Keywords: periodic heat transfer, computational model, computational procedure, implementation, user defined function.

The heating of an immersed object in a fluidised bed can be considered as heat transfer from two alternant heating sources-particle packet and bubbles [1,2]. Because there is a very large difference in geometrical size between the particle packet, the gas bubble and the heated object, there is an instant changing in fluid boundary which requires adequate mesh and grid generation to correctly model the physical phenomenon and develop an adequate computational strategy.

This paper computationally studies the alternant heat transfer induced by particle packets and gas bubbles on the surface of an immersed object in a gas fluidised bed. A DPPM model [3], which includes a porous medium close to bulk and two particle layers near the object surface, is proposed to model the particle packets. The bubbles in contact with the object are considered as being transparent and enclosed by the particles. They are modelled by a hemispherical model [4]. The radiative heat flux from the bubbles is calculated theoretically. The heat transfer within an immersed solid object - a bolt is also simulated.

The differences in the sizes among the DPPM model, hemispherical bubble model and the immersed object are very obvious and considered in building and meshing a corresponding 3-D geometry. The DPPM model is small mm (heightwidthdepth) for the 0.1 mm particles; the hemispherical bubble model is bigger than the DPPM model and is 15mm in radii; the immersed object is much larger than both DPPM and hemispherical models. For the DPPM model, the TGrid type of meshing scheme is applied and different mesh sizes ( ) are used. Most of the mesh elements are tetrahedral, some are hexahedral pyramidal and wedge. The hexahedral elements of a size of 2 mm are used in meshing the object. The meshes of bolt surface in depth of 2 mm are refined by a 0.05 mm in the first row and a growth factor of 1.05. In the centre of the bolt, the spacing size of 2 mm is used.

Two parallel processes are developed to perform the simulation of alternant heat transfer for different purposes. The first parallel process is designed to focus on computing the instantaneous heat-transfer rate on the surface of immersed object. It executes on two computers in a network. The heat diffusion within immersed object and heat transfer from bubble phase are performed on one computer, while the heat transfer from emulsion is simulated on the other. The temperature of immersed surface is shared by different models and the data interaction between the models is performed through a library built in computer one.

The second parallel process was focused on simulating the heating process of the immersed object in the fluidised beds. The simulation of heat transfer between the particle packet and immersed surface and the calculation of heat transfer from bubbles are carried out on one computer. The average heat transfer coefficient on the immersed surface is calculated, depending on heat transfer coefficient of the particle packet, the heat transfer coefficient of bubbles and bubble fraction. The result is proposed as convection boundary conditions in simulating the heat diffusion within the immersed object on another computer. Same as the first parallel process, the models are interacted via communicators and message stored in a library. The operations, such as printing, displaying messages, and writing to a file, can be performed both computers.

The simulations are implemented into Fluent CFD package. Due to the governing equations and boundary conditions in the present simulation being different from those available in the software, various user-defined functions (UDFs) are developed and integrated into a shared library linked to standard Fluent executables. The simulation methods proposed in present work are successfully used to study the heat transfer mechanism, calculate the instantaneous heat-transfer coefficient, and simulate the heating process of the immersed object in fluidised beds [5,6].

1

Toomey, R.D. and Johnstone, H.F., "Gas fluidization of solid particles," Chem. Eng. Prog., 48, 220-226., 1952.

2

Kunii, D. and Levenspiel, O., "Fluidization Engineering," Butterworth-Heinemann, Krieger, Huntington, New York, 1991.

3

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

4

Yoshida, K., et al., "Mechanism of bed-wall heat transfer in a fluidized bed at high temperature," Chemical Engineering Science, 29, 77-82, 1974. doi:10.1016/0009-2509(74)85032-3

5

Gao, W.M., et al., "Numerical simulation and mechanism of heat transfer in quenching fluidised beds," Proceedings of the 4th international conference on quenching and the control of distortion, Beijing, 137-142, 2003.

6

Gao, W.M., et al., "Numerical Study of the Conduction and Convection between Immersed Object and Gas Fluidised Bed," ESDA 2004, Manchester, United Kingdom, ESDA2004-58264, 2004.

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