Scott T. Sanders

Diode-laser absorption sensor for line-of-sight gas temperature distributions

Line-of-sight diode-laser absorption techniques have been extended to enable temperature measurements in nonuniform-property flows. The sensing strategy for such flows exploits the broad wavelength-scanning abilities (>1.7 nm approximately 30 cm(-1)) of a vertical cavity surface-emitting laser (VCSEL) to interrogate multiple absorption transitions along a single line of sight. To demonstrate the strategy, a VCSEL-based sensor for oxygen gas temperature distributions was developed. A VCSEL beam was directed through paths containing atmospheric-pressure air with known (and relatively simple) temperature distributions in the 200-700 K range. The VCSEL was scanned over ten transitions in the R branch of the oxygen A band near 760 nm and optionally over six transitions in the P branch. Temperature distribution information can be inferred from these scans because the line strength of each probed transition has a unique temperature dependence; the measurement accuracy and resolution depend on the details of this temperature dependence and on the total number of lines scanned. The performance of the sensing strategy can be optimized and predicted theoretically. Because the sensor exhibits a fast time response (~30 ms) and can be adapted to probe a variety of species over a range of temperatures and pressures, it shows promise for industrial application.

Diode-laser sensor for monitoring multiple combustion parameters in pulse detonation engines

Diode-laser absorption spectroscopy techniques have been adapted and, applied for in situ measurements of pertinent combustion parameters in pulse detonation engines (PDEs). A sensor employing five multiplexed diode lasers operating in the 1300–1800 nm spectral region has been developed for monitoring gas temperature, H2O concentration, liquid fuel concentration, and soot volume fraction. Gas temperature is determined from the ratio of H2O absorbances at different wavelengths: water mole fraction and fuel and soot volume fractions are determined from the measured gas temperature and absorbances at selected wavelengths. The sensors time response (0.5 μs) and non-intrusive, nature make it suitable for measurements in the hostile environment generated by PDEs. The sensor was used to monitor a 4 cm diameter PDE operating on a JP-10/oxygen aerosol. Measurements revealed charges of non-uniform equivalence ratio at ignition. Detonations processing such charges reached 95% of the Chapman-Jouget velocity and gas pressures predicted for a stoichiometric, uniform load. Gas temperature and H2O concentration, however, reached only ≈50% of the Chapman-Jouget predictions, as a result of the decreasing fuel concentration along the length of the engine. The sensor also revealed the presence of hot H2O for a long duration (>100 ms) relative to the duration of the pressure pulse (≈500 μs) in the blowdown following the detonation. The engine performance information recorded by the sensor is expected to enhance PDE modeling and optimization efforts, potentially enabling PDE combustion control.

In situ combustion measurements of CO 2 by use of a distributed-feedback diode-laser sensor near 2.0 mm

High-resolution absorption measurements of CO(2) were made in a heated static cell and in the combustion region above a flat-flame burner for the development of an in situ CO(2) combustion diagnostic based on a distributed-feedback diode laser operating near 2.0 mum. Calculated absorption spectra of high-temperature H(2)O and CO(2) were used to find candidate transitions for CO(2) detection, and the R(50) transition at 1.997 mum (the nu(1) + 2nu(2) + nu(3) band) was selected on the basis of its line strength and its isolation from interfering high-temperature water absorption. Measurements of spectroscopic parameters such as the line strength, the self-broadening coefficient, and the line position were made for the R(50) transition, and an improved value for the line strength is reported. The combustion-product populations of CO(2) in the combustion region above a flat-flame burner were determined in situ to verify the measured spectroscopic parameters and to demonstrate the feasibility of the diode-laser sensor.

Monitoring and control of a pulse detonation engine using a diode-laser fuel concentration and temperature sensor

Fuel measurements are needed to accurately tailor fued charges in pulse detonation engines (PDEs) to improve engine performance and to validate PDE models and computations. Here, we report simultaneous concentration and temperature measurements of C 2 H 4 fuel in a PDE using a newly developed diode-laser absorption sensor. These measurements enable characterization of the fuel loading and ignition timing of the engine. Based on these characterizations, a real-time control system that optimizes fuel consumption and maximizes specific impulse in the engine has been realized. Similar measurements of C 2 H 4 concentration and temperature were used to characterize pulse-to-pulse interference resulting from loading fresh fuel/oxygen reactants into hot combustion products. The sensor was used in a simple control scheme to minimize such interference, illustrating its potential role in control systems to maximize the engines operation rate. During these studies, the sensor demonstrated two valuable improvements over traditional absorption spectroscopic techniques: (1) increased robustness and accuracy and (2) simultaneous measurements of concentration and temperature. These improvements are enbled by broad wavelength scanning of the Q-branch spectra of C 2 H 4 near 1.62 μm. The success achieved in these small-scale tests provides strong support for expanded use of diode-laser sensors in propulsion applications.

Rapid temperature tuning of a 1.4-μm diode laser with application to high-pressure H 2 O absorption spectroscopy

Enhanced wavelength tuning of a distributed-feedback InGaAsP diode laser is demonstrated by use of rapid temperature cycling. The laser-active region is cycled from -10 to +50 degrees C (scanning the output from 1399 to 1403 nm) at kilohertz rates by pulsed heating with an auxiliary 532-nm laser. Such 4-nm scans represent a ten-fold increase in the wavelength-scanning range offered by standard current-tuning techniques and thus extend the capabilities of scan-wavelength sensors and systems. As an example application, we demonstrate absorption spectroscopy of H(2)O vapor at a pressure of 10 atm.

Diode Laser Sensor System for Multi-Parameter Measurements in Pulse Detonation Engine Flows

A diode laser sensor system has been applied to measure multiple performance-critical flow properties in Pulse Detonation Engines (PDEs). The sensor system monitors both the unburned and the burned flows in each PDE cycle. The unburnedflow sensors monitor O2 concentration, fuel vapor concentration, and liquid fuel droplet size and volume fraction. The burned-flow sensors monitor H2O concentration, gas temperature, soot temperature, and soot volume fraction. In both liquid- and gas-fueled PDEs, the system has been demonstrated to provide meaningful data that was previously unavailable to PDE researchers. The data provided by the system is expected to enhance both PDE modeling and design efforts. Additionally, by enabling PDE optimization, the system’s potential for use in active PDE combustion control systems has been established.

Multiplexed Diode-Laser Absorption Sensors for Aeropropulsion Flows

Diode-laser sensors based on absorption spectroscopy techniques have been developed for characterizing both reactants and products in aeropropulsion flows. We have demonstrated the sensors in pulse detonation engines (PDEs) operating on liquid-JP-10 and gaseous-C 2H4 fuels in air and oxygen. The measured properties include unburned and burned gas species concentrations, unburned and burned gas temperature, and burned gas velocity. The information provided by the sensors has been used to verify computational detonation models, thus advancing pulse detonation engine development. Because they provide critical flow properties that have been generally unavailable to aeropropulsion researchers, the sensors are expected to enable novel combustion control schemes.

Scanned-wavelength diode laser sensors for harsh environments

Diode laser absorption offers the possibility of high-speed, robust, and rugged sensors for a wide variety of practical applications. Pressure broadening complicates absorption measurements of gas temperature and species concentrations in high-pressure, high-temperature practical environments. More agile wavelength scanning can enable measurements of temperature and species concentrations in flames and engines as demonstrated by example measurements using wavelength scanning of a single DFB in laboratory flames or a vertical cavity surface emitting laser (VCSEL) in a pulse detonation engine environment. Although the blending of multiple transitions by pressure broadening complicates the atmospheric pressure spectrum of C2H4 fuel, a scanned wavelength strategy enables quantitative measurement of fuel/oxidizer stoichiometry. Wavelength-agile scanning techniques enable high-speed measurements in these harsh environments.