Reacting Pressurized Spray Combustor Dynamics: Part 1 — Fuel Sensitivities and Blowoff Characterization

Blowoff sets important operational limits on a combustor system. While blowoff is intrinsically a system-dependent phenomenon, it is also dependent on the chemical and physical properties of the fuel. This paper describes an experimental study of the lean blowout limits of eight liquid fuels in a swirl-stabilized combustor, with data for both a pressure atomizer and an airblast atomizer. Three of the fuels were traditional jet fuels (an average Jet-A, JP-5, and JP-8) and the remaining five fuels spanned a range of physical and kinetic properties. These experiments were performed at a combustor pressure of 345 kPa and an air temperature of 450 K. In addition to some sensitivity of blowoff conditions to the thermal state of the combustor, results also clearly show sensitivities to fuel composition. Strong correlations were observed for pressure atomizer blowoff with fuel physical properties, particularly for boiling point temperature, indicating that fuels less easily atomized and vaporized are harder to blow off. These results are consistent with the idea that delaying atomization and/vaporization, and therefore reducing the level of premixing that drives the local fuel-air ratio towards the very lean global fuel/air ratio, is advantageous in order to promote regions of locally elevated flame temperatures. We suggest that this behavior occurs when the air preheat temperature is above the fuel flashpoint, as a similarly good correlation with boiling point temperature but with the opposite trend, has been previously reported in a study obtained for preheat temperatures below the fuel flashpoint. In contrast, the airblast results do not show strong correlations with fuel physical properties. Rather, the best correlation of the airblast atomizer results is with the percentage of iso-paraffins in the fuel. We speculate that this reflects a sensitivity to kinetic properties of the fuel, as the superior atomization characteristics of the airblast atomizer may de-emphasize the importance of physical properties.