Fine particulate matter (PM) emissions from military gas turbine engines, associated with the formation of soot particles, are of increasing concern because of evidence linking these particles to direct health effects and their contribution as cloud condensation nuclei and to the formation of contrails in the upper atmosphere. Studies have been conducted in which empirical relations have been derived relating fuel composition and engine operating conditions to observed soot emissions; however, these studies have not provided enough detailed understanding to allow a predictive capability to be developed regarding specific fuel composition and engine operation effects. Similarly, a number of fundamental studies into soot formation of complex fuels in simple flames and flow arrangements have been conducted, but effective means of bridging this information into the practical turbulent flame environment of gas turbine engines have not been achieved.
The objective of this project was to develop a reduced chemical model and associated experimental data that permit accurate predictions by combustor models of engine-out fine PM emissions, dominated by soot, from military gas turbine engines.
Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel consisting of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to typical JP-8 fuel samples, demonstrating that the surrogate was representative of JP-8, with a tendency to strong soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation. Even in the higher temperature flames, the soot particles demonstrated liquid-like behavior. Doping of benzene into ethylene fuel and operating the burner on n-dodecane had little influence on the trends previously established for ethylene fuel. Significant quantities of aliphatic carbon were present in soot sampled from the premixed flames, increasing with flame temperature and height above the flame.
An array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners with well-defined boundary conditions. Mean and statistical soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and polycyclic aromatic hydrocarbons. Time records of local soot concentration-temperature were collected, as well as spatially resolved thermal radiation emitted from the flames. Measurements of red laser light extinction across the flames provided useful data for correcting the soot concentration measurements for signal trapping in these optically thick flames.
A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. An attempt was made to develop a detailed mechanism for the JP-8 surrogate, but the existing knowledge of m-xylene chemistry was found to be insufficient to yield suitable agreement with validation data. The reduced ethylene mechanism was incorporated into a high-fidelity large eddy simulation (LES) code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically thick radiation model to accurately predict gas temperatures and thus soot formation rates. When including such a radiation model, the LES model predicts mean soot concentrations within 30% in the ethylene jet flame.
The results of this project suggest that LES modeling, when incorporating suitably reduced chemical kinetics with fuel-rich chemistry and a suitable, optically thick radiation model, can predict soot formation with good accuracy in an ethylene nonpremixed jet flame (at 1 atm) when using a fairly simple soot model (developed explicitly for application to ethylene flames). This model for soot formation and oxidation is available for use by engine designers to reduce soot emissions in future engines and evaluate the effects of fuel composition and the use of fuel additives on soot emissions. Extension of this predictive ability to more complex fuels representative of JP-8 requires improvements in the understanding of aromatic oxidation and pyrolysis chemistry and may require further improvements to the soot model itself.