The Jet Engine thread

How not to test a jet engine:

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I've been thinking... what makes a jet engine spin?

On a piston engine, there's an small explosion that pushes the piston downward, and the crankshaft inertia pushes the piston upward... that up/down movement of the piston makes the crankshaft spin, and this movement is transferred to the wheels...

How the jet engine keeps spinning?

I've been thinking... what makes a jet engine spin?

 

...

 

How the jet engine keeps spinning?

it's an engine: the combustion of the combined compressed air/fuel spins a turbine in the back connected by a shaft to the compressor in the front, repeating the cycle.

it's an engine: the combustion of the combined compressed air/fuel spins a turbine in the back connected by a shaft to the compressor in the front, repeating the cycle.

But how?

There is no up/down movement that is transferred as a rotational movement...

The jet engine is rotational only. How does a fuel exploding makes it turn?

Short answer : airflow

Currently at work having issues with one that’s flaming out prematurely so too busy to write a more detailed explanation! Lol!

But how?

 

There is no up/down movement that is transferred as a rotational movement...

 

The jet engine is rotational only. How does a fuel exploding makes it turn?

Jacobs,

I'll try to explain the basics of it. Maybe I'll be a bit too technical, but that's me :D

Short answer: The turbine is responsible for extracting the kinetic energy of the gases coming out of the combustion chamber. This stream of airflow and gases, at a very high temperature, make the turbine spin. Just like your fan when the wind is blowing against it. And since the turbine is connected to the compressor rotor thru a shaft, the turbines are keeping the compressor working, so the compressor pulls the air, air is compressed, then ignited in the combustion chamber, they release a lot of energy and they pass thru the turbine rotors, around 2/3s of this energy in a turbojet is used to drive the compressor, and the rest goes into the atmosphere and pushes the aircraft forward.

So imagine if you could blow enough air onto the turbine rotor, the entire engine would spin. Compressor, accessory gearbox, everything.

Technical answer:

We could say that a turbine engine is composed of five parts, the compressor, diffuser, combustors, turbines, and nozzle.

The compressor is responsible for getting the air and rising its pressure with the minimum temperature possible. Compressor pulls the air from the atmosphere, increases the velocity of the air, and then the air is diffused by the stator vanes, that's what compresses the air in the engine, it's not the compressor rotor.

Now that we have this air under pressure, we need to send this air to the combustors, where the air and fuel mixture are going to be ignited, just like the fuel and gasoline in a reciprocating engine. Pressure and temperature are EXTREMELY important here because they are directly related to energy expansion, so higher the pressure and higher the temperature in the combustion chamber, more energy is released for the same amount of fuel. Therefore more energy can be extracted from the gases, and more energy can be used to produce thrust.

After the air is ignited in the combustion chamber, the combustors have the hard, and complicated job of releasing all this energy into the next stage, in a stable stream of airflow, at a temperature that is not going to melt the turbines.

But before the turbines, the hot gases are going to pass thru the turbine nozzles, which are stator vanes used, not to compress the air, but the opposite, to increase their velocity and guide the flow at the correct angle onto the turbine blades. So the hot gases go thru the turbine nozzle, velocity is increased a bit, they turn the turbines that drive the compressor and keeps the cycle working.

And finally, the nozzle works the same way as the turbine nozzle, it basically uses a specific geometry in order to increase the velocity of the gases coming out of the exhaust. That would be Bernoulli's principles. Take a look at De Laval Nozzle on the internet.

I hope that helps!

And one more of the JT15D-5:

 

That's a very cute engine! :D

But how?

 

There is no up/down movement that is transferred as a rotational movement...

 

The jet engine is rotational only. How does a fuel exploding makes it turn?

Turbine is a disk with small wings attached at the edge that make it spin when exhaust passes through it, just like wind mill. Fuel doesn't explode in jet engine, only expands. This expansion increases flow velocity and this increase in flow velocity is where thrust comes from. Combustor is like a liquid fuel rocket engine in principle (pressure vessel that has fuel and oxidizer coming in from one end and high velocity gas going out from the other) but the oxygen is taken in from the atmosphere and the power to compress air for burning is extracted by turbine from the exhaust gas.

I hope this doesn’t complicate things but I thought it was a neat gif. This is a bit more complex since it’s a dual spool high bypass turbofan (as opposed to a turbojet).

Source: https://commons.m.wikimedia.org/wiki/File:Turbofan_Labelled.gif

1. Nacelle

2. Fan

3. Low pressure compressor

4. High pressure compressor

5. Combustion chamber

6. High pressure turbine

7. Low pressure turbine

8. Core nozzle

An animation of a turbofan engine. This is a 2-spool, high-bypass turbofan. The high pressure spool (yellow) acts as a gas turbine generating a powerful jet of exhaust gas which drives the rotation of the low pressure spool. The low pressure spool rotation aids compression for the turbine and rotates the fan, the fan provides additional thrust. The two spools, yellow and turquoise, spin at different speeds - the high pressure spool rotates significantly faster than the low pressure spool. High-bypass refers to the ratio of air volume which goes through the turbine compared to the volume which goes through only the fan - far more air travels through the fan than the turbine in a high-bypass engine. Turbofan engines provide good thrust at speeds at up to approximately Mach 1.6, at higher speeds turbojet engines are more effective whilst turboprop engines are more effective at low speeds.

RD-33MK engine(MiG-29K):

Biggest turbine engine mounted on an aircraft. 15cm (5.9") bigger than the GE90!

All those thrust values are gross thrust though, they will, generally speaking, decrease quite a bit once the aircraft gets airborne. :)

Why didn't they use the TF39 on the B-52 however? I always wondered why that is.

Looks like the Air force isn't interested in the four engines per aircraft option, probably because it would require a lot of research and stuff. But I'm super interested in seeing how it will work out.

Regarding the gross and net thrust, I was mainly talking about ram drag. A high bypass turbofan like that would lose a lot of thrust due to the difference between V1 and V2 velocities.

Ok, nice graphs. But this thread is about jet engines, reciprocating engines would be for another topic. :)

Source: https://commons.m.wikimedia.org/wiki/File:Turbofan_Labelled.gif

For reference, in this animation, all stages except the final low pressure turbine have their blades arranged in the wrong direction. :)

:megalol: Just noticed it. That looks funny tbh!

http://www.chromalloy.com

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The forward edge of the blade is the desired turning direction.

I learned this with R/C engines...LoL...bloddy fingers make you pick up things rather fast.

Heat stress fractures come to mind. Explode like a grenade!

How often do they changeout the turbine blades?

It depends on the unit itself and how many times it was exposed to very high temperatures, sometimes beyond the ITT limits, GE engines (turbine blades and turbine nozzles actually) often have a ceramic coating on them, which increases the lifespan by a quite a bit. It highly depends on the temperatures so I'd say this needs to be looked at individually.

Things like FOD, loss of the ceramic coating or idling the engine without the cooling it down can decrease the time between changes/overhaul. But if everything goes well throughout its service time, it can be used safely for a long time. Other parts of the engine don't even have a lifespan though.

Heat stress fractures come to mind. Explode like a grenade!

 

How often do they changeout the turbine blades?

On modern engines the blades are being actively cooled by blowing cool air through orifices on the leading edge, which forms a protective layer of cool air around the blade. They may or may not have a ceramic protection layer as well, and the super alloys that the blades are made from are especially tailored to have good tensile strength at high temperatures to begin with.

Sirius, F/A-22 does not exist anymore. It's been 13 years already since its extinction.

Regarding the afterburner blueish flame, there is nothing secret about it :) It has a blueish/purple flame due to the combustion temperature and efficiency. Generally speaking, the russian engines have this characteristic, due to the bypass ratio and higher amount of O2. The flame is hotter and more efficient simply because it has more oxygen available. In this case, though, the engine is on the testbench, being run in a cell with a limited amount of light, if you go outside and run the engine in full AB, you'll see the color changing to yellow. But most of the time, regardless the efficiency and temperature of the afterburner, they will always look blueish at night/ambients with a limited amount of light.

Take a look at the AL-31 turbofan family, especially the ones mounted on Su-30s. They are blueish, they look stunning.

But the best one has to be the Tu-22; It has to be the most beautiful and most efficient afterburner ever! :)

If the F-35 had a blue afterburner, it would be amazing too.

Keep in mind that this has nothing to do with fuel!

Sure! F101 is a very nice engine. Beautiful afterburner too.

Love watching the B-1Bs taking off in full afterburner. :)