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Dave, one point made in the stuff I have read on diffusers is that at our
speeds the air density is considered constant - no meaningful increase in
air density occurs. I also believed at one time that was the reason we got
more cooling with lower velocity (I mean it makes sense, greater density =
more mass to carry away the heat). But having been disabused of that idea
from the material I have read, I now have a different understanding of
what's happening.
While there is no meaningful density change in a diffuser (subsonic
velocities), there IS a pressure increase as Tracy pointed out. Increased
pressure can result from either an increase in density (which we are told
does not happen to any meaningful extend in diffusers of interest) and/or an
increase in temperature (again, no significant variation of temps in the
diffuser) OR an increase in the air molecules momentum mV (mass * Velocity).
It is the latter that appears to happen in a diffuser, the average momentum
of the air molecules in the diffuser is increased by the high energy
airstream entering the diffuser and therefore average velocity of the air
molecules in the diffuser being increased (since the mass of the molecule
does not change).
Since in effect these air molecules are in a fixed space, their higher
momentum (which is in the form of an increase in air molecule velocity)
results in an increase in the average number of impacts with the core walls
per unit time. Higher velocity means the molecule transverse the same
distance in less time resulting in more impacts per unit time. It is these
contacts and the resulting transfer of heat energy from the core fins/tubes
to these air molecules that is the major heat transfer mechanism as I
understand it (radiation being a very minor contributor at these temps).
Mass flow (p*Area*Velocity) is indeed fixed and does not change once the
flow begins in the duct/core system. Since the density is considered
constant that leaves only the area and velocity as variables. Their product
(A*V) must be also be a constant through the system A1*V1 = A2*V2 and you
would have the same density air at both locations A1 or A2 position. But,
the pressure of the air (constant density) may indeed be different and if
greater at position A2 (core passage) than A1 (say duct inlet) then greater
heat transfer would occur at position A2 due to the higher average number of
molecules impacting the walls of the containment. This even though the mass
flow is the same at both locations. Greater pressure =>more air molecule
contacts with core fins/unit time => more heat transfer/unit time. Of
course, if you have no mass flow velocity. then the same molecules would be
involved time after time and would very soon be saturated with all the heat
they can carry. But, since we have those molecules constantly replaced with
fresh molecules, heat continues to be transferred from core fins/walls to
the air.
At least that is what I think I understood. Bill, we need you here
Ed A
----- Original Message -----
From: "David Carter" <dcarter@datarecall.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, April 03, 2005 5:42 PM
Subject: [FlyRotary] Re: Cooling -Learned a lot
> I wonder - is it not more correct to say: Behind the diffuser, the
velocity
> will be slower, density higher, therefore there will be "nearly the same
> mass flow of air" thru the radiator, with same cooling, BUT "at slower
> speed, higher air density, and therefore less cooling drag"?
> - Drag is a function of velocity squared. The air density factor is
> not squared, thus we seek a reduction of drag by cutting velocity thru the
> rad.
>
> Bernie, you are the SR-71 PW engine air duct man - am I even close to
> expressing any useful and true info above?
>
> David Carter
>
> ----- Original Message -----
> From: "Ed Anderson" <eanderson@carolina.rr.com>
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Sent: Sunday, April 03, 2005 4:15 PM
> Subject: [FlyRotary] Re: Cooling -Learned a lot
>
>
> You are absolutely correct, Tracy.
>
> I did not make it clear but the diffuser does the velocity reduction and
> increases the pressure in front of the core by recovery of (some) dynamic
> pressure component of the air flow. This higher pressure in front of the
> core then results in an increased pressure differential across the core.
> This increase in pressure differential across the core, as you stated,
> actually speeds up the air flow through the core itself.
>
> My apologies for being less than careful on that point.,
>
> Ed A.
> ----- Original Message -----
> From: Tracy Crook
> To: Rotary motors in aircraft
> Sent: Sunday, April 03, 2005 12:24 PM
> Subject: [FlyRotary] Re: Cooling -Learned a lot
>
>
> Excellent summary Ed, correlates with my experience as well. Only
> exception I would take is in the following excerpt:
>
> "A good diffuser will reduce airflow
> velocity through the core which will reduces cooling drag. Pressure
> across
> the core is increased which further enhances cooling."
>
> A good diffuser will reduce velocity but the reduction occurs IN the
> diffuser, not through the core. As counter-intuitive as it may sound,
the
> velocity through the core is HIGHER than it would have been without the
> diffuser's velocity decrease (and pressure increase).
>
> Think about it this way, How could velocity through the core be reduced
> by a pressure increase? It isn't. The velocity at this point (through
the
> core) is increased.
>
> This is the single most misunderstood detail in liquid cooled engine
> systems.
>
> Tracy
>
>
>
> Subject: [FlyRotary] Cooling -Learned a lot
>
>
> Too right, Jerry
>
> My first 40 hours or so were in the marginal cooling zone. {:>). As
> other
> things in this hobby, there are so many variables that interact, that
> what
> may appear simply at first, is almost always a bit more complex. I
> say(Cooling Axiom 1) if you have enough cooling surface area and air
> mass
> flow then it WILL cool. However, you may incur a high penalty in
> cooling
> drag - which may not be as important for draggy airframes (such as
> biplanes)
> as it is to sleeker airframes. Also a system that adequately cools
an
> engine producing 150 HP may not cool an engine producing 180 HP.
> Picking
> your cooling design point is important. Optimizing for cruise and
your
> will
> be less than optimum for take and climb. Optimize for climb and you
> will
> probably have more cooling drag than required at cruise. Compromise,
> compromise - cowl flaps are sometimes used to try to have the best of
> both
> worlds.
>
> Some folks advocate a thinner, larger surface area core -which is
great
> for
> slow moving automobiles stuck in traffic with low dynamic pressure
> potential, but I think is not the optimum for most aircraft. Once you
> trip
> the airflow and turn it turbulent you have incurred most of the drag
> penalty. Larger surface area cores disrupt a larger airstream and
incur
> more drag. Yes, thicker cores produce a bit more drag than the SAME
> frontal
> area thinner cores. But, with a thicker core you can use a core with
> smaller frontal area.
>
> The NASCAR radiator's average 3" thick and on the long tracks where
> speeds
> are higher some even go up to 7" thick. My contention is their
> operating
> environment is more akin to ours than regular automobiles moving at
> slower
> speeds. You know that the NASCAR folks will spend $$ for just a tiny
> advantage - so clearly they don't use thick cores because it is a
> disadvantage. But, some folks will continue to point to the large thin
> radiators designed for environments with much lower dynamic pressure
as
> being the way to go. Will it cool? sure it will (Cooling axiom 1
> above).
> Is it the lowest drag option for an aircraft of the RV/TailWind type,
I
> am
> convinced it is not.
>
> The diffuser makes a considerable amount of difference and can made
the
> difference between a system that cools adequately and one which does
> not.
> The biggest culprit that lessens cooling effectiveness is turbulent
> eddies
> that form inside the duct due to flow detachment from the walls.
These
> eddies in effect act to block effective airflow through part of the
> core.
> So keeping the airflow attached to the sides of the diffusers is
crucial
> for
> good cooling from two standpoints. A good diffuser will reduce airflow
> velocity through the core which will reduces cooling drag. Pressure
> across
> the core is increased which further enhances cooling.
>
> I have gone from a total of 48 sq inches opening (total) for my two GM
> cores
> and that provided marginal cooling - down to 28 sq inches (total) with
> adequate cooling with an engine now producing more HP. Experimenting
> with
> the diffuser shape made the difference.
>
> The K&W book (Chapter 12) really provided the insight to how and which
> diffuser shapes provided the better dynamic recovery. The Streamline
> duct
> was shown to be able to provide up to 82% recovery of the dynamic
> pressure.
> Some folks reading the chapter misinterpreted the chart to show only
42%
> recovery where there chart was actually only showing the pressure
> recovery
> contribution due to the duct walls and did not include the
contribution
> due
> to the core. On the same chart, an equation (which apparently gets
> ignored)
> clearly shows that the TOTAL pressure recovery is 82%.
>
> I have taken the Streamline duct as a starting point, but since I do
not
> have the space to provide the 12-14" for a proper Streamline duct, I
did
> some "creative" things to try to insure that there was no separation
> even
> though my walls diverge more rapidly than the Streamline duct. Won't
> claim
> mine are as good as a Streamline, but they clearly are much better
than
> the
> previous design which basically just captured the air and forced it
> through
> the cores.
>
> FWIW
>
> Ed Anderson
> RV-6A N494BW 275 Rotary Hours (Plugs Up)
> Matthews, NC
> eanderson@carolina.rr.com
>
>
> ----- Original Message -----
> From: "Jerry Hey" <jerryhey@earthlink.net>
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Sent: Sunday, April 03, 2005 9:27 AM
> Subject: [FlyRotary] Re: phase I flight restrictions was:N19VX flys
>
>
> > It was not long ago that "cooling" was the major issue. Now it
seems
> > that we have learned enough to make several different configurations
> > work. I can't lay my finger on what it is we have learned but my
> > recommendation is to use smaller radiators and EWPs. Jerry
> >
> >
> >
>
>
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
>
>
>
>
> >> Homepage: http://www.flyrotary.com/
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