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Date: Tue, 07 Aug 2007 09:17:18 -0400
From: Ernest Christley <echristley@nc.rr.com>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: RV -7A Cooling Update  8/6/07
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Ed Anderson wrote:
> From my understanding of K&W and your radiator orientation, I would 
> hazard a "Yes", I think you may have found a significant reason for 
> getting less cooling than you would initially think you would get with 
> that size radiator.
>  
> Rather than the larger, relatively more aerodynamic cross-tank tubes 
> turning the air, the air, as you point out,  is meeting the sharp edge 
> of the fins between the tubes.  This likely causes flow separation and 
> that turbulence at the entrance  could, in effect, act as a barrier to 
> good flow through the core channels.  So that may indeed be a reason 
> why manipulation of your airflow inside the duct
Just thinking out loud here, but I'm wondering what effect lanced-fins 
have on this dynamic.

History lesson as it was passed on to me.  Engineers attempted to push 
up the efficiencies of heat exchangers by using more but thinner fins as 
manufacturing technology progressed enough to make working with such 
thin material feasible.  They also attempted to use fin configurations 
that created micro-turbulence within the fin passages that would break 
the boundary layer and help transfer heat to the air.  They soon found 
that it choked the airflow to much.  They eventually figured out that 
they could cut holes in the fins to reduce the back pressure, while also 
increasing micro-turbulence.  The non-lanced fins would entrain a vortex 
and consume energy maintaining it.  The vortices would form on lanced 
fins, but then there is a hole that breaks it up.  The result was 
comparatively excellent efficiencies for both heat removal and low 
airflow restrictions.  Most newer cores are built using lanced-fins, and 
I believe the Mazda oil cooler does also.

The weed blower with a radiator in a box would give a quick answer, but 
I'm thinking it may not make much of a difference.  This would leave you 
searching for another restriction that is reducing airflow.  What did 
you say the pressure diff across the rad faces was, Dennis?
>  
> Ed
>  
> P.S. Really could use some of the wet stuff   
Nawh!  We've got plenty of water.  It's just hanging in the air as 
steam.  Good thing, too.  I've always wanted a sauna, now all I have to 
do is walk outside.