Quarter-Wave combustor

Based on the initial studies of unforced flows described above, inflow was selectively perturbed to investigate if the amount of particle dispersion and the location of enhanced dispersion within the combustor can be shifted as desired. Calculations were performed in which an acoustic perturbation was imposed from the back wall of the combustor with an amplitude of 0.5% of the initial chamber pressure and a frequency of 1380 Hz, 690 Hz, or 145 Hz, the characteristic frequencies of the system under study. The vortex-shedding frequency was 1380 Hz, the first-merging frequency was 690 Hz, and 145 Hz was the quarter-wave mode of the inlet. In addition, simulations were also performed with forcing at a frequency unrelated to the system, 1000 Hz. The particle size chosen for these simulations was 15 pm in diameter (St = 0.97), since this size particles were found to be optimally dispersed in the unforced flow case. All other parameters remain unchanged from the unforced case discussed above. 

Acoustic quarter-waves with an antinode at the upstream end of the combustor and RMS pressures up to 10 kPa have been shown to dominate the flows in the two combustors tested. The quarter-wave occupied the duct length upstream of the annular ring in the first arrangement and the entire duct length in the swirling flow. 

To determine the origin of the instability frequency acoustic analysis was performed which revealed that both the quarter-wave mode of the inlet and the Helmholtz mode of the combustor-inlet system occurred at 35 Hz. The phase... 

The combustor is naturally unstable under certain operating conditions. Figure 16.7 shows combustor pressure oscillations and the Fast Fourier Transform (FFT) spectrum under atypical, unstable operating condition. The fundamental mode at 39 Hz and its higher harmonics were observed. The fundamental-mode frequency corresponds to the inlet quarter-wave mode of acoustic oscillations. During stable operation as shown in Fig. 16.8, the amplitude of pressure oscillations is much less. Also, no significant peak was observed in the pressure spectrum. 

Pulse combustors may be categorized into three distinct classes according to the specific acoustic system on which their operation depends. These are the Quarter-Wave (or Schmidt) combustor, the Helmholtz combustor, and the Rijke-type combustor. 

The operation of the Schmidt pulse combustor is based upon the principle of the quarter-wave sound resonator formed of a tube closed at one end. The acoustic pressure oscillations excited within this tube experience their maximum value at the closed end (at the lid) and their minimum value (which is... 

Pulse combustors may be categorized into three distinct classes according to the specific acoustic system on which their operation depends (i) the Quarter-wave (or Schmidt) combustor (ii) the Helmholtz combustor and (iii) the Rijke-type combustor. In contrast to the Rijke combustor, which operates with solid fuels, both the Schmidt and Helmholtz combustors accept liquid and gaseous fuels. The Helmholtz combustor is preferred for drying applications because the larger volume of the combustion chamber and the smaller (but longer) tailpipe allows for multivalve assembly. Detailed information on these types of combustor is available in articles by Zinn (1985) and Kudra and Mujumdar (2009). Some combustors also exploit the resonance phenomenon these are referred to as frequency-tunable pulse combustors. 

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