Momentum Exchange Coefficients Research Articles

CFD Simulation of Air-Glass Beads Fluidized Bed Hydrodynamics

The hydrodynamic behaviour of air-glass beads bubbling fluidized bed reactor containing spherical glass beads is numerically studied, using OpenFoam v7 CFD software. Both Gidaspow and Syamlal-O'Brien drag models are used to calculate momentum exchange coefficients. Simulation predictions of pressure loss, bed expansion rate, and air volume fraction parameters were compared and validated using data, existing in the literature obtained experimentally and performed by other numerical softwares. Pressure loss and rate of bed expansion were calculated with relative root mean square error (RMSE) equal to 0.65 and 0.095 respectively; Syamlal-O'Brien model is considered more accurate than Gidaspow model. Hence, numerical model reliability developed on OpenFoam was also proved. The hydrodynamic aspect study of the fluidized bed reactor was then performed, to analyse the impact of inlet air velocity (U) on particles motion. It was revealed that with U increment, air and glass beads axial velocities increase in the reactor centre and decrease in the sidewalls. Thus, a greater particle bed expansion is induced and the solid particles accumulated highly on the reactor sidewalls. In general, with the increase of U, the solid volume fraction decreases from 0.63 to 0.58 observed at 0.065 m/s and 0.51 m/s, respectively.

Air–Sea Enthalpy and Momentum Exchange Coefficients from GPS Dropsonde Measurements in Hurricane Conditions

The intensity of tropical cyclones is highly dependent on air–sea enthalpy and momentum exchange. At extreme wind speeds, the values of the enthalpy, CK, and momentum, CD, exchange coefficients are characterized by high uncertainty. The present study aims to expand the previously used algorithm for CD retrieval to obtain the values of CK from wind speed measurements and the enthalpy profiles obtained from NOAA GPS dropsondes in hurricane conditions. This algorithm uses concepts from technical hydrodynamics, describing turbulent boundary layers on flat plates and pipes. According to this approach, the velocity (and enthalpy) defect profiles are self-similar in the entire boundary layer, including the layer of constant fluxes and the “wake” part, where the airflow adapts to the undisturbed flow region. By using the self-similarity property, the parameters of the constant flow layer (the roughness parameter, friction velocity, and the enthalpy and exchange coefficients CD and CK) could be obtained from measurements in the “wake” part for wind speeds from 20 m/s to 72 m/s. The estimates of the CK/CD ratio revealed values of 0.7 and 0.96 (depending on the self-similar approximation limits), and the results suggest that there are slight variations with the wind speed.

Modeling of Combustion Characteristics of Particles in Transient Gas–Solid Reacting Flow via a Machine Learning Approach

Particle group combustion presents a strong temporal and spatial inhomogeneity owing to the complicated interphase interactions. Based on the data set from the fictitious domain method, the recurrent fully connected and convolutional parallel neural network (R-FC&CNN) architecture and its two comparable simplified models, that is, the recurrent fully connected neural network (R-FCNN) and the recurrent convolutional neural network (R-CNN) architectures, were constructed for predicting the gas–solid momentum exchange coefficient, β (kg·s–1·m–3), average combustion rate per unit surface area of particles, rc¯/A (kg·s–1·m–2), and comprehensive NaCl release parameter, γ, selectively. A time sequence of average particle temperature, T̅ (K), and particle volume fraction, ε, which can be extended in the matrix form, were constructed as the features selectively according to their correlation with the target physical quantity. The average relative error, δ̅, and coefficient of determination, R2, were used as the evaluators. Through final testing, in the mild combustion domain, the R-CNN and R-FCNN models with simple structures showed good performance for β(δ̅ = 0.13, R2 = 0.8) and rc¯/A (δ̅ = 0.04, R2 = 0.84), respectively, while in the severe combustion domain, the R-FC&CNN model, with more complete features and functional structure, performed the best (for β, δ̅ = 0.12, R2 = 0.85; for rc¯/A, δ̅ = 0.05, R2 = 0.92; and for γ, δ¯=0.05,R2=0.96). A fine-tuning and interpolated prediction method was developed to investigate further the model’s expansibility. Both ensured acceptable performance on a new similar problem. In summary, the feasibility of physical meaning-oriented machine learning--based model(s) for predicting the combustion characteristics of nonuniformly distributed particles was confirmed.

New parameterization of air-sea exchange coefficients and its impact on intensity prediction under major tropical cyclones

Understanding and quantifying air-sea exchanges of enthalpy and momentum fluxes are crucial for the advanced prediction of tropical cyclone (TC) intensity. Here, we present a new parameterization of air-sea fluxes at extreme wind speeds from 40 m s−1 to 75 m s−1, which covers the range of major TCs. Our approach assumes that the TC can reach its maximum potential intensity (MPI) if there are no influences of external forces such as vertical wind shear or other environmental constraints.This method can estimate the ratio of the enthalpy and momentum exchange coefficient (Ck/Cd) under the most intense TCs without direct flux measurements. The estimation showed that Ck/Cd increases with wind speed at extreme winds above 40 m s−1. Two types of surface layer schemes of the Hurricane Weather and Research Forecast (HWRF) were designed based on the wind speed dependency of the Ck/Cd found at high winds: (i) an increase of Ck/Cd based on decreasing Cd (Cd_DC) and (ii) an increase of Ck/Cd based on increasing Ck (Ck_IC). The modified surface layer schemes were compared to the original HWRF scheme (using nearly fixed Cd and Ck at extreme winds; CTRL) through idealized experiments and real-case predictions. The idealized experiments showed that Cd_DC reduced frictional dissipation in the air-sea interface as well as significantly reduced sea surface cooling, making the TC stronger than other schemes. As a result, Cd_DC reduced the mean absolute error and negative bias by 15.0% (21.0%) and 19.1% (32.0%), respectively, for all lead times of Hurricane Irma in 2017 (Typhoon Mangkhut in 2018) compared to CTRL. This result suggests that new parameterization of Ck/Cd with decreasing Cd at high winds can help improve TC intensity prediction, which currently suffers from underestimating the intensity of the strongest TCs.

Spatializing the roughness length of heterogeneous urban surfaces to improve the WRF simulation-Part 2: Impacts on the thermodynamic environment

Roughness length (Z0) is a fundamental parameter in atmospheric boundary layer theory that affects the transfer of momentum and heat. However, this parameter in urban areas in the WRF model is typically assigned a fixed value. In a previous study, we developed a Z0 database for the heterogeneous urban underlying surface of Guangzhou and demonstrated that its application in the WRF model had significantly improved the simulations of wind speed. In this study, the impact of Z0 update on the dynamic and thermal environment in Guangzhou urban areas is investigated through two numerical experiments: the base experiment applies the model-default Z0 value (0.5 m) for the urban areas and the sensitivity one uses a heterogeneous Z0 map (with higher Z0 values in most urban areas) estimated in the previous study. The results show that the temperature and relative humidity at 2 m height are not sensitive to this Z0 modification. For the dynamic environment, the modification of Z0 decreases the wind speed at 10 m height and increases the friction velocity. The reduction of horizontal wind speed due to Z0 increases reaches heights of several hundred meters, especially below a height of 400 m (950 hPa). Additionally, the updated Z0 leads to an increase in the turbulent momentum exchange coefficient over the urban underlying surface, resulting in a more vigorous turbulent mixing and elevated planetary boundary layer height. For the thermal environment, the Z0 increases result in a higher turbulent heat exchange coefficient near the surface layer, which contributes to the vertical heat flux transport. Moreover, the Z0 increase enhances the sensible heat flux during daytime and reduces the land surface temperature and ground heat flux in Guangzhou central urban area. Due to the reduced heat storage during the daytime, the amount of heat released at night decreases accordingly, thus causing a reduction in the land surface temperature at night and a decrease in the sensible heat flux. This study provides a comprehensive understanding of the sensitivity of the thermodynamic environment to the urban Z0 increase and reconfirms the significance of a realistic Z0 dataset in atmospheric modeling.

Study of Suspended Sediment Diffusion Coefficients in Submerged Vegetation Flow

AbstractThe traditional profile model of the sediment diffusion coefficient (), which is parabolic along the water depth, cannot precisely predict the vertical distribution of in vegetated flow. Therefore, in this study, a series of flume experiments were conducted to clarify the influence of submerged vegetation on the diffusion characteristics of suspended load particles. First, the submerged canopy increases the efficiency of momentum exchange but restrains the vertical diffusion of suspended sediment, indicating that the presence of submerged plants promotes the siltation of solid particles. Second, the depth‐averaged sediment diffusion coefficient () monotonically decreases with increasing relative water depth, while the depth‐averaged momentum exchange coefficient () presents two opposite trends, likely owing to the dominant turbulence event changing from ejections to sweeps and the different variation tendencies of the velocity gradient and Reynolds shear stress with increasing vegetation density. Compared with the ratio () of to the momentum exchange coefficient in nonvegetated flow, in submerged canopy flow is significantly less than 1, which means that the solid particles diffuse less readily than liquid particles because of the stem‐scale vortices generated by the vegetation. In addition, vertical profile models of and suspended sediment concentration were proposed and validated by experimental data. The models exhibit a high accuracy, correlation and applicability and can provide critical information to promote research on riverbed deformation and nutrient dynamics in vegetated rivers, wetlands and estuaries.

Uncertainty in TC Maximum Intensity with Fixed Ratio of Surface Exchange Coefficients for Enthalpy and Momentum

Uncertainty in TC Maximum Intensity with Fixed Ratio of Surface Exchange Coefficients for Enthalpy and Momentum

Potential for New Constraints on Tropical Cyclone Surface-Exchange Coefficients through Simultaneous Ensemble-Based State and Parameter Estimation

AbstractThe tropical cyclone (TC) surface-exchange coefficients of enthalpy (Ck) and momentum (Cd) at high wind speeds have been notoriously challenging to estimate. This difficulty arises from many factors, including the difficulties in collecting observations within the turbulent TC boundary layer, and the complex coupled physical interactions between the TC boundary layer and ocean surface, which are challenging to accurately model. Motivated by recent studies highlighting the limited practical predictability of TC intensity as a result of uncertainty in the physical representation of the air–sea fluxes of momentum and enthalpy at high wind speeds, we investigate the potential to estimate the surface enthalpy and momentum exchange coefficients through ensemble data assimilation. Significant ensemble correlations between tangential wind, radial wind, and simulated infrared brightness temperatures with parameters controlling the enthalpy and momentum exchange coefficients suggest potential to use all-sky satellite and/or airborne radial velocity observations to estimate these unknown parameters. Using a series of observing system simulation experiments (OSSEs), simulated infrared brightness temperature observations, and a known truth, we demonstrate some potential for simultaneous state and parameter estimation with an ensemble-based data assimilation system to converge toward the correct known parameter values. In all OSSEs with either one or multiple unknown parameters, the initial parameter errors are reduced through simultaneous model state and parameter estimation. However, challenges still exist in converging to the precise true parameter values, as state errors during rapid intensification can project onto the parameter estimates.

Effects of interphase momentum exchange models on simulation of subcooled flow boiling

This research aimed to explore the accuracy of different momentum exchange models in the Eulerian multiphase method. Effects of different correlations for interaction forces on predicting of subcooled fluid flow boiling parameters in heated vertical tubes were studied numerically. Specifically, numerical simulations were carried out using several combinations of the momentum exchange coefficients separately. The results were compared with the experimental data of two benchmark test cases, and the error percentages were calculated. Results indicated that the accuracy of the two-phase simulations depended on the selection of the appropriate combination of momentum exchange models. However, it was found that modeling drag with Ishii and Universal, lift with Moraga and Saffman, wall lubrication with Antal et al. and Hosokawa, turbulent dispersion with De Bertodano, and turbulent interaction with Troshko-Hassan correlations generally led to higher accuracy. The results showed that the experimental benchmark data could be simulated numerically with the minimum relative error of 6.97%. The study also showed that wall lubrication and lift had negligible impacts on the numerical results for most working conditions.

Evaluating planetary boundary-layer schemes and large-eddy simulations with measurements from a 250-m meteorological mast

We evaluate the performance of six planetary boundary layer (PBL) schemes and a large-eddy simulation (LES) for the characterization of the neutral PBL through idealized simulations using the Weather Research and Forecasting model. The evaluation is performed by comparison with sonic anemometer measurements from a 250-m tall meteorological tower that observes close-to-homogeneous winds under the predominant westerlies. All simulations show similar behavior for the vertical temperature profile except for one that uses a non-local PBL scheme, which is known to produce excessive vertical mixing. Within the range of measurements of the tower and except for a PBL scheme that produces a too low jet, the simulations using PBL schemes, which were developed to simulate the nighttime atmosphere, generally show the largest deviations from the observed wind speeds. As expected within the surface layer, the LES shows excessive vertical shear. More importantly, we find that two PBL schemes that use their own surface-layer scheme are effectively reducing the surface roughness. When looking at the vertical profile of momentum exchange coefficient, we find very good agreement between a local and a non-local PBL scheme within the bulk of the PBL, the highest values for the simulation with the PBL scheme showing excessive vertical mixing and the lowest values for that most suitable for nighttime conditions. The comparison with the observed turbulent kinetic energy reveals a good match between the LES and a simulation using a local PBL scheme and a general underestimation (overestimation) of turbulence within the range of measurements by the PBL schemes (LES).

CFD modeling of a typical fluidized bed column

The hydrodynamic behavior of a 2-D gas–solid fluidized bed has been studied using computational fluid dynamics (CFD) software package ANSYS Fluent. The modeling results were validated using the experimental data carried out in a lab-scale perspex column based fluidized bed having a diameter of 0.1 m and height of 0.62 m. The modeling studies showed a very good resemblance with the experimental observations. Studies were carried out by using 0.0003 m biomass fine powder having a density of around 1242 kg/m3 at different inlet gas velocities. Eulerian-Eulerian model approach along with the kinetic theory of granular flow is used for simulating the gas–solid fluidization behavior. In the present work, the effects of inlet gas velocity, pressure drop and bed expansion ratio was studied. The minimum fluidization velocity condition was predicted by using the simulated contour plot of the computational software. The momentum exchange coefficients were calculated using Syamlal-O’Brien functions. The comparison study showed that the CFD simulation of a gas–solid fluidized bed can be used effectively for the designing and operation of a lab-scale fluidized bed column. Moreover the simulation studies would also help in the scaling up of the fluidized bed further for industrial usage.

Nonlinear Impacts of Surface Exchange Coefficient Uncertainty on Tropical Cyclone Intensity and Air‐Sea Interactions

AbstractTropical cyclone maximum intensity is believed to result from a balance between the surface friction, which removes energy, and a temperature/moisture (enthalpy) difference between the sea surface and the air above it, which adds energy. The competing processes near the air‐sea interface are controlled by both the near surface wind speed and the surface momentum (Cd) and enthalpy (Ck) exchange coefficients. Unfortunately, these coefficients are currently highly uncertain at high wind speeds. Tropical cyclone winds also apply a force on the ocean surface, which results in ocean surface cooling through vertical mixing. Using coupled atmosphere‐ocean and uncoupled (atmosphere only) ensemble simulations we explore the complex influence of uncertain surface exchange coefficients on storm‐induced ocean feedback and tropical cyclone intensity. We find that the magnitude of ocean cooling increases with storm intensity and Cd. Additionally, the simulated maximum wind speed uncertainty does not necessarily decrease when ocean feedback are considered.

Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique

In terms of gas holdup, liquid velocity, and volumetric mass transfer coefficient for oxygen (KLa), the hydrodynamic behavior of four configurations of an airlift reactor (ALR) with a net draft tube (NDT) of different net mesh sizes (ALR-NDT-3, 6, 12, and ALR) have been numerically simulated for a range of inlet air flow rates. The effect of various levels of ratio of height (H) to inner tube diameter (D) of the net draft tube (H/D: 9.3, 10.7, 17.5, and 20) and ratio of inner cross-sectional area of the riser (Ar) to the inner cross sectional area of the downcomer (Ad) (Ad/Ar: 1.3 and 7) for different air flow rates is also evaluated for each reactor configuration operating with an air–water system. The two-fluid formulation coupled with the k–ε turbulence model is used for computational fluid dynamics (CFD) analysis of flow with Eulerian descriptions for the gas and liquid phases. Interactions between air bubbles and liquid are taken into account using momentum exchange and drag coefficient based on two different correlations. Trends in the predicted dynamical behavior are similar to those found experimentally. A good agreement was achieved suggesting that geometric effects are properly accounted for by the CFD model. After a comparison with experimental data, numerical simulations show significant enhanced gas holdup, liquid velocity, and KLa for the ALR-NDTs compared with the conventional ALR. Higher gas holdup values are achieved for ALR-NDT-3 than that for the other ALRs because it acts like a bubble column reactor as the holes present in the NDT are large. Maximum liquid velocities are seen in ALR-NDT-12, which operates like a conventional ALR. Moreover, the interaction between the NDT and upward gas flow leads to cross flow through the net, small bubbles, and high interfacial area as well as good mass transfer. This was significant in ALR-NDT-6 with maximum KLa value of 0.031 s−1. The applied methodology provides an insightful understanding of the complex dynamic behavior of ALR-NDTs and may be helpful in optimizing the design and scale-up of reactors.

Drill cuttings transport and deposition in complex annular geometries of deviated oil and gas wells: A multiphase flow analysis of positional variability

In this study, the two-fluid Eulerian–Eulerian multiphase flow model in ANSYS Fluent (v17.1) is adopted for the simulation of cuttings transport in a deviated annular geometry using two different non-Newtonian drilling fluids (described using the power law and Herschel–Bulkley models). The Syamlal-O'Brien interphase momentum exchange coefficient is implemented to capture non-spherical effects of the drill cuttings. The analysis conducted is based on the hypothesis that a position-dependent profile evaluation is expected to yield more insight into the transport process compared to a volume-averaged analysis over the entire flow domain. This is because the adopted simulation approach takes into account the microscopic particle properties which significantly affect the overall particle transport mechanism. However, this requires the application of robust postprocessing functionalities for data extraction from desired annular regions. Particle sizes considered are in the range of 0.002m–0.008m with a sphericity range of 0.5 to 1.0. A rotational effect is incorporated in our model to describe the drillpipe motion in an annular wellbore with a vertical eccentricity of 0.6. The considered geometry contains a vertical, inclined and horizontal section with interconnecting upper and lower bends. The analysis of the particle velocity and concentration profiles revealed that the mud rheology, particle sphericity and particle sizes play vital roles in determining the cuttings removal process. It is also particularly observed that the lower annular region of the upper bend is the most susceptible to particle deposition with the lowest transport velocities observed. Our positional variability analysis has shown that the variable dominance of non-spherical and spherical particles’ velocities significantly depends on the nature of the flow (i.e. dense granular flow or dilute annular flow in the upper and lower sections, respectively).

An Observational Analysis of Ocean Surface Waves in Tropical Cyclones in the Western North Pacific Ocean

AbstractKnowledge of ocean surface waves in tropical cyclones (TCs) is necessary to calculate air‐sea exchanges of momentum and enthalpy for improved TC predictions. Here we use 24 years (1992–2015) of TC and significant wave height (SWH) observations in the western North Pacific to analyze storm‐following waves within the TC. The SWH was composited according to the TC translation speed, Uh, and TC intensity based on Vm, the maximum 10‐m wind speed. The results show that SWH is largely symmetrical for slow‐moving TCs when Uh is less than 3 m/s and is asymmetric for faster‐moving storms with larger waves in the right‐front quadrant referenced to the TC heading. For a class of TCs translating at a medium Uh between 3 and 7 m/s, a little less than the waves' group speed, near resonance leads to strong asymmetry with over 50% of waves with SWH exceeding 7–8 m on the right‐front quadrant. As Vm intensifies to 50 m/s and greater, extreme waves develop and become saturated: SWH ≈ 10–11 m, corresponding to when, as shown in previous studies, the momentum exchange coefficient CD levels off while the enthalpy exchange coefficient Ck may increase. Over 80% of the saturated waves then reside in the right‐front quadrant, indicating the quadrant's dominant influence on CD and Ck. The medium‐translating TCs were found to have a greater propensity for intensification than other types of TCs. Our results suggest a link between TC intensity, translation, and wave saturation and asymmetry in the right‐front quadrant.

Static pressure distribution along the main current channel of drip irrigation pipe

Based on the conservation law of mass and momentum, a mathematical model of variable mass flow was established. Combined with the pressure test data, the expression of static pressure distribution along the main current channel of the drip irrigation pipe was obtained. The results show that the static pressure variation along the main current channel of the drip irrigation pipe depends on the frictional resistance and the momentum exchange. The frictional resistance term diminishes the static pressure, and the momentum exchange term makes the static pressure tend to increase. The pressure test shows that the axial flow velocity distribution index m of the drip irrigation pipe is independent of the dripper characteristic parameters and linear with the drip number N . Based on the theoretical analysis and experimental data regression, the expression of the momentum exchange coefficient k is obtained, and the equation is solved by the Blasius resistance formula. The calculated value of the static pressure agrees well with the measured value. In the static pressure distribution model, the influence factors of static pressure are attributed to the length-diameter ratio of the drip irrigation pipe and the Reynolds number Re0 of the inlet, which is convenient for optimizing the structural design and determining the optimal operating conditions. This paper can provide a reference for the hydraulic calculation of drip irrigation pipe and the hydrodynamics study of porous pipes.

Development and verification of anisotropic drag closures for filtered Two Fluid Models

Over the past decade, filtered Two Fluid Models (fTFMs) have emerged as a promising approach for enabling fluidized bed simulations at industrially relevant scales. In these models, the filtered drag force is considered to be the most important quantity that requires closure. To date, such closures have typically relied on an isotropic interphase momentum exchange coefficient by applying a drag correction factor to the microscopic drag closures commonly used in resolved simulations. In the present study, both isotropic and anisotropic closures are developed for predicting the filtered interphase forces. The relative performance of these two approaches is then evaluated by means of an a priori assessment, considering data obtained from simulations in which all flow variables are resolved, which were also used for closure derivation. Also, an a posteriori assessment, which compares coarse grid simulation results to a benchmark resolved simulation of a bubbling fluidized bed, is presented. The primary conclusion from the present study is that it is essential to account for the anisotropy of the filtered momentum exchange coefficient. It is shown that this can be done by employing a drift velocity formulation of the filtered drag force and by considering a gravitational contribution that only acts in the vertical direction. Furthermore, it is found that for the large computational grid sizes that are typically required in industrial scale fluidized bed simulations, a closure for the meso-scale interphase force is essential. Finally, also for coarse grids, a non-linearity correction factor, which accounts for assumptions in deriving the drift velocity-based form of the filtered drag force, requires closure. The present study therefore highlights multiple avenues for improving drag closures used in fTFMs. Hence, these results may critically strengthen the predictive capabilities of fTFMs, as well as guide future modelling efforts.

Impact velocity-dependent restitution coefficient using a coupled Eulerian fluid phase-Eulerian solid phase-Lagrangian discrete particles phase model in gas-monodisperse particles internally circulating fluidized bed

A Coupled Eulerian fluid phase-Eulerian solid phase-Lagrangian discrete particles phase model (CEEL) is proposed to study of the hydrodynamics of gas and monodisperse particles in an internally circulating fluidized bed (ICFB). The impact velocity-dependent coefficient of restitution (CoR) used in the kinetic theory of granular flow (KTGF) is determined from the dynamic information of Lagrangian discrete particle phase using discrete element method (DEM). The interphase momentum exchange coefficient between Eulerian fluid phase and Eulerian solid phase gives back to the Lagrangian discrete particle phase. Numerical simulations show that the impact velocity-dependent CoR decreases with the increase of solid volume fraction and impact velocity of discrete particles. The predicted velocities and circulation mass fluxes of Eulerian solid phase and Lagrangian discrete particles phase are close to each other in the ICFB. The granular temperature and rotational granular temperature of Eulerian solid phase and Lagrangian discrete particles phase are predicted. Present CEEL model provides all fields of continuous fluid phase, Eulerian solid phase and the motion of discrete particles in the ICFB.

Verification of a One-Dimensional Model of $$\hbox {CO}_{2}$$ CO 2 Atmospheric Transport Inside and Above a Forest Canopy Using Observations at the Norunda Research Station

A model of (Formula presented.) atmospheric transport in vegetated canopies is tested against measurements of the flow, as well as (Formula presented.) concentrations at the Norunda research station located inside a mixed pine–spruce forest. We present the results of simulations of wind-speed profiles and (Formula presented.) concentrations inside and above the forest canopy with a one-dimensional model of profiles of the turbulent diffusion coefficient above the canopy accounting for the influence of the roughness sub-layer on turbulent mixing according to Harman and Finnigan (Boundary-Layer Meteorol 129:323–351, 2008; hereafter HF08). Different modelling approaches are used to define the turbulent exchange coefficients for momentum and concentration inside the canopy: (1) the modified HF08 theory—numerical solution of the momentum and concentration equations with a non-constant distribution of leaf area per unit volume; (2) empirical parametrization of the turbulent diffusion coefficient using empirical data concerning the vertical profiles of the Lagrangian time scale and root-mean-square deviation of the vertical velocity component. For neutral, daytime conditions, the second-order turbulence model is also used. The flexibility of the empirical model enables the best fit of the simulated (Formula presented.) concentrations inside the canopy to the observations, with the results of simulations for daytime conditions inside the canopy layer only successful provided the respiration fluxes are properly considered. The application of the developed model for radiocarbon atmospheric transport released in the form of (Formula presented.) is presented and discussed. (Less)

Temperature effects on hydrodynamics of dense gas-solid flows: Application to bubbling fluidized bed reactors

In this article, the effects of operating temperature on the hydrodynamics of dense gas-solid flow inside the fluidized bed reactor are investigated systematically. To this end, 3D simulations have been carried out by incorporating the appropriate model parameters and using the well-known Euler-Euler two fluid methodology in ANSYS FLUENT. The methodology is validated against the experimental results available in the literature. Temperature, air velocity and particle sizes are varied systematically. Results show that the variation of minimum fluidization velocity with temperature depends upon the particle size. For small particles (Geldart's group B), the minimum fluidization velocity decreases with an increase in temperature and the trend is reversed when large particles (Geldart's group D) are fluidized. This behavior is explained by analyzing the modification in inter-phase momentum exchange coefficient due to temperature variation. Voidage profiles, particle velocity and bubble characteristics are also seen to be influenced by the thermal conditions of the fluidized bed reactor. Particle axial velocity tends to decrease with an increase in the operating temperature. For group B particles, temperature has negligible effect on bubble size and expanded bed height. On the other hand, bubble size and expanded bed height decrease with an increase in temperature for group D particles.