Shawn Wehe

A diode laser sensor for rapid, sensitive measurements of gas temperature and water vapour concentration at high temperatures and pressures

A near-infrared diode laser sensor is presented that is capable of measuring time-varying gas temperature and water vapour concentration at temperatures up to 1050 K and pressures up to 25 atm with a bandwidth of 7.5 kHz. Measurements with noise-equivalent-absorbances of the order of 10−3 (10−5 Hz−1/2) are made possible in dynamic environments through the use of wavelength modulation spectroscopy (WMS) with second harmonic detection (2f) on two water vapour spectral features near 7203.9 and 7435.6 cm−1. Laser performance characteristics that become important at the large modulation depths needed at high pressures are accounted for in the WMS-2f signal analysis, and the utility of normalization by the 1f signal to correct for variations in laser intensity, transmission and detector gain is presented. Laboratory measurements with the sensor system in a static cell with known temperature and pressure agree to 3% RMS in temperature and 4% RMS in H2O mole fraction for 500 < T < 900 K and 1 < P < 25 atm. The sensor time response is demonstrated in a high-pressure shock tube where shock wave transients are successfully captured, the average measured post-shock temperature agrees within 1% of the expected value, and H2O mole fraction agrees within 8%.

Thermoelectrically cooled quantum-cascade-laser-based sensor for the continuous monitoring of ambient atmospheric carbon monoxide

We report the first application of a thermoelectrically cooled, distributed-feedback quantum-cascade laser for continuous spectroscopic monitoring of CO in ambient air at a wavelength of 4.6 microm. A noise-equivalent detection limit of 12 parts per billion was demonstrated experimentally with a 102-cm optical pathlength and a 2.5-min data acquisition time at a 10-kHz pulsed-laser repetition rate. This sensitivity corresponds to a standard error in fractional absorbance of 3 x 10(-5).

Measurements of Gas Temperature and Velocity in Hypervelocity Flows Using Diode-Laser Sensors

Rapid measurements of temperature, velocity, and H2O partial pressures have been obtained in hypervelocity air flows generated in the Calspan 96-inch Hypersonic Shock Tunnel (Buffalo, New York) using diode-laser sensor systems developed at Stanford University. The measurements were determined from high-resolution water (H2O) and potassium (K) absorption lineshapes recorded every 100 (is by tuning the wavelengths of two DFB (InGaAsP) diode lasers and a Fabry-Perot (AlGaAs) diode laser independently at 10-kHz rates across H2O transitions (v,+v3 band) near 1392 nm and 1393 nm and potassium D, transitions (SI/2-»PI/2) near 770 nm. Two hardened probes incorporated the electro-optics required to pitch and detect the laser energy and were installed near the nozzle exit to minimize complications due to facility boundary layers. The probe targeting potassium, is a newly developed compact probe, measuring only 5 cm in overall width. Gas temperature was inferred from the Doppler width of H2O lineshapes. H2O partial pressure was determined from the measured absorbance of a single transition and the gas temperature. Gas velocities were determined from Doppler-shifted H2O and K absorption lineshapes. The measured gas temperatures and velocities were consistent with calculated steady-state values (500 K and 4.2 km/s, respectively) based on standard tunnel measurements. The results obtained demonstrate the applicability of diode-laser absorption diagnostics for direct multi-parameter measurements in the hypervelocity flowfields of high-enthalpy facilities.

Tunable diode-laser absorption measurements of temperature, velocity, and H2O in hypervelocity flows

Tunable diode-laser absorption measurements of temperature, velocity, and H2O partial pressures have been recorded in hypervelocity air flows using a dual-wavelength tunable diode-laser system developed at Stanford and installed at the Calspan University of Buffalo Research Centers (CUBRC) Large Energy National Shock Tunnel (LENS Tunnel) in Buffalo, New York. The measurements were recorded using a hardened probe, which contained critical optical components and photodetectors, that was installed directly into the flowfield near the nozzle exit to minimize complications due to boundary layers and facility vibration. The wavelengths of the distributed feedback diode lasers were independently current-tuned at an 8-kHz rate across H2O transitions (v,+v3 band) near 1400 nm and 1396 nm to yield high-resolution absorption lineshape measurements every 125 |0,s. Values of rotational temperature, determined from the ratio of measured absorbances, were in excellent agreement with translational temperatures, determined from the measured (Doppler-broadened) lineshapes, over the measured range (400-900 K). H2O partial pressure was determined from the measured absorbance of one transition and the gas temperature. Gas velocity was determined from Doppler-shifted absorption recorded by directing the 1400-nm beam through the flowfield at a 54° angle relative to the bulk gas velocity. The measured gas temperatures and velocities were consistent with calculated steady-state values (650 K and 4500 m/s, respectively). The results obtained demonstrate the applicability of diode-laser absorption diagnostics for direct multi-parameter gas measurements in hypervelocity flowfields for improved characterization of high-enthalpy facilities.

PLIF imaging and thermometry of NO/N2 shock layer flows in an expansion tube

Planar laser-induced fluorescence (PLIF) imaging and thermometry of quasi-steady hypersonic flows over a wedge and circular cylinder in an expansion tube facility are reported. The facility performance as inferred from the PLIF measurements are compared with that determined from shock speeds, static pressure, pitot pressure and infrared absorption measurements. The collisional quenching corrections required in these high-temperature shock layer flows are explored and direct comparisons are made between theory and experiment for the quenching of nitric oxide fluorescence.

Room-temperature mid-IR lasers for on-line measurements of trace combustion generated pollutants

Measurements of NO and CO in combustion exhaust gases are reported using thermo-electrically cooled, mid-IR quantum cascade lasers operating near 5.26 and 4.62 /spl mu/m respectively. Measurements of NO are performed in an insulated exhaust duct of a C/sub 2/H/sub 4/-airflame at temperatures of approximately 600 K. CO measurements are performed above a rich H/sub 2/-air flame seeded with CO/sub 2/ and diluted with N/sub 2/ to 1150 K. Sensitivities on the order of 0.36 ppm-m are achieved in the case of NO and 0.21 ppm-m in the case of CO using a balanced ratiometric detection technique. Comparisons between measured and predicted water-vapor and CO/sub 2/ interference is discussed.

Tunable diode laser sensor for multiple species monitoring in harsh atmospheres

Energy intensive industries such as steel, aluminum, and glass require combustion processes that are characteristically at high temperature with high levels of particulate matter. Monitoring and control of these processes for improved efficiency, pollutant reduction, and product quality requires a sensor adaptable for such harsh environments. Traditional industrial monitoring relies on extractive sampling that requires frequent maintenance due to probe plugging or corrosion and routine calibration. In addition, capturing the temporal behavior of the process can be problematic with extractive sampling systems because of the slow response time associated with the sampling line lengths and slow response analyzers. To meet the demands of these harsh combustion processes the ideal sensor would perform in-situ process monitoring, require little or no maintenance and provide real-time process information The use of tunable diode lasers based on absorption monitoring overcomes many of the problems associated with conventional extractive sampling. However, the majority of industrial combustion processes will undergo temperature variations along with changes in the atmosphere oxidation or reducing state during normal operation. Therefore, temperature monitoring along with key combustion species monitoring that describes the atmosphere e.g., O2 and CO, is often necessary for optimal process control. The temperature is not only a useful parameter describing the state of the process, but is needed to accurately determine the species concentration since the absorption measured is dependent on temperature. To monitor both reducing and oxidizing combustion atmospheres in addition to gas temperature requires a diode laser system capable of multiple species monitoring. Here we describe an industrial prototype system operating in the near-infrared for simultaneous monitoring of O2 (.76 μm), CO (1.5 μm), H2O (1.5 μm) and gas temperature. The prototype system addresses the issues of added complexity with multiple species monitoring by using only two diode lasers and a beam launch and receiver optical design to discriminate the vastly different laser wavelengths while suppressing background radiation noise and beam steering from thermal gradients. Measurement results using the system for industrial process monitoring on a 100-ton/hr steel reheat furnace are presented. The measurements in this test were conducted at different zones in the furnace and at different heights relative to the processed material. The results show dynamic variations in concentration and temperature that could aid in improved atmosphere control.

Quantum-Cascade Laser-Based Sensor for CO and NO Measurements in Combustor Exhaust Flows

Measurements of CO and NO are reported using room-temperature, mid-IR quantum cascade lasers operating at 4.6 μm and 5.26 μm, respectively. Using a balanced ratiometric detection technique, sensitivities on the order of 10 ppbv are achieved for each species. Extensions to in-situ, high-temperature measurements for combustion control applications are described. Introduction and Motivation Advances in gas turbine combustor technology for aeroengine applications are driving combustor designs toward high pressure, ultra-lean operating points. Highpressure operation, up to 50 atm, is desired for improved thrust-to-weight ratio (i.e., a higher energy density heat release) and ultra-lean operation is desired to ameliorate the increase in NOx and particulate emissions associated with higher-pressure operation. The emissions reduction is also being driven by increasing regulatory pressure to limit NOx and particulate emissions during all flight phases, not just cruise conditions. Ultra-lean conditions can move the gas turbine toward stability margin boundaries, resulting in deleterious or potentially destructive combustion instabilities or flame blow-off. Combustor re-light at altitude is also more difficult at ultra-lean conditions, thereby substantially increasing the penalty associated __________ *Principal Scientist, Member AIAA Principal Research Scientist Principal Research Scientist, Senior Member AIAA Copyright

A novel variable power diode laser sensor for harsh process monitoring

Industrial applications using tunable diode laser technology for process gas monitoring are often faced with technical challenges because of dynamic operating conditions in the presence of high particle densities and high temperature. Furthermore, issues related to alignment stability and maintenance requirements must be overcome for industry acceptance of the sensing technology. To address these technical challenges a novel near infrared tunable diode laser system for monitoring CO, H2O and gas temperature is presented. The system incorporates balanced ratiometric detection and a variable laser power delivery scheme allowing the launch laser power to vary between 2-248 mW while maintaining a constant reference power. Feedback control is used to adjust the level of laser power delivered to the process based on the light transmission through the measurement zone. Results are presented using the system on a 500 kW oxy-fuel pilot furnace with controlled particle injection to simulate industrial conditions in preparation for field test campaign measuring the off-gas of an electric arc (EAF) steel-melting furnace. For the industrial test, monitoring on the EAF process can be considered one of the harshest environments to perform a measurement with particle densities rising above 100 g/Nm3 and temperatures up to 1800°C. In addition, special requirements are needed to integrate the sensor into the process because of the high level of mechanical vibration, high and varying ambient temperatures, EMF interference sources, and protection against flying debris.

Tunable Diode Laser Sensors for Combustion

High sensitivity monitoring of combustion species using compact semiconductor lasers began shortly after their invention in the mid 1970s, and included portable sensors for monitoring CO emissions from automobile exhausts and in-situ measurements in laboratory burners.1,2 Today, packages of multiple near-IR diode laser sensors are included by NASA as part of the atmospheric and environmental instrumentation suite of robotic Martian explorers. Significant attributes of Tunable Diode Laser (TDL) sensors based on absorption spectroscopy include: simplicity of design and operation, leading to fully autonomous sensors; high-speed wavelength tuning, leading to high bandwidth sensor response; and low-cost, rugged, and often fiber-coupled configurations, leading in turn to important applications in practical, industrial-scale combustor facilities. The combination of these attributes is direct monitoring of important combustion parameters such as temperature, velocity, mass flux, and individual species concentration levels.3–5