Flow and thermal performance analysis is an important area in the nuclear industry for the safe operation of reactors and the optimum design of fuel rod bundles with spacers in the reactor core. The spacer, an essential element in the nuclear fuel rod assembly, is used to support the fuel rods in the bundle and maintain suitable clearance between the rods. This paper represents a comprehensive review of the effects of spacer/mixing vane spacer on flow behavior and heat transfer performance in nuclear fuel rod bundles, annular channels, and circular tubes. This paper is based on the experimental and computational fluid dynamics (CFD) investigations performed by researchers in literature for both single-phase and two-phase fluid flow. In addition, the effects of some parameters, such as Reynolds number, hydraulic diameter, pitch-to-diameter ratio, blockage ratio, working fluid, and number of fuel rods in an assembly, on flow and thermal performance analysis have been reviewed. A detailed review has been carried out by considering both experimental and CFD methods used in the past for analysis of thermal-hydraulic performance in the fuel channels of nuclear reactors.