Jose Sinibaldi

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.

Fabrication of a microelectromechanical directional sound sensor with electronic readout using comb fingers

By emulating the hearing organ of the Ormia ochracea fly, a microelectromechanical sound sensor was fabricated which is able to determine the direction of incident sound despite an overall size much smaller than the wavelength of interest. The sensor consists of two wings that are coupled in the middle and attached to the surrounding substrate by two legs. The design incorporated interdigitated comb fingers on the wings and the substrate which enables electrostatic (capacitive) readout. Measured electrical response showed a strong dependence on the direction of incident sound.

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.

MEMS directional sound sensor with simultaneous detection of two frequency bands

Conventional directional sound sensing systems use an array of spatially separated microphones to achieve directional sensing by monitoring the arrival times and amplitudes at each microphone. The directional accuracy is then determined by the spatial separation of the microphones. In contrast, the Ormia ochracea flys ears are separated by merely 500 µm yet have remarkable sensitivity to the direction of sound even at wavelengths two orders of magnitude greater than the size of the flys hearing structure. The fly uses a unique system which consists of two identical wings hinged at the center to determine the direction of sound. Recently, our group demonstrated a MEMS-based directional sound sensor modeled on the hearing system of the fly. The sensor response has two resonant modes (rocking and bending) and a strong response was only achieved at the bending frequency since the amplitude of the rocking mode depends on the tiny pressure difference between the two wings. In this paper, we report achievement of dual-band sensing using asymmetric wings to enhance the response at the rocking frequency. The fabricated sensor showed nearly equal responses at the rocking and bending frequencies.

Investigation of Flow Field Properties on Detonation Initiation

The ignition characteristics for ethylene/air mixtures have been explored for varying refresh flow conditions in a simple pulse detonation combustor configuration. A 7.62 diameter combustor with four inlet arms 90 degrees to the combustor access was used to evaluate the effects of flow uniformity, turbulence, temperature, and wall spiral size on the ignition and detonation of ethylene/air mixtures. The effect of creating a more uniform flow field through the use of turbulence-generating screens demonstrated mixed results. The baseline configuration, without a turbulence screen installed, produced large recirculation zones which supported the initiation of high aggregate mass flow rates, but had longer ignition delay times. The 3.175 mm-hole turbulence screen generated the smallest turbulence scales and also produced the shortest ignition delay times, but could not support the reliable ignition of mass flow rates above 0.30 kg/s. The use of two turbulence-producing spirals resulted in detonation initiation for most conditions, but the 6.35 mm spiral produced detonations in shorter distances than the 3.175 mm spiral and was determine to be the preferred spiral. The total pressure loss associated with both spirals was determined and the total pressure loss for the 3.175 mm spiral was found to be approximately 30% of the value for the 6.35 mm spiral. The 6.35 mm diameter spiral had the best detonation initiation performance, but with the associated higher flow loss. A factor of two reduction in ignition delay and deflagration-to-detonation timescales was observed over an initial reactant temperature range of 300K to 480 K. This result directly benefits the development of PDC systems with elevated inlet temperature conditions. The substantial reduction will allow for a substantial increase in operating cycle frequency for these systems and should be applicable for supersonic and hybrid PDC applications.

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.

Design, Modeling and Evaluation of an Initiator Unit for a Split -Path JP -10/Air Pulse Detonation Combustor

The initiation of a de tonation in Pulse Detonation Engines (PDE) has been identified as one of the critical and enabling technologies for PDEs. In particular, the initiation of practical fuel -air mixtures containing liquid droplets without supplementary oxygen or other high lo ss mechanisms is a capability that could enable the PDE to exceed the performance of ramjets and expendable turbo -machinery based systems. Although many past engine concepts have relied upon sensitive fuel/oxygen initiator unit s or unrealistic gaseous fu els such as hydrogen and ethylene , the split -path PD E has been designed to enable the use of JP -10, a high -density liquid fuel. Flow through segments of th e combust or was both modeled with a compressible flow solver and experimentally evaluated in the lab oratory at simulated flight conditions . A s piral -lined initiator demonstrated a significantly lower total pressure loss when compared to a ring -lined version , and thus was the preferred initiator configuration. Experimental values for the turbulence were found to be significantly lower than the computed values at similar conditions when using a k -�model. Finally, successful ignition s of the JP -10/Air initiator were achieved up to refresh Mach numbers of 0. 13 at inlet pressures and temperatures of 150kPa and 550 K, respectively. The increase head -end recirculation/rotational regions with 60 -degr ee versus the 45 -degree side dump angles of the inlet arm s substantially improved the ignition success rate.

MEMS Scanning Diffraction Grating Spectrometer

A new type of grating spectroscopy, using a micromachined scanning grating spectrometer is presented. We achieve resolution of 17-70 nm in the spectroscopy using a HeNe laser as a source