I read that the velocity of the air is increased at the nozzle guide vanes of a turbine before it actually hits the turbine blades, then compresses, and the process repeats. My question is, what’s the reasoning behind this? The way I’m thinking of it is like this:
Let’s compare compressed air to a sumo wrestler.
And then let’s compare just a normal guy.
If the sumo wrestler (compressed air) runs at you with a somewhat slow speed, he’ll apply a lot of force to you, but if a normal guy runs at you with a much faster speed, you’ll feel a lot of force too. I’m wondering where the logic is behind this, because can’t the same force be applied both as compressed air and sped up air if both mass and velocity contribute to momentum?
Thanks
In: 1
It’s fluid dynamics and it’s a headache.
But basically:
You want to compress air: get the air in the compressor, this is not a volumetric compressor with a piston, you can’t compress air, all you can do is to make air go faster. Ready? Spin a fan to accelerate the air. now send that air into vanes that transform the speed into pressure (Venturi stuff). Rinse and repeat.
Now your multi stage axial compressor has made the air compressed.
Burn the fuel in that air.
Congrats, you have a lot of gas that’s hot and compressed. This is not a volumetric engine, you can’t use the gas against a piston. You can only extract speed from the air.
So, get your compressed gas a way out, in a convergent duct (Venturi stuff) your gas will decrease temperature and pressure and the energy will become speed energy. Use that speedy air to hit a blande and move that blade.
Congrats, you moved the turbine. Rinse and repeat, Venturi ti sped up the gas, hit another blade and so on.
Continue until the gas has little to none useful energy that can be transformed in speed.
Eject the gas behind the engine with some speed so it does push the plane forward.
This is true whatever turbine you use. The only difference is the ratio of the energy you use to spin a fan or to jet out gas. Early jets do rely only on jetting gas behind. Turbofans extract most of the gas energy and use it to spin the fan, only a small fraction of energy will go in the jet effect.
Turboprops (and turboshafts) will extract all the usable gas energy and vent out the exhaust gas with no usable energy.
Hope it’s clear to you now. I assume you are at least an aviation enthusiast or student.
I had aviation students that don’t get this stuff. No shame to ask or review this things over and over, it’s complex.
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