DOI: 10.25549/usctheses-c40-348218
title: Studies of atmospheric and subatmospheric combustion characteristics of neat and practical fuels

description: Studies of atmospheric and subatmospheric combustion characteristics of neat and practical fuels have been conducted. First half of the effort was done regarding a neat fuel or inother words, a single-component fuel. Laminar flame speeds of methyl formate/air mixtures have been measured at atmospheric and sub-atmospheric pressures, under conditions that limited data exist. The experiments were carried out in a counterflow configuration at an unburned mixture temperature of 333 K. Flow velocities were measured using particle image velocimetry and particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and gaseous phases increases. The data were modeled using two recently developed kinetic models of methyl formate oxidation, which showed significant disagreement in predicted values. Significant discrepancies between the experimental and predicted values of laminar flame speeds were also found for the entire pressure range considered, especially for fuel rich mixtures. Reaction path analysis revealed that the kinetics of one particular radical species, CH₂OCHO, which is produced directly from the fuel, does have a notable effect on the overall reactivity. The variation of laminar flame speed with pressure revealed a notably different behavior between experiments and simulations. Further insights into the sources causing the discrepancies were investigated through model-experiment comparisons of main branching species as well as sensitivity analysis of laminar flame speed with respect to kinetics is performed. ❧ The second half of the effort involves studies of more practical aviation jet and rocket fuels. The flame initiation generated by spark discharge and flame stabilization at a single fuel injector inside a gas turbine combustor together is referred to as relight. Due to an effort of decreasing nitrogen oxides (NOx) emission, new engine design using lean-direct-injection (LDI) concept has been developed. However, while this design is beneficial to reduce NOx emission, at the same time, it inhibits the ability to restart the engine at high altitude. Typical restart altitude ranges from 20,000 to 30,000 ft, which corresponds to combustor inlet pressure of 0.3 to 0.45 atm. As completion of engine restart process, the flame must spread around the combustor annulus to all fuel injectors. Therefore, failure to propagate at such condition, or susceptibility to flame extinction inside a gas turbine still remain evidence of failure to maintain relight conditions that could have serious safety implications. Experimental studies at such pressure conditions can be valuable to test the reliability of kinetic models at such high altitude conditions. Therefore, non-premixed flame extinction studies of aviation jet fuels are proposed at sub-atmospheric pressure conditions, where high altitude relight typically occur. ❧ Traditionally, surrogate fuel was used to test combustion kinetic models. However, not physical method guarantees such models‟ completeness. Not only it is impossible to match both physical and chemical properties with one specific surrogate mixture, but also its empirical approach requires exhaustive experimental validation. Therefore, more realistic and direct method is necessary. Such kinetic models can also be valuable when tested against relevant practical fuels. Studies have shown that large hydrocarbon fuel breaks down into about half a dozen intermediate molecules. Then, these fragments enter into flame zone to go through oxidation process to form combustion products. The overall jet fuel reaction kinetics can be, thus, approximated by combining an experimentally constrained fuel pyrolysis model along with a foundational chemistry model for the oxidation process of decomposition products. Jet and rocket fuel experimental data conducted at atmospheric and sub-atmospheric conditions will be used to evaluate the validity of such hybrid chemistry model.