Unsaturated hydrocarbon combustion chemistry and gasoline and diesel fuel ignition: Experimental and theoretical studies
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The primary objective of the work presented in this Thesis is to investigate the oxidation chemistry of unsaturated hydrocarbons. Butenes (1-, 2-, iso-butene) and 1,3-butadiene are the shortest olefins with structural isomers and are the simplest conjugated hydrocarbons and were thus selected as candidates to systematically elucidate their combustion chemistry using combined experimental, theoretical and kinetic modelling approaches. Experimental ignition delay time (IDT) measurements of the butenes were carried out in both a rapid compression machine (RCM) and in a high-pressure shock tube (HPST) at equivalence ratios of 0.5, 1.0, and 2.0 at engine relevant ranges of temperature (650 – 1400 K) and pressure (10 to 50 atm). A comprehensive chemical kinetic mechanism (AramcoMech 2.0) to describe the combustion of 1- and 2-butene has been developed. It has been validated using the IDT data as measured above in addition to a large variety of literature data: IDTs, species profile data as a function of time and temperature measured in jet-stirred reactors (JSRs), premixed flames, and flow reactors, and laminar flame speed data. Important reactions have been identified via flux and sensitivity analyses including: (a) H-atom abstraction from 1-butene by hydroxyl radicals and molecular oxygen from different carbon sites; (b) addition reactions, including hydrogen atom and hydroxyl radical addition to 1-butene; (c) allylic radical chemistry, including the addition reactions with methyl radical, hydroperoxyl radical and self-recombination; (d) vinylic radical chemistry, including the addition reaction with molecular oxygen; (e) alcohol radical chemistry, including the Waddington type propagating reaction pathways and alkyl radical low-temperature branching chemical pathways. Rate constants and thermodynamic properties for Ḣ-atom addition to 1,3-butadiene and related reactions on the Ċ4H7 potential energy surface were calculated using two different series of quantum chemical methods and two different kinetic codes. The calculated results including zero-point energies, single-point energies, rate constants, barrier heights, and thermochemistry were systematically compared among the two quantum chemical methods. Moreover, a systematic investigation of H-atom abstraction by molecular oxygen of primary, secondary and tertiary hydrogen atoms from a series of allylic radicals was carried out using four different quantum chemical methods and two different kinetic codes. Finally, the autoignition characteristics of two gasoline and diesel fuel blends (volume ratio: 75:25 and 50:50) were studied experimentally over a wide range of temperatures, equivalence ratios, and pressures using both a high-pressure shock tube (HPST) and a rapid compression machine (RCM). The reactivity of these two gasoline and diesel fuel blends were systematically compared with the other pure gasoline fuel samples. Additionally, three surrogates (Primary Reference Fuel (PRF), Toluene PRF (TPRF) and a multi-component surrogate) were carefully formulated for each fuel blend by matching various fuel characteristics (RON, MON, Octane Sensitivity and AKI) with two gasoline surrogate models from LLNL and KAUST being employed to simulate the experimental data.
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