Real-time equivalence ratio measurements in gas turbine combustors with a near-infrared diode laser sensor

A rapid (2 kHz) near-infrared tunable diode laser (TDL) absorption sensor is developed for real-time measurements of methane concentration (and thus equivalence ratio) in gas turbine combustors. The non-intrusive sensor is based on fixed-wavelength laser absorption of methane near 1.65 μm using a single fiber-coupled diode laser. This CH4 absorption wavelength is free of interference from the absorption of other species in air. Wavelength modulation spectroscopy (WMS) is combined with second-harmonic detection to improve the sensor sensitivity and accuracy. WMS-1f signal is used to normalize the 2f signal to remove the need for calibration and account for the laser transmission variations in gas turbine combustors. The near-IR TDL sensor is first calibrated in a static cell with CH4 and N2 mixture to determine the line-strength and find the laser setpoint. Test with a gas turbine combustor operating at atmospheric pressure is then conducted to demonstrate the sensor accuracy and response time. The TDL sensor is used to characterize equivalence ratio fluctuations in the combustor before the fuel/air mixtures enter the combustion chamber. The gain between heat release and equivalence ratio fluctuation is measured when the fuel flow rate is modulated at different levels. The combustion test results demonstrate the utility of the TDL sensor as a useful diagnostic tool to study flame characteristics and evaluate different combustor designs.