Real-time process control using diode-laser absorption sensors

A multiplexed diode-laser sensor system, based on absorption spectroscopy techniques and comprised of two InGaAsP diode lasers and fiber-optic components, has been developed to measure temperature and species concentration non-intrusively over a single path for closed-loop process control. The system was applied to measure and control the gas temperature in the post-flame gases 6 mm above the surface of a Hencken burner (multiple CH4-air diffusion flames). The wavelengths of the lasers were independently current-tuned across H2O transitions near 1343 nm (v(1)+v(3) band) and 1392 nm (2v(1), v(1)+v(3) bands). Temperature was determined from the ratio of measured peak absorbances, and H2O concentration was determined from the measured peak absorbance of one transition set at the measured temperature. A closed-loop feedback system was demonstrated to control the mean temperature and the amplitude of temperature fluctuations at particular frequencies by appropriately modulating the fuel flow rate. The results obtained demonstrate the potential of multiplexed diode lasers for rapid, continuous, in situ measurements and control of gasdynamic parameters in high-temperature combustion flowfields and other environments with difficult optical access.