Droplet Size Distribution Variation of Pendent Fire Sprinkler Spray Depending on the Measurement Location

Droplet size of sprinkler sprays is related to the rate of evaporation and penetration of a fire plume. However, sprinkler sprays have various droplet sizes even at one location. Therefore, it is essential to examine the droplet size distribution depending on the location to predict the fire suppression performance of the sprinkler spray. To examine the droplet size distribution of spray from a pendent sprinkler head, acrylic plates were installed around the sprinkler head and a gap was made on one side of the wall. A charge-coupled device camera was installed to capture the droplet images both on a plane parallel to the sprinkler frame arm and on a plane perpendicular to the frame arm. Droplet information was obtained by deriving the image from the brightness and gradient images extracted from the original image. Large droplets, exceeding 1.5 mm in diameter, were observed in the mainstream of the spray. The probability of observing small droplets decreased as the droplets moved downstream. Spherical droplets were observed in the mainstream of the frame arm direction, while nonspherical droplets were observed in the perpendicular direction to the frame arm because of high velocity. The number based cumulative distribution function (CDFs) fitted using the Rosin–Rammler distribution function provided the best fitting results. The volume CDFs fitted using the Rosin–Rammler distribution function yielded acceptable adjusted R2 values. In this case, the coefficient m related to Dv50 and the coefficient n related to the width of the distribution increased with increasing radial and vertical locations.