Hi Tracy,
As you and have long discussed, a thin
large radiator is (by definition) more "efficient" than a thick one due to the
DeltaT factor. Also if you have very low dynamic pressure available then you
have no choice but to use very thin radiators (as in modern traffic-jam
car types) as well as fans. Also if you are only going 45 mph
then cooling drag is not a very significant factor.
However, having one component more efficient does not
necessarily mean that makes the entire installation more efficient. This
is a total system and what we really want is more "effectiveness"
- which depends on the operating environment, constraints and
costs.
With aircraft we are in a different operating
environment. Here we want to:
1. capitalize on the high dynamic pressure
available
2. constrain installation size
3. minimize cooling drag.
All of these considerations swing the system
"effectiveness" equation (so to speak) to favor the thicker core - even
though it is slightly less "efficient" due to the delta T effect. You take
the far largest high in drag by the initial disturbance of the air flow, once
its disturbed you then have skin friction - but that is several orders less than
the frontal drag. So minimize frontal area to
minimize frontal drag, increase thickness to compensate for smaller
frontal area heat rejection loss.
Rather than looking at the 45 mph Soccer Mom car
radiator (large in frontal area, but thin and efficient), look instead at
the Nascar vehicles which travel at speeds approaching (or greater {:>)) than
ours. The average Nascar radiator is 3 1/2" thick(humm about the same as
the old GM core) for the average track, whereas for the longer higher
speed tracks they build them as thick as 7". See attached report.
Now, if those guys are spending $$$$$$ just to pick up a 0.5MPH advantage, then
I am confident that they have done the $$$ research to show that with those high
speeds and dynamic pressure thick radiators are more "effective"
- if not more "efficient".
".... Race radiators are all aluminum
and range anywhere from 3-1/2 inches thick to 7 inches thick in the core, with
the standard intermediate track radiator being 3-1/2 inches
thick..."
I have attached a pdf of the article as it has some other
nice information in it.
However, Tracy, I once heard you sum it up as
concisely as I have ever hear it stated . I think you summed it up nicely
when you said something to the effect of , "....disturb the smallest column of air you can - that is adequate
for cooling."
Ed
----- Original Message -----
Sent: Monday, November 12, 2007 10:09 AM
Subject: [FlyRotary] Re: Diffuser Configuration
Comparison
>I especially liked
the comment in the report where it says that the
> higher the pressure
drop across the core, the higher the diffuser
> efficiency. I
interpret this as 'thicker core is better than
> thinner!' Of
course there is a point of diminishing returns where
> flow is simply too
low as Ed has pointed out but in an application
> where diffuser
efficiency is such an important factor, this pushes
> that point in the
direction of 'thick'.
>
> Tracy (back from the dragon and sorry to
bring up the thick vs thin
> thing again :>)
>
>
>
On Nov 7, 2007 3:54 PM, Ed Anderson <eanderson@carolina.rr.com>
wrote:
>>
>> Here are two of the charts from the Naca
report. The one on the left shows
>> diffuser resistance without
a resistance (radiator) behind it and the one on
>> the right shows the
diffuser with a radiator core behind it.
>>
>> Note the
considerably increase in diffuser efficiency with a core. It
>> appears
that the while the pressure that builds up in front of a core
>>
eventually leads to boundary layer separation due to the adverse
pressure
>> gradient,.
>>
>> It in fact
actually helps the resistance diffuser pressure recovery
>> efficiency
over the open diffuser by preventing the separation until much
>>
later. The one on the right also shows that a shorter length L/D also
helps
>> efficiency as that apparently causes the expansion before the
boundary layer
>> has grown too thick with the tendency to
separate.
>>
>> Of course you could always get the report and
do your own interpretation
>>
{:>)
>>
>>
>> Ed Anderson
>> Rv-6A N494BW
Rotary Powered
>> Matthews, NC
>> eanderson@carolina.rr.com>>
http://www.andersonee.com>>
http://members.cox.net/rogersda/rotary/configs.htm#N494BW>>
http://www.dmack.net/mazda/index.html>> --
>> Homepage: http://www.flyrotary.com/
>>
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>>
>>
>
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