•Verification, validation, and application for the CFD-DEM method coupled with heat exchange are conducted.•Fluid velocity and particle diameter have a more remarkable influence on convection than ...conduction.•Particle temperature in the riser and the system presents a two-peak histogram distribution.
For a single isolated particle, the effect of overlap displacement on conduction and the effect of particle diameter/fluid velocity on convection are comprehensively studied using a CFD-DEM method coupled with heat exchange. Moreover, the model is further validated in a pseudo-2D bubbling fluidized bed and the influence of operating parameters (i.e., fluid velocity, particle diameter, and mass loading) on heat transfer contribution is deeply revealed. The results show that the fluid velocity and particle diameter have a more remarkable influence on convective heat transfer than conductive heat transfer. Besides, the particle-fluid convection dominates the heat transfer while the particle-particle conduction is almost negligible. The particle-fluid-particle conduction occupies about 10% of the total heat transfer. Finally, the model is successfully applied in a circulating fluidized bed. The average mass fractions of solid materials in the riser, cyclone, and dipleg are 0.737, 0.023, and 0.240, respectively. Solid internal circulations and lower solid velocities in the horizontal part of dipleg lead to a two-peak histogram distribution of particles temperature in the riser and the system.
•CFD-DEM model is applied to predict gas-solid flow dynamics in the full-loop CFB with multiple cyclones.•Pressure drop within middle components are slightly smaller than within other components.•The ...axial symmetry arrangement for the parallel cyclones is better than the central symmetry arrangement.
Multiple cyclones are adopted to enlarge the capacity of circulating fluidized bed (CFB). In this work, the effect of cyclone arrangements on the gas-solid flow dynamics in the three-dimensional full-loop CFB is investigated by the computational fluid dynamics coupled with discrete element method (CFD-DEM). Flow patterns, pressure distribution, and non-uniform distribution of the gas-solid flow in the CFB is comprehensively studied. Results show that the CFB with multiple cyclones gives rise to some unique characteristics, including: (i) solid back-mixing behavior mainly occurs close to the front and back walls and in the four corners of the riser; (ii) the spiral directions of the internal and external vortexes of gas flow in the cyclone are same; (iii) the closed-loop pressure is obtained and the pressure drop in the standpipe and L-valve are nearly four times of that in the cyclone; (iv) gas-solid flow shows non-uniform distribution in the multiple cyclones, and the middle cyclones have higher solid holdup and solid velocity than the corner cyclones; (v) the axial symmetry arrangement for the multiple cyclones is better than the central symmetry arrangement in terms of the uniform distribution of solid flux in the cyclone and solid inventory in the standpipe. The results provide meaningful understanding for the design, scale-up, and optimization of CFB apparatuses.
In this paper, a reactive MP-PIC model is built on the open-source software package OpenFOAM to explore biomass gasification in a fluidized bed reactor. In this model, gas phase is solved under ...Eulerian frame while solid phase is regarded as Lagrangian computational particles. The general flow patterns, gas-solid fluxes, distribution of syngas composition, and effect of operating parameters on gasification performance are both qualitatively and quantitatively analyzed. The results show that biomass particles are inclined to migrate and concentrate at the bed surface. Profiles of vertical gas fluxes show a single peak distribution while profiles of vertical solid fluxes present a “W” shape. The heterogeneous reactions mainly occur at the bed surface while the homogeneous reactions mostly take place above the bed surface. Higher temperature enforces the endothermic reactions for products formation. Increasing steam/biomass ratio increases H2 and CO2 formations but decreases CO formation. The four types of feedstock show a similar gasification performance with little variation in the syngas composition. As a priori test, it is demonstrated that the proposed model is reasonable to be used to investigate gas-solid hydrodynamics and reactive characteristics of biomass gasification in fluidized bed reactors.
General flow patterns and syngas composition in biomass gasification are presented, meanwhile, numerical results agree well with experimental data. Display omitted
•A reactive MP-PIC model is developed on OpenFOAM.•Gas-solid hydrodynamics highly relates with reactive characteristics.•Heterogeneous and homogeneous reactions occur in different regions.
•A coarse-grained CFD-DEM method is developed for modeling reactive fluidized beds.•The effects of operating conditions on a fluidized bed gasifier are analyzed.•New insight on relationships between ...mixing and gasification reactions is provided.
In the present study, a comprehensive coarse-grained computational fluid dynamics and discrete element method (CFD-DEM) is developed for modeling the fluidized bed gasifier. Based on the model validations against experimental measurements, a series of simulations are conducted to investigate the effects of different operating parameters on the coal gasification in a bubbling fluidized bed. This study offers new insights into the effects of gas-solid mixing on the gasification reaction. The results show the non-uniform spatial distributions of the gasification rate in both the horizontal and vertical directions, which reflects the preferential distribution of fuel particles. It is found that the gas back-mixing plays an important role in controlling of the pyrolysis gas combustion, which is undesired for producing gasification products. The results show that the sand particle size has the most remarkable effect on the gas back-mixing. Though the uniformity of the fuel particle distribution is not a key factor influencing the average product yields, the horizontal fuel particle mixing is found to be the main cause of the fluctuations of the gasification rate. Fundamentally, it is demonstrated that the fluctuations of the horizontal fuel mixing are mainly dominated by the bubble behaviors.
Supercritical water (SCW) gasification is regarded as the low pollution approach with great potential for biomass utilization. As one of the most important components of biomass, lignin's conversion ...greatly influences the final energy conversion efficiency of the wood. It is necessary to disclose the detail reactive mechanism of lignin's gasification in SCW. In this paper, studies on the lignin gasification process in SCW are carried out with the reactive force field molecular dynamics method, in which the Nimz model of the lignin molecule is adopted. The detailed chemical reactions and their occurrence frequencies are both obtained. The generation paths of the main products including CO2, H2, and CH4 are disclosed. The results show that both the yields of all the three main products and the corresponding H2 proportions in the products increase with the temperature.
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•The gasification process of a lignin macromolecule in supercritical water is studied with the ReaxFF MD method.•The evolution of the lignin macromolecule, the typical pathway of the CO2, CH4 and H2 generation is obtained.•The effect of temperature and SCW density on gasification are discussed.
•We report the direct numerical simulation of droplet breakup in forced homogeneous isotropic turbulence with a mass-conserving level set method.•Compared with the flow local topology in the ...carrier-phase turbulence, the local topology of the bi-axial strain is suppressed at the statistical stationary state inside the droplet.•During the droplet breakup process, the vorticity tends to be tangent to the large-scale interface at the early stage, and subsequently the alignment is mitigated for large Weber numbers.
We report the direct numerical simulation of droplet breakup in forced homogeneous isotropic turbulence with a mass-conserving level set method. In particular, the effect of the Weber number on the interface morphology of droplet breakup and the interaction between vortical structures and the interface are addressed. The densities and viscosities are the same for both phases in the present simulations. It is found that with increasing the Weber number, the initially large-scale spherical droplet tends to break down into dispersed small droplets. Compared with the flow local topology in the carrier-phase turbulence, the local topology of the bi-axial strain is suppressed at the statistical stationary state inside the droplet. During the droplet breakup process, the vorticity tends to be tangent to the large-scale interface at the early stage, and subsequently the alignment is mitigated for large Weber numbers, because the small droplets with relatively strong surface tension and small local Weber numbers can resist the deformation induced by local turbulent straining motion in the statistically stationary state.
In this paper, a systematic investigation of turbulence modulation by particles and its underlying physical mechanisms in decaying compressible isotropic turbulence is performed by using direct ...numerical simulations with the Eulerian–Lagrangian point-source approach. Particles interact with turbulence through two-way coupling and the initial turbulent Mach number is 1.2. Five simulations with different particle diameters (or initial Stokes numbers,
$St_{0}$
) are conducted while fixing both their volume fraction and particle densities. The underlying physical mechanisms responsible for turbulence modulation are analysed through investigating the particle motion in the different cases and the transport equations of turbulent kinetic energy, vorticity and dilatation, especially the two-way coupling terms. Our results show that microparticles (
$St_{0}\leqslant 0.5$
) augment turbulent kinetic energy and the rotational motion of fluid, critical particles (
$St_{0}\approx 1.0$
) enhance the rotational motion of fluid, and large particles (
$St_{0}\geqslant 5.0$
) attenuate turbulent kinetic energy and the rotational motion of fluid. The compressibility of the turbulence field is suppressed for all the cases, and the suppression is more significant if the Stokes number of particles is close to 1. The modifications of turbulent kinetic energy, the rotational motion and the compressibility are all related with the particle inertia and distributions, and the suppression of the compressibility is attributed to the preferential concentration and the inertia of particles.
In this paper, pulverized coal flames stabilized in a three-dimensional laminar counterflow configuration are simulated with detailed chemistry and the flame behaviors are analyzed in detail. Effects ...of radiation, coal particle mass flow rate and strain rate on the pulverized coal flame structure are investigated. The results show that the coal particles transported by the air stream tend to be ignited in a premixed combustion mode, which is followed by a non-premixed flame reaction zone, forming a typical double-flame type structure. The contribution of premixed combustion to the total heat release rate is sensitive to the studied operating conditions. Both volatile combustion and char off-gases combustion contribute to premixed combustion and their relative importance is influenced by the operating conditions. The pulverized coal combustion is significantly affected by radiative heat transfer. Without radiation, the reaction zone becomes thinner and the ignition is delayed. As the coal particle mass flow rate increases, the coal particles are ignited earlier and the combustion of char off-gases expands over a larger region. As the strain rate increases, both the premixed combustion share and the contribution of char off-gases combustion to the total heat release rate are decreased. For the extremely high strain rate case, the oxidizer can diffuse into the coal cloud from the oxidizer side to ignite the gas fuels at the fuel side (i.e. effect of oxidizer “leakage”). In order to properly consider the interphase heat transfers in gaseous flamelet models, a new tabulation method (compared to the conventional ones, e.g., Wen et al., (2017)) is proposed. The a priori analysis of the new tabulation method on different configurations shows that, compared to the conventional tabulation method, it better accounts for the heat transfers between the coal particle phase and gas phase, and can be applied to combustion systems with different oxidizer streams without introducing additional manifolds.
The augmented coarse-grained CFD-DEM contributes a significant improvement in computational efficiency with a little loss in numerical accuracy.
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•An augmented coarse-grained CFD-DEM ...is proposed.•The model improves efficiency with little loss in numerical accuracy.•The calculation speed approaches 5000 times faster than the traditional models.
Despite the wide range of applications, the traditional computational fluid dynamics-discrete element method (CFD-DEM) simulations have been running into bottlenecks when dealing with large scale systems. To overcome this issue, an augmented coarse-grained CFD-DEM approach which combines the reduced particle stiffness model and the coarse-grained CFD-DEM is developed to multi-dimensionally reduce the computation load. In spatial scale, several original particles are lumped into a computational parcel based on the coarse-grained ratio while the time step of solid phase is enlarged in temporal scale according to the reduced particle stiffness ratio. The accuracy and efficiency of the augmented coarse-grained CFD-DEM are quantitatively evaluated in fluidized beds, and different gas–solid characteristics are obtained via comparing with experimental measurements and traditional CFD-DEM. The results show that the augmented method contributes a significant improvement in computational efficiency while a little loss in numerical accuracy. Generally, the computational efficiency of the augmented coarse-grained CFD-DEM increases with the increase of coarse-grained ratio while it enhances with the decrease of the reduced particle stiffness ratio. This augmented coarse-grained CFD-DEM is expected to be a promising tool to optimize gas–solid flow dynamics in large-scale dense particulate systems.
