IGNITION THEORY BASIC INFORMATION AND TUTORIALS

What is ignition theory? How things ignite?

Most ignition theories are based on the idea that the transient ignition source, usually an electric spark, must supply sufficient energy to the combustible mixture to create a volume of hot gas that just satisfies the necessary and sufficient condition for propagation, namely, that the rate of heat generation just exceeds the rate of heat loss.

The work of Lewis et al. did much to clarify and improve knowledge of spark ignition in quiescent mixtures. The first major contribution to ignition theory for flowing mixtures was made by Swett who studied the influence on ignition energy of variations in pressure, velocity, equivalence ratio, and turbulence.

Swett’s theory is based on the ideas that (1) only a portion of the discharge length is important in the ignition process and (2) heat loss by thermal conduction is negligible compared with heat loss by eddy diffusion.

Both of these ideas were fully confirmed in subsequent experiments carried out by Ballal and Lefebvre on ignition in flowing mixtures. Unfortunately, Swett’s treatment of turbulence is very limited and much of his experimental data are suspect for reason.

Gaseous Mixtures
Ballal and Lefebvre analyzed the processes governing the rate of heat generation in an incipient spark kernel and the rate of heat loss by thermal conduction and turbulent diffusion. They conclude that, for the spark kernel to survive and propagate unaided throughout a gaseous mixture, its minimum dimension should always exceed the quenching distance.

Heterogeneous Mixtures
All the evidence obtained in the studies of Subba Rao, Rao, and Lefebvre on the ignition of flowing mixtures of fuel drops and air serves to suggest that passage of the spark creates a kernel in which high gas temperatures are attained, partly from the energy supplied in the spark, but also from the heat liberated by the evaporation and rapid combustion of the smallest fuel drops.