Kevin Michael Hinckley

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.

A WAVELENGTH-MULTIPLEXED DIODE LASER SENSOR FOR TEMPERATURE MEASUREMENTS IN PULSE DETONATION ENGINES

A wavelength-multiplexed diode laser sensor to measure temperature during the blowdown process in pulse detonation engines (PDEs) has been developed and demonstrated. The sensor utilizes direct-absorption spectroscopy, and gas temperature is determined from th e ratio of peak absorbance of two spectral lines. Three specific transitions near 1400 nm were selected to maximize temperature sensitivity in the range 300 to 1800 K, while maintaining adequate absorption levels. The sensor employs three fiber-coupled distributed feedback (DFB) diode lasers which are injection-current tuned 2-3 cm -1 over the selected transitions at 5 kHz, yielding temperature data with a temporal resolution of 200 µs. The sensor is compact, rugged and portable and has been applied to PDE test facilities at Stanford University and the Naval Postgraduate School in Monterey, CA. The sensor shows promise as a tool to aid in the evaluation of PDE performance, and has utility for high-temperature sensing in other harsh combustion environments.

3.39 µm Laser Absorption Sensor for Ethylene and Propane Measurements in a Pulse Detonation Engine

A fiber-coupled mid-infrared (mid-IR) laser absorption sensor was designed to monitor ethylene and propane concentration in a multiple-cycle pulse detonation engine (PDE). The engine was operated at repetition rates ranging from 5 to 20 Hz with near-stoichiometric fuel/air mixtures. A 3.39 µm helium neon laser was fiber-coupled to the engine, enabling continuous, time-resolved measurements of fuel concentration. Design criteria are discussed to overcome sensor noise from optical emission, spark ignition, and the harsh mechanical vibrations of the fired engine. The sensor provides quantitative measurement of the stoichiometry to compare to PDE simulations and allows optimization of fuel valve timing and fill pressure for improved engine performance. Pulse-to-pulse interactions during fired engine tests can perturb the fuel loading compared to unfired tests, illustrating the need for in situ fuel monitoring for PDE development and testing.

A Dispersive Ultra-Violet Absorption Sensor for Nitric Oxide Detection

There is a growing need for gas sensors that can monitor the concentration of nitric oxide (NO) in harsh reacting environments. In this study, we explore the entitlement of a simple fiber-optic coupled ultra-violet absorption sensor for determining the concentration of nitric oxide in a gas cell and downstream of an atmospheric flame. By monitoring the v=0 0 vibrational transition in the A 2+ X 2 electronic manifold of NO, the gas cell measurements show that the absorption process is insensitive to other common combustion reactants and products at room temperature. However, in an atmospheric flame, the smaller NO number density instigates a significant loss in the signal-to-noise ratio of the NO absorption spectra. Nevertheless, a correlation between NO concentration and optical absorbance was observed.

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.

An Experimental and Computational Study of Jet-A Fueled Pulse Detonation Engine Operation

An experimental and computational study regarding the operation of a Jet-A-fueled pulse detonation engine has been performed. In the experiments, liquid Jet-A fuel is injected into the head end of the engine in the form of atomized droplets. Preheated air serves to entrain and evaporate the fuel droplets, which are then spark ignited after traversing a finite length of pre-vaporizer section. The operation of the engine was investigated over a range of initial temperatures and pressures characteristic of high Mach-number operation or conventional gas turbine compressor discharge conditions. Pressure transducers and ionization probes were utilized to measure the absolute pressure rise and propagation speed of the combustion wave in order to verify detonation. Consistent and repeatable detonations have been observed in Jet-A / air mixtures at initial temperatures and pressures ranging from 225 - 312 F and 1.0 2.4 atm at frequencies up to 25 Hz. In addition, computational simulations of detonation propagation in an ideal-tube PDE fueled with Jet-A/air mixtures are presented.