Author Topic: exhaust stacks & thrust (Read 2197 times)

hitech · « **Reply #45 on:** December 05, 2005, 09:52:42 AM »

150 lbs dosn't sound like much until you see what it does at top speeds.

Lets say a 2000hp eng at 400mph TAS with a prop eff of 0.9 = 1690 lbs thrust

Now that 150 extra pounds is almost 10% of the total thrust.

HiTech

bozon · « **Reply #46 on:** December 05, 2005, 10:11:12 AM »

yes HT, I neglected inlet compression as it's only a factor of around 2 and I was doing order of magnitude estimate. It still comes out very supersonic. gripen sent me an interesting NACA report and they really claim that the flow is supersonic with speeds of 1000 - 1800 ft/s. Then if I add the manifold pressure and corrected air density I fall into this range.

I'll have to read it through to see what the effect will be at the exit. I'm not very well versed in supersonic aerodynamics.

Bozon

Badboy · « **Reply #47 on:** December 05, 2005, 10:37:27 AM »

Hi Bozon,

Quote

Originally posted by bozon
I didn't care where did the energy come from or how it was accelerated. Supersonic (several mach) exhaust airflow doesn't seem to be realistic.

The nozzle velocity I quoted comes from a report by some very well known engineers, one of the authors was Dr Stanley Hooker, (look him up) of Rolls-Royce and the results come from flight tests, that have been validated by separate tests carried out by the RAE and Hawker Aircraft Co, on a Merlin engine installed in a Hurricane, and also in the high altitude test plant at Wright field USA. Those sources provide a large series of results and very comprehensive analysis. I have overlays of the graphs of those results and they all agree within very close limits. I also only quoted you a single value at one altitude, and the exhaust velocity varies significantly from sea level to 35k, peaking at 1901 ft/s at 30k, which might seem even more unrealistic to you, but the data has been so well corroborated and validated it is incontrovertible. Also, the exhaust ejector nozzles used in these tests were not optimised for thrust augmentation, so it is safe to assume, as others in this thread have pointed out, even more was possible.

Badboy

HoHun · « **Reply #48 on:** December 05, 2005, 01:35:28 PM »

Hi Badboy,

>one of the authors was Dr Stanley Hooker, (look him up) of Rolls-Royce

Maybe you already have this, but I just bought a copy and think some of the experts here would enjoy it:

"The Performance of a Supercharged Aero Engine" by Stanley Hooker, Harry Reed and Alan Yarker, ISBN 1872922112, ca. EUR 10 :-)

Regards,

Henning (HoHun)

Badboy · « **Reply #49 on:** December 05, 2005, 03:30:25 PM »

Quote

Originally posted by HoHun
Maybe you already have this, but I just bought a copy and think some of the experts here would enjoy it

Yep, picked up my copy from the Rolls Royce Heritage Trust about seven years ago, it is basically just a copy of the original report. Very good reading for anyone wishing to inform an aero engine performance model

Badboy

bozon · « **Reply #50 on:** December 06, 2005, 06:16:37 AM »

After talking to someone with a better knoledge of supersonic flow, it appears that supersonic flow within the exhaust tube is plausible. Delicately depending on the shape of the tube it is possible to develop high speed flows without loosing a lot of the energy to shocks.

The piston itself, at ~3000 RPM, cannot be supersonic (unless it has a travel or a few meters), so the picture we get is relatively slow but hot, slightly compressed gas accelerating by expansion down the exhaust tubes up to supersonic speeds, and creating thrust. Thus utilizing both the energy invested in compressing it at the inlet and the residual thermal energy of the burning.

There will be however a shock wave at the exit of the tube where the fast gases meet the subsonic surrounding flow, the effect of which is hard to estimate. Perhaps this is one reason these engines are so noisy.

Thanks to the people contributing to the discussion and to gripen for the NACA report. It's always fun to learn new things.

Bozon

hitech · « **Reply #51 on:** December 06, 2005, 09:37:51 AM »

bozon:

How would the exaust temp effect things when
Speed of sound with an egt of 1300 deg is over 2000 fps.

HiTech

Angus · « **Reply #52 on:** December 06, 2005, 11:52:39 AM »

No barrier break on the inside then, but how does it look as soon as it gets out into the totally frozen air at -40-50 celcius then??

Anyway it is a known fact that the design of the stubs could affect the speeds, and the exhaust design is quite a thing nowadays with racecars and bikes. (the turbine being at the other end if you see what I mean).

And HiTech: If you're in engines a friend of mine works on the development on Total Combustion Technology, called TCT. If you want some info I can find it, just ping it on this thread

bozon · « **Reply #53 on:** December 06, 2005, 02:37:14 PM »

Quote

Originally posted by hitech
How would the exaust temp effect things when
Speed of sound with an egt of 1300 deg is over 2000 fps.

1300 deg?! hmmm... THAT is my problem right there. I assumed much much lower temperatures and since the speed goes like the square root of T was not that significant. I estimated up to a factor of around 50% to change in speed of sound that agrees with temperatues of a few hundred degrees. Ignorance require too many assumptions to compensate for...
In that case, this is enough to solve the needed 1700 fps velocity in the tubes problem.

Thanks HT.
Bozon

Badboy · « **Reply #54 on:** December 06, 2005, 04:59:03 PM »

Quote

Originally posted by bozon
1300 deg?! hmmm... THAT is my problem right there.

The exhaust temp in the tests I've quoted are in the order of 1100 degrees, for which the speed of sound is just over 2400ft/s.

Badboy