The whole area of packed-bed studies, including heat transfer, fluid flow and reaction, is the subject of considerable research efforts due to the numerous applications. Packed beds are of broad interest, due to their wide use in industrial processes such as gas-liquid absorbers, distillation columns, catalytic reactors, metallurgical processes such as a blast furnace, and heat storage units using phase change materials (Akiyama et al., 1993). When monosize spheres are packed in a cylindrical container, the radial distribution of voidage within the bed is not uniform due to the existence of the wall. In particular, the voidage variations very close to the wall are remarkably larger than in other regions: this is the so-called "wall effect."Some processes have suffered from a pronounced wall effect, since they have small aspect ratio [tube diameter (D)/particle diameter (d)]. Furthermore, the flow maldistribution caused by this larger voidage near the reactor wall can be an important secondary effect of wall. This flow pattern, which is generally referred to as "channeling," will decrease the overall efficiency of reaction and heat transfer.There have been many articles in the literature regarding the voidage of packed beds (for example, Roblee et al., 1958, Benenati andBrosilow, 1962;Kimura et al., 1955). The principle of their experimental techniques to obtain the radial variation of voidage was identical. They first filled all the interstices in the packed bed with a resin or a wax. Then, the solid cylinder was machined to a successively smaller diameter. By measuring the weight removed and the diameter after each cut, the voidage in each thin annular section was determined. According to their data, a large randomly packed bed of uniform spheres tends to take an average voidage of 0.39. However, the voidage varies locally. The voidage changes in a oscillatory manner near the wall, due to the geometry of the spheres and the curvature of the container wall. Roblee et al. (1958) reported that the oscillatory variations of local voidage near the wall are found from the wall to three-sphere diameter at least. Later, Benenati and Brosilow (1962) confirmed that the annular zone, within which the oscillations of local voidage occur, extends inward a distance of approximately five-sphere diameters. In addition, they showed that the voidage tends to unity on the wall due to point contact andCorrespondence concerning this article should he addressed to T. Akiyama.has a minimum at one sphere radius from the wall. Mathematical formulations have also been proposed to describe the voidage variation by exponential forms (Chandrasekhara and Vortmeyer, 1979) and by damped sinusoidal forms (Schulunder, 1977).The sharp rise in voidage at the wall reduces the resistance to flow in this region; therefore, the velocity tends to be larger in the boundary layer. This has been observed experimentally (Carman, 1937;Metha and Hawly, 1969;Schwartz and Smith, 1953) and also has been reproduced in numerical simulations (Akiyama and Yagi, 19...