And finally, what I feel SHOULD BE discussed.
Flow of stuff through an orifice(hole?), as basic concept; depends on....... inlet pressure, size of the hole, and pressure drop.
When pressure drop exceeds 50% flow does not increase further, it is CHOKED. Is choked sonic?? is sonic the hedgehog? Doesn't really matter. I'll call it choked past 50%.
The velocity of the stuff coming out of the hole depends on the inlet pressure and pressure drop, or past 50% drop, just the inlet pressure. Mainly the inlet pressure.
Yes the properties of the stuff come into play but that's more or less a constant.
The velocity of the stuff coming out of the hole depends on the inlet pressure, and drop%. (I said it twice, very important)
Why is this important? well, because during blowdown, if the valve is opened fast enough, the velocity coming out of the port is tremendous. Like a rocket!
Yall understand venturi.....right? venturi with flow, you measure at the small point........low pressure(uncle Bernoulli), then it expands at say 7 degree, and slows the flow down, now you have higher pressure. It recovers the pressure and pulls a bigger differential across the small part. Tis a venturi.......or the divergent part anyhow. 7 degree is a pretty good taper to use for a carburetor. If it diverges too fast the air won't follow, and it won't work, as you know.
Of course how it's working is the flow is HITTING the slower stuff in front of it and this is what slows the flow jet down, and it has to happen gradually enough to stay orderly. If the tail of the venturi was in SPACE, in vacuum, then the flow might just travel in a straight line, and not expand, and not follow the taper (I think???).........but if other end of venturi is in space.... then there would probably be more than 50% pressure drop across it also.
And.........what I am struggling to somehow explain, is taking the ROCKET THRUST coming out of the exhaust port and turning it into PRESSURE in the header tube, gets strange at high speeds. For reasons I'll claim are assumed (pipemax says so), the port is smaller than the header tube. So the JET, has to expand and slow down to produce pressure in the header tube. As a human, air breathing creature, I expect 7 degree taper would work great.
Actually no. Tests show it won't slow down that fast. In my simulation software, I can connect a little pipe to a big pipe and through the magic of make-believe the flow suddenly matches the size of the big pipe right away, but not in the real world. In the horrible book they found it does not slow down for, in some cases, a foot down the pipe. This still happens.
And that is why the first pressure traces in this thread below differ from predicted.
(open in new window please) viewtopic.php?f=15&t=37398&start=30
The pressure measures LOW halfway through blowdown because the JET of velocity in the pipe has not expanded and slowed to the header tube size!! Or the very high earlier velocity has accelerated the gas down the tube to high velocity, and so there is virtually nothing in the pipe TO slow the flow as it dischrges slower. Actually both. High velocity reads low pressure(the bernoodle affect). Down the pipe 6 or 12 inches it is finally slowing down and reading higher but there was no sensor hooked up there I guess. So what IS in the pipe during that dip of pressure? I guess there is just a jet following the walls and empty space around. Maybe. Once the cylinder pressure goes down then the thrust and velocity is lower and you see pressure come back up, as not reverse flow but certainly flow slows, and pressure creeps backward.
If a header tube gathers no pressure then It's not doing it's job very well, or you might say the pressure coming back UP again is......what they call "monkey motion." Some of that volume is not functioning positively here in the first half. Then again what's the harm? There could be other reasons you want the header size big all the way to the port(doppler), then again, it does not NEED to be big, so........why not keep it small 6 inches out, 8 inches out. Or approximate a very slow taper. Calvin Elston has found some good results staying small till a ways out I read. If you take the VOLUME that was taken out by staying small, and put that volume 6-12 inches down the pipe where the jet finally does slow down, then.........well, maybe you HAVE something. You have stored pressure, with reduced monkey motion, and higher velocity near the port. It is a small thing, but when deciding if the AR chamber is part of a well balanced breakfast we have to look deep.
The claimed inventor of the thing, did understand this reality and said it with much less words. I'm impressed.