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 autoignition. A two-phase model is established by embedding evaporation source terms into a well-stirred reactor. Evaporation Damkholer number (Da(eva)) 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 Da(eva). Both mode 1# and 2# are two-stage auto-ignition and their difference is whether the evaporation is completed before (mode 1#, Da(eva) <= 1.0) or after the lowtemperature auto-ignition (mode 2#, Da(eva) > 1.0). The feature of mode 1# is that the second auto-ignition delay is unchanged with increasing Da(eva). 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 Da(eva) at the 1#-2# boundary (Da(eva)approximate to 1.0), while an increased equivalence ratio slightly decreases the turning Da(eva). Furthermore, a decreased total autoignition 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%.