nobrakes
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| posted on 11/7/16 at 10:22 AM |
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Weird fan problem (airflow, not electrical)
I've got a 14" and 2 x 7" curved blade fans (from McGill Motorsport) on my radiator, they're set up as pushers and the
surrounds are on a strong rigid frame that holds them virtually touching the core - there's probably only a couple of thou clearance if that to
minimise air loss between the surround/core gap. The flanges on the surrounds were bigger than I thought so I've trimmed to fit round the top
and bottom tanks and I've filled the gaps with black polyurethane sealer.
When the fans are on you can feel air blowing out the back of the radiator and place a hand in front of the fan near the middle and you can feel it
sucking if you see what I mean.
Now here is the weird thing - move your hand to the front outside edge of the fan surround and air is being blown out! Its as if the blades suck air
in and resistance through the core means there is enough pressure to force air back out through the gap between the fan blade tips and the
surround.
Has anyone else had this happen? If so, what did you do?
I'm thinking, either lining the inside of the surround with something to reduce the blade-to-surround clearance (but by how much?) or block off
the outside edge of the fan grills to redirect the escaping air back into the fan (would that work?) or something else?
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Slimy38
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| posted on 11/7/16 at 10:36 AM |
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Won't the airflow when you're driving be different anyway? You're replicating the engine cooling itself down when you're stuck
in traffic and the temperatures are going up. When you're driving normally there should be (reasonably) equal atmospheric pressure on all parts
of the radiator.
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Bluemoon
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| posted on 11/7/16 at 10:37 AM |
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quote:
Originally posted by nobrakes
I've got a 14" and 2 x 7" curved blade fans (from McGill Motorsport) on my radiator, they're set up as pushers and the
surrounds are on a strong rigid frame that holds them virtually touching the core - there's probably only a couple of thou clearance if that to
minimise air loss between the surround/core gap. The flanges on the surrounds were bigger than I thought so I've trimmed to fit round the top
and bottom tanks and I've filled the gaps with black polyurethane sealer.
When the fans are on you can feel air blowing out the back of the radiator and place a hand in front of the fan near the middle and you can feel it
sucking if you see what I mean.
Now here is the weird thing - move your hand to the front outside edge of the fan surround and air is being blown out! Its as if the blades suck air
in and resistance through the core means there is enough pressure to force air back out through the gap between the fan blade tips and the
surround.
Has anyone else had this happen? If so, what did you do?
I'm thinking, either lining the inside of the surround with something to reduce the blade-to-surround clearance (but by how much?) or block off
the outside edge of the fan grills to redirect the escaping air back into the fan (would that work?) or something else?
Your hand is not a great indicator of flow, use smoldering cigarette or joss stick similar and check again. Your had is large enough to effect flow
etc..
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nobrakes
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| posted on 11/7/16 at 11:00 AM |
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quote:
Originally posted by Slimy38
Won't the airflow when you're driving be different anyway? You're replicating the engine cooling itself down when you're stuck
in traffic and the temperatures are going up. When you're driving normally there should be (reasonably) equal atmospheric pressure on all parts
of the radiator.
Agreed, it isn't a problem when driving and maybe I'm worrying about nothing because when I held it at 2000rpm for 20 minutes the fans
were running full blast and the temperature gauge was showing a steady 90°C.
What I'm wondering about is being on holiday stuck in heavy traffic up a steep hill in the south of France in the summer when I'll be
needing every last drop of air to keep a 4.6 V8 cool.
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nobrakes
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| posted on 11/7/16 at 11:23 AM |
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quote:
Originally posted by Bluemoon
Your hand is not a great indicator of flow, use smoldering cigarette or joss stick similar and check again. Your had is large enough to effect flow
etc..
A good point but air is definately being blown back out of the front outside edge, I also tried it with a leaf gradually pushed over the edge and it
was blown/bent forwards. On full speed you can actually feel it several inches away from the fans too, more so on the two 7" fans because they
are rated higher. The 14" says 90 watts/670cfm and the 7"s say 80 watts/1270cfm each which surprised me. The flows probably bear no
relation to my installation but I suppose they're useful as a comparison.
I think I'll knock up a crude manometer with some water in a plastic pipe and try taping over the outside front edge to see if it makes any
difference?
[Edited on 11/7/16 by nobrakes]
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Bluemoon
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| posted on 11/7/16 at 11:25 AM |
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quote:
Originally posted by nobrakes
quote:
Originally posted by Slimy38
Won't the airflow when you're driving be different anyway? You're replicating the engine cooling itself down when you're stuck
in traffic and the temperatures are going up. When you're driving normally there should be (reasonably) equal atmospheric pressure on all parts
of the radiator.
Agreed, it isn't a problem when driving and maybe I'm worrying about nothing because when I held it at 2000rpm for 20 minutes the fans
were running full blast and the temperature gauge was showing a steady 90°C.
What I'm wondering about is being on holiday stuck in heavy traffic up a steep hill in the south of France in the summer when I'll be
needing every last drop of air to keep a 4.6 V8 cool.
If the thermostat is working correctly the gauge should be solid close to the thermostat temperature. The thing to check is that the top and bottom
hoses have a good temperature difference, indicating the fan and radiator is cooling, you should also see the thermostat doing it's thing and
this will be reflected in the radiator heater hose temperatures (i.e. you will see fluctuations in the top and bottom hose temperatures as the
thermostat opens and closes). If the thermostat is always fully open (i.e hoses temperature stable) then the radiator cooling is insufficient.
If the fan thermostat temperature is set correctly this should cycle on and off as required and also indicates that the system is working, if the fan
is on all the time something is amiss...
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02GF74
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| posted on 11/7/16 at 12:24 PM |
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Not sure I follow all that, I prefer pictures to lots of words, so is the diagram below a summary of what you feel is happening?
The blue arrows is what you are concerned about and it would be strange if the radiator was in fee air. I assume it is fitted in a car that has
enclosed sides and top (i.e. bonnet) and engine behind it - the air has to exit somewhere and it appears to want to go back through the radiator.
Providing the temperatures is maintained at idle when fans are running there is nothing to worry about; ensure the hot air can get out
easily.
Due to the hysteresis in the fan switch, the fans usually will run for a while, then stop and then run again; if they don't stop at all, then
that is a sign the cooling is not up to scratch.
Unless you have the resources of a large manufacturer who has access to a test department and all data for engine, radiator etc. it is almost pure
guess work as to what radiator will suit each engine; generally people shout polo and that is fine in most cases and that is only based on the
experience of others. You won't know if the cooling is sufficient for extreme situations unless the car is subjected to them.
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nobrakes
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| posted on 11/7/16 at 12:26 PM |
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I don't know how good the ancient Smiths temperature gauge is but I used an 82° thermostat. The fan switch is low speed on at 88°/off at 83°,
high speed on at 92°/off at 87°.
Also, there's no engine driven fan so I suppose the electric ones are bound to come on sooner or later when stationary.
I was bedding in the cam running it at 2000rpm on the driveway (luckily the neighbours didn't mind the noise) so I think that's why all
the fans were running fast, normally there'd be no reason to run the engine like this.
Turner Engineering put 4 temperature sensitive stickers on the core plugs in the engine block, during this run the 88° bit changed to black but the
next bit labelled 99° was still white so the engine was over 92° but under 99 if the fan switch and indicator strips are accurate.
I've since left it on the driveway running on tickover (which was around 850rpm, I've since lowered it a bit) and eventually the fans did
cut in and out at low speed, they were on for longer than they were off but its a brand new engine so maybe I should expect it to run a bit hotter
than usual until its properly run in?
All in all, I'm reasonably confident that the cooling will be ok although I haven't been out on the road yet (first time on the road will
be a trip to the MOT station this week), its just that I'd like to try and get as much air from the fans as possible through the radiator for
those stuck in traffic in a heatwave moments.
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nobrakes
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| posted on 11/7/16 at 01:00 PM |
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quote:
Originally posted by 02GF74
Not sure I follow all that, I prefer pictures to lots of words, so is the diagram below a summary of what you feel is happening?
No, the fans are blowing through the radiator as per your red arrows, no air is following your blue arrows back through the rad.
Imagine it like this - lets say the fan blades are 7" across and the surround is 11" across (the gap isn't this big but it'll
do for this explanation).
Now slide a thin sheet of plywood between the fan and the radiator (my rad isn't blocked off like this but again, it'll do to help
explain)
Turn the fan on and what'll happen is the fan will suck air in, turn it through 180° and blow it out around the 2" gap between the fan
blade tips and the surround.
Does that help visualise it?
What I've actually got is a lot less than 2" gap and air gaps between the radiator tubes/gills so most of the air is going through the rad
but the fans are pressurising the surround to the point where its easier to blow back through the gap between the blades and surround than through the
rad.
No problems with bodywork hindering airflow because there isn't any, its in a trike and the rad sits in front of the engine (Reliant rather than
tadpole wheelplan).
I don't actually know what the radiator is from, this is one of the problems of working on something built by and probably altered by one or
more of the previous owners.
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02GF74
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| posted on 11/7/16 at 02:53 PM |
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See, that is why i like pictures.
So what i understand now is there is a gap of 2 inches between fan blades and shroud?
That is bad, the gap should be minimal as air will take path of least resistance, which is not through radiator..... as you seem to have
discovered.
You need to fit correctly sized shroud or larger fan so there is no gap.
Let me find a picture of what i mean. See on this page
http://calsonic.com/en/products/engine-cooling/fan-shroud
Two fans sitting in a hole in the shroud that are just a bit bigger than the fan blades.
[Edited on 11/7/16 by 02GF74]
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MikeRJ
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| posted on 11/7/16 at 03:30 PM |
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quote:
Originally posted by 02GF74
See, that is why i like pictures.
So what i understand now is there is a gap of 2 inches between fan blades and shroud?
That is bad, the gap should be minimal as air will take path of least resistance, which is not through radiator..... as you seem to have
discovered.
Absolutely this. An axial fan without a close fitting shroud waste a lot of airflow by moving the air outwards along the blades like a centrifugal
blower.
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nobrakes
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| posted on 11/7/16 at 03:33 PM |
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quote:
Originally posted by 02GF74
See, that is why i like pictures.
So what i understand now is there is a gap of 2 inches between fan blades and shroud?
That is bad, the gap should be minimal as air will take path of least resistance, which is not through radiator..... as you seem to have
discovered.
You need to fit correctly sized shroud or larger fan so there is no gap.
Let me find a picture of what i mean. See on this page
http://calsonic.com/en/products/engine-cooling/fan-shroud
Two fans sitting in a hole in the shroud that are just a bit bigger than the fan blades.
[Edited on 11/7/16 by 02GF74]
No no no, I was trying to make it clearer but appear to have failed.
Like I said, I don't have a 2" gap, just a small one where air is coming out between the fan and the surround/shroud.
I want to reduce this 'leak' of air from the fans just in case I ever need every last bit of cooling (like if I'm stuck in heavy
traffic going uphill in summer in the south of France and the temperature is up in the 30's)
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nobrakes
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| posted on 11/7/16 at 03:43 PM |
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quote:
Originally posted by MikeRJ
Absolutely this. An axial fan without a close fitting shroud waste a lot of airflow by moving the air outwards along the blades like a centrifugal
blower.
Yes, that's it! The shroud round the fan isn't close enough to the fan so this is whats happening.
Hence I'm thinking, either:
- line the inside of the shroud with something to reduce the blade-to-shroud clearance (but by how much, how close does it have to be?)
- block off the outer edge of the fan grills with a round baffle like a giant polo mint to redirect the escaping air back into the fan (would that
work?)
- something else?
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coyoteboy
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| posted on 11/7/16 at 03:43 PM |
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Fan recirc
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02GF74
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| posted on 11/7/16 at 04:26 PM |
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just had a look at one fan, distance between end of blade and shroud about 1.5 - 2 mm.
I see your picture now.
If the gap is as above, then I would leave it - we can probably assume the fan manufacturers know what they are doing?
If you start to cover the gap, then will that not restrict the flow of air into the fan as the open area is reduced?
Which of the two situations is worse I cannot say.
The radiator matrix should be covered by the cowl, as per the photo I posted and the fan(s) sit(s) in the holes, the air pushed by the fans will pass
through the entire matrix not just a part of it for maximum effieciency, again as done by car manufacturers..
[Edited on 11/7/16 by 02GF74]
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nobrakes
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| posted on 11/7/16 at 04:26 PM |
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Does this help? (Isuppose I should have shown all of the air arrows going through the radiator in the side view with the baffle)
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SPYDER
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| posted on 11/7/16 at 09:39 PM |
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So the fans are "set up as pushers".
This obviously needs the direction of rotation to be correct.
It also requires the fan to be the correct way round, otherwise the fan efficiency suffers.
Are your fans the right way round?
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Dusty
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| posted on 11/7/16 at 10:26 PM |
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A rotary fan will always impart a centrifugal(centripetal?) force to some of the air-mass so some goes straight through the rad and some goes
sideways. Regardless of how tightly the shroud fits some will be blown as you describe. Best answer is to fit the fan behind the rad as a sucker and
the problem goes away. It is more efficient in this configuration. Shrouding can be more imaginative also if its behind the rad without reducing
airflow through the rad although due to the increased efficiency it doesn't need to be.
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02GF74
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| posted on 12/7/16 at 07:55 AM |
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quote:
Originally posted by Dusty
Best answer is to fit the fan behind the rad as a sucker and the problem goes away. It is more efficient in this configuration. .
Hmm - how do you mean more efficient - better cooling for given power than front mounted? Can you provide figures?
Fan positioning was discussed not too long ago - I argued that a push fan will flow more than a pull fan (rear mounted), my argument being a pull
fan can only suck at most 1 atm whereas a push fan can go over that, and more air through a radiator will shift more heat so more cooling.
Regarding efficiency, the pull fan may be more efficient than push at low flows - for reasons you state (I don't know it that is a fact but it
sounds plausible), but eventually the pull becomes less efficient as it runs out of air to suck (its limited to 1 atm) whereas the push will keep
flowing air as long as it is available and I would guess is limited mainly by the power of the motor and design.... and I don't have figure for
that but is it obvious from the physics.
Other than many production cars have fan mounted at the front, that wouldn't be the case if it was significantly less efficient than rear
mounted, anyway I'm not saying anything more on this as we are heading off on a tangent and I don't want a repeat of previous
'argument'.
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nobrakes
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| posted on 12/7/16 at 05:26 PM |
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quote:
Originally posted by coyoteboy
Fan recirc
Nope, there isn't any significant amount of air escaping between the rad & shroud as per the grey curly arrows.
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nobrakes
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| posted on 12/7/16 at 05:30 PM |
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quote:
Originally posted by SPYDER
So the fans are "set up as pushers".
This obviously needs the direction of rotation to be correct.
It also requires the fan to be the correct way round, otherwise the fan efficiency suffers.
Are your fans the right way round?
They're pushers because of the lack of space between the engine & rad.
One of the reasons I bought these particular fans is because you can run the motor in either direction and also the fan blade can be flipped over so
its running in the right direction.
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nobrakes
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| posted on 12/7/16 at 06:10 PM |
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I had a measure up today and the gap between the blades and shroud on the 7" fans is around 4mm or so which looks like is just too big. Also the
fan is just blades sticking out of a hub, it doesn't have any sort of ring around the outside so all in all I think it's pretty poor
design.
I think the way forward is to reduce the inner diameter of the shroud to only slightly larger than the fan blade diameter.
Maybe a couple of lengths of uPVC tapered edge architrave, bent into a circle and with the tapered edges facing the front and back of the shroud would
do the job.
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coyoteboy
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| posted on 13/7/16 at 08:56 AM |
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Are you sure it's that bad? The amount escaping the sides will be orders of magnitude less than the amount going through?
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nobrakes
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| posted on 13/7/16 at 10:16 AM |
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quote:
Originally posted by coyoteboy
Are you sure it's that bad? The amount escaping the sides will be orders of magnitude less than the amount going through?
This is the thing, I'm not entirely sure. Remember the fans are quite powerful (1270 cfm each according to the maker's specs) and to me,
it feels like a strong draft but it is hard to quantify.
It can't hurt to increase the airflow in the right direction, especially if I can do it just by glueing some plastic inside the shroud.
(unfortunately, because I was worried about vibration loosening the push-fit fan brackets, I put Tigerseal in all the joints and brackets so got to
see if I can get it apart first...)
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r1_pete
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| posted on 13/7/16 at 12:43 PM |
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One simple thing to check is that the fins on the radiator are not distorted, if they are they will interrupt the airflow through the rad.
If you hold the rad up you should be able to see through it, and any patches of twisted / distorted fins will be obvious.
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