Effect of droplet size evolution and distribution of water mist on the thermal radiation attenuation

Water mist systems for fire protection are widely recognized for their ability to attenuate thermal radiation and suppress fires. This study used high-precision shadow imaging with a non-luminous radiant panel to investigate the water mist droplet evolution and distribution under varying nozzle pressures and radiant panel temperatures ( i.e. , ambient thermal conditions), and their effects on thermal radiation attenuation (TRA). Under non-thermal conditions, droplet growth with distance was dominated by coalescence, while under thermal exposure, both evaporation and coalescence were observed, with pressure governing the dominant mechanism: at low pressure, droplets transitioned from evaporation-dominant to a combination of evaporation and coalescence, whereas at high pressure, the mechanism sequence was reversed. A radiation-driven critical distance based on characteristic droplet size (CDS) and size variance distribution parameter (σ) was identified, beyond which the dominant mechanism changed. A simplified theoretical equation, validated against experiments, reproduced droplet size and velocity evolution, revealing the coupled mechanism arising from size variation with distance. Furthermore, the water mist radiation protection was significantly influenced by radiant panel temperature, thereby altering TRA. These findings provide theoretical and experimental insights for advancements of efficient water-mist technologies for thermal radiation protection and fire suppression.