Once again,Al, I agree that generalizations can
mislead - if taken too literally . But, on the other hand I
think generalization is very useful approach to making
points and conveying understanding.
I think each cooling system starts out as a
conceptual generalization. We generalize about how much power we
think we are going to produce (unless we decide to dyno the engine - certainly a
good move, but one seldom done) and therefore how much heat we need to get rid
of. we also probably start with a rough estimate of core size
(perhaps its a core we already have or readily available-such a GM cores or the
space we have allotted for it) , then we must consider the kinetic energy
available in the air stream for our critical cooling design point (should it be
climb or cruise).
I think generalizations are useful in this process.
For instance, I think its fairly safe to say that for our operational
environment that a 1" thick radiator core is probably not the most suitable
dimension. Can one be used, certainly, but its frontal area is going to be
large (space constraints may inhibit) and cooling drag is going to be
higher. Now if you are flying a bi-plane then cooling drag may be such a
small part of your total drag that you wouldn't gain much by reducing it.
But, at your speeds and even my slower speed, it starts to become an appreciable
portion of total drag.
Fin density, type fins, core thickness, turbulation or
not, core orientation of the air flow, duct losses, drop across the core,
hydraulic diameter, Reynolds number, etc, etc, all play a role in the final out
come. Then no matter how carefully you "design" you cooling system, leaks,
sharp edges on any turns, distance between back of core and obstructions,
boundary layer ingestion, etc, installation imperfections can mess up the
performance of the "perfect" system. Again this is all
generalizations, but that does not necessarily mean they are not
correct.
I have spent weeks, months (slow learner) reading
and studying all I could find and reasonably understand about the airflow
side this problem. It has really pointed out that effective cooling is
all about balancing conflicting aerodynamic and thermodynamic
factors. It is certainly beyond my ability to grasp it all, but
there are certain things that I have become convinced of and more will follow (I
hope) as I study further.
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 {:>)
Best Regards
Ed