###### Abstract  
In this study, experiments and numeric studies were conducted to analyze the causes of abnormal behavior in temperature, heat flux, and combustion efficiency that occurred before a compartment fire was converted to the under-ventilated condition. The fire experiments were performed using a 1/4 reduced-scale ISO 9705 compartment and propane fuel. Considering that the occurrence and termination of abnormal behavior are related to ventilation conditions, vertical openings of various shapes designed to have the identical ventilation factor (0.52A\(\sqrt{\text{h}}\)) were used. As a result, an abnormal phenomenon was observed in which the measured heat release rate did not increase despite the increase in the fuel flow rate under the over-ventilation condition in all opening shapes. Due to this phenomenon, the combustion efficiency was reduced to 70% regardless of the opening shape. In order to understand these abnormal behaviors and to analyze the causes of their occurrence, the temperature and chemical species concentrations of the opening discharge flow were measured. The results indicated that the abnormal behavior of thermal physical quantities, HRR, and combustion efficiency occurred because fuel that was not burned inside the compartment did not reach the lower combustion limit even outside the compartment and was discharged. compartment fire; ventilation condition; combustion efficiency; global equivalence ratio; fire behavior +
Footnote †: journal:  
the upper layer temperature (ULT) and incident heat flux were not proportional to HRR in the fire growth stage. Specifically, in studies by [PERSON] et al. [12] and [PERSON] et al. [13] that used ISO 9705 [16], the ULT and incident heat flux on the floor temporarily decreased despite the increase in HRR due to the boiling of liquid fuel, and then increased afterwards. Similarly, in the study by [PERSON] [14], the ULT was constant at approximately 250 \({}^{\circ}\)C despite the increase in HRR from 450 kW to 650 kW. After the transient abnormal behavior, ULT increased rapidly, showing behavior proportional to HRR. The HRR measured in the section where this phenomenon was observed was considerably lower than the ideal HRR. Considering this, it can be inferred that incomplete combustion is the cause of the abnormal thermal physical quantities. Incomplete combustion in the fire growth stage can increase the fire risk by generating more toxic substances than expected. However, the causes and effects of this phenomenon, which appears very temporarily, have not been analyzed in detail. Therefore, it is necessary to better understand it by analyzing the fire behavior of compartment fires that grow in stages. Staged fire growth can be implemented using the concept of global equivalence ratio (GER, \(\Phi_{\mathrm{g}}\)). GER, a concept similar to the equivalence ratio of a premixed flame, can be used to quantify the ventilation condition of a compartment fire assuming the complete mixing of fuel/air in the compartment from a macroscopic perspective. Fuel is locally mixed and diffused with the air in an actual compartment, so GER calculated based on the assumption of complete mixing may differ from the actual value. Nevertheless, GER can be used to predict CO yield through the fuel-specific correlation that varies with the ULT [17; 18]. [PERSON] et al. [19] examined chemical species concentrations and ULT in the range of \(\phi_{\mathrm{g}}\) = 0.5-3.0 under various ventilation conditions in ISO 9705 and confirmed that the chemical species concentration and ULT had a linear relationship with GER, and the predictability through correlation was suggested. In addition, [PERSON] et al. [20] investigated local and global combustion efficiency according to GER to analyze fire phenomena. These studies demonstrate that the GER concept is effective for analyzing the phenomena of fires that grow in stages. [PERSON] et al.