Gas burner making high pitch, flute like, noise ?? - myLargescale.com > Community > Forums


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Old 07-25-2017, 11:18 AM   #1 (permalink)
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Default Gas burner making high pitch, flute like, noise ??

I have been performing some simulation tests of the gas burner that I plan to use in my loco. The idea is to use the "open smokebox" approach that Bill described some time ago. The burner is a radiant, metal fiber type. The burner produces a bright red flame and it appears to perform really well

Cremador by joan lluch, on Flickr

However, I have an issue that I did not anticipate, and I canít figure out how to solve. The problem is that in most conditions the system produces a very loud, high pitch, resonant like noise that resembles the sound of a flute. See this video:

https://flic.kr/p/X62nzM

After some research, my understanding is that this is a relatively known problem that may happen sometimes while designing burner systems. The high pitch sound appears to be caused by some sort of combustion resonance that occurs with burners in constrained or long shaped combustion chambers. The actual phenomena is described here:

https://en.wikipedia.org/wiki/Rijke_tube

I wonder if someone has seen this issue before and whether you can point me with any tips or suggestions that I can try to avoid that.

I tried to make several changes, but so far I don't get with the proper solution.

Any ideas?
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Old 07-25-2017, 10:28 PM   #2 (permalink)
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Put an adjustable choke on the air intake holes. Once the burner warms up it doesn't seem to need as much air. At least mine behave that way.

vr Bob
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Old 07-26-2017, 02:58 AM   #3 (permalink)
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Put an adjustable choke on the air intake holes. Once the burner warms up it doesn't seem to need as much air. At least mine behave that way.

vr Bob
Hi Rob, thanks for your reply.

In my case, the hotter the burner becomes the louder is the noise and the greater is the vibration amplitude. Restricting air intake does indeed help, but then I found that a part of the flame is formed at the end of the tube (boiler), not in the firebox.

My testing setup is extremely prone to produce this phenomena. The sound is really loud and annoying. It seems to do it whatever changes I try. It makes me think that there's a fundamental flaw in my design or the whole setup. I wonder how common is that?.

Do you think that the size (or shape) of the pre-mix chamber has anything to do?
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Old 07-26-2017, 09:48 AM   #4 (permalink)
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Not sure of the design of your box
Is it just for testing or something you plan to use?
For sure, the sheet metal structure will amplify any noise and may be the cause of all of the noise

I would go ahead and build my firebox boiler and then try the burner in it

My ceramic burners sometimes howl for a second or two at the start and then go dead silent
As bob stated, poker burners do this all of the time and usually can be quieted down by choking off the air to a degree but this also reduces the efficiency and lighting ability of the burner
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Old 07-26-2017, 03:27 PM   #5 (permalink)
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Hi Bill,

Thanks for your reply.

This is only for testing. It's just simply made steel sheet box simulating the inner part of the boiler, to help me understand gas burner behaviour before I make the actual boiler. The box has about the same shape as the firebox and the same section for the flow gases towards the smokebox.

As you point out, the sheet metal structure amplifies the sound. The sound of the vibrating sheet is partly audible on the video above, this can be eliminated by just wrapping the metal surface with something rigid (preventing it to vibrate), but the louder sound comes from INSIDE, just like a flute. In fact, if you add rigidity to the system the sound becomes even louder and the pitch is purer (Like the sound of an organ musical instrument).

I would like to solve this problem (or understand how to solve it) before starting making the boiler. The boiler will be a full locomotive type with wet firebox for a big 5" gauge loco, so it will be quite expensive to make. I just want to adapt the design to (hopefully) work without any forced draft, as you do on the "G" scale. I am not aware of anybody who has attempted this on a 5" locomotive.

After posting my previous message I read somewhere on the internet that by adding a section of perforated tube to the end of a the "Rijte" tube, the thermo-acoustic phenomena can be stopped by breaking the sound waves resonance. So this made me think that maybe I can have something placed on the smokebox to break this cycle. (?)
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Last edited by joanlluch; 07-26-2017 at 03:49 PM.
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Old 07-26-2017, 05:11 PM   #6 (permalink)
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Joan
It is close to the boiler design but not the same. The boiler will have tubes where the hot gasses flow through to the smoke box.
The design of the boiler will be the same no mater what fuel system you use only the bottom of the firebox and the smokebox will be different so i would go ahead and build the boiler and then try your burner in it. It may have no noise after that.
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Old 07-26-2017, 06:55 PM   #7 (permalink)
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Quote:
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Joan
It is close to the boiler design but not the same. The boiler will have tubes where the hot gasses flow through to the smoke box.
The design of the boiler will be the same no mater what fuel system you use only the bottom of the firebox and the smokebox will be different so i would go ahead and build the boiler and then try your burner in it. It may have no noise after that.
Ok I see your point. I am still considering whether I should have (a) flue tubes in the traditional way as per the normal coal-fired boiler design, or (b) to have an enlarged kidney-shaped section with water tubes in it, before the hot gases enter a shortened flue tubes section, or (c) to have said kidney-shaped section with water tubes going all along the length of the boiler up to the smokebox with no flue gas tubes section.

I am a bit concerned that with the normal coal-fired boiler design (a), the pressure drop along the boiler may be too high for the open smokebox (without draft) to work properly. The safest design in this regard is of course (c), but I guess this would be at the expense of some efficiency loss. The primary reason for I decided to try with my metal sheet setup was to understand better how critical was the section available for gases through the tubes for the several possible scenarios. Then the "thermo-accoustic" noise issue came totally unexpected!. I guess I will continue with my testing of the several scenarios and will ignore the noise issue for a while.
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I'm currently partly disabled with a progressive disease affecting both my hands with pain and some disfunction. I attempt to work on my build as my health condition allows it, which means my progress is very slow. I used to post my build thread Here, but I am no longer able to type that much on the computer, so I stopped that.
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Old 07-26-2017, 07:01 PM   #8 (permalink)
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Quote:
Originally Posted by joanlluch View Post
Hi Rob, thanks for your reply.

In my case, the hotter the burner becomes the louder is the noise and the greater is the vibration amplitude. Restricting air intake does indeed help, but then I found that a part of the flame is formed at the end of the tube (boiler), not in the firebox.

My testing setup is extremely prone to produce this phenomena. The sound is really loud and annoying. It seems to do it whatever changes I try. It makes me think that there's a fundamental flaw in my design or the whole setup. I wonder how common is that?.

Do you think that the size (or shape) of the pre-mix chamber has anything to do?
Yes, that's what happens. Choke off some of the air reduces the noise. But then too much choke is inefficient. It's really a trial and error thing.

Talked with Bill at the steamup last week. He said the length of the mixing tube should be at least 6 times the inside diameter. That's very true for a poker burner otherwise they don'y light very well. Bill says its true for ceramic too. Don't know if that affects noise however.

vr Bob
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Old 07-26-2017, 07:53 PM   #9 (permalink)
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this is an excel file that I use for designing my boilers. I made it up so long ago that I forget where I got the formula. I think Martin Evans.
It is for coal burners but it works fine for ceramic too.
If the tube is too small and long, it will choke things out
If it is too big and short, it will lose heat out the stack


od id Ideal length Surface area per inch
0.311 0.250 4 0.977
0.375 0.311 6 1.178
0.500 0.436 12 1.571
0.625 0.495 16 1.964
0.625 0.561 20 1.964
0.750 0.620 25 2.356
0.875 0.745 36 2.749
1.000 0.870 49 3.142
1.250 0.120 1 3.927
1.375 1.245 101 4.320
1.500 1.370 122 4.712

This program lumps everything together so you can't see the columns but on the first line od is 0.311*** id 0.250*** ideal length 4 ***surface area 0.977
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Old 07-26-2017, 08:16 PM   #10 (permalink)
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I looks like the heated air and the cool air to support combustion are using the same path.

If you can raise the whole assembly up off the burner base about 1/2 an inch and see if it is now quiet.

Cold Air flow up from the bottom of the burner and heated air out the open end of the asm. Air movement all in one direction.
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