Multiple fans

[JohnD]

All,
My cooling radiator is out of the frontal airflow, and needs more fan to make it effective at max power. I already have a pair of 'puller' fans on the back of the rad and intend to put two 'pushers' on the front as they may work better.

If I leave the pullers in place, working, will the fans either side be additive?
Or will there be some interference effect, if they are not the same type of fan? The existing ones are both four bladed, and the new ones will be multi(10-12) bladed.

JOhn

[TwoWheelTerror]

sounds very inefficient..

Could you not redirect some airflow or relocate the radiator.

[JohnD]

No!
Or else I wouldn't ask!
John

[Chris Y]

I would think the two sets of fans would interfere with each other, if they weren't perfectly matched, creating turbulence and distubing the flow of air somewhat.

Of course, I can't be more scientific than that, but perhaps one of our aerodynamicists will chime in soon

Out of curiosity, what car is this? A rear-engined one?

On the subject of blades - more isn't always better. Some 4-blade fans are more efficient, particularly at high speeds. Multiblades are really designed to keep noise to a minimum, not provide more flow - they can really chop the air around, again creating more turbulence.

[JohnD]

When your car is shaped like a brick, any aero advantage is good.
This project minimises airflow under the car, by blanking the radiator grille, fitting an air dam/splitter and side skirts (Not in this image)
Radiator goes in the back, with intakes just behind the rear wheels. The puller fans are seen through the exhaust ducts.

IMHO this does work! Without knowing, I lost a skirt the other day and was wondering why I felt so unstable in fast corners for the rest of the race. Problem is, overheating when using max power. Cruising, it keeps it's cool, so more air flow through the rad is needed.

John

[phoenix]

I think you will find that even the best fans cannot generate the kind of airflow required to cool your car at maximum power. A car moving forward at even 30 mph with the rad in the airflow will cool more than fans can. A rough calculation shows that a radiator of frontal area of 2 square feet will be subject to a flow of 5280 cfm at 30 mph. Two Pacet Profans (PF107) will only flow 4720 cfm between them, so over about 26 mph they will not be contributing much if anything.

I would be inclined to investigate ducting the exit of the radiator to the area of lowest pressure behind the car. This will then pull the air through the radiator, which is what happens on most radiator installations anyway. I certainly wouldn't add two "pushers" as well - they will probably only restrict the flow to the radiator even more.

A quick question - are the fans fitted close up against the rad itself, or some distance away?

[Al Weyman]

Two points spring to mind and having just spent over 300 quid for a massive aluminuim rad they are prevelant in my mind because even with cooling fans my V8 was overheating..

A) To win you have to finish so no matter how good the aero effects are if they are hurting cooling you are going to have to compromise somewhat.

(B) Have you seen the size of a fan on a rolling road,? Well think along those lines for what it sounds like you need at full chat and that would just be totally impractical. Cooling fans are really only for when the car is stationary or moving very slowly, I switch mine off once the race starts as it makes not a jot of difference.

[JohnD]

Phoenix,
Thanks for the calculation.

If you have seen the pic, you will see that the radiator exit ducts are through the tailgate below the window - fairly low pressure area, I hope.

I don't understand your fan calculation - the PF107 is rated by Pacet at 980cfm, so two might blow 1986. And I had in mind a pair of Profans KPF1211, 12" x11blade, or KPF115 rated at 1640 and 1690 respectively. that should acheive a flow of up to 3320cfm.

Maybe a scoop on the intakes???

JOhn

PS sorry, should have added - 6 cylinder, 2500cc, about 140bhp. J.
PPS Al, I take the point you both make about the advantages of a front radiator. So how do Porches manage? J.

[phoenix]

Originally Posted by JohnD

Phoenix,
Thanks for the calculation.

If you have seen the pic, you will see that the radiator exit ducts are through the tailgate below the window - fairly low pressure area, I hope.

I don't understand your fan calculation - the PF107 is rated by Pacet at 980cfm, so two might blow 1986. And I had in mind a pair of Profans KPF1211, 12" x11blade, or KPF115 rated at 1640 and 1690 respectively. that should acheive a flow of up to 3320cfm.

Maybe a scoop on the intakes???

JOhn

PS sorry, should have added - 6 cylinder, 2500cc, about 140bhp. J.
PPS Al, I take the point you both make about the advantages of a front radiator. So how do Porches manage? J.

My mistake - PF127 is the one I used in my calculations with 2360 cfm which with two makes 4720 cfm.

I agree that your exit is in what should be a low pressure area - but is it the area of lowest pressure? idon't know.

However, if there is, say, a -2 psi pressure at the current location of the fans, it is only acting on the area offered by the cuttouts for the fans. If they are (guessing) 12" diameter that is 226 square inches. If you doubled the area open at the back (an area 30" x 15" would give you that) you would double the effect of the low pressure area sucking air through the radiator and increase the cooling as a result.

Larger intakes might help, as might scoops, but only if you can get the air out the back at least as fast as it is going in.

[phoenix]

Originally Posted by Al Weyman

(B) Have you seen the size of a fan on a rolling road,? Well think along those lines for what it sounds like you need at full chat and that would just be totally impractical. Cooling fans are really only for when the car is stationary or moving very slowly, I switch mine off once the race starts as it makes not a jot of difference.

In my experience even huge rolling road fans are not up to the job really - just enough to get away with a power run over a few seconds, but not much more, before you have to let the engine cool down.

[Al Weyman]

PPS Al, I take the point you both make about the advantages of a front radiator. So how do Porches manage? J.

Years of development and a very slippery shape no doubt and originally aircooled but I know nothing about their later liquid cooled engines, do they not have a rad up front? Also what about using the oil cooler to remove a lot of the engine heat, I see a guy with a little NSU in Classic Thunder runs a big one up front to stop it overheating.

[Al Weyman]

Originally Posted by phoenix

In my experience even huge rolling road fans are not up to the job really - just enough to get away with a power run over a few seconds, but not much more, before you have to let the engine cool down.

Presactly

[JohnD]

Great ideas, guys!

Oil cooler, Al - the concept is to reduce athe air going under the bonnet to the minimum. To that effect, there is an oil-to-water cooler under there. so ALL the engine cooling is by the water rad.

Phoenix, Hmmmmmm, yes, a bigger hole. The rad is full width across the back, so there is a lot of ti that is probably under used.

I'm grateful! Think I'll still try the extra fans first though!

J.

[phoenix]

Even in the air cooled porsches, oil cooling plays a major roll in engine cooling. A front mounted oil rad in the right hand front wing, together with a long pipe run and dry sumping was how it was mainly done, though on the 356 and 912 the oil cooler was enclosed withing the ducting around the engine, where the engine driven fan forced air past it.

The airflow over the body at the rear of a Porsche and the positioning of the large exit grill on the engine cover also means that the air cooling, which is fully ducted around the engine, is exhausted into a low pressure area.

On the water cooled cars the rad is at the front, there being no space at all to accommodate one at the rear.

[phoenix]

Originally Posted by JohnD

I'm grateful! Think I'll still try the extra fans first though!

If you don't make the exit area as large as the rad - or bigger - then you will make the radiator very inefficient.

And another thing - a hole is a lot cheaper than two more fans!

[Locost47]

Blimey, you don't half make things difficult for yourself with that layout! Putting another set of fans on is likely to make things worse, rather than better.

The airflow data given by fan manufacturers is just one point off a pressure rise vs flow rate graph and won't still be valid in your case where there's a huge resistance in the system.

Having the intakes in the rear wheelarches isn't a good idea as the pressure there will be very low and there simply isn't a fat lot for the fans to drag through themselves.

I appreciate that your set-up will give you very low lift or probably a reasonable amount of downforce but you could get very nearly the same with significantly lower drag if you did all the cooling up at the front. Properly ducted radiators/coolers at the front of the car, exiting up through the top of the bonnet is generally the best solution.

The drag benefit comes from not having a big bluff face at the front of the car for the air to stagnate against. Allowing it in, through a heat exchanger and back out again is sometimes the lesser of two evils if the car isn't particularly 'streamlined' in the first place (looking at the photo your's isn't!).

Not having the fans assisting with the evacuation of the underbody will increase the pressure there a bit so you'd lose some overall downforce but that pressure would be pulling you backwards, as well as down so you'd lose some more drag too. You may even regain the lost downforce from the other cooling system, come to think of it. It's win-win.

Rob

p.s Porsche are the dog's danglies when it comes to cooling systems. They put a lot of effort into their optimisation and often use the cooling airflow to actually improve flow in other areas and so give a net overall drag reduction. Very clever!

[GordonG]

And holes are light, too.

Fans are really, really not going to do it for you.

You need pressure differential across the rad - ie inlet in a higher pressura area than the exit. The exit position looks near optimal. An inlet behind the rear wheels may also be a low pressure area though. Consider testing with a manometer (either tube and coloured water job or a modern electric job) to see if there is a higher pressure area available for the inlet.

Consider a scoop on the inlet to increase pressure, but make sure cross sectional area is sufficient to supply the rad.

You may even find that the interior of the car is higher pressure on the move than any external area you can reach with reasonable ducting (remember that a long ducting run will have significant pressure drop in itself). If that's the case you may be better off taking the inlet from inside the cra, though you'd have to check the blue book and formula regs to make sure you dont fall foul of regs designed to stop you getting covered in boiling coolant.

Consider taking the inlet from under the car. By the time you get to the rear it's likely the pressure will be reasonably high, certainly c.f the rear panel and suciking air from here can only help downforce.

Fans, however, unless driven by a separate snowmobile engine (Brabham?) arent going to do the job and at some point someone will protest you for movable aerodynamic devices...

Or you could fit a RR Merlin engine in the back with a big prop, call it a cooling fan and top 200MPH down the straight. Maybe not.

G

[Dave Brand]

OK, here's the radical solution......reverse the direction of the airflow through the rad!

Silly idea? Not really - there's a precedent. Takashi Suzuki, in his excellent book 'The Romance of Engines' (recommended reading for anyone with the remotest interest in engines!) describes how Hino improved airflow through the mid-1960s Contessa's rad by doing just that, after employing Keio Universtiy to conduct wind tunnel tests. The Renault 8 also drew in air through the rear panel. If you think about it, an estate tends to draw water, dust, etc., onto the tailgate, indicating that there is an airflow into, rather than away from, that area.

[Locost47]

It is indeed the case that the flow would be naturally trying to head towards the rear of the car rather than away from it and that the existing fans are having to overcome that tendency to drive flow the opposite way.

In addition, the pressure in the rear wheelarch could actually be lower than that on the back face (which often isn't as low as people think - typically Cp >-0.25 if that means anything to you), so the flow would want to go 'backwards', without the intervention of the fans.

Reversing the fan direction may improve your current airflow but it would still be only a small proportion of what you'd get from the external airstream at the kind of speeds where you're actually using full engine power.

You've obviously decided to keep airflow out of the front of the car for a considered reason - what is it?

[phoenix]

The reason that muck and filth are drawn onto the back of the car is because it is a low pressure area.

If the low pressure has an ambient or higher pressure area to "fill" it other than from behind the car - in this case from the radiator exit, then it will draw the air from there, either instead or as well as behind/below/from the sides of the car. If there is a positive pressure (i.e. higher than the ambient pressure) in front of the rad then the low pressure area should be filled by the air passing throuh the rad, rather than by sucking in air from behind/below/or round the sides of the car.

This in itself should reduce drag, as I think someone mentioned earlier in the thread.

If the back of the car stays cleaner, then you will know for sure it is working!

[JohnD]

Gosh, overwhelmed with advice! All of it good.

To correct a misconception, the intakes are not IN the wheel arches but just behind them, between the fuel filler and the Davies Carig sticker. Not so easy to see on the black livery. The intakes are MR2 Mk1 engine compartment vents, square, louvered.

I did consider taking the air from under the car, but one of the reaosns the Brabham was banned was all the road debris that it spat out behind. Didn't want that going through the radiator.

And a bonnet vent with a scoop behind a front radiator would do nicely, BUT the straight six leaves almost no room for a radiator anyway, Extractor ducts would need to be sculpted around the front of the engine. Doable, but difficult. (more difficult than this??)

I'm writing this to show that I have put some thought into it, not just hacked it. Okay, you've all changed my mind. BIG hole in the tailgate, scoops on the intakes (The series scrutes will LOVE those), consider more holes in the rear windows, ducted down to the radiator.

Then, a bit ot of proper aero study. Can't afford a wind tunnel, so tufts all over the tailgate and sides - lets see where the air is going.

Thanks a lot guys - I've got a plan. I'll let you know!

John

[Locost47]

Oooops. Should have looked at the photo a bit closer. I see the intakes on the sides now. They're away from the wheel wakes so sticking a small scoop on the rearmost of them should help but equally important will be how you duct the air to and then from the rad because it looks like it will have to do a very tight 'S'shaped turn.

No sharp corners or leaks in the ducting and nowhere else for the air to go other than throught the rad (it will desperately try to go around if it can). Those are the main things.

[JohnD]

47,
The intakes go into sort of plenum chamber in front of the rad, sealed off from the rest of the car. Well, I say 'sealed', more enclosed, with alloy panels, that to date only seal where they touch the internal bodywork. As someone said, this chamber should be better sealed, both from the car interior and around the rad. That needs attanetion too,
The chamber is quite large, being wider than the rad and about ten times deeper, so should slow the air and promote flow.

John

[zac510]

I've a funny feeling I have seen that car before. Was it in a magazine?

[Al Weyman]

Were did you say the engine was John? With respect I can't understand why you do not run a conventional layout with the rad upfront, that seems over complex and all a bit OTT if you don't mind me saying so, nice to wind people up that you have a laterday Brabham or Chapperel fan car but I can't really grasp what you are trying to achieve. You have an estate body so surely the front to rear balance must be pretty good, on my black IROC it is a very respectable 53/47 and that is with a heavy old iron V8 up front. Just moving the battery to the back will probably achieve almost the same!

I tried all this sort of stuff years ago in HotRod racing and one of the reasons was to move the rad to a less vunerable position and to help with front to rear weight ratio but it never worked and I gave it all up in the end.