Joseph K. Lefkowitz

Plasma Assisted Low Temperature Combustion

This paper presents recent kinetic and flame studies in plasma assisted low temperature combustion. First, the kinetic pathways of plasma chemistry to enhance low temperature fuel oxidation are discussed. The impacts of plasma chemistry on fuel oxidation pathways at low temperature conditions, substantially enhancing ignition and flame stabilization, are analyzed base on the ignition and extinction S-curve. Secondly, plasma assisted low temperature ignition, direct ignition to flame transition, diffusion cool flames, and premixed cool flames are demonstrated experimentally by using dimethyl ether and n-heptane as fuels. The results show that non-equilibrium plasma is an effective way to accelerate low temperature ignition and fuel oxidation, thus enabling the establishment of stable cool flames at atmospheric pressure. Finally, the experiments from both a non-equilibrium plasma reactor and a photolysis reactor are discussed, in which the direct measurements of intermediate species during the low temperature oxidations of methane/methanol and ethylene are performed, allowing the investigation of modified kinetic pathways by plasma-combustion chemistry interactions. Finally, the validity of kinetic mechanisms for plasma assisted low temperature combustion is investigated. Technical challenges for future research in plasma assisted low temperature combustion are then summarized.

Species and temperature measurements of methane oxidation in a nanosecond repetitively pulsed discharge

Speciation and temperature measurements of methane oxidation during a nanosecond repetitively pulsed discharge in a low-temperature flow reactor have been performed. Measurements of temperature and formaldehyde during a burst of pulses were made on a time-dependent basis using tunable diode laser absorption spectroscopy, and measurements of all other major stable species were made downstream of a continuously pulsed discharge using gas chromatography. The major species for a stoichiometric methane/oxygen/helium mixture with 75% dilution are H2O, CO, CO2, H2, CH2O, CH3OH, C2H6, C2H4 and C2H2. A modelling tool to simulate homogeneous plasma combustion kinetics is assembled by combining the ZDPlasKin and CHEMKIN codes. In addition, a kinetic model for plasma-assisted combustion (HP-Mech/plasma) of methane, oxygen and helium mixtures has been assembled to simulate the measurements. Predictions can accurately capture reactant consumption as well as production of the major product species. However, significant disagreement is found for minor species, particularly CH2O and CH3OH. Further analysis revealed that the plasma-activated low-temperature oxidation pathways, particularly those involving CH3O2 radical reactions and methane reactions with O(1D), are responsible for this disagreement.

Nanosecond Pulsed Plasma Activated C2H4/O2/Ar Mixtures in a Flow Reactor

The present work combines numerical and experimental efforts to investigate the effect of nanosecond pulsed plasma discharges on the low-temperature oxidation of C2H4/O2/Ar mixtures under reduced pressure conditions. The nonequilibrium plasma discharge is modeled using a one-dimensional framework, employing separate electron and neutral gas temperatures, and using a detailed plasma and combustion chemical kinetic mechanism. Good agreement is seen between the numerical and experimental results, and both results show that plasma enables low-temperature C2H4 oxidation. Compared to zero-dimensional modeling, the one-dimensional modeling significantly improves predictions, probably because it produces a more complete physical description (including sheath formation and accurate reduced electric field). Furthermore, the one- and zero-dimensional models show very different reaction pathways, using the same chemical kinetic mechanism and thus suggest different interpretations of the experimental results. Two kine...

The Effects of Repetitively Pulsed Nanosecond Discharges on Ignition Time in a Pulsed Detonation Engine

An experimental investigation of the effectiveness of a nanosecond duration repetitivelypulsed plasma discharge device for ignition of a pulsed detonation engine was carried out. Ignition of C2H4/air mixtures and aviation gasoline/air mixtures at atmospheric pressure produced a maximum reduction in ignition time of 17% and 25%, respectively, as compared to an automotive aftermarket multiple capacitive-discharge spark ignition system. It was found that the ignition time is reduced as total energy input and pulse repetition frequency is increased. Further investigation of ignition events by Schlieren imaging revealed that at low pulse-repetition frequency (0-5 kHz), individual ignition kernels formed by the discharge do not immediately interact, while at higher pulse-repetition frequencies ( ≥ 10 kHz) ignition kernels combine and result in a faster transition to a self-propagating flame front.

Numerical and Experimental Investigation of Nanosecond-Pulsed Plasma Activated C2H4/O2/Ar Mixtures in a Low Temperature Flow Reactor

The present work combines numerical and experimental efforts together to investigate the effect of low temperature, nano-second pulsed plasma discharges on the oxidation of C2H4/O2/Ar mixtures at 60 Torr pressure. The non-equilibrium plasma discharge is modeled by a two-temperature framework with detailed chemistry-plasma mechanism. The model shows that 75%~77% of input pulse energy was consumed in electron impact dissociation, excitation and ionization reactions, which efficiently produces significant amount of important radical species, fuel fragments and several excited species. The trends of numerical and experimental results agree well. The results from 1D model are compared with 0D model and it show that 1D model in general agrees better with experiments than 0D model. The modeling results reveal that reactions between O(1D) and hydrocarbons are importantly affecting the formation of C2H6, CH2CO, CH2O, CO, CO2, H2O2, H2O, O2(aaΔΔgg) and O2(bbΣΣgg). Due to the persistent relatively high level of O2(aaΔΔgg) and O2(bbΣΣgg), C2H2 converts into HCO directly without the need of going through the intermediate species of HCCO, CH2* and CH2 in the case without plasma. Owing to the long lifetime of O2(aaΔΔgg), this effect can last to 3.1 sec after the finish of all 150 pulses.

Low temperature oxidation of methane in a nanosecond pulsed plasma discharge

A study of the low temperature chemistry of methane oxidation in a nanosecond repetitively pulsed (NRP) discharge is undertaken. Both laser absorption spectroscopy measurements of temperature and formaldehyde, as well as gas chromatograph sampling of major products are used to evaluate the kinetic mechanism of a stoichiometric methane/oxygen mixture with 75% helium dilution. In addition, a predictive tool for calculating electron collision reactions, excited and ionized species reactions, and combustion reactions has been built. A model for methane, oxygen, and helium plasma has been assembled for use with this new tool, and is incorporated with a low temperature combustion mechanism. Comparisons between the model and the measured species are in agreement for CH4 and O2 consumption, as well as production of H2O, CO, CO2, and H2. However, CH2O, CH3OH, C2H6, C2H4, and C2H2 are not accurately predicted. Path flux analysis reveals that methane oxidation proceeds through one of two intermediates: CH3 or CH2, which are created primarily by electron collision reactions and H-abstraction reactions by plasma-generated radicals. The major species resulting from CH2 oxidation are generally well predicted, while those resulting from methyl oxidation are poorly predicted. Thus, further investigation of low temperature (400-500 K) methyl reactions, particularly the consumption pathways of CH3O2, are needed to bring the model into agreement with the present measurements.

Plasma Assisted MILD Combustion

A new test platform for plasma assisted MILD combustion is developed. The burner consists of three coaxial channels: a center fuel jet, a coflowing dielectric barrier discharge (DBD) reactor, and a vitiated air section. The preheating burner upstream of the vitiated air section enables an increase of the oxidizer temperature up to 1300 K near the exit of the fuel jet nozzle. A nanosecond repetitively pulsed (NRP) DBD is formed surrounding the exit of the fuel jet nozzle, and the effect of the plasma discharge on the MILD combustion regime is investigated in a highly diluted CH4/air mixture. We successfully observe an extension of the MILD combustion regime by applying the plasma discharge at a significantly low preheating temperature of 1050 K. The changes of flame shapes and species distributions with/without plasma discharge are also investigated. A gas chromatography system is used to evaluate the effect of the plasma discharge on the species distribution after the plasma reactor section. Computational fluid dynamics (CFD) simulations are also carried out to understand and gain an insight into the mixing layer interactions with and without the plasma. Based on the species profile data and supported by the CFD simulations, we find that the reforming of a portion of the methane/air mixture at the exit of the fuel jet into intermediate species can have a significant impact on the ignition time scale. In addition, flow perturbations owing to the large density gradient induced by prompt gas heating in the plasma has a significant impact on the mixing process in the fuel jet. Thus, we demonstrate the potential of the newly developed experimental apparatus to advance our understanding of plasma assisted MILD combustion.

Towards Simultaneous Measurement of OH and HO 2 in Combustion Using Faraday Rotation Spectroscopy

Preliminary results from the development of a dual wavelength Faraday rotation spectrometer for simultaneous quantification of HO2 and OH in combustion research are presented.