Scramjet fuels autoignition study

A series of shock-tube experiments was conducted to measure and compare the ignition-delay times of several fuel candidates for scramjet propulsion and to evaluate the importance of fuel cracking on the autoignition of endothermic-fuel/product mixtures. Ignition delays of ethylene, heptane, and JP-10 were measured in dilute mixtures behind reflected shock waves for temperatures in the range 1100-1500 K, pressures of 3-8 atm, and equivalence ratios of 0.5-1.5. The experimental data were compared to results published for heptane and ethylene, and new ignition-delay correlations were determined from the combined data sets. The resulting expressions were also compared to prior work on methane and hydrogen. In addition, typical endothermic-fuel product mixtures (representing different degrees of cracking of the parent fuel) were simulated and their ignition-delay times measured fur equivalence ratios 0.5 and 1.0. The relative ignition-delay times for the different fuels were found to be methane > JP-10 congruent to heptane > reformed endothermic fuel > ethylene > hydrogen. The results support the premise that fuel cracking enhances ignition, However, autoignition of the endothermic reaction product mixture is not driven entirely by the constituent with the shortest ignition delay (i.e., ethylene or hydrogen). Furthermore, small changes in concentrations of the individual component species are not likely to make dramatic changes in the ignition-delay times of the fuel. The empirical data were compared to ignition-delay times predicted using detailed chemical kinetics models and they support the validity of published reaction mechanisms fur methane and heptane.