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Modeling of heat transfer in circulating fluidized beds Xie, Donglai


Suspension-to-wall heat transfer in circulating fluidized beds is modeled considering both the reactor-side and wall-side heat transfer processes. The overall flow structure in fast fluidized beds is represented by a core-annulus flow pattern with a stagnant particlefree gas gap between the wall and wall layer. Descending particles are assumed to enter the heat transfer zone with the same temperature as the core suspension. As particles descend in the wall layer, they lose heat to the gas by convection and gain heat from fresh particles arriving from the bulk core region. Gas is dragged downwards in the heat transfer zone by the rapidly-descending annular particles. The gas receives heat from the immersed particles by particle-to-gas convection and from the core by conduction. Heat is then conducted to the wall through the stagnant gas gap, and then through the furnace wall to the coolant. The model is the first to include the coolant-side heat transfer in the overall process. Particles also participate in radiation from the core to the wall through the wall layer. They are assumed to constitute a gray continuous absorbing, emitting and scattering medium. The radiation heat transfer process is solved by the two-flux model in a twodimensional model for CFB units with smooth walls, while the moment method is employed for the three-dimensional case when membrane walls are present. Under highdensity CFB operating conditions with smooth walls, the model is extended by allowing the suspension in the vicinity of the wall to travel intermittently downwards and upwards as is observed experimentally. The two- and three-dimensional models are validated using experimental results from the literature and both yield satisfactory predictions of the suspension-to-wall heat transfer. The influences of key parameters on the heat flux are analyzed and are found to be consistent with experimental trends where these are known. The simulation results suggest that the particles participate in a significant way in determining the radiation flux through the wall layer. Therefore radiation cannot be uncoupled from particle and gas conduction and convection without introducing significant error for high temperature systems. Experiments were conducted in the 76 mm diameter jacketed riser of a dual-loop high-density CFB facility with FCC particles of 65 pm Sauter mean diameter as bed material. The superficial gas velocity varied from 4 to 9.5 m/s and the solids circulation flux was as high as 527 kg/m²s. The suspension temperature and the average and local suspension-to-wall heat transfer coefficients were measured. The suspension temperature distributions indicate that the particles in the vicinity of the wall do not move in one direction only, but oscillate downward and upward, leading to higher local heat transfer coefficients at the ends of the heated section. Experimental results also show that suspension-to-wall heat transfer coefficients are strongly influenced by suspension density. However, they are not significantly influenced by superficial gas velocity at a constant suspension density. By superimposing the heat transfer results when the suspension in the vicinity of wall is allowed to move downwards and upwards separately, the model predicts the experimental results well.

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