Understanding the role of multifunctional nanoengineered particulate additives on supercritical pyrolysis and combustion of hydrocarbon fuels/propellants
Open Access
- Author:
- Sim, Hyung Sub
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 17, 2015
- Committee Members:
- Richard A Yetter, Dissertation Advisor/Co-Advisor
Richard A Yetter, Committee Chair/Co-Chair
Domenic Adam Santavicca, Committee Member
Adrianus C Van Duin, Committee Member
Robert Martin Rioux Jr., Committee Member - Keywords:
- Supercritical Pyrolysis
Supercritical Combustion
Particulate Fuel Additives
Graphene
Platinum Nanoparticles
Hydrocarbons - Abstract:
- This dissertation aims to understand the fundamental effects of colloidal nanostructured materials on the supercritical pyrolysis, injection, ignition, and combustion of hydrocarbon fuels/propellants. As a fuel additive, functionalized graphene sheets (FGS) without or with the decoration of metal catalysts, such as platinum (Pt) or polyoxometalates (POM) nanoparticles, were examined against conventional materials including nanometer sized fumed silica and aluminum particles. Supercritical pyrolysis experiments were performed as a function of temperature, residence time, and particle type, using a high pressure and temperature flow reactor designed to provide isothermal and isobaric flow conditions. Supercritical pyrolysis results showed that the addition of FGS-based particles at a loading concentration of 50 ppmw increased the conversion rates and reduced apparent activation energies for methylcyclohexane (MCH) and n-dodecane (n-C12H26) fuels. For example, conversion rates, and formations of C1-C5 n-alkanes and C2-C6 1-alkenes were significantly increased by 43.5 %, 59.1 %, and 50.0 % for MCH decomposition using FGS19 (50 ppmw) at a temperature of 820 K and reduced pressure of 1.36. In addition, FGS decorated with 20 wt % Pt (20wt%Pt@FGS) at a loading concentration of 50 ppmw exhibited additional enhancement in the conversion rate of n-C12H26 by up to 24.0 % compared to FGS. Especially, FGS-based particles seem to alter initiation mechanisms, which could result in higher hydrogen formation. Hydrogen selectivities for both MCH and n-C12H26 decompositions were observed to increase by nearly a factor of 2 and 10, respectively. Supercritical injection and combustion experiments were conducted using a high pressure and temperature windowed combustion chamber coupled to the flow reactor through a feed system. Supercritical injection/combustion experiments indicated that the presence of a small amount of particles (100 ppmw) in the fuel affected the injection, ignition, and subsequent combustion for hydrocarbon fuels. For instance, the addition of Pt@FGS reduced the core lengths of the supercritical jets by up to 56.7 % (MCH) and 68.8 % (n-C12H26) depending on flow rate. Supercritical combustion studies demonstrated that the addition of 100 ppmw 20wt%Pt@FGS to n-C12H26 fuel reduced ignition delay times by nearly a factor of 3 (12.4 to 4.1 ms), increased spreading angles by approximately 32.0 % (15.4 to 20.3o), reduced the flame lift-off length by 54.0 % (1.74 to 0.8 mm), and demonstrated an increase in conversion by 35.0 % relative to the pure fuel baseline at a volumetric flow rate of 5.0 mL/min. The enhancing mechanisms of FGS-based materials on pyrolysis and combustion were studied using ReaxFF molecular dynamics (MD) simulation. The simulation results were in good agreement with the experimental observations, showing enhanced conversion rates and lowered activation energies in the presence of the particles. A combination of Pt and FGS facilitated catalytic dehydrogenation forming a n-C12H25 radical. Another catalytic initiation step is hydrogenation of the fuel molecule resulting in production of an alkanium ion, as a free hydrogen atom is donated to the fuel molecule. Furthermore, hydroxyl radical (OH), which is dissociated from FGS, was found to participate in dehydrogenation of the fuel forming water molecules. The recovery of the catalyst was observed, resulting from dissociation of H2 molecule from the surface of the Pt. Using the ReaxFF embedded with nudged elastic band (NEB) method, it was found that the bicomposite structure of Pt@FGS served to lower the reaction energy and energy barrier for dehydrogenation, which could result in the enhanced conversion rates and increased product yields. These results demonstrate that a low mass loading of a high surface area material employed either as an additive or a means of distributing another additive can significantly enhance, and be used to tailor, the conversion of liquid hydrocarbon fuels/propellants under supercritical conditions, resulting in reduced ignition delay time and improved combustion. Such enhancements benefit practical propulsion systems which require high conversion efficiency in a short residence time.