## 3 Results and Discussion
### Effect of External Heat Flux on CO and Soot Yields  
Before examining the changes in CO and soot yields according to external heat flux, the mean mass flow rates of CO and soot according to the mean MLR, which were applied to calculate the yield, are illustrated in Figure 8. CO\({}_{2}\), which is a common complete combustion product, is also presented. The horizontal and vertical error bars in the figure indicate the standard deviation obtained during the process of calculating the average of the tests repeated at least three times. The mean mass flow rates of CO, CO\({}_{2}\), and soot measured in the exhaust duct clearly increased linearly with respect to the mean MLR. Quantitatively, the linear regression line in the figure shows that coefficients of determination (R\({}^{2}\)) were all above 0.96. That is, the amount of CO, CO\({}_{2}\), and soot generated all had a linear relationship with respect to the mean MLR. More specifically, the mean mass flow rates of CO and CO\({}_{2}\) were directly proportional to the mean MLR, considering that the y-intercept of the linear regression lines shown in Figure 8a,b is near 0. In contrast, the y-intercept of the linear regression line in Figure 8c is a negative value that cannot be neglected, which implies that the mean mass flow rate of soot simply had a linearly proportional relationship with the mean MLR instead of a directly proportional relationship. As presented in Figure 6a, the mean MLR increased linearly with respect to the external heat flux, while the yields of combustion products are expressed as the ratio of \(x\)-axis values to \(y\)-axis values shown in Figure 8. Therefore, the yields of CO and CO\({}_{2}\) can have a constant value when the external heat flux increases, whereas the soot yield can be expected to vary. In addition, differences in ignition methods such as auto and spark ignitions had almost no influence on the mass flow rates of CO, CO\({}_{2}\), and soot. Figure 7: Effective heat of combustion as a function of external heat flux for the auto and spark ignitions. Figure 9 illustrates the changes in CO, CO\({}_{2}\), and soot yields with respect to the external heat flux. As shown in Figure 8, the mean mass flow rates of CO and CO\({}_{2}\) were directly proportional to the mean MLR, while MLR was known to increase linearly as the external heat flux increased. Accordingly, CO and CO\({}_{2}\) yields were 0.0070 and 2.3202 g/s, respectively, regardless of the external heat flux or ignition method, as shown in Figure 9a. For reference, the CO and CO\({}_{2}\) yields measured in this study were quantitatively very similar to the results of a previous study [31]. Constant CO and CO\({}_{2}\) yields with respect to the external heat flux imply that the ratio of CO/CO\({}_{2}\) was also constant. The ratio of CO/CO\({}_{2}\) is used for predicting CO generation in the hazard assessment of some room fire models in which the value is known to have a relatively consistent value in the given fuel and fire environments [48]. The ratio of CO/CO\({}_{2}\) was constant because the same ventilation conditions (a cone calorimeter in an open space) of fire as black acrylic were applied. As mentioned in the introduction, an increase in the external heat flux can induce changes in the flame height and residence time through an increase in MLR. However, the changes in these factors related to CO generation did not significantly impact CO emission in a cone calorimeter where the air is sufficiently provided, which is a meaningful implication. As illustrated in Figure 9b, the soot yield increased linearly with respect to the external heat flux, unlike the CO yield. This result was deduced because the mean mass flow rate and mean MLR of soot shown in Figure 8b were not in a directly proportional relationship. Considering previous studies on soot formation, an increase in the residence time due to flame height, which increases linearly at the 2/3 power of MLR, has contributed most significantly to the increase in the soot yield.