And here is a chart showing the temperature, pressure and velocity of
the gas flow in a turbine engine, the J79.
Regards,
Gordon.
******************************************
Okay, here's a basic (probably dumb) question: Notice that the
pressure in
the middle of the combustion section is higher than the pressure at the
entry of the combustion section. Given this, why doesn't the gas flow
forward (to a section of lower pressure) to the compressor section?
Is it
maybe the fact that "pressure" shown in the chart is "static
pressure" and
that the static pressure PLUS the dynamic pressure of the gas moving aft
into the combustion section is higher than the static pressure in the
combustion section, thus the gas keeps moving aft?
Mark Waldron
--------------------------------------
No signature so I had to cut and past Mark Waldron name ;)
You guys are hard to train.
There are no dumb questions. In fact that is an excellent question.
Gases have velocity, mass and inertia, not shown in the diagram,
that also explain their behavior.
Ram jets and SCRAM jets work in exactly the same way.
Paul Lamar ...No rotor no motor.
--------------------------------------
Good question, Mark.
As Paul mentioned, the reason the gas doesn't back up is because it
has inertia due to its mass and velocity. In order to stop and
reverse its flow back into the compressor, the pressure rise in the
combustion chamber would have to be high enough to overcome the inertia.
Another way of stating this is momentum. If you run down a football
field you have a momentum equal to your mass times the speed you are
going. Inertia is the tendency of wanting to keep going in the same
direction and speed.
You mentioned static and dynamic pressure. If you take a bicycle
pump and pump up a tire you have increased the static pressure
inside the tire. But there is no dynamic pressure because the air is
not moving -- nor is there an object moving against the air.
Now when you unscrew the valve, you release a jet of air which has
dynamic pressure. If you put your hand against it you can feel a
force. This force is the result of the momentum that the moving air
imparts to your hand as it hits. This is what thrust is.
Now think what it would take to make that stream of air coming out
of the tire reverse course and flow back into the tire. The pressure
in the chamber in which the tire is sitting would have to increase
very quickly and dramatically in order to arrest that jet and force
it back into its hole.
So yes, you thought it out correctly that the dynamic pressure of
the air flow inside that engine is what keeps it from flowing in
reverse.
Regards,
Gordon.
Just to add a bit of clarification about the question of static and
dynamic pressure as it relates to the chart of the jet engine.
The air being pumped by the compressor into the combustion chamber
has dynamic pressure by dint of its velocity and its mass.
However, we do not add this dynamic pressure to the "static"
pressure inside the combustion chamber, as Mark speculated. The
total pressure is what is shown on the chart -- that is the pressure
exerted on the chamber walls perpendicular to the fluid. The total
pressure inside the combustion chamber is the result of the air that
is forced in by the compressor.
As we already explained, the gas does not back up because the
pressure increase from combustion is not enough to overcome the
inertia of the moving gas. Note that as the pressure increases at
the beginning of the combustion phase, the velocity decreases
correspondingly.
In order for the gas to back up, the pressure would have to be high
enough to bring the velocity to zero and then to negative velocity.
So the concept of dynamic pressure is relevant here only in the
sense that it means the moving gas has momentum and inertia, which
makes it resistant to backing up toward lower pressure. It does not
mean that the actual pressure inside the chamber is higher than what
is measured.
Think of tire with a hole in it. If you keep pumping to keep the
tire at a particular steady pressure of say 30 psi, you will have
dynamic pressure associated with the airflow coming into the tire as
well as the airflow blowing out of the hole. But the pressure inside
the tire will remain steady at 30 psi.
The jet of air going into the tire has dynamic pressure because it
is moving, but this pressure does not add to the overall pressure
inside the tire. This is similar to how an engine works, with gases
coming in one end, doing some work and then exiting at the other end.
Regards,
Gordon.
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