Hacker News new | ask | show | jobs
by chewbaxxa 1426 days ago
The pressure is high at the intake
1 comments

Not outside the intake (outside the engine). Basically, to answer the question fully, you also need to say why the pressure is high there, and why it is higher than on the other side of the engine.
Because the compressor is on the intake side of the combustion. The compressor increases the pressure more on the intake side than the resistance from the turbine does on the output side.
It's been so long since I took thermo and I have no engineering experience with gas turbines but.

You add heat at constant pressure to the air flowing through the engine. Which means the volume has to increase. Only way balance things out is the velocity of the air has to increase as well.

> Which means the volume has to increase

Ah, there's the key.

https://clubtechnical.com/brayton-cycle

If you look at the TS diagram state 1 is the intake. 2 is after the compressor. 3 is after the burners. 4 is the exit from the turbine. Temperature is decent proxy for total energy. You can see the vertical lines of constant entropy. That means the energy is being extracted as work. Note real compressors and turbines the lines are angled slightly which reflects inefficiency.

You'll note the difference between 3->4 is larger than 1->2. The turbine expanding the hot gas produces more power than the compressor does compressing the cold air.

So it boils down to the angle of the blades being steeper on the compressor side than on the turbine side, doesn't it?

Another related question: what happens if you don't rotate the engine before you ignite it?

There will be a fuel fire inside the engine. Just a regular fire at atmospheric pressure.

But jet engines can and do backfire spectacularly at times. You can find lots of cool footage from hobby turbine engine enthusiasts on the tubes.