A flow reactor study of pyrolysis and oxidation characteristics of N-dodecane [electronic resource]

The design of the next generation of energy efficient and fuel flexible air breathing propulsion systems requires the development of increasingly sophisticated turbulent combustion models that incorporate reduced but realistic finite-rate chemical kinetic mechanisms for fuel oxidation and pyrolysis. N-dodecane (n-C12H26) is a straight chain alkane that is often recommended as an important constituent of the 3-5 component surrogate blends that are used for modeling the combustion characteristics of aviation fuels like JP-8 or Jet-A. Therefore, an experimental investigation of n-dodecane oxidation and pyrolysis kinetics under engine relevant conditions is necessary for the development and validation of chemical mechanisms that seek to predict the combustion characteristics of jet fuels. In the current work, the Stanford Variable Pressure Flow Reactor (VPFR) facility was used to investigate n-dodecane pyrolysis and oxidation at the less studied intermediate temperature regime (950K - 1250 K) in a vitiated environment and at 1 atmosphere pressure. The initial stages of fuel decomposition were investigated in a 1000 K pyrolysis run and a 1050 K rich oxidation run, while the final oxidation and heat generation stages were investigated in 1170 K lean oxidation and 1220 K rich oxidation runs. The species time history data for n-dodecane and oxygen, as well as for 12 intermediate product species including C2H4, CH4, H2, C3H6, C2H2, CO, CO2, CH2O and several of the higher 1-alkenes, were measured using online Gas Chromatography over a 1-40 ms residence time. The experimental data were used to validate an optimized version of the detailed JetSurf 1.0 mechanism as well as to compare against several other detailed models that are used in the combustion community. The optimized JetSurf model performed relatively well against the flow reactor measurements, though the selected detailed mechanisms were observed to diverge significantly from each other in their species time history predictions. The cause of these divergences was identified to be uncertainties in several of the C1- C3 reaction rates and thermochemistry values that are responsible for radical build up and for ethylene generation and oxidation kinetics. The experimental results also pointed to a temporal scale separation between the initial stages of fuel breakdown that occur through essentially pyrolytic reactions into a mix of 1-alkenes, and the later stages where the alkenes undergo oxidation to form CO and CO2. A new methodology of kinetic mechanism reduction was proposed that lumped the pyrolysis stages into a small set of one-step reactions that directly convert the fuel into the final cracked products. An initial proof of concept validation of the new lumping scheme was done by comparing the predictions from a lumped version of the JetSurf mechanism with the flow reactor measurements. The results were found to be promising enough to warrant the further development of the lumping methodology in future investigations.

Keywords: Mechanical Engineering.
Advisor: Bowman Craig T. (Craig Thomas)
Hanson Ronald advisor. Thesis advisor
Wang Hai