The influence of the rib heights on heat transfer characteristics in U-shaped variable cross-section channels was experimentally studied in the steady state.The design parameters include:relative roughness pitch 15 mm

rib width 2 mm

and relative roughness heights 1 mm

1.5 mm

2 mm

and 2.5 mm

respectively

and Renault number from 12 000 to 56 000.Based on ribless channel of the same shape

the influence of rib heights on heat transfer effect and resistance of a channel with 15 mm pitch was studied

so an empirical relational expression comprising geometrical parameters was obtained.The experimental results show that

the heat transfer of ribbed channel will be improved;the influence of the rib heights on the heat transfer of U-shaped variable cross-section channel is not monotone

but distributed with multi peak values.The influence of rib heights on the resistance coefficient increases with the rib heights.