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
I am a bit rusty but I think you got that wrong. In a typical jet engine air first decelerates and compresses, then it is mixed with the fuel and burns, and then expands into the turbine
Air needs to be compressed for the combustion to work
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.
Whether the rotor uses velocity or pressure to drive it depends on the type of turbine: impulse or reaction respectively.
The overall aim is to extract energy from the working fluid. It arrives with non-zero pressure and velocity, and ideally leaves at zero pressure and velocity (relative to some baseline). Where these conversions occur within the many stages of the turbine (for gas and steam turbines) depends on the design constraints. A reaction turbine can be more efficient, but maintains higher pressure through the stages, and needs more of them, so is heavier overall. An impulse turbine trades pressure for velocity earlier and so can have a lighter body.
So in answer to your question, yes you can use either velocity or pressure to push your rotor at a given stage in the turbine, and there are various factors that go into that decision.
The way all planes generate thrust is by speeding up air.
A jet engine has a few important sections to make it generate it’s thrust. The first section is the inlet which is where the air enters the engine.
Next, is the section that controls how the air flows. This is to make the air flow smoothly so it doesn’t have random changes in air speed as well as make the air all flow in one direction. We do this so it doesn’t overload the compressor blades with extra stress from the changing air speeds.To do this we use guide vanes. This might be the section you refer to about the air speeding up before compression.
Next is the compression stage. This stage is used to pressurise the air for the combustion as this improves the efficiency of combustion. To compress the air we slow it down and constrict the area in which it can flow through.
After the compression stage we have the combustion stage which burns fuel to heat up the compressed air. Increasing the temperature of the air also increases the pressure of the air.
Finally we have the turbine section. This is where we convert the hot slow moving pressurised air into cool fast moving air. To do this, we pass air through blades with an expanding area for the air to pass through. This makes the air lose pressure (which for thermodynamic reasons also makes the air cooler). This decrease in pressure makes the air move faster, which generates the thrust the plane needs. To power the compression section, the turbine blades are connect by a strong metal rod to the compression section.
It’s just a big fan.
From the place it’s pulling air in, the air goes faster because it’s being pulled. From where the air is being spit out, the air slows down, because it runs into the slow moving air that’s on the other side.
Okay, first we need to clarify some things. Not all the blades in a gas turbine engine are “turbine blades”. Only the blades in the actual turbine section are referred to as such. The blades in the compressor section are compressor blades. Based on your question, I am assuming you are asking about ambient air entering the compressor section, and not exhaust gas entering the turbine section.
The compressor section compresses ambient air by first accelerating it and then decelerating it. The rotor blades accelerate the air, and then the stator blades slow it down, increasing pressure. As the air moves through the compressor, the cross-sectional area decreases in order to maintain an ideal Mach number.
Inlet guide vanes are used in multi-stage axial compressors to help manage the flow and allow for higher compression ratios than would otherwise be possible.
Jet engines work by accelerating gas, which pushes the engine forward. Burning fuel and oxygen creates an enormous volume of hot gas, which expands in whatever direction it can. Directing this gas through a tapered nozzle optimizes its velocity to produce maximum force for a particular volume of gas.
It does’t really matter how fast the air travels prior to ignition. What matters more is how compressed it is, because more compression means more mass available to feed the chemical reaction that generates the gass that actually moves the airplane.
In fact, the process of slowing intake air actually aids compression, which helps the engine produce more thrust.
At the end, the air needs to be moving out faster than it was moving in. That’s how the engine pushes the jet forward.
There’s two streams of air in a modern jet engine. One going through the fan, and one going through the turbine. The air in the fan is super simple – it gets compressed a bit by the fan and this pressure pushes the air back/the plane forward. Most of the thrust of the engine comes from this air.
The turbine air is where the engine gets its energy. Running a giant fan at high speed costs a lot of energy. At the center of the fan, behind it, there is a series of turbines that compress the air more and more. Once fully compressed, the air reacts with fuel and expands, before going out another turbine at the back. This turbine at the back gets spun by the air and gets the energy to power the engine.
The reason this air is compressed is sort of simple. The fuel burning cannot compress the air at all, since the jet engine is really just a big tube. The pressure would just escape. What it does is cause the air to expand. This generates much more energy when the air is under pressure.
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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