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Date: Tue, 13 Nov 2007 16:53:33 -0800 (PST)
From: Ron Springer <ron2369@sbcglobal.net>
Subject: Re: Thick vs Thin was : Diffuser Configuration Comparison
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
In-Reply-To: <list-2470424@logan.com>
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Message-ID: <394806.70373.qm@web81305.mail.mud.yahoo.com>

Al,

Yes, you are absolutely correct. That was an incorrect
statement that I made. Just delete my item #5 below
and replace it with this:

For turbulent flow,
1) Drag scales with velocity to the 1.8 power (takes
into account that skin friction coefficient changes
too, so it is no longer 2.0)
2) Heat transfer scales with velocity to the 0.8 power

So, you can increase velocity across a radiator to get
more heat transfer, but the drag goes up much faster
when you do that.

Ron

--- Al Gietzen <ALVentures@cox.net> wrote:

> Ron;
> 
> I'm with you all the way until you get to: " Then,
> because heat transfer and
> drag scale with
> velocity squared,"
> 
> I'm aware that the heat transfer is not linear with
> velocity, but how does
> it go as go as the square?
> 
> I have a ROT that a 16 fin/in rad with 1/2" tube
> spacing should be 2 3/4"
> thick; and I'm sticking to it:-)  Oh; I have another
> ROT; the diffuser area
> ratio should be in the range of 4 - 6.
> 
> Al G
> 
> 
> Subject: [FlyRotary] Thick vs Thin was : Diffuser
> Configuration Comparison
> 
> 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
> 
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