Measurements of ignition times, OH time-histories, and reaction rates in jet fuel and surrogate oxidation systems [electronic resource]
Keywords: Mechanical Engineering

Fossil-based hydrocarbon fuels account for over 80% of the primary energy consumed in the world - it is still expected to be about 70% in year 2050 - and nearly 60% of that amount is used in the transport sector. The basis for globalization is transportation and a driving force has been the growth in global air traffic. The current climate crisis magnifies the need for improving the performance of jet engines by introducing scientific designs in which the use of chemical kinetics will be essential and critical for better performance and reducing pollutant emissions. Most aviation fuels are jet fuels originating from crude oil and there are major gaps in our knowledge of the high-temperature chemistry of real liquid carbon-based fuels. There is a critical need for experimental kinetic databases that can be used for the validation and refinement of jet fuel surrogate mechanisms. To fill this need, experiments were performed using shock tube and laser absorption methods to investigate jet fuel and surrogate oxidation systems under engine-relevant conditions. Ignition times and OH species time-histories were measured and low-uncertainty measurements of the reactions of OH with several stable intermediates were carried out. The work presented in this study can be broken into three categories: 1) jet fuel oxidation, 2) surrogate oxidation, and 3) OH radical reactions with several stable combustion intermediates. Ignition delay times were measured for gas-phase jet fuel oxidation (Jet-A and JP-8) in air behind reflected shock waves in a heated high-pressure shock tube. Initial reflected shock conditions were as follows: temperatures of 715-1229 K, pressures of 17-51 atm, equivalence ratios (phi) of 0.5 and 1, and oxygen concentrations of 10 and 21 % in synthetic air. Ignition delay times were measured using sidewall pressure and OH* emission at 306 nm. The new experimental results were modeled using several kinetic mechanisms using various jet fuel surrogate mixtures. Normal and cyclo alkanes are the two most important chemical classes found in jet fuels. Ignition delay time experiments were conducted during high-pressure oxidation of two commonly used representative components for normal and cyclo alkanes in jet fuel surrogates, i.e., n-dodecane and methylcyclohexane (MCH), respectively. Fuel/air ignition was studied for the following shock conditions: temperatures of 727-1177 K, pressures of 17-50 atm, phi's of 0.5 and 1. OH concentration time-histories during high-pressure n-dodecane, n-heptane and MCH oxidation were measured behind reflected shock waves in a heated, high-pressure shock tube. Experimental conditions covered temperatures of 1121 to 1422 K, pressures of 14.1-16.7 atm, and initial fuel concentrations of 500 to 1000 ppm (by volume), and an equivalence ratio of 0.5 with O2 as the oxidizer in argon as the bath gas. OH concentrations were measured using narrow-linewidth ring-dye laser absorption near the R-branchhead of the OH A-X (0,0) system at 306.47 nm. Detailed comparisons of these data with the predictions of various kinetic mechanisms were made. Sensitivity and pathway analyses for these reference fuel components were performed, leading to reaction rate recommendations with improved model performance. Reactions of OH radical with two alkenes (ethylene and propene) and a diene (1,3-butadiene) were studied behind reflected shock waves. Measurements were conducted in the range of temperatures from 890-1438 K and pressures from 1.99-10.18 atm for three initial concentrations of fuels (500ppm, 751.1ppm and 1000ppm). OH radicals were produced by shock-heating tert-butyl hydroperoxide, (CH3)3-CO-OH, and monitored by narrow-line width ring dye laser absorption of the well characterized R1(5) line of the OH A-X (0, 0) band near 306.7 nm. OH time-histories were modeled by using a modified oxidation mechanism and rate constants for the reactions of OH with ethylene, propene, and 1,3-butadiene were extracted by matching modeled and measured OH concentration time histories in the reflected shock region. Detailed error analyses yielded an uncertainty estimate of ± 22.8% (OH+ethylene at 1201 K), ±16.5% (OH+propene at 1136 K), and ±13% (OH+1,3-butadiene at 1200K). Canonical and variational transition state theory calculations using recent ab initio results gave excellent agreement with our experimental measurements and data outside our range and hence the resulting expressions can be used directly in combustion models. In the current studies, a rate measurement for the decomposition of TBHP has been obtained in the range 745-1014 K using both incident and reflected OH data.