For the N1 fan and N2 compressor to move, the turbine needs to spin in an airliners engine. How does energy from the ignited gas spin this turbine? Do the blades on the turbine generate lift just like a wing and spin to spin the N1 fan and N2 compressor?
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7$\begingroup$ Find a pinwheel, and blow on it, the same way that rapidly expanding gas would move. Watch the pinwheel spin. $\endgroup$– abelenkyCommented Sep 8, 2015 at 21:51
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1$\begingroup$ Maybe it's not important to the question, but I have a feeling that the picture shows a zero bypass ratio engine, thus it does not have N1 and N2. $\endgroup$– Stelios AdamantidisCommented Sep 8, 2015 at 21:59
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$\begingroup$ I am asking about the exact process of how this spinning motion happens. $\endgroup$– EthanCommented Sep 8, 2015 at 22:00
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4$\begingroup$ In the shortest possible answer, yes. The blades are like any other aerofoil. $\endgroup$– SimonCommented Sep 8, 2015 at 22:01
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1$\begingroup$ The picture shows a pretty unusual engine, if such thing even exists: a radial flow geared turbofan. Many early turbojets were radial flow, but they had no fan, not to mention geared. While modern turbofans are axial flow, with gears only slowing coming in fashion. $\endgroup$– Jan HudecCommented Sep 9, 2015 at 7:11
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2 Answers
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A turbine is a machine that extracts energy from a moving fluid and converts it into work. In the turbine blades, the air is expanded, which produces work. In many ways, the turbines are the opposite of compressors.
"Two stage reaction turbine" by S.M.Yahya - Turbomachinery. Licensed under Public Domain via Commons.
The turbines used in the aircrafts are axial flow turbines, which employ multiple stages to extract work from the air coming out of combustion chamber.
The turbines have two parts- the stationary nozzles (or stators) and the moving rotors. The purpose of the nozzles is to turn the incoming flow so that that it impinges on the rotors at the correct angle, while the actual work is extracted out from fluid by the rotors.
"Velocity triangle for an axial turbine stage" by S.M.Yahya - Turbomachinery. Licensed under Public Domain via Commons.
Basically, the rotors are 'turned' by the incoming fluid as shown above which produces work to rotate the shaft (which runs the compressor). In case of turbojet engines (and to some extent turbofan), the air pressure decreases, while the velocity increases. This provides the required thrust.
The main mechanism by which the turbine blades spin is that the flow turns the rotor blades by hitting them, rather than any lift being produced.
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$\begingroup$ So it hits the blades to turn them. The blades don't generate any lift to turn. $\endgroup$– EthanCommented Sep 9, 2015 at 0:31
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1$\begingroup$ In general, the term 'lift generation' is not used to describe the method by which the turbine blades turn. Similar pressure forces are involved, but the method is not like the case of wind turbines, where the blades are rotated by the lift. $\endgroup$ Commented Sep 9, 2015 at 0:59
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$\begingroup$ So different pressures are created spinning. Not the same as lift though. Pressure develops in a different way is what I am getting out of it. $\endgroup$– EthanCommented Sep 9, 2015 at 1:26
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1$\begingroup$ @Ethan: The blades do indeed generate lift. The definition of lift is merely a force vector generated by having difference in pressure between two sides of an object. If the blades spin, they generate lift. If they don't generate lift then they don't spin. Now, there are many mechanisms that generate lift so turbine rotors may not necessarily generate lift the same way as windmill rotors. $\endgroup$ Commented Sep 9, 2015 at 4:04
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4$\begingroup$ @Ethan It is not the gas "hitting" the turbine blades. The blades generate a force like any other aerofoil and that force turns the turbine. People are hesitating when you say "lift" because this has a specific meaning which is the force generated to oppose gravity. In this case, it is not "lift" but is exactly the same mechanism of force generation. $\endgroup$– SimonCommented Sep 9, 2015 at 7:19
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I will attempt to answer the question though I'm not an engineer. Each spool (N1, N2 etc) has a compressor in the front and a turbine in the rear. Compressor is a set of rotor blades followed by stator vanes. This is a compression stage. Turbine has the opposite setup: stator vanes followed by rotor blades. They are either pure impulse turbines or impulse/reaction turbines. You may have a look in wikipedia to see the turbine working principle.
Now the high speed combusted air that comes out of the combustion chambers put the turbine in motion. The turbine moves the shaft, the shaft moves the compressor, the compressor feeds the combustion chamber with compressed air and that cycle goes on and on for as long as there is fuel going in the chambers.
Here is a diagram of how is the turbine "set to motion". Red arrows show how the airflow is guided from the nozzles to an impulsive turbine blades.
Image source: own work
The stator vanes are convergent and the air coming through the resulting nozzles is accelerated. As the accelerated air impacts the curved rotor blades, they cause aerodynamic forces on them, similar to those caused on a wing (as others already said). These forces set the turbine to motion.
EDIT: Added image and corrected compressor and turbine stages description
answered Sep 8, 2015 at 23:13
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1$\begingroup$ Slight addition from an engineer: you describe an
impulse turbine, which converts pressure into momentum through a nozzle, and momentum into work. The other end of the spectrum is areaction turbine, which converts pressure directly into work. In reality, jet turbines are a (computer-optimized) combination of the two. $\endgroup$ Commented Sep 16, 2015 at 10:17 -
$\begingroup$ @sanchises Sorry but I don't think I follow. I describe an impulse turbine but I mentioned the impulse/reaction turbine. The former was easier to draw. :) If I had something wrong or missing, let me know so I correct it. $\endgroup$ Commented Sep 17, 2015 at 7:05
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$\begingroup$ Sorry, I glanced over that. Perhaps you could add 'These forces, as well as some pressure differential across the blades, set the turbine to motion'. But I guess it's not really necessary given your first remark. $\endgroup$ Commented Sep 17, 2015 at 11:39