Ducting air from grille to radiator
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 |
Ducting would be ideal. However, its a huge pain in the butt to make which is why I don't think I've seen actual ducting work done.
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pics to follow
Ok, it'll be a little while before I get pics, this is still in the planning stage. On my '06 Dodge Dakota quad cab I have blocked the entire grille and removed the cooling fan from the engine.
Even as cold as it has been (30 F), I'm having trouble keeping the engine cool enough. (It has been the plan all along to install an electric cooling before spring.) I'm hoping to be able to use the two holes below the bumper to duct cool air thru the radiator. I'll keep you posted. |
Once I build my adjustable grill block, I plan to duct it. Not really ideal to do it while it's still all just packing tape.
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I don't know if you can call it 'ducting', but I boxed in the area between the lower grille opening and the radiator of my Vibe. You can see it in post #53 of my 'Vibe mods' thread. With the upper grille completely blocked, and the lower opening blocked roughly 90%, coolant temperature is never an issue. In fact, I think it's taking LONGER to warm up than before. I noticed this immediately after installing the radiator-bumper 'ducting', which was before winter hit hard and we still had comfortable ambient temps.
If your car has a similar gap between the grille and the radiator, I would highly recommend boxing it in or 'ducting' it. You can get away with a much smaller grille opening. Of course, keep an eye on the coolant temps, preferably with a Scangauge. |
huge pain
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Do you have any pics of your setups? Materials used? Tips on construction?
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Yeah I plan on ducting mine since I only have a 6 by 12" hole in the bottom. It is probably the #1 thing you can do to lower coolant temps again if you are doing a grill block, more than running the fans more or anything.
Plus it just makes the "system" (car) MUCH more efficient. Oh, one more thing that I have noticed (with a grill block and will probably notice it more with the ducting) is how clean my engine bay has been... it looks pretty much BRAND NEW. |
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Korff's Ideal
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http://ecomodder.com/forum/member-ae...ing-system.jpg
Note that aerohead said AREA, not height like the book's diagram mentions. More precisely, you should be figuring that ideal openings are 1/6th the AREA of the radiator behind. Most cars have too short a nose to have the 1/6th radiator slit up front. Unless you're going to fab up a nose cone like basjoos' Aerocivic, that would mean you'd have to have a larger than 1/6th opening, because of that shorter space. My car's the same as what trebuchet03 is modelling in his CFD thread, and I think that flow is typical for most cars: http://ecomodder.com/forum/attachmen...1&d=1228931035 Just look at the front where trebuchet03 did more detailed modelling of the opening with the grill slats, the bumper, and how the air eddies and has to move around the irregular shape of the engine before exiting out. And note that he didn't (yet?) try modelling how the fans (two on this model of Jetta) will make the airflow even more turbulent in the bay.
Here's a compromise idea: two inlet ducts - one fitting the upper half of the radiator, one fitting to the lower half. This reduces the area / height for each duct, which MIGHT allow for Korff's ideals of width/height/area to work. Splitting an 18" tall radiator into two 9" halves would mean an ideal 9" depth for a duct. For your own car... you'll have to measure to see! :) ** Walter P Korff's book: Designing Tomorrow's Cars: From Concept Step by Step to Detail Design (1980) ISBN: 9780960385003 |
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NACA did a lot of good work on ducting in the 1940s, with optimal inlet and outlet geometry.
The cooling air needs a chance to slow down and regain pressure (higher pressure air absorbs more heat) before it goes through the radiator, so the smooth inlet widens to that point. Inlet vanes help to distribute the cooling air evenly across the radiator, rather than just bunching up at the middle. Then, after it goes through the radiator and draws off the heat, the cooling air needs to be re-introduced to the airstream in a smooth, non-turbulent way, preferably as parallel to ambient flow as possible. Ideally, a good inlet geometry takes advantage of the stagnation point at the nose for intake, coupled with a low pressure outlet along the side of the body, where the fast-traveling slipstream sucks the cooing air out. There is a synergy here, if done right. Think: fish gill on a tuna or other fast swimmer. |
SO... since there is high pressure at the windshield, could you make ducting inside the hood's framework to duct that high pressure air back to the front of the radiator, then let it dissipate into the low-pressure area under the hood behind the radiator?
Low flow volume would be the only real concern there, I'd think. And it should be pretty easy to duct your hood, but with the negative volume area under the hood, due to the flow under the car pulling with no non-turbulent inlets, it should create enough flow to aid in cooling, no? Just a thought. |
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Ducting air through the hood is not a new concept, the 1990 Z24 Cavalier uses hood ducting for induction from the ram scoops on the hood. (yes, they're functional) What I had mentioned was to take the pressure from the high pressure area at the top of the hood/base of the windshield, and vent it through the hood, back to the radiator, which does not disrupt aero at all. Also, in the case of basjoos' Civic, which obviously doesn't allow alot of air through the front to the radiator, the engine bay would be a significantly lower pressure area than even post 11's diagram shows, meaning that it would be OK to vent that windshield pressure to that area, and could possibly create a slightly different aero profile for the car. The only downside, other than complexity, that I see to the whole thing, is that if there is no high pressure area in that "crook" where the windshield is angled, flow will have a higher tendency to attach there, and may actually hurt the aerodynamics of the car, by causing more drag. Without seeing a diagram of this modeled, I couldn't tell for sure. |
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That's what I was thinking, as the air would become attached at the windshield, and then detach at the windshield-roof bridge line.
Thank you for reinforcing my doubt on this one, though. |
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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 :thumbup: Going to borrow this image http://ecomodder.com/forum/member-ae...ing-system.jpg 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 :p Quote:
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 :thumbup: Plus, shrouding a fan increases it's efficiency :thumbup::thumbup: ------- 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*) :thumbup: |
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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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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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 :) |
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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"...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.. |
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. |
Hi,
I think exhausting out of the back of the hood, into a low pressure zone, works better? |
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. |
Hello,
The very front of the car (the grill and bumper) is a high pressure zone, and it deflects much of the air flow up and over much of the hood; making the hood a low(er) pressure zone than the front. On my xA, which has the hood sloped down, this is still the case. The upper half of the windshield is also high pressure; but the base of the windshield is a low(er) pressure zone, as well. The reason I know this the case on my car anyway, is when there is loose (light and fluffy) snow around the windshield wipers, it stays put; unless there is a gust of wind from the side. In any case, the back of the hood/base of the windshield is a lower pressure zone than under the car. There are other threads that quote aerodynamic reference books showing that intake down low on the front and the exhaust on the back of the hood is the most efficient way to move air through a front radiator. |
autospeed had a great article on bonnet vents.
however i'm inclined to think from their results and from the placements of functional vents on production cars half way the hood is a better place the pressure difference with the front is much greater there than just ahead of the windshield. the back of the windshield is generally the place where the interior cooling air INTAKES are. perhaps raising the hood at the back might still work because of the sheer size of the resulting gap, but relative small extractors early on the hood can do the same thing |
If I had time, money, and a workspace I'd be working on a way to duct my rad. I mean, look at all this space:
http://i443.photobucket.com/albums/q...9978403462.jpg If I could move the top supports out towards the front, I could angle it nicely to duct it out to the hood. Forget money, I just need time and a workspace. *sigh* |
Coroplast would make some nice ducting material, applied with duct tape.
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is that a concrete undertray? seems like any sort of ducting might include or double as one too :)
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Another article on Radiator ducts, is about aeroplanes, but principles are the same, thought it best to tag on this thread:
Contrails ! Radiator aerodynamics Regarding adjustment of mass flow, thought this comment was relevant: Quote:
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2x4s.........:rolleyes: |
:o sorry.......... :D
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