Advanced diode-laser absorption sensors for combustion monitoring and control

A multiplexed diode-laser absorption sensor system, comprised of two distributed feedback (DFB) InGaAsP diode lasers and fiber-optic components, has been developed to non-intrusively measure gas temperature and H2O concentration. over a single path in the combustion region of a 50-kW pulsed annular dump combustor. The wavelengths of the DFB lasers were independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm (v(1)+v(3) band) and 1392 nm (2v(1), v(1)+v(3) bands). Temperature and water vapor concentration were determined from the measured absorbances. In addition, measurements of CO, C2H2, and C2H4 concentrations in the exhaust were determined from absorption spectra recorded using a fast-sampling probe, a multi-pass absorption cell (33-meter path), an external cavity diode laser (ECDL), and a distributed feedback diode laser (DFB). The ECDL was tuned over the: CO R(13) transition (3v band) neat 1568 nm and the C2H2 P(17) transition (v(1)+v(3) band) near 1535 nm, and the DFB laser was tuned over selected C2H4 transitions (v(1)+v(9), v(5)+v(9) bands) near 1646 nm. A correlation was established between the magnitude of the measured temperature oscillations in the combustion region and measured concentrations of CO and hydrocarbons in the exhaust. Adaptive control strategies were applied to maximize: the coherence of the: temperature oscillations and thus optimize the combustor performance. The closed-loop control system was able to adaptively optimize the phase and amplitude of the applied forcing within 100 ms, and the forcing frequency within 10 seconds. These results demonstrate the applicability of multiplexed diode-laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, fn high-temperature environments.