Compressible flow and Destin Sandlin's 1050-MPH baseball cannon 2

[JoeFrickinFriday]

You've probably seen this by now, but just in case, last fall Destin Sandlin uploaded a video about a pneumatic baseball cannon he and some friends built that can launch baseballs at well over 1,000 MPH. There's a 23-minute video in which they describe the design, constructions, and early tests of it:

https://www.youtube.com/watch?v=cqidD7kVnxY

Destin uploaded a companion video a few months later, in which he and his fellows fire baseballs through a variety of items through which you should not be able to fire a baseball:

https://www.youtube.com/watch?v=4I-p8vjQ95s

My question concerns the first video. at 2:48, we see a 2-D CAD printout showing a critical flow venturi in the barrel just upstream of where the baseball begins its journey. This made sense to me, as it seems like a CFV would be needed to accelerate the driving gas past sonic velocity. Without a CFV, ISTM the tank and barrel together would form a CFV with an exit-to-throat ratio of 1 (so I guess something akin to a critical flow orifice), making it impossible to go faster than the local sonic velocity. But a little while later at 3:30, we see a SolidWorks model that shows no CFV in the barrel. So what gives? I asked this question in the comment section, but got no response (no hurt feelings on my parts, he's got a lot of videos with a lot of comments and can't respond to all of them).

So I'm wondering what the collective here has to say. Is a CFV necessary for achieving supersonic muzzle velocity in a pneumatic cannon like this, or am I missing something about compressible flow?

Thanks...

 

[SwinnyGG]

Did both the tank and the accumulator lose energy and there was no heat produced. The potential energy of the compressed gas just went into nothing?

For the tenth time:

This is a real process we are discussing. Not an ideal one. Real processes, especially this one, are not adiabatic.

I'd bet money that while the gas being expanded is cold, due to the physics we've already discussed, that the throttling valve on the bottle is hot. There's some energy right there. Along with the significant energy it takes to pump a reasonably large volume of gas across that field through all that small diameter plumbing, with all the associated friction and thermal losses along the way.

Even if you want to act like this process is wholly isenthalpic (which it isn't but we can approximate):

H = U + PV

One thing this system most definitely is is closed. No mass exchange- which means that PV term is a constant.

When the gas is expanded, enthalpy can remain constant despite the temperature change, because a portion of U is converted to kinetic energy. When the gas stagnates at the other end, it is correct that this kinetic energy is returned as temperature - but an increase in temperature vs the starting point cannot happen without an associated enthalpy increase.

Perpetual motion, man.

 

[TugboatEng]

If you break the firing down in to stages, some stages you can assume to be adiabatic. The compression of the air to the filling of the chamber takes substantial time so this part is not adianatic. We can take a snapshot right as the trigger is pulled to know start conditions. From the time the trigger is pulled to the ball leaving the barrel, very little time passes so it's safe to assume that stage is adiabatic. Once the ball has left the barrel the rest of the process is irrelevant.

 

[SwinnyGG]

From the time the trigger is pulled to the ball leaving the barrel, very little time passes so it's safe to assume that stage is adiabatic. Once the ball has left the barrel the rest of the process is irrelevant.

I agree with this 100% - but the filling stage is what we've been spending all our time talking about, for better or worse.

 

[TugboatEng]

Why is the filling stage relevant? You only need a snapshot of the conditions the moment the trigger is released.

 

[SwinnyGG]

Why is the filling stage relevant? You only need a snapshot of the conditions the moment the trigger is released.

The conditions at the moment the trigger is released are the result of the filling stage, and are what is being debated above ad nauseum.