We have developed a new aerosol loading technique to be used in shock tube measurements of combustion kinetics, in particular ignition times, of low-vapor pressure fuels. This technique provides a uniform spatial distribution of aerosol in the shock tube, which ensures well-behaved shock-induced flows and a narrow micron-sized aerosol size distribution that rapidly evaporates, thereby providing the capability to produce high-concentration vapor mixtures derived from a wide variety of fluids including low-vapor-pressure practical fuels and fuel surrogates. At present we utilize the incident shock wave to vaporize the fuel droplets, and the reflected shock wave to induce chemical reaction. We report here the first aerosol shock tube ignition delay time measurements of n-dodecane/O2/argon mixtures. These measurements are found to be consistent with those made in our heated shock tube facility.

This thesis describes a new facility and method of experimentation, which can be used to study the combustion chemistry of low-volatility fuels in the gas phase. Two main goals are described: first, the development of the aerosol shock tube and procedures; and second, a demonstration of its capabilities. There is a lack of high-quality, accurate chemical kinetics data for the oxidation of large hydrocarbons, which are important for modeling diesel, rocket, or jet engines among other combustion systems. While conventional shock tubes are very effective reactor vessels for low-molecular-weight gaseous fuels (n-alkanes up to five carbon atoms), larger fuel molecules exist as low-volatility liquids/solids, and the vapor-pressures of these fuels are not large enough for high or even moderate fuel loadings. Heating the shock tube has extended the use of shock tubes to carbon numbers of 10 to 12, but beyond that, the high temperatures prior to the shock initiation can decompose the fuel, and (for fuel mixtures like diesel) can cause fractional distillation. The question is then: how can we study low-vapor-pressure fuels in a shock tube? The solution presented here, which avoids the problems associated with heating, is called the aerosol shock tube. In the aerosol shock tube, the fuel is injected as an aerosol of micron-size droplets. Then a series of shock waves first evaporate the fuel and subsequently raise the resultant purely gas-phase mixture to combustion-relevant temperatures. With proper selection of the shock strength and timing, this process effectively decouples the mass and heat transfer processes associated with evaporation from the chemical mechanism of combustion. This enables the study of extremely low-volatility fuels, never before studied in a purely gas-phase form in a shock tube. The first application of this new facility was to measure the ignition delay time for many previously inaccessible fuels in the gas-phase. In this thesis, we have measured ignition delay times for the pure surrogate fuel components n-decane, n-dodecane, n-hexadecane, and methyl decanoate as well as for multi-component fuels such as JP-7 and multiple different blends of diesel fuel. Taken over a range of conditions, these measurements provide sensitive validation targets for their respective chemical mechanisms. These data showed agreement with past heated shock tube experiments for fuels in which premature fuel decomposition is not an issue (n-decane and low concentration n-dodecane). However, when comparing heated and aerosol shock tube ignition delay times for fuels that require significant heating, like n-hexadecane, the existing heated shock tube data demonstrated evidence of premature decomposition. The second application to the study of chemical kinetics was to measure the concentration of important species during the decomposition and oxidation of select low-vapor-pressure fuels. These species time-histories provide much more information for kinetic mechanism refinement. Experiments were performed to measure the important OH radical and the stable intermediate C2H4 for both n-hexadecane and diesel. The number of important low-vapor-pressure fuels that require high-quality validation targets is large, and our new method for providing this data has proven very effective. This work enables the development of the next generation of accurate chemical mechanisms and will be essential to their success.

We report results of high-temperature shock tube research aimed at improving knowledge of the combustion behavior of diesel, jet and related fuels. Research was conducted in four Stanford shock tube facilities and focused on the following topics: (1) development of the aerosol shock tube; (2) ignition delay time measurements of gaseous jet fuels (JP-8 and Jet-A) and surrogate components at high pressures and low temperatures; (3) laser absorption measurements of species time-histories for OH radicals and alkanes; (4) ignition delay times of n-dodecane, jet fuel and diesel using the aerosol shock tube technique; and (5) improving shock tube performance and modeling.

Several techniques have been developed for obtaining long, constant-pressure test times in reflected shock wave experiments in a shock tube, including the use of driver inserts, driver gas tailoring, helium gas diaphragm interfaces, driver extensions, and staged driver gas filling. Here, we detail these techniques, including discussion on the most recent strategy, staged driver gas filling. Experiments indicate that this staged filling strategy increases available test time by roughly 20 % relative to single-stage filling of tailored driver gas mixtures, while simultaneously reducing the helium required per shock by up to 85 %. This filling scheme involves firstly mixing a tailored helium-nitrogen mixture in the driver section as in conventional driver filling and, secondly, backfilling a low-speed-of-sound gas such as nitrogen or carbon dioxide from a port close to the end cap of the driver section. Using this staged driver gas filling, in addition to the other techniques listed above, post-reflected shock test times of up to 0.102 s (102 ms) at 524 K and 1.6 atm have been obtained. Spectroscopically based temperature measurements in non-reactive mixtures have confirmed that temperature and pressure conditions remain constant throughout the length of these long test duration trials. Finally, these strategies have been used to measure low-temperature n-heptane ignition delay times.

Chemical kinetic modelers make extensive use of shock tube ignition data in the development and validation of combustion reaction mechanisms. These data come from measurements using a range of diagnostics and a variety of shock tubes, fuels, and initial conditions. With the wide selection of data available, it is useful to realize that not all of the data are of all the same type or quality, nor are all the data suitable for simple, direct comparison with the predictions of reaction mechanisms. We present here a discussion of some guidelines for the comparison of shock tube ignition time data with reaction mechanism modeling. Areas discussed include: definitions of ignition time; ignition time correlations (with examples taken from recent n-heptane and iso-octane measurements); shock tube constant-volume behavior; shock tube diameter and boundary layer effects; carrier gas and impurity effects; and future needs and challenges in shock tube research.

Mathematical Modelling of Gas-Phase Complex Reaction Systems: Pyrolysis and Combustion, Volume 45, gives an overview of the different steps involved in the development and application of detailed kinetic mechanisms, mainly relating to pyrolysis and combustion processes. The book is divided into two parts that cover the chemistry and kinetic models and then the numerical and statistical methods. It offers a comprehensive coverage of the theory and tools needed, along with the steps necessary for practical and industrial applications. - Details thermochemical properties and "ab initio" calculations of elementary reaction rates - Details kinetic mechanisms of pyrolysis and combustion processes - Explains experimental data for improving reaction models and for kinetic mechanisms assessment - Describes surrogate fuels and molecular reconstruction of hydrocarbon liquid mixtures - Describes pollutant formation in combustion systems - Solves and validates the kinetic mechanisms using numerical and statistical methods - Outlines optimal design of industrial burners and optimization and dynamic control of pyrolysis furnaces - Outlines large eddy simulation of turbulent reacting flows