Flow visualization using a transient liquid crystal method is presented to study the effect of divider thickness on the local heat transfer distributions around a sharp 180-deg turn of a two-pass smooth square duct. Detailed local Nusselt number distributions on the bottom, blade-tip, and side walls are given for three divider thicknesses, W-d* = 0.10, 0.25, and 0.50, at a Reynolds number of 1.2 x 10(4). Complementary pressure loss measurements are presented in terms of variation of friction factor with W-d*. The thermal performance investigation shows that W-d* = 0.25 provides the highest total averaged Nusselt number ratios, both at constant flow rate and at constant pumping power conditions, and a moderate friction factor. Near-wall laser-Doppler velocimetry measurements are further performed to explain the liquid crystal measured surface heat transfer distributions. The results show that the direction and strength of the secondary flow with respect to the wall are most important fluid dynamic factors affecting the heat transfer distributions, followed by the convective mean velocity, and then the turbulent kinetic energy.