lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

Combustion [chamber] Questions

Sun Nov 28, 2004 2:33 pm

  • Is the fuel sprayed in an atomized liquid injection or like the pilot light in a home waterheater?


  • What is the size of the injector 'tap' and what is the approximate length of this 'flame'? (I know this may seem peculiar, I'm trying to mentally define chamber length as in how much distance a complete burn requires before it goes into the turbines.)


  • If it is a liquid injection, what happens if some unburned (or yet to be burned)fuel lands on the chamber's skin?


  • Does relatively cooler air pass in between the combustion process and the skin of the chamber in a sort of by-pass effect to reduce shear friction skin temps?


  • With regards to start-up, where is the igniter located with respect to the injector?


  • With regards to the injectors, does the compressed air flow into a side duct resevoir near the fuel spray or is the injector positioned in the middle and the compressed air flows around it, like a spike that bleeds fuel in the back to further mixing?


  • Wait, how is fuel properly mixed with air, it's got to be a wide spray to mix with all incoming air, right?


  • Most of all, because my basic aerodynamics book includes various methods of determining the area difference in combustion throat and exhaust, it does not mention how the nozzle must be shaped. Any ideas? Is it inverse function, i.e. since velocity changes with pressure?




  • I hope the detail may help you to understand, answer any to all, ask if still thrown off. Thank you and please don't slam my education.  Big grin


    [Edited 2004-11-28 06:35:29]
    The meaning of life is curiosity; we were put on this planet to explore opportunities.
     
    air2gxs
    Posts: 1443
    Joined: Mon Jun 18, 2001 1:29 pm

    RE: Combustion [chamber] Questions

    Sun Nov 28, 2004 10:40 pm

    1. The fuel is atomized
    2. The size of the hole of course varies with engine type. I don't stock any nozzle here so I can't measure them the orifice, but I seem to recall about 1/8 inch or so.
    3. The fuel is atomized and the secondary airflow through the engine is designed to prevent any unburned fuel from coming in contact with the combustor.
    4. What you are describing is secondary airflow. There are 2 airflows through the core engine. The priamry which is burned with the fluel and the secondary, which is used to cool the combustor and act as a buffer between the flame and the combustor.
    5. Again that is depended on engine type. 2 ignitors are usually installed just aft of the injectors/nozzles.
    6. Depending on design, some of the air may pass directly through the nozzle and mix with the fuel at the nozzle, but the majority of the air enters the chamber around the nozzles (there are fuel nozzles/injectors postioned all around the enigine i.e. PW2000 series has 24) and mixes with the fuel.
    7. Again. some of the air can go right through the nozzle and directly mix with and assist in atomizing the fuel.
    8. Not sure what you're asking, but the nozzle is a fixed orifice designed to atomize the fuel regardless of volume, pressure is constant at the nozzle.
     
    747NUT
    Posts: 77
    Joined: Sun Sep 05, 2004 12:24 am

    RE: Combustion [chamber] Questions

    Mon Nov 29, 2004 12:09 am

    Wait, how is fuel properly mixed with air, it's got to be a wide spray to mix with all incoming air, right?

    The nozzles have swirl vanes around the injectors that "swirl" the air helping in mixing the fuel with the air, and the injectors spray a fine mist of fuel.

    If it is a liquid injection, what happens if some unburned (or yet to be burned)fuel lands on the chamber's skin?

    Only about 10% of the air that goes through a core of an engine is actually used for combustion, and the rest is for cooling. There are small holes along the full length of the liners side that introduce cooler air that is suppose to prevent this.
    You do sometimes get clogged nozzles that do not give a nice fine spray, and this causes a "hot spot" on the liners, you can usually see this during a borescope inspection which looks like a dark spot on the liners
    If it's not broken, don't fix it !
     
    Dalmd88
    Posts: 2399
    Joined: Fri Jul 28, 2000 3:19 am

    RE: Combustion [chamber] Questions

    Mon Nov 29, 2004 1:11 am

    The nozzels can vary alot. On a PT6 they are a single hole on each nozzle. The spray pattern is in the shape of an onion. On a Garrett 331 each nozzle looks more like a kitchen sink spray head. They have mulitple holes. On any type if you get a stream of fuel to come out it can burn a hole through the chamber.

    There are also only a couple of igniters per engine. If I remember on both of these types they were about 3-4 inches downstream of the nozzles at about the 4 and 8 o'clock positions.
     
    lehpron
    Posts: 6846
    Joined: Tue Jul 10, 2001 3:42 am

    RE: Combustion [chamber] Questions

    Wed Dec 08, 2004 2:48 pm

    Nice, you guys pretty much answered all of my questions, thank you so much. I've been buggin 'bout those for a darn long time.

    Okay ,now I got another question; same topic but different category. With regards to the simple picture I made up below (which means is may be too general).



    In a ramjet's combustion chamber, the air is compressed but going subsonic in the chamber and I would assume fuel would combust with the in-coming air like any turbojet engine would at that point. What about scramjets? Their combustion process occurs entirely supersonic, so wouldn't there be shockwaves everywhere? If so, would it not interfere with the mixing of fuel? (I donno about the Hyper-X's engine specifics other than NASA being a bit jumpy w.r.t X-43's combuster.)

    For example, in the pix, I drew in th red shock lines coming off the intake spike and spreading about the combustion chamber. Could anybody give me an idea as how the fuel (I assume atomized) mixes at all with such a high speed flow of air?

    Thank you and good night.  Smile
    The meaning of life is curiosity; we were put on this planet to explore opportunities.

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