Fireball or thermal

The cloud of rising hot gas due to the sudden release of a finite amount of energy is called a thermal. If this occurs due to the sudden release of a finite volume of gaseous fuel that is ignited, the thermal then initially rises as a combusting region. This is depicted by the 

Sketches from photographs of flame development Initial fuel volume, VF= 185.4 cm3 Total burning time, tb = 0.702 s 

The relative, normal entrainment velocity is uniform over the fireball and proportional to the rising velocity, dz/dt, i.e. ve = a0dz/df, with a an entrainment constant. 

The pressure on the fireball is constant except for pressure effects due to buoyancy. 

The actual air entrained is n times the stoichiometric air needed for combustion. 

---