The stability of a water droplet in a gas stream is governed by the balance between the aerodynamic drag forces tending to shatter it and the surface tension forces holding it together. This relationship is quantified by the Weber number. In the high-velocity, turbulent gas flows characteristic of a fire environment, the aerodynamic forces are significantly increased. While a water droplet can be as large as 5000 microns in a standard, calm atmosphere, the intense turbulence within a fire plume will cause droplets larger than approximately 2000 microns to become unstable and break apart into smaller ones.

A. 1000 microns: This is a common droplet size found in fire suppression sprays but is not the maximum size that can be sustained before breakup in a turbulent fire.

B. 1500 microns: While droplets of this size are more stable than larger ones, they are still below the accepted maximum stable diameter in a turbulent fire environment.

D. 2500 microns: Droplets of this size are generally considered unstable in the high-velocity, turbulent conditions of a fire plume and would readily break apart.

1. SFPE. (2016). SFPE Handbook of Fire Protection Engineering (5th ed.). Springer. In Chapter 29, "Water-Based Fire Suppression: System Components," Section 29.3.2.1, "Droplet Size," it is stated: "In a turbulent fire environment, the maximum stable diameter is reduced to approximately 2000 μm (0.08 in.)." (p. 918).

2. SFPE. (2016). SFPE Handbook of Fire Protection Engineering (5th ed.). Springer. The underlying principle is explained in Chapter 11, "Interaction of Sprays with Fire and the Environment," Section 11.2.2, "Droplet Breakup," which discusses the critical Weber number (typically 10-20) above which droplets become unstable and shatter. (p. 352).