## 4 Conclusions  
In the present experimental study, the influences of discharge area and atomizing gas type in full cone twin-fluid atomizer on heptane pool fire extinguishment were investigated under the heat release rates of 1.17 and 5.23 kW in an enclosed chamber. With respect to the effect of discharge area, the large and small full cone twin-fluid atomizers were used. Nitrogen and air were used to examine the effect of the atomizing gas type. The Sauter mean diameter (SMD) of the water mist for both twin-fluid atomizers was measured and discussed. Then, the effects of the discharge area and atomizing gas type on the extinguishing times, peak surface temperatures, and minimum oxygen concentrations were analyzed via a series of fire-extinguishing experiments. The conclusions of the present study are summarized as follows. 1. With respect to the droplet size of the water mist, as the water and air flow rates decreased and increased, respectively, the droplet sizes 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. The SMD of the large atomizer was larger than that of the small atomizer. This is due to the fact that the discharge velocities of the atomizing gas in the large atomizer were reduced and the energy required to atomize the liquid decreased owing to its large annular gap area. 2. Increases in extinguishing times resulted in increasing the peak surface temperatures and decreasing the minimum oxygen concentrations. 3. With respect to the discharge area effect, the large atomizer exhibited longer extinguishing times, higher peak surface temperatures, and lower minimum oxygen concentrations than the small atomizer. One of the reasons for these trends is that the small atomizer with a small discharge area showed the higher velocity of the discharged flow and smaller droplet size of the water mist. This can lead to shortening of the extinguishing time, cooling of the flame and fuel surface, and preventing of fuel vapor retention in the reaction region. Additionally, the effect of the discharge area on fire-extinguishing performance was dominant under certain flow rate conditions. 4. With respect to the atomizing gas type effect, under the most flow rate conditions, the nitrogen and air appeared to show nearly similar extinguishing time, peak surface temperatures, and minimum oxygen concentrations. In the present study, water mist and atomizing gas were discharged directly and intensively into the fire source. Hence, heat extraction (i.e., cooling) of discharged water mist and atomizing gas can be a 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 the atomizing gas type on the fire-extinguishing performance is dependent on the relative positions of the discharged flow (i.e., water mist and atomizing gas) and fire source. In this study, it was found that the discharge area of twin-fluid atomizer significantly affects the fire-extinguishing performance, and the effect of atomizing gas type on fire suppression performance can be dependent on the discharge flow direction relative to fire source. The experimental data reported in this study were valuable and they were interpreted in the consideration of fire-extinguishing mechanism. The present experimental results can be utilized to improve the performance of water mist fire suppression systems under the given situation and to understand in more detail the influences of water mist and discharged gas in twin-fluid atomizers on fire suppression phenomena. However, the present experiments were performed under limited conditions. Hence, the future works recommended are summarized as follows.