## 4 Results and Discussion
Accordingly, actual GER (\(\phi_{\text{g}}\)) considering air inflow rate for each opening shape, was used in the following analysis of fire phenomena. Figure 10b presents the average HRR in the steady state as a function of the ideal HRR. As shown in the figure, under the condition of a low ideal HRR (<20 kW), HRR does not greatly differ from the ideal HRR regardless of the opening shape. However, average HRRs in specific sections were considerably lower than the ideal HRR despite the increase in fuel flow rate. This phenomenon occurs earlier as the opening height decreases. This abnormal behavior occurs when the ideal HRR is 20-35 kW and is attributed to incomplete combustion before flame ejection as mentioned earlier. Figure 9: Comparison of predicted air inflow rate according to opening shape under various nominal GER (\(\phi_{\text{g}}^{\text{n}}\)) conditions. Figure 11 shows the detailed analysis results of abnormal behavior according to the experimental conditions. In Figure 10(a), the combustion efficiency is presented as a function of actual GER, and the combustion efficiency predicted based on the correlation proposed by [PERSON] et al. [21] is shown together. This study refers to the abnormal behavior of combustion efficiency caused by incomplete combustion before flame ejection as the \"sinkhole phenomenon.\" The sinkhole phenomenon can be explained with the additional figure inserted at the top of Figure 10(a). The combustion efficiency of the compartment fire under low GER conditions does not show a significant difference from that of the open environment, but the combustion efficiency begins to decrease as the GER increases. However, this decreasing trend does not continue. When GER is further increased, the sinkhole phenomenon ends, and combustion efficiency increases again. Considering this process, looking at the compartment fire experiment, the combustion efficiencies in the range of \(\phi_{\text{g}}\) = 0.2\(\sim\)0.6 show a value close to 100% regardless of the opening shape. However, the combustion efficiency starts to decrease around \(\phi_{\text{g}}\) = 0.6 and reaches a minimum of about 70%. The combustion efficiencies were compared based on the actual GER, in which the change in air inflow rate according to the opening shape was taken into account. Nevertheless, differences according to the opening shape were observed in the condition (\(\phi_{\text{g}}\)) at which the decrease in combustion efficiency starts and the condition (\(\phi_{\text{g}}\)) showing the lowest value. Because of this, the correlation [21] does not seem to adequately reflect the shifting of the sinkhole phenomenon along the actual GER axis according to the opening shape. These differences can be attributed to the change in the height of the neutral plane according to the opening height and the change in fuel and air mixing time due to the corresponding change in air inflow path. Therefore, the influence of these factors should be further examined to specify when the sinkhole phenomenon occurs. After showing the lowest value, the combustion efficiencies increase rapidly to reach close to 100%. The increase in combustion efficiency is achieved at the same time as the flame is ejected through the opening. Figure 10(b) presents actual GER (\(\phi_{\text{g}}\)) at the time when the flame is first observed outside the compartment and a photo of the flame. In Figure 10(a), the combustion efficiency of DW05 begins to increase at the condition with \(\phi_{\text{g}}\) = 1.36, at which point flame ejection is first observed. The same phenomenon is observed in the conditions of DW10, DW15 and DW20. From these results, it can be seen that the sinkhole phenomenon has a close relationship with the flame ejection through the opening. In order to understand the relationship between flame ejection and the sinkhole phenomenon, an analysis of the opening exhaust flow according to the actual GER was performed. Figure 10: Change in fire phenomena according to the shape of openings with the constant ventilation factor.