This paper presents discussions on predicting turbulence and heat transfer in two types of square sectioned U-bend duct flows with mild and strong curvature by recent second moment closures. Batten et al.'s [AIAA J. 37 (1999) 785] modified version of Craft and Launder's [Int. J. Heat Fluid Flow 17 (1996) 245] two-component-limit (TCL) turbulence model and Shima's [Int. J. Heat Fluid Flow 19 (1998) 549] wall-reflection free model are presently focused on. They are low-Reynolds-number models totally free from geometrical parameters. The former model is realizable and called the TCL model. For turbulent heat flux, a higher order version of the generalized gradient diffusion hypothesis by Suga and Abe [Int. J. Heat Fluid Flow 21 (2000) 37] is applied along with the TCL model. The results suggest that although both second moment closures are generally good enough for predicting flow and heat transfer in the case of mild curvature, only the realizable TCL model is reliable in the strong curvature case.