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The performance of water-based fire suppression systems is governed largely by the spray discharge characteristics associated with the nozzle geometry and injection conditions. In many nozzle configurations such as sprinklers, this initial spray is produced by injecting a water jet onto an orthogonal deflector, resulting in thin, unstable, radially expanding streams. These streams ultimately disintegrate into a complex population of drops forming the spray. The initial spray is generated in distinct stages, which include sheet formation, sheet breakup, and ligament breakup. A Sprinkler Atomization Model (SAM) has been developed based on these physics to predict the initial drop velocity, location, and size based on the nozzle geometry and injection conditions. The initial spray from a simplified yet realistic sprinkler geometry has been quantified through detailed measurements to provide insight into these atomization processes and to evaluate SAM performance. Flow visualization revealed that the deflector produces a continuous radially expanding stream resulting from the flow directed over the tines and a connected underlying orthogonal stream resulting from the flow through the spaces. The measured and predicted breakup locations and drop sizes follow We -1/3 scaling laws, previously established by other researchers in similar canonical configurations. However, SAM over predicts the volume median drop diameter by as much as 40%, probably due to the absence of models to characterize the orthogonal stream underlying the radially expanding sheet. This orthogonal stream generated by the spaces was measured to consist of nearly 50% of the flow and produces smaller drops than the radially expanding sheet. The detailed breakup mechanisms for this stream are currently being characterized to improve fidelity of the atomization model. INTRODUCTIONAutomatic fire sprinklers are accepted as the fire protection system of choice for a wide variety of applications. To support the development of many types of fire protection systems and the design of fire safe environments, modern engineering practices are currently being established like performance-based design, which requires the prediction of fire behavior using physics-based analytical methods and tools. Despite the simplicity of the basic operating principles for fire sprinklers, the complex physics governing water-based suppression including multi-phase transport processes, flame sheet extinction, and extinction of condensed phase reactions, present profound analytical and modeling challenges. Even the physical mechanisms controlling the sprinkler's simple action to generate a dispersed spray are quite complex and do not yield readily to analysis. Yet, as advanced fire protection engineering practices continue to gain popularity, the need to model fire sprinklers for suppression system and even component analysis is inevitable. In this study, the challenge to characterize the initial spray from sprinklers is addressed to advance understanding of the atomization...

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.

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