Effect of wall-temperature ratios (TR) and channel orientations (beta) on heat transfer in rotating smooth square U-duct is experimentally investigated. The inlet Reynolds number and rotation number (Ro) range from 10,000 to 30,000 and 0 to 2.0, respectively. Three TR cases (0.08, 0.12, and 0.16) and two channel angles (0,45 degrees) are selected. The effect of TR on heat transfer distribution can be unified by considering the wall-to-bulk temperature ratio ((T-b/T-w)(n)) which the exponent n varies from stationary to rotation. Compared with TR = 0.16, smaller TR case has larger critical Ro on the leading surface of the first pass and larger exceed-Ro of the second pass. Compared with beta = 0 channel, beta = 45 channel shows negative effect on heat transfer as Ro increases over the critical Ro. The channel orientation also enlarges the critical Ro on the leading surface of the first pass. The correlations between the critical Ro (Ro(c)) and the location (X/D), including wall-temperature ratios (TR) and channel orientations (beta), are developed into simple expression. The correlations demonstrate that the minimum heat transfer in the whole channel can be determined by the specified rotational states. And thus the correlations for minimum Nu/Nu(s) ratio as a function of Ro (0 < Ro < 1.0) are developed for different wall-temperature ratios and channel orientations. (C) 2015 Elsevier Ltd. All rights reserved.