“UV Laser-Absorption Measurements in Jet Fuel and Surrogate Oxidation Systems
Under Engine-Relevant Conditions”
Dr. Subith Vasu, Stanford University
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. Current kinetic models to describe jet fuel ignition chemistry use surrogate jet fuel mixtures and detailed reaction mechanisms. 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 critical need, experiments were performed for the first time using shock tube and laser absorption methods to investigate jet fuel and surrogate oxidation systems under engine-relevant conditions. Shock tubes can provide a variety of kinetic data including ignition delay times, species concentration time-histories and direct measurements of specific reaction rates that can be effectively independent of fluid mechanics and other transport effects, thereby allowing relatively direct study of the chemistry involved in the fuel kinetics models. Ignition delay time is an important parameter in the combustor design of most engines. Ignition delay times were measured for jet fuel oxidation (Jet-A and JP-8) and surrogate components (n-dodecane and methylcyclohexane MCH) behind reflected shock waves in a heated high-pressure shock tube. The new experimental results were modeled using several kinetic mechanisms using various jet fuel surrogate mixtures.
Species time-history measurements of OH during the oxidation process can provide information about the reaction pathways that occur during ignition. OH concentration time-histories during high-pressure n-dodecane, n-heptane and MCH oxidation were also measured behind reflected shock waves using UV laser absorption. Sensitivity and pathway analyses for these reference fuel components were performed, leading to reaction rate recommendations with improved model performance.
Small alkenes are important intermediates in the oxidation of hydrocarbon fuels and are formed in large quantities in practical engines. Reactions of OH radical with several alkenes (ethylene, propene, butene), a diene (1,3-butadiene) were studied behind incident and reflected shock waves using OH UV laser absorption. Current transition state theory calculations using recent ab initio results gave excellent agreement with experimental measurements. The resulting expressions can be used directly in kinetic models to aid the development of future advanced engines.
The OH laser absorption strategy used in this work can also be used to study biofuels. The first high-temperature measurement of the reaction rate of OH radical with n-butanol was performed behind reflected shock waves. The current measurements provide useful input to theoretical development of hydrogen abstraction rates from higher alcohols.
Subith Vasu earned his BTech degree in aerospace engineering from the Indian Institute of Technology Madras in 2004 and his PhD in mechanical engineering from Stanford University this year. His research areas focus primarily (in the field of energy conversion and propulsion) on laser-diagnostics, combustion chemical kinetics and biofuels optimization. He has published over 15 journal papers (including a cover article in the journal Combustion and Flame) and is currently preparing a textbook in the area of non-equilibrium processes in high-temperature gases. He is a member AIAA and the Combustion Institute.