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Howling Burners
Posted by: johnhennessy (---.plus.com)
Date: April 1, 2010 06:51AM

Howling Burners

I read with interest the article by Geary Baese in the April edition of “the Steam Car”; at last a definitive explanation of an interesting phenomena that most of us have experienced at some time.
Sorry Geary I can not agree with your explanation.
In the opening statement “Stanley burners howl because the mixing tubes act like organ pipes”
I think not.
We can agree that, common with all wind instruments, an organ pipe has an oscillating device near where the air enters followed by a tube which acts as a resonating cavity (which determines the frequency of the note).
In the mixing tube assembly on most of the burners that I have come across, the air/fuel mixture is directed through the venturi straight down the tube. And I challenge anybody to blow straight down a 1 inch diameter tube 14 inches long and produce a note. Further, in my burners (Oterway type) the mixing tubes are closed at the end but have a series of holes along them; so the effective resonating cavity is only approximately 2 inches long, which would, in an organ pipe produce a note near the limit of audibility. But have you ever tried a burner on its own without a boiler on top? Howl they do not.(well mine didn’t)
So what is going on?
Here is my explanation for what it is worth.
First find your resonating cavity. Having eliminated the mixing tube we are left with the boiler tubes.
On Locomobile boilers they are around 13 inches (in my case 12.75) long. Based on Geary’s formula this would give a frequency of 258 (sounds familiar) organ note around C That sounds about right also considering that there are, in my case, 305 such tubes all making there contribution to the cacophony.
But now we come to the tricky bit and where I put my head above the parapet. What is triggering and maintaining the oscillation? I think it is instability in the combustion, or if you like “flame oscillation”.
And the cause? Partial combustion of the gasses in the burner flames. The flame oscillation is probably of a much higher frequency than the final pitch of the howl, and this is where the mixing tube parameters come in; this oscillation then maintains the fundamental of the boiler tube cavities.
Consider some of the conditions and parameters. Howling usually occurs at start up and then dies away. It is effected by, amongst other things, venturi size, jet to venturi gap, fuel type, fuel pressure. and jet size. In my case the last two are the most significant. At start-up the air/fuel mixture is relatively cold and often ‘wet’ after a while the mixture gets hotter and less dense, the combustion is improved and the flue gasses get hotter, and surpise,suprise, howling stops.
I like to run the fuel pressure at around 35psi. I can make my system howl continually, or not at all, by changing the jet size. I prefer to have howling at start up so when out on the road, and the burner cuts in, it is good enough it let me know we have hotness where it matters.
In fairness to Geary much of what he says about the characteristics of the howling phenomena I do indorse, but I think he is short of the mark.
So why do these burners howl? Answer, flame oscillation due to incomplete combustion coupled with the resonating cavities of the boiler tubes.
And when the pilot goes out I get my own back and howl instead.

John H

Re: Howling Burners
Posted by: Tom Dawson (---.cpe.metrocast.net)
Date: April 1, 2010 09:10AM

JOHN:
I agree. "Flame Oscillation" is the cause. Thanks for clearing that up. The magazine article made me wonder. But, you set it straight. Thanks!!
TOM

Re: Howling Burners
Posted by: Jeff Theobald (Moderator)
Date: April 2, 2010 09:30AM

Hi All,

Just to throw a in a little input, I don't think the boiler tubes play the main part in the howling although they will affect the sound, I have had a number of burners howl while out of a car on my test rig, (firing in the open).

I think what happens is the flame front exceeds the speed of the incoming gas in a small area of the burner, at some point across the burner the bottom of the flame collapses back to the burner surface, the process repeats and seem to propagate to the whole of the burning surface, the speed this happens produces a note which will be affected by many things, the venturi length, area under the plate, temperature and humidity of the day, type of fuel, mixture, etc etc.

Of course once the burner is under the boiler, the tubes, burning area, flue size, is the blower operating etc, all play a part. Over the years of operating the Brooks she's made some amazing sounds, the best being a very pleasant high pitch warbling, I think this is caused because the two half's of the burner are completely separate so each half oscillates at a different speed.

To get the best out of the very small boiler for the size and weight of the car, you have to fire as hard as possible which makes for a noisy burner.

all the best, Jeff.

Re: Howling Burners
Posted by: Mike Clark (---.bb.sky.com)
Date: April 3, 2010 07:43AM

John H - I hate to disagree but on the subject of burners howling when not under the boiler - they can and I have a bit of video of mine in action to prove it. Jeff has a copy of this and I have asked him to link it to the forum or put it on the web site.

We have debated this so many times before and I am sure it is all there on other threads on the forum but here is a short summary.

The resonance is in the mixing tubes and the mixing chamber rather in the boiler pipes.

The resonance is set off by high frequency vibration of the flame which is trying to burn back down through the holes in the burner plate against the flow of gas, gets so far, goes out and is pushed back up by the gas. Initially this flame vibration is random over all the 6 or 7000 flame holes but in the right conditions when by chance enough flames are trying to push the gas back at the same time this causes a minor hiccup in the total gas flow through the venturis - a pulse if you like. If the frequency of this pulse matches the natural resonant frequency of the mixing tubes (which is matter of physics and determined by the length of the tube) a wave of pressure is set up in the mixing tubes which has the effect of synchronising all the flame pulses together across the entire burner, augmenting the pulses in the mixing tubes and howling away.

Anything which alters the rate of gas flow will change the howling frequency by altering the battle between the flame going down inside the plate and the gas going up. This is why putting the boiler on top or putting in bigger jets or higher fuel pressure makes a difference. The degree of fuel vaporisation also matters as this too changes the flame properties. If Jeff manages to get my video up you can actually see that when the burner is howling there are tiny flashes of yellow among the generally clear flame which I suspect are globlets of unvaporised fuel which have got through the burner plate hole as liquid and so have a local shortage of oxygen as they burn. That must be why a cold burner howls more than a hot one. There is also the possibility that the well known propensity of paraffin to cause more howling than petrol is due to a difference in the speed of the flame front as it tries to go down the burner holes.

It's a very interesting subject!

Edited to add

Think of it as a trombone - the flames are the player's lips making a vibration while the mixing tubes are the slide and flared cone in which the vibration resonates. There is a further point - howling can warble, going up an octave and back down - this is known to the musician as overblowing which has just the same result.

Mike



Edited 4 times. Last edit at 04/03/10 06:01PM by Mike Clark.

Re: Howling Burners
Posted by: (---.wavecable.com)
Date: April 3, 2010 10:15AM

Dear Mike, A well presented explaination you have there. I fully agree. SSsssteamer

Re: Howling Burners
Posted by: Jeff Theobald (Moderator)
Date: April 3, 2010 05:03PM

Hi All,

The video is availible to view now, click on the link below, all the best, Jeff.


www.steamcar.net

Re: Howling Burners
Posted by: Rolly (---.hsd1.ma.comcast.net)
Date: April 4, 2010 08:30AM

Burner howl
No mater how you slice it the noise is still coming from the resonating venturi tube. An opened ended organ tube.

Calculating the frequency equals the speed of sound divided by twice the cavity length. This was all worked out by the Benedictine Monks in the 16th Century.

The cause and affect is the changing fuel pressure, changing combustion pressure. Speed at which the fuel and air pass through the tube.

You can eliminate the howl by changing the angle of the tube on the inside edge of the burner. It can be stepped or cut at a steep angle. The longer the tube the better, but it should be at least six times the diameter at the shortest edge.

Attachments: Original Venturi tube.JPG (151kB)   Venturi shape.jpg (23kB)  
Re: Howling Burners
Posted by: Mike Clark (---.bb.sky.com)
Date: April 4, 2010 08:57AM

Interesting solution Rolly. Mark Drake and I wondered if having one venturi shorter that the other would cause some interferance and prevent the howl - mine now runs with one tube shorter by 1 inch than the other - no difference in howl though. I also wondered whether putting a series of holes down the side of the tube, like the finger holes on a flute would change it but have not tried that.

By the way the howl on my video is at a higher pitch than the burner normally produces which must be due to the absence of the restriction with the boiler dismounted.
Mike

Re: Howling Burners
Posted by: (---.dynamic.dsl.maxnet.co.nz)
Date: April 4, 2010 04:27PM

[www.swtpc.com]

Thought I had remembered this from back in the past. A flame by itself can amplify a frequency. Possibly a burner can be considered a resonant chamber with a flame amplifier?
Cheers

Re: Howling Burners
Posted by: (---.wavecable.com)
Date: April 4, 2010 11:12PM

This is a new concept to me and I have to rethink my reasoning. Sounds like they may have it correct. The burner's mixing tubes make the tone heard and the flame in the burner amplifies the sound. The article's one test sounds convincing to me. It stated:

"...Stanford Research Institute came up with what its scientists dubbed a "dragon horn": a diaphragm type speaker horn (the kind used in public address systems) screened at the end and through which, when additional volume was required, methane and air were introduced and ignited. The screened horn-end, converted to a burner, belched flame (thus, "dragon horn"), and, SRI found, effectively boosted audio output 'by some 15 dB..."

That also makes sense because when you have the Stanley's smoke bonnet's lid open, the howling heard comes up through the fire tubes, and not hardly any howling from the front of the burner. When you close the smoke bonnet's lid, the howl is still in there but now it is very muted by the closed lid. It matches the "dragon Horns" example. A very interesting concept.

Thank you for sharing the "Popular Science" article with us.

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