Matthew A. Oehlschlaeger

An experimental and kinetic modeling study of the oxidation of the four isomers of butanol.

Butanol, an alcohol which can be produced from biomass sources, has received recent interest as an alternative to gasoline for use in spark ignition engines and as a possible blending compound with fossil diesel or biodiesel. Therefore, the autoignition of the four isomers of butanol (1-butanol, 2-butanol, iso-butanol, and tert-butanol) has been experimentally studied at high temperatures in a shock tube, and a kinetic mechanism for description of their high-temperature oxidation has been developed. Ignition delay times for butanol/oxygen/argon mixtures have been measured behind reflected shock waves at temperatures and pressures ranging from approximately 1200 to 1800 K and 1 to 4 bar. Electronically excited OH emission and pressure measurements were used to determine ignition-delay times. The influence of temperature, pressure, and mixture composition on ignition delay has been characterized. A detailed kinetic mechanism has been developed to describe the oxidation of the butanol isomers and validated by comparison to the shock-tube measurements. Reaction flux and sensitivity analysis illustrates the relative importance of the three competing classes of consumption reactions during the oxidation of the four butanol isomers: dehydration, unimolecular decomposition, and H-atom abstraction. Kinetic modeling indicates that the consumption of 1-butanol and iso-butanol, the most reactive isomers, takes place primarily by H-atom abstraction resulting in the formation of radicals, the decomposition of which yields highly reactive branching agents, H atoms and OH radicals. Conversely, the consumption of tert-butanol and 2-butanol, the least reactive isomers, takes place primarily via dehydration, resulting in the formation of alkenes, which lead to resonance stabilized radicals with very low reactivity. To our knowledge, the ignition-delay measurements and oxidation mechanism presented here for 2-butanol, iso-butanol, and tert-butanol are the first of their kind.

Shock Tube and Chemical Kinetic Modeling Study of the Oxidation of 2,5-Dimethylfuran

A detailed kinetic model describing the oxidation of 2,5-dimethylfuran (DMF), a potential second-generation biofuel, is proposed. The kinetic model is based upon quantum chemical calculations for the initial DMF consumption reactions and important reactions of intermediates. The model is validated by comparison to new DMF shock tube ignition delay time measurements (over the temperature range 1300-1831 K and at nominal pressures of 1 and 4 bar) and the DMF pyrolysis speciation measurements of Lifshitz et al. [ J. Phys. Chem. A 1998 , 102 ( 52 ), 10655 - 10670 ]. Globally, modeling predictions are in good agreement with the considered experimental targets. In particular, ignition delay times are predicted well by the new model, with model-experiment deviations of at most a factor of 2, and DMF pyrolysis conversion is predicted well, to within experimental scatter of the Lifshitz et al. data. Additionally, comparisons of measured and model predicted pyrolysis speciation provides validation of theoretically calculated channels for the oxidation of DMF. Sensitivity and reaction flux analyses highlight important reactions as well as the primary reaction pathways responsible for the decomposition of DMF and formation and destruction of key intermediate and product species.

The Decomposition Products of JP-10

Establishment of an accurate kinetic mechanism for JP-10 pyrolysis and oxidation will require identification of the initial products of JP-10 thermal decomposition. Recently we have developed a new high-speed UV absorption kinetic spectrograph that can to be used with a shock tube to rapidly acquire spectra to measure and identify these products at combustion temperatures. Using this kinetic spectrograph, the high-temperature UV absorption spectra for a variety of hydrocarbon species related to the decomposition products of JP-10 have been measured. These species include: benzene, cyclopentene, propene, ethylene, acetylene, 1,3butadiene, cyclopentadiene and JP-10. Few of these spectra have been measured at high temperature before. By comparison of these spectra to the absorption spectrum of JP-10 decomposition products, evidence of cyclopentene as an initial decomposition product with near unity yield was found: JP-10 -* C5Hg+ other products Measured formation rates of cyclopentene were found to be consistent with an initial C-C bond-scission step. Several previously suggested pathways for JP-10 decomposition can be ruled out using the same methods.

Temperature measurement using ultraviolet laser absorption of carbon dioxide behind shock waves

A diagnostic for microsecond time-resolved temperature measurements behind shock waves, using ultraviolet laser absorption of vibrationally hot carbon dioxide, is demonstrated. Continuous-wave laser radiation at 244 and 266 nm was employed to probe the spectrally smooth CO2 ultraviolet absorption, and an absorbance ratio technique was used to determine temperature. Measurements behind shock waves in both nonreacting and reacting (ignition) systems were made, and comparisons with isentropic and constant-volume calculations are reported.

Carbon Dioxide Thermal Decomposition: Observation of Incubation

Abstract Incubation prior to the thermal decomposition of CO2 is observed for the first time behind shock waves, confirming the expected bottleneck in collisional activation of this triatomic molecule. The thermal decomposition of carbon dioxide has been investigated behind reflected shock waves at temperatures of 3200–4600 K and pressures of 45–100 kPa. Ultraviolet laser absorption was used to monitor the CO2 concentration with microsecond time resolution, allowing observation of a pronounced incubation period prior to steady CO2 dissociation. Master equation simulations, with a simple model for collisional energy transfer, were carried out to describe the measured incubation times and dissociation rate coefficient. The second order rate coefficient for CO2 dissociation was found to be 3.14 × 1014 exp(−51300 K/T) cm3 mol−1 s−1. The number of incubation collisions was found to range from 7 × 103 at 4600 K to 3.5 × 104 at 3200 K. The master equation simulations suggest that the energy transferred per collision must have a greater than linear dependence on energy.

Passivation and stabilization of aluminum nanoparticles for energetic materials

In aircraft applications, fuel is used not only as a propellant but also as a coolant and improving both the thermal conductivity and combustion enthalpy of the fuel is beneficial in these applications. These properties can be enhanced by dispersing aluminum nanoparticles into the fuel; however, the nanoparticles require stabilization from agglomeration and passivation from oxidation in order for these benefits to be realized in aircraft applications. To provide this passivation and stabilization, aluminum nanoparticles were encapsulated with a coating by the plasma enhanced chemical vapor deposition (PE-CVD) method from toluene precursors. The thermal conductivity, combustion and ignition properties, and stability of the nanoparticles dispersed in RP-2 fuel were subsequently evaluated. In addition, the effect of dispersing aluminum nanoparticles in RP-2 fuel on the erosion rate of fuel nozzles was evaluated. The dispersion of PE-CVD coated aluminum nanoparticles at a concentration of 3.0% by volume exhibited a 17.7% and 0.9% increase in thermal conductivity and volumetric enthalpy of combustion, respectively, compared to the baseline RP-2 fuel. Additionally, particle size analysis (PSA) of the PE-CVD coated aluminum nanofuel exhibited retention of particle size over a five-month storage period and erosion testing of a 1 mm stainless steel nozzle exhibited a negligible 1% change in discharge coefficient after 100 hours of testing.

An Experimentally Validated Surrogate Fuel for the Combustion Kinetics of S-8, a Synthetic Paraffinic Jet Aviation Fuel

A surrogate fuel designed to emulate the gas phase chemical kinetic combustion phenomena of this target S-8 fuel is formulated in an a priori manner. The surrogate fuel is composed of n-dodecane/iso-octane and its performance is evaluated by the measurement of the same combustion phenomena as the target fuel under identical conditions. The performance of available kinetic models for S-8 surrogates is evaluated by analysis of their computations of this experimental data. Furthermore, an experimental study evaluating the significance of weakly isomerized alkanes as important components for surrogate fuels is presented.

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.

Diesel Engine Simulations and Experiments: Fuel Variability Effects on Ignition

The chemical composition and properties of fuels used in compression-ignition engines can influence engine performance significantly. Consequently, the modeling of fuel chemistry within computational fluid dynamics (CFD) simulations of diesel and other compression ignition engines is important. Modern detailed chemical mechanisms may provide predictive modeling of fuel chemistry; however, they are generally far too computationally expensive for use in CFD. We present simulations of diesel engine combustion, focusing on the prediction of ignition, using the CONVERGE CFD software package. A CFD simulation framework with models for turbulence and spray breakup and atomization is presented with a reduced global reaction model to describe fuel oxidation and ignition. The global reaction model incorporates a single parameter, the derived cetane number (DCN), to describe fuel reactivity variability. CFD simulations are compared to experiments carried out in a single-cylinder diesel engine for compositionally diverse conventional and alternative diesel and jet fuels. Model-experiment comparisons show general agreement for ignition timing and the influence of fuel variability on ignition timing. In addition, the sensitivity of CFD predictions on the chemistry, turbulence, and spray models is illustrated.Copyright