Does the Gas Port Control Pressure?

[mcb]

I will add two more reference to the thread that might be interesting reading for those that really want to get into the details. Both are from Aberdeen.

@luzyfuerza I agree the gas port is likely a choked super sonic nozzle for most of the time gas is flowing. See the first reference below I think it will confirm your choked flow assumption is a good one though it is far from a simple steady state choked flow for some parts of the cycle. The author, Spurke, also makes and argument in an addendum to the paper that treating the port as an area discontinuity rather than a nozzle might be a better way to model it.

http://www.dtic.mil/dtic/tr/fulltext/u2/704342.pdf
This one deal extensively with the flow through the gas port and some of the interesting super sonic flows in the gas tube.

http://www.dtic.mil/dtic/tr/fulltext/u2/880431.pdf
This one covers both the M-16 and the AR-18 so be aware of the two different gas system modeled in this paper.

 

[luzyfuerza]

The differential equations describing initial flow through the gas port were tough to solve using the computational power available in the 1970s. Fortunately, these initial flows are insignificant compared to the quasi-steady state flows that follow once the standing shock is formed in the gas port.

Another point that these papers make is that the time for the pressure signal to travel down the gas system from the gas port to the bcg (at the speed of sound) is very short compared to the total operating time of the system. This delay in pressure reaching the bcg is deemed to be immaterial in understanding the overall performance of the system.

Mcb, these documents really make it clear how these gas systems work! Thanks for posting them!

 

[Varminterror]

The function of the gas port is actually significantly different than I think has been described above.

When a compressible fluid flows through a given orifice (like an AR gas port) higher upstream pressures create higher mass flows (assuming constant downstream pressure). UNTIL the upstream pressure reaches the CRITICAL PRESSURE. At this critical pressure, a standing shock wave forms in the throat of the orifice, and mass flow cannot increase any further (the nozzle is choked) no matter how much higher upstream pressure is raised.

Of course, pressure downstream of the gas port in an AR isn't constant during the firing cycle. However, as long as the ratio of upstream and downstream pressures stay above the CRITICAL PRESSURE RATIO, the gas port will remain choked.

On page 10 of the excellent report referenced above, the little * marks above the parameters at the gas port indicate operation at critical pressures.

Bigger orifices, when choked, will pass more mass.

Mass flows through the gas port of an AR are steady as long as the gas port is operating above the critical pressure ratio, whatever that ratio is for a particular fluid temperature.

This fact creates a constant mass flow into the gas system for much of the firing cycle; a flow rate that does NOT vary with barrel gas pressure.

Wholly incorrect based on your own statements. Of most basic principles in thermodynamics, and equally of combustion of progressive powders:

Pressure and temperature are interrelated. Wave velocity is dependent upon temperature (and media density), such when the bore pressure varies, the density and temperature vary, as does the resulting maximal wave velocity.

The flow is limited by the port pressure differential, but it is not constant regardless of bore pressure, because the bore pressure dictates the speed of sound within the medium.

 

[luzyfuerza]

Pressure and temperature are interrelated. Wave velocity is dependent upon temperature (and media density), such when the bore pressure varies, the density and temperature vary, as does the resulting maximal wave velocity.

I can only guess what you might be referring to as "maximal wave velocity". I presume that you meant speed, since velocity always includes a vector (directional) component. Speed of sound? Then just call it the speed of sound. Speed of wave propagation in a tube? Who knows? To keep the conversation meaningful, use real, well-defined terms.

Wave propogation speed (say, down a barrel or a gas tube) is not the same phenomenon as flow through a standing shock wave in a nozzle (which is what happens at the gas port). Temperatures and pressures inside the barrel DO vary, but in the case of the AR gas port, these conditions appear to be more than sufficient to create a steady, standing shock in the throat of the port, at least during the quasi-steady state period of gas cycle operation. The differential equations described in the paper that mcb provided above describe exactly how conditions (temperature, pressure, and resulting density) of the gas in the barrel affects mass flows during the initial inflow through the nozzle and during the quasi-steady-state period of gas system operation.

Note that the steady state period while the standing shock is in place is referred to as "quasi" steady-state. This means that it isn't a perfect steady-state condition, but for practical purposes (for "government work"?) it can be considered to be just like true steady state.

If you're going to claim that the effects of changing barrel gas conditions have a significant effect on mass flows across the standing shock in the gas port during the quasi-steady period of gas port operation, and that these variations really do effect gas system operation (which is what I think you're claiming above) then you're going to have to solve these equations numerically (they're too complex to determine exact solutions), or find someone who already has solved them numerically and published the results to prove it. Something more technically defensible and less inflammatory than this:

Wholly incorrect based on your own statements. Of most basic principles in thermodynamics, and equally of combustion of progressive powders:

...etc.

 

[Varminterror]

Can’t have it both ways - you’re stating the wave speed is constant because of the properties of the system, but then say the speed is constant regardless of variations in the properties of the system. The speed of sound varies with the density of the medium, which varies with temperature, pressure, and composition, and in the case of AR’s, also changes with port location - and the velocity across the port varies with diameter of the port...

Again, can’t have it both ways. Either thermodynamic principles apply, or they don’t.

 

[luzyfuerza]

No, what I said is that you're describing a variation that doesn't practically matter. "How many angels can stand on the head of a pin" stuff.

I simply asked that you prove your assertion that the variation you're discussing has a significant impact on gas system operation through the use of engineering tools. The normal engineering tool to be used here is to provide a numerical solution to the underlying differential equations for various barrel gas conditions. This isn't tough at all, but I haven't done it for this specific situation. Your assertion, your burden of proof.

You are familiar with differential equations and have developed numerical solutions for them, right?

 

[mcb]

I have nearly 300 lines of python code that attempted to take Werner and Spurkes work and create my own functional dynamic simulation of an AR gas system based on their research. In the end I did get it to run and agree to some marginal measure with experimental data but never good enough to trust as a predictor of a new configuration, especially one involving another cartridge. I shelved the program and though it was sort of a waste of time since I did not have the fluid and thermal skills to make it really work I did appreciate the intuition the endeavor gave me. In reality it's simply easier to start with a small gas port and work up to what is already know to be good bolt velocities. We know the gas port is the "Big Dial" and that early research supports that assumption even if we still argue exactly how the gas port does it. You can empirically solve most of your gas issues by turning that "Big Dial".

 

[luzyfuerza]

Amen!

 

[luzyfuerza]

Mcb, in your model, do you recall how much mass flow through the choked gas port changed over the gas cycle?

 

[mcb]

Mcb, in your model, do you recall how much mass flow through the choked gas port changed over the gas cycle?

Unfortunately I do not off the top of my head. I found the code this morning when I was finding the two reference I posted above. I popped it open and realized that I never commented the code very well and it would take me a fair amount of time to parse that code again. I wrote that nearly three years ago and have not look at it since I shelved the idea. Sorry.

 

[Varminterror]

We know the gas port is the "Big Dial" and that early research supports that assumption even if we still argue exactly how the gas port does it.

This was largely the driver for this thread - there was a disagreement the gas block was a dial at all. We came together in this one around proof the port/block is the “big dial,” usually it doesn’t happen that way online.