  Effects of low-temperature chemistry induced by ultrafine water droplet
evaporation on reaction front development from an ignition spot with
temperature gradient are studied in this work. The Eulerian-Eulerian method is
used to simulate the gas-liquid two-phase reactive flows and the physical model
is one-dimensional spherical reactor with stoichiometric gaseous n-heptane/air
mixture and ultrafine monodisperse water droplets (initial diameter 5
micrometres). Homogeneous ignitions of two-phase mixtures are first simulated.
The water droplets can complete evaporation in the reactor prior to ignition,
and hence pronouncedly reduce gas temperature, which may induce the
low-chemistry reactions. It is found that the turnover temperature for negative
temperature coefficient range increases with droplet volume fraction.
Three-stage ignitions are present when the volume fraction is beyond a critical
value, i.e., low-temperature, intermediate-temperature, and high-temperature
ignitions. The chemical explosive mode analysis also confirms the low-chemistry
reactions induced by the evaporation of ultrafine water droplets. Then reaction
front development from an ignition spot with temperature gradient in two-phase
mixtures is analysed based on one-dimensional simulations. Different modes for
reaction front origin in the spot are identified, based on the initial gas
temperature and lower turnover temperature. Specifically, the reaction front
can be initiated at the left and right ends of the ignition spot, and inside
it. Detailed reaction front developments corresponding to the above three modes
are discussed. Besides, the pressure wave from high-temperature ignition is
important, compared to those from low- and intermediate-chemistries.
