Dead volume and the sound barrier
Posted: Thu Dec 19, 2013 8:34 pm
Given I've got into some discussions about supersonic pneumatics recently, I figured it was actually worth experimenting with the theory regarding the adiabatic heating in the dead volume.
I'm starting with rather exaggerated example - fitting HEAL with the extended (175cm) barrel, and having this "piston"... ... in the breech, and this sabot carrying a .177" pellet... ... 75cm further down the barrel. The space between the two provides a very large dead volume, which shouldn't mix with the cooling air from the chamber.
This could be looked upon as a variant of the spring piston airgun (or light gas gun), although without a distinct compression chamber and barrel (as they have the same diameter and the piston can continue chasing the projectile out of the barrel).
Test #1 was to have a shakedown run and try to chrony it via sound recording. While normally inferior to a light-gate chrony, having the microphone behind the target makes it easy to prove whether the projectile beat sound over the 10 metre distance and inherently takes into account the speed of sound not being a fixed velocity.
(And if I can't achieve a supersonic average over that distance... well, then I'd be too close inside the margins of error to be sure anyway).
[youtube][/youtube]
Unfortunately, the shot was not on target, so the velocity is uncertain. But based on the state of the recovered piston and sabot, I am defaulting to considering the test a failure - they do not appear to have been strong enough to have functioned as intended.
For future tests of this set-up, a slightly heavier and stronger version will be used. Given the gases in the chamber already mass around 18 grams, going a bit heavier on the piston and sabot won't dramatically affect the accelerating mass.
After that, I'll switch to smaller dead volumes and see what happens. Eventually, I'll reach a comparison of HEAL's normal barrel with the sabot in the normal breech and test with/without the "piston" (adjusting sabot mass to keep the overall mass the same) to separate the dead volume*
*Which, on HEAL, is full bore and straight, so it's easy enough to put a "piston" up against the valve sealing face to completely segregate the dead volume.
"With" should mimic a perfect case scenario, where the dead volume gases aren't at all mixed with the cooling chamber gases.
"Without" relies on the assumption that the flow without the piston is turbulent enough to mix the gases to the point of negating any temperature rise, but this should be at least strongly true.
The difference should be reasonably telling about how much a good "laminar" valve could reasonably capitalise on this compression effect.
I'm starting with rather exaggerated example - fitting HEAL with the extended (175cm) barrel, and having this "piston"... ... in the breech, and this sabot carrying a .177" pellet... ... 75cm further down the barrel. The space between the two provides a very large dead volume, which shouldn't mix with the cooling air from the chamber.
This could be looked upon as a variant of the spring piston airgun (or light gas gun), although without a distinct compression chamber and barrel (as they have the same diameter and the piston can continue chasing the projectile out of the barrel).
Test #1 was to have a shakedown run and try to chrony it via sound recording. While normally inferior to a light-gate chrony, having the microphone behind the target makes it easy to prove whether the projectile beat sound over the 10 metre distance and inherently takes into account the speed of sound not being a fixed velocity.
(And if I can't achieve a supersonic average over that distance... well, then I'd be too close inside the margins of error to be sure anyway).
[youtube][/youtube]
Unfortunately, the shot was not on target, so the velocity is uncertain. But based on the state of the recovered piston and sabot, I am defaulting to considering the test a failure - they do not appear to have been strong enough to have functioned as intended.
For future tests of this set-up, a slightly heavier and stronger version will be used. Given the gases in the chamber already mass around 18 grams, going a bit heavier on the piston and sabot won't dramatically affect the accelerating mass.
After that, I'll switch to smaller dead volumes and see what happens. Eventually, I'll reach a comparison of HEAL's normal barrel with the sabot in the normal breech and test with/without the "piston" (adjusting sabot mass to keep the overall mass the same) to separate the dead volume*
*Which, on HEAL, is full bore and straight, so it's easy enough to put a "piston" up against the valve sealing face to completely segregate the dead volume.
"With" should mimic a perfect case scenario, where the dead volume gases aren't at all mixed with the cooling chamber gases.
"Without" relies on the assumption that the flow without the piston is turbulent enough to mix the gases to the point of negating any temperature rise, but this should be at least strongly true.
The difference should be reasonably telling about how much a good "laminar" valve could reasonably capitalise on this compression effect.