Changsheng Bu

Development and test of CFD–DEM model for complex geometry: A coupling algorithm for Fluent and DEM

Abstract CFD–Discrete Element Method (DEM) model is an effective approach for studying dense gas–solid flow in fluidized beds. In this study, a CFD–DEM model for complex geometries is developed, where DEM code is coupled with ANSYS/Fluent software through its User Defined Function. The Fluent Eulerian multiphase model is employed to couple with DEM, whose secondary phase acts as a ghost phase but just an image copy of DEM field. The proposed procedure preserves phase conservation and ensures the Fluent phase-coupled SIMPLE solver work stable. The model is used to simulate four typical fluidization cases, respectively, a single pulsed jet fluidized bed, fluidized bed with an immersed tube, fluidization regime transition from bubbling to fast, and a simplified two-dimensional circulating fluidized bed loop. The simulation results are satisfactory. The present approach provides an easily implemented and reliable method for CFD–DEM model on complex geometries.

Modeling and Coupling Particle Scale Heat Transfer with DEM through Heat Transfer Mechanisms

The detailed heat transfer mechanisms particle interior, gas film around particles, gas gap between contact surfaces, and rough surface are considered to model heat transfer between particles. The validation of the heat transfer model is accomplished and the predicted results show good agreement with other experiments. From the quantitative comparison of four heat transfer paths, it is revealed that the heat transfer through gas gap and rough surface could be neglected for a particle diameter larger than 2 mm. Furthermore, the detailed heat transfer model is coupled with the discrete element method (DEM) to calculate macro effective thermal conductivity (ETC) of fixed beds, and the accuracy and applicability is verified by comparing with other estimated and experimental results. The influence of particle diameter, density, specific thermal capacity, and thermal conductivity on ETC is investigated. Results show that the proposed heat transfer model provides an effective and accurate way to couple with DEM in the particle system.

Experimental study on the controlled air oxidation of sawdust in a packed-bed reactor

The controlled air oxidation technology is a promising way of disposing medical waste, which has been a huge challenge in China. It converts waste through partial oxidation into a gaseous mixture, small quantities of char and condensable compounds. But operational performance of the primary chamber of the controlled air incinerator is poorly understood, leading to difficulty in control. In this paper, a packed-bed reactor was established to study the effect of O2 concentration on sawdust oxidation. The feed gas flow rate was kept constant at 0.6 m3/h at room temperature (26 °C) with O2 concentrations varied from 6% to 12%. Temperature profiles of the beds, product yields and gas compositions in the out-of-bed fuel gas were measured in detail. The results showed that the sawdust beds achieved low temperatures for the given O2 concentrations and leveled off in the oxidation processes. The bed temperatures increased and the solid yields decreased with the increase of O2 concentrations. When the O2 concentration was 10%, the gas yield reached a minimum and the liquid reached a maximum correspondingly. When the O2 concentration increased from 6% to 10%, the peak concentrations of CO and CH4 in the gas yield increased. However, when the O2 concentration exceeded 10%, CO and CH4 concentrations decreased. As O2 concentration varied from 6% to 12%, CO2 concentration increased continuously. This study provides a fundamental insight that the reaction processes could be well regulated by means of adjusting the feed air in practical units.

Typical Gaseous Semi-Volatile Metals Adsorption by Meta-Kaolinite: A DFT Study

Kaolinite can be used as in-furnace adsorbent to capture gaseous semi-volatile metals during combustion, incineration, or gasification processes for the purposes of toxic metals emission control, ash deposition/slagging/corrosion inhibition, ultrafine particulate matter emission control, and so on. In this work, the adsorptions of typical heavy metals (Pb and Cd) and typical alkali metals (Na and K) by meta-kaolinite were investigated by the DFT calculation. The adsorption energies followed the sequence of NaOH-Si surface > KOH-Si surface > PbO-Al surface ≈ CdO-Al surface ≈ NaOH-Al surface > KOH-Al surface > NaCl-Al surface ≈ Na-Si surface > Na-Al surface > KCl-Al surface > Pb-Al surface > PbCl2-Al surface > CdCl2-Al surface ≈ K-Si surface ≈ PbCl-Al surface > K-Al surface > CdCl-Al surface > NaCl-Si surface > KCl-Si surface > Cd-Al surface. Si surface was found available to the adsorptions of Na, K, and their compounds, although it was invalid to the adsorptions of Pb, Cd, and their compounds. The interactions between adsorbates and surfaces were revealed. Furthermore, the discussion of combining with the experimental data was applied to the subject validity of calculation results and the effect of chlorine on adsorption and the effect of reducing atmosphere on adsorption.

CFD–DEM Modeling of O2/CO2 Char Combustion in a Fluidized Bed

This paper describes a CFD–DEM modeling of char combustion in a bubbling fluidized bed (BFB) under both O2/CO2 and O2/N2 atmosphere. The char combustion model under O2/CO2 atmosphere is based on our previous work of oxy-fuel combustion of a single char particle. Different mole concentrations of O2 in the inlet gas mixture are studied. The result indicates that the small bed materials restrict the mass transfer from bed to the surface of char particles when they are deep into dense phase. The distribution of reactive gases (O2, CO2, CO) in the bed does not show much difference when the O2 mol concentration is 21 % in the inlet gas between O2/N2 and O2/CO2 atmosphere, but gasification of char should not be ignored when the inlet O2 mol concentration is up to 30 % in O2/CO2 atmosphere.