For fluid catalytic cracker (FCC) riser reactor, the diffusion pattern of feed spray and the flow features of catalysts in the feed injection zone were investigated in a cold-riser model that is made of 186-mm ID plexiglass pipe. In the feed injection zone, when a feed spray is introduced into the riser, a secondary flow of spray will occur. The secondary flow extends at first and then merges into the mainstream of spray. The occurrence of the secondary flow enhances the mixing of catalysts with feed. However, the extension of the secondary flow causes a violent catalyst backmixing: it is believed to he harmful to FCC reaction. The generation of the secondary flow of feed spray was theoretically analyzed by using the Kutta-Joukowski Lift Theorem. Furthermore, a FCC feed nozzle, which can control/utilize the secondary flow in riser, was proposed. The effects of the nozzle used in some commercial FCC units are quite desirable. (C) 2009 American Institute of Chemical Engineers AIChE J. 56: 858-868, 2010