Sponsor Message:
Aviation Technical / Operations Forum
My Starred Topics | Profile | New Topic | Forum Index | Help | Search 
Basic Combustion Question  
User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Posted (10 years 5 months 6 days 6 hours ago) and read 1537 times:

At least I think it should be really easy. All this time I've assumed that because it doesn't happen in reality, there is no problem, but:

ASSUMPTION: The combustion pressures are greater than intake pressures [after ignition].

QUESTION: If the above is correct, why doesn't the combustion go back up the intake and the outlet at the same time?


I would figure with engine systems that have a physical block (compressor turbine or a intake value in 4-cycle piston system) will prevent flow going out the front.

What about a ramjet? That is actually what I am getting at. What prevents forward outflow? The incoming dynamic pressure is less than combustion pressure such that a net force results over the outlet area, correct?


The meaning of life is curiosity; we were put on this planet to explore opportunities.
13 replies: All unread, jump to last
 
User currently offlineKlaus From Germany, joined Jul 2001, 21521 posts, RR: 53
Reply 1, posted (10 years 5 months 6 days 5 hours ago) and read 1516 times:

Lehpron : What about a ramjet? That is actually what I am getting at. What prevents forward outflow? The incoming dynamic pressure is less than combustion pressure such that a net force results over the outlet area, correct?

No, I don´t think so. The total pressure (static+dynamic) at the combustion chamber inlet is greater than the pressure exerted by the combustion process (at least partially due to inertia at high speeds). As far as I understand, that´s the whole deal with (SC)RamJets: At lower speeds you need a turbocompressor to prevent a stall; Only at very high speeds the ram air compression alone is sufficient to do that.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 2, posted (10 years 5 months 6 days 5 hours ago) and read 1508 times:

As I understand my physics, the intake pressure HAS to be greater than the combusion area pressure, or thrust would not be generated.

However, energy has to be added somewhere, and combustion does this, in both turbofans and ramjets.

I'm sure the more technical members can explain this better.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineKlaus From Germany, joined Jul 2001, 21521 posts, RR: 53
Reply 3, posted (10 years 5 months 6 days 5 hours ago) and read 1505 times:

In a SCRamJet, the combustion process is supersonic anyway; I don´t think there would be even a theoretical chance of a stall any more as long as the flow speed is kept that fast.

User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 4, posted (10 years 5 months 6 days 2 hours ago) and read 1488 times:

Most combustion processes in engines can be assumed to take place at constant pressure. See "brayton cycle": http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ThermLaw2/HeatEngines.html

User currently offlineKlaus From Germany, joined Jul 2001, 21521 posts, RR: 53
Reply 5, posted (10 years 5 months 5 days 23 hours ago) and read 1474 times:

MITaero: Most combustion processes in engines can be assumed to take place at constant pressure.

In the sense of "continuous" at constant load? Sure. The point here were the different (even if individually constant) pressures at different places within the engine.

And - correct me if I´m wrong - one of the reasons for (turbine- or ram-effect-driven) compressors in a jet engine was to keep the combustion exhaust from pushing forward through the compressor instead of aft through the nozzle or turbines, right?


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 6, posted (10 years 5 months 5 days 23 hours ago) and read 1465 times:

Right. There's no thrust produced if pressure isn't lower at the ass end.
.
.
.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6120 posts, RR: 14
Reply 7, posted (10 years 5 months 5 days 18 hours ago) and read 1443 times:

If I remember correctly, the pressure will be the same, but since the air that leaves the combustion chambers is of a higher temperature, it will need much more area to remain at the same pressure. This is why jet engines suck in less air than they put out, as the air that is taken in, is the same mass of air as exhuasted, but becuase it is heated, it expands in order to maintain it's original pressure, thus exiting at a higher speed.


Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 8, posted (10 years 5 months 5 days 17 hours ago) and read 1434 times:

>The point here were the different (even if individually constant) pressures at different places within the engine.

The combustor inlet and exit are two different places in the engine. But I understand what you're talking about.

Flows can still move from low to high pressure. This is called an adverse pressure gradient; it happens all the time. You guys are right to be concerned, as separation is much more likely in the compressor due to a higher pressure at the back end of the compressor. However, because the incoming flow has enough momentum to overcome the adverse pressure gradient, the flow doesn't come close to being reversed.

Starlionblue, there is always thrust produced if the momentum of the exiting air is greater than the momentum of the air coming into the engine. There are nozzles that expand the exhaust to atmospheric pressure (called "ideally expanded" nozzles), and thrust is definitely produced.

Goldenshield, there's no way for an engine to suck in less air than it puts out; this would violate conservation of mass. The inlet and exit mass flow rates are the same (ignoring the small fuel mass flow rate). The increment in momentum of the exiting air causes the thrust.

[Edited 2004-07-26 10:07:48]

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (10 years 5 months 5 days 8 hours ago) and read 1369 times:

Physics aside (for the benefit of those of us not well schooled in physics) don't forget the role of velocity in this. All jet engines have a velocity to the gas stream and it would take a lot of pressure to back up against this.

There was a variant of the ramjet, called a pulse jet. It used shutters at the intake which were momentarily closed by the combustion-induced overpressures. It would open, admit some air for the next combustion cycle, and close while that was taking place. Produced a characteristic pulse sound.

Extra points for anyone knowing where it was used.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 10, posted (10 years 5 months 5 days 7 hours ago) and read 1361 times:

>Physics aside (for the benefit of those of us not well schooled in physics) don't forget the role of velocity in this. All jet engines have a velocity to the gas stream and it would take a lot of pressure to back up against this.

That's right (that's what I meant by momentum, which is just the product of velocity and mass).

>Extra points for anyone knowing where it was used.

I saw an episode of JAG where they used it over Korea on a secret mission, and for some reason they let the (lawyer) main character guy fly it. Hilarious.


User currently offlineVorticity From United States of America, joined May 2004, 337 posts, RR: 5
Reply 11, posted (10 years 5 months 5 days 7 hours ago) and read 1358 times:

The germans used the pulse jet in their V-1 buzz bombs that they used to attack England. They had a very characteristic sound because of the pulse jet.

Anyway, compressors play a big role in all of this, compressing the air that goes into the combustion chamber. Ramjets and Scramjets attempt to compress the air without using compressor turbines. Lots of solutions for flight.



Thermodynamics and english units don't mix...
User currently offlineGoldenshield From United States of America, joined Jan 2001, 6120 posts, RR: 14
Reply 12, posted (10 years 5 months 5 days 6 hours ago) and read 1342 times:

If only I could edit what I wrote. > Sad

It should read, "This is why [it seems] that jet engines suck in more air than they put out."



Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 8
Reply 13, posted (10 years 5 months 5 days 5 hours ago) and read 1337 times:

Heh, it's ok, I knew what you meant, just didn't want other people to get confused.

Top Of Page
Forum Index

Reply To This Topic Basic Combustion Question
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...
Air Compression And Combustion Chamber Question posted Fri Oct 6 2006 01:29:43 by Blackened
Probably A Basic Question, But Please Answer! posted Wed Nov 7 2001 06:33:11 by 777-300 FAN
Very Basic Question On Hydraulics posted Thu May 24 2001 20:59:07 by Bio15
Airborne Express 767-200: Loading Question posted Wed Dec 6 2006 16:09:58 by Jonathan-l
An XB-70 AV-2 Question. posted Tue Dec 5 2006 23:59:56 by Starglider
Bombardier Q400 Field Performance: Question posted Sun Dec 3 2006 17:40:48 by A342
Continental 737-300 Cockpit Question posted Fri Nov 24 2006 08:26:31 by Artsyman
Headwinds Question posted Sun Nov 12 2006 10:36:39 by FCA787
B767-200 Fuel Consumption Question posted Sun Nov 12 2006 07:06:31 by Jetline
Concorde Gear Question posted Tue Nov 7 2006 18:38:10 by EHHO

Sponsor Message:
Printer friendly format