Ed;
I’m sure we are basically in
agreement on most of these things regarding coolant system design. Certainly
I agree with point number 1; if you haven’t done the calcs to determine
the amount of the heat load that you need to handle, and determined the mass
flow rates needed based on reasonable assumptions of temperature changes; then
you haven’t begun to design the system.
Beyond the basic points we could discuss
indefinitely; but I will say that discussing ‘thickness’ without
stating tube and fin density is like assuming, for example, that all metals
have the same density and strength. Similarly, discussing thickness
without some info about diffuser area ratios is also a bit nebulous. So I
learn very little about cooling from someone telling me that racing radiators
are 3.5 – 7” thick.
I will repeat my favorite R.O.T.
cooling mantra: Every CFM passing through the cooling system represents
drag. Unless I have missed an important point somewhere, more CFM will
always result in more drag. (Tracy)
This is true if you assume that you put
the air back into the free stream at a velocity negligibly small compared to
the velocity that went in. That may be true in many cases; but I could
have infinite CFM, and with zero pressure drop, or velocity change, have zero
drag. Drag is about the energy (velocity) difference between the air
going in and the air going out.
Al G
1. Mass flow through the core
is the most critical element of cooling. If there is insufficient
mass flow then it does not matter how good you ducting or core is , you will
not meet your cooling objective. Your air mass flow requirement is
dependent on your heat rejection needs.
2. The maximum duct mass
flow possible is a function of free stream kinetic energy available. This
means you cooling design point airspeed is as much (or more) a
crucial factor in your design as any other factor.
3. Many factors determine what
you actually mass flow will be, these include both design, fabrication,
installation, environmental and operational factors. A pretty
general statement, but valid just the same. Its the nailing down of the
factors in this area that to me represents the most beneficial (and the most
difficult) factors to understand in detail.
4. The maximum flow in the
ducts (and through the core) is a function of the free stream kinetic energy
and the pressure loss coefficient of the duct (and core).
5. Air Flow separation in the
diffuser is the most significant factor in degrading core effectiveness.
Separation reduces cooling by reducing mass flow, by creating
pressure losses, disrupting even velocity distribution across the core and
increasing drag.
6. Diffuser's performance
depend, in significant part, on the core characteristics.
7. It is a balancing and
optimization problem of opposing aerodynamic and thermodynamic
attributes.
8. If you had enough core and
enough air flow - you will cool, but the penalty in drag and weight may be
higher than you would like.
9. Few of us have the
knowledge, understanding, tools, time, $$ or inclination to do it the right the
first time , but always time to re-do-it after the first flight {:>)
Besides the generation that appeared
to bring this discussion about was that thicker radiators offer advantages at
higher airspeeds. I still stand by that generalization.
note. I did not say that 2 1/2"
was too thin or 7" was too thick. But, I do believe that the
Nascar crowd have the resources and inclination to do the research on radiator
size that none of us do have. There speeds are comparable to ours, so
again, I personally feel that a core in the vicinity of 3" thick sets a
bench mark that is probably as valid as anything we could afford to do.
Just because my GM cores
happen to be 3 1/2" thick has nothing to do with it {:>)
Appreciate you comments, Al. I
will try to hold my generalizations to an ...A'hem ... acceptable minimum
{:>)