Fire Safety Science Digital Archive

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.