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Ed,
I am more than a little confused here; are you saying that the air molecules
have a higher velocity through the Rad ( fin/walls) -because the energy
transfer from the incoming air molecule velocity. Even though the density
remains the same ( in the diffuser duct) - which is a constant for
temperature also.
If I have this right then it reminds me of the theory of 'For every action
there is an equal an opposite reaction' - demonstrated by the hanging balls
novelty, demonstrating the transfer of energy.
George (down under)
> 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/
> > >> Archive: http://lancaironline.net/lists/flyrotary/List.html
>
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
>
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