Burning of droplet clouds

Current understanding of how particle clouds and sprays bum is still limited, despite numerous studies—both analytical and experimental—of burning droplet arrays. The main consideration in most studies has been the effect of droplet separation on the overall burning rate. It is questionable whether study of simple arrays will yield much insight into the burning of particle clouds or sprays. 

An interesting approach to the spray problem has been suggested by Chiu and Liu [29], who consider a quasi-steady vaporization and diffusion process with infinite reaction kinetics. They show the importance of a group combustion number (G), which is derived from extensive mathematical analyses and takes the form 

The value of G was shown to have a profound effect upon the flame location and distribution of temperature, fuel vapor, and oxygen. Four types of behaviors were found for large G numbers. External sheath combustion occurs for the largest value and as G is decreased, there is external group combustion, internal group combustion, and isolated droplet combustion. 

Isolated droplet combustion obviously is the condition for a separate flame envelope for each droplet. Typically, a group number less that 10 2 is required. Internal group combustion involves a core with a cloud where vaporization exists such that the core is totally surrounded by flame. This condition occurs 

FIGURE 6.16 Stagnant film height representation for condensed phase burning. 

---