Spontaneous ignition of liquid droplets from a view of non-homogeneous mixture formation and transient chemical reactions

Abstract

Spontaneous ignition of isolated single fuel droplets in air is investigated. Special attention is given to the transient behavior of chemical reactions. Single-stage and two-stage ignition or cool flame behavior is investigated. A fuel droplet of n-heptane, n-dodecane, or iso-octane is suddenly exposed in a high-temperature, high-pressure air to ignite. The succeeding process is observed by a Michelson interferometer that visualizes the instantaneous temperature field around a droplet. Experiments are done with air of 500–1100 K in temperature and 0.1–2.0 MPa in pressure. Ignition regions are mapped on a temperature-pressure plane and the roles of the low-and high-temperature branches of chemical reactions are described. Induction times and their temperature or pressure dependence are analyzed. The diameter dependence of the induction times shows influences of physical process only on first induction time. Pressure dependence of the second induction time indicates that the dominating factor for the induction time is the nature of a cool flame during the first stage. Mixture conditions at the occurrence of a cool flame and the relation between mixture condition and second induction times are examined. Fuel and temperature dependence of second induction time and TC (zero-temperature coefficient) behavior of total induction time are explained by the dependence of the mixture condition on those parameters through cool flame temperature.