Another fact is a tangential muffler is a vortex generator.
This vortex is forced to flow aft out of the can due to exhaust
gas pressure build up in the front of the can.
Paul Lamar
"Robinson, Chad" wrote:
Could somebody please explain what the purpose of cooling the
exhaust
gases is? I understand the muffling aspects, but I don't understand
why the cooling effect is valuable.
Regards,
Chad
It is not the exhaust gases we are interested in cooling. It is the
steel the muffler is made out of :) Cooling the gases does not reduce
the noise. Something like the opposite happens. Due to viscosity and
friction
effects the steel is heated by the noise over and above the 1700
degree F
temperature of the exhaust gases and the noise is reduced in the
process.
In theory this design could reduce the noise by almost half exiting
the
can.
This is because of the concentric shape of the can doubling the steel
area the exhaust gases are rubbing against.
At the same time all the steel sheet exposed to the gas in the can
would
also be exposed to cooling air on it's opposite surface.
If this works it could be a revolution in low back pressure mufflers.
Paul Lamar
Mark Wrathall wrote:
Nicely drawn Bill.
I don't really buy the cooling effect. The greatest risk are the welds
where
the stacks enter the muffler. This design does nothing for lowering the
EGT
until the tailpipe, and no one is having issues with unrestricted
tailpipes
cracking.
How do you support that X inch long cantilevered pipe? If you weld
struts
into the muffler-tailpipe transition piece to support it, they will fail
(hot vibrating gas, thin section to avoid backpressure, strong thermal
gradient, corrosive environment). If you don't support it, a weld on the
cooling inlet end will fail, giving you a hell of a bad day (fire risk).
Do we have a single example of a muffler with any contents surviving the
Aviation duty cycle 13B gas path?
I thought centering the outlet on a tangiential muffler causes the
throat to
choke due to the vortex?
Mark
"I don't really buy the cooling effect." Do you mean the air flowing over
the
steel will not cool it or do you mean cooling the exhaust gas reduces the
noise?
Yes Tracy is having good luck with a .073 thick Spin Tech muffler but it
is outside the cowl and causing drag. His early tangential muffler lasted
650 hour and it was only .035 thick. Dave Atkins has flown a thin .035
Inconel tangential muffler for years.
Good point on the inner tube supports. Thick Inconel is the only way to
go on those. Perhaps they can be welded in at an angle to enhance the
vortex.
If they are not welded to the inner tube the thermal gradients would be
less
of a problem.
As far as outlet choking is concerned this is an improvement as the outlet
is annular and closer to the main rotating air mass in the vortex.
I think it is well worth making a test piece and I am prepared
to pay for it if Ed Klepeis would read his email once in an while :)
Paul Lamar
Mark wrote:
The cooling air is only flowing over and cooling metal which is part of the
inductor tube, and the tailpipe. The outer muffler case is going to be just
as hot as as with a conventional muffler. The most highly stressed weld
(stub to muffler) is not cooled. Based on gas turbine experience, the huge
temp gradients in this worry me. Great that someone has volunteered to make
one, and that the test bed is getting closer. This design is simple enough
that it should be feasible, but you may have to go through a few iterations
of inductor support, and isolate the flange and stubs from the weight of the
VAM.
As you have pointed out with the Cooling system. It is better to have an
overcool, safe system which is reliable from day one, than to have a pretty,
sleak cowl and reliablity problems. I think Ed and Tracy have applied the
same philosophy to the exhaust. Better to run striaght pipe from the flange
overboard, and then hange the muffler in the breeze, where a failure has
minimal consequences.
The VAM is an attempt at an optimized system, which should be developed on
the ground, analog to the LAEV Prop.
Tracy has pointed out that his first 650H were flown with ~140hp (Carbs) and
~170Hp) (EFI) and that he operated at very low cruise power settings at that
time. His 0.035" muffler did not survive the much higher HP of the current
installtion, which is more in line with what most are building.
Mark
On the other hand Ed Anderson is using very thick .140 SS 304 pipe that hardly
discolors so there must be some optimum thickness to life trade off going
on here. My guess is .065 would go through anything.
For the test piece I am advocating .049 SS 321 rather than .035.
Paul Lamar
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