In a jet engine, fuel is added at the center. Why doesn't the released energy flow out both ends equally, with no net thrust? The claim is that the front fan prevents this, but it gets its energy from the rear fan so there can be no net thrust or energy from the fan.
The ultimate example is a RAM jet with no fans at all. It seems symetrical, but the thrust is only forward. There must be some equation that says the increased presure from the expanding fuel, which must be the same in all directions, causes less wrong way thrust slowing the incoming air than it produces right way thrust accellerating the outgoing air. How does this happen?
The pressure from the incoming air is stronger than pressure from the expanding gas. This is because the incoming air is fed through a set of fans that compress the air, increasing the force of the air, but also priming it for an injection of fuel.
Ramjets work without a compressor because they are traveling fast enough that the shape of the engine acts as an adequate compressor.
Ramjets work without a compressor because they are traveling fast enough that the shape of the engine acts as an adequate compressor.
QUOTE (flyingbuttressman+)
..increasing the force of the air
?
?
QUOTE (Confused2+Jul 28 2009, 10:14 AM)
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High pressure gas produces a force when flowing into a low(er)-pressure area, no?
High pressure gas produces a force when flowing into a low(er)-pressure area, no?
Dude, look up HowThingsWork. It's there. And about a ram jet, look at the design.
What happens in a jet engine is this, the air is pressurized, then heated, using the flame using the fuel, and then the heated, pressurized, air shoots out the back shooting the engine forward. Why doesn't it go out both ways? Well, here is the thing. A ram jet engine is special because it has to be pushed off. A normal jet engine starts itself using no other help. So, the air is going out only one way in both engines. It is that simple.
What happens in a jet engine is this, the air is pressurized, then heated, using the flame using the fuel, and then the heated, pressurized, air shoots out the back shooting the engine forward. Why doesn't it go out both ways? Well, here is the thing. A ram jet engine is special because it has to be pushed off. A normal jet engine starts itself using no other help. So, the air is going out only one way in both engines. It is that simple.
The reason is that, by burning kerozene, the air is heated and expands.
Then, the turbine has more P*V available than the compressor must push. The power (not P*V but related) taken by the turbine is bigger than consumed by the compressor, and some is even left to get a thrust.
However, designing a jet engine that runs isn't easy. The maximum temperature allowed by the turbine is rather low, so the burnt air contains little more energy (enthalpy) than after compression. Add to it that turbines and compressors have a limited efficiency, especially on aeroplanes where the engine must be light, and then getting some overall efficiency from a jet engine is difficult.
The same (P*V increased by burning kerozene) holds for a Ramjet.
Just for fun: some Ramjets for M>6 don't even have a tube. The aeroplane's fuselage compresses and expands the air; ejecting fuel from the fuselage creates thrust.
About symmetry: jet engines aren't symmetrical in their construction. And anyway, once air has begun to flow in one direction thanks to a starter, symmetry is already broken.
Then, the turbine has more P*V available than the compressor must push. The power (not P*V but related) taken by the turbine is bigger than consumed by the compressor, and some is even left to get a thrust.
However, designing a jet engine that runs isn't easy. The maximum temperature allowed by the turbine is rather low, so the burnt air contains little more energy (enthalpy) than after compression. Add to it that turbines and compressors have a limited efficiency, especially on aeroplanes where the engine must be light, and then getting some overall efficiency from a jet engine is difficult.
The same (P*V increased by burning kerozene) holds for a Ramjet.
Just for fun: some Ramjets for M>6 don't even have a tube. The aeroplane's fuselage compresses and expands the air; ejecting fuel from the fuselage creates thrust.
About symmetry: jet engines aren't symmetrical in their construction. And anyway, once air has begun to flow in one direction thanks to a starter, symmetry is already broken.
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