The local heat transfer and pressure drop characteristics of developing turbulent ยฏows of air in three stationary ribbed square ducts have been investigated experimentally. These are: ribbed square duct with constant cross-section (straight duct), ribbed divergent square duct and ribbed convergent square duct. The convergent/divergent duct has an inclination angle of 1ยฐ. The measurement was conducted within the range of Reynolds numbers from 10 000 to 77 000. The heat transfer performance of the divergent/convergent ducts is compared with the ribbed straight duct under three constraints: identical mass ยฏow rate, identical pumping power and identical pressure drop. Because of the streamwise ยฏow acceleration or deceleration, the local heat transfer characteristics of the divergent and convergent ducts are quite dierent from those of the straight duct. In the straight duct, the ยฏuid ยฏow and heat transfer become fully developed after 2ยฑ3 ribs, while in the divergent and convergent ducts there is no such trend. The comparison shows that among the three ducts, the divergent duct has the highest heat transfer performance, the convergent duct has the lowest, while the straight duct locates somewhere in between.