Combustor Lean Blowout Performane Correlation with Pyrolysis Products from Jet Fuels - A Shock Tube Study

Understanding the impact of fuel characteristic differences on combustion behaviors such as cold start, high altitude relight, and lean blowout (LBO) limit can determine the application of alternative jet fuels. The LBO limit is one of the parameters considered when approving a new jet fuel. The physical and chemical properties of aviation fuels influence the combustion process near the blowout condition. This dissertation work has made an extensive effort to contribute towards the understanding of the role of chemical composition and resulting chemistry of jet fuels at the earliest stage of combustion on lean blowout. In addition, efforts were made to find a correlation between the measured LBO from the referee rig combustor and the pyrolytic products of alternative and conventional jet fuels. This study was conducted in the heated shock tube facility at the University of Dayton Research Institute (UDRI). In this work, attention was paid to studying the stable species from pyrolysis and their formation and consumption behavior by varying the experimental dwell time over a temperature range of 1050 - 1700 K. The data provides valuable information to comprehend the reaction kinetics and gain more insight on the decomposition pathway of fuels. Selected fuels include Jet A (A2), FT-Sasol, Gevo (C1), n-dodecane (n-C12), iso-dodecane (iso-C12), n-dodecane/ m-xylene (75/25 by Liq. vol.), and Jet A/ Gevo (80/20 by Liq. vol.). This list of fuels represents petroleum-derived jet fuel (Jet A), bio-derived alternative jet fuel (Gevo), synthetic alternative jet fuel (FT Sasol), petroleum/ bio-derived jet fuel blend (Jet A/ Gevo), long chain single compound paraffinic jet fuel surrogate (n-dodecane), branched single component alkane (iso-dodecane) and aromatic/ n-paraffin binary mixture (n-dodecane/ m-xylene). The measured emission indices of the light gaseous hydrocarbons (C1-C5) from the shock tube study were correlated with the measured LBO from a single nozzle combustor (SNC) study. A linear regression analysis was performed between each pyrolytic product emission index and the LBO for all the tested fuels. A strong correlation was observed between the LBO and ethylene EI measured for all the fuels at temperatures of 1050, 1250, and 1350 K at dwell times of 4 - 5 ms. The 1700 K condition showed a poor correlation between the LBOs and EIs for all pyrolytic products. LBO values were predicted using the obtained linear regression of the ethylene EI from the four temperature conditions in this study; despite the weak correlation with the ethylene EI from the shock tube at the 1700 K condition, all the predicted values are within ± 5% of the measured LBOs. The strong agreement between the calculated and measured LBOs suggests the possibility of predicting the LBO combustion metric performance based on the fuel composition and the primary decomposition products without the need for costly real combustor testing. For this study, the following kinetic mechanisms were used to gain a better understanding of the pyrolytic reaction kinetics: SERDP 2015 (used for n-C12 and n-C12/m-xylene blend), HyChem high temperature (used for A-2, C-1, and A-2/C-1), and RMG, mmc iso-dodecane (developed by Mao et al.) and LLNL iso-alkane high temperature (used for iso-C12). The modeling simulations were performed for all four temperature conditions and six dwell times at the shock tube experimental conditions. The pyrolytic modeling results show better agreement with the experimental results at 1050 K and 1250K than the 1370 K and 1700 K conditions. All four kinetic mechanisms have poorly predicted the acetylene behavior at all four temperature conditions