Fuel Pyrolysis at Atmospheric and Elevated Pressure Conditions in Micro-Flow Tube Reactor

Improved efficiency and reduced emissions are essential elements of more sustainable propulsion systems. In addition to providing energy, the fuel in propulsion systems can be also used as coolant for critical engine components. Both the choice of fuel and understanding the fuel pyrolysis and oxidation behavior is critical for design of future propulsion systems. Specifically, well validated chemical kinetic models are needed in designing such systems. A fundamental approach in developing detailed chemical kinetic models is to start with the simplest fuel molecule and progressively increase the complexity of the molecular structure. Unfortunately, real fuels, and jet fuels in particular, consist of hundreds of hydrocarbon species and therefore the development of appropriate chemical kinetic models is challenging. As part of development of chemical kinetic models for these complex fuels, several surrogate models have been identified in order to simplify the modelling effort. Despite the fact that the surrogate reaction models represent a major simplification, they are still too large and require further simplification before implementation in computational simulation of propulsion systems.

The present research work aims at solving this problem by decoupling the fuel pyrolysis and oxidation processes. The idea of semi-global model with a fast-thermal pyrolysis of large fuel molecules, in combination with more detailed H2/C1-C4 base model, is considered. The work described here is mainly focused on the experimental aspects of fuel decomposition into H2 and C1- C4 species. For this purpose, a novel micro flow tube reactor (MFTR) with a small mixing volume was designed and developed. Extensive investigations were performed to better understand the uncertainties associated with characterization of temperature of the reactor and reactant composition. In particular, traditional reactor temperature measurements by thermocouples were verified by chemical thermometry concept. The fidelity of the reactor was tested by conducting pyrolysis experiments with better understood fuel molecules such as ethane and n-butane. A range of temperatures (950-1100 K), pressures (1-15 atm) and residence times (10- 590 ms) was explored and the experimental results were compared with several predictive models available in the literature as well as experimental data. A key finding is that experimental speciation data generated by MFTR has a much lower uncertainty compared with current model uncertainty. This implies that the MFTR can be a valuable tool in developing accurate chemical kinetic models.

The experimental work was also extended to homogeneous pyrolysis studies of large hydrocarbon fuels such as n-dodecane and JP-8 for a range of temperatures and residence times. Similar to small hydrocarbon fuels, the pyrolysis results were compared with available chemical kinetic models. Additionally, a one-step fast thermal pyrolysis model for JP-8 was also developed based on present experimental results. The model was able to capture most of the key species from the experiments. Finally, homogeneous and heterogeneous catalytic pyrolysis studies of JP-10 were conducted to explore the applicability of reactor to conduct heterogeneous pyrolysis studies in cooling channels of hypersonic engines. The results indicated almost 200 K shift in fuel pyrolysis temperature which can lead to enhance cooling capacity due to endothermicity of pyrolysis process.