## 4 Results and Discussion  
Prior to the compartment fire experiments, experiments according to fuel flow were performed in an open environment, and the results are presented in Figure 5. In the figure, the average HRR and combustion efficiency measured in the steady state for each experimental condition are presented as a function of the fuel flow rate. Except for a slight difference in the high fuel flow rate condition, the average HRR measured with the LSC showed similar behavior to the ideal HRR (solid line) according to the fuel supply. This  
Figure 4: Results of grid sensitivity analysis for major fire physical quantities (temperature, O2, and CO concentrations). trend was identical in the investigation of combustion efficiency. The combustion efficiency was close to 100% at a relatively low fuel flow rate but gradually decreased as the fuel flow rate increased. Nevertheless, the lowest value of combustion efficiency in an open environment is 94%, indicating that the conditions considered in this study burn very smoothly in an open environment. Figure 6 shows a comparison of instantaneous flame photos under typical GER conditions to examine the effect of the opening shape on the fire phenomenon. As confirmed in previous studies [26; 27], the actual air inflow rate may differ from the theoretical value depending on the opening shape. Under identical fuel flow rate conditions, a change in air inflow rate causes a change in GER. Accordingly, in the analysis of compartment fire experiments, GER was expressed as nominal GER (\(\phi_{\mathrm{g}}^{\mathrm{n}}\)) based on fuel flow rate and theoretical ventilation factor. Figure 6a-c presents photos of instantaneous flames under the over-ventilated (\(\phi_{\mathrm{g}}^{\mathrm{n}}=0.2\)), stoichiometric (\(\phi_{\mathrm{g}}^{\mathrm{n}}=1.0\)), and under-ventilated (\(\phi_{\mathrm{g}}^{\mathrm{n}}\)=2.0) conditions for the DW05 opening. In Figure 6(a), the over-ventilated condition, the combustion reaction occurs only in the compartment, and the flame length is short. In the condition of \(\phi_{\mathrm{g}}^{\mathrm{n}}=1.0\) (Figure 6b), most of the combustion reaction takes place within the compartment, but the intermittent ejection of the flame tip is observed as the flame length increases. Figure 6c corresponding to \(\phi_{\mathrm{g}}^{\mathrm{n}}\)=2.0 shows the characteristics of under-ventilated condition in which flames are ejected outside the compartment. Through this, a change in the fire phenomenon according to the fuel flow rate is observed under the same opening condition. Figure 6d-e show instantaneous flame photos of different openings taken in the experiment under the condition \(\phi_{\mathrm{g}}^{\mathrm{n}}=1.0\). Figure 6d presents a photo of DW10. This condition did not show a great difference from DW05, and only the flame tip was intermittently ejected. Figure 6e shows the DW15 experiment, in which the flame volume ejected through the opening slightly increased. Finally, Figure 6f shows the DW20 experiment, where a substantial flame volume is ejected outside the compartment even under \(\phi_{\mathrm{g}}^{\mathrm{n}}=1.0\). Consequently, the ejected flame volume tended to increase as the width of the opening increased (as the height decreased) at the same nominal GER. This difference indicates that even if openings are designed to have a constant ventilation factor, the fire phenomena change according to the opening shape. Figure 5: Heat release rate and combustion efficiency in an open environment according to the fuel supply conditions. To investigate the internal environment according to flame behavior, Figure 7 shows a comparison of the average ULT according to nominal GER. The front and rear temperatures were measured via thermocouples installed at a height of 0.55 m from the compartment floor. When the fire source is located at the center of the compartment, the internal thermal environment undergoes the following changes as the fire grows.