Investigating the Sound Reflector of a Gas Inlet Tube

[pkc111]

TL;DR

Ive built a prototype Rubens tube from aluminium 50mm diameter and 1.2m long. It runs on natural gas. With a 1 inch flame I can get a nice node within the first 30cm of the speaker end of the tube only (only at certain frequencies that match the expected resonance frequencies). My problem is nothing happens at the other end (gas inlet) even at higher resonance frequencies. If I raise the volume they start to appear but then the ones at the speaker end roar and then blow out.

My theory is that the gas inlet end, which is supposed to act as a sound reflector may not be doing a great job because it is plastic and has a hole cut in the centre for the gas inlet port. But by that reasoning, the flames at the end most distant from it (the sound inlet end) should be the hardest standing waves to create, but the opposite is true, they are the easiest to create with the smallest sound volume required.
Ive attached a photo of the tube at low volume sound and you can see the reduction in flame height (node) at the left hand end only (sound inlet).
Any ideas would be appreciated.
Many thanks

 

[Tom.G]

Probably because you are not driving it at a multiple the tube resonant frequency (about 188Hz for an open-closed methane-filled tube 1.2m long).

See:
https://www.google.com/search?&q=resonance+in+tube+closed+at+one+end

Cheers,
Tom

 

[DaveE]

If you think of that tube as a resonant structure, then all of those holes along it's length would represent losses. I wonder if there's too much damping by the time you get to the end. It would be interesting to know what would happen if you had tiny holes, or maybe if you plugged a bunch of them.

 

[tech99]

I think you might need a perfect seal for the speaker, a flat heavy plate at the far end and a small gas inlet tube on the side. I have seen one operated with very large flames, but the one offered by an equipment supply company does show small flames.

 

[pkc111]

I think you might need a perfect seal for the speaker, a flat heavy plate at the far end and a small gas inlet tube on the side. I have seen one operated with very large flames, but the one offered by an equipment supply company does show small flames.

Thank tech99 I am thinking the same thing. I will change the gas entry to the centre of the tube and place a hard plate at the end. The speaker end is perfectly sealed witha balloon so there shouldn't be much problem there. I just tried with propane and its a lot better.
Although the flames now have a square sort of shape. Is it possible to get a square sound wave in a Ruben's tube??
My sound generator is sine wave.

 

[tech99]

Thank tech99 I am thinking the same thing. I will change the gas entry to the centre of the tube and place a hard plate at the end. The speaker end is perfectly sealed witha balloon so there shouldn't be much problem there. I just tried with propane and its a lot better.
Although the flames now have a square sort of shape. Is it possible to get a square sound wave in a Ruben's tube??
My sound generator is sine wave.

Regarding the square wave, I am wondering if the gas inlet needs to be large and the individual holes small.