## 4 Results and Discussion
Figure 12 presents the measurement results of the average temperature of the discharge flow through the opening. The temperature of the discharge flow was measured through experiments on DW05, and the measurement results were presented as a function of the actual GER. The location of thermocouples (T\({}_{\text{C}}\), T\({}_{\text{R}}\) and T\({}_{\text{L}}\)) for temperature measurement is shown in Figure 1. The three thermocouples are useful for considering the temperature deviation in the width direction of the opening. Figure 12 shows the average value and standard deviation for the three measurements. In the conditions before the sinkhole phenomenon occurs, the temperature of the discharge flow increases with GER. This behavior is the same as the result of a study [19] showing that the temperature of the fire flow field increases with GER in the over-ventilated fire condition. However, in the region where the sinkhole phenomenon occurs (shaded region), the temperature of the discharge flow shows abnormal behavior. In the section where the combustion efficiency decreases (\(\phi_{\text{g}}\) = 0.9-1.36), the temperature of the discharge flow does not change significantly despite the increase in the fuel flow rate. Through this, it can be seen that the amount of heat released inside the compartment is limited for some reason, and some of the supplied fuel is not burned. After that, starting with the condition of \(\phi_{\text{g}}\) = 1.36 where the flame was ejected out of the compartment, the temperature of the discharge flow increased. Because the temperature inside the compartment decreased under the same conditions (Figure 7a), the temperature increase of the discharge flow is attributed to contact between the ejection flame and the thermocouple beads. To verify whether some fuel was not burned in the section where the temperature of the discharge flow did not increase, the chemical composition of the discharge flow was analyzed. Figure 13 shows the concentration measurements of chemical species contained in the discharge flow in the DW05 experiments. In Figure 13(a), the O\({}_{2}\) concentration in the section where the combustion efficiency decreases remains constant and then decreases from when the flame is ejected. This indicates that the amount of fuel burned in the compartment is limited in the section where the combustion efficiency decreases. Moreover, in Figure 13b, the CO\({}_{2}\) concentration in the same section does not increase but slightly decreases. This behavior is attributed to the dilution effect of CO and UHC (products of incomplete combustion), along with the limitation of the combustion reaction occurring in the compartment. As shown in Figure 13c,d, the volume fractions of CO and UHC continuously increase from the time the sinkhole range is entered. The concentrations of O\({}_{2}\) and CO\({}_{2}\) do not greatly change, whereas the concentrations of CO and UHC increase, indicating that the amount of incomplete combustion continuously increases. In conclusion, it can be seen that the sinkhole phenomenon occurs because the residual fuel discharged  
Figure 11: Comparison of combustion efficiency and flame behavior according to opening shape in relation to the sinkhole phenomenon. out of the compartment after local fuel/air mixing and combustion in a real fire environment does not reach the lower combustion limit. Considering this, the reason why the actual GER exceeds 1 in the condition where the flame is first ejected in Figure 11b can be interpreted. Even in a real compartment environment, if fuel and air burn in a perfect mixture, the flame will be ejected to the outside under the condition of \(\phi_{\rm g}=1\). However, in the actual compartment environment, a phenomenon different from the theory is observed because flames are ejected after the concentration of the fuel discharged to the outside of the compartment reaches the lower flammable limit.