CFD - Fluidized bed thermo-chemical treatment technology
A fluidized bed allows fine solid particles to behave like a liquid by the flow of supporting gas from the bottom of the solid bed. Various heat and thermo-chemical treatment processes can be performed in a fluidized bed by modifying parameters such as temperature, gas mixture and solid/gas mixture. The advantageous features of a heat treatment fluidized bed are near ideal temperature uniformity through the whole gas-particle volume and fast heating rate of the parts treated. We have proven the capability to heat treat and developed many production processes utilizing a fluidized bed. The thermo-chemical heat treatment is a complex diffusion process of metallic and non-metallic elements (such as C, N and Cr) into a thin substrate surface to modify the surface chemistry and microstructure of components. It includes gas-solid flow, heat transfer, species transport, chemical reactions of fluidizing gases, decomposition of gaseous molecules, absorption of atoms, and diffusion of the elements in metallic components. The CFD group's mission is to comprehensively improve the understanding of the chemical reactions and mass transfer in fluidizing gases and between the fluidizing gases and immersed metallic parts, as well as the dynamics and heat transfer in fluidized beds, where multiple parts and a basket of parts are immersed. |
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Project: Modeling of Heat and Thermo-chemical Processes in a Fluidized Bed
Research Team:
- Dr. Weimin Gao
- Dr. Dan Fabijanic
- A/Prof. Lingxue Kong
- Prof. Peter Hodgson
A fluidized bed allows fine solid particles to behave like a liquid by the flow of supporting gas from the bottom of the solid bed. Various heat and thermo-chemical treatment processes can be performed in a fluidized bed by modifying parameters such as temperature, gas mixture and solid/gas mixture. The advantageous features of a heat treatment fluidized bed are near ideal temperature uniformity through the whole gas-particle volume and fast heating rate of the parts treated. We have proven the capability to heat treat and developed many production processes utilizing a fluidized bed. Typical processes are:
- Nitriding and nitrocarburising of tool steels
- Low temperature Cr deposition and other metal coatings onto tool steels
- Quenching using a fluidized bed
Previous studies have made good progress in the understanding of the dynamics and heat transfer in fluidized beds without the inclusion of immersed objects. However, in terms of heat treatment, the previous dynamic and heat transfer models have rarely concerned the effect of part geometries, multiple parts and a basket of parts, as well as the chemical reactions either between the fluidizing gases or between the fluidizing gases and immersed parts.
ObjectivesThe aim of this project was to establish a comprehensive knowledge of:
- gas-solid multiphase flow
- heat and mass transfer at the part surace and between solid particles and the gas phase
- solid-gas chemical reactions in various bed and atmosphere compositions
- element diffusion into parts through computational simulation of processes and experimental evaluation of models
Salient results of this project are highlighted below:
- To understand the effect of immersed parts on fluidization and heat transfer, we simulated the gas-solid flow in a fluidized bed of 0.1 mm alumina (Al2O3) particles. The simulation of the gas flow was implemented using computational fluid dynamics (CFD) method, while the motion of particles were described by Newton equation with additional forces and solved with the discrete element method (DEM) (Figure 1).
- A macroscopic model composed of two particle layers and a porous medium was developed to simulate the heat transfer at the surfaces of parts. A calculation algorithm of the model was coded into the above dynamic and heat transfer models to calculate the heat transfer coefficient, which is a function of solid fraction, thermal-properties of gas and particles, temperature, velocity of gas-solid mixture and residence time at the part surface.
- Molecule diffusion and chemical reactions in beds were simulated to improve the fundamental understanding of the functions of inert solid particles in fluidised beds. Both the finite rate model (FRM) and the eddy dissipation model (EDM) were employed to model chemical reactions. Figure 2 shows the simulation results for the reaction of methane and air in a fluidized bed.
- The carburizing process of a metallic component using a fluidized bed has been simulated. The carbon transfer at the interface of the heat-treatment atmosphere and the steel surface was expressed as the decomposition of gaseous molecules, the absorption of atoms and chemical reactions. These mass transfer processes were integrated into CFD codes and the carburizing rates at different part surfaces were modeled.
- Mass transfer coefficient in a heat treatment furnace can be extracted from the element concentration at the surface or the element profile in the surface of a sample treated in the furnace. Two methods to extract the mass transfer coefficient were developed through the simulation of the carburizing of a part and the use of theoretical analysis, and this method subsequently used for other heat treatment processes.
Figure 1. Solid particle distribution in a fluidized bed
Figure 2. Distribution of mass fraction for different gases (average particle mass concentration: 420 kg/m3)
