| OCR Text |
Show - 4 - Figure 2 shows a set of the pressure-time waveforms recorded of a turbulent flow(a) and methane-air turbulent premixed flames(b-d) at 4.5 m/s. The equivalence ratios of methane-air turbulent premixed flames were 0.75(b), 1.14( c) ,and 1.4( d). t and P in the figure represen t the time after the record-n ing in Ins and the sound pressure in P a, respectively. The pressure-time waveforms show no apparent periodicity. Close examination of the trace (a), the pressure waveform of a t ur bulen t flow shows irregular spikes of about 0.4 Pa(peak to peak) . The amplitudes of spikes decreased as the flow velocity decreased and no apparent spike was observed at 0 m/s. These spikes were confirmed to be not of electronic nor back ground noise. The pressure-time waveform of a turbulent premixed flame shows 2 to 5 ms duration period fluctuations which did not appear in the pressure-time waveform of a turbulent flow of the same velocity. Tlle amplitude of the fluctuations became about 1.3 Pa(peak to peak) at ¢ = 1.14(c). At ¢ = 0.75(b) and 1.4(d), the amplitude, 0.4 Pa, of the fluctuations was as large as that of irregular spikes. The amplitude of irregular spikes was independent of the equivalence ratio, although that of the fluctuations discussed above depended largely on the equivalence ratio . Frequency spectra of sound waveforms presented in Fig. 2 are shown in Fig. 3. f in the figure is the frequency in kHz. S pi is the mean square of sound pressure P in dB relative to 20 J-lPa. A turbulent flow sound spectrum is seen to be a s typical one associated with aerodynamic noise, containing no distinct frequency peaks, extending over 10 kHz and rising to a single broad maximum. The peak around 0 Hz in the turbulent flow sound spectrum is mainly due to the contribution of background noise of this sound record system. The major difference between the spectra of sound with turbulent flames and that without flame can be found at the low frequency components below 2 kHz. At 250 Hz, intensities of the sound pressure with turbulent flam es are 60 dB, 73 dB, and 58 dB for ¢ = 0.75, 1.14, and 1.40, respect ively, while the intensity of the sound pressure without flame is 35 dB. The intensiti s of the sound pressure with turbulent flames are at least 20 dB higher than that without flame in these cases. At 5 kHz, intensities of the sound pressure with turbulent flames and that without flame are almost of the same value, 50dB. These spectra in the frequency range |
