Jeff Spitzer wrote:

Paul Lamar wrote:

Here is my $.02. I think a lot depends on the velocity of the airflow
and the pressure difference across the core. If the velocity is real low
and the pressure difference is real high there will be flow through
the core no matter what. It would matter little if the air had
to make a ninety degree turn through the core.

I'm with you up to here.  Keep going.

With  an ideal diffuser and a limit condition the pressure would
be infinite the velocity near zero.

Right.  And that would make the face area of the HX infinite and the
thickness near zero.

Core drag would be near zero and the system would make a
damn fine engine or waste heat energy recovery system when the
air was finally speeded up in the exit nozzel to provide thrust.

Right you are.

Hoerner Fig 17 suggests just that.
Look at the left side of the chart.

w can be taken as the velocity through the core and
V the velocity of the airplane. C sub da is the overall
drag coefficient of the system.

Perhaps this is an argument for thick.

 The thick radiator will require greater velocity through the core
for the same heat rejection.  The max. temp rise of the air is limited to
the temperature differential between air and coolant.  Therefore, a minimum
mass flow is required for a given heat load.  Therefore, a smaller thicker
core must have the same mass flow (actually more due to efficiency loss) as
a larger thinner one.  Therefore, the velocity through the core must be
higher.  

Jeff Spitzer

 
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