Radu M. Mihalcea

Diode laser sensor for measurements of CO, CO 2 , and CH 4 in combustion flows

A diode laser sensor has been applied to monitor CO, CO(2), and CH(4) in combustion gases with absorption spectroscopy and fast extraction-sampling techniques. Survey spectra of the CO 3nu band (R branch) and the 2nu(1) + 2nu(2)(0) + nu(3) CO(2) band (R branch) near 6350 cm(-1) and H(2)O lines from the nu(1) + 2nu(2) and 2nu(2) + nu(3) bands in the spectral region from 6345 to 6660 cm(-1) were recorded and compared with calculated spectra (from the HITRAN 96 database) to select optimum transitions for species detection. Species concentrations above a laminar, premixed, methane-air flame were determined from measured absorption in a fast-flow multipass absorption cell containing probe-sampled combustion gases; good agreement was found with calculated chemical equilibrium values.

A diode-laser absorption sensor system for combustion emission measurements

A diode-laser sensor system has been developed to measure the concentrations of NO, O, CO, and in combustion gases using absorption spectroscopy and fast-extraction sampling techniques. Measured survey spectra of the NO 3 band (the R branch) and O lines from the band in the spectral region from 5556 to 5572 were recorded and compared with calculated spectra to select optimum transitions for detection of NO. Similarly, measured survey spectra of the O 3 band from 6535 to 6600 were used to identify optimum transitions for detection of O. High-resolution NO absorption measurements ((7.5) and (7.5) lines) were recorded in a fast-flow multipass cell containing probe-sampled combustion gases to determine NO concentrations in a laminar, premixed -air flame, seeded with . For fuel-lean conditions, the measured NO mole fractions corresponded to 68% of the injected . For fuel-rich conditions, the fraction of converted to NO decreased with increasing equivalence ratio. In additional experiments, CO, and absorption measurements (the R(13) line of the 3 band, the R(16) line of the band and the RQ(7, 8) line of the b band, respectively) were used to determine species concentrations above a laminar, premixed -air flame. Good agreement was found between measured CO, and concentrations and calculated chemical equilibrium values.

Diode-laser absorption measurements of CO 2 near 2.0 μm at elevated temperatures

A diode-laser sensor system based on absorption spectroscopy techniques has been developed for nonintrusive measurements of CO(2) in high-temperature environments. Survey spectra of the CO(2) (20 degrees 1,04 degrees 1)(I)-00 degrees 0 and (20 degrees 1,04 degrees 1)(II)-00 degrees 0 bands between 1.966 and 2.035 mum (4915-5085 cm(-1)) were recorded at temperatures between 296 and 1425 K in a heated static cell and compared with calculated spectra (by using the HITRAN 96/HITEMP database) to find candidate transitions for CO(2) detection. High-resolution measurements of the CO(2) R(56) line shape [(20 degrees 1,04 degrees 1)(II)-00 degrees 0 band] were used to determine the transition line strength, the self-broadening half-width, and the coefficient of temperature dependence of the self-broadening half-width. The results represent what are believed to be the first measurements of CO(2) absorption near 2.0 mum with room-temperature diode lasers. Potential applications of the diode-laser sensor system include in situ combustion measurements and environmental monitoring.

Advanced diode laser absorption sensor for in situ combustion measurements of CO2, H2O, and gas temperature

A diode-laser sensor system based on absorption spectroscopy techniques has been developed to measure CO2, H2O, and temperature nonintrusively in high-temperature combustion environments. An external-cavity diode laser operating near 2.0 μm was used to scan over selected CO2 [(12°1)–(00°0) band] and H2O transitions [(011)–(000), (021)–(010) bands] near 1.996 and 1.992 μm for measurements of CO2 and H2O concentration and gas temperature. Gas temperature was determined from the ratio of integrated line intensities. Species concentration was determined from the integrated line intensity and the measured temperature. The system was applied to measure temperature and species concentrations in the combustion region of a premixed C2H4-air flat-flame burner operating at fuel-lean conditions. The laser-based temperature measurements were in agreement with values determined using a (type S) thermocouple to within 3%. In addition, the measured CO2 and H2O concentrations agreed to within 6% and 3%, respectively, with calculated equilibrium values at measured temperatures. The minimum CO2 detectivity was 200 ppm (for =0.51, 1470 K, a l-m path length, 200-Hz detection bandwidth). These results represent the first in situ combustion measurements of CO2 concentration using room-temperature near-IR diode lasers. Furthermore, the results demonstrate the utility of diode-laser absorption sensors, operating near 2.0 μm, as attractive diagnostic tools for in situ combustion measurements of temperature and the concentrations of CO2 and H2O.

Tunable diode-laser absorption measurements of NO 2 near 670 and 395 nm

Two single-mode diode lasers were used to record high-resolution absorption spectra of NO(2) (dilute in Ar) near 670.2 and 394.5 nm over a range of temperatures (296 to 774 K) and total pressures (2.4 × 10(-2) to 1 atm). A commercial InGaAsP laser was tuned 1.3 cm(-1) at a repetition rate of 1 kHz to record the absorption spectra near 670.2 nm. In separate experiments with a prototype system, an external-cavity GaAlAs laser was frequency doubled with a quasi-phase-matched LiNbO(3) waveguide and tuned 3.5 cm(-1) to record absorption spectra near 394.5 nm. Variations of the spectral absorption coefficients with temperature and pressure were determined from measured spectra.

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.

Diode laser absorption sensor system for measurements of combustion pollutants

A diode-laser sensor system has been applied to measure the concentrations of NO, N2O, CO, and CO2 in combustion gases using absorption spectroscopy and fast extraction- sampling techniques. Measured survey spectra of the NO 3v band and H2O lines from the v2 + v3 band in the spectral region from 5556 cm-1 to 5572 cm-1 were recorded and compared to calculated spectra to select optimum transitions for NO detection. Similarly, measured survey spectra of the N2O 3v3 band from 6535 cm-1 to 6600 cm-1 were used to identify optimum transitions for N2O detection. High- resolution NO absorption measurements were recorded in a fast-flow multipass cell containing probe-sampled combustion gases to determine NO concentrations in a laminar, premixed CH4/air flame, seeded with NH3. For fuel-lean conditions, the measured No mole fractions corresponded to 68 percent of the injected NH3. For fuel-rich conditions, the fraction of NH3 converted to NO decreased with increasing equivalence ratio. In additional experiments, CO and CO2 absorption measurements were used to determine species concentrations above a laminar, premixed CH4/air flame. Good agrement was found between measured CO and CO2 concentrations and calculated chemical equilibrium values.