Autoignition Characteristics of Hydrocarbon Fuels at Elevated Temperatures and Pressures

Abstract

A problem to be treated in the design of prevaporizing/premixing combustors for use in advanced air-breathing engines is autoignition of the fuel-air mixture prior to injection into the primary combustion zone. The high combustor inlet temperatures and pressures of these engines can promote preignition and flame stabilization in premixing passages. However, data regarding the ignition delay characteristics of hydrocarbon fuels of interest, e.g., JP-type fuels and fuel oils, under conditions representative of those encountered in operating engines are not presently available in the literature. Therefore, a research program was undertaken to develop design data and criteria which will permit evaluation of the autoignition characteristics of hydrocarbon fuel-air mixtures at elevated pressures and temperatures. Measurements of the autoignition characteristics of JP-4, No. 2 fuel oil, and No. 6 fuel oil were made in dry air at temperatures in the range 750°F (400°C) to 1100°F (593°C) and at pressures in the range 100 psia (6.8 atm) to 240 psia (16.3 atm). Tests were performed in a steady-flow apparatus in which the pressure, temperature, and mixture flow rate were adjusted to induce autoignition and maintain a stationary flame front. The ignition delay time was considered equivalent to the residence time of the fuel-air mixture between the point of injection and the axial position of the flame, and it was computed from the average flow velocity. The effects of a number of physical factors, including air pressure and temperature, fuel temperature and concentration, and initial spray characteristics (e.g., droplet size and size distribution), upon the ignition characteristics were evaluated.