ok im getting towards fitting the radiator - itsa going to be fitted underneath the belly pan similar to helbus has his.
BUT i have a quaestion about fans - should I use blowing fans which blow the heat down to the road dispersing it into the airflow or sucking fans which pull the airflow up through the radiators. I have thought this through so much over christmas and cant seem to get a logical answer - there are pros and cons to both set ups.....

Blowing - will get rid of the air but heat rises so it will work against the natural convection.

Sucking - relies on good heat dispersion through the top of the radiator and the ability to get rid of the heat /warm air which will hopefully have a lot of airflow between the chassis rails. THis may mean I have to mount the radiator slightly lower than in the above set up to ensure enough room above to ensure easy air movement backwards.

am I worrying about nothing? who is running what set up? any words of wisdom? put me out of my misery!!
Its a lowered 1970 Bay with 2ltr WRX turbo by the way

In your case I would install the fans behind the radiator and let them suck the air up. The fans under the rad would cover a part of the cooling surface. With the WRX engine you need good cooling.

Jörg

dude, of coarse you need to suck through the rad, its the only way that works best, jusat look at every other watercooled car on the road and you will understand...

The (angled) front of the radiator will be an area of high pressure, and the (stagnant) back an area of low pressure. Thus you want to mount the fans 'behind' the radiator (to assist in) drawing air through the radiator as Helbus has done.

Convection effects are trivial compared with aerodynamic pressure at anything over 5 kph, so ignore those.

hth

I agree with suck. The fan is a 16" ICE unit and seems to cope pretty well. On the 42 degree day we had here on New Years Eve day, the bus ran it's hottest yet.

The guage had the needle just sitting on and just a couple of times the width of the needle over 100 degrees.

Trust me I was watching it continuously while Fleur dragged off Hyundai's and Volvo's, even an old HQ.

I think the only improvements I could make would be to drop the plain water that is in there at the moment and actually put some coolant in, and maybe open up the vents on the lower cover a bit. On normal days up to 36 degrees, the guage never gets over 90.

ok cheers guys - i think you misunderstand my poor explanation!!
The aim is to have the radiator mounted flat underneath hanging below the chassis rails.
Whether i use fans to blow or suck they will still be mounted above the radiator.

subaruboxer - the fans will still be mounted above whether blowing down or sucking up.

kevo - every other watercooled car has a rad in the front with the fans either blowing through (eg peugeot/citreon/) or sucking though (most others) and vauxhall omega has both!!! one either side??!!
this is what has prompted my question as I can get both types of fans easily.


Modulus - i agree on the pressure but what if the rad is mounted flat? the major air flow will be under the van acrooss the lower side of the rad creating a vacuuming air down through the rad rather than scooping it up if it was angled. in this scenario surely I should blow the heat down into to the airflow.

I do appreciate your responses and dont mean to be argumnetative perhaps i will draw some simple diagrams of my thoughts to make sure we are not talking at cross purposes cheers
Rich

I was just using water for the first few weeks while testing a few things and as I was dropping the water to change thermostat and drilling holes in it and so forth, it was economical sense to use just water. I ended up emptying the system at least 4 times. I then had my accident which put me out of action for a while.

I plan on doing a 33% coolant mix.

Rich,
If are worried about heat dissipation you could make a sort of reverse scoop on the trailing side of the radiator. The negative pressure caused by air running over it will cause suction in the top fan radiator area. Should assist in removing warm air. Same as as sheeted area in front of the radiator to grab and direct air into it. Same concept as wing lift on a plane. (I'm doing this on mine)

Regards
Brendan

I think the fans should go on the air out side of the radiator. That is, pulling the air through it. Then the fans create extra air flow in the low pressure zone.

Can I also just ask, are you using a shroud for the fans?

That is a panel that the fan mounts on a small distance away from the radiator core (say 1.5 or 2 inchs). I've loaded up a pic of mine below so you can see what I mean. I had a sheetmetal tradey friend of mine make me one up, but it'd be to make your own. The value of such a shroud is great. Firstly, it means that the fans draw air through the whole radiator, not just the bits covered by the fan/fans. Secondly, you don't get the hot spots and cold spots on the core that you do when the fan is directly mounted to the core. Thirdly, the air is slowed down a bit as it has to go out the fan holes and so takes more heat with it.

Between having one of these on my car, and having a proper scoop and ducting, my buggy with a rear mount radiator ( ) barely gets over 82deg now, topping out at 90deg on the freeway at 110km/h. It used to struggle to stay under 100deg at 100km/h.

Generally speaking laminar flow absorbs less heat than does turbulent flow. Sucked air is more laminar than blown air. Put your hand either side of a houshold fan to see what I mean. Turbulent air is more efficient at absorbing heat.

It's the same when dissolving sugar into a lemon drink. A much better job is done more quickly by moving the tea spoon back and forth to create turbulence rather than around in the same direction, creating laminar flow.

So, blow if you can.

Seems just about every theory has been mentioned and all good stuff too... except one key point maybe?

The shape of a fan's streamtube, or a propellor's which it is.

Th effect of laminar or turbulent flow on cooling mentioned above are correct, turb is better but on average we are talking V**.73 ish where V is velocity. So whilst laminar gives V**0.5 and turb about linear with V (V to power 1) no-one has mentioned V itself...

And as a propellor's streamtube is shaped like a wine glass in cross-section with the wide part upstream and the narrow part downstream, continuity theory says that the velocity downstream must be greater than upstream.

In simple terms, a fan will pull air in from a very wide cross-sectional-area and discharge some distance behind the blades into a much smaller csa. Put your hand behind a fan and you can feel quite draft, and in front?- hardly any at all! A BIG difference. A natural effect, if the fan or propellor is not shrouded. Maybe 3:1 in CSA or more, far outweighing any differences in laminar or turbulent (which are much more complex and not easily amenable to detrmination, hence settling on a mean value of V^0.73)

I'm not voting for either suck or blow here, just adding some more facts. The fact is though that an upstream fan will add a propensity for turbulent flow with its greater heat transfer as well as doubling or tripling the airspeed (and cooling is a function of this, before we even bear in mind what power of Velocity we might get for the cooling effect)

The factor that might determine all this is the physical possibilities of layout, and any shrouding that is used, which tends to ameliorate the streamtube effect described somewhat - and of coures, how close the fan is to the rad (saving grace of rear mounted fans?)

As with all things aero and thermodynamic, a good grounding in basic theory, thoughtful design, plenty of scepticism of 'logical' assumptions and wihtout recourse to heavyweight CFD software, actual testing of ideas and layouts is required, with instrumentation, to an ideal arrangement.

We then have the countering factor that a smaller area of the rad will be getting that additional cooling velocity - isn't engineering such an interesting subject, full of greys instead of black and whites :-)

I like Modulus' suggestion of differential manometers! Without knowing what the air wants to do anyway (natural or artificial low pressure areas always being the driving force for volume flow rate around vehicles of any sort) very often, people using just commensense and not much theory or empiricism as well, are pissing in the wind, trying to turn back the tide, so to speak! Find that low pressure region, or create a good strong one (spoiler/air dam) and then use it, work with it, not agin it...


Happy New Year to all Aussies (and Kiwis too, oops!), hehe :-)

Harry
Up and Overland

PS. Great forum!

[ Edited on 5-1-2006 by HarryMann ]

Hi

Great 1st post Harrymann.

I tend to look at the way carmakers do things for a guide, they don’t always get it right.

On my old bug I 1st used an Audi 100 fan and shroud, waste air blowing on the ground would blow leaves etc. The Audi shroud along with all other VW & Audi shrouds used rubber air flaps, these allowed air flow at road speeds to blow open the flaps and allow air to be forced over 100% the radiator and when the fan came on the suction of the fan closed the flaps so that all the air was forced through fan shroud.

The Audi fan had a huge motor hanging out the back which made things a little messy at the back of the shroud so I went to a huge V8 style Davies Craig fan, still at the rear of the radiator and made my own rubber flaps just like VW. This fan worked fine but never had the leaf blowing power like the old Audi fan.

Steve

this is great guys - keep it coming - i havent got to draw any pics yet but i will and I may even do a manometer experiment using my mates van i think the info may be of use on here for others too.

For info tho I will definatly be shrouding the Rad fans.
im currently wavering towards sucking because the rad will be mounted behind the front xmemeber so this will act as a dam creating low pressure above the rad BUT the decision is by no means made! for example i have also been toying with the idea of reprofiling the front cross member by an inch or 2 to improve airflow above the rad so in this scenario i may be better blowing??!!

whatever happens nothing is irreversible but i dont want to make work !! Thanks again for all the replies
Cheers
Rich

Hi

On my old 1302 bug the radiator was mounted in the front behind a vented A/C front apron, the bottom of the apron was open, I made small deflector for the bottom of the pan which angled the air up and it seemed to help. You could always do a wool tuft test and film the results at different speeds.

Steve

Another trick that might help cooling (in your very hot weather) is keeping the radiator return hose out of the hot air stream behind the rad. I've use this new allyfoil covered 2" wide bubble wrap tape (plumber's pipe insulation) to spirally wrap the colder return feed where exposed to the full blast of radiator heat (that almost burns you!) on my Transporter T3 Syncro Diesel - these actually have a wider cooling tunnel than the petrols - that's how VW saw the problem - even making a special body just for the diesel's cooling requirements and they also heavily insulated the underfloor area to maintain cab comfort in the summer.

Other mfrs might have just stuck a denser core radiator in and hoped for the best - not VW; same rad, bigger fan, in a bigger duct and a 500W motor. Really impressed with the final iteration of their rear-engined vans, but I can see why they decided the parts count and production cost had got out-of-hand - just took me a week to refurb the g/c from one end to the other, but wow, I can find all the gears now!

I've also insul. wrapped both heater hoses together to get the heater working quicker, as that is the bypass circuit when the thermostat's closed - it's brass monkey's here in up and overland at the moment, alternating -2 one day, + 2 and rain the next, for weeks on end, with a bit of snow thrown in for fun. Enjoy your summer...

Tufts wouild be good, but actually watching or photographing them in any meaningful way is not easy unless in a wind tunnel - and again, would urge circumspection unless you've experience in flow dynamics labs or wind tunnels. Best use tuft flows hand-in-hand with pressure readings, just find that natural stagnation point at the front and any true low pressure region underneath and try to always focus on the exhaust side of a system rather than the inlet - if it can't get out due to area restrictions or -ve pressure gradient it won't matter what shape the entry is - that's what VW did with the diesel T3 vans.


[ Edited on 6-1-2006 by HarryMann ]

have a look under a few jap vans they have a/c radiator/condenser mounted under them in all funny places I think fans on top sucking , check out any ducking, scoops mite give ya a idea
:jesus

i was just about to say the same thing as ian.mezz.

This is quite common in forward steering controlled vans (i.e. engine under the front seats and steering wheel in front of engine).

Instead of having a vertical rad up the front like the T3's or the new hiaces, some of them have them laying flat/horizontally up under in-font of the front wheels.

[ Edited on 8-1-2006 by kombi kountry ]

Could be right....

I remember having a quick look at a friends pre 89 (the shape before the bubble shape) tarago, but one of the older versions (early 80's) 2.0 ltr carbie as oppossed to later 80's with 2.2 fuel injection, and from memory the fan was the first thing i saw when looking from underneath, so like you said, it must be sucking in through the front and down and out.

[ Edited on 8-1-2006 by kombi kountry ]

Not automotive but the same idea...

Just had a look at a few aircon heat exchangers outside houses. They were both draw through setups with the fan sucking air through the core into the box and then out through the fan into the atmosphere. i think the reason is, it pulls air across the whole core so the largest part of the core possible has air pulled through it. Blowing air through means you are more like to have bits of the core missed by the fan's direction.

That's my thinking too, an even cooling effect rather than a strong chill just over the fan area.

And of course, for horz cooling fans on flat roofs etc. the flow would be naturally upwards - hot air and all that jazz plus installation practicalities.

This does not mean that the flow under those flat rads in front-gear vans necessarily goes upwards though - I still reckon it would be exhausting downwards - unless some special ducting arrangements were in place.

I once saw a feature on a big block powered sports sedan in Zoom years ago and they had the radiator mounted flat. The air was ducted from the front air dam into the top of it and then the hot air went down and out under the car. Don't know what he did for fans, didn't really look as it wasn't a concern at the time. Be worth having a look at a similar setup and seeing what they do. I think they also mentioned that the setup was part of the aerodynamics package on the car, not sure how exactly though.

so what you are saying is that it creates a low presure effect under the car and holds it down. right?

That reminds me of a similar type set-up i saw in a High Performance Imports mag from a couple of years ago.

It involved one of Top Secret's (Famous Japanese Tuning House) in-house late model Toyota Supra's (All their cars are painted Gold)

The car is set-up for high speed (300+ k/ph) street racing (I forget what name they give this type of class) and instead of using the standard 3 litre straight six, they use the 3sgte (2 litre 4 cylinder) engine (highly tuned of course). This leaves lots of room at the front of the engine bay (which is big to start with), so from memory they have the radiator (and possibly the intercooler) laying flat/horizontally in front of the engine.

Can't remember the specific reason they gave for this (If any) but must of been partly to do with aerodynamics.

Anyway, i'm sure you can google "Top Secret" and check for yourself if wanted.

The Japs don't quite get the idea of top secret then if it's all over the web.

• getting the engine block out of the hot air stream;
  
• moving the c.g. back a bit
  
• keeping hotter exhaust air away from any engine air inlets
  

Can you explain the 'Meredith effect' please?

it's ok, I found it here,

http://www.airandspacemagazine.com/ASM/Mag/Supp/JJ99/Mustang.html 

Sadly, I don't think I could safely control my buggy at 400mph, otherwise I might try this.:P

[ Edited on 9-1-2006 by pete wood ]