Species Transport Model Research Articles

CFD Study on Effect of the Air Supply Velocity on Contaminant Transport within an Road Vehicle Cabin

The RNG turbulence model and species transport model were employed to simulate the dispersion of CO2 contaminant in the road vehicle cabin. Three different air supply velocities were analysed, the results indicates that CO2 concentration at middle zone is higher, CO2 concentration at seat zone is higher and easy to gather at lower air velocity. When air velocity is smaller, the height concentration appear distinctly layering. The concentration layering gradually weaken when the air supply velocity increase 5m/s. Increasing the air supply velocity the air velocity is higher than the levels and energy cost.

Numerical Simulations of Combustion and Decomposition Processes in Precalciner with Two Different Heights of Raw Meal Inlets

To reduce energy consumption, numerical simulations of combustion and decomposition processes in a precalciner were carried out with two different heights of raw meal inlets. In Euler coordinate system the gas phase is expressed with k-ε two-equation model, in Lagrange coordinate system the solid phase is expressed with discrete phase model (DPM), the chemical reaction is expressed with species transport model, and the radiation is expressed with P1 radiation model. The results show that when the raw meal inlets are near the jetting coal pipes, there is much better dispersing condition of CaCO3 but a much poorer coal combustion condition, resulting in a much higher CaCO3 decomposition rate but a lower coal burn-off rate than that when the raw meal inlets are far away from the jetting coal pipes. It is advised to install both the two heights of raw meal inlets in order to obtain not only high CaCO3 decomposition rate but also high coal burn-off rate.

Optimization for Water Outlet of an Ozone Contactor by Computational Fluid Dynamics

Geometry optimization is an effective method to improve the hydraulic efficiency of an ozone contactor. The enhancement of hydraulic efficiency can lead to a smaller dose of ozone and thus minimizing the potential risk caused by disinfection by-products. Unlike adding some extra structures in geometries in previous studies, optimization for water outlet was applied to improve the hydraulic efficiency in this paper. The water outlet was modified to several parallel flumes arranged uniformly on the top of the contactor from overflow weir. In a typical contactor with diffusers, the effect of water outlet optimization on system performance was studied based on computational fluid dynamics employing a two dimensional Euler model coupled with species transport model and discrete particle model. The T10/HRT, the most common indicator of hydraulic efficiency, was improved by 10%-24% with less short-circuiting after optimization. Compared to the original geometry, optimization for water outlet brought about a 19.6% increase in log inactivation, which represents the disinfection efficiency. To maintain the same Cryptosporidium inactivation efficiency after the water outlet optimization, ozone dosage could be reduced by 18% from 2.0 mg/L to 1.64mg/L, which resulted in a minimization of bromate formation by 8.90%.

Numerical Simulations of Combustion and Decomposition Processes in Precalciner with Two Types of Locations of Jetting Coal Pipes

To reduce energy consumption, numerical simulations of combustion and decomposition processes in a precalciner were carried out with two types of locations of jetting coal pipes. In Euler coordinate system the gas phase is expressed with k-ε two-equation model, in Lagrange coordinate system the solid phase is expressed with discrete phase model (DPM), the chemical reaction is expressed with species transport model, and the radiation is expressed with P1 radiation model. For the base case the predicted burn-off rate of pulverized coal was 86%, and the decomposing rate of calcium carbonate was 92.9%, which are in accordance with actual measurement engineering data. The results show that when the jetting coal pipes are above the tertiary air inlet, the flow pattern of coal streams is more rational in the precalciner, being beneficial to the coal burn fully, resulting in a high temperature zone, around which the calcium carbonate decompose rapidly, than that when the jetting coal pipes are far away from the tertiary air inlet.

Continuous species transport and population balance models for first drying stage of nanosuspension droplets

The present contribution reports on comparison and verification of two different modelling approaches to intra-droplet mass transfer for nanosuspension droplet drying in the constant-rate period. The first approach is continuous species transport (CST) modelling coupling external gas-droplet heat and mass transfer to a species transport equation of intra-droplet diffusion of nanoparticles. The second approach is a population balance (PB) model with similar description of external heat and mass transfer from gas to droplet. In contrast to the CST model, the PB approach deals with dispersed particles as a population and accounts for the change of nanoparticle distribution by possible aggregation. Both CST and PB models have been successively validated using published and new experimental drying data on single silica nanosuspension droplet. A parametric study revealed insignificant differences in the predicted temporal evolutions of solid volume fraction profiles and values of locking point between the two models when aggregation was “turned off” in the PB model. These small differences can be explained by different mathematical formulations and numerical implementations of the two modelling approaches. A larger contrast between the CST and PB models is the predicted duration of the first drying stage, which has been found to be longer in the case of CST approach. Such divergence is explained by the absence of a shell shrinkage period in the current PB formulation. When applied with aggregation, the PB model can predict the experimentally observed decrease in the diffusion coefficient after the gelation point.

Modeling ozone penetration through the wall assembly using computational fluid dynamics

Outdoor pollutants, such as ozone, can penetrate into the indoor environment through the wall assemblies and influence the indoor air quality (IAQ). Ozone can also react with chemical compounds (i.e., d-limonene, α-pinene, β-pinene, styrene, etc.) within the wall assemblies to create secondary emissions causing IAQ concerns. This study developed a modeling framework for predicting ozone penetration through building envelope systems. The transport process of ozone through leakage paths was numerically simulated by two approaches: (1) a species transport model plus a chemical reaction model and (2) a user-defined scalar (UDS) transport model plus a user-defined deposition model. A simplified method to simulate ozone transport through fibrous media was also developed in a UDS model based on the analysis of two deposition mechanisms: (1) transport-limited deposition and (2) surface uptake of the fiber. The model was successfully applied to a typical residential wall assembly, assuming crack heights of 1 mm (0.04 in.) in the vertical direction and 3 mm (0.12 in.) in the horizontal direction and with fiberglass insulation width of 0.14 m (5.5 in.). Theoretical analysis showed that gas phase reaction between ozone and unsaturated volatile organic compounds emitted from wall materials in leakage paths and fiberglass insulation could be neglected compared with the surface reaction. The calculated penetration factors through leakage paths by the species transport model and UDS transport model were in good agreement. Fiberglass insulation media greatly reduced ozone penetration by more than 60% under almost all circumstances when reaction probability was larger than 10−8.

Numerical Simulation of Two-Phase Liquid–liquid Flow in a New Static Mixer Structure

In this article, Spiral belt static mixer with changing diameter was taken as the object. The numerical simulation method was used to investigate the mixing process of two-phase liquid–liquid flow in water treatment by a commercial CFD code,namely Fluent.The k-ε model and species transport model were established to research this project. Then the mixing effect was compared with that of HEV static mixer. The result showed that spiral belt static mixer with changing diameter promote the mixing effect greatly. The mixing effect relative to that of HEV static mixer increased 10 times and the the pressure loss only increased 3 times.

Structure of n-heptane/air triple flames in partially-premixed mixing layers

Results of a detailed numerical analysis of an n-heptane/air edge flame are presented. The equations of a low-Mach number reacting flow are solved in a two-dimensional domain using detailed models for species transport and chemical reactions. The reaction mechanism involves 560 species and 2538 reversible reactions. We consider an edge flame that is established in a mixing layer with a uniform velocity field. The mixing layer spans the equivalence ratios between pure air and 3.5. The detailed model enables us to analyze the chemical structure of the n-heptane edge flame. We identify major species profiles, discuss reactions causing the heat-release, and exploit Computational Singular Perturbation (CSP) to discuss the main fuel-consumption pathways and the structure of explosive modes in the edge flame. This analysis is performed for several regions in the edge flame to discuss the different processes at work in the premixed branches and the trailing diffusion flame. We compare different cuts through the 2D edge flame to canonical 1D premixed and diffusion flames. We also analyze the accuracy of a skeletal mechanism which was previously developed using CSP from homogeneous ignition calculations of n-heptane and show that a significant reduction in size of the mechanism can be achieved without a significant decrease in accuracy of the edge flame computation. This skeletal mechanism is then used to study the effects of increasing the equivalence ratio in the partially-premixed fuel stream.

Effective Transport Properties of LiMn2O4 Electrode via Particle-Scale Modeling

The extension of Li-ion batteries, from portable electronics to hybrid and electric vehicles, is significant. Developing a better understanding of the role of material properties and manipulating the morphology of the particle clusters comprising Li-ion electrodes could lead to potential opportunities for attaining higher performance goals, for which the effect of both material properties and morphology needs to be considered in a physics-based model. In this work, different particle packing arrangements are analyzed for the calculation of effective transport properties and reaction density that appear in the porous-electrode formulation due to the volume-averaging process. Surrogate-based analysis is used to systematically construct and validate reduced-order models for species transport at the particle-electrolyte interface. The low effective solid transport predicted through microscale modeling indicates the effect of packing arrangement and tortuosity, an aspect not captured by the Bruggeman’s relation. Particle cluster simulations reveal a Li-ion flux quantitatively different than that predicted by the porous-electrode model due to the variation of overpotential at the microscale. The present study offers a first-step towards integration of the effect of microstructure into a macroscale simulation.

A semi-analytical model for species transport in combined electroosmotic and pressure driven microflows with surface adsorption–desorption reactions

We derive semi-analytical solutions for solutal concentration distribution in microfluidic channels under the combined action of pressure-driven and electroosmotic transport, considering the effects of adsorption–desorption reactions at the substrate–fluid interface. Within the pertinence of common bio-microfluidic applications, we obtain the spatio-temporal evolution of analyte concentration by solving the corresponding eigen value problems on species transport with non self-adjoint operators, yielding non-orthogonal eigen functions. We analyze the implications of the relative strengths of the pressure-driven and electroosmotic flow components towards dictating the strongly interconnected reaction–advection–diffusion transport. We also discuss various interesting limiting cases, including the situations in which the solutal concentration band effectively propagates in the microchannel as a shock front. For such cases, we obtain the propagation velocity of the band as an explicit function of the relative strengths of the respective flow actuation mechanisms and the other significant parameters pertinent to the surface reactions.

Properties of lean turbulent methane-air flames with significant hydrogen addition

We examine the combustion of mixed H2-CH4-air fuels using two-dimensional simulations that incorporate detailed kinetics and a mixture-averaged model for differential species transport. The mixtures range from lean H2-air at ϕ=0.37 to lean CH4-air at ϕ=0.7. For each mixture, we compute the quasi-steady propagation of a flame into flow with superimposed low-level turbulent fluctuations, so that the resulting flames are in the laminar flamelet regime. We examine the resulting global flame characteristics, and quantify how the chemistry depends on local flame curvature. We then examine in more detail how the methane chemistry is modulated by the presence of hydrogen. In particular, we find that the local methane burning speed shows a strong positive correlation with local flame curvature when sufficient hydrogen is added to the mixture. Moreover, for higher hydrogen concentrations, the mixtures exhibit cellular burning patterns that are traditionally associated with thermodiffusively unstable fuel mixtures. Various pathways for the oxidation of methane are identified and are shown to be amplified considerably in the presence of H2 combustion, which varies considerably along the flame surface.

From 0D to 1D modeling of tubular solid oxide fuel cell

An analysis of tubular solid oxide fuel cell fuelled with humidified hydrogen is presented in the paper. Numerical calculations have been performed by means of 0D and 1D levels of the electrical current density vector prediction together with 3D level of continuity, momentum, energy and species transport modeling. It has been shown that constant current density assumption throughout the fuel cell, as in 0D model, gives some overestimation of voltage, power and temperature in comparison with 1D model. However, obtained characteristics are similar for both cases. Additionally, an influence of discretization segments number on the model prediction has been analyzed. The local changes of current density along the fuel cell tube are also presented. It is shown that for increasing values of average current density throughout the cell, relevant local values of current density decrease linearly with tube length.

A complete numerical model for electrokinetic flow and species transport in microchannels

A theoretical model is developed to investigate electrokinetic driven fluid flow and species transport. A typical cross-channel geometry is chosen with applications of species on-chip injection-separation analysis. The electrical potential, fluid flow and species concentration fields are presented in detail. These governing equations are non-dimensionalized uniquely to identify the controlling factors in microchannel performance. For the unsteady species transport equation, two different non-dimensionalization methods are compared. A preferred way to non-dimensionalize time in the concentration transport equation provides superior insight and understanding. Using a non-uniform staggered grid, a finite control volume method and line-by-line iterative procedure are implemented for the simulations. The Semi Implicit Method for Pressure Linked Equations (SIMPLE) algorithm is employed to solve the discrete equations. Grid independence and convergence studies are performed.

Solute Transport Modeling for Urban Drainage Structures

Solute transport and dispersion processes affect the performance of a wide range of water engineering structures. Some urban drainage network models transport the pollutants by advection only, whereas others also account for the effects of dispersion, although there is only limited knowledge regarding appropriate values for dispersion parameters. Computational fluid dynamics (CFD)-based software tools enable engineers to simulate flow patterns and associated pollutant transport mechanisms within both natural and engineered hydraulic structures. It is feasible to use CFD to represent solute transport using two contrasting approaches, an unsteady species (scalar) transport model or a discrete phase (Lagrangian particle tracking) model. This paper outlines these two approaches, using the example of a storage tank to demonstrate, compare, and validate the two approaches, and to explore a number of issues associated with interpretation of the simulation outputs. It is concluded that both CFD-based approaches m...

SU-GG-J-25: Aerosols/Nanoparticle Therapy for Malignancy Involving the Lung - a Transport Phenomenon Study

Purpose: Study the nanoparticles/aerosols localized deposition patterns in morphometrically realistic lung bifurcations geometry using computational fluid dynamics code FLUENT®. Method and Materials: Chest CT scan data from an unidentified patient, with 3 mm slices in the axial direction are used to reconstruct the lung airway bifurcation geometry using Amira 4.0 (Mercury Inc.) program. The 3D rendered airway bifurcations are exported to FLUENT® preprocessor GAMBIT where further meshing was performed and exported for FLUENT® computation. Species Transport and Reaction model in FLUENT® was used to explore the local deposition patterns of nanoparticles/aerosols. At this time, only the inspiratory phase of the breathing cycle has been studied in realistic bifurcation geometry for 3rd and 4th generations. Modeling using the CT scan data is currently in progress. N2 has been assumed to be the bulk carrier gas in the flow fields carrying arbitrary particles uniformly sized (smoke, radon daughters, radiopharmaceuticals, etc.) with defined physical and diffusional properties. In this initial study a mass fraction of 10−8 was used for particle mass concentration. The particles interacting with the the airway wall surface are assumed to be deposited. Results: Results from preliminary studies of localized particle deposition velocities and deposition fractions using unrealistic symmetric airway bifurcations geometries for 3rd and 4th generations using FLUENT® model for Po-218 nanoparticles are in close agreement with the experimental data of Kinsara et al. (Health Phys. 68(3), 1995). Further, results obtained with the realistic bifurcation geometry for 3rd and 4th generations, suggest that the particle deposition is maximum at the carinal region of the bifurcations. Conclusion: Maximum particle deposition concentration was observed to occur at the carinal regions in the bifurcations. Further, the deposition patterns provide very useful information for the design of the inhalation drugs, with detail on where the particles end up after one breath in.

OS-D1: A Simulation Framework for 0D Engine Combustion and Pollutant Formation Combined with 1D Exhaust Gas Aftertreatment : Control of Gasoline Engine Emissions During Drive-Cycle(OS-D Advanced engine simulation (prediction of performance & emissions, transient simulation),Organized Session Papers)

The introduction of more stringent standards for engine emissions requires a steady development of engine control strategies in combination with efforts to optimize in-cylinder combustion and exhaust gas aftertreatment. This study presents a simulation approach combining an engine and vehicle model. A well established 1D gas dynamics and engine simulation model is extended by five key features. These are models for (1) general species transport, in-cylinder (2) combustion and (3) pollutant formation, (4) catalytic pollutant conversion and (5) manifold wall wetting. The model is used to investigate the effect of retarded spark timing on engine out and tailpipe emissions during the first 60 seconds of a NEDC. The results lead to the conclusion that the applied simulation approach has the potential to reduce the experimental effort on a roller dynamometer in future.

Thermodynamically constrained averaging theory approach for modeling flow and transport phenomena in porous medium systems: 4. Species transport fundamentals

This work is the fourth in a series of papers on the thermodynamically constrained averaging theory (TCAT) approach for modeling flow and transport phenomena in multiscale porous medium systems. The general TCAT framework and the mathematical foundation presented in previous works are built upon by formulating macroscale models for conservation of mass, momentum, and energy, and the balance of entropy for a species in a phase volume, interface, and common curve. In addition, classical irreversible thermodynamic relations for species in entities are averaged from the microscale to the macroscale. Finally, we comment on alternative approaches that can be used to connect species and entity conservation equations to a constrained system entropy inequality, which is a key component of the TCAT approach. The formulations detailed in this work can be built upon to develop models for species transport and reactions in a variety of multiphase systems.

Modelingof Acetylene Pyrolysis under Steel Vacuum Carburizing Conditions in a Tubular Flow Reactor

In the present work, the pyrolysis of acetylene was studied under steel vacuum carburizing conditions in a tubular flow reactor. The pyrolysis temperature ranged from 650 degrees C to 1050 degrees C. The partial pressure of acetylene in the feed mixture was 10 and 20 mbar, respectively, while the rest of the mixture consisted of nitrogen. The total pressure of the mixture was 1.6 bar. A kinetic mechanism which consists of seven species and nine reactions has been used in the commercial computational fluid dynamics (CFD) software Fluent. The species transport and reaction model of Fluent was used in the simulations. A comparison of simulated and experimental results is presented in this paper.

Computational fluid dynamic modeling of two passive samplers

To effectively use a passive sampler for monitoring trace contaminants in the gas-phase, its sampling characteristics as a function of ambient wind conditions must be known. In this study two commonly used passive samplers were evaluated using computational fluid dynamics. Contaminant uptake by the polyurethane foam (PUF) was modeled using a species transport model. The external–internal flow interactions in the sampler were characterized, and the uptake rates of contaminant species were quantified. The simulations show that flow fields in the samplers have strong velocity gradients, and single-point velocity measurements do not capture flow interactions accurately. Sampling rates calculated for a PUF in freestream are in good agreement with sampling rates for PUFs in the passive samplers studied for the same average velocity over the PUF. The calculated sampling rates are in general agreement with those obtained experimentally by other researchers.

NUMERICALL DYE WASHOUT METHOD AS A TOOL TO CHARACTERIZE THE HEART VALVE FLOW

CFD methods are widely used for the development of artificial valves. However, choosing the proper flow parameters to characterize the flow to predict the valve function in vivo, is still a big challenge. There is a set of parameters, which are used for flow characterization: pressure drop, maximal velocity magnitude, calculation of the residence time along path line and maximal values or histogramms of wall shear stress, shear stress and turbulence intencity. We propose to use the numerical dye washout method for flow characterization. This allows the assessment of regions with a long or short residence time and therefore predicts the regions of high or low thrombus risk. This successful experimental method is modeled now numerically. This allows a 3D-resolved flow characterization and a possibility to validate the results experimentally. In order to simulate the dye washout, an unsteady flow simulation is performed with the flow solver FLUENT. Furthermore, a species transport model simulating the convective diffusion mixing of species (entering/outflowing blood) is applied. The results are distributions of the fluid fraction for different time steps. These data were used for the dye washout visualization, which was realized using the software AMIRA. Results are images, which represent the projection of the absorbed light intensity. The analysis of the washout process was done by quantifying the time course of the gray value. The method was applied in 5 different mechanical valves. The results show that dye washout flow characterization seems to be a very useful parameter.