# 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 [PERSON]'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. water-mist; droplet diameter; computational fluid dynamics; flux density distribution; FDS 202220212 January 202202202202202202202202202220222022202  
an optical spectrolpuviometer. They also investigated the effect of working pressure and nozzle size on the size distribution of droplets. [PERSON] et al. [13] conducted experiments to gain information about its droplet size distribution in the wind, using the laser precipitation monitor. [PERSON] [7] conducted a study to investigate the characteristics of a water-mist spray produced by a typical atomiser at high operative pressure (in the range 60-80 bar). They used laser-diffraction-based instruments to evaluate drop size, and particle image velocimetry (PIV) technique to determine the spray's velocity field and spray-cone angle. The specialised techniques for determining spray characteristics are also reported by [PERSON] et al., 2008 [14], which generate data on the multiple scattering attenuation of light in dense sprays of water-mist. The first concern is that these laser-based methods are very expensive and require specialised techniques. The second concern is that the size distribution of water droplets and their initial velocity are susceptible to the pressure at the nozzle head [15]. Therefore, with regard to experimental studies, the drop size of sprays has represented a challenge for decades as a parameter to be quantitatively determined [7]. Furthermore, research in the domain of water spray systems remains important due to the complexity involved in predicting the interaction between the water sprays and the fire environment [15]. In recent years, computational fluid dynamics (CFD) models have been increasingly used to investigate the growth and suppression of fires, including the dynamics of sprays [3]. There can be an alternative way to use a CFD-based model to determine the spray parameters of a water mist nozzle. The median diameter (VMD) of droplets is one of the spray parameters used to define the spray characteristics. Therefore, this paper suggests a relatively simple technique for determining the median droplet size from a nozzle spray. This technique requires only two simple experiments to determine the mass flux distribution generated by a spray nozzle operating at two known pressures and a CFD model to estimate the median diameter of the water spray under these conditions. In this work, a Fire Dynamic Simulator (FDS) is used to simulate the spray.