Further experimental results on the structure and acoustics of turbulent jet flames

Abstract

The structure of open turbulent jet flames is experimentally studied in the context of their noise emission characteristics. The differences between premixed and (co-flow) non-premixed flames are explored. Recent experiments repeated in an anechoic chamber complement earlier results obtained in a hard-walled bay. The reactants (methane and enriched air) are burned in the premixed, or non-premixed, mode after a length of pipe flow (t/D> 150). The thick-walled tubes anchor the flames to the tip at all of the velocities employed (maximum velocity, well over 300 ft/sec), thus eliminating uncertainties associated with external flameholders. The time-averaged appearance of the flames is obtained with still photographs (1160 sec). The detailed structures are revealed through high-speed ("" 2500 frames/sec) motion pictures. The acoustic outputs of the flames are mapped with a condenser microphone. The recorded data are played back to obtain the amplitude, waveshapes, directionalities, and frequency spectra of the noise. Profound differences are found between the premixed and non-premixed flames in their structures and noise characteristics.

Acoustic radiation from weakly wrinkled premixed flames

“…It is also generally experimentally observed that flame noise levels peak in the frequency range 200-1000 Hz [6,11]. Some data also indicate the presence of multiple peaks in the acoustic spectra [12]. A number of analyses have assumed a Strouhal number scaling (based on burner diameter and jet exit velocity) for this peak frequency [12,13].…”

“…Some data also indicate the presence of multiple peaks in the acoustic spectra [12]. A number of analyses have assumed a Strouhal number scaling (based on burner diameter and jet exit velocity) for this peak frequency [12,13]. Noting the shortcomings of these Strouhal number scalings, others [14,15] have suggested that this frequency is kinetically controlled.…”

Further Experimental Results on the Structure and Acoustics of Turbulent Jet Flames

“…Also, an attempt is made to minimize the usual uncertaintie s as s ociated with external flameholder s. Premixed flames have also been studied here, under otherwise identical conditions, with the intention of having a ready and meaningful basis of comparison with non-premixed flames. Some of these results were presented earlier (Kumar 1974). Those experiments, done in a hard-walled bay, have now been repeated in an anechoic chamber, and the two results are compared.…”

“…The Experimental Details As described earlier (Kumar 1974), the details are as follows. Figure 1 shows the simple apparatus constructed.…”

“…The judgement on whether the similarities are more profound or whether the dis similarities are more profound is left to the competent reader, who should also evaluate the earlier statement " ... comparison between results for the premixed and non-premixed flames should be valid ... II in the hard-walled bay (Kumar 1974).…”