Posted by: Mike Clark
Googled for Rayleigh Criterion in combustion - as you say there's a lot of info.
This link [home.earthlink.net
] sums it up pretty well.
The various steamcar forums and (going back 40 years Light Steam Power) have been speculating about howling when there was already a scientific explanation well known to combustion engineers. Several point made in the article and in others fit very well with our Stanley experience. Good for you for pointing it out.
To summarise, the combustion chamber, boiler flues and smokehood/exhaust flue form an oscillation chamber just like an organ pipe while the burner flames have a natural tendency to high frequency pulses as the flame front velocity and gas flow interact (and here no doubt the length of the venturi, the fuel flow and vaporisation all have their effect). When the two frequencies more or less coincide the flame oscillations reinforce the natural frequency of the combustion chamber in a sustained resonance.
Your comment about the influence of temperature on the speed of sound - maybe this explains why we get a howl in situations where vaporisation is incomplete, if the jet is too big, or when forcing the burner during firing up from cold, or after a short stop - perhaps the temperature of the fuel/air mix in the venturi is lower than when the fuel is fully vaporised. This would change the speed of sound in the venturi and hence alter the resonance frequency. If the fuel is fully vaporised it is a hot gas mixing with the air to arrive at a combined temperature. If it is hot but still partially in droplet form presumably these are evaporating frantically as they pass through the venturi and the temperature of the mixture may be lower.
Our messing about with a shield at the venturi or deflactors at the flue obviously alter the natural frequencies of the inward and outward ends of the system or possibly by changing the pressure difference slow down the gas flow and change the frequency of the flame oscillation.
My maths is not up to using the formulae in the Seebold paper but it really gives us food for thought.