Christopher Brophy

Transient plasma ignition of quiescent and flowing air/fuel mixtures

Transient plasmas that exist during the formative phase of a pulse-ignited atmospheric pressure discharge were studied for application to ignition of quiescent and flowing fuel-air mixtures. Quiescent methane-air mixture ignition was studied as a function of equivalence ratio, and flowing ethane-air mixture was studied in a pulse detonation engine (PDE). The transient plasma was primarily comprised of streamers, which exist during approximately 50 ns prior to the formation of an equilibrated electron energy distribution. Results of significant reduction in delay to ignition and ignition pressure rise time were obtained with energy costs roughly comparable to traditional spark ignition methods (100-800 mJ). Reduction in delay to ignition by factors of typically 3 in quiescent mixes to >4 in a flowing PDE (0.35 kg/s), and other enhancements in performance were obtained. These results, along with a discussion of a pseudospark-based pulse generator that was developed for these applications, will be presented.

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.

Fuel Distribution Effects on Pulse Detonation Engine Operation and Performance

This research was supported by the Office of Naval Research with Gabriel Roy as program manager. The authors would also like to thank Jose Sinibaldi at the Naval Postgraduate School and Ma Lin from Stanford for their contributions to this effort.

Compact Pulsed-Power System for Transient Plasma Ignition

The use of a compact solid-state pulse generator and compact igniters for transient plasma ignition in a pulse detonation engine (PDE) is reported and compared with previous results using a pseudospark pulse generator and threaded rod electrode. Transient plasma is attractive as a technology for the ignition of PDEs and other engine applications because it results in reductions in ignition delay and has been shown to ignite leaner mixtures which allows for lower specific fuel consumption, high-repetition rates, high-altitude operation, and reduced NOx emissions. It has been applied effectively to the ignition of PDEs as well as internal combustion engines. Nonequilibrium transient plasma discharges are produced by applying high-voltage nanosecond pulses that generate streamers, which generate radicals and other electronically excited species over a volume. The pulse generator used is in this experiment is capable of delivering 180 mJ into a 200-¿ load, in the form of a 60-kV 12-ns pulse. Combined with transient plasma igniters comparable with traditional spark plugs, the system was successfully tested in a PDE, resulting in similar ignition delays to those previously reported while using a smaller electrode geometry and delivering an order of magnitude less energy.

Transient Plasma Ignition of Hydrocarbon-Air Mixtures in Pulse Detonation Engines

A transient plasma ignition system has been demonstrated to substantially reduce the ignition delay and detonation-to-detonation transition times for ethylene-air and propane-air mixtures under dynamic fill conditions. The effects initial conditions including equivalence ratio, a temperature range of 280K to 430K, and pressure range of 1 to 6 atm were evaluated. Ignition delays were reduced by up to a factor of 5 and the corresponding deflagration-to-detonation time scales were observed to decrease accordingly when compared to conventional capacitive discharge systems. The substantial reduction of the ignition delay times resulted in the generation of strong pressure waves which inherently steepened into shock waves quickly and in a short distance. Although direct initiation of a detonation wave was not obtained, the sub sequential use of a Shchelkin spiral was able to rapidly and reliably accelerate the combustion driven shock waves to detonations within practical distances. The efficiency and performance of the transient plasma ignition strategy will likely contribute to the development of fuel-air detonation initiators.

High-bandwidth scanned-wavelength-modulation spectroscopy sensors for temperature and H2O in a rotating detonation engine

The design and use of two-color tunable diode laser (TDL) absorption sensors for measurements of temperature and H2O in a rotating detonation engine (RDE) are presented. Both sensors used first-harmonic-normalized scanned-wavelength-modulation spectroscopy with second-harmonic detection (scanned-WMS-2f/1f) to account for non-absorbing transmission losses and emission encountered in the harsh combustion environment. One sensor used two near-infrared (NIR) TDLs near 1391.7 nm and 1469.3 nm that were modulated at 225 kHz and 285 kHz, respectively, and sinusoidally scanned across the peak of their respective H2O absorption transitions to provide a measurement rate of 50 kHz and a detection limit in the RDE of 0.2% H2O by mole. The other sensor used two mid-infrared (MIR) TDLs near 2551 nm and 2482 nm that were modulated at 90 kHz and 112 kHz, respectively, and sinusoidally scanned across the peak of their respective H2O transitions to provide a measurement rate of 10 kHz and a detection limit in the RDE of 0.02% H2O by mole. Four H2O absorption transitions with different lower-state energies were used to assess the homogeneity of temperature in the measurement plane. Experimentally derived spectroscopic parameters that enable temperature and H2O sensing to within 1.5–3.5% of known values are reported. The sensor design enabling the high-bandwidth scanned-WMS-2f/1f measurements is presented. The two sensors were deployed across two orthogonal and coplanar lines-of-sight (LOS) located in the throat of a converging-diverging nozzle at the RDE combustor exit. Measurements in the non-premixed H2-fueled RDE indicate that the temperature and H2O oscillate at the detonation frequency (≈3.25 kHz) and that production of H2O is a weak function of global equivalence ratio.

Mid-IR Laser Absorption Sensor for Ethylene in a Pulse Detonation Engine

A flber-coupled ethylene sensor based on optical absorption of the 3.39 „m transition of a Helium-Neon laser has been designed and demonstrated. The sensor exploits the fundamental vibrational mode of the C{H stretch of ethylene which has absorption features between 3.2 and 3.4 „m. Cell measurements were performed to determine the pressure, temperature and composition dependences of the absorption coe‐cient at this wavelength. The sensor was demonstrated in an air-breathing pulse detonation engine where it was used to measure fuel concentration versus time 5 cm downstream of the initiator. Timeresolved (100 „sec resolution) fuel measurements enabled optimization of valve timing and evaluation of engine performance at difierent stoichiometries.

Real-Time Measurements of C2H4 Concentration with Application to PDEs Operating on Oxygen and Air

An existing C2H4 sensor is enhanced by extending its range to low concentrations of C2H4 as well as by increasing its temporal resolution. The spectral absorption coefficient of C2H4 near 1626 nm is measured at various temperatures and mole fractions. The dependence of absorption coefficient on mole fraction is described in terms of broadening parameters. This database is used for fast (10 kHz) measurements of fuel concentration in a PDE which burns C2H4 with air. Two techniques of determining mole fraction are compared. Non-uniform distribution of fuel across the PDE diameter is quantified using a known difference in path length. The minimum measurable fuel mole fraction is determined and compared with practical limits on C2H4/air detonation.

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.

The role of water in transient plasma ignition for combustion

Pulse detonation engines offer the potential for a single, air-breathing, propulsion system that can operate from subsonic speeds (including take-off) to high Mach number flight. Critical to this technology is rapid repetition rate operation. Plasma-based ignition systems based on short pulse, high voltage pulsed power have been shown to decrease the ignition time in experimental system. The presence of water in the fuel-air mixture, however, has been shown to adversely affect the performance of these ignition systems. Herein, we report on simulations of the plasma chemistry from the application of the pulsed power (10s of ns) through the ignition time (100s of microsecond). These simulations suggest a long-lived neutral species created by the transient plasma ignition process that are strongly affected by water, and can play a role in both combustion processes. Comparison with experimental results, potential mitigation schemes for the water effects, and implications for internal combustion engines will be discussed.