Keywords: heat transfer, fluid dynamics, computational simulation, modelling, fluidized beds, conduction, convection.

The fluidised bed has been widely used in many industrial applications for various purposes by transforming fine solids into a fluid-like state through contact with a gas phase. The high heat-transfer coefficient of the bed is one of the most important characteristics. However, due to the lack of theoretical study of heat transfer to, or from, the gas and particles in contact with the immersed surface in the fluidised beds, the interpretation on the heat transfer depends greatly on the researchers' understanding on the process [1].

To improve the understanding of the heat transfer mechanism and find a reliable and simple heat-transfer model, the gas flow and heat transfer between fluidised beds and immersed object surfaces was numerically simulated by using a double particle-layer and porous medium model [2].

The double particle-player and porous medium has the ability to simulate the gas flow and the heat transfer near the surface of immersed object in fluidised beds, and was successfully used in calculating the dynamic characteristics of the gas phase, the temperature change of particles, and the radiative parameters of particle group. The results provide sufficient information to improve the understanding of heat transfer process near the immersed surface.

Heat transfer between immersed surface and emulsion phase is strongly affected by the gas flow near the heating surface. The gas velocity determines the type of heat transfer. In the stifling regions where the gas is almost at rest, the heat transfer is dominated by conduction. About 75% of the total heat flux transferring to the immersed object can be contributed to conduction.

The change of gas velocity with the distance from the immersed surface is consistent with the variation of the voidage. The gas velocity between the immersed surface and the first layer particles is . The result is significant to calculate the convective heat transfer.

The temperature of the gas near the immersed surface depends mainly upon the temperature of the immersed surface and the first layer particles. For small particles, the temperature of the gas near the immersed surface changes rapidly from bed temperature, , to immersed-surface temperature, , within the distance of a half particle diameter in a very short residence time. For Large particles, the change of gas temperature only occurs within a distance of approximately . The thermal penetration depth is less than one particle diameter for the longest residence time, 10 second.

The temperature of the particles in contact with the immersed surface decreases markedly with an increasing residence time. The change of temperature only occurs in the two particle layers. The double particle-layer model is therefore accurate enough to simulate the heat transfer between the immersed surface and the emulsion. For large particles, the particle temperature changes slightly for all residence times.

1

Molerus, O. and Wirth, K.-E., "Heat Transfer in Fluidized Beds", Chapman & Hall, London, 1997.

2

Gao, W.M., et al., "Numerical simulation of heat and mass transfer in fluidised bed heat treatment furnaces", Journal of Materials Processing Technology, 125-126, 170-178, 2002. doi:10.1016/S0924-0136(02)00372-2

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