The laminar convective flow and heat transfer in trapezoidal ducts with a rectangular cross-section are studied numerically. Under certain conditions, the temperature difference distribution and pressure drop in trapezoidal ducts are better than that of in rectangular ducts, which leads to an advantage of heat transfer enhancement. At the same heat transfer area, the maximum Colburn (j) factor in trapezoidal ducts increases by 8% when compared with a rectangular duct, and the minimum friction (f) factor decreases by 22.6%. The numerical data also shows that the thermal-hydraulic characteristics for trapezoidal ducts first increases and then decreases while slope angle (6) increases. The airflow is disturbed at the wall showing recirculation flow and separation when beta = 30 degrees, but the thermal-hydraulic performance starts to decrease when beta > 40 degrees. In a word, the thermal-hydraulic characteristics of trapezoidal ducts are better than that of rectangular ducts when beta < 40 degrees. On the contrary, When beta > 40 degrees, the rectangular duct has a better performance than that of trapezoidal ducts. The applicability of the above conclusions is proved by using the taguchi method. (C) 2017 Elsevier Ltd. All rights reserved.