Scramjet Fuels Autoignition Study

Abstract

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 re ected 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 for equivalence ratios 0.5 and 1.0. The relative ignition-delay times for the different fuels were found to be methane > JP-10 = » » 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 for methane and heptane. Introduction I N scramjets rapid spontaneous ignition and reaction of the fuelair mixture are required to achieve ef cient combustionin a practical length. The relatively long ignition-delay times of hydrocarbons relative to hydrogen present a key obstacle to development of storable-fueled scramjets. In addition, autoignition affects heatrelease rates, and, if too rapid, it can promote dynamic instabilities or choking.High-speed propulsion systems are being designed to utilize storable hydrocarbon fuels for cooling and endothermic decomposition of the fuel to increase the available heat sink. Implementation of endothermic reaction technology involves cracking the fuel into a mixture of small hydrocarbons and hydrogen in a heatexchanger/reactor prior to combustion. This reaction will result in the formation of some species (e.g., ethylene and hydrogen) that are more reactivethan the parenthydrocarbonand others(e.g., methane) that are less reactive. Therefore, the design and development of endothermic-fueledcombustors require knowledge of the autoignition characteristics of typical cracked product mixtures as well as the component species.Consequently, shock-tube experiments were performed to measure the ignition-delay times of several candidate scramjet fuels, namely ethylene,heptane,and JP-10, as well as model endothermicfuel/cracked product mixtures. Testing of ethylene...

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“…Such correlations are historically Deleted: again used to provide estimates of ignition delay times over the range of validity of the given relation [8,11,12,[46][47][48].…”

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