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

Combustion Process, Fuel Air Burn?

Mon Jan 05, 2004 12:56 pm

i'm trying to understand the atmosphere of the combustion chamber of an axial flow turbine. What is going on in there? Is the fuel injector discharging the fuel as:

  • a liquid spray or

  • a vaporized form mixed with compressed air or

  • some kind of pilot light that ignites like in a home water heater or gas stove?


  • Give me an idea of how the fuel behaves as it leaves the injector as it burns.
    The meaning of life is curiosity; we were put on this planet to explore opportunities.
     
    Staffan
    Posts: 3879
    Joined: Tue Mar 18, 2008 9:21 am

    RE: Combustion Process, Fuel Air Burn?

    Mon Jan 05, 2004 8:42 pm

    Fuel is sprayed in liquid form with very high pressure. This together with the heat from the compressed air and the motion of the air entering the flame tube, breaks up and vaporizes the fuel before burning.

    Staffan
     
    air2gxs
    Posts: 1443
    Joined: Mon Jun 18, 2001 1:29 pm

    RE: Combustion Process, Fuel Air Burn?

    Mon Jan 05, 2004 9:51 pm

    The "pilot light" is an ignitor (similar in concept to a apark plug) that produces a spark which lights the fuel/air mixture. After the engine is started the ignitor is turned off. The ignition system is again turned on only when called for in a checklist (icing, precip, etc.)

    The burning process is self-sustaining after the engine reaches idle.
     
    MD11Engineer
    Posts: 13916
    Joined: Sun Oct 26, 2003 5:25 am

    RE: Combustion Process, Fuel Air Burn?

    Mon Jan 05, 2004 11:30 pm

    The airflow from the compressor is too fast to sustain a combustion. The combustion chamber liner dividesthe airflow into several parts. The first part, called primary air, is being let through swirl vanes around the injector nozzles to provide a good proportional, chemicaly correct mix with the fuel and to create a vortex which will prevent the flame from being blown away. Addition air is being let into the liner through holes in the wall in this zone to prevent direct contact of the very hot flame with the the liner. Almost the complete combution takes place in this zone.
    The gases coming from the combution zone aremuch too hot to pass trough the turbine (about 2000°C), so additional air is being mixed through holes in the liner further downstream to cool the combutsion gases to temperatures around 800°C, which the turbine materials can handle.

    BTW, on early jet engines the fuel was vaporized before injection.

    Jan
    Je Suis Charlie et je suis Ahmet aussi
     
    SlamClick
    Posts: 9576
    Joined: Sun Nov 23, 2003 7:09 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 1:47 am

    MD11Engineer

    That was a good explanation. I think even a lot of pilots and mechanics are suprised to learn how the airflow is managed inside a turbine engine. Not sure if it is still being done but some used to port bleed air radially out through turbine blades to cool them from within.

    I would always tell new students that in a healthy jet engine, flame never touches metal. This helps explain why an engine may be able to sustain higher EGT at high RPM than during start. Picturing the difference between the managed flame and airpaths versus puddled fuel burning in the combustor section without proper airflow it seemed to help them catch on about start abnormals.

    I also recall seeing a JT-8D cutaway that showed 13th stage compressed air temperatures as high as 900 degrees C (at SL standard day) Not hard to imagine getting Jet-A to ignite in that atmosphere.

    Happiness is not seeing another trite Ste. Maarten photo all week long.
     
    MITaero
    Posts: 485
    Joined: Mon Jul 28, 2003 8:00 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 2:37 am

    Nice explanations. It's funny that the "cool" air that is used to cool the turbine blades is actually air from the compressor exit. I believe the reason is that the pressure must be high enough in the coolant to force it through the holes in the turbine blades, but I'm no expert in this area - maybe someone who works on them can give more details.
     
    MD11Engineer
    Posts: 13916
    Joined: Sun Oct 26, 2003 5:25 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 2:59 am

    The compressor outlet air is somewhere in the 400-500°C temperature range. You wouldn´t want to use too cold air for cooling the turbine blades and vanes because it can lead to thermal stresses inside the blade and vane structure leading to cracks. The thing to force the air out trough all the small holes in the blades and vanes is the pressure difference. The area with the highest pressure in a gas turbine engine is the diffusor area between the compressor outlet and the combustor, where the airflow coming from the compressor getsslowed down and acc. to Bernoulli´s law the pressure rises. In the turbine energy is removed from the hot gas leading to a temperature and pressure drop. This is why the diametre of the turbine rotors and casing increases with each stage. Also usualy only the first two turbine stages are cooled.

    Jan
    Je Suis Charlie et je suis Ahmet aussi
     
    MD11Engineer
    Posts: 13916
    Joined: Sun Oct 26, 2003 5:25 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 3:01 am

    BTW, Rolls-Royce offers a very nice book called "The Jet Engine" It is a very nice introduction into the theory and design of gas turbine engines. I used it myself for exam preparation long ago.

    Jan
    Je Suis Charlie et je suis Ahmet aussi
     
    Staffan
    Posts: 3879
    Joined: Tue Mar 18, 2008 9:21 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 4:50 am

    The cooling holes in the turbine parts are so small that they have to be made in a special way using electric sparcs, it's quite fascinating!
    BTW, does anyone know if the newer single crystal blades needs this type of cooling too?

    Staffan

     
    MD11Engineer
    Posts: 13916
    Joined: Sun Oct 26, 2003 5:25 am

    RE: Combustion Process, Fuel Air Burn?

    Tue Jan 06, 2004 6:30 am

    Yes the monocrystaline blades are cooled as well. BTW the holes are usualy drilled using high power lasers.

    Jan
    Je Suis Charlie et je suis Ahmet aussi
     
    darkblue
    Posts: 227
    Joined: Thu Sep 11, 2003 10:27 pm

    RE: Combustion Process, Fuel Air Burn?

    Wed Jan 07, 2004 1:00 am

    The key to cooling turbine blades is to use compressor bleed air that is at a higher pressure than the turbine pressure in the region you are trying to cool. If the cooling air pressure is lower than the hot region pressure then the hot gas will flow back through the cooling holes creating a mighty big problem!

    In the first stage of the high-pressure turbine, you must use the compressor discharge bleed, while in later stages of the turbine, air from the interstages of the compressor can be used. It is interesting to note that combustors are designed not only to mix fuel and air efficiently, but are also designed to have pressure losses that are greater than the pressure losses associated with the cooling air through the passages it must take from the compressor to the turbine. This allows the cooling air to remain at a higher pressure than that in the turbine.

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

    RE: Combustion Process, Fuel Air Burn?

    Sat Jan 10, 2004 2:50 pm

    Very very interesting, you guys really know your stuff, MD11Engineer, I am impressed.

    "breaks up and vaporizes the fuel before burning"

    Like a cleaning solution mistifier or perfume bottle spray?

    "After the engine is started the ignitor is turned off"

    I heard this process is continuous during landing to prevent a flameout, I image that would be pretty bad if the engines shutdown at that particular instance.

    "The airflow from the compressor is too fast to sustain a combustion."

    If this is the case with a jet, how does a ramjet operate at all?

    "Addition air is being let into the liner through holes in the wall in this zone to prevent direct contact of the very hot flame with the the liner"

    Maybe it is me, but this gives me the visual of a vacuum within the engine core and the fuel ends up getting sucked into the flow, like a venturi tube, am I right?


    This stuff give real insight to what I was thinking, I was trying to apply this to the ideas of pulse detonation enignes with respect to the burn process, I figured thy would have injectors that sprayed similarly even though one was continuous and the other was pulsed continual.

    The thing about the vaporizing with high pressure confirmed my belief about an experiment I did with gasoline in a Windex cleaner bottle and I sprayed it into the air and ignited it with a lit-match at the end of a yard stick, the result was noisy but made sense to me. If this explosion was captured in a cone there should be a force resultant perpindicular to the base.

    I have yet to do that, all I have done was run a dopey ramjet made of twin shell aluminum tubes and a vacuum cleaner fan and a few ounces of Octane -- a ducted rocket if you will. I have had this pulsed ramjet engine concept for a while and I'm trying to test it before I graduate.

    Anyway, thank for all the responses and I will try to get that one book.  Big thumbs up
    The meaning of life is curiosity; we were put on this planet to explore opportunities.
     
    A/c train
    Posts: 674
    Joined: Sun Jun 10, 2001 7:57 am

    RE: Combustion Process, Fuel Air Burn?

    Sun Jan 11, 2004 1:23 am

    The fuel is not a vapourised spray/delivery to the combustion chamber, it is an atomised spray. Think about a Vapour mask, you cant see Vapour but you can smell it, you can see an atomised spray, the delivery is atomised fuel mixing with HP air and being igntited,
    regards ,
    a/c

    Who is online

    Users browsing this forum: Baidu [Spider], Google [Bot] and 12 guests