Laminar Flame Speed, Ignition Delay Time, and CO Laser Absorption Measurements of a Gasoline-like Blend of Pentene Isomers.

The combustion properties of a gasoline-like blend of pentene isomers were determined using multiple types of experimental measurements. The representative mixture (Mix A) is composed of 5.7% 1-pentene (1-C5H10), 39.4% 2-pentene (2-C5H10), 12.5% 2-methyl-1-butene (2M1B), and 42.4% 2-methyl-2-butene (2M2B) (% mol). Laminar flame speeds were measured at equivalence ratios of 0.7-1.5 in a constant-volume combustion chamber, and ignition delay times (including both OH* and CH* diagnostics) as well as CO time-history profiles were performed in shock tubes, in highly diluted mixtures (0.995 He/Ar), at a stoichiometric condition for temperatures ranging from 1350 to 1750 K, and at near-atmospheric pressure. Two additional unbalanced mixtures removing either 2M2B (Mix B) or 2-C5H10 (Mix C) were studied in a shock tube to collect CO time histories, representing the most stringent validation constraints, as these two pentenes constitute the biggest proportions in Mix A and exhibit opposite behaviors in terms of reactivity due to their chemical structure differences. Numerical predictions using a recent validated chemical kinetics mechanism encompassing all pentene isomers from Grégoire et al. ( Fuel2022, 323, 124223) are presented. The use of a complex blend of four pentene isomers in the present paper provided a capstone test of the current mechanism's ability to model pentene-isomer combustion chemistry, with very good results that reflect positively on the current state of the art in pentene isomer kinetics modeling.

Impact of Methanol and Butanol on Soot Formation in Gasoline Surrogate Pyrolysis: A Shock-Tube Study.

The influence of methanol and butanol on soot formation during the pyrolysis of a toluene primary reference fuel mixture with a research octane number (RON) of 91 (TPRF91) was investigated by conducting shock-tube experiments. The TPRF91 mixture contained 17 mol % n-heptane, 29 mol % iso-octane, and 54 mol % toluene. To assess the contribution of individual fuel compounds on soot formation during TPRF91 pyrolysis, the pyrolysis of argon diluted (1) toluene, (2) iso-octane, and (3) n-heptane mixtures were also studied. To enable the interpretation of the TPRF91 + methanol and TPRF91 + butanol experiments, the influence of both alcohols on soot formation during the thermal decomposition of toluene and iso-octane was also investigated in a separate series of measurements. Pyrolysis was monitored behind reflected shock waves at pressures between 2.1 and 4.2 bar and in the temperature range of 2060-2815 K. Laser extinction at 633 nm was used to determine the soot yield as a function of reaction time. For selected experiments, the temporal variation in temperature was also measured via time-resolved two-color CO absorption using two quantum-cascade lasers at 4.73 and 4.56 µm. It was found that soot formed during TPRF91 pyrolysis is primarily caused by the thermal decomposition of toluene. Adding methanol to TPRF91 results in a slight reduction of soot formation, whereas admixing butanol results in shifting soot formation to higher temperatures, but in that case, no overall soot reduction was observed during TPRF91 pyrolysis. Measured soot yields were compared to simulations based on a previous and an updated version of a detailed reaction mechanism from the CRECK modeling group [Nobili, A.; Cuoci, A.; Pejpichestakul, W.; Pelucchi, M.; Cavallotti, C.; Faravelli, T. Combust. Flame 2022; 10.1016/j.combustflame.2022.112073]. Rate-of-production analyses for reactions involving BINS at different experimental conditions were carried out. Although in the case of TPRF91 and toluene pyrolysis, no quantitative agreement was obtained between the experiment and simulation, the comparison nevertheless shows that the new version of the CRECK mechanism is a significant improvement over the previous one. In the case of n-heptane decomposition and iso-octane pyrolysis with and without alcohols, the updated reaction mechanism shows excellent agreement between simulation and measured soot yields.

A new high-pressure aerosol shock tube for the study of liquid fuels with low vapor pressures.

A new shock-tube facility for studying gas-phase and condensed-phase mixtures has been developed for the investigation of various hydrocarbon species at the Qatar campus of Texas A&M University. At present, the facility is intended for studying combustion behaviors of long-chain hydrocarbon molecules and mixtures thereof under realistic engine conditions. Equipped with an aerosol generation and entrainment apparatus, the facility also possesses an enlarged driver section and double-diaphragm interface between the driver and driven sections. The driver section diameter is 19.37 cm with a configurable length of 1 m-6 m. Additionally, the stainless-steel driven section has an inner surface with a mirror finish and internal diameter of 15.24 cm. The driven section is also configurable in length up to approximately 5.2 m. As with most modern shock tubes, this shock tube is equipped for use with current methods of shock velocity detection, optical diagnostics, and other diagnostic techniques. In addition to the study of aerosolized liquids (fuels and non-fuels) related to combustion chemistry, reaction kinetics, evaporation studies, and particle-fluid interactions, among others, the facility is capable of investigating traditional gas-phase mixtures like those previously undertaken in a similar facility in the Petersen Group Laboratory at Texas A&M University. The operating limits of the title facility include temperatures and pressures up to 4000 K and 100 atm, respectively. The design and characterization of a novel aerosol introduction method is also presented, which yielded measured aerosol loading uniformities of 92%-97%. Finally, ignition delay time measurements of stoichiometric mixtures of decane in air are presented, which show excellent agreement with those recently recorded in heated shock tubes.

A Shock-Tube Study of the Rate Constant of PH3 + M ⇄ PH2 + H + M (M = Ar) Using PH3 Laser Absorption.

Phosphine (PH3) is a highly reactive and toxic gas. Prior experimental investigations of PH3 pyrolysis reactions have included only low-temperature measurements. This study reports the first shock-tube measurements of PH3 pyrolysis using a new PH3 laser absorption technique near 4.56 µm. Experiments were conducted in mixtures of 0.5% PH3/Ar behind reflected shock waves at temperatures of 1460-2013 K and pressures of ∼1.3 and ∼0.5 atm. The PH3 time histories displayed two-stage behavior similar to that previously observed for NH3 decomposition, suggesting by analogy that the rate constant for PH3 + M ⇄ PH2 + H + M (R1) could be determined. A simple three-step mechanism was assembled for data analysis. In a detailed kinetic analysis of the first-stage PH3 decomposition, values of k1,0 were obtained and best described by (in cm3·mol-1·s-1) k1,0 = 7.78 × 1017 exp(-80,400/RT), with units of cal, mol, K, s, and cm3. Agreement between the 1.3 and 0.5 atm data confirmed that the measured k1,0 was in the low-pressure limit. Agreement of the experimental k1,0 with ab initio estimates resolved the question of the main pathway of PH3 decomposition: it proceeds as PH3 ⇄ PH2 + H instead of PH3 ⇄ PH + H2.

Ultrashort-pulse laser-induced breakdown spectroscopy for detecting airborne metals during energetic reactions.

Ultrashort-pulse laser-induced breakdown spectroscopy (LIBS), specifically using a femtosecond laser, has certain advantages over longer-pulse, nanosecond-duration lasers, in that they typically have kilohertz repetition rates and reduced background noise along with little-to-no laser-plasma interaction, all of which lead to a better chance of detecting LIBS signals from trace particles. In this work, femtosecond-LIBS is investigated for the detection of metallic particles in the hot flame zone of solid propellant strands burning in the atmosphere. The metallic particles doped into the solid propellants were aluminum (Al), copper, lead, lead stearate, and mercury chloride, which are all either typically found in energetic formulations as additives or impurities. Using an 80-fs-pulse-duration, amplified Ti:Sapphire laser operating at 1000 Hz, single-shot concentration measurement experiments were performed. The femtosecond-LIBS apparatus could detect all metallic additives, whereas a previous nanosecond-LIBS scheme with comparable conditions was able to detect only higher concentrations of Al. The single-shot concentration study, conducted with the Al-doped propellants, indicated that there is a linear relationship between the percentage of laser shots detecting a LIBS signal and the mass percentage of Al initially present in the strands. The present results illustrate the advantages of using a femtosecond laser over a nanosecond laser for LIBS detection during energetics material reactions.

Laser-induced-breakdown-spectroscopy-based detection of metal particles released into the air during combustion of solid propellants.

Numerous metals and metal compounds are often added to propellants and explosives to tailor their properties such as heat release rate and specific impulse. When these materials combust, these metals can be released into the air, causing adverse health effects such as pulmonary and cardiovascular disease, particulate-matter-induced allergies, and cancer. Hence, robust, field-deployable methods are needed to detect and quantify these suspended metallic particles in air, identify their sources, and develop mitigation strategies. Laser-induced breakdown spectroscopy (LIBS) is a technique for elemental detection, commonly used on solids and liquids. In this study, we explored nanosecond-duration LIBS for detecting airborne metals during reactions of solid propellant strands, resulting from additives of aluminum (Al), copper, lead, lead stearate, and mercury chloride. Using the second harmonic of a 10-ns-duration 10-Hz, Nd:YAG laser, plasma was generated in the gas-phase exhaust plume of burning propellant strands containing the target metals. Under the current experimental conditions, the ns-LIBS scheme was capable of detecting Al at concentrations of 5%, 10%, and 16% by weight in the propellant strand. As the weight percentage increased, the LIBS signal was detected by more laser shots, up to a point where the system transition from being nonhomogeneous to a more-uniform distribution of particles. Further measurements and increased understanding of the reacting flow field are necessary to quantify the effects of other metal additives besides Al.

Experimental and Chemical Kinetics Study of the Effects of Halon 1211 (CF2BrCl) on the Laminar Flame Speed and Ignition of Light Hydrocarbons.

In this study, the effect of Halon 1211 (CF2BrCl) on the ignition delay time and laminar flame speed of CH4, C2H4, and C3H8 were investigated experimentally for the first time. The results showed that the effects of Halon 1211 on the ignition delay time are strongly dependent on the hydrocarbon: the ignition delay time of CH4 is significantly decreased by Halon 1211 addition, while a significant increase in the ignition delay time was observed with C2H4 for the lowest temperatures investigated. Ignition delay times for C3H8 were slightly increased, mostly on the low-temperature side and for the fuel-rich case. A significant reduction in the laminar flame speed was observed for all of the fuels. A tentative chemical kinetics model was assembled from existing models and completed with reactions that have been determined in the literature or estimated when necessary. The experimental results were reproduced satisfactorily by the model, and a chemical analysis showed that most of the effects of Halon 1211 on the ignition delay times of C2H4 and C3H8 are due to the consumption of H radical through the reaction HBr + H ⇄ Br + H2. In the case of methane, the CF2 radical promotes the formation of H via CF2 + CH3 ⇄ CH2:CF2 + H, which then promotes the branching reaction H + O2 ⇄ OH + O. The laminar flame speed results can be explained using catalytic cycles involving Br atoms that are similar to those reported in the literature for CF3Br. This study exhibits the need for a better estimation of the chlorine atom chemistry during the combustion of hydrocarbons in the presence of fire suppressants.

Experimental and Kinetic Modeling Study of 2-Methyl-2-Butene: Allylic Hydrocarbon Kinetics.

Two experimental studies have been carried out on the oxidation of 2-methyl-2-butene, one measuring ignition delay times behind reflected shock waves in a stainless steel shock tube, and the other measuring fuel, intermediate, and product species mole fractions in a jet-stirred reactor (JSR). The shock tube ignition experiments were carried out at three different pressures, approximately 1.7, 11.2, and 31 atm, and at each pressure, fuel-lean (ϕ = 0.5), stoichiometric (ϕ = 1.0), and fuel-rich (ϕ = 2.0) mixtures were examined, with each fuel/oxygen mixture diluted in 99% Ar, for initial postshock temperatures between 1330 and 1730 K. The JSR experiments were performed at nearly atmospheric pressure (800 Torr), with stoichiometric fuel/oxygen mixtures with 0.01 mole fraction of 2M2B fuel, a residence time in the reactor of 1.5 s, and mole fractions of 36 different chemical species were measured over a temperature range from 600 to 1150 K. These JSR experiments represent the first such study reporting detailed species measurements for an unsaturated, branched hydrocarbon fuel larger than iso-butene. A detailed chemical kinetic reaction mechanism was developed to study the important reaction pathways in these experiments, with particular attention on the role played by allylic C-H bonds and allylic pentenyl radicals. The results show that, at high temperatures, this olefinic fuel reacts rapidly, similar to related alkane fuels, but the pronounced thermal stability of the allylic pentenyl species inhibits low temperature reactivity, so 2M2B does not produce "cool flames" or negative temperature coefficient behavior. The connections between olefin hydrocarbon fuels, resulting allylic fuel radicals, the resulting lack of low-temperature reactivity, and the gasoline engine concept of octane sensitivity are discussed.

High sensitivity of diesel soot morphological and optical properties to combustion temperature in a shock tube.

Carbonaceous particles produced from combustion of fossil fuels have strong impacts on air quality and climate, yet quantitative relationships between particle characteristics and combustion conditions remain inadequately understood. We have used a shock tube to study the formation and properties of diesel combustion soot, including particle size distributions, effective density, elemental carbon (EC) mass fraction, mass-mobility scaling exponent, hygroscopicity, and light absorption and scattering. These properties are found to be strongly dependent on the combustion temperature and fuel equivalence ratio. Whereas combustion at higher temperatures (∼2000 K) yields fractal particles of a larger size and high EC content (90 wt %), at lower temperatures (∼1400 K) smaller particles of a higher organic content (up to 65 wt %) are produced. Single scattering albedo of soot particles depends largely on their organic content, increasing drastically from 0.3 to 0.8 when the particle EC mass fraction decreases from 0.9 to 0.3. The mass absorption cross-section of diesel soot increases with combustion temperature, being the highest for particles with a higher EC content. Our results reveal that combustion conditions, especially the temperature, may have significant impacts on the direct and indirect climate forcing of atmospheric soot aerosols.