Studies of combustion characteristics of heavy hydrocarbons in simple and complex flows

The main focus of this dissertation was the experimental and numerical investigations of laminar flames of heavy liquid and solid hydrocarbons under simple (one-dimensional, steady state flow field using canonical configuration) and complex (two/three-dimensional, transient flow at high Karlovitz number) flow conditions. A number of theories that have developed based on simplified assumptions and asymptotic analysis and more important for light fuels such as methane, were examined both experimentally and numerically in two steady state and canonical configuration, namely counter-flow configuration and Bunsen flame configuration. The counter-flow configuration was used to determine laminar flame speeds and extinction strain rates over a wide range of heavy hydrocarbons including normal alkanes (up to carbon number 16), practical gasolines and jet fuels and aromatics (cyclopentadiene). The analytical solution derived from asymptotic analysis provides good agreement for laminar flame speeds for fuel lean conditions. However notable discrepancies have been identified for fuel rich conditions due to lack of consideration of fuel-oxygen differential diffusion especially for heavy fuels for which the molecular weight disparity between oxygen and fuel is large. For the Bunsen flame configuration, the area and angle methods were examined to measure laminar flame speeds of methane/air flames (representative of light fuel) and propane/air flames given that propane is the lightest hydrocarbon with distinctly higher molecular weight than oxygen. The results indicated that apart from issues raised from inlet boundary condition, flame extinction induced complex flow distribution at burner edge and flame tip effect, such configuration can’t produce quantitative results for fuels heavier than methane due to lack of consideration of flame speed variation to stretch for fuel/air mixtures with non-unity Lewis number. Based on the understanding of the propagation of flames of heavy fuels, accurate measurements of laminar flame speeds were carried out using the counter-flow configuration at atmospheric pressure for a variety of complex fuel molecules for which data are non-existing and which are of direct relevance to practical fuels. The interaction between a flame and turbulence is a fundamental aspect of combustion. To further illustrate the difference of flame behaviors between light and heavy fuels, the vortex laminar flame interaction was studied numerically in a canonical two-dimensional configuration for methane and n-dodecane flames. The n-dodecane exhibits early decomposition prior entering the flame due to the local temperature rise caused by the vortex, and such phenomenon is not observed in methane/air flames. In summary, the main conclusion of this dissertation is that the fuel complexity that has been frequently ignored in flame research needs to be accounted for in simple and complex flows. It was shown that the fuel effects are both of physical and chemical nature