DROPLET INTERACTIONS WITH BURNING SURFACES

En’Urga Inc. conducted an experimental and numerical study of the interaction of droplets with burning surfaces.  The study was part of the Next Generation Fire Technology Program, funded by the SERDP.  The major objective of the project was to study the interaction between droplets and burning surfaces so as to engineer better fire suppressants.  The project consisted of two parts.  The first part determined the effect of water mist on the suppression of flame spread over a solid fuel.  The second part examined the role of buoyancy and evaporation on the droplet size and velocity distributions as they approach a hot surface. 

The effect of water mist on the reduction in flame spread velocity was obtained by experiments conducted in a wind tunnel.  Flame spread velocities with and without the water mist were obtained for a solid fuel, polymethyl methacrylate (PMMA) using Focal Plane Array Imaging. PMMA sheets of different thickness were burned in an opposed flow configuration, with varying wind velocities.  A spray nozzle, suspended over the PMMA sheets, was used to vary the water loading at the flame front.  

In the presence of water mist, the flame spread rate is a strong function of the water loading on the fuel surface. As the loading increases, the flame spread rate decreases.  The effect is more pronounced at low values of opposing wind speeds.  The effect of mean drop size of the spray on flame spread rate was small for the range of droplet sizes used in the experiments.  An empirical correlation of the water loading required to retard the spread of unwanted fires and to extinguish them was developed.  The results of the study will be directly applicable to designing fire suppressant devices so that the minimum mass of suppressant is applied directly to the fire. 

The total water loading at the fire location depends on the trajectory of droplets through the fire.  Two physical effects that change the trajectory and size of droplets are evaporation and buoyancy induced local air velocity.  A non-evaporating spray (Dow Thermal Fluid 510) was used to assess the effects of buoyancy induced air velocity on droplet size and trajectory.  An evaporating spray (water) was used to study the combined effects of buoyancy induced flow as well as evaporation on the droplet trajectories.  The results show that coalescence is the dominant mechanism by which droplets change size during their flight to the hot surface.  

The data was used to evaluate two models for droplet coalescence and evaporation.  The results showed that these models have to be improved significantly before they can be directly used in fire suppression studies.