VOF Simulations Research Articles

Mixing and recirculation characteristics of gas–liquid Taylor flow in microreactors

The effects of operating parameters (capillary and Reynolds numbers) and microchannel aspect ratio (α=w/h=[1;2.5;4]) on the recirculation characteristics of the liquid slug in gas–liquid Taylor flow in microchannels have been investigated using 3-dimensional VOF simulations. The results show a decrease in the recirculation volume in the slug and an increase in recirculation time with increasing capillary number, which is in good agreement with previous results obtained in circular and square geometries (Thulasidas et al., 1997). In addition, increasing the aspect ratio of the channel leads to a slight decrease in recirculating volumes but also a significant increase in recirculation times.

Simulation of two-dimensional droplet collisions based on SPH method of multi-phase flows with large density differences

In this paper, the corrective continuity equation proposed by Ott and the momentum equation improved by Adami combine to solve two-dimensional simulation problems of droplet collisions in air. To effectively improve the calculation accuracy, the artificial viscosity equation and the artificial stress equation are derived which are suited for multi-phase flows with large density differences. This method is validated to be effective via examples of an initially square droplet under surface tension and in evolution process of two droplets in air. Droplet collisions for different Weber numbers (8.8, 19.8) and different impact parameters (0, 0.5) are simulated, all of which are compared with the results of VOF simulation. Through further calculation, distribution map of the two-dimensional droplet collision outcomes in air is obtained, which is in agreement with the experimental results. It is demonstrated that this method can be effective for solving problems of droplet collisions which are involved in multi-phase flows with large density differences, and is easily extended to three-dimensional simulation; thus it lays a foundation for further simulation of the secondary atomization in liquid rocket engines.

Low-pressure thermal bonding

Polymer-based micro/nano fluidic devices are becoming increasingly important for biological applications and fluidic control. In this paper, we propose a new method for the enclosure of the large-area channel by thermal bonding at low pressure using a flexible PET film material as bonding substrate. The flexible film allows bonding of micro/nano pattern features at such a low pressure primarily due to “sequential” bonding made possible by the mold flexibility and the conformal contact made between the bonding layer and the structure layer in large area. According to the bonding mechanism investigated using VOF simulation and experiments, high bonding temperature makes melt bonding polymer be filled into the trenches inevitably and achieve the chain entanglement of the polymer over the boundary by the capillary force. This fabrication technique has great potential for low-cost mass production of polymeric-based micro/nano fluidic devices.

A mesh-dependent model for applying dynamic contact angles to VOF simulations

Typical VOF algorithms rely on an implicit slip that scales with mesh refinement, to allow contact lines to move along no-slip boundaries. As a result, solutions of contact line phenomena vary continuously with mesh spacing; this paper presents examples of that variation. A mesh-dependent dynamic contact angle model is then presented, that is based on fundamental hydrodynamics and serves as a more appropriate boundary condition at a moving contact line. This new boundary condition eliminates the stress singularity at the contact line; the resulting problem is thus well-posed and yields solutions that converge with mesh refinement. Numerical results are presented of a solid plate withdrawing from a fluid pool, and of spontaneous droplet spread at small capillary and Reynolds numbers.

Height functions for applying contact angles to 3D VOF simulations

AbstractA rigorous methodology is presented for applying a contact angle as a contact line boundary condition within a 3D volume‐of‐fluid‐based flow algorithm, based on the recently developed height function methodology that yields accurate interface normals and curvatures from volume fractions. We demonstrate that the approach yields accurate estimates of curvature and surface tension at a contact line, values that converge with spatial refinement. We then study the efficacy of this approach via examples of both static and dynamic contact line phenomena. Copyright © 2008 John Wiley & Sons, Ltd.

Height functions for applying contact angles to 2D VOF simulations

AbstractRecent work on a consistent representation of surface tension and on the accurate computation of interface curvature has extended the applicability of the volume‐of‐fluid, or VOF, method to surface tension‐driven phenomena. We have extended these methodologies to interfaces near solid surfaces; specifically, we show how height functions can be used to enforce a contact angle boundary condition at a contact line, for the full range of contact angles. As such, this work may be viewed as following up on the methodology of Renardy et al. (J. Comput. Phys. 2001; 171:243–263). This approach yields accurate values of curvature and the surface tension force at contact lines, values that converge with mesh refinement. The efficacy of the methodology is demonstrated using 2D examples of static fluid configurations in equilibrium and dynamic contact line phenomena that approach known equilibrium configurations. Copyright © 2007 John Wiley & Sons, Ltd.

流体体積 (VOF) 法を用いた気中へ噴霧された2流体ジェットからの液滴生成・分裂のシミュレーション

流体体積 (VOF) 法を用いた気中へ噴霧された2流体ジェットからの液滴生成・分裂のシミュレーション

Studies of gas–liquid flow through reactors internals using VOF simulations

Two-phase flow through reactor internals have been experimentally and numerically studied. Experiments have been carried out with a setup running under ambient pressure for two configurations. The first configuration consists of a mixing box orifice inlet through which liquid flows as a film sheared by a gas flow. The liquid height at orifice inlet is documented over a wide range of liquid and gas flowrates. The second configuration consists of the two-phase flow through a downcomer of a distributing tray. Two and three dimensional computational fluid dynamic (CFD) simulations using the volume of fluid approach have been used to compute both flows for similar flow conditions as used in the experiments. It is shown that the agreement between experiments and calculations is very good. Based on this good agreement, it is finally discussed how CFD can be used to achieve better design rules for gas liquid reactor internals via simulations carried out for industrial process conditions.

Comparison of SPH simulations of high pressure die casting with the experiments and VOF simulations of Schmid and Klein

SPH simulations of die filling for three geometries previously studied by Schmid and Klein are compared to their experimental and numerical results. The agreement with experiment for both numerical methods is very good, but the SPH solutions tend to capture free surface and void shapes modestly better than the VOF and also predict small scale structure better. These result from the Lagrangian nature of SPH and the superior mass conservation properties of this particle method.