Received-SPF: none
 receiver=logan.com; client-ip=211.29.132.191; envelope-from=lendich@optusnet.com.au
Message-ID: <001301c538b5$427cfa50$6e441fd3@george>
From: "George Lendich" <lendich@optusnet.com.au>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-853756@logan.com>
Subject: Re: [FlyRotary] Re: Cooling -Learned a lot
Date: Mon, 4 Apr 2005 11:25:59 +1000
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

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
>