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Old 04-14-2012, 01:18 PM   #15 (permalink)
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Originally Posted by TOOSTUBBORN2FAIL View Post
Thanks mort, that was my thought, if ones going to the back, I didnt think it made sense to keep the drag from the radator at the front and then add another heat exchanger in the rear, even if it just breaks even. I cant help but think I'll get a little something though, using the cars wake to pull the air through.

Just curious, your saying that from the inlet to the outlet, there should be a 15 degree difference in temps? I could probably do a rough test of that pretty easily.
The 15F air temp differential is just an educated guess. If you can think of a way to measure the air temp. in the radiator outlet flow, maybe 5 different spots averaged, then you'd know.
There isn't really much vacuum behind the car. Imagine the back of the car is just the trunk lid height and width, say 7.5 sq ft, and the total aero drag force on the car is about 90 lbs (at 65 mph). If all the aero is suction in the wake on just that tail part, that's about 0.08 psi. So you build your radiator to fill the back of the trunk and the air is sucked through, that air comes from somewhere, duct work from under the car? Is that air at a high enough pressure to push 10000 cfm through the radiator? I think you need a fan.
However, this is a good idea. If you can insert air into the wake volume, using a few hp fan, and remove the radiator from the front of the car then there should be some reduction in aerodynamic drag.

You compared removing the radiator opening with the flat license plate. The license is smaller, but if it is centered it adds almost 0 to the cd. Somewhere on the front of any blunt object is a small flat area, and it can be quite a large percent of the frontal area before it makes much difference. Also the standard cd for a flat plate, considering it is on the front of a long object is 1.0. The opening for the radiator is a different problem. The radiator dumps heat by causing the air to be turbulent as it passes through. Basically radiators have the worst cd of anything on purpose. So the cd for a radiator opening maybe 2 or 3. Modern cars reduce the drag due to having to have a radiator by using slat vent like openings and setting the radiator at the back of a plenum, then suck air through with a fan when needed. It helps, but the biggest single contributor to drag at the front of the car is still the radiator.
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