## 4 Results and Discussion
In particular, the volume fraction of UHC exceeded the measurement limit under some conditions during the experiments on DW15 and DW20, which have relatively low heights. Considering that CO and UHC are incomplete combustion materials, these results mean that the lower the opening height, the more inefficient combustion occurs under the same fuel supply condition. Changes in the chemical environment according to the opening shape can lead to changes in the overall characteristics of the compartment fires. Therefore, the cause of the change in chemical environment according to the shape of the opening should be examined. Figure 7: Average upper layer temperature of the front and rear regions in the compartment according to nominal GER (\(\phi_{\mathrm{g}}^{\mathrm{n}}\)). In order to analyze the cause of the change in the fire phenomenon according to the opening shape, Figure 9 compares the average air inflow rate in the steady state under representative nominal GER (\(\phi_{\rm g}^{\rm n}\)) conditions. There are various difficulties in directly measuring air inflow through an opening. These typically include flow derangement caused by the measurement instrument, real-time changes in flow due to interaction with neutral plane height, and difficulties in deriving representative values for non-uniform flow distributions. Accordingly, Figure 9 compares the average air inflow rate in the steady state predicted through FDS. Figure 9 presents a comparison of the predicted air inflow rate according to the opening shape at \(\phi_{\rm g}^{\rm n}=0.2\), the over-ventilated condition. As a result, although the openings had the same ventilation factor, the lower the opening height, the lower the air inflow rate. As a result of comparison under the condition of \(\phi_{\rm g}^{\rm n}=1\), the air inflow rate was also increased for all opening shapes due to the increase in the emission of combustion products. Nevertheless, the trend of decreasing air inflow with the height of the opening was the same. This trend was also observed under the \(\phi_{\rm g}^{\rm n}\)=2.0 condition (Figure 9c), which is the under-ventilated condition. The cause of the changes in fire phenomena shown in Figures 7 and 8 can be explained by these results. As the opening height decreases, the air inflow rate decreases, causing differences in the thermal and chemical environments of compartments with the same ventilation factor. This examination suggests that errors occur in the air inflow rate predicted from 0.52A\(\sqrt{\hbar}\) depending on the opening shape. Figure 8: Comparison of species (O\({}_{2}\), CO\({}_{2}\), CO and UHC) volume fraction according to opening shape at the upper layer as a function of the nominal GER (\(\phi_{\rm g}^{\rm n}\)). In order to examine the effect of the difference in air inflow rate on the fire phenomenon, Figure 10 compares the GER and average HRR according to the opening shape under the same fuel flow condition. In Figure 10a, the actual GER (\(\phi_{\text{g}}\)) based on the air inflow rate predicted through FDS is presented as a function of the nominal GER (\(\phi_{\text{g}}^{\text{n}}\)). As a result of the comparison, the actual GER, considering the air inflow rate according to the opening shape has a higher value than intended under all experimental conditions. Under identical fuel flow rates, actual GER increased as the opening height decreased. In particular, it was confirmed that DW20 had an actual GER exceeding 3.0 under the condition of fuel flow rate corresponding to \(\phi_{\text{g}}^{\text{n}}\)=2.0. This difference is closely related to the change in the fire phenomenon according to the opening shape observed in Figures 6-8. Under the same fuel supply condition, a relatively high actual GER can have a very large effect on fire characteristics. Specifically, as confirmed in Figure 6, it causes an increase in the ejected flame volume and also brings a large change to the temperature distribution and chemical environment inside the compartment, as shown in Figures 7 and 8.