## 3 Results and Discussion
### Influence of Discharge Area  
#### 3.2.1 Extinguishing Time  
In Figure 4, the extinguishing times of large and small atomizers under different HRR conditions are shown. The fire-extinguishing time was estimated using the interval between the timing of the water mist discharge (or only gas discharge) and fire extinguishment.  
Figure 4: Extinguishing time for large and small atomizers under different HRR conditions (note that the scales of \(y\)-axis in both figures are different): (**a**) large atomizer and (**b**) small atomizer.  
Figure 3: Droplet size of twin-fluid atomizers: (**a**) large atomizer and (**b**) small atomizer.  
As shown in Figure 4a, at the only nitrogen discharge condition (i.e., water flow rate = 0 g/min), as nitrogen flow rates increase, the extinguishing times decrease. This may be due to the fact that the high-velocity nitrogen flows contribute to cooling the flame and fuel surface and preventing the retention of fuel vapor in the reaction region. The extinguishing times for a higher HRR of 5.23 kW were shorter than those for a lower HRR of 1.17 kW, in particular, at the low nitrogen flow rate condition. A reason for this is that a fire with a higher HRR consumes the oxygen more quickly in the enclosed chamber and is extinguished within a shorter time when compared with a lower HRR. However, the difference in extinguishing times between two HRR cases decreased with increases in the nitrogen flow rates. This implies that the effect of the discharged nitrogen flow rate on the extinguishing time became predominant.  
Under the water mist discharge conditions in Figure 4a, the extinguishing times gradually decreased up to a water flow rate of 85 g/min for all nitrogen flow rate conditions. However, with respect to the water flow rate conditions of 160 and 240 g/min, an increase in the water flow rate led to a slight increase in the extinguishing time at the low nitrogen flow rate condition (e.g., 20 L/min). This may be due to the decreased extinguishing performance caused by the increased droplet size of the water mist. The larger water droplet can penetrate into the fire and reach the fuel pool. In such a case, in the reaction region of fire, the effects of water mist (e.g., chemical reaction and heat and mass transfer caused by the fine water droplets) can be reduced. With increasing the water flow rate under the constant nitrogen flow rate condition, the droplet size of water mist becomes larger, and the impact of water mist in reaction region becomes smaller. This is one possible reason that under the low nitrogen flow rate condition, the extinguishing time increases with increasing water flow rates. This result implies that there are optimal flow rate and droplet size conditions for maximizing the fire-extinguishing performance of the water mist. Under low nitrogen flow rate conditions (e.g., 20 L/min), the extinguishing time was sensitively affected by the water flow rate. Conversely, at the high nitrogen flow rate condition (e.g., 60 L/min), the extinguishing time remained almost constant and was nearly independent of the water flow rates. Additionally, the differences in extinguishing times between two HRR cases under water mist discharge conditions became smaller than those under the only nitrogen discharge conditions. In other words, under the water mist discharge conditions, the extinguishing times for an HRR of 5.23 kW were quite similar to those for an HRR of 1.17 kW.  
These general trends of large atomizer are observed as nearly similar to those of small atomizer, as shown in Figure 4b. The higher HRR case exhibited shorter extinguishing times than the lower HRR case in certain flow rate conditions. Additionally, as the nitrogen and water flow rates increased, the extinguishing time decreased.