粉粒体層における熱移動機構

抄録

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 D_s 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.