Three-dimensional modelling on the hydrodynamics of a circulating fluidised bed

The rapid depletion of oil and the environmentalimpact of combustion has motivated the search for cleancombustion technologies. Fluidised bed combustion (FBC)technology works by suspending a fuel over a fast air inletwhilst sustaining the required temperatures. Using biomassor a mixture of coal/biomass as the fuel, FBC provides alow-carbon combustion technology whilst operating at lowtemperatures. Understanding the hydrodynamic processes influidised beds is essential as the flow behaviours causing heatdistributions and mixing determine the combustion processes.The inlet velocities and different particle sizes influence theflow behaviour significantly, particularly on the transitionfrom bubbling to fast fluidising regimes. Computationalmodelling has shown great advancement in its predictive capabilityand reliability over recent years. Whilst 3D modellingis preferred over 2D modelling, the majority of studies use2D models for multiphase models due to computational costconsideration. In this paper, two-fluid modelling (TFM) isused to model a 3D circulating fluidised bed (CFB) initiallyfocussing on fluid catalytic cracker (FCC) particles. Thetransition from bubbling to fast fluidisation over a rangeof velocities is explored, whilst the effects on the bubblediameter, particle distributions and bed expansion for differentparticle properties including particle sizes are compared. Dragmodels are also compared to study the effects of particleclustering at the meso-scale

Improved three-dimensional color-gradient lattice Boltzmann model for immiscible multiphase flows

In this paper, an improved three-dimensional color-gradient lattice Boltzmann (LB) model is proposed for simulating immiscible multiphase flows. Compared with the previous three-dimensional color-gradient LB models, which suffer from the lack of Galilean invariance and considerable numerical errors in many cases owing to the error terms in the recovered macroscopic equations, the present model eliminates the error terms and therefore improves the numerical accuracy and enhances the Galilean invariance. To validate the proposed model, numerical simulation are performed. First, the test of a moving droplet in a uniform flow field is employed to verify the Galilean invariance of the improved model. Subsequently, numerical simulations are carried out for the layered two-phase flow and three-dimensional Rayleigh-Taylor instability. It is shown that, using the improved model, the numerical accuracy can be significantly improved in comparison with the color-gradient LB model without the improvements. Finally, the capability of the improved color-gradient LB model for simulating dynamic multiphase flows at a relatively large density ratio is demonstrated via the simulation of droplet impact on a solid surface.Comment: 9 Figure

Pore-scale study of coke formation and combustion in porous media using lattice Boltzmann method

In-situ combustion (ISC) has long been recognized as a promising technique for heavy oil recovery. However, ISC includes multiple physicochemical processes, which are still poorly understood and difficult to predict and control. This study establishes a lattice Boltzmann (LB) model to simulate the two important aspects of ISC at the pore scale: coke formation and combustion. The LB model includes thermal expansion effects and solves the reactive air-coke interface without iterations. Moreover, this model improves upon previous models by considering both coke formation and two-step coke combustion, as well as the growth of solid geometry. Results show that the LB model correctly captures coke combustion properties. Meanwhile, the newly introduced coke formation and two-step combustion yield important findings. As heat released from combustion transfers downstream, oil cracking and coke formation ahead of the combustion front are successfully tracked. The generated coke fuels the upstream combustion, making the system self-sustained. During coke formation and combustion, four coke transition states are identified. In addition, a parametric study demonstrates that the large inlet oxygen content and driving force are desirable, while too high a driving force should be avoided as it causes high burning temperature. Furthermore, it suggests that the inlet air temperature should be set appropriately. On one hand, a high temperature may promote coke formation and retard the front propagation. On the other hand, a low temperature may slow down the combustion of coke 2, even though it is high enough to ensure the ignition of coke 1. The decelerated coke 2 combustion may further cause the insufficient heat release and the failed coke formation, thus inducing the early termination of combustion. Such effects of the inlet temperature indicate the necessity of considering coke formation and two-step coke combustion. These results help to improve the understanding and facilitate the development of ISC

Mesoscopic model for soft flowing systems with tunable viscosity ratio

We propose a mesoscopic model of binary fluid mixtures with tunable viscosity ratio based on the two-range pseudo-potential lattice Boltzmann method, for the simulation of soft flowing systems. In addition to the short range repulsive interaction between species in the classical single-range model, a competing mechanism between the short range attractive and mid-range repulsive interactions is imposed within each species. Besides extending the range of attainable surface tension as compared with the single-range model, the proposed scheme is also shown to achieve a positive disjoining pressure, independently of the viscosity ratio. The latter property is crucial for many microfluidic applications involving a collection of disperse droplets with a different viscosity from the continuum phase. As a preliminary application, the relative effective viscosity of a pressure-driven emulsion in a planar channel is computed.Comment: 14page

Unified lattice Boltzmann method with improved schemes for multiphase flow simulation: Application to droplet dynamics under realistic conditions

As a powerful mesoscale approach, the lattice Boltzmann method (LBM) has been widely used for the numerical study of complex multiphase flows. Recently, Luo et al. [Philos. Trans. R. Soc. A: Math. Phys. Eng. Sci. 379, 20200397 (2021)] proposed a unified lattice Boltzmann method (ULBM) to integrate the widely used lattice Boltzmann collision operators into a unified framework. In this study, we incorporate additional features into this ULBM in order to simulate multiphase flow under realistic conditions. A nonorthogonal moment set [Fei et al., Phys. Rev. E 97, 053309 (2018)] and the entropic-multi-relaxation-time (KBC) lattice Boltzmann model are used to construct the collision operator. An extended combined pseudopotential model is proposed to realize multiphase flow simulation at high-density ratio with tunable surface tension over a wide range. The numerical results indicate that the improved ULBM can significantly decrease the spurious velocities and adjust the surface tension without appreciably changing the density ratio. The ULBM is validated through reproducing various droplet dynamics experiments, such as binary droplet collision and droplet impingement on superhydrophobic surfaces. Finally, the extended ULBM is applied to complex droplet dynamics, including droplet pancake bouncing and droplet splashing. The maximum Weber number and Reynolds number in the simulation reach 800 and 7200, respectively, at a density ratio of 1000. The study demonstrates the generality and versatility of ULBM for incorporating schemes to tackle challenging multiphase problems

Improved three-dimensional thermal multiphase lattice Boltzmann model for liquid-vapor phase change

Modeling liquid-vapor phase change using the lattice Boltzmann (LB) method has attracted significant attention in recent years. In this paper, we propose an improved three-dimensional thermal multiphase LB model for simulating liquid-vapor phase change. The proposed model has the following features. First, it is still within the framework of the thermal LB method using a temperature distribution function and therefore retains the fundamental advantages of the thermal LB method. Second, in the existing thermal LB models for liquid-vapor phase change, the finite-difference computations of the gradient terms ∇ · u and ∇T usually require special treatment at boundary nodes, while in the proposed thermal LB model these two terms are calculated locally. Moreover, in some of the existing thermal LB models, the error term ∂t0 (T u) is eliminated by adding local correction terms to the collision process in the moment space, which causes these thermal LB models to be limited to the D2Q9 lattice in two dimensions and the D3Q15 or D3Q19 lattice in three dimensions. Conversely, the proposed model does not suffer from such an error term and therefore the thermal LB equation can be constructed on the D3Q7 lattice, which simplifies the model and improves the computational efficiency. Numerical simulations are carried out to validate the accuracy and efficiency of the proposed thermal multiphase LB model for simulating liquid-vapor phase change

Atomistic insight into enhanced thermal decomposition of energetic material on graphene oxide

Graphene oxide (GO)-based nanocomposites are promising additives for practical applications of cyclotrimethylenetrinitramine (RDX). GO is not only an excellent support for nanoparticles, but also has independent catalytic activities, which have not been well understood. In this study, the reactive molecular dynamics simulation method is employed to investigate the kinetics and fundamental catalytic mechanisms of the thermal decomposition of RDX on GO. The RDX decomposition reaction is found to be enhanced in the presence of GO and the catalytic effect is better at low than at high temperatures. Additionally, GO addition lowers the activation energy by 11.35% compared with the thermal decomposition of pure RDX. The study shows that the catalytic capabilities of GO primarily originate from its functional groups that promote both the initiation and intermediate reactions. Furthermore, the H exchange process between the functional groups on GO and RDX/RDX intermediates plays an important role in the reaction. GO is further oxidized with more functional groups during the reaction, which are also involved in the catalytic activities. Finally, the energy barrier of functional group-participated reactions is found to be lower than their corresponding unimolecular decomposition leading to enhanced thermal decomposition of RDX. The proposed catalytic mechanisms in the present research should also be applicable to other energetic materials of the same class with a similar structure as RDX

Droplet Collision Simulation by a Multi-Speed Lattice Boltzmann Method

Realization of the Shan-Chen multiphase flow lattice Boltzmann model is considered in the framework of the higher-order Galilean invariant lattices. The present multiphase lattice Boltzmann model is used in two-dimensional simulation of droplet collisions at high Weber numbers. Results are found to be in a good agreement with experimental finding

elcome@12Impact of oxygen and nitrogen-containing species on performance of NO removal by coal pyrolysis gas

Coal pyrolysis gas is considered a promising reburn fuel with excellent NO reduction performance because of the present of nitrogen-containing species (HCN and NH3) in the pyrolysis gas. In this study, we explored the effects of oxygen and nitrogen-containing species on NO removal performance with HCN and NH3 by reactive force field (ReaxFF) molecular dynamics (MD) simulations. Results indicate that appropriately reducing O2 concentrations and increasing the amount of nitrogen-containing species can benefit the NO reduction performance by coal pyrolysis gas. In addition, the effects of oxygen and nitrogen-containing species content on the NO removal and mechanisms of NO consumption and N2 formation are illustrated during NO reduction with HCN and NH3, respectively. Finally, based on the simulations results, practical operating strategies are proposed to optimize the NO reduction efficiency. In summary, this study provides new insights into NO reduction performance, which may contribute to optimizing the operating parameters to decrease NOx emissions during coal combustion