The Impact of Preferential Vaporization on Lean Blowout in a Referee Combustor at Figure of Merit Conditions

Abstract

As alternative jet fuels continue to be developed, their impact on combustor performance remains of utmost importance. Alternative jet fuels generally contain few aromatics and differ in alkylated compositions, yielding different chemical and physical properties from those of conventional jet fuels; understanding how these property differences impact combustor performance near limiting conditions is important in certifying their use in blending with petroleum derived fuels or as complete substitutes. Ignition and extinction properties that are associated with Lean Blowout (LBO) are areas of focus for jet fuel certification as they are important safety metrics bounding combustor stability. Previous results for 23 different test fuels in a referee combustor show a strong correlation of Lean Blowout (LBO) with fuel Derived Cetane Number (DCN). This previous study involved fuels with compositions similar to conventional fuels. However, fuels with properties differing significantly from conventional fuels were found to have a weaker correlation with DCN and higher LBO equivalence ratios overall. The surrogate fuels and blends that show the largest discrepancy from the earlier correlation were blends involving highly volatile, low DCN components such as iso-octane prevalent in the early stages of distillation, and less volatile, high DCN normal alkane components such as n-hexadecane, prevalent in the final stages of distillation. Thus, significant differences in fuel reactivity along the distillation curve from those of conventional petroleum derived fuels appeared to exhibit differing LBO character. From these observations, three hypotheses, preferential vaporization, relative droplet lifetimes, and thermal quenching, are proposed and investigated by utilizing the available data. Using normalized power law regressions, distillation simulation methods and Quantitative Structure Property Relation (QSPR) results, the DCN at 34% distillation recovery show a stronger correlation with LBO than the DCN determine for the fuel itself. In this paper, we apply findings to propose fuel compositions to investigate the noted hypotheses by utilizing reactive low molecular weight molecules and a less reactive high molecular weight fuel. The suggested fuel to stress test this hypothesis is a blend of 30 (molar)% n-heptane and 70 (molar)% Gevo Alcohol-to-Jet (ATJ), which is essentially composed of (primarily) 2,2,4,6,6 iso-dodecane and isocetane. If preferential vaporization is significant, then this fuel should be more stable than the “DCN-Law,” i.e. fuels are no more stable than the corresponding DCN allows, would predict.