## 4 Results and Discussion
At a very low GER condition, the flame forms in the vertical direction of the fire source, and the front and rear temperatures do not greatly differ. As the fuel flow rate (i.e., GER) increases, the mass of combustion products and air entering and exiting the compartment also increases. At this time, the increased flow rate of the inlet causes the flame to move to the rear of the compartment, thus increasing the temperature at the rear. As GER further increases, the oxygen inside the compartment is depleted, and the flame moves to the front. In this process, the front and rear temperatures become reversed, and the front exhibits a higher temperature. From this perspective, examining the experimental results of DW05 in Figure 7a, the front and rear temperatures in the range of \(\phi_{\text{g}}^{\text{n}}=0.2\) to 0.8 do not show a large difference (\(\text{T}_{\text{front}}\approx\text{T}_{\text{T}_{\text{T}_{\text{T}_{\text{ rear}}}}}\)). However, at \(\phi_{\text{g}}^{\text{n}}=0.85\), it shows a relatively high temperature at the rear side (\(\text{T}_{\text{front}}<\text{T}_{\text{T}_{\text{T}_{\text{T}_{\text{T}_{ \text{T}}}}}}\)). At \(\phi_{\text{g}}^{\text{n}}\)=2.0, the temperature difference between the front and rear decreases due to the forward movement of the flame, and at a higher \(\phi_{\text{g}}^{\text{n}}\), the front will show a higher temperature (\(\text{T}_{\text{front}}>\text{T}_{\text{T}_{\text{T}_{\text{T}_{\text{T} \text{T}}}}}\)). The experimental results of the other openings in Figure 7b-d also exhibit similar behavior. However, the timing at which the relationship between the temperatures measured at the front and rear changes varies depending on the shape of the opening. The experimental results of DW10 in Figure 7b do not greatly differ from DW05. On the other hand, in the experiment of DW15, the rear temperature showed a higher value at \(\phi_{\text{g}}^{\text{n}}=0.8\), and the temperatures were reversed at \(\phi_{\text{g}}^{\text{n}}=1.5\). This trend is intensified in the DW20 experiment. Specifically, the rear temperature is higher than under the condition of \(\phi_{\text{g}}^{\text{n}}=0.7\), and the temperature reversal is observed under the condition of \(\phi_{\text{g}}^{\text{n}}=1.1\). Figure 6: Comparison of instantaneous flame photos according to the opening shape in representative nominal GER (\(\phi_{\text{g}}^{\text{n}}\)) conditions. That is, as the opening height decreased, the thermal environment changed at a lower nominal GER. Through this, it can be seen that even if the ventilation factor is the same, the transition condition (fuel flow rate, GER) to the under-ventilated fire varies depending on the opening shape. Similarly, the same result can be obtained in comparison with the condition in which the maximum temperature is measured. In general, the temperature of the fire environment shows the maximum value under stoichiometric (\(\phi_{\mathrm{g}}^{\mathrm{n}}=1.0\)) conditions [19]. The nominal GER at which the maximum temperature was measured in the experiment of each opening was 0.95, 0.90, 0.85, and 0.70 in the order of increasing opening width. That is, as the height of the opening decreases, the stoichiometric ratio is reached under the condition of a relatively lower fuel flow rate. Figure 8 compares the average volume fractions of species measured in the upper layer according to the nominal GER under various opening shape conditions. Under the same nominal GER conditions, the volume fraction of O\({}_{2}\) decreases with decreasing opening height in Figure 8a. Conversely, CO\({}_{2}\) exhibits a higher volume fraction as the height of the opening decreases. Through this, it can be seen that the change in the opening shape affects the chemical environment inside the compartment under the same fuel flow condition. As a result of examining CO volume fraction (Figure 8c), under the same fuel supply condition, the lower the opening height, the higher the value, and the UHC also exhibits the same behavior with CO.