## 3 Results and Discussion
### Influence of Atomizing Gas Type
#### 3.3.4 Comparison between Nitrogen and Air  
In Figure 15, the effects of nitrogen and air on the extinguishing times, peak surface temperatures, and minimum oxygen concentrations are compared. The differences in all parameters are defined as \"Nitrogen-Air\". With respect to the differences in extinguishing times in Figure 15a, under relatively low gas and water flow rate conditions, a few  
Figure 14: Extinguishing time vs. minimum oxygen concentration for gas type effect (note that the scales of \(x\)-axis in both figures are different): (**a**) HRR = 1.17 kW and (**b**) HRR = 5.23 kW.  
Figure 13: Minimum oxygen concentration for nitrogen and air (note that the scales of \(y\)-axis in both figures are different): (**a**) HRR = 1.17 kW and (**b**) HRR = 5.23 kW.  
experimental data on the differences in extinguishing times appeared to be negative, but largely scattered. However, under high gas and water flow rate conditions, their values were close to zero. The differences in peak surface temperatures and minimum oxygen concentrations between both atomizing gases are shown in Figure 15b,c, respectively. Their average values were mostly close to zero, with the exception of a few data with large error bars. Given the scattering of experimental data, the effect of the atomizing gas type on the peak surface temperatures and minimum oxygen concentrations was not clearly observed. The experimental results in the present study appear to be different from those of a previous study [29]. [PERSON] et al. [29] compared the fire-extinguishing performances of nitrogen and air, as atomizing gases, in a twin-fluid atomizer. They reported that nitrogen reduces the fire-extinguishing times by 30-50% when compared with air because nitrogen leads to a faster oxygen dilution effect than air. However, in this study, no significant enhancement of the fire-extinguishing performance due to nitrogen was observed. This inconsistency between the present and previous studies can be due to the differences in water mist and atomizing gas discharge directions towards fire sources in two studies. In a previous study [29], the water mist and atomizing gas from the twin-fluid atomizer were not discharged and penetrated directly into the fire source. However, in the present study, a twin-fluid atomizer was installed vertically above the fire source, and the water mist and atomizing gas were discharged directly and intensively into the fire source. Hence, in a previous study [29], the oxygen dilution can be an important fire-extinguishing mechanism, and thus the effect of the atomizing gas type is large. Conversely, in the present study, the heat extraction (i.e., cooling) of discharged water mist and atomizing gas is potentially the predominant fire-extinguishing mechanism wherein the effect of the atomizing gas type becomes small. Based on the present and previous studies, it was observed that the effect of atomizing gas type on fire-extinguishing performance is dependent on the relative positions of the discharged flow (i.e., water mist and atomizing gas) and fire source.  
Figure 15: Comparison of differences in measurement parameters between nitrogen and air: (**a**) extinguishing time, (**b**) peak surface temperature, and (**c**) minimum oxygen concentration.