Combustion roar

Another source of noise in a combustion system comes from the burner and is sometimes referred to as combustion roar. 33 This noise is a combination of the gas flow through the burner nozzles and also from the combustion process itself. There are many factors that affect the noise level produced by the combustion system. These include the firing rate, oxidizer-to-fuel ratio, turbulence intensity of the gas flows, combustion or mixing intensity, amount of swirl, preheat of the oxidizer or fuel, type of fuel and oxidizer, number of burners, geometry of the combustion chamber, insulation used in the combustor, and even the dampening effects of the material being heated. 

Noise is often defined as unwanted sound. There are many possible sources of noise in industrial combustion testing (see Chapter 8). Some of these include combustion roar, jet noise for flow through orifices, flow noise for fluids flowing fhrough piping, and equipment noise such as from fhe combustion air fan. Acoustic resonance can exacerbate the problem by magnifying the noise. 

Low Frequency Noise Sources Combustion roar and instability Fan noise... 

It has been recognized for a long time that the noise emitted from a normal operating flare has two mechanisms at work, namely, combustion roar and gas jet noise. Combustion roar typically resides in the lower frequency region of the audible frequency spectrum while gas jet noise occurs in the higher frequencies as illustrated in Figure 8.16. 

The combustion roar emitted from a flare flame is not highly directional and is considered to be a monopole source. That is, it is analogous to a spherical balloon whose surface is expanding and shrinking at various frequencies and emitting uniform spherical waves. 

Figure 8.12 is a plot showing a typical noise spectrum emitted from a burner operating under normal conditions in a furnace. Notice that the noise spectrum has two peak frequencies associated with it. The high frequency noise confribufion is from the fuel gas jets while the low frequency confribufion is from the combustion roar. Like the combustion roar emitted from flares, burner combusfion roar is associated with a smooth... 

The combustion roar associated with flares typically peaks at a frequency of approximately 63 Hz while combustion roar associated with burners can vary in the 200-500 Hz range. Burner noise can have a spectrum shape and amplitude that can vary with many factors. Several of these factors include the internal shape of the furnace, the design of the burner muffler, plenum and tile, the acoustic properties of the furnace lining, the transmission of the noise into the fuel supply piping, and the transmissive and reflective characteristics of the furnace walls and stack. 

As previously discussed, the two main sources of noise emitted from industrial flares is combustion roar and gas jet noise. Inhibiting the rate at which the air and fuel streams mix can reduce the level of combustion roar however, this noise abatement technique generally tends to reduce the smokeless performance, increase thermal radiation, and flame length. Reducing the mixing rate of the air and fuel stream in order to lower combustion roar levels usually cannot justify the accompanying sacrifices in the performance of a flare. 

The most important of fhe above sound sources, typically, are the combustion roar from the combustion process itself, which resides in the frequency range of approximately 100 to 1000 Hz, and the gas jet noise, which typically ranges between 4000 and... 

Jet Mixing Noise Screech Noise Combustion Roar Total... 

The screech noise would not exist if the flare operated below the critical gas pressure. Below the critical gas pressure shock waves do not form, which causes screech noise. The summation of the combustion roar, gas jet mixing noise, and screech noise provides the total SPL prediction emitted from the flare. 

The flameless oxidation was discovered in 1989 during trials aimed at NO reduction in combustion with highly preheated air [5,6]. No flame could be detected or heard in the combustion chamber and the NO abatement was above all expectations. Figure 23.2 shows pictures of these trials. On the left-hand side, the burner is shown in flame mode the stable high velocity flame is pale blue and the high temperature at the burner nozzle causes a local bright luminosity. On the right-hand side, the same burner is shown in flameless oxidation mode. The hot zone at the burner nozzle has disappeared and no flame is visible or any combustion roar is audible. 

A candle flame is a miniature example of a type F long, luminous, laminar flame. Author Reed has often demonstrated some of the features of type F flames with a candle—polymerization soot formation, flame quenching, flame holders, starved air incineration, natural convection, particulate emission, streams in laminar, transition, and turbulent flows, aeration (by exhaling through a tiny straw across the blue base of the candle flame) changes it to a compact, all-blue flame that demonstrates combustion roar. Some of these demonstrations were recently found to have been alluded to in Professor Michael Faraday s famous candle lectures of the 1850s r erence 19). 

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