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Such debates certainly helps me to focus on where my
understanding diverges from that of others {:>).
Yes, I understand that IF you keep the mass flow the
same between thin and thick radiator (with 1/2 less frontal area and twice as
thick) that the air velocity through the thicker core will increase and
the time spent in the cores will be essentially the same for both
- and under those conditions the detalT would be approximately the same for
both cores. So we are all (I think) in agreement on those
points.
However, if you put in a thicker radiator, then presumably
it would be because it could provide some benefit over the thin one in some
area. I would certainly have to modify the duct if for no other
reason, to have the duct cross sectional area match the smaller frontal area of
the core. To fully benefit from using the thicker radiator then I would
naturally have a duct matched to achieve those benefits - not for the
thinner radiator.
So IF in installing the thicker core, I slow down the mass
flow through the duct by 1/2 (which I can easily do - increasing it might
be more difficult) then the air velocity through the core will be reduced.
With the velocity decreased, the air will take longer to transverse the
core. The longer the air takes to move through the core the more heat it
will absorb, the more heat the air absorbs the greater the DeltaT of the
air. The greater the deltaT of the air the more heat will be removed even
with the slower velocity and mass flow ( the drag would also be reduced, just
had to throw that one in {:>)).
Q = m cp DeltaT. Clearly shows that heat
removal is proportional to BOTH mass flow AND deltaT (as well as the
specific heat but that is fixed by nature). Yet, for some reason
that I still don't understand, you skeptics seem to fixate on the mass flow
factor {:>) and ignore DeltaT as if it were a factor you can do nothing
about. Certainly you can not cool without some mass flow but then the same
is true about the deltaT factor. As long as the product of the mass flow
and the deltaT equals the heat (Q) you need to remove - then you have achieved
your cooling objective.
However, I can see that we have about rung any usefulness
out of this discussion (for the time being {:>)) and I'm certain we are
all getting a bit tired of it not to mention others on the list.
Again, appreciate your viewpoints and counter arguments
- it has given me something to sit and think about - I mean it just
borders on the possibility of reality that I might be incorrect with my
viewpoint {:>), so I am going to go do some more research and thinking.
Onward to other topics - ones which I refrain from
suggesting.
Best Regards
Ed
----- Original Message -----
Sent: Tuesday, November 13, 2007 10:39
PM
Subject: [FlyRotary] Re: Rebutal to the
rebutal {:>) Thick vs Thin was : Diffuser Configuration Comparison
Thanks Al,
Saved me the trouble. That last bit
about pressure recovery on exit is a nice touch, and you are right it is
important - with either radiator.
I know where this discussion is
going, and we have there (nowhere) before.
And I think we all realize
that the difference (if there is one) is too small to be significant in our
applications, so everyone just keep building and install what you
can.
Ed, your jedi mind trick smoke and mirrors will not work on
me. The mind trick only works on intelligent life forms....
I
think the disconnect there is that you are envisioning a guy with a thick
radiator, who changes nothing other than the radiator and minimal duckting
changes. But clear your mind, and picture this:
Two guys (twins)
Two Airplanes identical in every way except the radiator, inlet ducting
and exit ducting. Frontal drag is the same, needed heat rejection is the
same etc.
The ducting systems are designed such that the mass airflow is
identical. ie, the thin radiator version has smaller inlet and outlets
but a larger plenum or whatever is necessary so that the airflow through the
two radiators is identical.
GIVEN identical airflow, identical rad volume,
inlet air temp and identical coolant temp... the Delta T on average is the
SAME.
The air, although going slower in the thinner rad, spends exactly the
same amount of time in each rad. (same amount of time for heat transfer and
same average temp rise of the air and same average temp drop of the coolant)
Now, I will admit that heat transfer is more efficient in the higher
velocity thick rad. But it is not squared, or even linear. It is
less than linear. (ie, as velocity increases in both rads heat transfer
becomes less dependent on velocity - they are both turbulent flow and you
cannot achieve a lower temp than the ambient air. At some point the
faster speeds become detrimental due to heating from friction) I submit
that the the speeds of most of our aircraft and relative dimensions of
radiators, both radiators are in turbulent flow and increased efficiency of
heat transfer is nearly negligible.
So in the end the difference
between these two systems is that the thin rad has smaller velocities across
the rad, smaller pressure drop, improved pressure recovery, higher exit air
velocity, same delta T, same heat transfer, same exit air temp, smaller inlets
and outlets, and larger volume sucked by ducting. All of which is
relatively negligible in our applications.
Thanks for the fun
discussion Ed. Anxiously awaiting reason for Delta T difference between
rads but will probably not buy it. (but still realize there is a very real
possibility that I am wrong).
Dave Leonard
On Nov 13, 2007 8:45 PM, Al Gietzen < ALVentures@cox.net> wrote:
Ed;
I swore after the
last one, it was my last on this subject. But, OK; one more. This is all a
bit like the three blind men describing an elephant. None are
necessarily wrong, just different points of view.
Let's look at it
from a sort of systems approach: First I calculate
the heat load. Then I determine the mass flow rate of air I need using
a certain delta T; one that I know is 'reasonable' - or even optimum
(radiator size and weight, etc.), based on other analyses which we
haven't/won't go into, but a number between 50 and about 80F is good based
on the OAT and coolant temps we deal with. We already know how to
compute the mass flow rate.
1st addressing your
statement regarding the inlet sizing.
Assuming that selecting your
inlet opening size controls mass flow is incomplete. It is the
total pressure loss for the entire duct (and core) which combined with the
available freestream kinetic energy (due to velocity) that
determines mass flow. I can make changes to any of the intake,
diffuser, core, and outlet and make changes in the mass flow - so its not
just the inlet.
Then I compute
the required opening area of my ram scoop based on the velocity of the
incoming air; the speed of the airplane at the design point; generally a
fast climb. Area x velocity = mass flow rate. The ram scoop is not
necessarily 100% efficient, so add maybe 10% to the area. Now, yes,
that is incomplete; but on first order, unless I do something wrong that
causes the scoop to spill air (like poor diffuser design or too thick a
radiator), then it is complete - that is the mass flow rate. Only
changes I make that cause air to spill around the scoop is going to change
that; so on this macro view I can ignore the details of the diffuser of
which you are more expert than I.
Now if I have a
well designed diffuser with an area ratio of say about 4+ I know that the
pressure recovery will be greater than the pressure drop through the rad, so
all that air is going through. Right? Well; here is where we have to start
thinking about both the thickness and the density of the core because the
pressure drop through the core must be less than the amount of pressure
recovery – and is why I say that talking thickness without specifying core
density and diffuser ratio is 'incomplete' because they depend on one
another.
But back to fist
order; given the conditions of adequate pressure recovery the flow rate is
fixed.
2nd regarding the deltaT "swap"
I made:
I do not agree that you need to
compare on the basis of sam e mass flow or deltaT - the only factor that
really matters is that the needed heat be removed. There are a large
combinations of mass flow and deltaT that will remove X amount of heat.
True,
but:
The design point
heat load is fixed, the mass flow rate is fixed; therefore the delta T is
fixed (see formula used for computing the the flow rate in the first
place.)
Now, given these
conditions; we can look at thick vs thin. We can have a large frontal
area, slow velocity through the core, and have pressure left over for
accelerating the air back toward free stream (along with the heat energy
that we pick up which gives velocity by expansion); or we can make the core
thicker up to the limit of the pressure recovery that we have achieved; and
have no remaining pressure at the core exit. But keep in mind that the have
a fixed scoop area for the speed we designed for; and the frontal area of
the rad is the other area in the diffuser ratio; so making the rad smaller
and thicker both cut into the available pressure recovery. When we reach
that limit where we have used all the available pressure recovery; we have
no pressure left over to accelerate the air back to something closer to that
at which it came in.
Because of the
effects of the velocity on heat transfer, as well as pressure drop, there
does happen to be a rad pressure drop (thickness) that results in minimum
drag – just as there is a corresponding delta T that results in minimum mass
of the core (all other things equal; i.e., properly
designed).
Now the big
caveat - It is clear here that to take advantage of the less pressure drop
in the thin rad to reduce drag, we have to have an exit configuration that
efficiently re-accelerates the air. If not, or if we going to release
the exit air into the free stream at a negligibly small velocity, then it's
a different ball game. Then from a drag standpoint there may be little
difference, and that using a radiator thickness (pressure drop) that
exhausts the pressure recovery, may be the way to go for fitting into a
confined space.
So there ya go;
cooling system design in a nutshell; minus all the magicJ
.
Al
G
--
David Leonard
Turbo Rotary RV-6 N4VY
http://N4VY.RotaryRoster.net
http://RotaryRoster.net
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