NUMERICAL INVESTIGATION ON THE AUTO-IGNITION OF C4-C7 n-ALKANES: MECHANISMS OF TRANSITION FROM FIRST-STAGE TO SECOND-STAGE IGNITION

The two-stage auto-ignition phenomena of normal alkanes—i.e., first-stage and second-stage ignition controlled by low-temperature and high-temperature chain-branching pathways, respectively—were investigated by numerical simulation using a closed homogeneous reactor model and incorporating detailed chemical kinetics. The underlying mechanism for the transition from first- to second-stage ignition was particularly emphasized. Four n-alkanes (n-butane, n-pentane, n-hexane, and n-heptane), all manifesting two-stage ignition behavior, were employed under extensive operating conditions. By considering the convective heat transfer between the reacting gas and the cold reactor wall, distinct ignition modes were distinguished, including one- and two-stage ignition. Two different mechanisms (i.e., thermal and chemical) were identified as being responsible for the transition of the ignition modes. A comparison of the computational results obtained for the four n-alkanes suggested that the lowest temperature for the chemical transition decreased with an increase in the carbon number of n-alkanes. Moreover, the computational results obtained under varying pressure conditions indicated that the transition from first- to second-stage ignition was sensitive to pressure, with increasing pressure significantly promoting the transition irrespective of the controlling mechanism.