Additives to Improve Fuel Heat Sink Capacity in Air/Fuel Heat Exchangers

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Hypersonic air-breathing vehicles travel at high speeds and generate heat loads that are greater than can be met from sensible heating of the fuel. However, the fuel can provide about 50% more heat sink capacity if it undergoes endothermic thermal cracking reactions before combustion. Unfortunately, thermal cracking reactions require high temperatures, in excess of 100°F, to obtain useful reaction rates. However, a fuel additive has been identified that increases the rate of thermal cracking reactions, allowing the same chemical endotherm to be obtained at lower fuel temperatures. This paper describes work in which we conducted extensive laboratory bench-scale calorimetric tests to directly measure the effectiveness of the cracking initiator compound with several prospective fuels, including JP-7 and normal decane. We then used the data obtained from the laboratory experiments to generate rate models, which we then used in the design of a pilot scale fuel/air heat exchanger. Finally, we tested the pilot scale unit at heat fluxes approaching 100,000 Btu /ft 2 ·h with JP-7 and n-decane with and without the initiator. At each test point, the data clearly indicated that the initiator produced significant increases in the rate of cracking and fuel heat sink capacity.