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Exhaust Velocity  
User currently offlineBruce From United States of America, joined May 1999, 5046 posts, RR: 15
Posted (12 years 2 months 1 week 2 days 19 hours ago) and read 1403 times:

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
8 replies: All unread, jump to last
 
User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 6
Reply 1, posted (12 years 2 months 1 week 2 days 15 hours ago) and read 1390 times:

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


User currently offlineTomcat From Belgium, joined Sep 2000, 161 posts, RR: 0
Reply 2, posted (12 years 2 months 1 week 2 days 5 hours ago) and read 1371 times:

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.


User currently offlineNikes From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 2 months 1 week 2 days 5 hours ago) and read 1368 times:


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


User currently offlineBroke From United States of America, joined Apr 2002, 1322 posts, RR: 3
Reply 4, posted (12 years 2 months 5 days 10 hours ago) and read 1313 times:

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.

User currently offlineDc10hound From United States of America, joined Dec 2000, 463 posts, RR: 5
Reply 5, posted (12 years 2 months 5 days ago) and read 1288 times:

>>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.."
User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 6
Reply 6, posted (12 years 2 months 4 days 15 hours ago) and read 1273 times:

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


User currently offlineLeezyjet From United Kingdom, joined Oct 2001, 4041 posts, RR: 53
Reply 7, posted (12 years 2 months 4 days 1 hour ago) and read 1252 times:

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"
User currently offlineSailorOrion From Germany, joined Feb 2001, 2058 posts, RR: 6
Reply 8, posted (12 years 2 months 3 days 16 hours ago) and read 1244 times:

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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