Abstract: An apparatus and a process for experimentally determining a heat transfer coefficient of a surface which includes a duct having an outer passage for water flow and an inner passage for air flow, the two passages separated by a thin wall membrane such that the water flow establishes a datum temperature on the thin wall membrane, and where air flowing through the second passage and over the thin wall membrane surface which can include heat transfer enhancements features will be heated by the features, and where the heat transfer coefficient can be determined from the surface temperature of the thin wall membrane and the change in temperature of the air flow. The duct with the thin wall membrane and heat transfer enhancement features is produced using a plastic or metallic additive manufacture process for low cost and quit turnaround time.