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From: "Chris Lowery" <chris.lowery@sbcglobal.net>
Hi All,
I have found that putting vanes in front of the radiators lowers the
coolant temperatures. It seems that it straightens out the airflow so that
it goes through the radiator more easily.
Carefully executed, this could work very well. It will work much better on thin radiators with a low fin count (distant fin spacing) than it will on a thick radiator with a high fin count. (If the radiator had the correct fin spacing and thickness for the air flow and available pressure drop, you wouldn't need to add the vanes.)
It is nearly certain that the additional cooling is because of increased turbulence, not "straightening" of the air flow. Breaking up the stream lines and tumbling the air can greatly increase the heat transfer from the surface of the radiator fins/tubes.
When the flow is laminar between the fins, heat has a hard time going from the fin surface to the core of the airflow. It is forced to first heat the closest air layer, which then heats the air next to it, and so on. (If the vanes are far apart, and the air does not spend enough time in the radiator, the core streamlines will not pick up heat.)
If the air is turbulent instead of laminar, the air mixes up as it flows past the surface of the fin. Blobs of air that would have slipped though in the center of the core of a laminar flow are likely to be hurled against the fin surface and forced to pick up heat if the flow is turbulent.
If the radiator is thick with close fin spacing, introducing turbulence might help some, but will not make a large difference in the heat transfer. (It could actually reduce the cooling ability of the radiator, in some cases.) The turbulent flow will return to laminar after is has traveled a distance of something like 10 to 30 fin spacings into the radiator. (This distance depends on the Reynold's number between the fins.) If the radiator thickness is less than, say, 20 fin spacings, (most car radiators are) introducing turbulence might help a lot.
Of course, there will be a balance between reducing the air flow and increasing the turbulence. As you increase the turbulence, you will restrict the airflow. There is an optimum point, either side of which will be reduced cooling. Putting in too many vanes will result in reduced, rather than enhanced, cooling. If the radiator is designed correctly for the application (most aren't designed for airplanes) then adding vanes will reduce the cooling.
I suspect that typical automotive or motorcycle radiators would be designed for much lower air flow (less air pressure drop) than is available in a speeding airplane. This is probably why the evaporator cores work so well. In the car they came out of, they have a very strong fan forcing air through them. They have closely spaced fins and are quite thick to take advantage of the high pressure drop available to force the air flow. They may not be quite thick enough or have fins that are spaced quite close enough to be optimal for a 200 mph airplane. They will be a much closer match than the typical car or motorcycle radiator, however.
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