It is my understanding that jet engines work via the ,”suck squeeze, bang blow process”. The main fan introduces air into the engine, and the compressor blades squeeze this air into a higher pressure region as it moves through the compressor stages toward the combustion chamber. If each subsequent compressor stage produces a region of pressurised air higher than the previous section, why does the air not move forward back towards the fan and area of lower pressure? What keeps the air flowing rearward?
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1$\begingroup$ I would think the fact the compressor simply never stops running is enough to stop reverse flow. $\endgroup$– DKNguyenJul 14, 2022 at 13:30
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$\begingroup$ A jet engine is continuous combustion, so where is the "bang"? $\endgroup$– Solar MikeJul 14, 2022 at 14:09
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2 Answers
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Well, it can, and sometimes it does. That is called a compressor stall.
https://www.youtube.com/watch?v=7MYh2KtEUQk
So if you waded through the video, let me explain a little bit more if I can.
The compressor section has rotors and stators. The shaft powered rotors add kinetic energy to the incoming air, and the stators between the rotors transform the kinetic energy into a pressure gain.
The combustion chamber adds heat. It doesn't add pressure. I'm going to repeat that - the combustion chamber doesn't add pressure, just heat which expands the air. In a piston engine, the combustion products can't expand - that's why it creates pressure. In a jet engine, we want the combustion products to expand and not create pressure.
The turbine section then takes kinetic energy out of the hot gasses and drives the compressor rotors with it. It is designed so that the back pressure is less than the compressor head pressure. On a power basis, the turbine works on higher volume and lower pressure change than the compressor. That's why the pressure is higher in front of the combustion chamber than behind it, and air can accelerate rearwards.
The highly cambered foils in the compressor rotors and stators mean that the thing just doesn't work going backwards. But sometimes, it doesn't work going forward either. All the vanes on the rotors and stators are lift devices and subject to stall, where either they don't add the kinetic energy, or they don't transform the kinetic energy to pressure. Once there is a local stall, it tends to rapidly expand to involve the entire compressor.
The following video shows some hardware and explains the stator control and bleed air systems used to manage stall. The stators are shown around 20 minutes in.
https://www.youtube.com/watch?v=yaFeO6Jze64
In a jet engine, there is no suck, nor squeeze, nor bang, nor blow. None of those things happen. Piston engines work that way.
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$\begingroup$ Hey thanks so much for the comment. How can the combustion only add heat and not pressure. Surely if the heat increases the hot gasses would be more energetic and be colliding causing a pressure increase? If you increase the temperature of air in a tire the pressure increases also?? $\endgroup$– RichardJul 16, 2022 at 20:58
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$\begingroup$ You're used to looking at constant volume behaviors, or quasi static problems that can be modeled as constant volume systems. The jet is an open system. Heating the air in a hot air balloon doesn't increase pressure, it reduces density (increases volume, which spills out the bottom and changes the pressure gradient within the balloon). $\endgroup$ Jul 16, 2022 at 23:51
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The compressor works on the fluid to increase pressure (a rotor row adds kinetic energy and the following stator row decelerates fluid for a corresponding rise in pressure). You probably wouldn't ask why water doesn't go in the reverse direction when a pump pushes water from low elevation to a higher elevation because pumps are a more common experience than compressors.
If the compressor doesn't work as designed, reverse flow is possible.
The suck, squeeze, bang, blow comment is not necessarily wrong if you consider the history of a distinct fluid particle moving from the inlet to the compressor, combustor, turbine, and nozzle. The bang is still in the combustor although with negligible pressure change (constant pressure combustion).
