I concur with Bill's assessment regarding the vanes.  There is one
additional thing to consider, while vanes may assist cooling during the
cruise phase, they can adversely affect cooling during the climb phase
depending on a number of factors.  Basically, if the incoming air is hitting
the vanes at a relatively high angle of attack (like during a climb), the
vanes can create eddies and disturb the  airflow.  While turbulent flow is
best for removing heat as Bill pointed out, disturbed airflow such as caused
by eddies actually impede heat transfer by blocking efficient flow through
parts of the core.

So again, like so many things, it depends on the implementation.  I find
that with the proper shaped ducts/diffusers that the two GM cores cool just
fine across my operational regime.  Right after take off on hot days, the
temps will climb to around 200F-210F for a short duration but stabilizes at
170F for both oil and coolant at cruise power setting.



Ed Anderson
----- Original Message -----
From: "Bill Dube" <bdube@al.noaa.gov>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, September 30, 2004 4:19 PM
Subject: [FlyRotary] Re: FW: Smoothing Flow Through Radiators


 

From: "Chris Lowery" <chris.lowery@sbcglobal.net>

Hi All,
      I have found that putting vanes in front of the radiators lowers
     

the
 

coolant temperatures.  It seems that it straightens out the airflow so
     

that
 

it goes through the radiator more easily.
     

        Carefully executed, this could work very well. It will work much
better on thin radiators with a low fin count (distant fin spacing) than
   

it
 

will on a thick radiator with a high fin count. (If the radiator had the
correct fin spacing and thickness for the air flow and available pressure
drop, you wouldn't need to add the vanes.)

        It is nearly certain that the additional cooling is because of
increased turbulence, not "straightening" of the air flow. Breaking up the
stream lines and tumbling the air can greatly increase the heat transfer
from the surface of the radiator fins/tubes.

        When the flow is laminar between the fins, heat has a hard time
going from the fin surface to the core of the airflow. It is forced to
first heat the closest air layer, which then heats the air next to it, and
so on. (If the vanes are far apart, and the air does not spend enough time
in the radiator, the core streamlines will not pick up heat.)

        If the air is turbulent instead of laminar, the air mixes up as
   

it
 

flows past the surface of the fin. Blobs of air that would have slipped
though in the center of the core of a laminar flow are likely to be hurled
against the fin surface and forced to pick up heat if the flow is
   

turbulent.
 

        If the radiator is thick with close fin spacing, introducing
turbulence might help some, but will not make a large difference in the
heat transfer. (It could actually reduce the cooling ability of the
radiator, in some cases.) The turbulent flow will return to laminar after
is has traveled a distance of something like 10 to 30 fin spacings into
   

the
 

radiator. (This distance depends on the Reynold's number between the
   

fins.)
 

If the radiator thickness is less than, say, 20 fin spacings, (most car
radiators are) introducing turbulence might help a lot.

        Of course, there will be a balance between reducing the air flow
and increasing the turbulence. As you increase the turbulence, you will
restrict the airflow. There is an optimum point, either side of which will
be reduced cooling. Putting in too many vanes will result in reduced,
rather than enhanced, cooling. If the radiator is designed correctly for
the application (most aren't designed for airplanes) then adding vanes
   

will
 

reduce the cooling.

        I suspect that typical automotive or motorcycle radiators  would
be designed for much lower air flow (less air pressure drop) than is
available in a speeding airplane. This is probably why the evaporator
   

cores
 

work so well. In the car they came out of, they have a very strong fan
forcing air through them. They have closely spaced fins and are quite
   

thick
 

to take advantage of the high pressure drop available to force the air
flow. They may not be quite thick enough or have fins that are spaced
   

quite
 

close enough to be optimal for a 200 mph airplane. They will be a much
closer match than the typical car or motorcycle radiator, however.