More GEEKNESS on the sonic choke theory .... AKA: TAPERED delivery passages ?

[Motorhead]

  I'm not even going to begin trying to share formulas or math or accepted physics here 

Just wish to have those that can ... elaborate on something I have been fooling with with much success & really can't say as to the why it works so well beyond the obvious Less drag, lower turbulence.

That is the use of a transfer port leaving the valve that starts off larger than specs of a standard straight hole port ( having a valve also above required flow specs * big throat and exit from it ) so that the amount of air & pressure within throat now gets forced into a tapering orifice to the pellet. ( transfer tube / port as we call them )

In other words ... lets say we have a valve utilizing a .235" throat & a stem of .118" which if using a straight diameter transfer tube to pellet would support a diameter of @ .200" ( This being of approximate equal area threw it all )
* Velocity of the air moving threw said spaces should be fairly consistent being area stays @ the same.

Now what happens if we simply decrease transfer tube I.D. with a straight wall drilled hole of less diameter ( Common practice ) ... flow drops and pellet speed drops too 

What happens if we decrease the transfer tubes area by miens of a TAPERED hole threw the transfer tube having at valve the area of the valve throats exit diameter ... but the exit at barrel being reduced to a smaller diameter value ?

The SPEED of the air exiting valves throat with the greater area is ( As I see it ) forced to accelerate progressively as the cross sectional area of the tapered tube diminishes / reduces.  In doing so Speed of the air is increased and the DURATION of the pressure pulse ( Fed by greater volume down stream ) increases.

Now take this idea of thought and make the proportions of valves throat much greater than the flow actually required being throttled down via a tapered transfer passage shape.   * Could one not set up the dwell cycle of the actual valve poppet VERY short yet get benefit of the tapered delivery passage shape to give more duration of pressure & flow exiting behind pellet ?


Anywho ... hope this makes sense 
Scott

[Rocker1]

 Makes lots of sense spent hours on end with a flow bench and do have a understanding of it, sometimes bigger is not better its the way its delivered, Vortex my friend.  David

[customcutter]

It makes a lot of sense.  I spent 5 hours last Tuesday night with a builder/mfg that argued that very point.  I'm new to the hobby and couldn't follow everything he was explaining, due to nomenclature. But I do know enough about water flow to know that when you reduce pipe size, velocity goes up.  Same thing if you constrict the flow on the end of a hose.  He says he gets higher velocities, and higher shot counts and you don't do that by wasting air drilling out all the contrictions and making air hogs.  Think about it, all those holes that get drilled out are just more volume that has to be "pressurized" in order to propel the pellet.   

[rsterne]

If I understand Scott correctly, he is suggesting that it may be possible to get more power from a smaller passage if the tapers are correct?.... I really doubt if that is the case, but only carefully controlled experiments could prove it one way or the other.... Using his 0.200" equivalent diameter as an example, I can see where a taper down to 0.140" could be more efficient, and deliver more power, than a sharp step down from 0.200" to 0.140".... However, I would really doubt that any carefully developed taper could deliver more power than a straight through port of 0.200"....

All that matters, IMO, is the force that is available to accelerate the pellet.... That force is pressure times area.... and for a given caliber, that means the pressure at the base of the pellet is the important thing.... While a carefully designed venturi could increase the velocity of the airflow at the narrowest point in the system, once the air flows past that point it expands and slows again.... The pressure at the base of the pellet, therefore, should depend on the reservoir pressure and how efficiently that air can get delivered to the pellet....

I can see where a tapered port could be better than a stepped one for a gun where you are reducing the power anyway.... but not for the best overall power....

Bob

[SpiralGroove]

I'm not even going to begin trying to share formulas or math or accepted physics here 
Just wish to have those that can ... elaborate on something I have been fooling with with much success & really can't say as to the why it works so well beyond the obvious Less drag, lower turbulence.

Hey Scott,
What are the actual results you have found, reflecting "much success".
Thanks ........ Kirk

[Motorhead]

Hey Scott,
What are the actual results you have found, reflecting "much success".
Thanks ........ Kirk

All I will disclose at this point is .... my WARP R&D dealing with massive valve flow throttled back to run smaller caliber / weight projectiles.

[Motorhead]

. While a carefully designed venturi could increase the velocity of the airflow at the narrowest point in the system, once the air flows past that point it expands and slows again.... The pressure at the base of the pellet, therefore, should depend on the reservoir pressure and how efficiently that air can get delivered to the pellet....

I can see where a tapered port could be better than a stepped one for a gun where you are reducing the power anyway.... but not for the best overall power....

Bob

What is my premise on this .... is that a compressible medium such as air changes speed in which it travels within a confined & under compressed space exits at a speed based upon the area seeking ambient pressure to equalize with. 

A straight port with X factor pressure & volume feeding will flow fairly consistent until pressure feeding it falls enough the flow volume reduces too.

My thoughts ... For what there worth ?   sees that when said pressure & volume is present UPSTREAM of proposed tapered delivery port
that the acceleration of the flow leaving valve being backed up with Much more pressure to drive it towards atmospheric pressure creates at its front more energy potential in a way solid liquids accelerate out a restricted hose outlet 
But ... being we're talking a gas and not liquid ... as pressure is raised ( across any given area of restriction ) Thus a stall of volume is created increasing DURATION said volume & flow exits from the smallest diameter in the system.

Again ... trying to verbiage my thoughts here.
 

[rsterne]

I think I understand what you are talking about, Scott.... but as you point out, air is compressible and water is not.... Bernoulli's principle, which applies to gasses, states that when air flows through a restriction it speeds up but the pressure DROPS.... The flow RATE stays the same.... and on the downstream side of the venturi the velocity slows down again and the pressure returns to normal.... Water, flowing from the nozzle in a hose, accelerates in the restriction, but remains focused when it exits.... The classic example is a "Hydraulic Monitor" as used in Placer Mining to move tons of dirt by simply squirting water at it....

I have no question that using a gentle taper will reduce turbulence, and therefore frictional losses.... Compared to a sudden step in diameters (from larger to smaller), that should increase the efficiency.... However, when we are talking overall efficiency (ie in FPE/CI) we need to examine PCPs set up for constant FPE.... As you know, there are several ways to get a certain FPE level in a PCP.... You can vary the pressure, barrel length, port sizes, and valve lift/dwell.... Even if we fix two of the quantities, pressure and barrel length, we still have two major ways to "tune" a PCP for a given FPE level.... port sizes and valve lift/dwell.... They both affect the amount of air that gets to the pellet and accelerates it.... You can either have wide open ports and not release much air to travel through them.... OR you can have more restricted ports and release more air, which will arrive at the pellet more slowly.... spread out over time if you wish....

Most of the acceleration of the pellet occurs in the first few inches of travel.... Generally speaking, lots of flow for a short period of time is more efficient than spreading the same amount of air over a longer time period.... That is why we can use less pressure with larger ports to get the same FPE.... Having said that, when you are tuning to a specific FPE level.... as you are, tuning to <20 FPE for FT.... there are a ton of combinations that will work.... You could probably use full bore-size porting and VERY short dwell, and it might be very efficient.... only trying that would tell you one way or the other.... At the other extreme, you could use extremely long duration, so that the valve was still open when the pellet left the barrel, and tiny ports to stay under 20 FPE.... I think we both know that would be terribly inefficient.... Only by trying a large number of combinations in between could you determine the most efficient setup.... and that is further complicated by the fact that when you change the dwell (hammer strike) you will change where on the "curve" you are operating; on the plateau, the knee, or the downslope.... Again, we both know that operating near the knee will give you the best shot count.... and you won't have to worry about the velocity increasing below the setpoint and putting you over 20 FPE if you take a few too many shots before refilling....

If we accept the premise that sudden steps in diameter are inefficient because of turbulence.... then the only way I can see to prove if a tapered (but smaller) port is more efficient is to make a series of them of different exit diameters, and then change the valve dwell to keep the FPE constant, and check the shot count.... For example, use a regulated PCP, and use a straight through port of 0.200" equivalent area all the way from the valve throat to the pellet to start....Tune for 20 FPE and record your shot count for a given pressure loss in your reservoir (eg. 500 or 1000 psi), using an accurate gauge.... Make a series of tapered ports, say every 0.010" in exit diameter, down to as small as you can still achieve 20 FPE with, using maximum hammer strike (ie running on the plateau).... Each time you install a smaller port, increase the hammer strike to get back to 20 FPE, and record the shot count for the same pressure loss.... Keep going smaller on the port until you can't hit 20 FPE no matter how hard you hit the valve.... Then graph shot count vs. port exit diameter, and see what you get.... Two measurements are critical to insure consistent, and therefore useful, results.... You MUST tune to the same FPE for each test.... and you MUST only count shots within the same pressure drop range (eg. 3000 psi down to 2000) and of course you must stop above your regulator setpoint....

My bet is that you will get the most shots with the largest port and the shortest dwell, for a given pressure, and pressure range.... but I've been wrong before.... IMO, only a careful test series can confirm or refute that idea.... If you manage to prove that a smaller port can give more shots at the same FPE from the same amount of air.... then we need to try and figure out why.... Until then, we are dealing with anecdotal evidence....

Bob

[Motorhead]

Some more HARD DATA on testing a STRAIGHT transfer passage to one that is tapered.

* Now understand my R&D is based off using a valve that is capable of providing sufficient air for a Hot .22 or mid power .25 )  The Exit from valves throat is @ .230" and we're plumbed to a .177 cal barrel.

So we have our test gun ( WARP ) set up with a .140" I.D. straight transfer tube with a small bevel at valve throat end.  JSB 10.3 adjusted to shoot @ 880 fps.  ES is running +/- 8 fps or so ( 872-888 ) good but not great.

Changing NOTHING but transfer tube to one having a TAPERED passage ( .230 at throat to .140 at barrel ) we pick up 25 FPS and the ES values nearly halfs to +/- 5 fps ( 875-885 )

Added benefit if my ears don't lie ... dwell cycles shorter because shots are a tad quieter too.

My take away on this is one of turbulence  and resistance to flow over distance.  A tapered delivery tube when used with a big valve flows freer with lower dwell cycle when the outflow of air is uninterrupted with radical changes in cross sectional area of the passages.

FWIW ...
Scott   

[K.O.]

I think there will be a perfect port size for each pellet weight... .140" is fine for 10.3g but say a 15g sniper or 13g monster or 21g piledriver will want what it wants...

If you have .230" feeding it... then for the heavies a heavier charge is needed to get the pellet going...so having an oblong port  say .140" by .180" is needed... so to me the smooth transition from circular to oblong would be the important thing if you are interested in eking out every bit of efficiency you can get...

point is what is most efficient will vary with pellet weight, pressure and barrel length... so we set up for a favorite pellet and the other weights will be a compromise...

[K.O.]

But then again maybe bore size porting could allow a short enough duration that  the lighter stuff stays efficient as the .140" port ..?  I think Bob has done that..?

[rsterne]

I would agree 100% with your conclusions.... Get rid of the turbulence, and the efficiency goes up.... You can use that to get more velocity from the same amount of air.... or the same velocity by reducing the hammer strike and valve dwell.... When you increase the efficiency, generally the report decreases, because more of the energy goes into driving the bullet and less into residual muzzle pressure, which ends up as noise....

I plugged your numbers into Lloyd's spreadsheet, and without a plenum volume and efficiency to work with (FPE/CI) I'm only guessing.... but a 3% increase in overall efficiency is worth about 25 fps in a 12" barrel at 1800 psi if the valve is closing when the pellet has gone about 6".... I would agree with K.O. that if you had straight (and smooth) bore-area porting right through, and then reduced the dwell to get back to your current velocity.... you should get even higher efficiency (and quieter).... That was the point of my initial replies in this thread....

Bob

[Motorhead]

If you have .230" feeding it... then for the heavies a heavier charge is needed to get the pellet going...so having an oblong port  say .140" by .180" is needed... so to me the smooth transition from circular to oblong would be the important thing if you are interested in eking out every bit of efficiency you can get...

In an ideal world if say we have a .177 bore ... feeding it with a Equal size passage from valve would give the greatest efficiency. ( Tho one would like to keep transfer distance short as is practical.)
Sadly @ 80% is a maximum an intersecting transfer hole can be in keeping a loaded pellet from tipping into the transfer hole as it passes by.

Which does support OVAL porting within barrel.
So long as a pellets skirt is seated ahead of hole before firing and probe seating it is out of the way on other end or retracted ... OVAL / ELONGATED works & quite well.

While my R&D here is to get simple parts with a simple to manufacture design working well together.
In such a large valve feeding a small caliber the IDEAL Big flow at near 100% to bore area is very doable using an oval barrel port shape.

[K.O.]

I also think the smooth transition is important to the E.S. as your experiment seems to show... the tighter the E.S. means accuracy at longer range...

Basically any turbulence increases chaotic flow which leads to more variance in flow from shot to shot...

[rsterne]

Scott, if the barrel port is 0.8 calibers wide and 1.2 calibers long, that will end up being very close to full bore area.... In .177 cal., with pellets, you can use a probe as small as 1/16", and that only reduces the bore area to the equivalent of a 0.166" port.... That is still 88% of the bore area, while your current 0.140" port is just 63%.... To get the absolute maximum flow you need a retractable probe.... but an oval barrel port and a 1/16" loading probe will still give you a 40% increase in area over what you have now.... I wonder how much you could reduce the dwell and still get your <20 FPE ?? .... or alternately drop the regulator setpoint.... or BOTH.... 

K.O.... Not so sure I buy the argument that turbulence is chaotic and leads to more variance shot to shot.... It may be on a molecular level, but FT guys have been using small transfer ports for years to succesfully flatten shot strings and reduce the ES....

Bob

[Motorhead]

Scott, if the barrel port is 0.8 calibers wide and 1.2 calibers long, that will end up being very close to full bore area.... In .177 cal., with pellets, you can use a probe as small as 1/16", and that only reduces the bore area to the equivalent of a 0.166" port.... That is still 88% of the bore area, while your current 0.140" port is just 63%.... To get the absolute maximum flow you need a retractable probe.... but an oval barrel port and a 1/16" loading probe will still give you a 40% increase in area over what you have now.... I wonder how much you could reduce the dwell and still get your <20 FPE ?? .... or alternately drop the regulator setpoint.... or BOTH.... 

K.O.... Not so sure I buy the argument that turbulence is chaotic and leads to more variance shot to shot.... It may be on a molecular level, but FT guys have been using small transfer ports for years to succesfully flatten shot strings and reduce the ES....

Bob

Bob,
We're in luck ... the WARP does indeed have a retractable probe 
And YES aware of the area math to get transfer / bore ratio near zero which with such large area in around breech ( being these guns typically sport .250" I.D. ports ), elongation of port in barrels easy peezy.

Scott

[rsterne]

I'll be interested in seeing if you can further improve the FPE/CI by going to full bore area porting and then backing off the hammer strike to get back to <20 FPE.... You may, of course, need to reduce the regulator setpoint to get the absolute best setup.... Before you go modding it again, please determine the FPE/CI of the existing setup....

Bob

[Tomg]

Just my two cents, the pressure differentials we are dealing with would largely negate the need for huge flow considerations, turbulence reducing geometry and such. And that is because the pressure wave propagation that imparts it's energy to the pellet is an expanding pressure wave much more than a flowing volume which have mass/volume considerations, such as in an engine, where flow and volume is a huge concern, but interestingly starts to get less egregious when supercharged or turbo'ed (positive pressure) Flow characteristics completely change when air goes from high pressure to low pressure, the change is on an order of magnitude different in PCP's since the pressure differentials are enormous.
As Bob said, air is compressible, but here is the rub, under no circumstance after the valve has been struck, does any of the released highpressure air get into a situation where it will be re-compressed in corners, cavities, or edges that would cause any concern of its efficiency to push a pellet, meaning the benefits would be marginal, and hard to measure. I am all but convinced, unless I am do not understand physics, which is possible, that during the decompression of the released air, there would be no pressure spikes anywhere behind the pellet until the pellet has left the building.

[Motorhead]

I'll be interested in seeing if you can further improve the FPE/CI by going to full bore area porting and then backing off the hammer strike to get back to <20 FPE.... You may, of course, need to reduce the regulator setpoint to get the absolute best setup.... Before you go modding it again, please determine the FPE/CI of the existing setup....

Bob

Hate to do this ... * Cross forum linking
But we got into the subject over on the Guild dissecting some performance & efficiency figures that sadly would not Copy & Paste well while making much sense.

So for those who wish to go down the rabbit hole some more ... the data we discussed was based upon the WarP gun with 100% porting done.  .230" at valve, .180 at barrel into an elongated barrel port.

See: http://airgunguild.com/ask-bob/plenum-size-and-fpe/msg11813/#msg11813