droplet and spray ignition

The quality of combustion processes is often pre-determined through the conditions at the moment of ignition. The ignition process is a complex interplay between physical processes of heating and vaporization and the chemical kinetic including low- and high temperature reactions.

The role of chemical precursor reactions is often underestimated by assuming that the chemical induction time is short and long lasting mixture formation is instantaneously leading to combustion. Research at ZARM revealed that the chemical induction starts very soon and happens almost entirely in parallel to the physical processes of heating and vaporization of droplets.

Another important finding is, that the induction time of droplets and sprays is much shorter than those of homogeneous pre-mixtures of the same overall equivalence ratio.

Aiming for mostly pre-vaporized and premixed combustion conditions, this results in the finding, that once vaporization could be achieved without autoignition there is an extra time for turbulent mixing of the fuel vapor with air. This the more, the fuel richer the system is.