  Autoignition of n-heptane droplet/vapor/air mixtures behind an oblique shock
wave are studied, through Eulerian-Lagrangian method and a skeletal chemical
mechanism. The effects of gas/liquid equivalence ratio (ER), droplet diameter,
flight altitude, and Mach number on the ignition transient and chemical
timescales are investigated. The results show that the ratio of chemical
excitation time to ignition delay time can be used to predict the oblique
detonation wave (ODW) transition mode. When the ratio is relatively high, the
combustion heat release is slow and smooth transition is more likely to occur.
In heterogeneous ignition, there are direct interactions between the
evaporating droplets and the induction/ignition process, and the chemical
explosive propensity changes accordingly. The energy absorption of evaporating
droplets significantly retards the ignition of n-heptane vapor. In the
two-phase n-heptane mixture autoignition process, the ignition delay time
decreases exponentially with flight Mach number, and increases first and then
decreases with the flight altitude. As the liquid ER increases, both ignition
delay time and droplet evaporation time increase. With increased droplet
diameter, the ignition delay time decreases, and the evaporation time
increases. Besides, for Mach number is less than 10, the ratio of the chemical
excitation time to ignition delay time generally increases with the flight
altitude or Mach number. It increases when the liquid ER decreases or droplet
diameter increases. When Mach number is sufficiently high, it shows limited
change with fuel and inflow conditions. The results from this work can provide
insights into spray ODW initiation. The ODW is more likely to be initiated with
a smooth transition at high altitude or Mach number. Abrupt transition mode
tends to happen when fine fuel droplets are loaded.
