## 4 Conclusions  
This study experimentally analyzed combustion properties according to irradiance level (external heat flux) using a cone calorimeter. Black acrylic having similar constituents as PMMA was used as a combustible, and external heat fluxes within the range of 15-65 kW/m\({}^{2}\) were considered. Auto and spark ignitions were all applied as ignition methods. The ignition time, MLR, HRR, EHC, CO, CO\({}_{2}\), and soot yields were measured. In particular, the need to re-examine the trend of changes in CO and soot yields was highlighted when there were fluctuations in the external heat flux as the primary parameter. In addition to the external heat flux, the effects of changes in the exhaust flow rate and hood height on CO and soot yields were also examined. The major findings of this study are as follows.  
Auto and spark ignitions that can be applied to a cone calorimeter exhibited diverse ignition phenomena depending on the occurrence of gas-phase reactions in the initial stage of pyrolysis under a relatively low external heat flux condition. However, the difference in the ignition method did not affect global parameters such as the MLR, HRR, and EHC, as well as CO and soot yields.  
As the external heat flux increased, the mean MLR and HRR linearly increased, while the EHC remained constant. When the external heat flux increased, the mean mass flow rates of CO and CO\({}_{2}\) had a directly proportional relationship with the mean MLR. Consequently, CO and CO\({}_{2}\) yields remained constant regardless of the external heat flux. In contrast, the mean mass flow rate and mean MLR of soot were linearly proportional instead of directly proportional, and the soot yield thus increased linearly with the external heat flux.  
The changes in the exhaust flow rate and hood height that can alter the velocity and temperature fields in post-flame and plume regions had almost no impact on the changes in CO and soot yields, as well as MLR and HRR. Accordingly, it can be deduced that there was no dependency of an individual researcher, who would be available during the experimental process or setup of exhaust flow rate and hood height, and the reference values proposed in ISO 5660-1 were sufficiently representative. The results of this study are expected to provide improved insight into conventional approaches on recognizing CO and soot yields as unique properties of each combustible, which are crucial for evacuation and personal safety.  
Conceptualization, S.-Y.M., J.-H.C. and C.-H.H.; methodology, S.-Y.M., J.-H.C. and C.-H.H.; software, S.-Y.M.; validation, S.-Y.M. and C.-H.H.; formal analysis, S.-Y.M. and C.-H.H.; resources, C.-H.H.; data curation, S.-Y.M. and J.-H.C.; writing--original draft preparation, S.-Y.M.; writing--review and editing, C.-H.H.; visualization, S.-Y.M.; supervision, C.-H.H.; project administration, C.-H.H.; funding acquisition, C.-H.H. All authors have read and agreed to the published version of the manuscript.  
Figure 12: Variations in CO, CO\({}_{2}\), and soot yields with changes in exhaust flow rate and hood height (h).  
**Funding:** This research was supported by the National Nuclear R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education Science and Technology (NRF-2018M2A8A4013721).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available upon request to the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest.