Sonic Choke? Maybe not. 2162 FPS

[lloyd-ss]

Temperature effect of a fast fill rate on a PCP gun's reservoir.

There has been some concern with these high velocity shots about heat transfer within the system and it possibly leading to inflated velocity numbers.  The concern is, I think, that filling the reservoir at a fast rate will heat the air to a point where the molecular velocity (?) is higher than would be seen in a normal PCP.  Darn, where are all the thermodynamics experts when you need them? There is nobody on this forum who professes to having aced there thermo courses, so we'll have to do the best we can.

I am sure that anybody who fills from a tank knows that a guns reservoir will heat up if it is filled very fast.  Then, after a short period of time, the heat dissipates and the reservoir pressure drops a little.  Scott and others mentioned this and Scott suggested that I fill the Max Vel gun's reservoir quickly, record the pressure, then let it cool down for a few hours an see how much the pressure dropped.  The idea is that the more the pressure dropped during the cooling phase, the more the air was "super heated" during the fill, and thus the more extra power could have been extracted from the superheated air IMMEDIATELY after the fast fill.  Something along those lines, and Scott can clarify what I have mis-stated.

So I ran an experiment last night and did a photo-documentation of it, viewable as a slide show in photobucket
http://s226.photobucket.com/user/loyd500/slideshow/MaxVel%20Test%20Gun/Max%20Velocity%20Gun%20Temperature%20Test
The slide show isn't as slick as I would have liked, but there are photos at various time intervals to show the pressure and thermal changes.

Basically, I filled the sealed breech block of the Version 2 Max Vel gun in about 20 seconds and then monitored the temperature and pressure as it cooled. Total monitoring period was 7 hours and 50 minutes.
Here is the intro page for the Photobucket slideshow and it explains the important points:

Max Velocity PCP Version 2.
Breech block fill temperature test.

55 cc, 1 lb 10 oz aluminum block.
55 cc chamber.

Fill started with block at 79.3*F (room temp).
Block was filled in 20 seconds to 3530 psi.
Temp immediately rose 3.7*F.
After 1 hour, block had returned to room
temp and pressure had dropped 30 psi.
After 2 hours pressure had dropped 40 psi total.

After 7 hrs 50 mins, room temp had dropped
7.2*F and pressure had dropped  only 100psi total.


Conclusion-
This fast fill rate had, at the most, only a  trivial effect on the velocity.
This shows that there was very little heating of the air that could have contributed to a velocity increase in the high velocity shots.  The fill was done at a very fast rate, but the overall affect was that the system only lost 100 psi (out of 3,.530 psi) due to the thermal losses.  If the air were excessively heated during the fill to the point that significant velocity gains could be made, the pressure loss would have to have been on the order of 1000 psi, not 100.
Therefore, the fill rate had nothing to do with the high velocities.

Anyone, please add your thoughts.
Lloyd

[lloyd-ss]

Wow, just wow.... You guys ROCK! Great Job Lloyd! Kudos to Scott and Bob for their analysis and support for Lloyd.

It amazes how much work got done when the three of you worked together, debated, argued yet always acted civilly... Quite the contrast to other places.... And this whole discussion, the objectives and the results, have made me realize a number of things. First, it is possible to gather smart folks and let them work toward a common goal, second, the current airgun setups are flawed in any number of ways (but had to be given the state of technology when they are/were designed, customer base and efforts to ensure reliability), third out of this research and experimental test will come the next/new generation of air gun mechanisms. 

While reading the entirety of this thread (and the references to the yellow and other forums) I had the thought(s) rolling around in my head about how to transliterate Lloyd's setup into something I could shoulder and take to the range. It may take some time, but I wont be surprised to see upgrades and improvements that is rooted in this research and experiments.

Again, you all deserve a huge round of applause

Dan,
Thanks from all of us for the kind words.  It really makes it so much more fun and productive when we all work together.  The synergy is pretty phenomenal.  You look at something like Bob's SSG hammer thread and the group effort is pretty unbelievable.
So Dan, a shoulderable version of a HV airgun?   We'd have to throw the pellets away, though, LOL.   
Airgunning at the next level!
Lloyd

[rsterne]

Lloyd, I'm very glad that I am on a short holiday and had limited time to respond to what you are referring to in the last couple of days.... I was delighted that you took the time to document the limited amount of heating that occurred in the shot reservoir and how quickly it disippated.... After reading the proposal that Joule-Thompson heating could raise the temperature by 250*F through the heat absorbed from the environment while the air passed through the fill hose for 20 seconds.... and then the completely contrary denial that the reservoir could cool because air was an insulator and had poor heat conductivity.... which are two completely contradictory versions, chosen to support the authors position of the moment.... I was about to make a fool of myself.... It is soooooooooooo much better to dazzle with data then baffle with BS....

It is a pleasure to see a true professional at work, Sir.... and it is an honour to know you....

Bob

[Buldawg76]

Lloyd
I am so glad to see you perform this test since while I agree the air when filling a PCP gun is heated if done so fast, but I myself never felt it was enough to be of any consequence or have a significant effect on pressure. The fact that it only lost 100 psi after cool down of an excessive time frame IMO and did not even gain 10*F is very informative and proof in my belief that it had no true value or effect on the velocity of your numerous test shots.

I think we can safely say it is time to put the theory of heated air molecules contributing to the velocity of any shot up this point to rest for good and for any further test shots as well.

Mike.

[Scotchmo]

I'm on my iPhone for the time being so posting is more difficult but will still do.

Thanks for all the work Lloyd. Keep in mind that different fill hoses/tanks/valves/etc. will have different amounts of isothermal gains or entropy gains. But after your test, I doubt they could be near enough to make much difference in your test velocities.

I have been thinking about the original rms theory. And what basis for it. That line of thinking also gave me some other ideas. I'm going to avoid the math, as merely understanding of the process is sufficient for now.

First, the basis. If I were asked, without knowing how an airgun  is configured, what is the maximum speed of the air, I would have said Mach 1. People knew soon enough that was not a limit for pre-charged guns. So another theory of molecular velocity came about. That might be worse than the first theory, but since it was harder to reach that 1640fps velocity, it was harder to refute with real evidence.  For 70F air, Mach 1 is 1128fps. That is the max air velocity from a tank into a straight pipe, regardless of pressure. That max flow happens when the flow becomes choked. With no pellet or very light pellet, I think the choke occurs when pipe length reaches around 50 diameters. That assumes typical friction coefficients. That choke establishes the max flow rate of the system, which occurs at Mach 1 inlet velocity.

Let's forget the 1640fps molecular velocity theory, as that one is a stretch. Let's see why the Mach 1, 1128fps muzzle velocity limit does not hold up in an airgun. The 1640fps theory falls away for the same reason. So, how does the muzzle velocity exceed that 1128fps inlet velocity?

Once the air passes the choke point, it appears to be undergoing a constant pressure expansion. Even as the mass flux (mass flow rate) falls off a little, the exit velocity increases. So the 1128fps inlet velocity limit holds up. But what about the muzzle velocity?

Some have argued that there is a temperature drop in the pipe. And there well may be a drop in "static" temperature. Me, with a basic mechanical engineering background, I prefer to use what I consider to be the real/measured/perceived/total temperatures and pressures. Someone designing ram jet engines might call that the "stagnation" temperature or "stagnation" pressure.  And they will deal with each type of pressures and temperatures  as needed. For my type of work, I don't need to.

The static temperature might be what you would measure if you could pluck a single molecule out of the air stream, and measure the temperature. The stagnation temperature would be what you might measure if you stuck a sensor/probe into the air stream.

Back to the flow. With a slight simplification. Assume the inlet air flow reaches Mach 1, and maintains that inlet flow once the pellet passes the choke point. After that, air friction comes into play and some energy goes to heating the air. That energy is forever lost from the possibility of doing work inside the system. That's called entropy. The extra heat expands the air while maintaing "real" pressure. Volumetric density falls. The expanding air adds more velocity. Mass flux is constant while velocity increase when moving downstream from the choke point. The process becomes less and less efficient as it proceeds down the pipe.

Our airguns have adiabatic, friction flow, in a pipe. Closest to our airguns would be "fanno" flow in pipes or ducts.

All this work has given me some other ideas for experimental, supersonic air guns.

[Bryan Heimann]

Have ya'll thought of advancements in the form of the projectiles?  Maybe something many times smaller than the bore, in an ultralight sabot?

[Scotchmo]

Wow, just wow.... You guys ROCK! Great Job Lloyd! Kudos to Scott and Bob for their analysis and support for Lloyd.
...

Dan,
I feel like part of Lloyd's support crew, but he's the driver. What I get out of it, is it answers questions for me definitively. I'm making some future plans to play with this new found info. I started designing my "Entropy Gun". A .375 caliber 2kfps monster. But before actually building something like that, I might design an "entropy generator" that can slip into an existing PCP. I won't need the extra long barrel. One shot to prime it, and then it's good for 2000fps for quite a few more shots. With light pellets of course. I think it could work.

My "flux capacitor" didn't pan out however.

[lloyd-ss]

Wow, just wow.... You guys ROCK! Great Job Lloyd! Kudos to Scott and Bob for their analysis and support for Lloyd.
...

Dan,
I feel like part of Lloyd's support crew, but he's the driver. What I get out of it, is it answers questions for me definitively. I'm making some future plans to play with this new found info. I started designing my "Entropy Gun". A .375 caliber 2kfps monster. But before actually building something like that, I might design an "entropy generator" that can slip into an existing PCP. I won't need the extra long barrel. One shot to prime it, and then it's good for 2000fps for quite a few more shots. With light pellets of course. I think it could work.

My "flux capacitor" didn't pan out however.

Scott, 
I like the sound of your plans.       I remember that crazy Monty Python TV comedy series from years ago, and one of the lines they always used was, "And now, for something completely different."   That's what you have to do to make leaps in progress.  I am scratching my head a little at what you are describing, so I know it is going to be good.  But I also know that it is hard work and takes a long time with a lot of dead ends. 
Lloyd
P.S. And I definitely appreciate the support and enthusiasm of you and everyone else here.   

[rsterne]

High velocity shots from another source....

http://www.network54.com/Forum/79537/message/1460082362/UNI+doesn%27t+know+there+is+a+%26quot%3Bspeed+limit%26quot%3B

Nearly 1900 fps with a 2.75 gr. Delrin ball at only 3000 psi.... I think you can now stamp a big CONFIRMED INDEPENDANTLY on Lloyd's data....

Bob

[lloyd-ss]

High velocity shots from another source....

http://www.network54.com/Forum/79537/message/1460082362/UNI+doesn%27t+know+there+is+a+%26quot%3Bspeed+limit%26quot%3B

Nearly 1900 fps with a 2.57 gr. Delrin ball at only 3000 psi.... I think you can now stamp a big CONFIRMED INDEPENDANTLY on Lloyd's data....

Bob

Bob,
Slowly whittling away at it, but it certainly isn't going down without a fight. 
Lloyd

[lloyd-ss]

New 2035 fps  shot with extensive documentation.

EDIT ===========
April 10, 2016.  IMPORTANT NOTE ABOUT VIEWING THIS VIDEO ON YOUR PHONE-
All of the documentation in this video is in the form of annotation text boxes inserted into the video via the YouTube editor.  Unfortunately, the YouTube annotations do not show up on most mobile devices.  It is a YouTube problem.  So, if you want to see all the documentation, you have to watch this on your computer.
Lloyd
 ==================== 


Here is the latest high velocity shot, 2035 fps.  This shot was taken with particular attention given to thorough documentation  that will answer some previous questions and concerns.
Lloyd

[Scotchmo]

There is "sonic choke" in fanno flow.

It limits mass flux to Mach 1 at the inlet, but does not limit velocity in the tube downstream of the choke point.

Realize that and everything is good.

Edit: fixed the title. exits exists

[madeInLV]

This was interesting, but now it's EXCITING!!!!


Loyd,
loads of useful data-thank you.
As for your next valve set up-a picture tells a thousand words:) A graphic representation of the internals would help a lot. I think i get your idea, but some parts are confusing. Here is  a link to a "Pif-Paf" valve.
forum.guns.ru/forummessage/30/78330.html
It's a pilot activated plunger valve. Pilot valve being very small for easy opening. There is one pressure in the entire system-simplifies the setup i think.

Word of advice (from experience): area imbalance on the plunger/piston causes force imbalance as you described. Ideally you want the acceleration of the plunger/piston=acceleration of the projectile, since at one point the pressure will be acting on both the  projectile and the piston/plunger pushing them in the opposing directions.
It is very doable with heavy bullets but might be a challenge for light pellets.
Thus the valve must be made in such a way that pressure imbalance during the pilot " pressure drop" is enough to generate needed acceleration of the piston/plunger.

That might allready be thecase in your design, I'm just sharing my 0.02$ to possibly save time in the future.

[rsterne]

Very well documented, Lloyd... whittling away, indeed....

Bob

[Motorhead]

Lloyd,
have a Q

Being we know O-rings deform under pressure as there shape shifts against there retention groove, it is a safe assumption the same is happening to the o-ring on your pellet ?

What I'm alluding too is the VERY high frictional losses that likely are happening as the ring is being forced to seal against the pressure, it is "Putting on the brakes" as it is pushed down length of the bore.

Could you Not make a pellet from Peek or similar material having a skirt or waist that creates the seal required for a single trip down the barrel that would mitigate these frictional losses I assuming are present ?

[rsterne]

I think the problem, Scott, is that the pellet has to have huge structural strength because the "skirt" is clamped by the release mechanism to allow the full pressure to build on the O-ring prior to firing.... Lloyd has tried some plastics and none has survived past about 1500 psi, IIRC....

Bob

[lloyd-ss]

Lloyd,
have a Q

Being we know O-rings deform under pressure as there shape shifts against there retention groove, it is a safe assumption the same is happening to the o-ring on your pellet ?

What I'm alluding too is the VERY high frictional losses that likely are happening as the ring is being forced to seal against the pressure, it is "Putting on the brakes" as it is pushed down length of the bore.

Could you Not make a pellet from Peek or similar material having a skirt or waist that creates the seal required for a single trip down the barrel that would mitigate these frictional losses I assuming are present ?

Scott,
I agree about the deficiencies of o-rings.  I made a Teflon U-cup seal in posts 321 and 332 and it performed well, but making it was a chore.  I think with the o-rings, the amount of crush  they have in the gland depth has as much to do with their performance as anything else.  At 4500 psi they need enough crush to seal, but not enough to cause high friction losses like you are talking about.  Its a balancing act to meet both requirements.
The current plan is to eliminate the current clamp-and-release mechanism so that the projectile will no longer be a structural element.  Then, a more conventional style of pellet or bullet can be used.  That will open up the possibilities for the types of projectiles, and their weight.
Lloyd

[lloyd-ss]

This was interesting, but now it's EXCITING!!!!


Loyd,
loads of useful data-thank you.
As for your next valve set up-a picture tells a thousand words:) A graphic representation of the internals would help a lot. I think i get your idea, but some parts are confusing. Here is  a link to a "Pif-Paf" valve.
forum.guns.ru/forummessage/30/78330.html
It's a pilot activated plunger valve. Pilot valve being very small for easy opening. There is one pressure in the entire system-simplifies the setup i think.

Word of advice (from experience): area imbalance on the plunger/piston causes force imbalance as you described. Ideally you want the acceleration of the plunger/piston=acceleration of the projectile, since at one point the pressure will be acting on both the  projectile and the piston/plunger pushing them in the opposing directions.
It is very doable with heavy bullets but might be a challenge for light pellets.
Thus the valve must be made in such a way that pressure imbalance during the pilot " pressure drop" is enough to generate needed acceleration of the piston/plunger.

That might allready be thecase in your design, I'm just sharing my 0.02$ to possibly save time in the future.

LV,
The concept of the pif-paf valve (I have never heard it called that) is similar to what I am trying to do for the version 3 gun.  Very simple in concept, but many details to work out to get it to function well.  Your concern regarding the acceleration of the valve vs the acceleration of the pellet has to do with the mass of the valve being too large to open quickly enough when light weight pellets are used, correct?  I can see how that would be a problem, but here is a possible way to overcome that, that I am hoping will work.

The valve piston, instead of being a straight sided rod, will have a step in it such that the front end (toward the pellet) will be larger than the back end.  The diameters are sized so that the annulus of the front end of the valve (the area outside of the sealing diameter of the barrel) will be approximately 80% of the area of the rear end of the piston.  The pressure imbalance will provide enough force to seal the valve, but when the pilot air is released the pressurizer imbalance is heavily biased to blow the valve open immediately.  The step in the valve piston means that it needs a second set of o-rings with an atmospheric bleed for the step so that the piston doesn't build up pressure in the stepped area when it is moving.  If the full high pressure air is bled back into the rear piston area, it will close the valve automatically, but it will not (usually) be able to close the valve until the bullet has left the muzzle and the barrel pressure drops enough allow the force of the rear piston to close the valve.

The idea of using the full reservoir pressure to operate the valve piston is nice, because it eliminates the need for a regulator.  It also keeps the valve piston forces in proportion to the reservoir pressure for more consistent operation.

I will post a sketch shortly.
Lloyd

[phoebeisis]

Good stuff-pretty fast
and your temperature drop agrees with your pressure drop .007-.008  drop    3.7/520 and 30/3500-
might be worth 15-20 fps  at most-probably less

Like MOTORHEAD SUGGESTS- the O-ring is a speed killer
Might be possible to  have the O-Ring  JUST  in the chamber- while it is being held
not  attached to the projectile-but set in a machined groove in chamber

The 2000 fps  foil ball   pellet made over 2000 fps-with no  effort made to get a good seal

[lloyd-ss]

Charles,
If the build of the Version 3 Hi Velocity gun is successful, it will accommodate conventional projectiles and eliminate the need for o-rings.  My hopes are to just make the custom  built projectile and o-ring friction problems go away.
Lloyd