A spray is formed by breaking up a volume of liquid into
small drops. This process is referred to as atomization.
Sprinklers use atomization to facilitate the dispersion of
water over a large area to protect commodities not yet
involved in the fire. The spray also delivers water to
burning materials and decreases the burning rate by
reducing heat feedback to the fuel surface. Moreover,
atomization greatly increases the surface area of the
injected volume of water. In the case of finely atomized
water mist sprays, this increased surface area results inenhanced evaporative cooling of the hot smoke from the
fire and displaces air with inert water vapor. These effects
result in abatement or even extinguishment of the fire.
For sprinkler sprays, the atomization consists of three
distinct stages. These stages are clearly illustrated in Fig. 1.
First, the jet formed at the exit of the injection orifice
impinges on a deflector plate to form a thin film that flows
along the plate. This film travels beyond the surface of the
plate to form an unconfined expanding sheet. This sheet
breaks up more readily than the relatively large-diameter
jet formed at the orifice exit. Next, aerodynamic waves are
established on the liquid sheet, resulting from the inevitable
small disturbances within the flow. These aerodynamic
waves are unstable and grow to a critical amplitude which
causes the sheet to break into ring-like ligaments. These
ligaments are also subject to disturbances and the formation
of aerodynamic waves. Finally, the waves on these
ligaments grow to a critical amplitude and break the
ligaments into small fragments which contract to form
spherical droplets.