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Date: Tue, 7 Aug 2007 6:39:11 -0700
From:  <cardmarc@charter.net>
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: RV -7A Cooling Update  8/6/07
Cc: Ernest Christley <echristley@nc.rr.com>
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---- Ernest Christley <echristley@nc.rr.com> wrote: 
> 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.
As a follow up, all the third gen radiators are mounted at a top forward almost 330 degree or so angle, but the fins are parallel with the tanks. So the fins are angled relative the the "relative" wind. Not to mention the electric fans. Not sure about any of this being relative...so to speak. Begs the question about angled fins being necessary? I do belive the fins are punched however.
Marc