Venturi

[mobilehomer]

Just wondering, has anyone ever tried a venturi transfer tube? What would be the effects?

[TPL]

I haven't but I think you mean de Laval nozzle?

[mobilehomer]

I haven't but I think you mean de Laval nozzle?

I'm not sure, butI don't think so. No heat involved. More like the throat on a carburetor than a rocket engine.

[HunterWhite]

I have wondered the same thing, but probably for a different reason than your musing.
Bernoulli and Venturi did research and theory that has changed the world.  Since their work is directly related to pneumatic rifles it only makes sense to constantly reapply those discoveries.

My musing:
Could a transfer port be designed to regulate (within reason) the velocity of the pellet?
I think so for two reasons:
First because we already have a lot of data about the peak velocity for different transfer port (TP) sizes, and because some air rifles have a velocity metering screw (VMS) before the transfer port. That screw is adjustable to permit tuning, but I would think that it would disrupt the airflow past the screw. It does clip the high velocity more than the low pressure velocity. The VMS and TP together provide a acceptable level of regulation without a dedicated pressure regulator and plenum.

It would seem that the flow would remain laminar at lower pressures, but at higher pressures the velocity increase may drop the pressure through the port until the air is so thin that it goes supersonic. This would create a barrier to limit more airflow, thus limit the velocity of the pellet. I guess that that's what the TP does anyway, in a clumsy way. A venturi TP may round out the shot curve a little better than a standard cylindrical TP.

What do you think?

Hunter

[HunterWhite]

I guess that there are three levels of air restriction:
Valve porting,
VMS,
TP.
None of these seem to be true venturi tubes.
Hunter

[TPL]

Some kind of regulation might be possible but I think any choke would do the same effect.

If thought velocity increase this is eye opening picture:



And tempereature, pressure and velocity are bind together in gas flow, that cannot be avoided. Pressure is what it takes to accelerate bullet.

Besides, there is a bullet to block free flow after whatever TP.

[HunterWhite]

I'm thinking chocked flow.
Choked flow is a fluid dynamic condition associated with the Venturi effect. When a flowing fluid at a given pressure and temperature passes through a constriction (such as the throat of a convergent-divergent nozzle or a valve in a pipe) into a lower pressure environment the fluid velocity increases.
. . .
The choked flow of gases is useful in many engineering applications because the mass flow rate is independent of the downstream pressure, and depends only on the temperature and pressure and hence the density of the gas on the upstream side of the restriction. Under choked conditions, valves and calibrated orifice plates can be used to produce a desired mass flow rate.
. . .

Keep in mind that I have a VERY limited understanding of any of this

Hunter

[HunterWhite]

And I found this, I'm not sure exactly how this relates to standard transfer ports.

Thin-plate orifices

The flow of real gases through thin-plate orifices never becomes fully choked. The mass flow rate through the orifice continues to increase as the downstream pressure is lowered to a perfect vacuum, though the mass flow rate increases slowly as the downstream pressure is reduced below the critical pressure. Cunningham (1951) first drew attention to the fact that choked flow will not occur across a standard, thin, square-edged orifice.

Hunter

[HunterWhite]

Just wondering, has anyone ever tried a venturi transfer tube? What would be the effects?

Ken, what were you hoping for?

Hunter

[mobilehomer]

Just wondering what the effect, if any, would be for efficiency, power or both. Just thinking.

[HunterWhite]

I don't think that it would have any effect on power or efficiency,  but may have some effect on extreme spread in an unregulated rifle.
To be realistic, I could have this fuzzy idea all wrong.

Hunter

[rsterne]

Venturi have not been well explored in airguns, although I have read the odd article about trying it.... As far as the idea of there being ANY laminar flow in any of the ports or barrel, I really doubt that.... The air has to turn corners just to get past the poppet and seat of the valve, which would surely trip the flow to turbulent.... and once that occurs, it never returns to laminar.... There have been (at least) two threads here in the Workshop which argued at length what is happening inside a PCP at higher velocities, and I would suggest that for those interested you do some background reading in those, and add your contributions to them if you like....

https://www.gatewaytoairguns.org/GTA/index.php?topic=102604.0

https://www.gatewaytoairguns.org/GTA/index.php?topic=94054.0

There is one thing not well understood about the flow through the valve and ports.... At he beginning of the shot, when the valve is opening, all that is really happening is that the volume between the valve seat and the base of the pellet is being pressurized.... That occurs very rapidly, as the molecules of air are moving randomly at about 1650 fps and it only takes a few microseconds for the pressure at the pellet to reach nearly reservoir pressure.... There is no real "airflow" velocity during this time.... Once the pellet starts to accelerate, the real flow of air begins, and it accelerates through the valve, ports and barrel as the pellet accelerates away from the breech.... During the latter part of the shot cycle, just before the valve closes, is when the flow is the fastest.... Even if the flow through the ports remains Subsonic, there is one location where eventually it will, indeed, choke.... That is at the valve seat, as the poppet approaches the seat on closing....

When that choke occurs, the pressure differential across the poppet is at least 47% of the air pressure, as that is a condition of choked flow.... This begins the cessation of flow, and the pressure differential slams the poppet shut.... You can think of the point where the flow chokes as the effective end of the shot cycle....  Remember than when calculating the Speed of Sound, and therefore the velocity where the flow will choke, you cannot just use 1126 fps.... At the very high pressures inside a PCP, the SoS increases.... That information is in the threads linked above....

Bob

 

[mobilehomer]

Thanks, Bob. I was just curious. And we all know what happened to the cat.   

[HunterWhite]

Thanks Bob, sometimes I need a wakeup slap. While I've been reading your posts since 2016, I rarely understand all of it.
I feel silly that I wanted to try some experiments on the Condor.

Hunter

[rsterne]

All our theories are great.... until somebody comes up with an experiment that blows them away.... then it's time for a new theory.... It wasn't very long ago that I believed the statement that a PCP running on air at room temperature could never exceed 1650 fps because that was the average molecular speed, so a column of air could never expand faster than that.... I saw one guy make a shot at over 1700 fps and started to question it.... Then Lloyd did over 2100 with a purpose built experimental PCP, and I knew the theory was wrong.... Since then I have done 3 shots averaging 2092 fps doing nothing more outlandish than using a 1.8 gr. Airsoft BB in a 6mm PCP that would shoot a 73 gr. lead bullet at 175 FPE on 4200 psi.... 

Bob

[HunterWhite]

Thanks for the encouragement Bob. Maybe I will try my silly idea after all!

Hunter