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Bruce
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Exhaust Velocity

Mon May 20, 2002 1:01 pm

Here's one for the die-hard technical types. On a typical 757, what is the velocity of the air/exhaust as it comes out of the cone in the engine? At cruise power level, and at takeoff thrust level?


bruce
Bruce Leibowitz - Jackson, MS (KJAN) - Canon 50D/100-400L IS lens
 
SailorOrion
Posts: 1959
Joined: Tue Feb 06, 2001 5:56 pm

RE: Exhaust Velocity

Mon May 20, 2002 4:46 pm

Normally, the flow speed in the throat of the nozzle is Mach 1 for takeoff and cruise power, although I'm not 100% positive that this is true for the bypass flow also, but I'd say yes.

SailorOrion
 
tomcat
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Joined: Thu Sep 28, 2000 4:14 am

RE: Exhaust Velocity

Tue May 21, 2002 2:23 am

Hi SailorOrion,

I'm not so positive about the fact that the speed of the bypass is sonic at the exhaust. Given the relatively low compression through the fan, I would say the speed is only subsonic (high subsonic).

Nicolas.
 
nikes
Posts: 38
Joined: Mon Aug 27, 2001 9:26 am

RE: Exhaust Velocity

Tue May 21, 2002 2:47 am


Well the fan tip speeds in todays airliners can approach in excess of Mach 1.2,
because the 757 nozzle is convergent, the speeds will either be Mach one (choked) or less.

Nikes
 
broke
Posts: 1299
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RE: Exhaust Velocity

Fri May 24, 2002 9:28 pm

On turbojet engines and in the hot stream of turbofan engines, the air leaving the exhaust can approach Mach 1 at takeoff power (Mach 1 is the ideal). The air leaving the by-pass never gets near Mach 1; that is one of the reason hi-pass ratio turbo-fans are so much quieter than turbojets. The tips of high by-pass ratio engines can get to Mach 1 are take-off, you can hear the sound if you know what to expect.
 
dc10hound
Posts: 460
Joined: Fri Dec 15, 2000 4:18 pm

RE: Exhaust Velocity

Sat May 25, 2002 7:29 am

>>I'm not so positive about the fact that the speed of the bypass is sonic at the exhaust. Given the relatively low compression through the fan, I would say the speed is only subsonic (high subsonic).<<

I can't find any info on jet blast in the B757 MM, but the A300 MM, Chapter
09-21-00-0 contains charts with the following data:

Take Off Power, 60 feet aft of Fan Nozzle Exit = 400 Knots (741 Km/Hr)

Take Off Power, 240 feet aft = 100 Knots (185 Km/Hr)

Take Off Power, 400 feet aft = 50 Knots (93 Km/Hr)

I'll try to scan the charts at home and post them.
"Eagles soar. But weasels never get sucked into jet intakes.."
 
SailorOrion
Posts: 1959
Joined: Tue Feb 06, 2001 5:56 pm

RE: Exhaust Velocity

Sat May 25, 2002 4:18 pm

As I said, I am not sure about the fan stream, but for the core stream, most modern aircraft engines have three 'critical cross sections' (cross section with an fluid Mach number of EXACTLY 1).

HPT inlet guide vanes
LPT inlet guide vanes
nozzle throat.

The reason for this is simple. A turbine running between two critical cross sections is running at a totally contant pressure ratio (at least when running above a certain thrust setting like during cruise, climb and takeoff), and can therefore be highly optimized for that pressure ratio. A compressor has a variety of different inlet pressures, velocities and temperatures which it has to work with, thats one of the reason why compressors have effiencies of 'only' about 85% to 90%, whereas latest generation turbines (running on next-generation engines) are around 97% or 98%.

For the fan stream, I think that with a high cruise speed, the exhaust velocity can be Mach 1 also, remember that the inlet Mach number is already around M 0.6 and cruise speed of a 747SP can be around 0.92. one should also consider, that if the Mach numbers for both core and bypass a 1.00, the velocity of the core can be easily 40 or 50% higher, usually around 500 meters / second

Exhaust mach number for a civil aerospace engine can never exceed Mach 1, since the nozzles cannot accelerate the steam further than Mach 1.

SailorOrion

P.S. Please excuse my crappy engieering english, hope it was understandible  Smile
 
Leezyjet
Posts: 3541
Joined: Sun Oct 14, 2001 7:26 am

RE: Exhaust Velocity

Sun May 26, 2002 7:11 am

I'd also like to know what the temperatures are of the exhaust gas at differant distances behind the a/c and differant engines. If anyone has this info, could you e-mail it to me or post it, I'd like it for a kind of project I have in mind.

Thanx.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"
 
SailorOrion
Posts: 1959
Joined: Tue Feb 06, 2001 5:56 pm

RE: Exhaust Velocity

Sun May 26, 2002 3:55 pm

They are described in the Aircraft Characteristics for Airport Planning manual, section 6.1 for each aircraft.

These can be found:
for the A380: http://www.airbus.com/products/a380_AC.asp

for Boeings and MDDs: http://www.boeing.com/assocproducts/aircompat/

SailorOrion

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