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Old 12-18-2008, 03:38 AM   #21 (permalink)
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Quote:
Originally Posted by fud2468 View Post
When doing a partial grille block, wouldn't it make sense to duct air directly from the grille opening to the radiator? Otherwise it would seem there's too much chance for turbulence enroute. A duct might allow a smaller opening in the grille to get the same amount of cooling effect.
Of course there'd still be turbulence for air passing through the radiator and into the engine compartment, but that's unavoidable.
Ray Mac
I did a quick scan of the thread and didn't see this addressed.

The worst possible thing you can do, from a heat transfer perspective, is to have laminar flow through a heat exchanger (such as a radiator). You want nice, slow, turbulent flow passing through that radiator.

why?
Turbulence increases heat transfer coefficients

Going to borrow this image


Diffuser slows down flow - due to increasing cross section.... Length and lower velocity should help the fluid go turbulent faster... Tapering diffuser behind radiator should increase velocity to be reintroduced with external flow.

Turbulence is your friend - everyone give your turbulence a hug, we've been bashing it too much

Quote:
* The bumper splits the airflow to the radiator
* The nose to radiator distance is FAR shorter than needed for Korff's Ideal Radiator Duct
* No clean exit for airflow from the radiator
* Fans and fan shrouds restrict and "dirty" the airflow after the radiator (but ARE necessary for additional airflow, like when you are travelling at low speeds after cruising on the highway for a length of time)
1. Yep
2. Totally
3. Not at all - there's other bits in the engine bay to cause problems too (hoses, et. al.)
4. It's not in front of me, but I do recall Hucho's book talking about fan shrouds (as in the Jetta model) being a compromise. All air entering the grille should pass through the radiator and never travel around. The actual car has foam sealant between the plastic shroud and radiator Plus, shrouding a fan increases it's efficiency


-------
If you're looking for aero gains in the cooling department.... I don't think you'll find what you're looking for - for anything short of a redesign. Keep in mind that the maximum aero gains will be less than the gains from completely blocking off your front grille(s). A function of limits, unfortunately.

If you can measure.... Attempt to measure the gains from blocking the grille. Then, decide if your time/money is worth gains less than what you measured. I'm willing to bet that your time/money are not worth it and better spent elsewhere for higher gains (*cough* - complete block; with adjustable slots for the worried - *cough*)

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Old 12-19-2008, 03:36 PM   #22 (permalink)
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Rich Taylor,Director of General Motors Truck Division, is the guru who advocated grille-blocking for the Popular Science team,modifying a pickup for better mpg.It's one of the few things he claimed would show up at the fuel pump.A photograph of the article can be found in the archives,under the Phil Knox aerodynamic photo albums.
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Old 12-19-2008, 05:11 PM   #23 (permalink)
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would it be worth it building an extraction ducts behind the radiator?

i've got quire some clearence but i don't think i could get an ideal hight length ratio as shown in the drawing above...


is there a similar ratio for the exit duct and is this ratio constant ?

like when the front duct length would be 1/2 of the radiator height would the ideal intake height be 1/3 of the rad height?
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Old 12-20-2008, 03:00 PM   #24 (permalink)
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extraction ducts

Quote:
Originally Posted by lunarhighway View Post
would it be worth it building an extraction ducts behind the radiator?

i've got quire some clearence but i don't think i could get an ideal hight length ratio as shown in the drawing above...


is there a similar ratio for the exit duct and is this ratio constant ?

like when the front duct length would be 1/2 of the radiator height would the ideal intake height be 1/3 of the rad height?
My short answer is no,I don't know.The person to ask would be Professor Alberto Morrelli in Italy.When he did the R&D for the CNR "banana-car" he used the Pininfarina windtunnel to determine extractor outlets,such that as the air left the radiator duct,it blended with the surrounding airstream at the same velocity so as not to trip the flow.I believe Morrelli published a paper on the project however I never tried for a copy.If anyone knew,he'd be the guy.The Rutan Brothers,at Scaled Composites,Mojave,California probably know.A letter or e-mail to them might lead to a reference.Sorry,I'm not much help.
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Old 12-21-2008, 03:57 AM   #25 (permalink)
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thanks anyway.

i'll continue my search for information.
time permitting i might rig up a carboard prototype, although the exit duct would probably call for a redesign of my undertray.

given the overall crudeness of the average cooling setup on a car perhaps a simple shroud could improve things already, but that's speculation and airflow is one of these things that can confuse the uninformed mind
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Old 12-21-2008, 10:19 AM   #26 (permalink)
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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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Old 12-21-2008, 12:32 PM   #27 (permalink)
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"...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..."


I disagree. NACA et al found that the ducting downstream from the heat exchanger and back into the slipstream is just as important as inlet design. In other words, inlet ducting is only half the job, which is not finished until efficient outlet ducting is in place. This is why NACA et al developed cowl flaps, etc..
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Old 12-22-2008, 02:30 AM   #28 (permalink)
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i think the main advantage of ducting is that one would have control of where and how post radiator air exits and how it merges with the existing flow around the vehicle.

who can really say what the air does when it exits the radiator? with so many subtle differences in engine bay layouts, and all the different pressures in and around it i think it's hard to come up with a generic picture. so in real world imperfect design i think it's better to have some sort of duct that guides the exit flow than nothing at all. the only danger i think is that a poorly designed duct (and very few existing cars will allow a mathematical correct duct to be retrofitted) might restrict airflow, especially at slow speeds when the airflow trough the radiator might not be so well defined, and hot air might remain trapped in the duct.
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Old 12-22-2008, 06:02 AM   #29 (permalink)
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Hi,

I think exhausting out of the back of the hood, into a low pressure zone, works better?
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Old 12-22-2008, 01:43 PM   #30 (permalink)
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The hood-windshield area (cowl, herein) is a high pressure zone on just about every car I've ever seen in my life, hence the reason for cowl induction hoods.

Radiator relocation would net the best testing application result, so that one could avoid the constraints of an engine bay.

Alternatively, get one of the Fiero guys to test something.. they still have the rad in the front, already angled, and no engine to contend with.

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