EXPERIMENTAL CHARACTERIZATION OF WILDFIRE SPRINKLER SPRAYS USING HIGH-SPEED VIDEOGRAPHY

摘要

External sprinkler systems are one of a relatively small number of measures that are frequently recommended for the protection of houses from wildfires. However, very little scientific work appears to have been undertaken to evaluate their effectiveness. Numerical simulation techniques such as computational fluid dynamics (CFD) could be used to investigate spray performance in the conditions of a wildfire; however, detailed characteristics of the sprays typically implemented in wildfire sprinkler systems must first be known so that they can be accurately represented in such simulations. This paper presents the results of an in-depth experimental investigation into the spatiotemporal distributions of droplet mass flux, diameter, and velocity in six water sprays used in wildfire sprinkler systems. The sprays were produced using (1) a flat-fan misting nozzle, (2) a hollow-cone nozzle, (3) a deflector-plate sprinkler, (4) a butterfly sprinkler, (5) an impact sprinkler main nozzle, and (6) the auxiliary nozzle of the same impact sprinkler. The experimental and video-analysis methodologies developed have also been described in detail, to serve as a guide for future investigations. A single-camera, back-illuminated, high-speed videography technique was adopted, and droplets within a specific measurement volume were identified in the video footage using a focal criterion based on the point-spread function half-width of droplet images. A new technique was developed to separate overlapping droplet images, which was found to perform better than existing methods when applied to noncircular droplet images. Procedures for the tracking of droplets between video frames and statistical correction of sampling biases are also described in detail.