This research seeks to identify how changes in temperature and oxygen concentration in the reaction zone affects liftoff heights for moderate or intense low-oxygen dilution (MILD) combustion of a jet fuel and a surrogate fuel. MILD combustion offers a method of burning fuels with reduced CO and NOx production by burning in a high temperature (e.g. 1300K) and low O₂ environment (e.g. 3%). Jet fuels and other large hydrocarbon fuels are significant because of their high energy density and widespread use in transportation industry. A surrogate fuel is a mix of one or more simple fuels, designed to match the combustion characteristics of a more complex fuel. Surrogate fuels, with significantly less constituents, can be more readily modeled than practical transportation fuels, allowing direct comparisons between simulations and experiments. In this study, MILD conditions are created above a jet in hot coflow burner by injecting preheated and vaporized fuel into a high temperature, low oxygen concentration coflow. The temperature of the coflow was varied between 1300K and 1500K, and the O₂ concentration was varied between 3% to 9% by volume. The liftoff height was measured using the chemiluminescence of OH* and used a metric of the combustion behavior. Spatial information about the formation of CH₂O was collected using PLIF. It was found the liftoff height for both fuels is highest in coflows with the lowest temperature and O₂ concentration. As the coflow temperature increases, or O₂ concentration increases, the both fuel’s liftoff height tends to decrease. Both fuels displayed similar characteristics at higher temperature coflows, as well as at higher O₂ concentrations.
However, at a coflow temperature of 1300K and O₂ concentration of 3%, larger liftoff heights were measured for the surrogate fuel. It is hypothesized that this discrepancy is caused by differences in fuel chemistry and changes in the formation of CH₂O. Jet-A is more sensitive to changes in the coflow temperature, as evidenced by increased OH* and CH₂O formation at a coflow temperature of 1500K. Additionally, a strong dependency on mixing was discovered for both fuels, as the liftoff height decreased with increased Reynolds number of the central jet.