Combustion control using a multiplexed diode-laser sensor system

Abstract

A multiplexed diode-laser sensor system, comprised of two InGaAsP diode lasers and fiber-optic components, has been developed to nonintrusively measure temperature and species concentration over a single path for closed-loop process control using laser absorption spectroscopy techniques. The system was applied to measure and control the gas temperature in the postflame gases 6 mm above the surface of a Hencken burner (for multiple CH 4 -air diffusion flames). The wavelengths of the lasers were independently current tuned across H 2 O transitions near 1343 nm ( ν 1 + ν 3 band) and 1392 nm (2 ν 1 , ν 1 + ν 3 bands). Temperature was determined from the ratio of measured peak absorption coefficients. H 2 O concentration was determined from the measured absorption coefficient of one transition set at the measured temperature. The mean temperatures recorded compared well with those measured by a thermocouple. Temperature fluctuations up to 250 Hz were measured accurately with the sensor system, demonstrating the high bandwidth of the measurement technique. A computer-controlled, closed-loop feedback circuit controlled the flame temperature in the probed region by adjusting the fuel flow in response to the difference between the measured and the desired gas temperature. The results obtained with this first-generation system demonstrate the potential of multiplexed diode lasers for rapid, continuous, nonintrusive, in situ measurements and control of combustion environments.