Every generation of advanced military aircraft fly higher and faster than previous generations. With these leaps in performance, aircrafts develop enormous heat loads which can exceed aircraft material limitations. To relieve these heat loads, aircraft can utilize the endothermic heat sink capacity of jet fuel realized through pyrolytic decomposition. Improved understanding of the effect of fuel chemical composition on supercritical pyrolytic reactivity under conditions relevant to advanced aircraft operation can assist with the successful development of viable cooling methodologies. The goal of the current study was to compare the pyrolytic reactivity, primary decomposition products, and global reaction rates of fuels with varying chemical composition.
A flowing reactor system was used to explore the pyrolytic chemistry of a Synthetic Paraffinic Kerosene (SPK) and specification jet fuels JP-8 and JP-7. The SPK was comprised solely of iso- and n-paraffins, with negligible cycloparaffin and aromatic content, while the specification fuels had chemical compositions consistent with typical petroleum-derived fuels. The pyrolytic studies were performed using stainless steel tube reactors which were 37.5 cm long and 0.5 mm inside diameter, with inlet flow rates of 0.2 to 0.6 mL/min at a pressure of 3.54 MPa. External reactor wall temperatures ranged from 500C to 650C. The liquid to gas conversion by mass was used as a metric for evaluating the pyrolytic reactivity due to the complex multicomponent composition of the test fuels. SPK averaged 45% higher conversion than JP-7 and 75% higher conversion than JP-8 at each respective temperature. All fuels followed similar reactivity trends with respect to controlling reaction chemistry, such as such as decomposition of long chain nparaffins, olefin formation, cycloparaffins formation, aromatic formation, and gas (e.g.,low molecular weight compound) production.
Characterization of the relative reactivity of the fuels was performed by assuming the fuels decomposed via a first order, irreversible reaction pathway with respect to the gravimetric liquid to gas conversion. The calculated reaction rates and temperature data were used to develop Arrhenius plots which yielded the following kinetic perimeters: SPK--pre-exponential (A) factor of 2.3 x 10^12 s-1 and activation energy (Ea) of 223 kcal/mol, JP-7--A of 2.1 x 10^12 s-1 and Ea of 226 kcal/mol, and JP-8--A of 4.6 x 10^12 s-1 and Ea of 235 kcal/mol. These parameters can be used to estimate the initial reactivity and decomposition of these fuels under endothermic conditions.
SPK fuels are more pyrolytically reactive compared to JP-7 and JP-8 using the liquid to gas conversion metric due to the variation in the neat chemical compositions. The mildly branched paraffins of the SPK with negligible cycloparaffins and aromatics, which can act as hydrogen donors reducing propagation rate, limited the reaction pathways resulting in a high liquid to gas conversion. JP-7 and JP-8 had a lower liquid to gas conversion due to the significantly higher initial concentrations of cycloparaffins and aromatics, thereby enabling these fuels to participate in a greater number of hydrogen donor reactions which lowers the extent of propagation reactions. Implications of these results can vary depending on the heat sink design and endothermic fuel cooling strategy. The propensity of SPK to react at lower temperature can enable SPK fuels to more readily reach the endothermic heating value. Unfortunately, a fuel with a higher pyrolytic reaction rate can also produce carbon deposition more readily. Further development into the design of a hypersonic heat exchanger system and the determination of the acceptable amount of liquid to gas conversion will dictate the optimal endothermic fuel.
Keywords: pyrolysis, endothermic jet fuels, synthetic paraffinic kerosene, hypersonic, endothermic heat sink, pyrolytic decomposition, JP-7, fischer tropsch, FT