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Continuing to do a bit of research on diffusers, I came across an extract
from the K&M bible on cooling that provides rationale for using thicker
radiators for aircraft. Interestingly, I have never heard it mention by
those who have studied K&M on the topic of cooling. Perhaps it was
overlooked.
Generally, I am a bit cautious ({:>) about interpreting
such techncial stuff - as it is not difficult to draw an incorrect
inference.
However, in this case the authors of the study provide the interpretation
(see attachment)
In essence, they make the statement that thicker radiators with closer fin
spacing provides for more heat dissapation into an airstream. The
refer to the thickness as "L" length of the cooling block (heat exchanger)
and the "D" the hydraulic wetted area spacing (basically related to the fin
area/spacing) . Re is the Reyonlds number which is smaller with lower
velocity.
So a low velocity airflow through a thick radiator (L) with close fin
spacing (D) is the way I interpret their words and equation for the heat
transfer coefficient "k". This would mean you need good pressure recovery
from a diffuser (low velocity out of your diffuser). So long as you have
the dynamic pressure (read that as having adequate airspeed through your
inlet) for the thickness you are using, it appears that you are ahead of the
game using a thicker radiator.
This makes sense (to me) in that a thicker
radiator continues to dump heat into the airstream as long as the air is
passing through it - even thought it becomes less efficient at this transfer
as the air through it heats up (thereby reducing the "Delta T"), it is still
transfering heat to the air. True a thinner radiator is more
thermodynamically efficient, but you have to have a considerable increase
in frontal area to get the same approx core volume as a smaller thicker
radiator. In both cases, the total metal surface to air contact is
proportional to core volume.
Since drag is proprotional to frontal area and to the square of the
velocity, it would seem that small frontal area (less drag) heat exchangers
and good diffusers (providing lower velocity through the heat exchanger) are
an idea combination. All else being equal - which it seldom is {:>)
Keep in mind that nearly anything will work to cool - provided you have
sufficient surface area and air mass flow across it. However, not
everything that would provide adequate cooling would be appropriate for an
aircraft and some approaches would impose severe drag penaltities.
One more thing on diffusers, it appears from my research that most diffusers
have an exhaust/inlet area ratio (Area_out/Area_in) of between 3 and 5. So
perhaps "4" would be a good target. Which if we apply it to the GM
evaporator core were Aout = approx 90 in^2, then the inlet area would be
90/4 = 22.5 in^2. My inlets happen to be 24 in^2 (just luck of the
constraints - not any preplanning) for my GM cores and I have no cooling
problem even with pretty dirty airflow ducts (cleaned up one and reduced it
to 9 in^2). 90/9 = 10. My cooling temps went up 5F when I reduced one
inlet from 24 to 9 in^2 - perhaps indicating that the resulting 10:1 ratio
resulted in excessive bondary layer separation. It appears that going
higher than a ratio of 5:1 that boundary layer separation is a problem.
Going less that 3:1 appears to unnecessarily reduce the pressure recovery
factor (meaning the air velocity stays higher = more drag).
FWIW
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
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