometries, namely we derive an analytical formula for the volume of the specific prismatoids obtained when decomposing the cell using the planes that are parallel to the interface and passing through all the cell nodes. This formula is used to bracket the interface plane constant such that the volume-matching problem is rewritten in a single prismatoid in which the same formula is used to find the final solution. The proposed method is tested against an important number of reproducible configurations and shown to be at least five times faster.
Phase change calculations for film boiling flows Tsui, Yeng-Yung; Lin, Shi-Wen; Lai, Yin-Nan ...
International journal of heat and mass transfer,
March 2014, 2014-3-00, 20140301, Volume:
70
Journal Article
Peer reviewed
A new VOF based interface tracking method, termed CISIT, developed by the present authors recently is extended to include heat and mass transfer due to phase change. In this method, the interface is ...represented by the contour surface of VOF value 0.5. The interface is advanced in a conservative prediction-correction manner to ensure that the distribution of the VOF is a good approximation to the Heaviside function. An important issue related to the mass transfer across the interface is the treatment of the jump condition for the energy equation. The interface is tackled as an internal boundary for temperature field. The heat flux at the boundary is calculated separately in individual phase. However, unlike other studies, the energy equation for the two phases is solved simultaneously in an implicit way. This method is validated through tests on model problems for which theoretical solutions are available. Calculations are then performed to simulate boiling bubble flows emerging from a planar film and a circular film. The latter case also serves to demonstrate that the present methodology is applicable to phase change flows with irregular geometry. Different boiling modes are identified according to the superheated temperatures.
To investigate the fluid-rigid body interaction issues with free surfaces, a numerical approach has been developed. This algorithm is in an arbitrary Lagrangian–Eulerian description and Volume of ...Fluid (VOF) framework, using dynamic unstructured mesh to solve the coupled system. The fluid–solid interface uses partitioned Dirichlet–Neumann iterations with Aitken’s relaxation. For the two-phase fluids part, an interface geometric reconstruction approach has been applied to accurately capture the free surfaces. This piecewise linear interface calculation (PLIC) based method uses Newton’s iteration to efficiently reconstruct interfaces on an unstructured mesh, and applies an un-split scheme to transport variables. The algorithm has been successfully implemented in open source code OpenFOAM®, and was compared with the latter’s built-in solver using interface compression method to deal with free surfaces. Numerical results suggest that our solver has better accuracy on multiphase flow problems, while the previous solver fails to obtain correct interfaces. Moreover, the capacity of accurately solving fluid-rigid body interaction problems with free surfaces has been achieved. Validation cases are provided for fluid–structure interaction problems with and without free surfaces, and results are in accordance with analytical and experimental data from the literature. The algorithm and solver in this paper, can be applied on fluid–structure interaction cases with free surfaces in the future, such as sloshing and water entry problems.
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