Ducting air from grille to radiator

[ChrstphrR]

Originally Posted by fanamingo

Can you elaborate? My google searches have turned up empty.

I remember Korff's book ** that Aerohead keeps on bringing up, and conveniently, he scanned in a part where Korff made a diagram with a summary of the details of "ideal" sealed ductwork for a radiator.

• Inlet duct should be equal to the radiator height.
• (inlet) entrance height need only be 1/6th of the radiator height IF duct length is equal to radiator height
• Duct must be sealed airtight on both sides of radiator to hold pressure.
• No air leaks between duct and radiator.

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:



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.

• 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)

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

[Tango Charlie]

Originally Posted by aerohead

...the pictures I have are mostly Kodachrome slides and the window of opportunity to transfer these to prints or digital is something I need to investigate...

A good camera store in your area can do that for you. In my area, we have Meijers stores (similar to Wal-Mart) that also provide this service.

[aerohead]

Originally Posted by fanamingo

Can you elaborate? My google searches have turned up empty.

fanamingo,what ChrstphrR posted( Thank you very much!!!!!!!!) should illustrate the ducting tech..Chrysler Aerospace Division used this technology when developing the nose for the 1969 Dodge Daytona Charger,which swept the NASCAR field for,I think two years.------------ Both Korrff and Chrysler' work can be traced to Sighard F.Hoerner's "Aerodynamic Drag",translated into English in 1951.In the section on radiators,a series of cooling systems are depicted for I think,a Messerschmitt fighter,and the system with the lowest drag is a dead-ringer for what Korff depicts,and what Dodge built.The P-51 Mustang used a system like this,but with a scoop,which I believe is credited to Chrysler.Thrust generated by the air expanding across the heat exchangers was actually enough to cancel the 2% drag of the cooling system and even provide a little net thrust to the plane.Truly delicious engineering! If you can track these materials down,you'll be rewarded with some great reading.

[Otto]

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.

[Christ]

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.

[aerohead]

Originally Posted by Christ

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.

While the pressure at the windshield cowl is relatively high,it is not as high as at the forward stagnation point of the car.NASCAR uses the cowl area for engine induction air,as when drafting,there's virtually no pressure against the front of the car and they'd be losing ram air,however,there is some pressure at the cowl.The pressurized cooling system offers a "buffer" against melt-down during drafting,and you may see team crews either adding or removing duct-tape from the "grille" to tailor engine temp during the race.Funny-Cars may have the bug-catcher for the blower protruding right through the windshield,however,they are usually elevated above the cowl deck.actually "lead" the windshield,and the scoop itself provides the stagnation.----------------- My opinion is that for aero optimization we should stick with the front of the car.

[Otto]

Originally Posted by Christ

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.

Better to go back to post #11 of this thread, and have another look at the colored picture showing highest pressure at the nose and lowest pressure at the roof and sides. Inlet at the highest pressure stagnation point at the nose, then good ducting, then outlet at the low pressure area along the sides around or behind the wheels. (The roof has wonderful low pressure, but ducting to that point would be too much trouble--go with the sides. Actually, ducting out through the fender wells and around the wheels would work pretty good, and also be handy to draw off engine heat.

[Christ]

Originally Posted by Otto

Better to go back to post #11 of this thread, and have another look at the colored picture showing highest pressure at the nose and lowest pressure at the roof and sides. Inlet at the highest pressure stagnation point at the nose, then good ducting, then outlet at the low pressure area along the sides around or behind the wheels. (The roof has wonderful low pressure, but ducting to that point would be too much trouble--go with the sides. Actually, ducting out through the fender wells and around the wheels would work pretty good, and also be handy to draw off engine heat.

I said nothing of a roof... and that post was meant for a car more like basjoos' Civic, where there probably isn't as much pressure build up at the nose, but likely still a nominal high pressure area at the base of the windshield.

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.

[aerohead]

Originally Posted by Christ

I said nothing of a roof... and that post was meant for a car more like basjoos' Civic, where there probably isn't as much pressure build up at the nose, but likely still a nominal high pressure area at the base of the windshield.

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.

If you pull that kind of volume of air from the windshield area you'll cause separation over the roof.If anything you want to eject air at this location.This is what Kamm did,and its what made his K-Car work.You'll never see a car with this and there is a critical reason for it.

[Christ]

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.