P.1 

OAR Box 1878 

Prepped by Charmelle Mathews 

Document Number: 

72) 11-1-2 

Docket Number: 

A-90-19 

Printed 11/28/20081:21:24 PM Sheet 72 of 145 
EPA AIR DOCKET 

North American 

COMBUSTION HANDBOOK 

A Basic Reference on the 

Art and Science of Industrial Heating 

with Gaseous and Liquid Fuels 

Second Edition 

Notfc* This material may be protected ftf 

Copyright Law 'T f 2 17 U.S. Code) 

NORTH AMERICAN Mfg. Co. 

CLEVELAND. OHIO 44105 USA 

© mi 
FUEL-BURNING EQUIPMENT 269 

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The maximum sound level is usually at stoichiometric fuel/air ratio or 

just slightly rich; and this peak sound level moves to a lower frequency 

(pitch) and lower amplitude (volume) for all other fuel/air ratios. If a constant 

total flow rate were maintained through a premix flame, the noise production 

would be cut in half (3 dB less) at the lean and rich limits of flammability. 

For premix flames, turbulence intensity can disturb a flame and cause 

it to make more noise: so orifices, valves, and fittings should be located far 

upstream to allow the turbulence to decay before the stream enters the 

flame. 

For nozzle-mix flames, the minimum noise and most compact flame are 

usually achieved when the combustion area and the zone of maximum 

turbulence are the same, while using the largest possible air and fuel ports. 

If the available momentum is not used for effective mixing within the combustion area, the volumetric rate of combustion (intensity) will be low; but if 

the turbulent zone exceeds the combustion area, excess noise will result. 

The geometry of a burner's refractory tile may set up strong natural 

modes of oscillation, amplifying combustion roar, and causing peaks at 

the natural acoustic frequencies of the gas in the tile. The natural acoustic 

frequency of a larger tile is lower. The use of sound-absorbing material for 

the tile may lower the sound level bv 5 or 10 dB at frequencies above 600 Hz. 

The basic sound spectrum generated by a flame can be altered by the 

acoustic characteristics of the fluid piping systems, the furnace, and/or 

the room. 

Superturbulent combustion noise is a special case of combustion roar 

associated with unstable flames and evidenced by a sudden jump in the 

sound pressure level. This results from the increased turbulence in the 

unstable operating mode. An example of such an unstable mode occurs 

when a flame fluctuates between type 2 and type 3 of Figure 6.2. Also, a 

5 dB jump was noted when a reduction in swirl of a type 3 flame caused it 

to detach from the tile. Changing the oil spray angle relative to the burner 

tile angle has had similar results. Firing a burner beyond its rated range 

may do likewise. 

Combustion-driven osci/Jnfions result when some feedback mechanism 

produces oscillations that lead to a variable heat release, the timing of 

which is such that it adds energv to subsequent oscillations. As an example, 

a pulsating blower not only generates noise, but provides a mechanism for 

combustion driven oscillations. Another is the alternate vortex shedding 

behind a bluff body. 

Combustion-driven oscillations have a higher frequency than combustion 

roar and they have narrow peaks of frequency as opposed to the broad 

spectrum of combustion roar.