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Aerospace How exactly does a gas turbine engine produce thrust?

  1. Oct 24, 2011 #1
    Hi
    I have been trying to figure out the force experienced by individual components of a gas turbine engine.

    Generally, the thrust of the engine is defined / calculated as the change in the momentum of the fluid. Usual formula is mass flow X (C_jet - C_intake) + the pressure difference in case the nozzle is under/over expanded.

    What I am interested to know is, how is this thrust transferred to the engine mounts? The only way the fluid can interact with the hardware is through pressure (& shear, but let’s forget it here). I reckon if pressure is integrated all over the surface of the engine, we should get the thrust value.

    Now let’s go through various parts of the engine(let’s assume a single spool turbojet):

    1. Intake
    More or less, the intake is cylindrical, so the pressure integral will not have any large component in the axial direction. If compressor is supported on 2 bearings, front bearing housing (covered by nose cone) will be pushed backwards, ie it is a dragging component.

    2. Compressor
    Compressor rotor blades can be thought of as being airfoils. Lift & drag produced by the airfoil can be thought as being equivalent to forward force & torque (strictly force about the shaft) requirement respectively. This forward force is transferred to the shaft, to the bearings, to the casing & to the engine mounts.

    Similarly, compressor stator blades due to their orientation will also experience a forward force & counter torque.

    3. Combustion Chamber
    Due to the geometry of the combustion chamber, it also is a dragging component.

    4. Turbine
    Similar to the compressor rotor & stator blades, turbine blades will experience backward force; turbine is also a dragging component.
    Exhaust cone though experiences force in forward direction.

    5. Jet pipe & nozzle
    Owing to their shape & continuously decreasing pressure, jet pipe does not contribute to any force and pressure distribution on the nozzle pushes it backwards.

    From these considerations, only major component which pushes the engine forward is the compressor. Rest all are dragging components. Turns out that gas ‘turbine’ engine is more of a misnomer, compression propulsion system would be a better choice it seems. Turbine is merely driving the compressor. So the question is, what exactly is the nozzle doing?

    From the control volume point of view, formula relating change in momentum to the net force makes perfect sense, but how is that force transferred to the engine mounts?

    Regards,
    Ankit
     
  2. jcsd
  3. Nov 3, 2011 #2

    russ_watters

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    Staff: Mentor

    The nozzle increases the pressure.
     
  4. Nov 3, 2011 #3
    1. If the compressor powers the combustion, and the turbine powers the compressor...then isn't the name ok?

    2. The components being propelled are connected to the fuselage, that's how the thrust is transmitted.
     
  5. Nov 3, 2011 #4

    AlephZero

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    Science Advisor
    Homework Helper

    Correct, and jet engine designers do the sort of modelling that you did to find the stress levels in the engine casings, as part of the design process.

    You forgot to take account of the forces on the rotor and stator blades in the compressor and turbine separately, but that doesn't affect the general principle.

    I think you also forgot that the pressure acts normal to the surface of the casings, so it produces a resultant force on the conical compressor and turbine casings independent of the aerodynamic forces on the blades.

    The resultant force on the rotor gets into the engine outer casing through the thrust bearing and the structure that supports it. Mechanically, you can think of the stator vanes as cantilever beams fixed to the outer casing, so the force on the vanes is transmitted into the casing through the reaction force where the stators are fixed.

    FWIW getting all the forces to add up the "correct" thrust for a real jet engine is hard to do. The total axial force on the compressors and the turbines is much bigger than the net thrust, which is the difference between those two big numbers.
     
  6. Nov 4, 2011 #5
    Hi russ,

    You mean to say that nozzle increases the back pressure. Right?

    If yes, I get that part, that is to match the turbine & compressor characteristics.

    Hi Travis,

    name is just fine. It was secondary.

    I am not talking about the transmission of force to the aircraft structure. I am concerned about the engine component loading.

    Hi AlephZero,

    Yes I did take all that into account. I mentioned both in OP. That's how I am analyzing the force.

    Correct.

    My question was about the purpose of nozzle. The standard 'change in momentum' formula uses the jet velocity. Hence it is assumed that faster the jet leaves, more the thrust, hence the impression that nozzle is used to increase the velocity of the jet.

    However, the sole purpose of nozzle is to match the characteristics of turbine & compressor, as russ pointed out.

    My next question was to be(had i received replies for OP), how does the afterburner increase the thrust?

    As per the Thermodynamics, afterburner affects nothing upstream. Lets us take a simple model with flame holder, jet pipe & nozzle. Heat addition does not affect the pressure at the flame holder plane(pressure loss is negligible), the expansion is still between the same pressures. However the temperature at the flame holder plane increases, simple equations would show that nozzle outlet temperature is also increased, which implies that the jet velocity would be higher with the afterburner. Using the 'change in momentum' formula, we see an increase in the thrust. But how is this increased thrust 'felt'?

    As I understand it, the static pressure distribution change due to the change in velocity along the axis. This is the only possible way thrust can increase, but this has to be proved quantitatively.

    I hope we can build up a discussion & gain a better understanding.

    Regards
    Ankit
     
  7. Nov 10, 2011 #6
    no no no no no, the nozzle increases velocity and reduces pressure!!!
     
  8. Nov 10, 2011 #7
    ...So the question is, what exactly is the nozzle doing?

    the nozzle increases velocity which increases thrust. thrust is defined as mdot x V so increasing v increases the thrust.
     
  9. Nov 10, 2011 #8
    ank, thrust can be boiled down to the difference in the exhaust vs. intake velocity. Faster exhaust jet results in more thrust. The nozzle makes more exhaust velocity, thus creating more thrust. Same with the afterburner.

    I mean all of these components do other things to ensure proper functioning and efficiency of the engine, but basically the more you speed up the exhaust, the more thrust you get.
     
  10. Nov 10, 2011 #9
    @navier1120, nozzle provides higher back pressure. Lets take an example, consider a pipe with P1 at one end. If pipe exhausts to the atmosphere, static pressure drops linearly from P1 all the way to the Pa. If a nozzle is provided at the outlet, P2 at pipe outlet(& nozzle inlet) is increased. This is equivalent to say that nozzle has increased the back pressure.

    In the nozzle, yes the bla bla happens. But that is not relevant to my question.

    @Travis, I get the momentum equation & I understand it perfectly. What I am trying to figure out is the force distribution (or contribution) of separate components.

    The way I understand it is that forward thrust is "felt" by the compressor. Everything else is dragging along, but everything else is needed for the compressor to work. But what happens when afterburner is switched on? I reckon nothing changes upstream, only the exit plane jet velocity increases. But how is this "extra" thrust transferred to the engine mounts?
     
  11. Nov 10, 2011 #10
    In basically the same way the air is transferred when the afterburner is not on.
     
  12. Nov 11, 2011 #11
    Hi Travis,

    what changes when afterburner is switched on?
     
  13. Nov 11, 2011 #12
    The afterburner adds fuel to the exhaust which ignites due to the exhaust temperatures. This increase in the exhaust jet velocity provides more thrust.

    Afterburners
     
  14. Nov 11, 2011 #13
    Which one; the inlet or exhaust? There are two, ya know.
     
  15. Nov 15, 2011 #14
    Hi Travis,

    The momentum equation is one way of calculating thrust at the surface of the control volume. It replaces the messy fluid-solid interaction with a simple formula. I am not contending the validity of this method. I am trying study the events inside the blackbox.

    To put it into perspective, would you say that power of an IC engine increases because you pressed the accelerator?
     
  16. Nov 17, 2011 #15
    Yes. The power output of an engine increases and decreases. They have curves (hence power bands).
     
  17. Nov 17, 2011 #16
    umm.. what?? :(
     
  18. Nov 18, 2011 #17
  19. Nov 18, 2011 #18
    Travis, you can stop now
     
  20. Dec 5, 2011 #19
    An engine may have one, two, or three shafts to which the blades are attached.

    Each shaft has one thrust bearing located near the front end.

    The thrust loads are transmitted from the shaft to the case structure through the thrust bearings.

    Vanes are stationary, and those loads are transmitted directly to the case.

    The case is bolted to the air frame through the engine mounts.

    Think of the augmentor as a separate rocket engine bolted to the back of the gas turbine. Those thrust loads apply to the back of the low speed shaft, and from there to the thrust bearing at the front of the same shaft.

    The big push in design for the last couple of decades is to minimize exhaust exit speed. More efficient that way, with much less noise. Most of the design of the nozzle is for effecient mixing with ambient air with minimum noise. Such engines depend on a higher mass flow rate rather than velocity for the thrust. That is the purpose of that really big fan in the front, and the incredibly high bypass ratios seen in the newer designs. That fan is responsible for most of the thrust in civilian engines. Military engines typically have lower bypass ratios, but the principle is the same.
     
  21. May 28, 2012 #20
    First of all it is so great to stumble upon this forum. Especially that I am looking for the answer to the same question, which ank_gl is trying to find.

    Second of all I am faced with the same amount of misunderstanding that ank_gl is finding within this thread. Google and people I ask keep talking about the "momentum change" and the Mdot*deltaV being the thrust.

    The equation above is a simple way to calculate net thrust.
    It is not, however, what produces the thrust, what pushes on the surface of the metal to create forward force.

    If you think of a released balloon, you can calculate its thrust measuring the jet velocity and mass flow, but the actual force propelling the balloon forward is the pressure imbalance created by the fact that on the rear face of the balloon you have one less spot (the opening), which has a pressure drop on it (pressure difference between the high pressure inside the balloon to the lower atmospheric pressure). In this case you can easily point to/calculate the surface, for which delP*A ~ thrust.

    In a propeller engine, there is a pressure difference developed accross the propeller blades, which is transferred to the shaft, to the bearing, to engine frame, to the thrust link, and then to the wing.

    In a turbojet or turbofan engine, an easy argument to make would be that the compressor and/or the fan act the same way as the propeller.
    I was, however, told that the shafts in a jet engine are "balanced" i.e. the shaft is pulled with about equal forces in the forward and back directions, and it is often pulled in the backward direction more than fwd - it is creating drag for the engine.

    What remains to push on the engine structure in the fwd direction would be the stators/cases. In this case, which stators and how much are contributing to the thrust in a jet engine? ...This may require an intimate knowledge of a given engine design, down to pressure balance cavities, etc. and may be also specific to a given engine model.

    I hope somebody out there could shine more light on this.
     
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