Daniel W. Mattison

Wavelength-agile diode-laser sensing strategies for monitoring gas properties in optically harsh flows: application in cesium-seeded pulse detonation engine

The rapid, broad wavelength scanning capabilities of advanced diode lasers allow extension of traditional diode-laser absorption techniques to high pressure, transient, and generally hostile environments. Here, we demonstrate this extension by applying a vertical cavity surface-emitting laser (VCSEL) to monitor gas temperature and pressure in a pulse detonation engine (PDE). Using aggressive injection current modulation, the VCSEL is scanned through a 10 cm-1 spectral window at megahertz rates - roughly 10 times the scanning range and 1000 times the scanning rate of a conventional diode laser. The VCSEL probes absorption lineshapes of the ~ 852 nm D2 transition of atomic Cs, seeded at ~ 5 ppm into the feedstock gases of a PDE. Using these lineshapes, detonated-gas temperature and pressure histories, spanning 2000 - 4000 K and 0.5 - 30 atm, respectively, are recorded with microsecond time response. The increasing availability of wavelength-agile diode lasers should support the development of similar sensors for other harsh flows, using other absorbers such as native H2O.

Pulse Detonation Engine Characterization and Control Using Tunable Diode-Laser Sensors

One of the phenomena limiting the performance of pulse detonation engines (PDEs) is detonation failure due to pulse-to-pulse interference. To better understand and control such interferences, two novel laser diagnostic techniques, based on absorption spectroscopy, have been developed and then used to demonstrate effective realtime control. The e rst technique utilizes a tunable diode-laser (TDL) sensor to measure H 2O temperature and concentration in the tube tail end at the Naval Postgraduate School’ s (NPS) PDE facility and in the tube head end at Stanford University’ s (SU) PDE facility. This sensor, capable of measuring temperatures from 300 to 1300 K at 3.33 kHz, reveals the temporal history of temperature for multipulse engines. In its application to the NPS facility, the sensor shows a distinct change in temperature proe le when the engine pulserate is changed from 5 Hz, where successful detonations are achieved, to 7 Hz, where interference produces undesirable e ame holding and subsequent dee agrations on some pulses. We observed that the geometry evaluated possessed excess recirculation at the higher pulse rates resulting in e ame holding at or near the point of injection. In its application to the SU PDE, this sensor reveals a temperature proe le characteristic of detonation failure that could be used in future control schemes. The second diagnostic technique developed is used to monitor fuel and is employed in an active, real-time control scheme. For this sensor, we monitor the C 2H4 (ethylene) concentration at the tail end of the NPS PDE initiator tube, which is operating at 20 Hz. When fuel is detected at the tail end, the sensor sends a signal to e re the ignitor. Compared to e xed-timing ignitor actuation, this control promotes more consistent detonation initiation and reduces mise re events. These two new laser diagnostic techniques provide useful tools for studying pulse-to-pulse interference and lay the groundwork for future, more advanced TDL-based PDE control strategies.

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.

Sensors for high-pressure, harsh combustion environments using wavelength-agile diode lasers

Practical combustors often produce a hostile environment for optical sensors, owing to elevated pressures, multiple phases, and unsteady behavior. Fortunately, new lasers and wavelength-tuning strategies have produced a class of wavelength-agile diode-laser sensors appropriate for such environments. Here, we demonstrate the extended capabilities of wavelength-agile sensors by applying a vertical cavity surface-emitting laser (VCSEL) to monitor gas temperature and pressure in a pulse detonation engine (PDE). Using aggressive injection current modulation, the VCSEL is scanned through a 10cm −1 spectral window at megahertz rates—roughly 10 times the scanning range and 1000 times the scanning rate of a conventional diode laser. The VCSEL probes absorption line shapes of the ∼852 nm D 2 transition of atomic Cs, seeded at ∼5 ppm into the feedstock gases of a PDE. Using these line shapes, detonated-gas temperature and pressure histories, spanning 2000–4000 K and 0.5–30 atm, respectively, are recorded with microsecond time response. To facilitate similar measurements using traditional spectroscopic targets such as H 2 O near 1.4 μm, where wavelength-agile lasers are not yet commercially available, we demonstrate a novel wavelength-tuning strategy. A standard distributed-feedback diode laser is thermally cycled from −10 to +50°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 10-fold increase in the wavelength-scanning range offered by standard current-tuning techniques. Measurements of H 2 O in a static cell at 10 atm provide the groundwork for future measurements in aeropropulsion and piston engines.

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.

PROPANE FUEL MONITORING IN PULSE DETONATION ENGINES USING A DIODE-LASER SENSOR

Characterization of fuel charging in pulse detonation engines (PDEs) is of fundamental importance to improve engine performance, to realize active engine control, and to enable comparison between experimental data and simulations. Here we report the development of a diode-laser-based C3H8 (propane) sensor and its applications in PDE facilities at Stanford University (SU) and at the Naval Postgraduate School (NPS). In its application to the SU PDE, this sensor measures the C3H8 concentration history quantitatively and facilitates the optimal valve and ignition timing. The quantitatively measured fuel concentration also enhances the value of our database for C3H8-detonation for validating simulations. In its application to the NPS PDE, this sensor monitors quantitative C3H8 concentration, demonstrating the sensor’s utility in high-repetition-rate engines. The success achieved in these applications shows the promise of diode-laser sensors as a tool for both fundamental PDE research and practical engine development.

PULSE DETONATION TUBE CHARACTERIZATION USING LASER ABSORPTION SPECTROSCOPY

The time-resolved OH concentration and gas temperature are measured in the Stanford University pulse detonation tube facility using CW, UV laser absorption. OH is monitored by direct absorption of the R21(5) and S21(1) transitions in the 0, 0 band of the A-X system. Gas temperature is determined from UV CO2 absorption. Results from these diagnostics are useful in verifying computational simulations and in advancing PDE design and development.

UV Optical Diagnostics for PDE Applications

The time-resolved OH concentration and gas temperature are measured in the Stanford University pulse detonation tube facility using UV absorption spectroscopy. OH is monitored by direct absorption of the R21(5) and S21(1) transitions in the 0, 0 band of the A-X system near 306.5 nm using a CW ring dye laser source. A new technique based on a kinetic spectrograph is used to measure burned gas temperature from broadband UV CO2 absorption. Results from these diagnostics are useful in verifying computational simulations and in advancing PDE design and development.

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.