Keith Moore wrote:
Paul/Perry:
Perhaps you may wish to consider the "heat pipe" concept. Instead of the
tubes filled with water, seal the tubes and add just a little distilled
water and ammonia. Pull a vacuum until you get down to the vapor pressure
of the mixture.
Now you have a very high efficiency (transfers heat at the speed of sound)
&
"light weight" heat transfer device!
Position the engine water manifold at the bottom (low side - rotate your
sketch 90 degrees, clockwise). The water would enter one side and return
in
the other side. The engine water manifold should have some surface area
contact with the heat pipes. The heat from the engine will quickly boil the
liquid to steam which rises up the pipe. And as the steam cools
(condense), the liquid returns to pick up more heat.
A similar but simpler fabrication scheme would be formed from two sheets of
aluminum, each with a series of dimples to keep them apart, rolled into the
diameter you want, and then seal welded on each edge.
If you are serious, there is some heat transfer analysis necessary.
Regards,
Keith "BlackHat" Moore
Long Easy
----------------
Having worked with heat pipes in the past, a couple of comments.
1. The achilles heel of this scheme is the heat transfer from the water
manifold into the heat pipe. You will likely be surface area limited.
2. You don't want to use a water-ammonia mixture. Water ammonia makes a good
refrigerant for an absorption heat pump, because you boil the fluid in one
location, and recombine them in another. The recombination *raises* the
temperature of the fluid, which is then cooled in a separate heat exchanger.
Working in a heat pipe this would be counter productive. You want pure fluids
in a heat pipe -- ammonia is used in the 70-80 foot long heat pipes for the
Alaska pipeline, but it is *anhydrous* ammonia.
3. The heat transfer up the heat pipe is at the speed of the flow of the
fluid (vapor) through the pipe, not at the speed of sound. It is however,
impressively fast.
Bill Schertz
KIS Cruiser # 4045
Paul,
Thanks so much for bringing up the surface area calculations for radiators.
Being a solar engineer I had been thinking of efficient heat exchangers,
ammonia and other more exotic working fluids, like Perry, Bill and Keith
here but....
Your common sense and George's remarks brought me back down to earth.
"Don't fix what ain't broke." IMHO Rotaries will be accepted by the
general aviation community, and the makers of the SeaWind:) more readily
when they are proven to have a effective and reliable cooling system. Don't
get me wrong, I love a sleek design. We all do. A safe one will bring us
home easier though.
I think I'll wait until I am flying before playing with alternative cooling
systems.
Perry perhaps you can save yourself a lot of time by if you don't have
access to an aerodynamics modeling program to build a cheap plywood wind
tunnel and l what happens to the air flow characterics within the duct if
lined with turbulenace producing pipes? Smoke and pipe dreams:o) Sorry. \
Doug in Japan
The cooling system design has been a problem for all liquid cooled aircraft engines
built since WW II. Continental has a fine liquid cooled aircraft piston
engine in the Voyager series. Good enough to get Dick and Jenna around
the world un refueled. However, Walter Extra could not figure out how to
get the six cylinder version to cool in the Extra 400. That remains a big
stumbling block for the rotary as well. We just need to keep plugging away
on the problem and get some airplanes flying with perfectly adequate cooling
systems that work under all ambient conditions and that are extra reliable.
Any commercial or certified liquid cooled aircraft engine manufacturer piston or
Wankel will have to address these problem for each type of aircraft because you cannot
expect the aircraft manufacturer to take on the task when they can buy an
air-cooled engine and simply bolt it in. They will also have to prove
the liquid cooled engine is superior enough to over come the drawbacks
of installing a liquid cooling system. I think the Wankel engine has that
superiority. I don't think a liquid cooled AC piston engine is enough
superior. There is no doubt it is somewhat superior.
As far as surface heat exchangers are concerned they suffer the same problems
as thick heat exchanger suffer from. As the air flows along the surface it picks
up heat reducing the all important difference in temperature between the
air and the metal. A further problem is the boundary layer build up
as the air flows along the surface. Air has adhesion and viscosity.
The air molecules immediately adjacent to the surface move at the same
speed as the surface. As you get further and further away from the surface
the speed of the molecule picks up. This stationary or slow moving layer
of air gets thicker and thicker along the surface as the airplane moves
through the air. This drastically reduces the overall heat transfer
for a given surface area. The ideal heat exchanger would be infinitely thin
and have infinite frontal area.
For these and other reasons surface heat exchangers are not too promising.
Paul Lamar
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