ADVANCES IN CHARACTERIZING FIRE SPRINKLER SPRAYS

Abstract

Knowledge of the initial spray characteristics of sprinklers is critical for fire suppression performance analysis. Although numerous tests and studies have been conducted on fire sprinkler sprays, measurements were mostly conducted in the far-field due to spray diagnostics limitations. Although these far-field measurements are useful for evaluating the ultimate sprinkler performance, they are convoluted by the dispersion process and yield little useful information regarding the initial sprinkler discharge characteristics. With the development of advanced non-intrusive spray diagnostics, high fidelity initial spray measurements are possible, providing sprinkler discharge characteristics which are useful alone for nozzle development or together with analytical tools for prediction of suppression performance.

In this study, a laser diagnostic technique based on Shadowgraphy was used to characterize the initial spray for actual fire sprinklers and nozzles having more basic configurations. The shadowgraphs revealed important information on the effect of nozzle geometry on sheet formation (from the injected jet) and sheet fragmentation into drops. Three breakup modes were observed depending on the injection conditions quantified through the We and the geometric details of the nozzle. Based on these breakup modes, scaling laws were developed to quantify the effect of nozzle geometry and injection condition on sheet breakup distance and drop size. The sheet breakup location followed a We <super>-1/3</super> power law for all observed breakup modes. However, drop sizes followed a We <super>-1/3</super> power law only for the ligament breakup mode which was observed to occur at very high We (We > 10<super>4</super>). The shadowgraphs also provided spatially resolved measurements of drop size and velocity on a hemisphere 0.3 m away from the nozzle. Based on these detailed measurements, a comprehensive spray initiation model was developed for the purpose of providing a high fidelity analytical description of the initial spray useful for spray modeling. A simple dispersion analysis, accounting only for drag forces on the droplet in a quiescent environment, was performed to compare with volume density measurements taken 1 m below the sprinkler. Predicted and measured volume densities compared favorably providing some validation of the initial spray measurements and simple dispersion analysis.