Blowout Sensitivities in a Liquid Fueled Combustor: Fuel Composition and Preheat Temperature Effects

The objective of this paper is to identify the influence of fuel composition on blowoff limits in a liquid fueled combustor. In premixed, gaseous systems, blowoff is a kinetically limited phenomenon, possibly with additional heat loss effects. In liquid fueled systems, the situation is far more complex, as a variety of processes can influence blowoff, including kinetics, atomization, vaporization, mixing, and heat transfer. Which one of these processes is controlling is a function of fuel and air preparation and premixing, approach flow temperature and pressure, and fuel physical and kinetic properties. This paper extends our prior work on this problem by presenting blowoff results from ten liquid fuels at two air inlet temperatures, 450 K and 300 K. At 450 K, blowoff appears to be limited by kinetics and/or radiation losses, while it seems vaporization limited at 300 K. Specifically, strong negative correlations were observed between blowoff limits and the cetane number of the fuel at the 450 K conditions. Similarly good correlations are observed with the fuel smoke point, and its percentage of aromatics. This supports two different hypotheses: (1) it is the fuels with the shortest ignition delay, and therefore the fastest reaction rates, that are the most resistant to blowoff, or (2) it is the fuels with the greatest radiation losses (presumably vaporizing/preheating the approach flow fuel droplets) that are the most blowoff resistant. Additional measurements with other fuels that decouple ignition and radiative characteristics are needed to differentiate these effects. At air inlet temperatures of 300 K, the governing physics seem quite clear from the data. Strong positive correlations were observed with LBO and boiling point temperature across the entire distillation range. As long as the air inlet temperature is above the fuel flash point, the easiest to vaporize fuels are the hardest to blowoff. It is suspected that difficult to vaporize fuels blowoff easily due to locally non-flammable regions in this low temperature regime.