The heat transfer rate from a smooth wall in an internal cooling passage can be significantly enhanced by using a convex patterned surface on the opposite wall of the passage. This design is particularly effective for a design that requires the heat transfer surface to be free of any augmenting features (smooth). Heat transfer coefficients on the smooth wall in a rectangular channel, which had convexities on the opposite wall were experimentally investigated. Friction factors were also measured to assess the thermal performance. Relative clearances $δ/d$ between the convexities and the smooth wall of 0, 0.024, and 0.055 were investigated in a Reynolds number $ReHD$ range from 15,000 to 35,000. The heat transfer coefficients were measured in the thermally developed region using a transient thermochromic liquid crystal technique. The clearance gap between the convexities and the smooth wall adversely affected the heat transfer enhancement $NuHD.$ The friction factors (f ), measured in the aerodynamically developed region, were largest for the cases of no clearance $δ/d=0).$ The average heat transfer enhancement $Nu¯HD$ was also largest for the cases of no clearance $δ/d=0,$ as high as 3.08 times at a Reynolds number of 11,456 in relative to that $Nuo$ of an entirely smooth channel. The normalized Nusselt numbers $Nu¯HD/Nuo,$ as well as the normalized friction factors $f/fo,$ for all three cases, decreased with Reynolds numbers. However, the decay rate of the friction factor ratios $f/fo$ with Reynolds numbers was lower than that of the normalized Nusselt numbers. For all three cases investigated, the thermal performance $Nu¯HD/Nuo/f/fo1/3$ values were within 5% to each other. The heat transfer enhancement using a convex patterned surface was thermally more effective at a relative low Reynolds numbers (less than 20,000 for $δ/d=0)$ than that of a smooth channel.

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