Studies of biodiesel surrogates using novel shock tube techniques [electronic resource]
Keywords: Mechanical Engineering.

In light of the finite supply of fossil fuels, concerns about the impact of combustion engine emissions, and increasing efficiency requirements for on-road vehicles, a search has begun for alternative energy resources. Such fuels would ideally have characteristics such as a high energy density, lower pollutant (hydrocarbon, soot, nitrogen oxide, etc.) emissions, domestic origins, and high miscibility with comparable existing fuels. With the possible exception of lower nitrogen oxide emissions, biodiesel fuel realizes all of these qualities and as such has become a leading candidate to blend with, supplement, or in some cases replace traditional fossil diesel fuel. To facilitate such changes in fuel stock, a comprehensive understanding of biodiesel oxidation chemistry is needed. In the work described herein, the Stanford University Aerosol Shock Tube (AST) was used to measure ignition delay times for six biodiesel surrogate molecules (called Fatty Acid Methyl Esters, or FAMEs) as large as methyl oleate (C19H36O2) at high temperatures (1100-1350 K), moderate pressures (3.5 and 7 atm), and both lean and rich equivalence ratios. This facility was able to make gas-phase measurements of these very-low-vapor-pressure fuels by loading the fuel as a spatially uniform mixture of fuel droplets suspended in an oxidizer/diluent gas blend; the droplets rapidly evaporated behind the incident shock wave. Temperatures behind both incident and reflected shock waves were calculated using a computer code that accounted for the enthalpy loss by the gas as the aerosol droplets changed phase. Moreover, a Fourier Transform InfraRed (FTIR) spectrometer was used to measure the spectra of eleven FAMEs, allowing quantitative fuel measurements during shock experiments. In addition to these aerosol-based experiments, novel shock tube techniques have been developed. The first involves confining the reactive test gas mixture to a small section of the tube using a sliding gate valve (called Constrained Reaction Volume (CRV) experimentation), thus enabling the measurement of ignition chemistry under constant-pressure conditions. The second involves expanding available shock tube test times by filling a low-sound-speed gas near the driver section end cap (called staged driver gas filling) to allow moderate-temperature, low-pressure oxidation experiments. Using these techniques, ignition delay times and time histories of intermediate/product species and temperature were measured during n-heptane (C7H16) oxidation at low temperatures (650-800 K) under constant-pressure conditions near 6.5 atm. The results revealed that many existing biodiesel kinetic mechanisms either under- or over-predict high-temperature ignition times for large biodiesel surrogates; subsequent modifications to high-temperature thermochemistry of biodiesel molecules resulted in more realistic fuel unimolecular decomposition rates and significantly better agreement between model and experiment. Measurements of FAME spectra showed that the absorption cross sections of saturated molecules at 3.39 microns, corresponding to the monochromatic output of a helium-neon (HeNe) laser, can be extrapolated using a simple relationship based on the number of C-H bonds in the molecule. CRV experimentation was shown to almost completely eliminate the pressure changes associated with both first and second stage ignition events during Negative Temperature Coefficient (NTC) region n-heptane oxidation. Comparisons between model and experiment indicated that at low temperatures, constant enthalpy-pressure (H-P) constraints are more appropriate for zero-dimensional shock tube simulations than constant internal energy-volume (U-V) constraints. Finally, the staged driver filling technique was found to increase shock tube test times by approximately 20%, while reducing necessary driver helium consumption by up to about 85%.