E. R. Furlong

Scanned- and fixed-wavelength absorption diagnostics for combustion measurements using multiplexed diode lasers

A multiplexed diode-laser sensor system comprising two diode lasers and fiber-optic components has been developed to nonintrusively monitor temperature and species mole fraction over a single path using both scanned-and fixed-wavelength laser absorption spectroscopy techniques. In the scanned-wavelength method, two InGaAsP lasers were current tuned at a 2-kHz rate across H 2 O transitions near 1343 nm and 1392 nm in the 2v 1 and v 1 + v 3 bands. Gas temperature was determined from the ratio of single-sweep integrated line intensities. Species mole fraction was determined from the measured line intensity and the calculated line strength at the measured temperature. In the fixed-wavelength method, the wavelength of each laser was fixed near the peak of each absorption feature using a computer-controlled laser line-locking scheme. Rapid measurements of gas temperature were obtained from the determination of peak line-intensity ratios. The system was applied to measure temperature and species concentration in the postflame gases of an H 2 -O 2 flame. The good agreement between the laser-based measurements obtained using scanned- and fixed-wavelength methods with those recorded with thermocouples demonstrates the flexibility and utility of the multiplexed diode-laser sensor system and the potential for rapid, continuous measurements of gasdynamic parameters in high-speed or transient flows with difficult optical access.

Real-time adaptive combustion control using diode-laser absorption sensors

A wavelength-multiplexed, diode-laser absorption sensor system, comprised of two distributed feedback (DFB) InGaAsP diode lasers and fiber-optic components, has been developed to nonintrusively measure gas temperature and H2O concentration over a single path in the combustion region of a 5-kW, atmospheric-pressure, non-premixed, annular dump combustor. The wavelengths of the DFB lasers were independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm (m1 ‘ m3 band) and 1392 nm (2m1, m1 ‘ m3 bands). Temperature was determined from the ratio of measured peak absorbances. Water-vapor mole fraction was determined from the measured absorbance and gas temperature values. The diode-laser sensors were applied for closed-loop control of the combustor, which utilized the concepts of acoustic forcing to improve the fuel-air mixing and thus the combustion efficiency. Adaptive control strategies were investigated that used the measured mean H2O concentration and the instantaneous temperature as sensed parameters. The relative phase between the acoustic forcing of the central air and the fuel (C2H4) flow, as well as the fuel-forcing amplitude, was adjusted to maximize the coherence and the extent of reaction in the combustion region and thus optimize the combustor performance. The closedloop control system was able to adaptively optimize the combustor performance within 100 ms, which corresponded to seven characteristic time constants of the actuator. The results obtained demonstrate the applicability of multiplexed diode-laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters in high-temperature combustion environments.

Diode-Laser Sensors for Real-Time Control of Pulsed Combustion Systems

A diode-laser based, closed-loop control system has been developed to nonintrusively optimize a pulsed, 50-kW dump combustor. The adaptive control system used temperature and water mole fraction measurements obtained at 10-kHz rates from the peak absorbance values of H 2 O features near 1.4 μm. In addition, measurements of CO, C 2 H 2 , and C 2 H 4 concentrations in the exhaust, determined from diode-laser absorption spectra recorded using a fast-sampling probe and a multipass absorption cell (nominal 33-m-long path), were used to evaluate the effectiveness of the control strategies. A correlation was established between the magnitude of the observed temperature oscillations and the measured CO concentration in the exhaust. Adaptive control strategies were then applied to maximize the coherence of the burning vortices in the combustion region and thus optimize the combustor performance. The closed-loop control system was able to adaptively tune the phase and amplitude of the applied forcing within 100 ms and the forcing frequency within 10 s. 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, in high-temperature combustion environments.

DIODE-LASER SENSOR SYSTEM FOR CLOSED LOOP CONTROL OF A 50-kW INCINERATOR

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 model pulsed incinerator. The wavelengths of the DFB lasers wee independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm. Temperature was determined from the ratio of measured peak absorbencies and used for closed-loop control of the combustor. In addition, measurements of CO, CO2, and C2H4 concentrations were determined from absorption spectra recorded in the incinerator exhaust using a fast-sampling stainless steel, water-cooled probe and a multi-pass absorption cell. An external cavity diode laser was tuned over the CO R(13) transition near 1568 nm and the CO2 R(16) transitions near 1572 nm, and a DFB laser was tuned over selected C2H4 transitions near 1646 nm. A correlation was established between the magnitude of the observed temperature fluctuations and the measured CO concentration in the exhaust. The amplitude of temperature fluctuations was controlled in a feedback loop by adjusting the relative phase between the primary and secondary forced air flows. The results obtained demonstrate the applicability of multiplexed diode laser absorption sensors for rapid, continuous measurements and control of multiple flowfield parameters, including trace species concentrations, in high- temperature combustion environments.

Advanced diode-laser absorption sensors for combusion 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 purposed annular dump combustor. The wavelengths of the DFB lasers were independently current-tuned at 10-kHz rates across H2O transitions near 1343 nm and 1392 nm. 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, an external cavity diode laser (ECDL), and a distributed feedback diode laser (DFB). The ECDL was tuned over the CO R(13) transition near 1568 nm and the C2H2 P(17) transition near 1535 nm, and the DFB laser was tuned over selected C2H4 transitions 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 frequently 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, in high-temperature environments.

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. The wavelengths of the lasers were independently current-tuned across H2O transitions near 1343 nm and 1392 nm. 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 gas dynamic parameters in high-temperature combustion flowfields and other environments with difficult optical access.

Recent Advances in Laser-based Diagnostics for Gaseous Flows

Laser-based diagnostic techniques offer unique capabilities for experimentation on gaseous flows. In this paper, we overview recent progress of two concepts: spectrally resolved absorption and planar laser-induced fluorescence (PLIF) imaging. The absorption measurements utilize tunable diode lasers (TDLs) as light sources. Recent TDL applications include a wavelength-multiplexed system for rapid temperature sensing for use in combustion control, and absorption probes for time-resolved measurements of temperature, velocity and species concentrations in a hypersonic shock tunnel. Recent PLIF work includes applications to supersonic, exothermic flowfields relevant to ram accelerators, and development of a method for imaging temperature in air flows using acetone seeding.