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Date: Tue, 13 Nov 2007 11:54:08 -0800 (PST)
From: Ron Springer <ron2369@sbcglobal.net>
Subject: Thick vs Thin was : Diffuser Configuration Comparison
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
In-Reply-To: <list-2470308@logan.com>
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Message-ID: <284267.53651.qm@web81313.mail.mud.yahoo.com>

Assume you have two radiators:

1) Radiator 1 is twice the frontal area of Radiator 2
2) Same wetted fin area and fin spacing, so Radiator 2
is twice as thick as Radiator 1.
3) Same air inlet with same mass flow rate
4) Different diffusers, but both have no flow
separation and perfectly uniform flow arrives at the
radiator face.
5) Both radiators have the same average skin friction
coefficient and average heat transfer coefficient in
the boundary layers that develop across the fins (this
is a small stretch and could be refined, but it is not
going to change the trends in this example).

Then, because heat transfer and drag scale with
velocity squared, and velocity is twice as high for
the thick radiator, and the thick radiator would have
four times the heat transfer, but also four times the
drag. There is always a tradeoff between heat transfer
and drag.

That is my take on the situation.

Ron