Single droplet ignition: Theoretical analyses and experimental findings

Spray ignition represents a critical process in numerous propulsion and energy conversion devices. Compared to a gaseous mixture, ignition in a spray is significantly more complex, as the state of ignition in the latter case can be defined by three distinct ignition modes namely, droplet ignition, droplet cluster ignition, and spray ignition. Ignition for an individual droplet represents the appearance of a flame surrounding the droplet or in the wake region, with a dimension on the order of droplet diameter. The cluster or group ignition refers to the ignition around or inside a droplet cloud, while the spray ignition implies the appearance of a global flame with a characteristic dimension few orders of magnitude larger than a droplet. In all three modes, ignition is preceded by the evaporation of fuel droplets, formation of a combustible gaseous fuel–air mixture, and initiation of chemical reactions producing sufficient radical species. The identification of the dominant ignition mode for given two-phase properties represents a problem of significant fundamental and practical importance. Research dealing with laminar and turbulent spray ignition has been reviewed by Aggarwal [1] and Mastorakos [2], respectively, while Annamalai and Ryan [3] have provided a review of droplet group combustion/ignition. In the present review, we discuss experimental, theoretical, and computational research dealing with individual droplet ignition. Topics include the quasi-steady and unsteady models for the ignition of a fuel droplet in a stagnant environment, the droplet ignition in a high-pressure environment, the convective effects on droplet ignition, and multicomponent fuel droplet ignition. Studies dealing with the two-stage and NTC ignition behavior for a droplet are also discussed. Finally, relationship between the droplet ignition mode to droplet cluster and spray ignition modes is briefly described. Potential topics for further research are outlined.