Ignition Delay Times of a Range of Alternate Jet-Fuels and Surrogate Fuel Candidate Hydrocarbons under Fuel-Lean Conditions: a Shock Tube Study

Abstract

The ignition delay times in the combustion of jet fuels under fuel-lean conditions provide critical information with regard to the high altitude relight and lean blowout limit (LBO). In this study, the ignition delay times were measured behind reflected shock waves under high pressures and fuel-lean conditions for conventional (JP-8), bio-, and Fischer−Tropsch��~ FT) processed alternate jet fuels. In addition, the ignition delay of several hydrocarbons proposed as components in surrogate jet-fuels - n-heptane, n-dodecane (the normal paraffinic component), m-xylene (the aromatic component), and a blend of n- dodecane and m-xylene (77:23 (liquid vol. %)) - was investigated. One of the primary goals of this study was to evaluate the feasibility of the tested alternate fuels as drop-in replacements for conventional jet fuels. This required an understanding of how differences in fuel composition could affect the ignition characteristics. As a first step, only the chemical ignition delay was considered, and therefore, the fuels were pre-vaporized and pre-mixed prior to ignition. Two single-pulse shock tubes, one heated and another non-heated, were used to obtain the current data set. The experimental conditions covered a temperature range of approximately 1000-1600 K, at a pressure of about 18 (±10%) atm, and at an equivalence ratio of 0.5, using argon as the diluent (93%, volume). The preliminary results show an indiscernible difference between ignition delay times of JP-8, the bio-and FT- processed jet-fuels, n-heptane, n-dodecane and the n-dodecane / m-xylene blend under the fuel-lean conditions of this study over the temperature range investigated. However, the ignition delay times of pure m-xylene were significantly longer under identical conditions. The SERDP kinetic model developed under the Strategic Environmental Research and Development Program (SERDP) was used to simulate the ignition delay times for the single component hydrocarbons and the n-dodecane / m-xylene blend. The SERDP model includes the JetSurf 0.2 mechanism as one of the sub-mechanisms. The sensitivity and reaction path analyses indicate the dominance of light hydrocarbon oxidation chemistry (mainly, C 1-C3) in the pre-ignition reaction domain under fuel-lean conditions irrespective of the parent reactant alkane tested. Preliminary results for sensitivity analysis of the blend indicate that even in the presence of an aromatic like m-xylene, the pre-ignition chemistry of the blend is essentially that of oxidation of n-dodecane fragmentation products since large n-alkanes are a major component in most of these fuels. Therefore, fast fragmentation and oxidation of