Vertical Dispersion Coefficient Research Articles

Coal-fueled chemical looping gasification: A CFD-DEM study

Chemical looping gasification (CLG) is an emerging technology for reducing greenhouse gas emissions, yet the complex physical-thermal-chemical behaviour in the CLG unit has not been well understood. This work developed a high-fidelity computational fluid dynamics-discrete element method (CFD-DEM) reactive model considering four heat transfer modes (e.g., conduction, convection, radiation, and reaction heat) and complex heterogeneous and homogeneous reactions. The hydrodynamics and thermochemical characteristics in a CLG unit operating under several key operating parameters are numerically studied. The contribution from each heat transfer mode is quantified and the relationship between particle-scale behaviour and mesoscale bubble structures is quantitatively illuminated. The results show that the solids vertical dispersion coefficient is one order of magnitude larger than the horizontal one. At a low solid holdup, the interphase drag force plays a dominant role and particles in the bubble phase have higher vertical slip velocities. The ratio of particle-averaged heating rates for char particles through conduction, convection, radiation, and reaction take 5.41%, 14.91%, 14.39%, and 65.29%, and that for oxygen carriers take 7.77%, 23.46%, 20.33%, and 48.44%, respectively. For char particle and oxygen carriers, the reaction heat dominates the heat transfer process. Increasing gas inlet velocity promotes particle mixing, alleviates dead zone, and finally increases gas products while increasing the char to oxygen carrier mass ratio decreases gas products. The present work provides a cost-effective tool for the in-depth understanding of heat and mass transfer mechanisms in the CLG process.

Reactive simulation of gas-catalyst thermochemical properties in a pilot-plant catalytic MTO fluidized bed

The methanol-to-olefin (MTO) process enables economical production of light olefins thus decreases the reliance on petroleum resources. The reactive multiphase flow patterns and reaction behaviors inside pilot-plant MTO fluidized reactor are numerically simulated with multiphase particle-in-cell model. Based on the model validation, the thermochemical properties of catalytic particle (e.g., heat transfer coefficient (HTC), temperature) and gas phase thermal characteristics are comprehensively investigated with the discussion on several crucial operating parameters. The results reveal that large HTC of catalyst particle is located in the crucial reaction zone and freeboard zone. The HTC of catalyst material covers a range of 50 to 150 W/(m2·K). Gas-solid hydrodynamics and gas concentrations exhibit spatial non-uniform distribution. Moreover, the reactant gas flow rate is more positively sensitive to the product distributions and gas thermal properties. Enlarging the reaction pressure significantly enhances the heat transfer. The vertical dispersion coefficient of catalyst is two orders of magnitude larger than the horizontal ones.

Maximum ground level concentration for Gaussian plume formula using standard and Briggs methods

AbstractThis work aims to determine the maximum concentration and downwind distance at the earth's surface in two methods Briggs and standard for two conditions are slightly unstable and neutral by using the Gaussian equation at different effective heights and wind seeped at (8.9, 8.5 and 8 m/s) under lateral and vertical dispersion coefficient measurement.

Modifying Elder’s Longitudinal Dispersion Coefficient for Two-Dimensional Solute Mixing Analysis in Open-Channel Bends

Elder’s equation for the longitudinal dispersion coefficient in two-dimensional solute transport analysis cannot be applied to curved channels because the vertical distribution of the longitudinal velocity does not obey the logarithmic law in the bends of an open channel. In this study, a two-dimensional longitudinal dispersion coefficient based on an equation that can appropriately describe the vertical distribution of flow velocity in open-channel bends is derived theoretically. The proposed equations for the vertical velocity distribution and dispersion coefficient are compared and verified with values measured from two different types of open channels, i.e., a laboratory channel and a natural-like channel. The increase in the longitudinal dispersion coefficient based on the difference in the vertical distribution of the flow velocity is evaluated quantitatively. In terms of the longitudinal dispersion coefficient, no significant difference is observed between the observed dispersion coefficient based on the concentration data and the coefficient value calculated using the equation proposed in this study. The dispersion equation proposed in this study can be easily applied to assign the value of the longitudinal dispersion coefficient for the two-dimensional mixing modelling in bends using basic hydraulic factors.

CFD study of the reactive gas-solid hydrodynamics in a large-scale catalytic methanol-to-olefin fluidized bed reactor

Lacking the understanding of multiphase flow in the fluidized bed methanol-to-olefin (MTO) process hinders the reactor design, operation, and optimization. Accordingly, a three-dimensional multiphase particle-in-cell model is established to study hydrodynamics and thermochemical characteristics during the MTO process in a fluidized bed reactor. After model validation, the influences of several critical operating parameters on reactor performance are discussed. The results show that particles in the medium part and freeboard of the reactor have heterogeneous velocity, temperature, and heat transfer coefficient (HTC) distributions. Thermal quantities of gas and solid phases are uniformly distributed in the medium part and freeboard of the reactor. The particle-averaged HTC under ranges from 60 to 120 W/(m2·K). Increasing wall temperature enlarges the particle HTC due to the enhanced exothermic reactions. Decreasing the particle diameter gives rise to a larger particle HTC. The vertical particle dispersion coefficient (Dz) is in the range of 0.01 m2/s to 0.03 m2/s. Methanol converts into olefin in a short period. Increasing methanol to catalyst ratio and wall temperature increases olefin including C2H4, C3H6, C4H8, and C5H10, and promotes the gas thermal properties. Particles with multi-sizes show a better MTO conversion performance than those with mono-sizes regarding the particle HTC and gas products.

A microscopic study on the mixing process of binary solids in bubbling fluidized bed

A novel method for measuring the micro mixing ratio of binary solids in dense fluidizations is developed. The mixing process within a bubbling fluidized bed that can be assumed as one mixing cell is resolved. Dispersion coefficient and mixing index methods are applied to characterize the mixing process. Results show that diffusion has an important effect on mixing at the bottom and top of the fluidized bed. Meanwhile, convection is the principal mechanism of mixing at the center. The lateral micro dispersion coefficient is between 0.005 m/s and 0.03 m/s. The vertical micro dispersion coefficient is approximately twice as that in the lateral direction. A new mixing index is proposed to evaluate the micro mixing quality of binary solids. The time required to reach uniform mixing in the lateral direction is more than twice as that in the vertical direction.

Particle behaviours of biomass gasification in a bubbling fluidized bed

Biomass gasification in a bubbling fluidized bed (BFB) reactor is numerically studied based on a particle-scale computational fluid dynamics-discrete element method (CFD-DEM), with thermochemical and polydispersity effects featuring. After model validation, the particle-scale information (e.g., particle motions, particle mixing, solid dispersion, and heat transfer contribution) are thoroughly explored with the discussion of the effects of several critical operating parameters on particle behaviours. The results show that the middle dense region has the highest biomass pyrolysis reaction rate due to the vigorous particle motion. Sand and biomass particles show synchronous horizontal motions, and the solid vertical dispersion coefficients are much higher than the solid horizontal ones, denoting that the vertically introduced gas flow dominates bed hydrodynamics. A higher operating temperature causes a higher solid dispersion coefficient. Elevating temperature and steam to biomass ratio (S/B) first increases and then decreases the particle mixing index. Convection plays a dominant role during the biomass gasification process, followed by the radiation and heat of reaction. The conduction accounts for the smallest proportion and can be neglected. Increasing operating temperature promotes chemical reactions, biomass temperature, and all heat transfer modes. Increasing S/B promotes biomass motions and gasification reactions, leading to more heat consumed and biomass temperature decrease. Decreasing biomass temperature results in a larger temperature difference between biomass particles and bed material, which enhances the conduction, convection, and radiation.

Development and Evaluation of SLINE 1.0, a Line Source Dispersion Model for Gaseous Pollutants by Incorporating Wind Shear Near the Ground under Stable and Unstable Atmospheric Conditions

Transportation sources are a major contributor to air pollution in urban areas, and the role of air quality modeling is vital in the formulation of air pollution control and management strategies. Many models have appeared in the literature to estimate near-field ground level concentrations from mobile sources moving on a highway. However, current models do not account explicitly for the effect of wind shear (magnitude) near the ground while computing the ground level concentrations near highways from mobile sources. This study presents an analytical model (SLINE 1.0) based on the solution of the convective–diffusion equation by incorporating the wind shear near the ground for gaseous pollutants. The dispersion coefficients for stable and unstable atmospheric conditions are based on the near-field parameterization. Initial vertical dispersion coefficient due to the wake effect of mobile sources is incorporated based on a literature review. The model inputs include emission factor, wind speed, wind direction, turbulence parameters, and terrain features. The model is evaluated based on the Idaho Falls field study (2008). The performance of the model is evaluated using several statistical parameters. Results indicate that the model performs well against this dataset in predicting concentrations under both the stable and unstable atmospheric conditions. The sensitivity of the model to compute ground-level concentrations for different inputs is presented for three different downwind distances. In general, the model shows Type III sensitivity (i.e., the errors in the input will show a corresponding change in the computed ground level concentrations) for most of the input variables using the ASTM (American Society for Testing and Materials) method. However, some recalibration of the model constants is needed using several field datasets to make sure that the model is acceptable for computing ground-level concentrations in engineering applications.

Experimental Study on Dispersion of Unconfined Aquifer in a Site of Jilin City

Dispersion parameter is an important parameter for the establishment of groundwater solute transport model.The dispersion test uses sodium chloride as a tracer,which was conducted in a site in Jilin City.The standard curve comparison method was used to solve the dispersion parameters of the aquifer under the natural flow field.The test results show that under the natural flow field,the longitudinal dispersion of unconfined aquifer in Jilin City is 0.400m,and the lateral dispersion is 1.933×10-5~6.557×10-3m;while the vertical dispersion coefficient is 0.246m2d,the lateral dispersion coefficient is 1.191×10-5~4.039×10-3m2d.The above results can provide an important parameter basis for the establishment of groundwater solute transport model,the accurate prediction of temporal and spatial variation of pollutant concentration in groundwater and the formulation of groundwater pollution prevention and control scheme.

Particle-scale evaluation of the biomass steam-gasification process in a conical spouted bed gasifier

Based on the multiphase particle-in-cell approach, the steam gasification of biomass in a lab-scale spouted bed gasifier is simulated to explore the particle-scale transport behavior in three distinct regions of the bed. The numerical results are firstly validated with the experimental data, followed by assessing the particle-scale features of sand and biomass species in the gasifier. The results demonstrate that biomass particles in three regions of spouted bed behave with different constituent content, heat transfer coefficient and temperature. The intensive heat transfer occurs in the spout region and fountain region. Biomass particles in the spout have a small temperature, large mass, small carbon fraction, and large volatile fraction. Heterogeneous reactions of the biomass particles mainly occur in the lower part of the fountain region but can be negligible in the spout and annulus region. The water-gas reaction is two-order of magnitude faster than the methanation reaction and Boudouard reaction. The spatial distribution of particle-scale level information of both particle species is nonuniform due to the presence of three regions and biomass accumulation. The horizontal and vertical dispersion coefficient of both sand and biomass species are at the scale of 10 −4 m 2 /s and 10 −2 m 2 /s, respectively. Spatial distribution of biomass particles colored with the carbon and volatile content in the spouted bed • Particle-scale transport behavior of biomass in spouted bed gasifier revealed. • Most intensive heat transfer occurs in the spout region. • Heterogeneous reactions of biomass mainly occur in the lower part of the fountain. • Operating parameters influence the biomass particles released from the bed outlet.

Evaluation of the Relationship between Momentum Wakes behind Moving Vehicles and Dispersion of Vehicle Emissions Using Near-Roadway Measurements.

A parameterization of initial vertical dispersion coefficient (σz,init) was developed for incorporation into California line source dispersion model, version 4 (CALINE4) and AMS/EPA regulatory model (AERMOD) to better predict pollutant concentrations near roadways. The momentum wake theory of moving vehicles indicates that both vehicle-induced turbulence (VIT) and dispersion occur in the vehicle wake. Based on a literature review, it is postulated that σz,init near roadways can be estimated using a "wake area model" concept of effective wake area defined as the vehicle height times the wake length, vehicle density, and vehicle type. A total of 523 5-min near-roadway simultaneous measurements (2016-2018) of pollutant concentrations and meteorological and traffic information were used to evaluate the model. Two roadways with distinct fleet composition and simple road configurations were selected for monitoring. The near-roadway σz,init ranged from 1 to 4 m for light-duty vehicles (LDVs) and from 3 to 7 m for fleet-mix (LDVs and heavy-duty vehicles (HDVs)). The results demonstrate that the dispersion contribution from one HDV was 31 times larger than that from one LDV. Calculated pollutant dispersion using the wake area model compared favorably with measurements (R2 = 0.91, slope = 1.07). These results indicate that σz,init varies with vehicle density and HDVs. Pollutant dispersion related to the vehicle wakes can be used to correctly parameterize dispersion models and improve prediction of pollutant concentrations near roadways.

Effect of surface roughness on turbulence, ventilation and pollutant dispersion over hypothetical urban area

Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO® bricks, a series of wind tunnel experiments are performed to investigate the effect of aerodynamic resistance on the ventilation ability and pollutant dispersion over rough surfaces. Water vapour is used as the tracer to examine the passive, inert pollutant dispersion from a line source in crossflows. Velocity and tracer concentrations are measured by hot-wire anemometers (HWAs) with X-wire probes and humidity sensors, respectively. The results show that the near-wall streamwise fluctuating velocity 1/2 , vertical fluctuating velocity 1/2 and momentum flux − are enhanced by the aerodynamic resistance induced by the surface-mounted roughness elements. The spatially averaged turbulence properties 1/2 , 1/2 and − are found peaked in the measurement cases AR = 1/12 (rib-type elements) and h:2l-T (cube-type elements). Both turbulent air exchange rate ACHʹ and vertical dispersion coefficient σz increase with increasing drag coefficient Cd, suggesting the prototypes of new parameterisations of ventilation and pollutant dispersion over urban areas.

Effect of surface roughness on turbulence, ventilation and pollutant dispersion over hypothetical urban area

Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO® bricks, a series of wind tunnel experiments are performed to investigate the effect of aerodynamic resistance on the ventilation ability and pollutant dispersion over rough surfaces. Water vapour is used as the tracer to examine the passive, inert pollutant dispersion from a line source in crossflows. Velocity and tracer concentrations are measured by hot-wire anemometers (HWAs) with X-wire probes and humidity sensors, respectively. The results show that the near-wall streamwise fluctuating velocity 1/2 , vertical fluctuating velocity 1/2 and momentum flux − are enhanced by the aerodynamic resistance induced by the surface-mounted roughness elements. The spatially averaged turbulence properties 1/2 , 1/2 and − are found peaked in the measurement cases AR = 1/12 (rib-type elements) and h:2l-T (cube-type elements). Both turbulent air exchange rate ACHʹ and vertical dispersion coefficient σz increase with increasing drag coefficient Cd, suggesting the prototypes of new parameterisations of ventilation and pollutant dispersion over urban areas.

Application of an Adapted Version of MT3DMS for Modeling Back-Diffusion Remediation Timeframes

Simulation of back-diffusion remediation timeframe for thin silt/clay layers, or when contaminant degradation is occurring, typically requires the use of a numerical model. Given the centimeter-scale vertical grid spacing required to represent diffusion-dominated transport, simulation of back-diffusion in a 3-D model may be computationally prohibitive. Use of a local 1-D model domain approach for simulating back-diffusion is demonstrated to have advantages but is limited to only some applications. Incorporation of a local domain approach for simulating back-diffusion in a new model, In Situ Remediation-MT3DMS (ISR-MT3DMS) is validated based on a benchmark with MT3DMS and comparisons with a highly discretized finite difference numerical model. The approach used to estimate the vertical hydrodynamic dispersion coefficient is shown to have a significant influence on the simulated flux into and out of silt/clay layers in early time periods. Previously documented back-diffusion at a Florida site is modeled for the purpose of evaluating the sensitivity of the back-diffusion controlled remediation timeframe to various site characteristics. A base case simulation with a clay lens having a thickness of 0.2 m and a length of 100 m indicates that even after 99.96 percent aqueous TCE removal from the clay lens, the down-gradient concentrations still exceed the MCL in groundwater monitoring wells. This shows that partial mass reduction from a NAPL source zone via in situ treatment may have little benefit for the long-term management of contaminated sites, given that back-diffusion will sustain a groundwater plume for a long period of time. Back-diffusion model input parameters that have the greatest influence on remediation timeframe and thus may warrant more attention during field investigations, include the thickness of silt/clay lenses, retardation coefficient representing sorbed mass in silt/clay, and the groundwater velocity in adjacent higher permeability zones. Therefore, pump-and-treat systems implemented for the purpose of providing containment may have an additional benefit of reducing back-diffusion remediation timeframe due to enhanced transverse advective fluxes at the sand/clay interface. Remediation timeframes are also moderately sensitive to the length of the silt/clay layers and transverse vertical dispersivity, but are less sensitive to degradation rates within silt/clay, contaminant solubility, contact time, tortuosity coefficient, and monitoring well-screen length for the scenarios examined. ©2015 Wiley Periodicals, Inc.

Chloride Dispersion across Silt Deposits in a Glaciated Bedrock River Valley.

Soil and groundwater from the Neponset River floodplain deposit that receive high concentrations of deicing agents from nearby highways were investigated. The silty sand floodplain is separated by a silty aquitard from the underlying aquifer that serves as a public water supply. We made a transport-based assessment of the capacity of the aquitard to protect the underlying aquifer. One hundred seventeen soil samples and 469 groundwater samples collected during a period of 4 yr from boreholes and 10 wells grouped in two well clusters were analyzed for dissolved Cl concentration. The soil characterization and groundwater monitoring results agreed, showing a very slow change in subsurface Cl contamination with time. These data also calibrated a vertical one-dimensional advective-dispersive transport model across the deposits. Advective transport dominated only in the top 3.37 m of the floodplain deposit, with dispersion being the main transport mechanism below this depth. Due to the silty nature of the aquitard, dispersion rather than diffusion was the main transport mechanism into the floodplain-aquitard system. Soil and groundwater quality data confirmed a Cl concentration at the floodplain surface near the highway runoff drainage outlets of 2450 mg L. The model estimated a vertical dispersivity at the site of 8 mm and a vertical hydrodynamic dispersion coefficient of 3.71 × 10 m s. These data confirmed the aquitard's capacity to contain deicing agents, protecting the underlying aquifer from contamination.

Large-eddy simulation of flows over two-dimensional idealised street canyons with height variation

A series of large-eddy simulation (LES) models consisting of two-dimensional (2D) idealised street canyons with building height variability (BHV) are examined. Building blocks with two different heights are placed alternately in the computational domains, constructing repeated street canyons of building-height-to-street-width (aspect) ratio (AR) = 1, 0.5, 0.25 and 0.125 together with BHV = 0.2, 0.4 and 0.6. LES results show that the air exchange rate (ACH) increases with increasing aerodynamic resistance. Apart from AR, BHV is another factor affecting the aerodynamic resistance and thus the ACH. The (vertical) dispersion coefficient σz of plume transport is also closely related to the aerodynamic resistance, suggesting that introducing BHV in urban areas could help improve the air quality.

Impact of thin aquitards on two-dimensional solute transport in an aquifer

The influence of aquitards on solute transport in an aquifer is an important and often overlooked process for subsurface contaminant transport. In particular, slow advection (leakage) into an aquitard is often neglected in previous analytical treatment of solute transport, making such analytical solutions unsuitable for benchmarking numerical simulations of transport when aquitard leakage exists. In this study, a semi-analytical solution to the two-dimensional conservative solute transport in an aquifer bounded by thin aquitards is derived in the Laplace domain. The governing equation in the aquifer (not aquitard) incorporates terms accounting for advection, longitudinal dispersion, and transverse vertical dispersion. Both one-dimensional vertical advection and molecular diffusion are considered for aquitard transport. The solutions are derived under conditions of steady-state flow and the first- and third-type transport boundary conditions in the aquifer along with assuming the continuity of concentration and vertical mass flux at aquifer and aquitard interfaces. The solutions in the real time domain are obtained by numerically inverting the solutions in the Laplace domain using the Stehfest (1970) algorithm. The semi-analytical solutions are compared with those from Zhan et al. (2009b), which considered aquitard leakage in infinitively thick aquitards. The concentration profiles, breakthrough curves and distribution profiles in the aquifer are different from those of Zhan et al. (2009b) at small ratios of the aquitard/aquifer thickness; whereas, the results of both are consistent for thick bounding aquitards. This study reveals that the residence time distribution (RTD) in the main aquifer is related to the aquitard/aquifer thickness ratios, Peclet numbers and porosities of adjacent aquitards. The results also suggest that MT3DMS (a commonly applied transport code) cannot successfully simulate solute transport at the aquifer-aquitard interfaces. The presented solutions improve available solutions for transport processes in an aquifer bounded by thin aquitards with leakage. The developed solutions can be directly extended to cases when the vertical hydrodynamic dispersion of the aquitards is considered by simply replacing the effective molecular diffusion coefficients of the aquitard by the vertical hydrodynamic dispersion coefficients of the aquitards.

Particle Dispersion and Circulation Patterns in a 3D Spouted Bed with or without Draft Tube

Solid dispersion and circulation properties in a 3D spouted bed with or without a draft tube are reported based on the CFD-DEM coupling approach. The distribution characteristics of the local and global solid dispersion coefficients are studied. Meanwhile, the effects of tube configuration and bed diameter on these two aspects are investigated. The results show that the large lateral dispersion coefficient concentrates in the spout–annulus interface and the lower part of the fountain region, while the vertical one lies in the central region of the spout. Insertion of a draft tube results in not only large systematic vertical and lateral dispersion coefficients but also more regular particle circulation patterns in the lateral and vertical directions. Furthermore, the larger the entrainment distance of the draft tube, the higher the local and systematic dispersion coefficients are and also an increasing particle circulation rate appears. Extension of the draft tube above the bed surface leads to a larger p...

Pollutant Plume Dispersion in the Atmospheric Boundary Layer over Idealized Urban Roughness

The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness.

Pollutant dispersion over two-dimensional idealised urban roughness: a large-eddy simulation approach

A series of two-dimensional (2D) street canyon models with a wide range of building-height-to-street-width (aspect) ratios are employed in this study to elucidate the pollutant transport over idealised urban areas. The large-eddy simulation (LES) is used to resolve the turbulent flows and pollutant transport in the urban boundary layer (UBL) over the street canyons. The results show that the pollutant removal is governed by atmospheric turbulence in both skimming flow and wake interference regimes. In the UBL, the vertical pollutant profiles illustrate self-similarity behaviours in the downstream region. The pollutant disperses rapidly over the buildings, exhibiting a Gaussian-plume shape. The maximum vertical pollutant dispersion coefficient is observed at aspect ratio equal to 1/10. A strong correlation between friction factor and dispersion coefficient is revealed, implying that the downstream air quality could be improved by increasing the roughness of urban areas.