Autoignition and detonation characteristics of n-heptane/air mixture with water droplets

The present study addresses the autoignition and detonation characteristics of n-heptane/air mixture with water droplets in a confined one-dimensional reactor. A Eulerian-Eulerian formulation for gas and liquid phases is employed to simulate multi-component, fully compressible and reactive multi-phase flows. The parametric investigations covering a range of droplet diameters and number densities are conducted to understand the reaction front development in gas phase and droplet evaporation characteristics under different gaseous combustion conditions. Four modes of autoignition behaviours in the reactor are identified and they are found to greatly depend on both droplet diameter and number density. At a relatively small droplet diameter and/or number density, detonation is initiated by hot spot but no autoignition occurs at the right boundary. When both or either of them increase, autoignition occurs at the right boundary and the reaction front may further evolve into detonative or deflagrative waves. This is because the temperature inhomogeneity in that region is considerably enhanced. Furthermore, droplet diameter and number density are used to quantify the different modes of autoignition and detonation development. For the droplet evaporation dynamics during the reactive front development process, various mechanisms are observed, related to the different effects dominated by the velocity difference between two phases (characterized by the droplet Reynolds number), high local gas pressure and also the droplet temperature. This results in non-monotonic spatial distributions of droplet evaporation rate in the reactor, e.g. M-shaped in the detonated or shocked regions.