STRUCTURE, VOIDAGE AND EFFECTIVE THERMAL-CONDUCTIVITY OF SOLIDS WITHIN NEAR-WALL REGION OF BEDS PACKED WITH SPHERICAL PELLETS IN TUBES

The structure of the near-wall region of the beds packed with spherica l pellets in tubes has been investigated experimentally at various rat ios of the tube-to-pellet radii. It has been found that more than 98% of all the pellets, which are immersed within the wall-adjacent bed la yer with the thickness equal to the pellet radius, contact the wall. O n the ground of this experimental finding, a relation has been derived between the number of pellets contacting 1 m2 of the wall surface are a and the average voidage within that bed region, and the ratio of the tube-to-pellet radii. The voidages have been then computed by means o f this relation for a set of the pellets numbers experimentally determ ined for several beds with different ratios of the tube-to-pellet radi i. The results have been verified with experimental data reported in l iterature, and satisfactory agreement has been observed; they have als o been employed in formulation of an empirical correlation between the average voidage within the wall-adjacent bed region extending up to a distance of pellet radius and the ratio of the tube-to-pellet radii. The experimentally obtained information concerning the structure of th e bed within its near-wall region has been utilized in deriving a rela tion expressing the effective thermal conductivity of the solids withi n that bed region as a function of its voidage and its geometrical par ameters, and the ratio of the true thermal conductivity of the solid m aterial and that of the fluid. Its predicted values can be used in the wall-boundary condition imposed on the heat flux within the solids ac cording to the two-phase and two-dimensional mathematical model often recommended for numerical simulation of performance of tubular reactor s packed with beds of spherical catalyst pellets.