S.L. Manzello and J.C. Yang
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899 USA
Typically, these new liquid fire suppressants are delivered to the fires in the form of droplets. Under practical conditions, the dispersion of the liquid agent results in the generation of numerous droplets that can be difficult to study systematically. Therefore, a simpler approach may be adopted to understand the influence of droplet impingement on a heated surface. One such approach is the study of single droplet impingement upon heated surfaces.
A dilemma exists for water droplet studies when viewed from a fire suppression perspective. Water has a freezing point of 0°C. In many practical fire suppression systems, the fire suppressant must be stored under temperatures below 0°C. Such low temperatures may result in freezing of the liquid fire suppressant. To prevent water from freezing, additives must be used to reduce the freezing point of water.
An experimental study is presented for water droplet impingement containing an additive upon a heated stainless steel surface. A solution of 30 % (mass fraction) sodium acetate trihydrate, CH3COONa3H2O was used for the experiments. The impaction process was recorded using a high-speed digital camera at 1000 frames per second. Three different impact Weber numbers were considered, namely 15, 80, and 181. Collision dynamics were investigated for each impact Weber number with the temperature of the stainless steel surface varied from film evaporation to film boiling. The temporal variation of the liquid film diameter was measured as a function of temperature for each impact Weber number. Experiments were also performed using water for direct comparison of the collision dynamics to the additive containing droplets. The collision dynamics were observed to be different for water droplets containing an additive at low impact Weber number but became increasingly similar as the impact Weber was increased.