Effects of dilution by aromatic hydrocarbons on staged ignition behavior of n-decane droplets

Spontaneous ignition of isolated two-component fuel droplets has been experimentally studied. The components were n-decane (ND)/1-methylnaphthalene (MN), or ND/1,2,4-trimethylbenzene (TMB). Both are n-alkane/aromatic mixtures, and therefore are candidates for model fuels of multicomponent commercial fuels refined from crude petroleum. ND showed two-stage ignition behavior, while the other two fuels (aromatic hydrocarbons) did not and were less reactive. A suspended droplet was suddenly brought to a high temperature for ignition. Observation by a Michelson interferometer detected cool-flame appearance as well as hot-flame appearance. The ambient gas was air, and the droplet diameter was 0.7 mm. The experimental conditions that were varied were ambient temperature, T_a, (500-1000 K), ambient pressure, P_a, (0.1-2.0 MPa), and the initial mole fraction of ND in the liquid phase, Z (0-1). In the case Z ≠ 1, P_a was fixed to 0.3 MPa. Ignition delays for cool-flame and hot-flame appearance (t₁ and t_tot) and the difference between them (t₂) were measured, and ignition types (no ignition, cool-flame only, singlestage ignition, and two-stage ignition) were classified on a "Z-T_a" map. It is common to ND/MN and ND/TMB that two-stage ignition region is narrowed and finally vanishes as Z decreases, and that t₁, t₂, and t_tot all increase monotonically as Z decreases. However, TMB was affected more than MN because of its higher volatility. (Normal boiling point: ND 447.3 K, MN 517.9 K, TMB 442.5 K) Numerical simulation with a fully transient one-dimensional model was employed to help the interpretation.