Rolf, Mark and All,
I have to correct my previous post. Mark was right. The total pressure
in the turbine is the sum of the static and dynamic pressure.
This is exactly what Bernoulli's Equation tells us:
Along a streamline: static pressure + dynamic pressure = total pressure
Expressed mathematically:
static pressure + (density * Velocity^2 / 2) = total pressure
In reference to that turbine chart, the pressure obviously shows the sum
of hte static and dynamic pressure.
The reason the flow does not back up is still explained by inertia, but
it is true that the total pressure equals the dyanmic and static
pressure, as Bernoulli tells us. No doubt about it.
Regards,
Gordon.
*/Rotary Engine
<rotaryeng@earthlink.net>/* wrote:
> 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.
>
Well said Gordon.
Another way to look at it is that the ignited and combusted gas goes the
ways of least resistance.
There is a lot less resistance going out the exhaust nozzle as there is
going back against the compressor.
Rolf Pfeiffer
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