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
The compressed air makes combustion a lot more efficient. In order to burn, the fuel needs oxygen. That means molecules of fuel have to literally be touching molecules of O2. Big balls of fuel mean that a lot of fuel is in the middle touching other fuel, so the fuel is *atomized* and sprayed into the combustion chamber as a fine mist. The next problem is how much air is there in the combustion chamber? If a molecule of fuel doesn’t have a chance to interact with a molecule of oxygen before all the other fuel molecules start burning, then the fuel will be ejected out the back and be wasted.
You have to limit the amount of fuel you put into the chamber to match the amount of oxygen available, and it will still only burn as fast as it can based on how much oxygen their is. So, why not shove more oxygen in there? And if you shove the oxygen close together, the fuel molecules have a lot more chances to interact with it before everything is burning and exhausting out of the engine. It makes everything burn *faster* because the fuel molecules don’t have to float around a bit before finding the oxygen, it’s already *right there*.
A typical piston engine does this during the compression cycle as the piston rises up inside the cylinder, compressing the fuel/air mixture until the spark plug ignites the mixture (or the heat from the pressure ignites it, in the case of a diesel engine). A jet engine doesn’t have that compression cycle because it doesn’t have pistons. Instead, the air coming into the engine is compressed by the fans at the intake, and some of the energy of the expanding exhaust is used to power the turbine to keep the compression fans going.
Critically, very fast jets have to *slow down* the air coming into the engine. Supersonic air does not play well with…well, much of anything. It will basically just bounce off the front of the engine and create a shock cone that blasts air away, so nothing can get in. It’s also just *really bad* for the pretty fragile fins of the fan. Supersonic jets have engine intakes designed in special ways to slow air down to subsonic speeds as it enters the engine.
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