I'm Stumped - Please help me pass gas!

[les]

Yes, I am expecting the title to attract attention and comments.
I'm not certain this is kosher, but I ran into a problem on my Tri-Trident thread and have not seen any suggestions, so I decided to start another thread to discuss the specific issue.

The Tri-Trident is an attempt to make a triangular based, ducted ejection, similar to the original Estes Trident kit.
So I plagiarized a bit from my other thread to explain things...
NOTE - these were test pieces to try before doing the real rocket

And to complicate it, I'm trying to mate the tubes angle to angle.


Multiple launch lugs will be used as "pipes" to connect the small outer tubes to the larger central tube (shown in blue). The joint will be reinforced with rocketpoxy (reddish) and a vertical matboard (green)


The launch lugs glues up as "pan pipes"


Notched the tubes and installed the pan pipes


Reinforced with the poxy and matboard


Now for the issue.....

I tried blocking the one end of the small tube and blew into the other end. I could not get air to move through the pipes.


OK - maybe I messed up and got glue or poxy in the pipes.
So I decided to cut open the large tube to look.

Appears clear. If you look closely, you can see the purple exacto handle I held in the other tube



I can even pass a file handle through all 6 pipes



So the pipes are open. But still, if I block one end of the small tube and blow into the other end of the small tube, the air does NOT go into the large tube!

I also tried the opposite direction. Blocked one end plus the cutout on the large tube and blew into the other end. When I maintained a good seal on both openings, no air moved.

From my point of view, this violates multiple physics and engineering principles.
But I can't figure out what the heck is going on!!
With the one end blocked and blowing air into the other end, I am pressurizing the small tube. The higher pressure should bleed through the pipes into the larger tube.
Oh - I also grabbed a loose lug and blew through it like a straw. Lots of air moved. With 6, I would expect even more to move.

WHAT AM I MISSING?
Is it time to hang up my slide rule???

 

[Raythain]

Since the opening of your pan pipes is orthogonal to the flow, there isn’t anything directing the gas to flow that way. Therefore vast majority will continue to flow straight towards the blocked end, until the air in the blocked end is compressed enough to start forcing new gas into the pan pipes. Over a long enough time frame, the pressures in the 5 and 50 tubes would equalize, but that time frame isn’t going to pop the nose or deploy a chute.

I can think of 4 things you can do to get more flow through your pan pipes.
1) Cut the ends in the incoming flow direction at an angle so they “catch” some of the incoming flow, and the wall can redirect the flow towards the larger tube. This means you’d need each upstream tube to be a little longer than the one downstream, so it is exposed to fresh flow.
2) Extend the input side of your pan pipes more into the center of the flow - the closer to the middle of the smaller tube the pipes are, the more smooth and laminar the incoming flow will be. That should be easier to get more flow redirected.
3) Install the pan pipes at an angle in the direction you want the flow traveling. This effectively accomplishes action 1, with the added benefit of having lower pressure loss because the angle you are changing the flow is lower.
4) Make a funnel. This would probably be the most work, but if you had a funnel to redirect the full flow, you’d get the most throughout on your pan pipes if you can redirect all of the flow.

I’ll try to get some diagrams when I have a minute to sit down with my computer.

 

[les]

I hear what you are saying, but....
With the one end blocked, there is very little actual flow. It quickly becomes a pressurized cylinder. If there is a vent, in this case the pan pipes, with a lower pressure on the other side (since just open to regular atmosphere), then air should flow through the pipes.

My thoughts for an example, if I try to use an air compressor to fill a tire, but the tire has a hole in it (caused by running over some pan pipes). It doesn't matter where the hole is with respect to the air valve, it is still going to leak.

The concept of the ducted ejection gases is from the Estes Trident


It just used slots, without the pipes, and works fine....
Note that the slot is 3/32 x 1 1/8. The launch lugs are 1/8" diameter and I have 6 in line. But there are losses in space due to the multiple circles vs rectangular opening

 

[les]

CHEEEZZE - I'm DUMB!!!!!!!!!!!

Issue was how I was blowing into the small tube

Trying to blow into the small tube, I wanted a tight seal. So I pressed the tube tightly against my lips.
Turns out the tube was sealing my lips closed.
When I instead put my lips over the outside of the tube, instead of against the end, lots and lots of airflow!!!

DUH

 

[Raythain]

Issue was how I was blowing into the small tube

Trying to blow into the small tube, I wanted a tight seal. So I pressed the tube tightly against my lips.
Turns out the tube was sealing my lips closed.
When I instead put my lips over the outside of the tube, instead of against the end, lots and lots of airflow!!!

Whelp, close the ticket as a setup issue. Looks like you have sufficient pressure gradient between the tubes to turn the flow. I was trying to solve an issue for lower pressure gradients.

Glad you've got it working.

 

[ThirstyBarbarian]

Now that you’ve successfully learned to pass gas, please remember not to do it in the kitchen.

 

[K'Tesh]

I'm a bicyclist... I pass gas every time I ride past the pumps.

 

[kuririn]

 

[jqavins]

But...

There will be considerably greater pressure inside the large tube when the ejection charge blows than when you do. I think. Probably.

Six tubes with 1/8" ID give a total of 0.0736 sq-in area. Compare that to the Trident's 3/32" by 9/8" opening, which has 0.1122 sq-in, which is a bit over 50% more. Add then the aerodynamic effects of the air going through small tubes, and I would be very nervous about getting the ejection gas out of the BT50 fast enough to relieve that pressure and avoid blowing the tube out. If I were you, I'd ditch the pan pipes and use all the vent area I could get, more like the Trident.

View attachment 598078

I think you've missed the point; he wants to pass gas.

 

[kuririn]

I think you've missed the point; he wants to pass gas.

OK

 

[les]

Six tubes with 1/8" ID give a total of 0.0736 sq-in area. Compare that to the Trident's 3/32" by 9/8" opening, which has 0.1122 sq-in,

Two things

First, and I did the math multiple times, but after my last whoops I'm not trusting myself too much. I calculate 3/32 x 9/8 as 0.1055, not 0.1122

Second, I measured the ID of a 1/8" lug. It measured 0.156". Which makes sense that there has to be some slop to fit on a metal rod that itself is 0.125.

Based on this, my calculations show the 6 pipes are 0.1147 sq-in vs the slot of 0.1055 sq-in.
The big difference is 3/32 = 0.0938" vs the 0.156 of the lug ID. The extra "width" (diameter) compensates for the surface area loss due to being a circle....

And if I have not messed up again, since the area is a bit bigger, there should not be an issue.

 

[jqavins]

Two things

First, and I did the math multiple times, but after my last whoops I'm not trusting myself too much. I calculate 3/32 x 9/8 as 0.1055, not 0.1122

Second, I measured the ID of a 1/8" lug. It measured 0.156". Which makes sense that there has to be some slop to fit on a metal rod that itself is 0.125.

Based on this, my calculations show the 6 pipes are 0.1147 sq-in vs the slot of 0.1055 sq-in.
The big difference is 3/32 = 0.0938" vs the 0.156 of the lug ID. The extra "width" (diameter) compensates for the surface area loss due to being a circle....

And if I have not messed up again, since the area is a bit bigger, there should not be an issue.

OK, that all sounds reasonable. Maybe. You're certainly right about the tube ID. And you're right about 0.1055, I screwed that up. Calculator user error, probably.

What does give me pause is the "The extra "width" (diameter) compensates for the surface area loss due to being a circle" bit. How long are the tubes? I don't know much about fluid dynamics (with apologies to Sam Cook) but I do know that, in discussions about tube fins, more knowledgeable people than I have stated that when L/D is too great, a tube might as well be a closed cylinder. 10:1 if I remember right, though I've never seen a tube fin design which comes close to that. Obviously, that's a speed and pressure dependent figure, as one certainly can blow through a straw which greatly exceeds 10:1.

How big is the opening you've cut and glued the tubes into? What if you used exactly the same hole and just left the tubes out? That's more open area and less surface area; is there a reason for the added complication of the tubes?

 

[les]

is there a reason for the added complication of the tubes?

In my main thread, there was this suggestion
Suggestion: Holes instead of a slots.
Short segments of launch lugs for gas conduits between tubes.
My first attempt the jugs were 0.5" long. I felt these were a bit tight and thinking of growing them to 0.6" long

To join the tubes point to point, I create a gap. The lugs help seal the gap but keep the poxy from plugging things up. They also add some strength to the joint. Maybe I'll try just 2, one at each end with a piece of card stock in between.