###### Abstract  
In this study, the effects of discharge area and atomizing gas type in a twin-fluid atomizer on heptane pool fire-extinguishing performance were investigated under the heat release rate conditions of 1.17 and 5.23 kW in an enclosed chamber. Large and small full cone twin-fluid atomizers were prepared. Nitrogen and air were used as atomizing gases. With respect to the droplet size of water mist, as the water and air flow rates decreased and increased, respectively, the Sauter mean diameter (SMD) of the water mist decreased. The SMD of large and small atomizers were in the range of approximately 12-60 and 12-49 \(\upmu\)m, respectively. With respect to the discharge area effect, the small atomizer exhibited a shorter extinguishing time, lower peak surface temperature, and higher minimum oxygen concentration than the large atomizer. Furthermore, it was observed that the effect of the discharge area on fire-extinguishing performance is dominant under certain flow rate conditions. With respect to the atomizing gas type effect, nitrogen and air appeared to exhibit nearly similar extinguishing times, peak surface temperatures, and minimum oxygen concentrations under most flow rate conditions. Based on the present and previous studies, it was revealed that the effect of atomizing gas type on fire-extinguishing performance is dependent on the relative positions of the discharged flow and fire source. t 2021 31 March 2021 5 April 2021 31 March 2021 5 April 2021 31 March 2021 31 March 2021 31 March 2021 31 April 2021 3  
attenuation, and fuel-oxygen mixture dilution [1; 2]. Additionally, when the water mist and atomizing gas are discharged into the fire, they can affect the chemical reaction kinetics, molecular transport, heat and mass transfer, and flow field (e.g., [3; 4]). These coupled phenomena can be involved in the fire phenomena and extinguishing performance. The spray characteristics of water mist are closely related to the performance of the water mist fire suppression system. In this view, the atomizer in the water mist fire suppression system is a critical component to generate a controlled and desired water mist for improving its fire suppression performance. Investigations on fire suppression via water mist have been performed extensively (e.g., [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17]). Additionally, research on water mist fire suppression has been summarized thoroughly and reported in the literature [18; 19; 20; 21; 22; 1]. Based on previous studies [1; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21], investigations on water mist fire suppression using single-fluid atomizers have been performed actively. The single-fluid atomizer generates water mist via an atomization process, in which the liquid is discharged through the small orifice with relatively high velocity and pressure [23]. However, studies on water mist fire suppression using twin-fluid atomizers have been more limited. The twin-fluid atomizer produces water mist via the atomization process, in which the low-velocity liquid is finely broken by a relatively high-velocity atomizing gas [23]. When compared with water mist fire suppression systems using single-fluid atomizers, those using twin-fluid atomizers are potentially more complex because of the additional supply system of atomizing gas [1; 20; 22]. However, the water mist fire suppression systems of the twin-fluid atomizers can reduce the operating pressure of the overall system and produce a smaller water mist than those of single-fluid atomizers [1; 20; 22]. In the previous study [20], it was reported that the single-fluid atomizers can generate the droplets as small as 90-100 \(\upmu\)m at the pressure conditions of approximately 5-6 bar, but to produce the tinier droplets (e.g., down to approximately 30 \(\upmu\)m), the twin-fluid atomizers are required.