Improved CFD modeling of full dissolution of alumina particles in aluminum electrolysis cells considering agglomerate formation

The full alumina dissolution process in aluminum electrolysis cells was investigated using an improved computational fluid dynamics(CFD)model based on the previous researches by consideration of agglomerate formation.The results show that the total mass of alumina agglomerate and its maximum size are mainly dependent on the feeding amount and increase with increasing it.Higher superheat can effectively inhibit the agglomerate formation and thus promote the full alumina dissolution behavior.The full alumina dissolution process mainly includes a fast stage and a slow stage,with an average dissolution rate of 17.24 kg/min and 1.53 kg/min,respectively.About 50%(mass percentage)of the total alumina particles,almost all of which are the well-dispersed alumina fine grains,dissolve within the fast dissolution stage of about 10 s.The maximum values of the average dissolution rate and final percentage of the cumulative dissolved alumina mass are obtained with a feeding amount of 1.8 kg for a superheat of 12℃.The formation of the alumina agglomerates and slow dissolution characteristics play a dominant role in the full dissolution of alumina particles.

A CFD Model to Evaluate Near-Surface Oil Spill from a Broken Loading Pipe in Shallow Coastal Waters

Oil spills continue to generate various issues and concerns regarding their effect and behavior in the marine environment,owing to the related potential for detrimental environmental,economic and social implications.It is essential to have a solid understanding of the ways in which oil interacts with the water and the coastal ecosystems that are located nearby.This study proposes a simplified model for predicting the plume-like transport behavior of heavy Bunker C fuel oil discharging downward from an acutely-angled broken pipeline located on the water surface.The results show that the spill overall profile is articulated in three major flow areas.The first,is the source field,i.e.,a region near the origin of the initial jet,followed by the intermediate or transport field,namely,the region where the jet oil flow transitions into an underwater oil plume flow and starts to move horizontally,and finally,the far-field,where the oil re-surface and spreads onto the shore at a significant distance from the spill site.The behavior of the oil in the intermediate field is investigated using a simplified injection-type oil spill model capable of mimicking the undersea trapping and lateral migration of an oil plume originating from a negatively buoyant jet spill.A rectangular domain with proper boundary conditions is used to implement the model.The Projection approach is used to discretize a modified version of the Navier-Stokes equations in two dimensions.A benchmark fluid flow issue is used to verify the model and the results indicate a reasonable relationship between specific gravity and depth as well as agreement with the aerial data and a vertical temperature profile plot.

Numerical aerodynamic analysis of bluff bodies at a high Reynolds number with three-dimensional CFD modeling

This paper focuses on numerical simulations of bluff body aerodynamics with three-dimensional CFD(computational fluid dynamics) modeling,where a computational scheme for fluid-structure interactions is implemented.The choice of an appropriate turbulence model for the computational modeling of bluff body aerodynamics using both two-dimensional and three-dimensional CFD numerical simulations is also considered.An efficient mesh control method which employs the mesh deformation technique is proposed to achieve better simulation results.Several long-span deck sections are chosen as examples which were stationary and pitching at a high Reynolds number.With the proposed CFD method and turbulence models,the force coefficients and flutter derivatives thus obtained are compared with the experimental measurement results and computed values completely from commercial software.Finally,a discussion on the effects of oscillation amplitude on the flutter instability of a bluff body is carried out with extended numerical simulations.These numerical analysis results demonstrate that the proposed three-dimensional CFD method,with proper turbulence modeling,has good accuracy and significant benefits for aerodynamic analysis and computational FSI studies of bluff bodies.

Simulation of a semi-industrial pilot plant thickener using CFD approach

Thickeners are important units for water recovery in various industries. In this study, a semi-industrial pilot plant thickener similar to the tailing thickener of the Sarcheshmeh Copper Mine was simulated by CFD modeling. The population balance was used to describe the particle aggregation and breakup. In this population balance, 15 particle sizes categories were considered. The Eulerian-Eulerian approach with standard k-e turbulence model was applied to describe two phases of slurry flow in the thickener under steady-state condition. The simulation results have been compared with the experimental measurements to validate the accuracy of the CFD modeling. After checking the numerical results, the effect of important parameters such as, feed flow rate, solid percentage in the feed, and solid particle size on the thickener performance was studied. The thickener residence time distribution were obtained by the modeling and also compared with the experimental data. Finally, the effects of feedwell feeding on the average diameter of aggregate and turbulent intensity were evaluated.

On numerical modeling of low-head direct chill ingot caster for magnesium alloy AZ31

A comprehensive 3D turbulent CFD study has been carried out to simulate a Low-Head(LH)vertical Direct Chill(DC)rolling ingot caster for the common magnesium alloy AZ31.The model used in this study takes into account the coupled laminar/turbulent melt flow and solidification aspects of the process and is based on the control-volume finite-difference approach.Following the aluminum/magnesium DC casting industrial practices,the LH mold is taken as 30 mm with a hot top of 60 mm.The previously verified in-house code has been modified to model the present casting process.Important quantitative results are obtained for four casting speeds,for three inlet melt pouring temperatures(superheats)and for three metal-mold contact heat transfer coefficients for the steady state operational phase of the caster.The variable cooling water temperatures reported by the industry are considered for the primary and secondary cooling zones during the simulations.Specifically,the temperature and velocity fields,sump depth and sump profiles,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles at the center and at three strategic locations at the surface of the slab are presented and discussed.

A coupled CFD simulation approach for investigating the pyrolysis process in industrial naphtha thermal cracking furnaces

In the steam thermal cracking of naphtha,the hydrocarbon stream flows inside tubular reactors and is exposed to flames of a series of burners in the firebox.In this paper,a full three-dimensional computational fluid dynamics(CFD)model was developed to investigate the process variables in the firebox and reactor coil of an industrial naphtha furnace.This comprehensive CFD model consists of a standard k-εturbulence model accompanied by a molecular kinetic reaction for cracking,detailed combustion model,and radiative properties.In order to improve the steam cracking performance,the model is solved using a proposed iterative algorithm.With respect to temperature,product yield and specially propylene-toethylene ratio(P/E),the simulation results agreed well with industrial data obtained from a mega olefin plant of a petrochemical complex.The deviation of P/E results from industrial data was less than 2%.The obtained velocity,temperature,and concentration profiles were used to investigate the residence time,coking rate,coke concentration,and some other findings.The coke concentration at coil exit was1.9×10^(-3)%(mass)and the residence time is calculated to be 0.29 s.The results can be used as a scientific guide for process engineers.

Simulating Urban Flow and Dispersion in Beijing by Coupling a CFD Model with the WRF Model

The airflow and dispersion of a pollutant in a complex urban area of Beijing, China, were numerically examined by coupling a Computational Fluid Dynamics （CFD） model with a mesoscale weather model. The models used were Open Source Field Operation and Manipulation （OpenFOAM） software package and Weather Research and Forecasting （WRF） model. OpenFOAM was firstly validated against wind-tunnel experiment data. Then, the WRF model was integrated for 42 h starting from 0800 LST 08 September 2009, and the coupled model was used to compute the flow fields at 1000 LST and 1400 LST 09 September 2009. During the WRF-simulated period, a high pressure system was dominant over the Beijing area. The WRF-simulated local circulations were characterized by mountain valley winds, which matched well with observations. Results from the coupled model simulation demonstrated that the airflows around actual buildings were quite different from the ambient wind on the boundary provided by the WRF model, and the pollutant dispersion pattern was complicated under the influence of buildings. A higher concentration level of the pollutant near the surface was found in both the step-down and step-up notches, but the reason for this higher level in each configurations was different： in the former, it was caused by weaker vertical flow, while in the latter it was caused by a downward-shifted vortex. Overall, the results of this study suggest that the coupled WRF-OpenFOAM model is an important tool that can be used for studying and predicting urban flow and dispersions in densely built-up areas.

CFD Model of Dense Gas-solid Systems in Jetting Fluidized Beds

A CFD code has been developed based on the conservation principles describing gas and solid flow in fluidized beds. This code is employed to simulate not only the spatiotemporal gas and solid phase velocities and voidage profiles in a two dimensional bed but also fluid dynamics in the jet region. The computational results show that gas flow direction is upward in the entire bed accompanied with random local circulations, whilst solid flow direction is upward at the center and downward near the wall. The radical reason of strong back mixing of solid particles and good transfer behavior between two phases is that the jet entrains solid particles. Numerical calculation indicates that gas velocity, solid velocity and pressure profile have a significant change when the voidage is 0 8. The simulated time averaged voidage profiles agree with the experimental results and simulated data reported by Gidaspow and Ettehadieh(1983). Therefore, CFD model can be regarded as a useful tool to study the jet characteristics in dense gas solid fluidized beds.

CFD modelling of Iongwall goaf gas flow to improve gas capture and prevent goaf self-heating

CFD models have been developed to investigate the Iongwall goaf gas flow patternsunder different mining and geological control conditions.The Iongwall goaf wastreated as porous regions and gas flow was modelled as a momentum sink added to themomentum equation.Gas desorption from the caved goaf and destressed coal seamswithin the mining disturbed area was modelled as additional mass sources in the continuityequation.These CFD models were developed according to specific Iongwall layoutsand calibrated against field monitoring data.Two case studies were presented demonstratingthe application of CFD modelling of goaf gas flow characteristics for improved goafgas capture and the reduction of oxygen ingress into the goaf areas for self-heating prevention.Results from the case studies indicate that the optimum goaf drainage strategywould be a combination of shallow (near the face) and deep holes to improve the overalldrainage efficiency and gas purity.For gassy Iongwall faces retreating against the seam dip,it is recommended to conduct cross-measure roof hole drainage targeting the fracturedzones overlying the return corner,rather than high capacity surface goaf drainage deep inthe goaf.

Design and Characterization of an Aerosol Test Chamber for Emergency Response Patient Contamination Control Simulation and Research

Contaminated or infected patients present a risk of cross-contamination for emergency responders, attending medical personnel and medical facilities as they enter a treatment facility. The controlled conditions of an aerosol test chamber are required to examine factors of contamination, decontamination, and cross-contamination. This study presents the design, construction, and a method for characterizing an aerosol test chamber for a full-sized manikin on a standard North Atlantic Treaty Organization litter. The methodology combined air velocity measurements, aerosol particle counts and size distributions, and computational fluid dynamics modeling to describe the chamber’s performance in three dimensions. This detailed characterization facilitates future experimental design by predicting chamber performance for a variety of patient-focused research.

Modelling Fire in Tunnels:A Large Scale Validated Two-Step Modelling Method

Fire is a quite common phenomenon in tunnel and being able to model its consequences with a good precision is crucial to design adapted safety measures.Modelling the fire behaviour in tunnel is quite challenging.Managing large-scale experiment for all the possible configurations is however economically unrealistic.This paper presents an experimental real scale fire test that was used not only for demonstrating the fire behaviour but also for evaluating the capabilities of the FDS(fire dynamics simulator)fire code to model fire consequences too.It enables highlighting the importance of wall and inlet boundary condition treatment.Keeping in mind that predicting fire development using a CFD(computational fluid dynamics)code is quite impossible,a two-level approach is discussed with an analytical model to predict the fire curve and a CFD model for predicting smoke propagation,temperature and toxic gases distribution inside the tunnel.The comparisons show a good agreement between experimental fire test and CFD modelling but also let appear requirements when using CFD.

Semi-continuous casting of magnesium alloy AZ91 using a filtered melt delivery system

A 3-D numerical simulation of an industrial-sized slab caster for magnesium alloy AZ91 has been carried out for the steady state operational phase of the caster.The simulated model consists of an open-top melt delivery system fitted with a porous filter near the hot-top.The melt flow through the porous filter was modeled on the basis of Brinkmann-Forchimier-Extended non-Darcy model for turbulent flow.An in-house 3-D CFD code was modified to account for the melt flow through the porous filter.Results are obtained for four casting speeds namely,40,60,80,and 100 mm/min.The metal-mold contact region as well as the convective heat transfer coefficient at the mold wall were also varied.In addition to the above,the Darcy number for the porous media was also changed.All parametric studies were performed for a fixed inlet melt superheat of 64℃.The results are presented pictorially in the form of temperature and velocity fields.The sump depth,mushy region thickness,solid shell thickness at the exit of the mold and axial temperature profiles are also presented and correlated with the casting speed through regression analysis.©2015 Production and hosting by Elsevier B.V.on behalf of Chongqing University.

Effects of Street-Bottom and Building-Roof Heating on Flow in Three-Dimensional Street Canyons

Using a computational fluid dynamics （CFD） model, the effects of street-bottom and building-roof heating on flow in three-dimensional street canyons are investigated. The building and street-canyon aspect ratios are one. In the presence of street-bottom heating, as the street-bottom heating intensity increases, the mean kinetic energy increases in the spanwise street canyon formed by the upwind and downwind buildings but decreases in the lower region of the streamwise street canyon. The increase in momentum due to buoyancy force intensifies mechanically induced flow in the spanwise street canyon. The vorticity in the spanwise street canyon strengthens. The temperature increase is not large because relatively cold above-roof-level air comes into the spanwise street canyon. In the presence of both street-bottom and building-roof heating, the mean kinetic energy rather decreases in the spanwise street canyon. This is caused by the decrease in horizontal flow speed at the roof level, which results in the weakening of the mean flow circulation in the spanwise street canyon. It is found that the vorticity in the spanwise street canyon weakens. The temperature increase is relatively large compared with that in the street-bottom heating case, because relatively warm above-roof-level air comes into the spanwise street canyon.

Numerical Study of Flow and Gas Diffusion in the Near-Wake behind an Isolated Building

To assist validation of the experimental data of urban pollution dispersion, the effect of an isolated building on the flow and gaseous diffusion in the wake region have been investigated numerically in the neutrally stratified rough-walled turbulent boundary layer. Numerical studies were carried out using Computational Fluid Dynamics （CFD） models. The CFD models used for the simulation were based on the steady-state Reynolds-Average Navier-Stoke equations （RANS） with κ-ε turbulence models; standard κ-ε and RNG κ-ε models. Inlet conditions and boundary conditions were specified numerically to the best information available for each fluid modeling simulation. A gas pollutant was emitted from a point source within the recirculation cavity behind the building model. The accuracy of these simulations was examined by comparing the predicted results with wind tunnel experimental data. It was confirmed that simulation using the model accurately reproduces the velocity and concentration diffusion fields with a fine-mish resolution in the near wake region. Results indicated that there is a good agreement between the numerical simulation and the wind tunnel experiment for both wind flow and concentration diffusion. The results of this work can help to improve the understanding of mechanisms of and simulation of pollutant transport in an urban environment.

Analysis of diffusion behavior of harmful emissions from trackless rubber-wheel diesel vehicles in underground coal mines

To define the diffusion behavior of harmful exhaust substances from diesel vehicles and support safety risk assessments of underground coal mines,we performed a multi-species coupling calculation of the emission and diffusion of harmful substances from a trackless rubber-wheel diesel vehicle.A computational fluid dynamics(CFD)model of the diffusion of harmful emissions was hence established and verified.From the perspective of risk analysis,the diffusion behavior and distribution of hazardous substances emitted by the diesel vehicle were studied under 4 different conditions;moreover,we identified areas characterized by hazardous levels of emissions.When the vehicle idled upwind in the roadway,high-risk areas formed behind and to the right of the vehicle:particularly high concentrations of pollutants were measured near the rear floor of the vehicle and within 5 m behind the vehicle.When the vehicle idled downwind,high-risk areas formed in front of it:particularly high concentrations of pollutants were measured near the floor and within 5 m from the front of the vehicle.In the above cases,the driver would not breathe highly polluted air and would be relatively safe.When the vehicle idled into the chamber,however,high-risk areas formed on both sides of the vehicle and near the upper roof.Forward entry of the vehicle caused a greater increase in the concentration of pollutants in the chamber and in the driver’s breathing zone compared with reverse entry.

Effects of a Building’s Density on Flow in Urban Areas

The effects of a building＇s density on urban flows are investigated using a CFD model with the RNG k - ε turbulence closure scheme. Twenty-seven cases with different building＇s density parameters （e.g., building and street-canyon aspect ratios） are numerically simulated. As the building＇s density parameters vary, different flow regimes appear. When the street canyon is relatively narrow and high, two counterrotating vortices in the vertical direction are generated. The wind speed along streets is mainly affected by the building＇s length. However, it is very difficult to find or generalize the characteristics of the street-canyon flows in terms of a single building＇s density parameter. This is because the complicated flow patterns appear due to the variation of the vortex structure and vortex number. Volume-averaged vorticity magnitude is a very good indicator to reflect the flow characteristics despite the strong dependency of flows on the variation of the building＇s density parameters. Multi-linear regression shows that the volume-averaged vorticity magnitude is a strong function of the building＇s length and the street-canyon width. The increase in the building＇s length decreases the vorticity of the street-canyon flow, while, the increase in the street- canyon width increases the vorticity.

Modeling of Fluidized Bed Gasification:Assessment of Zero-dimensional and CFD Approaches

In modeling fluidized bed gasification experiments,equilibrium and CFD models are valuable options.The existence of multi-dimensional effects inside the reactor vessel due to the kinetics of the process and the fluid dynamics phenomena could result in deviation from the zero-dimensional assumption.Complex models integrating kinetics and hydrodynamics are being developed by using a computer fluid dynamics(CFD)approach.The objective of this investigation is to assess and compare the adequacy of zero-dimensional and CFD approaches in modeling fluidized bed gasification regarding a semi-industrial scale(numerical results are validated under experimental runs).Results show that the zero-dimensional model based on the approach of dual stage equilibrium performs reasonably well in adequately predicting the product gas composition at different operating conditions and for different feedstocks,although with quantitative discrepancy.Furthermore,the discrepancy depends on the oxygen content of the oxidation agent and on the steam-to biomass ratio decreasing when these parameters increased.CFD models provide deeper information being able to estimate the syngas composition or other operating parameter at any point of space and time.Despite of some quantitative discrepancy,the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the determining design conditions simulation.

CAVITATION MODELING WITH A CFD APPROACH

A CFD model of two-dimensional cavity flow is developed using the full Navier-Stokes equations. Based on pseudo-compressibillty and time marching techniques, the cavity surface evolves with the flow field during the time marching process. The cavitation boundary conditions are enforced on the cavity surface. Numerical computations are carried out for a wide range of two-dimensional cavity flows over various hydrofoils, including leading edge cavities and midchord cavities. Extensions to three dimensional nows are, in principle, straight forward.

DPM simulation in an underground entry: Comparison between particle and species models

The diesel particulate matter(DPM) emission from diesel powered equipment in underground mines can cause health hazards including cancer to the miners. The understanding of the DPM propagation pattern under realistic mining condition is required for selecting proper DPM control strategies and to improve working practices in underground mines. In this paper, three dimensional simulations of DPM emission from the exhaust tail pipe of a load-haul-dump(LHD) vehicle and its subsequent distribution inside an isolated zone in the typical underground mine are carried out using two different solution models available in Ansys Fluent. The incoming fresh air into the isolated zone is treated as a continuous phase and DPM is treated either as a continuous phase(gas) or as a secondary discrete phase(particle). Species transport model is used when DPM is treated as gas and discrete phase model is used when DPM is assumed to behave like a particle. The distributions of DPM concentration inside the isolated zone obtained from each method are presented and compared. From the comparison results, an accurate and economical solution technique for DPM evaluation can be selected.

Effects of Wind Fences on the Wind Environment around Jang Bogo Antarctic Research Station

This study investigated the flow characteristics altered by Jang Bogo Antarctic Research Station using computational fluid dynamics(CFD) modeling. The topography and buildings around Jang Bogo Station were constructed with computeraided-design data in the CFD model domain. We simulated 16 cases with different inflow directions, and compared the flow characteristics with and without Jang Bogo Station for each inflow direction. The wind data recorded by the site’s automatic weather station(AWS) were used for comparison. Wind rose analysis showed that the wind speed and direction after the construction of Jang Bogo Station were quite different from those before construction. We also investigated how virtual wind fences would modify the flow patterns, changing the distance of the fence from the station as well as the porosity of the fence. For westerly inflows, when the AWS was downwind of Jang Bogo Station, the decrease in wind speed was maximized(-81% for west-northwesterly). The wind speed reduction was also greater as the distance of the fence was closer to Jang Bogo Station. With the same distance, the fence with medium porosity(25%–33%) maximized the wind speed reduction.These results suggest that the location and material of the wind fence should be selected carefully, or AWS data should be interpreted cautiously, for particular prevailing wind directions.