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A TRAnsparent cylindrical (TRAC) shock tube test section has been developed to explore the ignition structure behind reflected shockwaves via simultaneous lateral and endwall high-speed imaging using three fuel/oxidizer mixtures: 0.1% n-C7H16/1.10% O2/4.14% N2/94.66% Ar, 0.1% iC8H18/1.25% O2/4.69% N2/93.96% Ar, and 5% CH4/10% O2/85% CO2. During mild ignition, the first exothermic centers were located outside of the observable test section. It is shown that with increasing temperature the far-wall ignition events move closer to the endwall, until a strong ignition is achieved. The temperature gradients that promote mild ignition were quantified and were found to have a slight dependence on fuel with the RMS temperature gradients being 0.80% for n-heptane and 1.07% for isooctane. The impact of temperature gradients on metrics used to improve kinetic models, such as species time histories, is noteworthy. From this temperature gradient, an experimental correlation is provided to estimate the transition temperature from mild to strong ignition for a mixture. The effect of diagnostic location in non-homogeneous reactions is quantified in reference to ignition delay. The unique and insightful perspective on the ignition process under heavy shock bifurcation is also discussed.

Ethanol oxidation was studied by measuring CO time-histories and ignition delay times behind reflected shockwaves at elevated pressures. In this study, experimental conditions included a temperature range of 960–1580 K, pressure range of 17.8–23.9 atm, and initial fuel concentrations of 6.54% and 0.25% with nitrogen and argon used as bath gasses, respectively. The equivalence ratio for high fuel loading was kept constant at 1.0, and for low fuel loading equivalence ratios were 1.0 and 0.5. For high fuel loading, early heat/energy release was observed in nearly all ignition delay time measurements, indicative of preignition; however, data collected are in good agreement with model predictions. These events are interpreted as a transition from mild to strong ignition. For the low fuel loading cases (ϕ = 1.0 and 0.5), no early heat/energy release was observed. Comparisons of measured CO concentration profiles with the predictions from kinetic mechanisms of Metcalfe et al. (2013), Mittal et al. (2014), and Zhang et al. (2018) were made assuming constant internal energy and volume for the test gas. Such comparisons in addition with performed sensitivity and pathway analyses for CO formation revealed lower temperature sensitivity to the bimolecular reaction of methyl and hydroperoxyl radicals given by CH3 + HO2 CH3O + OH, and H-atom abstraction by the hydroperoxyl radical at the alpha site on ethanol given by elementary reaction C2H5OH + HO2 sC2H4OH + H2O2. The current study provides important validation targets for ethanol chemical kinetic mechanisms and highlights the benefits of time-history measurements at various temperatures and pressures in a shock tube, which are scare in the literature.

To reduce the reliance on fossil fuels in the transportation sector and increase combustion efficiency, the Co-Optima initiative from US Department of Energy identified the top 10 biofuels for downsized, boosted, spark-ignition engines. Most of these biofuels have detailed reaction mechanisms available in literature developed based on studies at temperatures lower than 1700 K and an equivalence ratio of less than five. As such, the performance of these detailed mechanisms at high temperature and extremely rich conditions are unknown. It is important to validate kinetic mechanisms at these conditions because they are conducive to soot formation. Prediction of soot by chemical kinetic models relies on the prediction of underlying benchmark species like carbon monoxide and ethylene. In this work, we conduct high temperature (1700–2050 K) and high equivalence ratio(Φ=8.6) oxidation of these biofuels, namely 2,4,4-trimethyl-1-pentene (α-diisobutylene), ethanol, cyclopentanone, methyl acetate, and 2-methylfuran, blended in ethylene behind reflected shock waves at 4–4.7 atm pressure. Carbon monoxide and ethylene time histories are measured simultaneously using a continuous feedback quantum cascade laser near 4.9 µm and a tunable CO2 gas laser at 10.532 nm, respectively. Results show that ethanol blend forms more carbon monoxide than other biofuels and consumes ethylene faster than the biofuel blends in the temperature range considered. The performance of different mechanisms in literature are evaluated against the experimental results. The novel reaction mechanism ‘the Co-Optima model’, which includes the sub mechanisms for all the biofuels in this study, was found to be the best mechanism for the experimental conditions studied.

In this work, the effects of pre-ignition energy releases on H 2 O 2 mixtures were explored in a shock tube with the aid of high-speed imaging and conventional pressure and emission diagnostics. Ignition delay times and time-resolved camera image sequences were taken behind the reflected shockwaves for two hydrogen mixtures. High concentration experiments spanned temperatures between 858 and 1035 K and pressures between 2.74 and 3.91 atm for a 15% H 2 \18% O 2 \Ar mixture. Low concentration data were also taken at temperatures between 960 and 1131 K and pressures between 3.09 and 5.44 atm for a 4% H 2 \2% O 2 \Ar mixture. These two model mixtures were chosen as they were the focus of recent shock tube work conducted in the literature (Pang et al., 2009). Experiments were performed in both a clean and dirty shock tube facility; however, no deviations in ignition delay times between the two types of tests were apparent. The high-concentration mixture (15%H 2 \18%O 2 \Ar) experienced energy releases in the form of deflagration flames followed by local detonations at temperatures

Common definitions for ignition delay time are often hard to determine due to the issue of bifurcation and other non-idealities that result from high levels of CO2 addition. Using high-speed camera imagery in comparison with more standard methods (e.g., pressure, emission, and laser absorption spectroscopy) to measure the ignition delay time, the effect of bifurcation has been examined in this study. Experiments were performed at pressures between 0.6 and 1.2 atm for temperatures between 1650 and 2040 K. The equivalence ratio for all experiments was kept at a constant value of 1 with methane as the fuel. The CO2 mole fraction was varied between a value of X C O 2 = 0.00 to 0.895. The ignition delay time was determined from three different measurements at the sidewall: broadband chemiluminescent emission captured via a photodetector, CH4 concentrations determined using a distributed feedback interband cascade laser centered at 3403.4 nm, and pressure recorded via a dynamic Kistler type transducer. All methods for the ignition delay time were compared to high-speed camera images taken of the axial cross-section during combustion. Methane time-histories and the methane decay times were also measured using the laser. It was determined that the flame could be correlated to the ignition delay time measured at the side wall but that the flame as captured by the camera was not homogeneous as assumed in typical shock tube experiments. The bifurcation of the shock wave resulted in smaller flames with large boundary layers and that the flame could be as small as 30% of the cross-sectional area of the shock tube at the highest levels of CO2 dilution. Comparisons between the camera images and the different ignition delay time methods show that care must be taken in interpreting traditional ignition delay data for experiments with large bifurcation effects as different methods in measuring the ignition delay time could result in different interpretations of kinetic mechanisms and impede the development of future mechanisms.