It has been widely reported that traditional circulating fluidized beds (CFBs) suffer from core-annulus flow structure which is characterized by severe solid back-mixing and non-uniform distribution of solids in both axial and radial directions. In this work, an S-shaped design of risers is proposed and its performance is examined first by numerical and physical experiments. Both experiments show that the flow pattern in the S-shaped risers is significantly different from that in traditional straight risers. Generally speaking, the uniformity of the distribution of solid concentration is improved in the S-shaped risers and the high solid concentration regions are mainly in the center of the riser instead of the regions near the riser wall, with the core-annulus flow structure of a traditional design almost eliminated. The reason for the improvement is that in an S-shaped riser, the high fluid velocity regions move back and forth along the riser wall, which can prevent the boundary layer of solids from being established. This S-shaped design could be useful to improve the solid-fluid contacting in vertical particle-fluid flow systems. (C) 2014 Elsevier B.V. All rights reserved.