Also:
Would it be a reasonable understanding, that the
flow will separate for lack of airspeed?
One reason for the expanding duct is to slow down
the air to be able to do some aork in the radiator. True increased pressure
may "help" seperation, but I would think if you keep the pressure lower with
exhaust augmentation you still will see separation once you get the airspeed
below a certain energy level. Like a wing will eventually stall, not for AOA,
but lack of airflow - you need a certain flow to keep the airstream attached
to the airfoil even at 0º AOA.....
Am I way off??
Thomas J.
I don't see anything unreasonsable about your
viewpoint, Thomas. While I think there is little question that less back
pressure will permit more flow through the radiator, I am not certain how it
will affect the separation point. Would less back pressure mean less
recovery pressure in front of the core? If there is no effect on
pressure buildup and maintenance before the core then the pressure gradient
there might remain the same -but the pressure differential across
the core increase. There is also the airmass flow
(which actually does the cooling) factor and even if the pressure build up in
front of the core remains constant the lessen back pressure may permit
more through-flow in the core.
There is no question that there are many factors
at work here and many of them conflicting with others - so balance rather than
elmination is the key. One reason there is no cut/dry answer - it all
depends on so many factors.
Here is an example from K&W chapter 12 The
heat flow coefficient Kp for turbulent flow in smooth passages (radiator
core passages) Kp = 1/2*L/D* 0.326/(Re)^-4
.
If Kp is a measure of goodness, then clearly if L
increases and D gets smaller Kp increases. Or if the Reynolds number Re
gets smaller Kp goes up. So what does this mean? It basically
shows that for the heat transfer to be large, the Reynolds number should be
low (I.e. the airflow through the core should be slow), the core should be
deep(large L) and the hole's hydraulic diameter (D) should be
small.
This makes sense as the thicker the core the more
heat transfer (although the further into the core the less efficient the heat
transfer), the holes should be smaller (area exposed area - with large
holes some of the cooling air in the center will simply not have as much
contact with the hot metal of the core) and the air velocity should be
slow (dwell time adds to heat transferred to the unit volume of air per unit
time).
However, if you make the core too thick or the holes too
small or slow the air too much - then your KP factor may be high - but
your over all cooling will suck because you have too little mass flow through
a too restrictive core. This is just one example of where optimizing on
one set of factors can play havoc with the overall system
function. One way of looking at it is that you have to suboptimize
a lot of factors in order to get an optimum system {:>)
My 0.02
Ed