###### Abstract
Additionally, as the twin-fluid atomizer discharges the atomizing gas along with the water mist for fire suppression, the atomizing gas can significantly affect the enhancement of fire suppression performance. In other words, if the inert gas is used as the atomizing media, it can contribute to improving the fire suppression performance at the certain conditions. Hence, extensive experiments on fire suppression performance of water mist using twin-fluid atomizers should be conducted. A few investigations on water mist fire suppression systems using twin-fluid atomizers have been performed and reported. [PERSON] et al. [24] reported that the fire suppression performance of water mist is influenced by the fire and ventilation conditions in the compartment and types of the water mist fire suppression systems (i.e., single-fluid and twin-fluid water mist fire suppression systems). Additionally, water mist fire suppression is more effective under natural ventilation conditions than under forced ventilation conditions. [PERSON] et al. [25] examined the effect of continuous and cycling discharge modes of water mist on fire-extinguishing performance. The cycling discharge mode of the water mist leads to a shorter extinguishing time and lower water requirements when compared to its continuous mode. Furthermore, [PERSON] et al. [25] reported that the enhancement of fire-extinguishing performance is due to the increased depletion and dilution rate of oxygen and the recurrent dynamic mixing. [PERSON] et al. [26] conducted n-heptane pool fire suppression of water mist at different heat release rates (HRRs) in a large enclosure. They reported that the fire suppression of water mist was attributed to the evaporation cooling of fine water droplets and the oxygen displacement by water vapor. Additionally, the fire-extinguishing time decreased with a decrease in droplet size, and the total amount of water mist required for fire suppression decreased with an increase in fire size. [PERSON] et al. [27] conducted pool fire suppression experiments of water mist using a pair of twin-fluid atomizers to examine the effects of nitrogen pressure and fire source location. They measured and reported the characteristics of water mist (e.g., droplet diameter and velocity) and variations in temporal temperature and gas concentration. [PERSON] and [PERSON] [28] conducted fire-extinguishing experiments using water mist discharged from a twin-fluid nozzle. They measured and analyzed various spray characteristics and heptane pool fire suppression performancesof a twin-fluid nozzle. Then, their experimental data were compared and discussed in detail with those reported in a previous study wherein a single-fluid nozzle was used. [PERSON] et al. [29] investigated the effects of discharge mode, fire size, pre-burn period, and atomizing fluid type on fire suppression characteristics by using a twin-fluid water mist fire suppression system. They measured the fire-extinguishing time, water consumption quantity for fire suppression, and temperature variation with respect to time. Based on the experimental results, a fire suppression performance index was developed and discussed. The experimental results indicated that the use of nitrogen, as the atomizing gas for water mist, led to a shorter fire suppression time by approximately 30-50% when compared to that of air. However, in a previous study by [PERSON] et al. [29], the water mist was not directly discharged and penetrated the pool fire. The effect of atomizing gas type during discharge of the water mist is potentially dependent on the discharged flow direction relative to the fire source position. However, based on previous studies on twin-fluid atomizers [24; 25; 26; 27; 28; 29], there are limited studies that perform experiments to examine the effects of the discharge area and atomizing gas type of the twin-fluid atomizer which can significantly affect the fire-extinguishing performance.