Numerical Simulation Of Two-phase Flow Research Articles

Pressure and volume changes of an air bubble in a liquid water flow through a heated micro-channel

Purpose – The purpose of this paper is to study a gas bubble flowing in a micro-channel filled with liquid and to quantify the compressibility effects induced in the bubble by a heat supply at the walls of the channel. Design/methodology/approach – The paper presents a model and its numerical implementation. A hybrid method combining front-tracking techniques and a Heaviside step function is introduced to ensure an accurate satisfaction of the mass and energy conservation laws. Findings – Compressibility effects in the bubble are quantified. Test cases for numerical simulations of two-phase flows involving heat transfer are proposed. Originality/value – The authors present original test cases in which expansion or compression of a gas bubble flowing in a liquid are induced by heat transfer at the wall.

Numerical analysis of evaporation in microchannel under capillary pumping

Abstract The paper presents numerical simulation of two-phase flow in a heated capillary with evaporation on the meniscus. To solve the problem, a model of evaporation from meniscus was developed in which the dynamics of liquid-vapour interface is described by the Cahn-Hilliard equation. The numerical simulations were performed using commercial software for 2D axially symmetric case. The flow evolution was analysed for different values of heat transfer coefficient at the capillary wall and inlet liquid mass flow rate.

The response of two-phase hydrothermal systems to changing magmatic heat input at mid-ocean ridges

We develop numerical simulations of two-phase flow in a NaCl–H2O fluid subject to time varying basal heat flux boundary conditions in order to understand the response of hydrothermal vent temperature and salinity to changing magmatic heat input. The results show that periodic changes in basal heat input on a time scale of several years will not be detected in a continuous time series record of temperature measurements. Fluctuations in vent salinity may be recorded, however. For models with monotonic decay of the magmatic heat flux, a decline in vent temperature may not be observed for several years; however, once single phase conditions are established at the base of the system, a pulse of brine-derived fluid is expected to appear at the surface, followed by a gradual decline of salinity to the seawater value. The pulse of brine salinity is expected to occur before an observed decline in vent temperature. Observed rapid changes in vent temperature and salinity associated with either eruptive or non-eruptive magmatic events are not likely a result of changes in basal heat flux.

Numerical Simulations of Two-Phase Flow in a Self-Aerated Flotation Machine and Kinetics Modeling

A new boundary condition treatment has been devised for two-phase flow numerical simulations in a self-aerated minerals flotation machine and applied to a Wemco 0.8 m3 pilot cell. Airflow rate is not specified a priori but is predicted by the simulations as well as power consumption. Time-dependent simulations of two-phase flow in flotation machines are essential to understanding flow behavior and physics in self-aerated machines such as the Wemco machines. In this paper, simulations have been conducted for three different uniform bubble sizes (db = 0.5, 0.7 and 1.0 mm) to study the effects of bubble size on air holdup and hydrodynamics in Wemco pilot cells. Moreover, a computational fluid dynamics (CFD)-based flotation model has been developed to predict the pulp recovery rate of minerals from a flotation cell for different bubble sizes, different particle sizes and particle size distribution. The model uses a first-order rate equation, where models for probabilities of collision, adhesion and stabilization and collisions frequency estimated by Zaitchik-2010 model are used for the calculation of rate constant. Spatial distributions of dissipation rate and air volume fraction (also called void fraction) determined by the two-phase simulations are the input for the flotation kinetics model. The average pulp recovery rate has been calculated locally for different uniform bubble and particle diameters. The CFD-based flotation kinetics model is also used to predict pulp recovery rate in the presence of particle size distribution. Particle number density pdf and the data generated for single particle size are used to compute the recovery rate for a specific mean particle diameter. Our computational model gives a figure of merit for the recovery rate of a flotation machine, and as such can be used to assess incremental design improvements as well as design of new machines.

Numerical simulations of two-phase flow in an anode gas channel of a proton exchange membrane fuel cell

In this work, the two-phase flow in an anode gas channel of a PEM (proton exchange membrane) fuel cell is numerically investigated using the VOF (volume of fluid) method. Water movement in the gas channel is analyzed and the effects of hydrogen inlet velocity, operating temperature and channel walls wettability are investigated. Results reveal that for hydrophilic channel walls water moves as films in the upper surface of the channel (surface opposite to the GDL (gas diffusion layer)) whereas it moves as a droplet when the channel walls are hydrophobic. Moreover, increasing hydrogen inlet velocity, operating temperature and channel walls wettability results into a faster water removal. However, for the case when hydrogen velocity is increased, a considerable increment on pressure drop is also observed. Results from the present work provide important quantitative information that complements experimental data from literature.

LentFoam – A hybrid Level Set/Front Tracking method on unstructured meshes

Hybrid algorithms for Direct Numerical Simulations of two-phase flows are designed to provide better solutions with respect to absolute accuracy, error convergence, numerical boundedness and volume (mass) conservation errors, compared to the combined original algorithms themselves. The algorithms developed so far support structured and Cartesian meshes as well as octree-based and block-structured local mesh refinement in the vicinity of the fluid interface.Using unstructured meshes for domain discretization represents a standard way to deal with flow domains of arbitrary geometrical complexity. Local dynamic Adaptive Mesh Refinement (AMR) enables efficient simulations of two-phase flows by providing accuracy where it is most needed: near the fluid interface.In this work we present a first to our knowledge hybrid Level Set/Front Tracking DNS method of two-phase that supports both accurate and very efficient computations on unstructured meshes. Additionally, calculations in both two-and-three dimensions are supported. The implementation has been developed within the OpenFOAM framework and is to be released as Open Source.

Numerical simulation of two-phase flows with heat and mass transfer

We present a finite element method for simulating complex free surface flow. The mathematical model and the numerical method take into account two-phase non-isothermal flow of an incompressible liquid and a gas phase, capillary forces at the interface of both fluids, Marangoni effects due to temperature variation of the interface and mass transport across the interface by evaporation/condensation. The method is applied to two examples from microgravity research, for which experimental data are available.

Modelling of two-phase flow in a minichannel using level-set method

Today there is a great interest in micro-scale multiphase fluid flow. In the paper, the numerical simulation of two-phase flow inside 3 mm minichannel was carried out. The liquid- gas interface was captured using the level-set method. During the calculation, the stabilization and reinitialization of level set function was performed in order to obtain the proper accuracy of the simulation. Incompressible Navier-Stokes equations were solved using the COMSOL Multiphysics® on a two-dimensional mesh. The process of formation of different two-phase flow patterns in the minichannel has been investigated. During the simulation it has been analysed three flow patterns: the bubbly flow and two kinds of slug flow with short and long slugs. It has been shown that unsteady flow at the inlet of the minichannel is responsible for the chaotic character of changes of the slug and bubble sizes. Such unsteady flow modifies the distance between the bubbles and slugs. It has been shown that for the low water inlet velocity the two-phase flow pattern becomes more stable.

A comparison of numerical methods on the two-phase flow in PEMFCs

Numerical simulation of two-phase flow is important for the performance evaluation of proton exchange membrane fuel cells (PEMFCs). Exchange of water between vapor and liquid phases in the gas diffusion layer (GDL) was considered by Nguyen and co-workers using the evaporation and condensation rate constants. Such an approach has been widely employed by many followers on the two-phase modeling of PEMFCs. However, the effect of water exchange rate parameters on the modeling results has never been evaluated. Another two-phase flow approach was proposed by Hsuen and Yin, in which a pseudo-phase-equilibrium function is used to approximate the vapor/liquid phase equilibrium. This method gives reasonable species profiles in the GDL and the expected polarization curves at various degrees of inlet humidification. The present study compares the numerical results of these two approaches in detail. Both 1-D and 2-D results reveal that these two methods provide accurate predictions of the overall cathode performance as long as the rate constants for water evaporation/condensation in the Nguyen's method are large enough. It was shown that profiles of liquid water saturation, water evaporation rate, water vapor flux and liquid water flux evaluated by these two methods exhibit quite different characteristics particularly in the region nearby the water front. However, the effects of such variations are averaged out within the GDL such that the polarization curves calculated using these two methods are almost identical. Although 2-D numerical results cannot be validated by any analytical means, a simplified 1-D exact solution is feasible to justify the accuracy of the proposed numerical approaches.

Two-phase flow numerical simulation and experimental verification of falling film evaporation on a horizontal tube bundle

In this study, a two-phase flow investigation on falling film evaporation phenomenon is described. To track liquid–vapor interface, the volume of fluid multiphase model based on the piecewise linear interface construction method in curvilinear coordinate is used. A finite volume method code based on the SIMPLE algorithm and non-orthogonal discretization grid system is used for solving the governing equations including continuity, energy, and Reynolds averaged Navier–Stokes equations with the k–ε turbulence model. Also, the energy and mass transfer during the phase change is taken into account. The effects of inlet mass flow rate and temperature difference between the tube temperature and the saturation temperature on the local and average heat transfer coefficient and the net vapor production are presented. Results show the average heat transfer coefficient increases/decreases by increasing in the inlet mass flow rate/the temperature difference at a constant temperature difference/inlet mass flow rate. Results also demonstrate at a constant value of ΔT/m˙f (MED-TVC design approach), as the inlet mass flow rate increases, the heat transfer coefficient may be decreased or increased. This increase or decrease depends on the values of ΔT/m˙f and ΔT.

The Design and Research on a Gas-Water Mix-Type Marine Fire Hydrant Based on Fluid Mechanics

The existing fire-extinguishing plants have a lot of problems. For example, the amount of extinguishant is very large, mix is uneven, extinguishant is easy to splash, harmful to the human, the system structure is complex, spray nozzle is demanding and so on. And the open literature sources on the marine fire hydrant are very few. Besides because of the difficulty of numerical simulation of two-phase flow, the research on numerical simulation of the marine fire hydrant is hardly any. In this thesis, a new-style marine fire hydrant is designed to be small volume, easy to use and low pressure lost. It blends the gas to water through its special structure. It regards the gas-water mixture as the extinguishant. In this way, the water flow can be soften, and reduce the splash. The extinguishant can overlay the burnable body uniformly. At last, reach the better extinguishant under the dual effect of the oxygen exclusion and cooling.

Two-phase flow numerical simulation with real-gas effects and occurrence of rarefaction shock waves

A discrete equation method (DEM) for the simulation of compressible multiphase flows including real-gas effects is illustrated. A reduced five equation model is obtained starting from the semi-discrete numerical approximation of the two-phase model. A simple procedure is then proposed for using a more complex equation of state, thus improving the quality of the numerical prediction. Classical test-cases well-known in literature are performed featuring a strong importance of thermodynamic complexity for a good prediction of temperature evolution. Finally, a computational study on the occurrence of rarefaction shock waves (RSW) in a two-phase shock tube is presented, with dense vapors of complex organic fluids. Since previous studies have shown that a RSW is relatively weak in a single-phase (vapor) configuration, its occurrence and intensity are investigated considering the influence of the initial volume fraction, initial conditions and the thermodynamic model.

Three-dimension simulation of bubble behavior under nonlinear oscillation

An air–water two-phase flow numerical simulation model under nonlinear oscillating condition is established based on the open source code OpenFOAM, and the bubble behavior under oscillating condition is simulated. The three-dimensional oscillating two-phase flow field is simulated using the VOF method and the moving grid of the Arbitrary Lagrangian–Eulerian (ALE) method. At first, the simulation results are compared with the experimental results and are shown to be in reasonable agreement, and the simulation model is validated. The validated model is then used to analyze the bubble motion. Bubble shapes, induced velocity and displacement versus time are investigated under different oscillating conditions. Rising bubbles under oscillation and non-oscillation conditions are also compared. It is found that a bubble, especially a rising bubble, is influenced significantly by oscillation.

Numerical Simulation for Two-Phase Flow Field of FGD Scrubber in Ship

Discrete phase model was used for three-dimensional numerical simulation of two-phase flow in the ship FGD scrubber. The κ-ε model and SIMPLE algorithm were adopted in the calculation. The results showed that adding porous baffles improved the distribution of flow field in the scrubber. The gas velocity in the scrubber became uniformity and the flue gas resistance decreased when the sprays worked. Under the action of the spray, the differential pressure of spray area changed greatly. The simulation plays a certain role in guiding the structural optimization design of scrubber.

Numerical Simulation of Two-Phase Flow under High Acceleration in SRM with Submerged Nozzle

The two-dimension axisymmetric and two-phase flow in a full-size solid rocket motor with submerged nozzle under high acceleration condition has been simulated with Euler-Lagrange model. Without acceleration and under high axial acceleration on particle trajectories, the influences of different particle diameters were analyzed. The difference between gas flow field and two-phase flow field is significant. The particle accumulation zone above the inner wall of chamber and nozzle is mainly concentrated in two regions. The axial acceleration will intensify the impaction to the end of the chamber. The accretion of the particle phase diameter will increase the inertia of the particle phase, which may cause the following property worse, and the particles can easily form a highly-concentrated aggregation flow.

Numerical simulation of two-phase flow in a Wurster coating chamber and comparison with experimental results

Pellet coating is commonly performed in a fluid bed coater such as a Wurster coater. Computational fluid dynamics (CFD) simulations and experimental measurements of laboratory-scale Wurster coating were performed. CFD simulations were performed in a 2D axisymmetric space using the two-fluid model. The effect of the Wurster gap and fluidizing airflow rate on gas–solid flow was analyzed in both simulations and experiments. Experimentally, the draft tube two-phase flow was evaluated by measuring the volume fraction of pellets within the draft tube and local particle velocities at the tube exit and by monitoring the local pressure drop fluctuations. A rather good agreement between the simulated and experimental results was observed and thus it can be assumed that this particular simulation setup represents a computationally reasonable tool that provides a valuable insight into the Wurster coating process due to the impact of two-phase flow properties on the process.

Numerical simulation of two-phase flow with bubble break-up and coalescence coupled with population balance modeling

A computational fluid dynamic (CFD) model was developed with an improved source term based on previous work by Hagesaether et al. [1] for bubble break up and bubble coalescence to carry out numerical prediction of number density of different bubble class in turbulent dispersed flow. The numerical prediction was based on two fluid models, using the Eulerian–Eulerian approach where the liquid phase was treated as a continuum and the gas phase (bubbles) was considered as a dispersed phase. Bubble–bubble interactions, such as breakage due to turbulence and coalescence due to the combined effect of turbulence and laminar shear were considered. The result shows that the radial distributions of number densities of lower bubble classes are more than its higher counterpart. The result also shows that the Sauter mean diameter increases with the increase of height up to 1m and then become steady. Simulated results are found to be in good agreement with the experimental data.

Numerical simulation of two-phase flow on step-pool spillways

The step-pool spillway is inspired by rhythmic sequences of steps and pools in steep mountain streams. By mixing the hydraulic process of stepped spillways and mountainous river step-pool systems, a new kind of stepped spillway, called the step-pool spillway, was designed for a noticeable increase in energy dissipation. The step-pool spillway has increasingly turned into an effective energy dissipator. However, until now, most studies on step-pool spillway overflow have been based on the model test. In this study, the two-phase flow over two types of step-pool spillway was investigated using two-phase schemes (Volume Of Fluid (VOF) and mixture) and various turbulence modeling. Numerical simulation of two-phase flow was carried out on two types of step-pool spillway with various slopes. Comparison of the energy dissipation rates and flow field variables of the present simulation with those of experimental models is presented. Results show that the mixture model with the Reynolds Stress turbulence Model (RSM) is suitable for simulation of two-phase flow over spillways.

Numerical Simulation for Transient Flow of Field of Water Jet Based on Euler Method

It is hard to accurately predict the shape and the characteristic of jet because of the dynamic characteristic of water jet in the atmosphere. The Eulerian model was used to calculate the water jet numerical simulation of two-phase flow. The distribution of the velocity, pressure and the component of the nozzle flow field were obtained under the condition of the initial pressure of nozzle is 100 MPa and the outlet diameter of nozzle is 0.2 mm. The results show that fluid velocity increases rapidly in the nozzle contraction section and appears the isokinetic core area after leaving the nozzle; the fluid dynamic pressure rapid rises in the nozzle contraction section and keeps invariant at the isokinetic core area; the ratio of the contraction flow radius and the nozzle radius is 7:10.

A parallel adaptive mesh method for the numerical simulation of multiphase flows

Methodology for performing large scale parallel numerical simulations of two-phase flow is presented that uses a three-dimensional (3-D) finite element method (FEM) solver, PHASTA, to perform the simulation and a parallel mesh adaptation code, phParAdapt, to iteratively refine and coarsen regions of the solution domain. This version of PHASTA uses a stabilized FEM to discretize the 3-D Incompressible Navier–Stokes (INS) equations plus a level set method to capture the two-phase interface. The phParAdapt code takes the current parallel mesh along with the PHASTA flow solution and, using various user-specified correction indicators as input, performs a parallel mesh refinement/coarsening procedure. Utilizing local mesh modification operators, element sizes are adjusted to equally distribute the errors across the distributed mesh during the mesh refinement/coarsening procedure. Numerical simulations ranging from simple canonical test problems up to an experimental annular steam/water flow condition were performed to illustrate this methodology. The annular flow simulation captured the instabilities that generate both the ripple and roll wave like structures on the steam/water interface and the droplet entrainment mechanisms expected in annular two-phase flows.