[Bicycle Bob]

Radiators have evolved to maximize the area for heat transfer with minimum weight and cost. A few ships have used the type of pipe you describe, but they are transferring to water. The 1911 Napier had its engine surrounded by copper pipes to make a streamlined enclosure with cooling surface. Generally, you want to protect the pressurized coolant from casual damage.

Theoretically, a duct with a rad can work as a low-temperature jet engine and add thrust. Many texts refer to the P-51 Mustang aircraft achieving negative drag from the rad housing, but it wasn't copied on the Voyager. As is usual on aircraft, it had a variable exit, rather than a variable intake, but that made little difference to the air. With the flow restricted, the ring of stagnant air replacing the stagnation point would just reduce its diameter, while the high pressure zone handled the flow shaping adequately.

A snowmobile has a underbody panel system like the one you describe, but if it is not sprayed with snow, as when running on ice or roads, it overheats.