by brouser
 P: 3 I am putting a water cooled engine in a Porsche and want to design a custom radiator. How do I calculate the area necessary, air flow across this area, and temperature under different conditions assuming the power rating of the motor is 250 HP and is probably 35% efficient at best ? I have a mechanical engineer who works for me but the size he came up with does not seem to match what I see in production vehicles.
 Sci Advisor P: 5,096 The basic heat exchanger equation is $$Q = UA \Delta T_{lm}$$ where Q = heat transfer rate U = overall heat transfer coefficient A = Area of heat transfer $$\Delta T_{lm}$$ = log mean temperature difference between the two media exchanging heat. The difficult part is coming up with U. It is usually found by coupling this equation with the energy balance for the system and iterating a result. It's not a straight forward process. Chances are the reason why the estimated area your ME came up with is because a radiator has an incridible amount of perforations and bends in it to increase the overall heat transfer area while not increasing the overall envelope of the radiator.
 PF Patron Sci Advisor P: 2,792 More things to consider: Your engine might be 35% efficient "at best". However, have you considered "at worst"?! I'm not being facetious, but you won't have much problem when the engine is really working (140mph down the autobahn). You're going to come unstuck when you hit the traffic, you have no ram-air effect, and the poor thing is trying to dump an insane amount of heat out through an inadequately sized rad. Have you had an opportunity to do a heat balance on the engine, or are you still at the drawing board stage? When we specify a new radiator, it's pretty much a full-time job for the thermo engineer to do the calculations and predictive work. I wouldn't set about trying to calculate the air flow across the radiator. This is going to be something decided for you, essentially by the package constraints and the heat rejection at varying speeds, based on the possibility of (say) a 45 deg C ambient.
P: 1