In a message dated 8/27/2005 10:36:07 PM Central Standard Time,
JohnWise@alumni.pitt.edu writes:
> It just struck me that one on the big issues in general aviation
> engine cooling design is the airflow aft of the engine. Some
> colleagues of mine (all aero engineering professors) spent months
> trying to improve the air intake to keep an experimental engine temps
> within limits. In the end, all it took was a tweak of the exhaust
> side of the system. A little better aero that created the
> appropriate low pressure to suck on the back side &the problem was
> solved.
>
> I would surmise that total flow would be impacted by both sides of
> the radiator.
>
Very well put. The air flowing through the radiator has to come from
somewhere but it also has to go TO somewhere. When you place an object in an
airstream it will create an air pressure depression (see carburetor theory) and
reduce air flow. But the object will also create a pressure pocket ahead of it
also decreasing air flow. Most low profile fans have the motor in the hub
which
has a diameter of 3 to 4 inches (8 to 10 cm). This is a significant
restriction of air flow. If this fan is bolted up intimate with the radiator
there
will be no air flow through the radiator in the area abuting the fan hub.
Likewise the fln blades will restrict air flow in a reduced but similar manner.
But the inertia of the air will cause the height of the air pressure bubble
leading the obstruction to be lower than the height of the depression pocket
trailing. So the distance required to reduce the effect of this restriction is
much less for a fan following a radiator than leading.
Look at any modern car and you will see a fan shroud. The shroud does two
things: 1: it spaces the fan away from the radiator reducing the effect of the
obstruction and 2: ducts air from the full face of the radiator to the fan.
This second effect means that when the fan is running it will pull air
through the whole of the radiator and not just the part in front of the fan
blades.
This means that the air is passing through the radiator at a slower speed
creating less pressure drop, it means it has less inertia and can change
directions more easily, and for a given amount of air flow there will be more
temperature rise which means the air is extracting more heat from the coolant.
Bolting a hayden fan to a radiator with the through-the-fins fasteners is
truely convenient but not an optimum solution.
The same can be done with a pusher fan but the distance between the fan and
the coil must be much larger and there isn't room in any practical car
configuration. But one advantage to placing the fan in front is that the fan
will
move more air because the air is cooler and more dense.
One advantage to mounting a pusher fan is that you can leave the stock fan in
place and get the advantage of both fans (and save the work of removing the
stock one). But if you have problems with overheating at speed (where ram
effect predominates) then perhaps the radiator needs servicing. Or enhancing*.
Dave Massey
57 TR3
71 TR6
80 TR8
P.S. * removal of the crank hole.
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