Sensing and control of combustion instabilities in swirl-stabilized combustors using diode-laser absorption

Thermoacoustic instability and lean blowout are monitored in propane/air flames in a swirl-stabilized combustor using a novel tunable diode-laser sensor for gas temperature using wavelength-scanned laser absorption of two neighboring water vapor transitions near 1.4 mu m. Although the gas composition and temperature are not uniform along the sensor line of sight, temperature oscillations and fluctuations are clearly identified in the fast Fourier transform power spectra of the time-resolved data. Detailed experiments are conducted to optimize the position of the sensor line of sight in the flame for thermoacoustic instability and lean-blowout sensing. The output of the sensor is used to suppress the thermoacoustic oscillations in a phase-delay feedback control system, and acoustic modulation of the intake air flow suppresses the temperature oscillations by more than 7 dB. The low-frequency temperature fluctuations measured by the tunable diode-laser sensor increase sharply As the flame approaches lean blowout. The intensity of these low-frequency fluctuations is used to detect the proximity to lean blowout and can be used as a control variable for active lean blowout suppression without knowing the lean-blowout fuel/air ratio limit. The tunable diode-laser sensor results are compared with traditional pressure and emission sensors, and the tunable diode-laser sensor is found to offer several advantages, including better spatial resolution and insensitivity to background acoustic noise and flame emissions.