GEEK Alert - Sonic Choking!

[Cal]

Curiously,  While watching the hockey game,  I was also doodling with port designs that include multiple tangential drillings that would allow  vastly increased port area without pellet problems.  (Two stroke piston port technology in play ;-)

I wonder about situations where the poppet valve face is the dominant feature in the gas flow equation.  Small poppets, light springs, long travel/dwell etc.

At present,  Typical designs seem to focus on the probe and port interface.  Should that just "go away"?

I could see a time when tuning is accomplished with valve face inserts to control port area, and/or travel limiters (Lift)  With everything down stream dictated by bore size.  Not to disregard reservoir pressures of course.

[rsterne]

The multiple port designs I have seen are inferior to the single, larger port.... In the RWS 850 you drill out the bottom barrel port if you want it to flow.... In the Hatsan with their 6-port valve outlet, plug 5 of them and enlarge the one that lines up with the flow and you gain efficiency.... Just my observations on what HAS been done, not that some other alternative can't be made to work....

If you want maximum power, make all the ports bore-size or a bit larger, and make the valve throat area about 10% larger than the bore area.... After that, I don't think it matters where you decide to throttle the flow, or how.... You can reduce the FPE by using a smaller transfer port, barrel port, larger probe, smaller valve exhaust port, choking one of the ports off with a screw, or as I did on my probeless, retractable bolt, prevent it from drawing clear of the barrel port... Any of those work to reduce the flow, be it by choking, adding resistance, whatever the method.... You can also reduce the FPE by reducing the valve lift and/or dwell, and you can do that by backing off on the hammer strike, adding more valve spring, or an O-ring buffer (which is really a very stiff, progressive valve spring), or by physically limiting the lift with a hammer stop... You can also cut the power by using less pressure, and a few other ways I've forgotten to mention or I'm not aware of.... I guess it boils down to what is the easiest way to shed power without giving up too much efficiency (or even by gaining it).... Some of these methods will work on regulated guns (eg. lift limiting like on a Korean PCP), but if you use them on an unregulated PCP they don't work well because you are taking away the valve's ability to self-regulate.... Others will work equally well on unregulated PCPs.... Some of them hurt the efficiency, and some help....

My opinion is that having everything as unrestricted as possible (including valve lift >1/4 throat diameter) and varying the amount of air released by the valve is the best from an efficiency point of view.... Trying to do that on an unregulated gun is VERY complicated, much less so if regulated, because you are just working with one variable, the dwell.... For a given pressure, barrel length and caliber, the dwell then represents the balance between power and efficiency.... more of one, less of the other, pure and simple.... Want more power, increase any of the above (no replacement for displacement).... I think of the product of the pressure (turbo boost) and the barrel volume (length x area = displacement) as the potential the gun has to produce power.... Fix those quantities, and the heavier you make the pellet, the more dwell you can stand (and need) to get more FPE from that heavier pellet.... The limiting factor on weight is when the SD of the bullet overcomes the pressure's ability to produce the velocity you want in the barrel length you have.... but I'm getting wayyyyyyyyyyyy off track here....

Given a choice, I would have wide open porting, on a regulated PCP.... To vary the power, I would change the setpoint pressure, and then find the best hammer strike to operate the gun so that the valve is closing at about 25-33% of barrel length (the knee of the curve).... Then we can forget all about choking....

Bob

[Cal]

Bob

You always catch me with comments like "then we can forget all about choking"
And yet it is one of the very first phenomena that comes to play on valve opening.


But,  perhaps it is insignificant in the over all scheme.....   Time wise any way.

It would be interesting to make use of flow inertia in the quest for efficiency.  The cooling of the reservoir must offer some energy to exchange "somewhere".
Are you aware that normally aspirated I.C. engines can have volumetric fill ratios greater than 100%?  All due to "tuned port" inertia.  I bet you did ;-)
If this same utilization of energy can be applied to air guns,  We should get more shots/fpe from the same fills.  The examples you have mentioned in testimony of the theme.

Yes,  Porting that did not raise more questions than knowns would be a real boon. 

Have you ever played with a flow bench?  Amazing what a 8mm diameter ball on a stick can do to the flow through a 30 mm tube.

[rsterne]

Yes, Cal, you're 100% correct, the valve is choked from the moment it opens until the pressure in the exhaust port reaches 53% of the reservoir pressure.... How long does that take, 0.00001 seconds?.... How much flow would you lose compared to if the valve was not choked during that time period?.... So there you go, you're right, OK, we can't get away from choking, it's part of a PCP....

but we CAN forget about the effect it (valve choking on opening) has on the operation of the gun, IMO.... better?.... The CONTEXT of my statement was regarding using bore size porting to eliminate choking in the part of the valve cycle that is actually DOING anything....

Never had the opportunity to play with a flow bench, but porting and polishing cylinder heads and all the accompanying tricks are quite familiar to me.... and also the multi-transfer ports in 2-strokes from the (wide open) crankcase.... and the use of tract length tuning (intake and exhaust).... quite a bit different than what takes place in an airgun.... again IMO.... YMMV....

Bob

[Cal]

Yes, Cal, you're 100% correct, the valve is choked from the moment it opens until the pressure in the exhaust port reaches 53% of the reservoir pressure.... How long does that take, 0.00001 seconds?.... How much flow would you lose compared to if the valve was not choked during that time period?.... So there you go, you're right, OK, we can't get away from choking, it's part of a PCP....

but we CAN forget about the effect it (valve choking on opening) has on the operation of the gun, IMO.... better?.... The CONTEXT of my statement was regarding using bore size porting to eliminate choking in the part of the valve cycle that is actually DOING anything....

Never had the opportunity to play with a flow bench, but porting and polishing cylinder heads and all the accompanying tricks are quite familiar to me.... and also the multi-transfer ports in 2-strokes from the (wide open) crankcase.... and the use of tract length tuning (intake and exhaust).... quite a bit different than what takes place in an airgun.... again IMO.... YMMV....

Bob

Are you going to tell Steve in NC?   ;-)

ps,  piston port two strokes at 10K rpm,  everything happens in .006 seconds, power stroke in .003!  Not far off from air gun internals. A volume and flow, BMEP  comparison might be in order. hmmmm   Another day.......? ;-)

pss  I always stuffed the crankcases.  makes for higher compression, and better flow ;-)

[Jim_Hbar]

With your comment:

"Since I regard "efficiency" to be the most important factor, I'm happy just staying away from the choked condition!!

Have you designed out choked flow conditions on poppet valve opening?   by what mechanism of efficiency?

What poppet valve??

 

[rsterne]

Or do you mean.... what valve?.... as I assume whatever it is, there would still be a greater than 2:1 pressure ratio across it when closed.... but now I'm being silly as like you said, Jim, other than a transient on valve opening, just keep the ports boresize and the velocity subsonic and we can disregard (better word than forget) choking....

Bob

[K.O.]

Ok guys  the deal is to use a convergent cone to feed the barrel Bore size from behind the pellet and to used sonic choking to produce sonic packing....

as the pellet travels down the barrel after a certain point it becomes a faux  de laval/C.D. nozzle and flow does go super sonic and the shock waves cause sonic packing of the charge behind the pellet which is already accelerating...

the charge behind the shock wave fall back to subsonic as it travels up the column of air but packs it in front till the shock wave  hits the back of the pellet...

post # 123 (instead of copy and paste) http://www.gatewaytoairguns.org/GTA/index.php?topic=94054.msg994804#new

?

[rsterne]

I have never heard of "sonic packing".... can you please supply a reference to it?.... Google returned zero hits....

Bob

[K.O.]

well that's my term for the compression of a gas in front of the shock wave...

scroll down to in supersonic flows.

https://en.wikipedia.org/wiki/Shock_wave

[rsterne]

The interesting thing I got from that reference is that the length of the shockwave is about 200 nm.... or about 0.00001".... Not much "packing" can be done in that length, IMO....

Bob

[K.O.]

a nitrogen  molecule  is .3 nm so the scale is 200mn pushing .3nm... and I think (?) I read using grams law oxy molecules  are smaller...

the sky is blue because the molecules are  in tenths of nanometers, smaller than the  wavelengths of visible light...


edit*
I looked here and 0-two has a  molecular weight slightly higher than N-two...

http://www.engineeringtoolbox.com/molecular-weight-gas-vapor-d_1156.html

[K.O.]

so scaled to inches that is 50' and .9"


?

[rsterne]

and compared to the size of the pellet, or the length of the barrel?.... I guess I just have no idea what you are suggesting.... Perhaps if you can show me where something like this has been measured, or the real effect of a shockwave in a barrel, as opposed to just expanding gasses, I will understand better.... but at the moment I am completely in the dark.... Sorry....

Bob

[K.O.]

https://en.wikipedia.org/wiki/Blast_wave

https://en.wikipedia.org/wiki/Shock_tube


thanks for the patience...

[rsterne]

Unless I misunderstood those two articles, we do not have any conditions inside a PCP which could even come close to creating a blast wave.... In fact, I do not think even a conventional firearm would have such conditions, as the propellant gas is created by rapid combustion, not detonation, which is required to produce a blast wave.... Even the gas-driven shock tube requires a rupture disc to allow a very short rise time for the pressure, I would think orders of magnitude faster than we get by opening a valve....

In Lloyd's experiment, there is NO rapid rise I pressure, only a simple "uncorking" of the high pressure by releasing the bullet.... and so far that is producing the highest velocities we have been able to attain.... These conditions are just about as far from a blast wave as you can get, IMO....

Bob

[Scotchmo]

Sonic choking, sonic packing, sonic horizon...

Why? So far, conventional mechanical engineering principals have been used to model and predict PCP velocities of >1700fps. If they hold to 2000fps, then all this talk of "sonic ?" is needless.

The diminishing returns build quickly at the levels we are seeing. I think we will see 2000fps, but not much more. Unless we want to go the great length (20 foot long barrels, and carbon fiber pellets).

The phenomenon being referred to as "sonic packing" is the radial velocity gradient resulting from fluid friction.

Maybe a CD nozzle would serve to promote the gradient sooner, but it will happen on it's own, even with a straight inlet.

[K.O.]

Sonic choking, sonic packing, sonic horizon...

Why? So far, conventional mechanical engineering principals have been used to model and predict PCP velocities of >1700fps. If they hold to 2000fps, then all this talk of "sonic ?" is needless.

The diminishing returns build quickly at the levels we are seeing. I think we will see 2000fps, but not much more. Unless we want to go the great length (20 foot long barrels, and carbon fiber pellets).

The phenomenon being referred to as "sonic packing" is the radial velocity gradient resulting from fluid friction.

Maybe a CD nozzle would serve to promote the gradient sooner, but it will happen on it's own, even with a straight inlet.

It matters because the design can be tuned and it matters where the shock wave occurs to possibly tune to a Mach Stem formation...

[K.O.]

"Unless I misunderstood those two articles, we do not have any conditions inside a PCP which could even come close to creating a blast wave...."

for a pellet to exit the barrel at super sonic speed there has to be super sonic flow right?

super sonic flow itself causes a shock wave..... It does not matter  what is moving thru the air it can be a column of air or a plane wing... as soon as it hits supersonic it causes a shock wave...

ok so how does a shock wave become a blast wave..? right?

A blast wave is simply a sharply peaked shock wave.

so

How does the supersonic shockwave become sharply peaked?

it  is in a focusing device called a barrel   quoting the article "Blast waves cause damage by a combination of the significant compression of the air in front of the wave (forming a shock front)"

for the pellet to beat the speed of sound of the starting pressure 4500 psi means there is a blast wave/shock wave...

in fact it has to cause the pressure to rise behind the pellet to accelerate the charge to the pellets MV 1750 fps...

how can that happen..?

The pressure has to rise  the point that the speed of sound is 1750 fps in the charge...
but pressure drops by the time the pellet is near the muzzle because of Boyles law... so pressure should be way down...


I can see your statement about the first shockwave not being strong enough to cause that rise... with your chart in the other thread that ends at 5000psi I make the guess it needs the charge to be about 5500-6000 psi or so... a 20-25% rise

but if it becomes stronger then maybe


that is why I wanted you to see is the bit about Mach stem formation and constructive interference cause that could be one of the things happening...

Quoting the article again " Anything in this area experiences peak pressures that can be several times higher than the peak pressure of the original shock front."

with your help we could find about when super sonic flow happens it has to be about the time the pellet is at high trans sonic near supersonic fps for the psi in the barrel/reservoir

but the pellet is a ways down the barrel which you can chart with your spreadsheet... velocity at barrel length

and the shock wave is near the nozzle which is also near the closed end of the tube...

so what happens when the reflected shock wave catches up to the shock wave headed to the pellet...

constructive interference/Mach stem...

[rsterne]

Well, as usual, a little knowledge is a dangerous thing.... I have never heard of a "Mach Stem", so when I Googled it and the very first (and in fact most of the) results talked about Nuclear explosions I immediately know I was in wayyyyyyyyyyyyyy over my head.... I have no intention of going back to University to learn about this stuff, and I definitely won't understand enough to intelligently discuss it any other way.... Therefore I will just have to stumble along with good ol' Newtonian Physics as I understand it.... and let those much more capable and knowledgeable than I debate the intricacies that may (or may not) be happening inside a PCP barrel....

Mach Stems?.... Blast Waves?.... Sonic Packing?.... Sorry, not a snowball's chance in H E double hockey sticks I will live long enough to understand them completely enough to apply them to a PCP....

Bob