Snips...

if you

want to keep the velocity low for good heat transfer

More snips...

I don't understand this. Why is heat transfer helped by low velocity???

Jon

You are right Jon!  I mis-spoke / mis-typed.  What was I thinking?  I wrote
the following stuff after reading your initial posting, but before seeing
what Tracy, Paul, and everyone else wrote in trying to devine the mysteries
of a fogged up head.  That said, it still seemed relevant, so I'm posting it
anyhow.  The one posting that still has me a bit befuddled had a statement
to the effect that after passing through a diffuser (and being slowed,
picking up static pressure/density), the air actually moves *faster* through
the rad.  I don't follow that one at all.

Everything is a trade-off.  Certainly you want to have the velocity high
enough (or else disrupted enough) that you maintain as turbulent flow
through the rad as possible.  This keeps the heat transfer rate up by
minimizing the thermal resistance of the boundary layer (keeps it as thin as
possible).

You also want to keep the air side temperature down as low as possible as it
flows through the rad so that the Delta-T (difference between water side and
air side temperatures) stays as high as possible.  This keeps heat flowing
through the tube walls at as high a rate as possible.

So those two considerations suggest as high an air side velocity as
possible:  turbulent, and lots of air flowing through to pick up very little
heat (and therefore temperature) per pound of air mass.

On the down side, higher velocities come with higher drag.  Paul well knows
the details a lot better than I, but IIRC, drag is a function of the square
of the velocity.  There will come a point where the effects of the high drag
outweigh the benefits for heat transfer.  I'm beginning to realize this is
why the optimum design requires a small frontal area scoop that then bells
out to reduce the incoming velocity and reduce the drag through the rad
itself.

A lower air side velocity is also beneficial for the size efficiency of the
rad (space under a cowl is at a premium):  It permits each pound of air to
pick up more heat as it passes through.  A longer loiter time in the heat
exchanger (by mozying through at a slower rate) will cause the air to heat
up more giving you more BTUs removed per sq ft of heat exchanger area... but
also offsetting the previously mentioned benefit of having a high Delta-T.

So you are right:  all other things being equal, a higher velocity is better
for heat removal.  I was fixated on the overall concept of how to
size/plumb/arrange a radiator in a small airframe and was thinking of
slowing the incoming flow first and recovering static pressure.

Thanks for giving me (yet another) learning experience!

Tom Kendall

 
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