Shock Tube Studies Of Hydrocarbon Fuels At Elevated Pressures

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.

Supercritical carbon dioxide (sCO2) cycles are being investigated for the future of power generation. These cycles will contribute to a carbon-neutral future to combat the effects of climate change. These direct-fired closed cycles will produce power without adding significant pollutants to the atmosphere. For these cycles to be efficient, they will need to operate at significantly higher pressures (e.g., 300 atm for Allam Cycle) than existing systems (typically less than 40 atm). There is limited knowledge on combustion at these pressures or at the high dilution of carbon dioxide. Nominal fuel choices for gas turbines include natural gas and syngas (mixture of CO and H2). Shock tubes study these problems in order to understand the fundamentals and solve various challenges. Shock tube experiments have been studied by the author in the sCO2 regime for various fuels including natural gas, methane and syngas. Using the shock tube to take measurements, pressure and light emissions time-histories measurements were taken at a 2-cm axial location away from the end wall. Experiments for syngas at lower pressure utilized high-speed imaging through the end wall to investigate the effects of bifurcation. It was found that carbon dioxide created unique interactions with the shock tube compared to tradition bath gasses such as argon. The experimental results were compared to predictions from leading chemical kinetic mechanisms. In general, mechanisms can predict the experimental data for methane and other hydrocarbon fuels; however, the models overpredict for syngas mixtures. Reaction pathway analysis was evaluated to determine where the models need improvements. A new shock tube has been designed and built to operate up to 1000 atm pressures for future high-pressure experiments. Details of this new facility are included in this work. The experiments in this work are necessary for mechanism development to design an efficient combustor operate these cycles.

Combustion, whether of fossil fuels or renewable fuels, is the dominant way the overwhelming demand for energy is currently met and will be for the foreseeable future. This has led to the production of harmful emissions that have a significant impact on the health of both the planet and humans. In order to improve the efficiency and cleanliness of combustion processes, improved knowledge of fundamental combustion chemistry is required. Recognizing the importance of understanding how intermediate species behave in combustion processes, particularly as related to the species that have been linked to forming harmful emissions, the work covered in this dissertation can be divided into two categories. The first is the development of diagnostics to measure key species that are intermediates in some combustion chemistry, and the second is experimental studies of ringed and small hydrocarbon decomposition chemistry employing those diagnostics, and the kinetic insights arising from that data. Arising from the need to characterize the decomposition chemistry of exo-tetrahydrodicyclopentadiene (JP-10), a diagnostic capable of measuring cyclopentadiene (CPD) in combustion conditions was developed. Using room temperature spectra and high temperature cross-section measurements, a diagnostic wavelength balancing high CPD absorbance with the existing knowledge base of other absorbing species was selected. This diagnostic was developed over a wide temperature range, and the absorbance of CPD characterized at other relevant wavelengths. The new CPD measurement tool was then employed in conjunction with a plethora of other laser diagnostics to characterize JP-10 pyrolysis chemistry between 1166 K to 1522 K at 2.5-3 atm. Using multiple measurement schemes and additional constraints, values for ethylene, propene, cyclopentadiene, methane, benzene, toluene, 1,3 butadiene, allene, and JP-10 fuel were reported, along with the overall decomposition rate constant for JP-10. Select portions of this data were used to constrain a HyChem model of JP-10 chemistry. Allene and propyne, two C3H4 isomers, were found to be intermediate species in the decomposition of many real fuels and contributors to forming polycyclic aromatic hydrocarbons, so it was desired to be able to measure their species time-histories. Employing the new capability of rapid scanning MIRcat-QCL lasers, the high temperature spectra of allene and propyne were measured for the first time. Using the measured spectra, wavelengths for allene and propyne diagnostics were selected carefully to minimize interference. Once selected, the diagnostics were characterized over 1196-1502 K, at 1.3-1.6 atm. These two tools were then used to measure the isomerization rates of allene to propyne and propyne to allene. These rate constants were found to be in good agreement with the average of past experimental determinations and recently computed rate expressions. Finally, small ringed hydrocarbon decomposition was studied, as these species are known to produce resonance-stabilized radicals that lead to PAH formation. Using the CPD diagnostic, improved and coupled with several other measurement tools, acetylene, ethylene, and cyclopentadiene time-histories were measured in cyclopentadiene and cyclopentene pyrolysis. Measurements for CPD pyrolysis were made over 1319 K to 1678 K at 1.2 - 1.5 atm, while cyclopentene measurements were made from 1220 K to 1681 K at pressures between 1.3 and 1.6 atm. Comparisons were made to multiple chemical kinetic models of different approaches, and the differences were examined. The work detailed in these studies illustrates the extensive capabilities that laser diagnostics employed on a shock tube provide for measuring combustion chemistry. The two new diagnostic tools developed for this work will enable future measurements of important combustion intermediates in real fuel chemistry, while the species time-histories and rate constants presented can be used to constrain and validate kinetic models.

We report results of basic research aimed at improving knowledge of the combustion behavior of diesel and jet-related fuels. The work is intended to develop a reference database of gas-phase chemical kinetics and two-phase spray measurements applicable to engine modeling. Research is being conducted in three Stanford shock tube facilities and focuses on two topics: (1) shock-induced ignition time and species time-history measurements and comparisons with current detailed kinetic models of jet fuels and cyclo-alkanes at both high and low pressures; (2) fundamental studies of fuel spray evaporation rates and ignition times of low-vapor pressure fuels such as JP-8, diesel fuel and normal alkane surrogates in a new aerosol shock tube using state-of-the-art optical diagnostic and imaging techniques.

Increasing energy demands, and the subsequent need for cleaner energy conversion to combat climate change, creates a challenge that requires both short- and long-term solutions. To that end, new energy conversion cycles such as the Allam-Fetvedt cycle uses the combustion products (CO2) as the working fluid to increase efficiency and reduce emissions. There are several challenges regarding the implementation of these cycles, namely the extreme combustor conditions required (approximately 300 bar). The new High Pressure, Extended Range Shock Tube for Advanced Research (HiPER-STAR) was designed, built, and characterized to study combustion at these conditions to aid in the development of these sCO2 systems, among other extreme environments such as rocket chamber conditions. Further, development of chemical kinetics models used to predict combustion in these conditions typically assume reactions only in the homogeneous bulk gas region, while in these systems there are stagnation regions where hot gases are in contact with a heated wall for extended durations. Heterogeneous reactions are historically difficult to study, as typically there are coupled gas dynamic and transport-related complications that affect the reactions. A shock tube is an ideal location to mitigate and decouple these effects. The current work explores reactive and non-reactive end wall effects at high pressure, an area of interest for implementation by industry and resultantly where better efficiency can be achieved. Further designs have been completed and fabrication is underway to improve the capabilities of the facility to better decouple thermal wall effects and catalytic surface effects, as well as improve other combustion diagnostic capabilities of the facility.