Shock tube studies of hydrocarbon fuels at elevated pressures
Keywords: Mechanical Engineering.

The recent International Energy Outlook 2018 reports that approximately 80% of current global energy consumption is contributed by fossil fuels. Because of this, improving fuel usage efficiency and reducing fossil fuel pollutant emissions become two important requirements for modern engine design in order to address environmental problems and avoid climate crisis. In these efforts, chemical kinetics plays an essential and critical role. Yet, there are major gaps in our knowledge of the oxidation chemical kinetics of real fuels, and hence a critical need exists for ignition delay time (IDT), species time history, and reaction rate constant databases for the validation and refinement of the oxidation mechanisms, especially at elevated pressures. This dissertation covers improved methods and data for the three types of measurements in a high-pressure shock tube. The first part of this dissertation describes IDT measurements. A necessary requirement for the simple modeling of shock tube studies of auto-ignition behind reflected shockwaves is the assumption of a zero-dimensional homogeneous ignition process. However, there is growing evidence of inhomogeneous ignition occurring under certain conditions. Thus, homogeneous and inhomogeneous ignition modes of n-heptane were studied using high-speed imaging in a high-pressure shock tube (HPST) over a wide temperature range (700-1250 K), and elevated pressures (> 10 atm). Ultraviolet (UV) OH* images were captured through a sapphire shock tube end wall using a high-speed camera and a UV intensifier. The study identified an intermediate temperature zone for reactive n-heptane mixtures that is susceptible to inhomogeneous ignition. We also observed that conventional sidewall diagnostic signals can, in many cases, be sufficient to identify inhomogeneous ignitions that are not accurately modeled by spatially uniform chemistry. Followed by the study of ignition homogeneity, ignition delay time datasets were reported for several small to large hydrocarbons. Ignition delay times of small hydrocarbons at elevated pressures provide valuable constraints for the refinement of the core small-hydrocarbon sub-mechanisms used in all combustion kinetics. Current knowledge of these core mechanisms is based largely on low-pressure data, as only limited high-pressure data is available. To remedy this, the present study provided ignition delay times in methane, ethylene, propene and their blends at elevated pressures. IDT measurements were performed in 4% O2, balance Ar mixtures, over the temperature range of 950--1800 K, at pressures of 14--60 atm and equivalence ratios of 1 and 2. IDT was determined from sidewall pressure, OH* emission measurements and fuel time-histories measured using laser absorption at 3.39 μm. These measurements extended the test conditions of earlier studies, with the advantage that they have all been performed at similar conditions and with the same facility and provided a uniform set of kinetics targets for the evaluation of core small-hydrocarbon mechanisms. This dataset also allowed the temperature variation of the pressure and equivalence ratio scaling for methane and ethylene IDT to be investigated. To mimic the operating conditions of a super-critical CO2 turbine, the IDT measurements of small hydrocarbons were extended to higher pressures up to 300 atm. The need for more efficient power cycles has attracted interest in super-critical CO2 (sCO2) cycles. However, the effects of high CO2 dilution on auto-ignition at extremely high pressures have not been studied in depth. As part of the effort to understand oxy-fuel combustion with massive CO2 dilution, we have measured shock tube ignition delay times for methane/O2/CO2 mixtures and hydrogen/O2/CO2 mixtures using sidewall pressure and OH* emission near 306 nm. Ignition delay time was measured behind reflected shock waves over a range of temperatures, 1045--1578 K, in different pressures and mixture regimes, i.e., CH4/O2/CO2 mixtures at 27--286 atm and H2/O2/CO2 mixtures at 37--311 atm. Besides small hydrocarbons, the IDT measurements of large hydrocarbons were also extended to elevated pressures. Additionally, IDT measurements of real distillate fuels were reported in this work and in earlier studies. The kerosene fuels, gasoline and diesel fuel exemplars were found to correlate well with a single expression at high temperatures. Similar studies were conducted at lower temperature and higher pressures; the fuels tested included gasoline, gasoline with oxygenates, and two surrogate fuels, one dominated by iso-octane and one by toluene. RON/MON for the fuels varied from 101/94 to 106.5/91.5. Measurements were conducted in synthetic air at pressures from 30 to 250 atm, for temperatures from 700 to 1100 K, and equivalence ratios near 0.85. This dataset provided a critically needed set of IDT targets to test and refine boosted gasoline models at high pressures. The second part of this dissertation describes speciation measurements. At sufficiently high temperatures in real distillate fuel combustion, the pyrolysis and oxidation processes are effectively separable in time or spatial scales. The number of significant fuel decomposition products or intermediates is small, typically six to ten. The composition distribution of these rapidly-formed thermal decomposition products determines the combustion properties of the original, multi-component real fuel. At engine operating conditions, the dominant thermal decomposition product in many distillate fuels is ethylene (C2H4), while measurements of methane (CH4) yields can be related to the aromatic content of the original fuel. The development of advanced kinetics mechanisms for real distillate fuels requires datasets of pyrolysis product yields to constrain the model and of kinetic targets to evaluate the model. To this end, we have measured selected species time-histories during fuel pyrolysis using laser absorption behind reflected shockwaves. Measurements were performed for three different jet fuels diluted in air or argon over a temperature range of 1000--1400 K, a pressure range of 12--40 atm, and equivalence ratios of 0.5--1. Fuel loading was measured using an IR He-Ne laser at 3.39 μm; ethylene with a CO2 gas laser at wavelengths of 10532 nm and 10674 nm; and methane with a tunable diode laser at wavelengths of 3175 nm and 3177 nm. A similar study was conducted for a single-component fuel, JP-10. Methane and ethylene time histories during pyrolysis and ignition delay time (IDT) during oxidation were investigated behind reflected shockwaves using a heated, high-pressure shock tube for JP-10 (exo-tetrahydrodicyclopentadiene, C10H16). The pyrolysis experiments were performed over the temperature range of 1200-1370 K, pressure range of 16.5-18.3 atm and fuel concentration of 0.7%. The IDT experiments were performed over the temperature range of 960-1230 K, pressure range of 15.5-18.3 atm and stoichiometries of ϕ = 0.5, 1.0, 2.0. The third part of the dissertation describes reaction rate constant measurements. Ignition delay time is mainly used for the kinetics mechanism validation and refinement, but it can also be used for reaction rate constant determination in some operating conditions. After addressing impurity and non-ideal shock behavior issues and having a clear criterion for homogenous ignition, ignition delay time measurements of hydrogen mixtures were used to infer the reaction rate constant of H+O2+M ⇔HO2+M. The rate constants for the reaction H+O2+M ⇔ HO2+M were investigated at elevated pressures from 12 to 33 atm using ignition delay time measurements behind reflected shockwaves in H2/O2/M mixtures with different collision partners M = Ar, N2 and CO2. The temperature and pressure ranges where the rate constants of the reactions H+O2+M⇔HO2+M and H+O2⇔OH+O dominate the IDT sensitivity were selected as optimum test conditions using the detailed H2/O2 mechanism of Hong et al. (2011). The current study thus provided a quantitative and relatively direct method for determining the rate constants for H+O2+M⇔HO2+M using simple IDT measurements. The rate constants found are consistent with earlier studies, but with reduced uncertainties. A more typical apparent measurement is to use species time history measurement. As an example, the rate constant for the reaction C2H4+H⇔C2H3+H2 was studied behind reflected shockwaves at temperatures between 1619 and 1948 K and pressures near 10 atm in a mixture of C2H4, CH4, H2, and argon. C2H4 time histories were measured using laser absorption. Experimental mixtures were designed to optimize sensitivity to the title reaction with only weak sensitivity to secondary reactions. Two mechanisms, FFCM1 and ARAMCO v2, were used for data analysis. The well-selected operating conditions and Monte Carlo sampling data analysis procedure resulted in mechanism-independent reaction rate constant measurements with a 2σ uncertainty of ±35%. The current data disagree with a broadly used theoretical calculation (Knyazev et al. (1996)), but are in good consensus with the review study of Baulch et al. (2005). To the best of our knowledge, this work provided the first high-temperature study of the C2H4+H⇔C2H3+H2 reaction rate constant with well-defined uncertainty. In summary, this dissertation reports ignition delay time, speciation, and reaction rate constant studies in a high-pressure shock tube. The data reported should be useful as kinetic targets for chemical kinetics mechanism validation and refinement.