Mustapha Fikri

Syngas is a desirable fuel for combustion in the Allam-Fetvedt cycle, which involves combustion under supercritical-CO2 conditions. While some work has been conducted in collecting ignition delay times (IDT) at the extreme pressures required by these systems, significant model deficiencies remain. Additionally, considerable barriers in terms of nonideal gas dynamic effects have been shown for these experiments in shock tubes. Further investigation into the fundamental combustion kinetics of H2/CO/CO2 mixtures is required. Time-resolved speciation measurements for target species have been shown to better aid in improving the understanding of underlying chemical kinetics than global ignition delay time measurements. Therefore, laser absorption measurements of CO were measured behind reflected shock waves during combustion of syngas at 5 and 10 bar and temperatures between 1080 and 2100 K. The mixtures investigated utilized H2-to-CO ratios of 1:1 and 1:4, respectively, each at stoichiometric conditions, allowing for discussions of the effect of initial fuel composition. A ratio of fuel to CO2 of 1:2 was also utilized to represent commercially available syngas. The mixtures were diluted in helium and argon (20% He, 76.5% Ar) to minimize thermal effects and to expedite CO thermal relaxation during the experiment. The resulting CO time histories were then compared to modern chemical kinetics mechanisms, and disagreement is seen for this system, which is assumed to be fairly well known. This study elucidates particular chemistry that needs improvement in moving toward a better understanding of syngas combustion at elevated pressures.

The interaction of fuel and lubricant droplets with gaseous fuel/air mixtures close to autoignition is relevant in the context of unwanted early autoignition in spark-ignition internal combustion (IC) engines. To study the influence of droplets on the ignition of fuel/air mixtures independent from the in-cylinder pressure/temperature history, the shock-tube technique in combination with an injection system was established, which enables the generation and injection of single droplets or droplet clusters of n-dodecane and lubricant base oil behind reflected shock waves at pressures and temperatures representative for the compression phase of IC engines. Injected droplets were imaged by high-repetition-rate laser-induced fluorescence. The ignition process was observed by imaging in the visible and UV simultaneously through the shock-tube end wall with a combination of color- and UV-sensitive high-repetition-rate cameras. It was found that the amount and composition of the injected liquid are important factors determining the extent of the interference with the ongoing autoignition of the premixed fuel/air bath gas. For a stoichiometric mixture of primary reference fuels (PRF95) in air, the droplets significantly accelerate ignition especially in the negative temperature coefficient regime at around 760 K. The comparison of the timing of local ignition and the occurrence of volumetric ignition indicates that only in cases where the surrounding gas is close to autoignition, the droplets can trigger early autoignition. This required temporal and spatial coincidence might explain the high level of randomness of early autoignition in engines.

Multiphase-induced ignition is frequently discussed as a trigger for early ignition in internal combustion engines. In this context, we investigated the ignition process of single lubricant-oil droplets and their interaction with the bulk air/fuel mixture in a high-pressure shock tube, mimicking oil-fuel interaction in turbocharged internal combustion engines at the end of the compression stroke. A fast micro-dispensing injector released single fuel or lubricant oil droplets with a diameter of 200±50 µm into shock-heated fuel/air mixtures consisting of PRF95 and synthetic air. The injector was flush-mounted in the sidewall of the shock tube. The droplets were released into the gas after the passage of the reflected shock waves at post-shock conditions of 2 MPa and 750-950 K. With a high-frame-rate color camera, the entire evolution of droplet injection and ignition was traced in space and time through a large sapphire window in the endwall of the shock tube. A high-repetition-rate laser at 532 nm was used to illuminate the droplets. The light scattered by the droplets was detected in the green channel of the camera, whereas ignition and soot luminosity were detected in the blue and red channel, respectively. From the camera images the time for the appearance of first detected kernel related to local ignition (originating from either droplet or gaseous ignition) was determined. Additionally, the volume ignition that proceeds after an ignition delay time was determined from the OH* emission time-trace from a photomultiplier tube. It was found that the oil-droplet injection had an influence on the local ignition whereas the ignition delay time was reduced in the low temperature range only.

The unimolecular decomposition of diethyl ether (DEE; C₂H₅OC₂H₅) is considered to be initiated via a molecular elimination and a C–O and a C–C bond fission step: C₂H₅OC₂H₅ → C₂H₄ + C₂H₅OH (1), C₂H₅OC₂H₅ → C₂H₅ + C₂H₅O (2), and C₂H₅OC₂H₅ → CH₃ + C₂H₅OCH₂ (3). In this work, two shock-tube facilities were used to investigate these reactions via (a) time-resolved H-atom concentration measurements by H-ARAS (atomic resonance absorption spectrometry), (b) time-resolved DEE-concentration measurements by high repetition-rate time-of-flight mass spectrometry (HRR-TOF-MS), and (c) product-composition measurements via gas chromatography/MS (GC/MS) after quenching the test gas. The experiments were conducted at temperatures ranging from 1054 to 1505 K and at pressures between 1.2 and 2.5 bar. Initial DEE mole fractions between 0.4 and 9300 ppm were used to perform the kinetics experiments by H-ARAS (0.4 ppm), GC/MS (200–500 ppm), and HRR-TOF-MS (7850–9300 ppm). The rate constants, k_total (k_total = k₁ + k₂ + k₃) derived from the GC/MS and HRR-TOF-MS experiments agree well with each other and can be described by the Arrhenius expression, k_total(1054–1467 K; 1.3–2.5 bar) = 10^12.81±0.22 exp(−240.27 ± 5.11 kJ mol^–1/RT) s^–1. From the H-ARAS experiments, overall rate constants for the bond fission channels, k₂₊₃ = k₂ + k₃ have been extracted. The k₂₊₃ data can be well described by the Arrhenius equation, k₂₊₃(1299–1505 K; 1.3–2.5 bar) = 10^14.43±0.33 exp(−283.27 ± 8.78 kJ mol^–1/RT) s^–1. A master-equation analysis was performed using CCSD(T)/aug-cc-pvtz//B3LYP/aug-cc-pvtz and CASPT2/aug-cc-pvtz//B3LYP/aug-cc-pvtz molecular properties and energies for the three primary thermal decomposition processes in DEE. The derived experimental data is very well reproduced by the simulations with the mechanism of this work. With regard to the branching ratios between bond fissions and elimination channels, uncertainties remain.