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I have been thinking about putting together some cooling airflow guidelines, but I need to do more research. Currently, I am leaning towards recommending a race-car type configuration like the illustration from Korff whenever possible. From my experiments back in '88 - '90, I know that inlets with frontal area recover much more of the freestream total pressure (ram) than "bottom breathers"; inlets that project no frontal area and rely on an airdam. However, I do not know where the optimum trade-off is between ram-recovery, fan power, and overall vehicle drag is. I discovered the day before yesterday that Jack Williams and my former advisor, Walt Oler wrote an updated paper in 2002 - Cooling Inlet Aerodynamic Performance and System Resistance (sae 2002-01-0256). I ordered it in electronic form from sae.org for $14. I have not had time to fully 'digest' it, especially since the notation has changed from what I used. It does say that the pressure loss from the fan to the engine bay is fairly minor compared to the other loss mechanisms and its not that sensitive to the spacing between the fan and the engine block. Having not thought about very long, my guess is the geometry of the duct downstream of the heat exchanger is not that critical - its sure to be better than the crude practice of directing the airflow at the engine block which results in most of it being deflected down. My research does support that ducting the frontal inlet(s) to the heat exchangers should be done whenever possible. Apparently non-uniform flow to the heat exchangers detracts significantly from the cooling system performance - yet another reason to favor a race-car configuration. I also believe that the fin design of high effectiveness heat exchangers (dimples, waves, etc.) generates sufficient turbulence on the proper scale to promote good heat transfer. I would keep the flow upstream of the heat exchangers smooth and clean.
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