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I agree, Al.
I must admit that I have a tendency to try and generalize
concepts - at least in part because it may have taken me a while to grasp them
and I hope to help others understand the basics. However, there is no
doubt we are on the same sheet of music. I certainly appreciate you
pointing out anytime you feel I have made a perhaps misleading "over
generalizations".
I am the first to admit my knowledge is very limited in
this area. Every once in a while I find something that appears to turn on
the "light bulb" and I am guilty of rushing to share it with
others.
But, I will be the first to say that there is much of it I
do not fully understand and anytime you (or anyone else) feels that I have
misunderstood/misinterpreted or mischaracterize something, I want to be the
first to be informed.
I think I'll wait for my next little nugget, I finally
found a study that actually addressed the effects of core thickness and its
effect on cooling and drag. But, I'm going to try out some calculations
using the information and see if I can get them to make sense , before I share
(what I think I've found).
Ed
----- Original Message -----
Sent: Monday, November 12, 2007 6:20
PM
Subject: [FlyRotary] Re: Generalizations
was Re: Diffuser Configuration Comparison
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
{:>)
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