Hi,
I think I understand quite well how does a turbine jet engine work, But there is something i don't understand.
When the copmpressed air is being ignited, more pressure is made in the combustion chamber. Why does the (more) copressed burned gas doesn't flow back into the compressor direction.
Or, How does the compressed air can get into the combustion chaber since the pressure there is (or should be) much higher and there is a direct path between the compressor and the combustion chamber?
Thanks in advance,
Zvika
How does a turbine jet engine work
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leo
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re: How does a turbine jet engine work
There isn’t produced more pressure in the combustion chamber, but more volume.
And the pressure has to be even lower to get it to the turbines, but because of the bigger volume there is more energy in it to drive the turbines.
And the pressure has to be even lower to get it to the turbines, but because of the bigger volume there is more energy in it to drive the turbines.
re: How does a turbine jet engine work
First, Thank for the reply,
This is exactly the thing I don't understand.
By burning the fuel and increasing the gas volume, according to the basic lows of thermodynamics (I don't recall which one of them), increasing the volume in a constant container (And I know that this isn't the exact case, since there is a not fully open path through the turbine), pressure increasing as a result.
As well as I understand it, the all idea of a jet engine is to greate a higher pressure at the rear side of the engine, and i thought that the pressure increment (by increasing the volume, of course) is made in the combustion chamber (and uses to drive the turbine and supply the thrust).
Please try to be more specific, since as I said before, this is exactly the thing that I'm getting confused by.
Thanks again,
Zvika
This is exactly the thing I don't understand.
By burning the fuel and increasing the gas volume, according to the basic lows of thermodynamics (I don't recall which one of them), increasing the volume in a constant container (And I know that this isn't the exact case, since there is a not fully open path through the turbine), pressure increasing as a result.
As well as I understand it, the all idea of a jet engine is to greate a higher pressure at the rear side of the engine, and i thought that the pressure increment (by increasing the volume, of course) is made in the combustion chamber (and uses to drive the turbine and supply the thrust).
Please try to be more specific, since as I said before, this is exactly the thing that I'm getting confused by.
Thanks again,
Zvika
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skyfrog
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re: How does a turbine jet engine work
Well said Ben, I totally agree your points.
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larry cottrill
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re: How does a turbine jet engine work
Blast! I wish I had a way to scan stuff into a graphic file:
In my old jet design book, there's a graph of intake air Compression Ratio vs Distance for a centrifugal impeller, its vortex, and a diffuser (which is divided into 'diffuser' and 'outlet cone' sections with turning vanes in-between). Several 'tip speed' graphs are superimposed, but the highest speed (1500 ft/sec) is the most interesting, of course:
Near the impeller's center, the comp ratio is right around 0.8; the graph arcs geometrically upward (like a parabola) to a value of about 2.4 as it leaves the impeller tip and throws into the vortex. Within the vortex, it begins to increase at an ever-lower rate, reaching 2.9 just as it leaves the vortex and heads into the diffuser. There is a fairly sharp break shown in this transition; i.e. the intake of the diffuser is suddenly a bit larger than the vortex outlet area (presumably where the two housings bolt up to each other). There is an immediate sharp rise in comp ratio in the diffuser, but again the rate of increase rolls downward; it is a smooth, almost circular arc through the diffuser and outlet cone and seems to be unperturbed by the 'corner vanes'. At the end of the diffuser, where the air would be entering the combustion chamber, the ratio reaches well over 3.9! Unfortunately, the air velocity is not shown, but would be decreasing radically in the diffuser as the comp ratio rises, of course.
What it amounts to is that what you want is a rise in air pressure and a decrease in air velocity as you approach the intake of the chamber. You need a reasonably low velocity going into the combustion zone of the chamber for good combustion. There will be a significant DROP in pressure as the air passes into the chamber (a large part of this drop will be in 'nozzling' through the holes in the flame tube). This ensures that there will never be backflow from the chamber forward. Similarly, there is a significant pressure drop (and a LARGE velocity increase) through the chamber exhaust nozzle into the turbine zone, then a final drop to atmospheric pressure beyond the turbine, as Ben has said above.
The turbine acquires energy (used just to drive the compressor, in a jet propulsion application) basically from the chamber exhaust velocity, rather than from excess pressure! The function of any jet exhaust nozzle is to "convert" pressure into high gas jet velocity as efficiently as possible.
L Cottrill
In my old jet design book, there's a graph of intake air Compression Ratio vs Distance for a centrifugal impeller, its vortex, and a diffuser (which is divided into 'diffuser' and 'outlet cone' sections with turning vanes in-between). Several 'tip speed' graphs are superimposed, but the highest speed (1500 ft/sec) is the most interesting, of course:
Near the impeller's center, the comp ratio is right around 0.8; the graph arcs geometrically upward (like a parabola) to a value of about 2.4 as it leaves the impeller tip and throws into the vortex. Within the vortex, it begins to increase at an ever-lower rate, reaching 2.9 just as it leaves the vortex and heads into the diffuser. There is a fairly sharp break shown in this transition; i.e. the intake of the diffuser is suddenly a bit larger than the vortex outlet area (presumably where the two housings bolt up to each other). There is an immediate sharp rise in comp ratio in the diffuser, but again the rate of increase rolls downward; it is a smooth, almost circular arc through the diffuser and outlet cone and seems to be unperturbed by the 'corner vanes'. At the end of the diffuser, where the air would be entering the combustion chamber, the ratio reaches well over 3.9! Unfortunately, the air velocity is not shown, but would be decreasing radically in the diffuser as the comp ratio rises, of course.
What it amounts to is that what you want is a rise in air pressure and a decrease in air velocity as you approach the intake of the chamber. You need a reasonably low velocity going into the combustion zone of the chamber for good combustion. There will be a significant DROP in pressure as the air passes into the chamber (a large part of this drop will be in 'nozzling' through the holes in the flame tube). This ensures that there will never be backflow from the chamber forward. Similarly, there is a significant pressure drop (and a LARGE velocity increase) through the chamber exhaust nozzle into the turbine zone, then a final drop to atmospheric pressure beyond the turbine, as Ben has said above.
The turbine acquires energy (used just to drive the compressor, in a jet propulsion application) basically from the chamber exhaust velocity, rather than from excess pressure! The function of any jet exhaust nozzle is to "convert" pressure into high gas jet velocity as efficiently as possible.
L Cottrill