## 1 Introduction
[14] verified that significant changes are observed in the ignition time, HRR, and smoke specific extinction with respect to the external heat flux (20-75 kW/m\({}^{2}\)) for 25 plastic materials. The researchers also proposed that the examination of the heat flux levels for responding to a developing and a developed fire is important when considering the environment in which materials are applied. As shown in previous studies mentioned above, the importance of the effects of external heat flux are sufficiently known in a fire risk evaluation using a cone calorimeter in which the flammability, pyrolysis, HRR, and smoke production rate are taken into account. However, there is a lack of research on the effects of the external heat flux on the generation of carbon monoxide (CO) and soot, which are major hazards in a fire. CO is a typical incomplete combustion product of hydrocarbon fuels, and the inhalation of CO is a major cause of death in fires. The generation of soot reduces visibility, which hinders the evacuation and fire extinguishing activities of firefighters, while the increased radiant heat owing to soot may increase the flame spread and burning rate [15; 16]. Therefore, it is important to predict the amount of CO and soot generated from various combustibles in a fire environment when attempting to accurately evaluate the associated fire risks. The yield, which is defined as the ratio of the mass production rate of chemical species to the mass reduction rate of fuel, is often used to determine the amount of CO and soot generated in a fire, in addition to the generation factor and emission index, which have the same meaning [17; 18]. The product emission yields for various combustibles obtained from a bench-scale cone calorimeter are directly used to evaluate the quantitative risks of hazardous materials, while providing useful information for extrapolating full-scale fire scenarios [1]. In general, the emission of CO in buoyant nonpremixed (diffusion) flames found in fires is generated from the imbalance of the production rate and oxidation rate. When the reaction pathway of hydrocarbon fuels is examined, CO is generated through the very complicated process of oxidation reactions, and then CO is converted to CO\({}_{2}\) mainly by reacting with OH. The reaction of CO+OH is relatively slower than other radical reactions,and therefore, the CO that is not oxidized is eventually discharged [18]. Conceptually, the emission of CO may increase if there is an insufficient amount of additional air required for the generated CO to be oxidized or an insufficient residence time for the oxidation reaction to occur [19]. If this principle of CO generation is applied to an experimental environment of a cone calorimeter, where the air is sufficiently supplied to an open space, the mixing of CO and air may not have a significant impact on CO emission. In contrast, the changes in MLR and flame height according to the external heat flux [13] may influence the residence time in the region, satisfying the oxidation reaction conditions (temperature and oxygen concentration) of CO.  
For soot, which has more complicated generation characteristics than CO, the flame temperature and residence time in the high-temperature region are known to be dominant parameters for controlling soot formation in a flame [20]. In fires involving hydrocarbon fuels, the flame temperature has a critical influence on soot formation starting from fuel pyrolysis, which corresponds to an endothermic reaction, to soot oxidation. When the flow rate of fuel changes, the changes in air entrainment and flame strain rate near the stoichiometric flame surface may cause the flame temperature to change, while the soot and flame temperature affect each other owing to radiation heat transfer.