Studies Of Flame Propagation And Extinction Characteristics Of Heavy Hydrocarbon Fuels

Jet and diesel fuels are used extensively in both the civilian and militarysectors worldwide. Due to the chemical complexity of these fuels, thecombustion characteristics of practical fuels cannot be modeled from firstprinciple and understood. The first step towards that goal is to performfundamental combustion studies for both the practical fuels and the attendantsurrogate compounds.In this paper, flame propagation and extinction characteristics of fourkinds of heavy hydrocarbons(n-hexadecane、n-tetradecane、iso-cetane、decalin)、two kinds of jet fuels(JP-5、HRJ)、and two kinds of diesel fuels(F-76、HRF)were studied in the counterflow configuration under atmospheric pressure andat evaluated temperature of443K. Digital particle image velocimetry (PIV)was used to measure the axial flow velocities along the stagnation streamline.The effect of carbon number and chemical structure were also considered. Thesimulations results were also shown for comparsion issues. This study alsomake improvement of kinetic mechnisms. The main conclusions are as follows:(1) The laminar flame speeds of heavy hydrocarbons are dominate by C0-C4mechanisms. Heavy n-alkanes exhibit similar laminar flame speeds.Compared to n-hexadecane, the lower reactivity of iso-cetane is attributedto the production of resonantly stabilized intermediates that result fromfuel decomposition and which inhibit the overall reactivity. Additionally,the differences of laminar flame speeds of iso-octane/air andiso-cetane/air mixtures are caused by the distinctions of the fuel molecularstructures.(2) At near limit state of premixed flames, the heavy hydrocarbon fuels areaffect by C0-C4small hydrocarbon mechanisms, the effect of fueldiffusivity are enhanced. As the chemical kinetics dominate the extinctionof premixed flame, the extinction strain rates of n-teterdecane andn-hexadecane premixed flames are similar to each other.(3) Non-premixed n-teterdecane flames were found to exhibit notably greater resistance to extinction compared to non-premixed n-hexadecane flamesdue to the dominate effect of fuel diffusivity. The extinction strain rates ofnon-premixed iso-cetane flames are lower than those of non-premixedn-hexadecane flames as more stabilized intermediate fragments weregenerated during the oxidation of iso-cetane. The correction of the massdiffusivity of the heavy fuel has great effct on the prediction of theextinction limts of the non-premixed flames.(4) The laminar flame speeds and extinction strain rates of JP-5、HRJ、F-76and HRF flames are lower than those of n-hexadecane flames. The laminarflame speeds and extinction strain rates of petroleum-derived fuels arelower than those of bio-derived fuels. The discrepancies of combustioncharacteristics of practical fuels are caused by the differences of chemicalcompositions.(5) The improvement of the C0-C4mechanism effectively improved theprediction accuracy in laminar flame speeds and extinction limits forheavy hydrocarbon premixed flames, while the improvement of fueldiffusity effectively improve the prediction accuracy in extinction limitsfor heavy hydrocarbon non-premixed flames.