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All right. Here's what I understand about axial turbines:

  • It is an axial compressor in reverse.
    • An axial compressor forces air to flow in an increasingly tight space, where there is not enough pressure gradient (due to the high pressure combustion chamber beyond) for Bernoulli's effect to speed up the flow and choke it, as happens with nozzles.
  • An axial turbine allows gas to flow in an increasingly wide passage down the pressure gradient from combustor to exhaust.

But here's what I do not understand. Normally, we use nozzles to speed up flow. In subsonic flow, as happens in the turbines, a constriction speeds up the flow, giving a good directional jet. A widening, will reduce the velocity an raise the pressure - not what we want for a turbine. Fine - the pressure gradient will overwhelm this in the jet engine, but the resulting flow will be less directional than that from a nozzle.

So, why do we use divergent turbine passages, instead of convergent nozzles to speed the flow?

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As the air coming through the engine gets heated by burning fuel in the combustors, its volume increases greatly. In order to maintain mass flow continuity, those hot gases have to accelerate to a speed greater than the speed of the air entering the engine. So all parcels of gas flowing through the engine experience a change in their momentum, which requires the application of a force, and the resulting reaction force applied to the engine is its thrust.

To drive the compressor on the inlet side of the engine, a turbine is built into the tailpipe of the engine behind the combustors. the first stage of the turbine extracts a little power from the flow through it, which slows down the flow. Again, to maintain mass flow continuity, the next stage of the turbine must have a slightly larger diameter and bigger blades, and it extracts a little more power from the (slightly slower) flow.

This means as the hot gas flows through all the successive stages of the turbine, the cross-sectional area of the turbine throat must get progressively larger at each stage, which makes the turbine section of the engine look like a diverging nozzle.

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  • $\begingroup$ does the throat have to be as small as possible while being larger than whatever was before it? That is, for example, what would happen if we expanded gas from the combustor volume straight to tailpipe volume with this diverging passage, with no turbines? would we get all the kinetic energy, or would there be a loss? $\endgroup$
    – Abdullah
    May 16, 2020 at 18:53
  • $\begingroup$ there's be no way to run the compressor if you did that! $\endgroup$ May 16, 2020 at 22:08
  • $\begingroup$ why is that so? $\endgroup$
    – Abdullah
    May 17, 2020 at 6:43
  • $\begingroup$ the turbine extracts work from the mass flow and sends it through a shaft to spin the compressor. without the compressor, the engine isn't an engine any more. look up brayton cycle on wikipedia. $\endgroup$ May 17, 2020 at 17:24
  • $\begingroup$ Not only will the turbine slow down the flow, it lets it expand, so density drops and volume goes up. Speed doesn't change as much as density does in the turbine. $\endgroup$ May 18, 2020 at 6:48
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The jet engine as a whole is a convergent nozzle: The diameter of the inlet is much higher than the diameter of the outlet. Air is compressed, as it flows through, and the pressure difference between inlet and outlet pushes the aircraft forward.

Most of the air, that enters a modern jet engine, is only compressed a little by the huge rotating fan, that can be seen from the front of the engine. This air is used to push the aircraft forward. You can even think of this fan like a huge propellor. Only a fraction of the inlet air is compressed further in a series of additional compressor stages and then led through the burning chamber. The dense hot air exiting the burning chamber is then decompressed in the turbine, so that most of the mechanical energy is transferred to the turbine. From the turbine, the energy goes to the compressor, which then pushes the aircraft forward.

Why this indirect procedure? Well, using the hot gas directly to propel the aircraft would result in an inefficient system with a high pressure difference between inlet and outlet.

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  • $\begingroup$ This answer is actually about turbojet vs turbofan. I meant, why use a diverging passage in the turbine section after combustor to accelerate subsonic flow? $\endgroup$
    – Abdullah
    May 16, 2020 at 11:47
  • $\begingroup$ Well, as laid out in the answer: The hot, compressed air from the burning chamber is decompressed in the turbine to transfer its mechanical energy to the turbine. Accelerating the hot air is also a good idea, as this increases the momentum out of the back of the engine, and thus the force on the aircraft. $\endgroup$
    – Kai Petzke
    May 17, 2020 at 19:46

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