The interaction between evaporation and chemistry on two-stage auto-ignition of an n-heptane droplet

This paper investigates the interaction between droplet evaporation and chemical reactions on two-stage auto-ignition. A two-phase model is established by embedding evaporation source terms into a well-stirred reactor. Evaporation Damkholer number (Daeva) is first proposed to investigate auto-ignition characteristics because it controls the evaporation-induced decrease of gas temperature and the increase of available fuel vapor. Three auto-ignition modes are observed with increasing Daeva. Both mode 1# and 2# are two-stage auto-ignition and their difference is whether the evaporation is completed before (mode 1#, Daeva ≤ 1.0) or after the low-temperature auto-ignition (mode 2#, Daeva > 1.0). The feature of mode 1# is that the second auto-ignition delay is unchanged with increasing Daeva. However, the second delay sharply increases for mode 2#. The mode 3# has a slow evaporation and it is a single-stage auto-ignition due to the disappearance of low-temperature auto-ignition. Ambient pressure does not influence Daeva at the 1#-2# boundary (Daeva≈1.0), while an increased equivalence ratio slightly decreases the turning Daeva. Furthermore, a decreased total auto-ignition delay is observed in two special areas with increasing evaporation timescale due to the shortening of the first or second auto-ignition delay. In the end, a new estimation method for auto-ignition delays is proposed for three auto-ignition modes and its error is controlled in the range of ±10%.