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Every time I see explanations about jet engines, I have the same basic question that never seems to get addressed. I think I know what the answer, but anyhow: w
by joelthelion 4y ago
Every time I see explanations about jet engines, I have the same basic question that never seems to get addressed. I think I know what the answer, but anyhow: why do the exhaust gases flow in the direction that we want? What prevents them from exiting from the intake?
- chewbaxxa 4y agoThe pressure is high at the intake
- joelthelion 4y agoNot 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.
- exDM69 4y agoBecause 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.
- joelthelion 4y agoSo 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?
- exDM69 4y agoThere 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.
- deleted 4y ago[deleted]
- Gibbon1 4y agoIt'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.
- dogline 4y ago> Which means the volume has to increase Ah, there's the key.
- Gibbon1 4y agohttps://clubtechnical.com/brayton-cycle 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.
- rwmj 4y agoIt happens - compressor surge (https://en.wikipedia.org/wiki/Compressor_stall https://en.wikipedia.org/wiki/Compressor_stall). Concorde infamously suffered from this during development. The modern answer is that there's an electric starter motor which starts the compressor before the fuel is injected, and better computers controlling the fuel flow.
- GuB-42 4y agoGenerally, it is because it is harder for the air to exit from the intake than it is to exit through the exhaust. It can be because the inlet into the combustion chamber is larger than the outlet, because the blade pitch is finer on the compressor side than it is on the turbine side, because of the specific geometry of various components, ... Often all of that. The intake is optimized for low speed high pressure (lots of elements, small space, fine pitch, ...) , the exhaust is optimized for high speed low pressure (less elements, more space, coarser pitch), and naturally, the air will flow from the high pressure zone to the low pressure zone.
- joelthelion 4y agoThank you!