Study on sooting behavior of premixed C1-C4n-alkanes/air flames using a micro flow reactor with a controlled temperature profile

Sooting behavior of premixed C1-C4n-alkanes/air mixtures at maximum temperatures lower than 1400 K is studied experimentally using a micro flow reactor with a controlled temperature profile. Flame and soot responses to equivalence ratio (1.5-4.0) are observed for all mixtures. Critical sooting equivalence ratio observed for methane, propane and n-butane is 1.8, whereas, for ethane it is 1.9. Sooting behavior at similar critical equivalence is further interpreted qualitatively by observing relative axial locations of upstream edges of sooting region, where relative downstream location indicates delayed soot formation. The order of axial location of upstream edge of soot at equivalence ratio 1.8 is: propane < n-butane < methane; and, the order at equivalence ratio 1.9 is: propane ∼ n-butane ∼ ethane < methane. Sooting region shifts upstream for ethane, propane and n-butane with increasing equivalence ratio. However, it does not change for methane with increasing equivalence ratio above 2.0. One dimensional computations with detailed chemistry are performed to study observed flame and soot responses. Ethane is found to be most reactive fuel due to coupling of H abstraction from fuel and H production in the flame. Computed flame positions vary widely among mechanisms for methane. Most exothermic reaction at flame is CH3 + CH3 = C2H6 for all mechanisms and for all the fuels. Computations using KAUST mechanism are performed for all the fuels at various equivalence ratios in the range 1.35-4.0 to discuss overall sooting behavior. Maximum computed pyrene mole fractions are used to study relative sooting behavior qualitatively as they correlate well with soot. The trends of maximum computed pyrene mole fractions qualitatively explain observed trends of critical sooting equivalence ratios and overall sooting behavior of fuels in the current experiments.