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#1
08-01-2006, 11:13 AM
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Anyone willing to do a cooling experiment?
On the subject of how far into the shroud the fan should be placed...traditional thinking is that the blades should be halfway into the shroud.
The more I think about it, I think the blades should be fully into the shroud; every ducted fan I've seen has the fan fully into the duct. The experiment would be for someone who has a shroud on their car to move the fan into the shroud while keeping a safe clearance to the radiator, of course, and compare results. If clearance is a problem, perhaps adding some sheet metal around the circumference of the shroud to encase the fan would work also. Who knows? Maybe this would be a idle speed airflow improvement that a lot of owners are looking for. I think I remember some full size chevies had this deep shroud arrangement. I would do the above, but my GTO is not streetable at the present time. George |
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#2
08-01-2006, 01:44 PM
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Axial fans throw off a lot of air in the radial direction. Pushing the fan fully into the shroud would restrict the outflow from the fan, thus reducing overall airflow. I would suggest adding to your experiment by trying the fan only 1/3 into the shroud, that's my bet where it will work best based on my experience.
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#3
08-01-2006, 02:44 PM
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My Buddies 72 G/P – stock, has one of those extended fan shrouds, fan is about a 1/3 of the way in. No cooling issues.
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#4
08-01-2006, 03:13 PM
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Wolf: But as the fan comes out of the shroud, the performance will approach that of a unshrouded fan, no?
I agree, it seems to me the unshrouded portion of the blade will move air in the radial direction, also axially. Not only that, the unshrouded portion will take air from the engine side of the shroud and displace it radially. When confined in the shroud the fan will displace air more efficiently from the radiator side. At least that's my gut feeling. I don't see how it would restrict the outflow, rather redirect it. That's what experimentation will help determine... george |
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#5
08-01-2006, 03:37 PM
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Any smokers out there? Couldn't you use smoke to see where the air is going? Granted, you might need to use a lot of smoke, due to the cfm of air, but I think the general idea still applies.
__________________
How many of you have driven over 340? 
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#6
08-02-2006, 04:00 AM
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George, when I used to work at the proving grounds, we did a lot of cooling tests. I once spent several days in a city bus running on the track until the temps stabilized, then pulling it in the shop, removing the radiator, and shimming the fan in or out before repeating the process. Our best results always came with the fan right around 1/2 way into the shroud. (Some setups liked the fan 1/2+ in the shroud, and some liked just under 1/2 in the shroud; I don't know if that was dependant on fan-shroud clearance, or blade pitch, or some other variable.)
Apparently as the air is "thrown" off the fan blades radially, it picks up enough additional velocity to create more vacuum on the suction side of the fan than the blades alone would create if they were all the way in the shroud.
__________________
Mike 
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#7
08-02-2006, 11:57 AM
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Mike, thanx for the info. it's good to have real world data to back up accepted theory.
I still wonder how this fan/shroud issue differs from a ducted fan design where the fan is well within the duct. I need to do some investigation of ducted fans. Watch yourself out there.... George |
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#8
08-02-2006, 01:06 PM
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My brother was talking about the Benini or Berllini principle, some science guy. any way having the affect of what happens to air or whatever moving in a direction until forced to move another. Does anyone know of this. Any way this could apply to what you are thinking. Cannot get a hold of him for further clarifacation.
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#9
08-02-2006, 02:02 PM
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One example of the Bernoulli principle is a carburetor venturi, which is a area in the carb throat which has a reduced diameter. The pressure of the air in the venturi is lower than in the larger diameter of the throat which produces a vacuum signal to draw fuel out of the bowl. Another is an aircraft wing. The air pressure on the top of a wing is lower than on the bottom due to it's shape, causing a net force lifting the wing.
A better definition and examples can be found at wikipedia.com and howstuffworks.com, etc. george |
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#10
08-02-2006, 02:45 PM
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Thanks George now at least I know how to spell. He uses this in irragation for water through bends in pipes for pressure. (I think).
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#11
08-02-2006, 06:42 PM
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I could see a "steep" pitched fan working better if it was placed farther in the shroud since it would "throw" the air out the back of the shroud and create a strong vacuum in front of it. I think the idea of the fan being part in and part out works best though since most fan blades are not 100% effiecent in throwing all of the air out the back. I think some of the air would be pushed "up" and recirculated if the fan were in the shroud therefore creating less of a vacuum? Don't know ...just tossing the ball here.
P.S. I like the smoke idea!
__________________
Buy a GOAT....they eat anything! |
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#12
08-04-2006, 10:29 AM
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nobody mentioned the turbulance if you place the fan all the way in. The air wants to spin off the tips of the fan so if you have the fan half way in, the air can in essence fling off the back half of the blades with no interference. i bank 1/2 to a a 1/3 in the fan.
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#13
08-04-2006, 10:30 AM
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Nobody mentioned the turbulance if you place the fan all the way in. The air wants to spin off the tips of the fan so if you have the fan half way in, the air can in essence fling off the back half of the blades with no interference.I bet 1/2 to a a 1/3 in the fan will be the best.
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#14
08-04-2006, 11:07 AM
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Good theory, guys but a modern high bypass jet engine has a ducted fan all the way in the housing......That seems to be the most efficient design since it is being widely used.
I googled "ducted fan" and found a number of setups used for model aircraft use...all obviously with the fan well in the housing. So I keep asking myself if that's the best setup for providing thrust, why would it not work with a fan-in-shroud setup? I'm on a mission..got to get a shroud for my car. george |
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#15
08-04-2006, 07:56 PM
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the jet engine has a blade with a different pitch and tighter no. of blades. for the Automotive fan, if not half in and out, the air on the downstream side will be recirculated back to the upstream side of the blade and therefore reduce total air flow. the shroud prevents that air from recirculating back. Think about a ceiling fan where air is recirculating around and around. with a shroud it would be flow out in one direction, but only if you would be directly under the fan. No experiment is necessary.
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#16
08-06-2006, 07:20 AM
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George,
We know the shroud works, & that the consenus seems to be that the fan needs to be about half in/half out of the shroud. Where I see a problem is with the gap between the fan blades & the shroud. It affects shroud efficiency. Obviously, if this gap kept increasing, you would get to the point where the shroud contributed nothing. Why not try making the shroud work harder by reducing this gap to a safe minimum by gluing some shaped rubber pieces in the shroud opening? Some shroud openings are huge & could certainly be reduced. |
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#17
08-06-2006, 02:03 PM
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Geoff: The clearance can't be too small because the engine shifts when a load is applied. Too small a clearance and the fan will contact the shroud. I've never measure the amount of movement, tho.
George |
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#18
08-08-2006, 12:55 AM
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thats the point
Quote:
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#19
08-08-2006, 02:15 AM
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George,
What about this: A ducted fan needs all the airflow in a coaxial direction (if I understand correctly turbulent airflow away from the axis causes tons of drag), but that's not important with a radiator fan. Maybe there's a little less total airflow with a completely ducted fan, but it's going in the right direction. Maybe it's a good thing to have some turbulent airflow under the hood to cool the fuel system. Pure speculation. |
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#20
08-08-2006, 07:27 AM
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George,
Not sure what model car you have, but my 66 GTO engine mounts have built in movement limiters so that the rubber does not split from engine torque. Left side mount wants to come apart with engine torque. I welded an additional 5/16" rod to the left mount to limit engine movement. I have about 3/4" gap from blade to bottom of the shroud. I have about a 1" gap from the blade to the top, right side & left side of the shroud. Some fan-shroud gaps are huge & at some point efficiency must suffer. |
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