An Approach to Determine the Median Diameter of Droplets in a Water-Mist Spray

Abstract

The physical characteristics of water sprays profoundly influence the efficacy with which fires are extinguished. One of the most important physical characteristics of water sprays is the median diameter of the water droplets. However, this parameter is difficult to measure without resorting to the use of specialised equipment. Furthermore, the distribution of the size of water droplets and their initial velocity are profoundly sensitive to the pressure at the nozzle head. This paper presents a simple technique to determine the median droplet size of a water spray produced by a nozzle. The method required only two experiments to determine the mass flux distribution generated by a nozzle operating at two known pressures. A computational fluid dynamics (CFD) program was then used to estimate the median diameter of the water spray under these conditions. The median droplets generated when the nozzle was operating under a different pressure can be calculated using an established empirical relationship. The approach advocated in this paper is supported by invoking Whewell’s principle of consilience of inductions. This was achieved by observing that the CFD software accurately predicts the mass flux distribution when the new pressure and estimated median diameter of the droplets were used as inputs. This provides independent evidence that the proposed approach has some merit. The findings of this research may contribute to establish a technique in calculating the median diameter of droplets when direct measurement of droplet diameter is not available.