Alternative Fuels and Combustion Strategies for Emissions Reductions: Experimental Studies of C3 Fuel Kinetics

Abstract

Identification of alternative fuel sources and combustion strategies are crucial to realizing emissions reductions in the global power and transportation sectors necessary to combat climate change. The use of alcohols produced from biomass, low temperature (<1300 K) combustion, and propulsion systems driven by detonation are three potential emissions reduction strategies driving recent research topics in combustion science. Each faces specific implementation challenges requiring improved understanding of the underlying combustion science to achieve their long-term emissions reduction potential. This dissertation presents fundamental studies of the chemical and physical mechanisms of three fuels of interest to advanced power and propulsion sectors. The studies leverage novel experimental methods to fill gaps in the knowledge of each fuel’s chemistry or gas dynamics. The first study focuses on understanding the reaction pathways important during iso-propanol (a potential biofuel with advantages in low-carbon transportation applications) pyrolysis at low temperatures and moderate pressures. The technical approach used the University of Michigan rapid compression facility (UM-RCF) to achieve desired state conditions while fast-gas sampling and gas chromatography quantified concentrations of iso-propanol and seven stable intermediate species at temperatures of 965 – 1193 K and pressures of 4.4 – 10.0 atm. The results validated dominant decomposition reactions but identified discrepancies in expected rate of iso-propanol decomposition at the highest temperature and in the expected branching pathways producing and consuming ethane at the lowest temperature. The second and third studies focused on ignition characteristics of propane at low temperatures and moderate pressures. Propane is an important fuel for heating and processing and an important alkane for developing hierarchical combustion chemistry. Two studies on propane ignition behavior within the UM-RCF and Tsinghua University rapid compression machine (TU-RCM) were conducted to measure the impacts of localized thermal gradients and thermal boundary layers on ignition characteristics both within and outside of the negative temperature coefficient (NTC) region for temperatures from 744 – 1070 K and pressures from 8.9 – 25.4 atm at equivalence ratios of ϕ = 0.25, 0.5, and 1.0. High speed imaging and pressure measurements were used to identify ignition characteristics and their impact on ignition delay times (IDT). Inhomogeneous, or “weak/mixed”, ignition exhibited meaningful differences between observed IDT and model predictions. Imaging data of thermal boundary layers spanning the NTC region appear to show a stratification of ignition behavior within the reaction chamber but did not demonstrate notable irregularities when compared to model predictions. The results reveal the complexity of interpreting experimental data in weak ignition regimes and complications introduced by NTC behavior. Lastly, the study of iso-propyl nitrate (IPN) as a sensitizer for detonation of propane focused on quantifying the detonation transition characteristics of IPN and propane mixtures, which is important for development of practical pressure-gain engines. The experiments were conducted using the detonation tube at the Centre National de la Recherche Scientifique (CNRS) in Orléans, France. Wave speeds, detonation cell sizes, and critical conditions required for detonation were quantified over a range of conditions. The data showed the addition of 10% IPN increased sensitivity to detonation of propane-oxygen mixtures in both dilute and non-dilute mixtures by decreasing the critical conditions by ~5% and ~10% respectively and decreasing the cell size observed in non-dilute mixtures by ~20%, demonstrating that IPN is a promising detonation sensitizer for applications relevant to the development of pulse and rotating detonation engines.