Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
How Are Compressors 'made' To Work?  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (8 years 4 months 3 weeks 1 day 16 hours ago) and read 4592 times:

If compressors are being able to take a larger cross section of air and reducing down while also reducing the velocity of the air, such that the inlet to burner is on the order of 50m/s while the pressure ratio increases upto 20 times...I get the impression of a divergent section while most cut-away views I've seen appear to me as convergent.

What am I missing?


The meaning of life is curiosity; we were put on this planet to explore opportunities.
8 replies: All unread, jump to last
 
User currently offline727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0
Reply 1, posted (8 years 4 months 3 weeks 1 day 13 hours ago) and read 4558 times:

Quoting Lehpron (Thread starter):
I get the impression of a divergent section while most cut-away views I've seen appear to me as convergent.

Do you mean you think the volume between stages should increase rather than decrease? I'm curious as to what gives you that iIMpression as COMpression involves making a constant mass of gas fit into a space with a decreasing volume. The fact that the velocity of the gas is reduced is explained nicely by Bernouli.


User currently offlineDC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 2
Reply 2, posted (8 years 4 months 3 weeks 1 day 13 hours ago) and read 4551 times:

Its very simple, what you see in the engine cut away views is that each stage of compression comprises a larger blade disc with smaller blades. The overall shape of the compressor discs are divergent while the airflow path is convergent.

In layman terms, as you pass each stage of engine compression, you're stuffing the air into a smaller opening.



Forums.AMTCentral.com
User currently offline777236ER From , joined Dec 1969, posts, RR:
Reply 3, posted (8 years 4 months 3 weeks 1 day 8 hours ago) and read 4501 times:

Quoting Lehpron (Thread starter):
If compressors are being able to take a larger cross section of air and reducing down while also reducing the velocity of the air, such that the inlet to burner is on the order of 50m/s while the pressure ratio increases upto 20 times...I get the impression of a divergent section while most cut-away views I've seen appear to me as convergent.

If you reduce the speed of the flow through the compressor, stages will surge. But yes, you generally want to recover as much as the dynamic head as possible before the combustor.


User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 4, posted (8 years 4 months 3 weeks 1 day 5 hours ago) and read 4455 times:

Convergent vs divergent ducts, nozzles vs diffusers, these do nothing but trade dynamic pressure (velocity) for static pressure. Total pressure remains constant (minus any losses). The entire point of a compressor is to increase total pressure, which can only be done by using a work input. A divergent duct will not increase total pressure.

You ask why is a compressor convergent from front to back. This is simply answered that since mass flow is constant, but density increases, then the exit area must be smaller compared to the inlet area. Now as DC-10Tech pointed out, if you look at the individual compressor stages, the flow path actually does expand, just not in the axial direction.

Why?

Compressors work by using alternating rows of rotor blades and stator vanes.
Every row of stators will turn the flow away from the axial direction. Because the flow is turned to the side tangentially, the exit area of each stator is actually larger than the inlet area, which means you get a rise in static pressure and a drop in velocity. However, stators don't move so no work is put into the flow, so total pressure remains constant (once again, minus any losses).

Rotors then turn the flow back to the axial direction. But because they are moving, work is put into the flow. Two things now happen in the rotors. The rotating blades increase the dynamic pressure of the flow. Rotors also turn the flow in a way that like the stators, relative to the flow they have exit areas larger than inlet areas, which increases the static pressure. The sum of the these two pressure rises is your increase in total pressure.

Here's a diagram comparing the diffuser that you are thinking of to the cascade type of diffuser in a row of rotor blades. Both have exit areas larger than inlet areas and will increase the static pressure.
Big version: Width: 496 Height: 358 File size: 30kb



The alternating rows of rotor blades and stator vanes work by allowing you to put work in to increase total pressure (rotor), then convert this total pressure rise to an increase in static pressure (stator). Net result of the compressor is an increase in pressure and a decrease flow velocity.
Big version: Width: 320 Height: 346 File size: 8kb


[Edited 2006-06-13 16:59:39]

User currently offlineEasternSon From United States of America, joined Jun 2006, 669 posts, RR: 1
Reply 5, posted (8 years 4 months 3 weeks 1 day ago) and read 4362 times:

Thanks, DarkBlue. An excellent, thorough and well-worded response.

I have to say this is a pretty good example of someone asking a relatively simple question and getting a great response from some knowledgeable people, rather than being ridiculed.

Regards,



"The only people for me are the mad ones...." Jack Kerouac
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 6, posted (8 years 4 months 3 weeks 9 hours ago) and read 4280 times:

Thanks DarkBlue, very nice explanation! When Lehpron asked the question, I didn't know how it worked either.  Smile

I'd like to add something...

From a thermodynamic point of view, here are the formulae that happen in the process:

Big version: Width: 736 Height: 300 File size: 21kb
Thermodynamic formulae for compressor process


I tried to solve to see what happens. Solving for P2 = 100000, T2 = 270, w2 = 100, A2 = 2, A3 = 1, gamma = 1.4, cp = 1005, l (compression ratio) = 20, gives:

P3 = 2129078, T3 = 647, w3 = 22.5

I don't know if these values resemble to real values, but the equations are correct I believe. The ''t'' subindex means stagnation (or total, however you call it). Note that ideal gas has been supposed, and the efficiency is the unity (that is, isentropic process. Actual efficiency is about 0.9 I think). Here the areas and the compression ratio have been fixed, but you could use other ones as known variables, such as the work. If work rate is 0 (diffuser), T3t = T2t, and with the same areas you see a drop in static pressure, P3 = 62271 and an increse in velocity, w3 = 280.5, which is what you could expect for a convergent section. The input of work is what makes total pressure rise, while velocity can drop.



Where there's a will, there's a way
User currently offlineGrandTheftAero From United States of America, joined Nov 2003, 254 posts, RR: 5
Reply 7, posted (8 years 4 months 3 weeks ago) and read 4201 times:

Quoting DarkBlue (Reply 4):
Here's a diagram comparing the diffuser that you are thinking of to the cascade type of diffuser in a row of rotor blades. Both have exit areas larger than inlet areas and will increase the static pressure.

Looks like you've taken Dave Wisler's class too!  Wink


User currently offlineDarkBlue From United States of America, joined Sep 2003, 233 posts, RR: 10
Reply 8, posted (8 years 4 months 2 weeks 6 days 6 hours ago) and read 4106 times:

Quoting GrandTheftAero (Reply 7):
Looks like you've taken Dave Wisler's class too!

Yep, after posting I realized I probably should have given him credit for the picture (and for the education in compressor design)  Smile


Top Of Page
Forum Index

Reply To This Topic How Are Compressors 'made' To Work?
Username:
No username? Sign up now!
Password: 


Forgot Password? Be reminded.
Remember me on this computer (uses cookies)
  • Tech/Ops related posts only!
  • Not Tech/Ops related? Use the other forums
  • No adverts of any kind. This includes web pages.
  • No hostile language or criticizing of others.
  • Do not post copyright protected material.
  • Use relevant and describing topics.
  • Check if your post already been discussed.
  • Check your spelling!
  • DETAILED RULES
Add Images Add SmiliesPosting Help

Please check your spelling (press "Check Spelling" above)


Similar topics:More similar topics...
Are Wings Shapes Made To Store Fuel? posted Tue Mar 15 2005 23:53:37 by A380900
Advice On How To Work For UAL Management? posted Sat Aug 18 2001 01:30:48 by *HighFlyah*
How Are (difficult) Liveries Applied To An A/c? posted Thu Jul 5 2001 14:13:57 by Aerokid
How Much Longer Are VORs Going To Last? posted Tue Feb 6 2001 11:03:31 by Chopper
How Easy Is It To Change Config. Of A Plane? posted Thu Oct 19 2006 04:08:48 by Brenintw
Subjects Needed To Work In Aviation posted Mon Oct 9 2006 10:41:10 by PureKiwi
Most Annoying Bizjet To Work MX On? posted Fri Feb 3 2006 05:38:54 by KDTWFlyer
Are Trijets Easier To Fly In Engine Failure posted Fri Jan 27 2006 02:44:05 by Newagebird
How Are Airplanes "washed" posted Sun Dec 4 2005 15:44:27 by JAGflyer
How Are Planes Tested After Maintenance Checks? posted Sun Aug 21 2005 00:06:26 by Comorin

Sponsor Message:
Printer friendly format