The Behaviour of Water-Mists in Hot Air Induced by a Room Fire: Effect of the Initial Size of Droplets

Abstract

This paper presents work on investigating the effect of the initial size of water mist droplets on the evaporation and removal of heat from the fire-induced hot gas layer while travelling through the air in a compartment. The histories of the temperature, diameter and position of droplets with different initial diameters (varied from 100 µm to 1000 µm) are determined considering surrounding air temperatures of 75 °C and 150 °C and a room height of 3.0 m. A water droplet evaporation model (WDEM) developed in a previous study (Fire and Materials 2016; 40:190–205) is employed to navigate this work. The study reveals that tiny droplets (for example, 100 µm) have disappeared within a very short time due to evaporation and travelled a very small distance from the spray nozzle because of their tiny size. In contrast, droplets with a larger diameter (for example, 1000 µm) reached the floor with much less evaporation. In the case of this study, the relative tiny droplets (≤200 µm) have absorbed the highest amount of energy from their surroundings due to their complete evaporation, whereas the larger droplets have extracted less energy due to their smaller area/volume ratios, and their traverse times are shorter. One of the key findings of this study is that the smaller droplets of spray effectively cool the environment due to their rapid evaporation and extraction of heat from the surroundings, and the larger droplets are effective in traversing the hot air or smoke layer and reaching the floor of the compartment in a fire environment. The findings of this study might help in understanding the behaviour of water-mist droplets with different initial diameters in designing a water-mist nozzle.

Experimental Study on the Effectiveness of Water Mist Extinguishing Systems for Protecting Paintings in Art Gallery Museums: Developing an Initial Methodological Model

“…Field testing is necessary to accurately capture the real behavior of water mist systems in response to a fire event [15]. Previous evaluations of the combined use of water mist and fire suppression mechanisms primarily focused on the general characteristics of the fire plume, such as its geometry, temperature, gas flow speed, and smoke quantity [16][17][18] and found that larger droplets are more effective in penetrating a smoke layer.…”

Full-Scale Experiments of Water-Mist Systems for Control and Suppression of Sauna Fires

“…Mitigation of explosions was investigated as an additional action by water mist in highly hazardous storage facilities [12]. Its success can be mainly attributed to a discharge of generally less water than that by sprinkler systems, since fragmentation into tinier droplets yields a faster evaporative action [13,14]. Dedicated studies were also focused on water-mist action in suppressing compartment fires, mainly emphasizing the need for controlled discharge [15] and systems that govern activation on a detection-based feedback [16].…”

On the Large Eddy Simulation Modelling of Water Suppression Systems Droplet Impact and Coverage Area

“…To date, extensive numerical studies have been performed to study the suppression behaviour of water-mist systems under different configurations and fire scenarios. Wang et al [23], Liu et al [24] and Mahmud et al [25] have studied the effects of different droplet sizes offered on droplets' penetrability and the evaporating rate at the flame region. Other spray characteristics such as spray angle [26], nozzle type [27], flow rate and operating pressure [28] have also been investigated through numerical studies.…”