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Hi George,
Well, that is almost the idea I was trying to convey. There are two
velocities I am talking about here and I need to make that clearer.
But, before I attempt it - remember I am not an aerodynamic or
thermodynamic engineer nor do I play one on TV. This is my understand ing
based on what I have read and the result of my duct experiments. Experts are
welcome to jump in and correct any misunderstandings.
The two velocities are: 1. The velocity of the air mass flow and 2. The
velocity of the individual air molecules.
First, the velocity of the air mass flow in a duct system. The mass air
flow (p*A*V) is constant and since p (air density) is constant at these
airspeeds that means the product of area and velocity (A*V) must be constant
ANYWHERE in the system. So once the diffuser does its thing (the Area of
the diffuser (Ad) is larger than its inlet (AI), so if Ad>AI, then Vd<Vi....
Vd (velocity diffuser) must be lower than Vi(velocity of air at inlet).
Basically if area increases then velocity decreases and vice versa. This
is the mass flow velocity which changes throughout its flow through various
parts of the system. As the systems flow area changes so must the mass flow
velocity
The second velocity I was speaking of was associated with individual air
molecules which is responsible for pressure. Pressure is the result of the
average momentum (air molecule Mass*Velocity) of the air molecules. The
velocity of individual air molecules is much higher than the mass flow
velocity (high in the mach numbers). This is the velocity of the individual
air molecule.
Think of a bag of air molecules flowing at velocity X (Mass flow velocity)
while each molecule inside the bag are whizzing around at velocity
4*X(molecule velocity). The average velocity of the molecules inside the
bag is what contributes to the pressure on the walls of the bag - or fins of
the core. The mass flow velocity is the velocity of the bag through the
core. Two different velocities.
Higher (molecular velocity) pressure means more contacts (per unit time)
between the air molecules responsible for carrying away the heat from the
core fins/walls. The number of air molecules (density) has not changed,
just their velocity which has increased due to the energy in the incoming
airstream. Remember the diffuser slows the velocity but increases the
pressure and does nothing to the density (as subsonic speeds).
Have I confused everyone enough for one evening? This is my interpretation
and no other party should be blamed.
Ed
----- Original Message -----
From: "George Lendich" <lendich@optusnet.com.au>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Sunday, April 03, 2005 9:25 PM
Subject: [FlyRotary] Re: Cooling -Learned a lot
> 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
> >
>
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
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