ATOMIZATION AND DISPERSION MEASUREMENTS IN FIRE SPRINKLER SPRAYS

Abstract

Water sprays are commonly used in fire suppression applications for cooling the fire environment. This cooling is achieved through the evaporation of droplets (dispersed in the fire gases) and through the wetting of surfaces (from hot or burning materials), inhibiting both the growth and spread of the fire. The suppression performance of these sprays is determined by their ability to penetrate the fire (i.e., the induced flow) to reach burning surfaces below, while dispersing water throughout the hot environment. Spray penetration and dispersion are governed by the initial drop size and velocity characteristics of the spray, which depend on the injection conditions and nozzle configuration. In many fire suppression devices, such as sprinklers, a jet is injected onto a deflector to generate the water spray. Although there are many variations on this basic concept, most sprinklers include a central boss surrounded by a deflector having both tines and spaces. To study the essential physics of the atomization process, discharge characteristics from simplified nozzles were measured. These measurements were compared with those from a more realistic sprinkler configuration. Flow visualization experiments revealed that the canonical impinging jet configuration produces a radially expanding sheet. While similar atomization mechanisms were observed, the realistic sprinkler configuration produces a three-dimensional sheet with two distinct flow streams generated by the tines and spaces of the nozzle. Comprehensive experiments were conducted to describe atomization (e.g., sheet breakup locations and initial drop sizes) and dispersion (e.g., volume density and local drop size profiles) in these sprays.