Correlations For Effective Thermal Conductivity Research Articles

A Numerical Investigation of Heat Transfer Mechanisms in Gas-Solid Fluidized Beds Using the Two-Fluid Granular Temperature Model

In this work, the two-fluid granular temperature model is used to investigate the heat exchanged between a heated wall and a gas-solid fluidized bed. Numerical simulations were performed in 2-D and 3-D fluidized beds using a solid phase effective thermal conductivity correlation based on the granular temperature. The heat exchange in the bubbles' wake is investigated by tracking the train of bubbles that rises along the heated wall. Large heat transfer coefficients were obtained in the rear wake region of bubbles due to relatively larger granular temperature there and intense particle circulation.

Mechanisms of Heat Transfer in Solid Particle Beds

Radial heat transfer rates in the fixed bed have been obtained by several investigators from the radial temperature traverses measured. Results are reported in terms of effective thermal conductivities and wall heat-transfer coefficients of the fixed bed. These values can be divided into two groups, one independent of fluid flow and the other dependent on the lateral mixing of the fluid in the bed interstices. The effective thermal conductivity caused by the lateral mixing of fluid has been expressed by Eq. (14), which is derived from the concept of a “random walk” theory, and the general correlation of effective thermal conductivity in the fixed bed has been obtained in the form of Eq. (16). Further, the authors have carried out the experiments of heat transfer in the bed of solid particles mixed by means of a paddle-typed impeller and have measured the values of effective thermal conductivities and wall heat-transfer coefficients of four kinds of powders, such as sand, coke, silica-gel, and cement clinker. It has been found that the heat transfer mechanisms in the bed of solid particles mixed are asociated with two processes operating in parallel, one of which is the heat transfer in the quiescent bed of solid particles, taking place in the same manner as in the fixed bed with motionless fluid and the other the heat transfer due to the mixing of solid particles corresponding to the heat transfer caused by lateral mixing of fluid flow in the fixed bed.Accordingly, by following the general correlation of effective thermal conductivity in the fixed bed and by introducing the apparent diffusivity Ds of solid particles mixed, in place of the effective diffusivity (E)td in the fixed bed, the correlation of effective thermal conductivity in the bed of solid particles mixed has been obtained in the form of Eq. (20). The correlation of wall heat-transfer coefficient in the bed of solid particles mixed has been expressed by Eq. (25) and a new expression for estimating the wall heat-transfer coefficient in the quiescent bed of solid particles has been presented as Eq. (23).Applying these correlations to the experimental values of effective thermal conductivities and wall heat-transfer coefficients, it has been found that these correlations are in good agreement with the experimental values.The authors have ascertained that the results obtained in both the fixed bed and the bed of solid particles mixed can be handled with the help of the same concept of a “random walk” theory.