Autoignition characteristics of aircraft-type fuels

Abstract

An applied research program was undertaken to evaluate the autoignition characteristics of five liquid hydrocarbon fuels in air over ranges of air temperature, pressure, and equivalence ratio appropriate to advanced aircraft gas turbine engines. Ignition delay times were measured using a continuous flow test apparatus which permitted independent variation and evaluation of the effects of temperature, pressure, flow rate, and fuel/air ratio on ignition delay time. A multiple conical tube fuel injector, consisting of 19 parallel venturi elements with independent fuel control to each element, was used for the testing. Measurements of the spray distribution produced were made by isokinetically sampling the flow at reduced temperatures with water injection, and indicated that a nearly uniform fuel/air mixture distribution would be obtained. Parametric tests to map the ignition delay characteristics of Jet-A, JP-4, No. 2 diesel, cetane, and an experimental referee broad specification (ERBS) fuel were conducted at pressures of 10, 15, 20, 25 and 30 atm, inlet air temperatures up to 1000K, and fuel/air equivalence ratios of 0.3, 0.5, 0.7, and 1.0. Ignition delay times in the range of 1–50 msec at freestream flow velocities ranging from 20 to 100 m/sec were obtained. In accord with classical chemical kinetics, the ignition delay times for all fuels tested appeared to correlate with the inverse of pressure and the inverse exponent of temperature, viz., t= A pn exp E RT In general, the data were very repeatable. With the exception of pure cetane, which has the shortest ignition delay times, the difference between the fuels tested did not appear to be significant. The apparent global activation energies for the typical gas turbine fuels ranged from 38 to 43 kcal/mole, while the activation energy determined for cetane was 50 kcal/mole. In addition, the data indicate that, for lean mixtures, ignition delay times decrease with increasing equivalence ratio. It was also noted that physical (apparatus dependent) phenomena, such as mixing (i.e., length) and airstream cooling (due to fuel heating, vaporization, and convective heat loss) can have an important effect on the ignition delay.