An experimental investigation into the global external heat transfer coefficients for circular U-bends (180 degrees bends) in a cross-flow air stream was performed using six different curvature ratios (delta) covering a Reynolds number range of 3000-80 000 while imposing a uniform-wall-temperature (UWT) boundary condition. The measurements specifically focused on a zero-degree angle-of-incidence, in which the external flow direction is parallel to the bend plane. A straightforward method to measure the global external heat transfer that is not dependent on geometry but limits the testing to UWT was employed. The external heat transfer coefficients for circular U-bends in cross-flow were found to be consistently higher than those for corresponding straight cylinder flows. Also, a strong dependence on the curvature ratio, delta, was observed. Measurements presented demonstrate a peak enhancement in the external heat transfer for curvature ratios close to an optimum value of delta(opt) approximate to 0.23. A geometrical analysis was performed to explain the experimental finding of a distinct optimum curvature ratio (delta(opt)) as a self-inflicted wake effect. U-bend Nusselt numbers were found to be over four times larger than those for straight tubes of identical diameter and Reynolds number. An engineering correlation for the global external Nusselt number is offered.