| [ Total Views: 1904 | Total Replies: 30 | Thread Id: 47531 ] |
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 richbaker78
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| posted on January 3rd, 2006 at 08:08 PM |
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Rad fans - suck or blow?
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
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subaruboxer
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| posted on January 3rd, 2006 at 08:15 PM |
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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
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kevo
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| posted on January 3rd, 2006 at 08:34 PM |
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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...
76 baywindow kombi - V8 & Weddle racing diff and gearbox
Daily driven beach cruiser
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modulus
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| posted on January 3rd, 2006 at 08:43 PM |
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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
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 helbus
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| posted on January 3rd, 2006 at 11:28 PM |
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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.
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richbaker78
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| posted on January 3rd, 2006 at 11:46 PM |
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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
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subaruboxer
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| posted on January 4th, 2006 at 03:38 AM |
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| Quote: | Originally
posted by richbaker78
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.
Hi Rich,
i got you right. I understood that you want to put the rad underneath the car. Sorry, that i caused a misunderstanding. But put it at a little angle
not flat. I would do that overhere in Germany, you might need more ground clearence in the countryside of Australia than i would need here.
Hi Helbus, you should put coolant into your system, it is not a good idea to use pure water! Your engine will thank you for that!
Jörg
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helbus
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| posted on January 4th, 2006 at 07:44 AM |
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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.
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71EJVan
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| posted on January 4th, 2006 at 08:48 AM |
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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
A VW for everyone!!
1972 1302 German Look
1971 Bay Panel with Subaru EJ20T
1970 Type 3 squareback in progress
1971 Type 3 notchback in progress
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pete wood
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| posted on January 4th, 2006 at 11:18 AM |
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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.
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modulus
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| posted on January 4th, 2006 at 02:01 PM |
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| Quote: | Originally
posted by richbaker78
... 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.
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I agree with the theoretical approach, but it only holds true for stable laminar flow; I believe the air flow under the car will be fully
turbulent.
Fortunately, there is an easy way to determine the best approach, just measure the pressure at different points, create a "dummy radiator" (out of
perforated mesh?) and mount it, measure again. The pressure measurement can be done easily with a differential manometer, easily constructed with a
bit of tubing and a ruler; then just drive at your target speed and measure the relative pressure points. It has to be better than guessing; I have
never measured the pressure patterns *under* a car so I confess that I'm guessing!
hth
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shaihulud
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| posted on January 5th, 2006 at 08:10 PM |
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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.
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HarryMann
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| posted on January 6th, 2006 at 07:46 AM |
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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 ]
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1303Steve
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| posted on January 6th, 2006 at 12:41 PM |
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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
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richbaker78
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| posted on January 6th, 2006 at 07:37 PM |
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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
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1303Steve
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| posted on January 6th, 2006 at 11:36 PM |
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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
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HarryMann
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| posted on January 6th, 2006 at 11:52 PM |
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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 ]
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ian.mezz
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| posted on January 7th, 2006 at 08:52 AM |
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fan
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
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kombi kountry
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| posted on January 7th, 2006 at 09:47 AM |
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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 ]
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HarryMann
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| posted on January 7th, 2006 at 10:34 PM |
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| Quote: |
Instead of having a vertical rad up the front like the T3's some of them have them laying flat/horizontally up under in-font of the front wheels.
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Would expect the flow to be through front and down and out in those cases?
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kombi kountry
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| posted on January 8th, 2006 at 11:41 AM |
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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 ]
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pete wood
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| posted on January 8th, 2006 at 04:21 PM |
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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.
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HarryMann
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| posted on January 8th, 2006 at 09:31 PM |
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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.
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pete wood
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| posted on January 9th, 2006 at 09:12 AM |
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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.
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HarryMann
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| posted on January 9th, 2006 at 12:16 PM |
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| Quote: |
I think they also mentioned that the setup was part of the aerodynamics package on the car, not sure how exactly though.
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Apart from the obvious of having to get aerod. things right at high speeds, reducing drag and creating downforce (or just minimising lift maybe, in
those zoom days) there's a maybe an unrecognised effect at work here
Hotter air is less dense and somewhat less viscous - to the extent that certainly here, the difference in both performance and fuel consumption can be
clearly noticed between cold winter and hot day summer driving. Despite reduced engine power due to hotter air temps, acceleration above 80mph can be
noticeably better on a hot summer day and fleet fuel consumption figures can be seen to degrade 10% (ish) during winter months, albeit some of this
being accounted for by wet, snow and generally poor road conditions.
Nevertheless, the effect is there theoretically and enough to be noticed by someone tuned in to their vehicle through the seasons.
At a rough guess, a 200 HP engine might be putting out 150KW in waste heat, and if it were only 100 KW, that's a lot! At high speed, any underbody
flow heated up by that, maybe 20 or 30 degrees C or more above ambient once it's mixed and dispersed, might well be able to subtly lower overall
drag, even though the flow there is often considered less important once an air dam is fitted.
[ Edited on 9-1-2006 by HarryMann ]
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pete wood
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| posted on January 9th, 2006 at 02:21 PM |
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so what you are saying is that it creates a low presure effect under the car and holds it down. right?
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kombi kountry
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| posted on January 9th, 2006 at 03:30 PM |
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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.
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pete wood
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| posted on January 9th, 2006 at 04:55 PM |
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The Japs don't quite get the idea of top secret then if it's all over the web. 
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HarryMann
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 posted on January 9th, 2006 at 10:33 PM |
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No, if you're referring to my last post...
This might be an outcome, but I was suggesting that if a lot of heat is being dumped under the front then the underbody would effectively be in a flow
field of thinner and less viscous air, probably lowering drag.
As for the hilariously 'secret' Japanese experiments there might be a couple of reasons for looking at this layout
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
As well as the above aerod. effects, yet another (theoretical) one being the possibility that a very small amount of thrust might be achieved by
exhausting the hot air through a properly shaped duct - in the way that the best and last of the prop-driven warbirds did e.g. the P51 and Mosquito
originally and the last of the Griffon engined Spitfires. In very thin, very cold air at high altitudes, the P51 when cruising, would be deriving more
thrust from the exhaust mass flow and it's Meredith effect radiator exhaust than the propellor itself!
But don't hold you breath expecting a big kick in the back, unless you're aiming at 400 mph or more and have 1 Megawatt of waste heat to do
something useful with!
[ Edited on 9-1-2006 by HarryMann ]
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pete wood
A.k.a.: figure itout
23 Windows of Awesome
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| posted on January 10th, 2006 at 08:57 AM |
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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 ]
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