## 2 Experimental Method and Conditions
### Experimental Conditions  
To examine the effects of changes in the external heat flux, exhaust flow rate, and hood height on CO and soot yields for black acrylic, an open-cone calorimeter illustrated in Figure 1 was produced based on ISO 5660-1 ([PERSON] et al., 2019). This apparatus was designed to be utilized as a controlled atmosphere cone calorimeter (CACC) ([PERSON] et al., 2019) to examine the variation in the ignition characteristics of solid combustibles with the oxygen concentration. Specifically, an open-cone calorimeter can be embodied by opening the front and rear back sides. Compared to the original open-cone calorimeter, by performing numerical simulations [40,41] and experiments comparing results with previous studies, it has been adequately proven that a limited open area due to external housing has no effect on measurement data (ignition time, HRR, yield, etc.). The HRR was measured based on the oxygen consumption technique [42,43], and was calibrated up to 8 kW using propane fuel. The MLR of black acrylic was measured using a load cell. To minimize the measurement errors of a load cell due to radiant heat, the apparatus was designed so that coolant is supplied outside the box in which the load cell is installed. The external heat flux, which is the primary parameter of this study, was varied within a wide range of 15-65 kW/m\({}^{2}\). The calibration of the external heat flux was performed by a Schmidt Boelter gauge at the specimen location. The surface area of the specimen was 100 mm \(\times\) 100 mm, and the actual surface area receiving radiant heat from the specimen holder was 0.0088 m\({}^{2}\) (94 mm \(\times\) 94 mm). The maximum value of HRR, the penetration time of heat in the depth direction, and the CO yield can be markedly affected by the thickness of the specimen [44]. Thus, the specimen thickness was fixed at 10 mm in this study. The distance between the specimen surface and the bottom of the cone heater was fixed at 0.025 m. The cross-sectional mean velocity inside a duct was measured using a blade-shaped pitot tube (Debimo 125, KIMO) and a differential pressure transmitter (MS-311-LCD, Dwyer). To ensure that the volume flow rate of the exhaust hood was consistently supplied, the hood was operated for 10 min before the experiment, and then a gas analyzer was calibrated. The experiment was repeated at least three times for each specimen to obtain the mean and standard deviation of the measurement data. To ignite solid combustibles in a cone calorimeter, the spark ignition and auto-ignition methods can be applied. The pilot ignition process is controlled by pyrolysis, while the auto ignition process is known to be dependent on the gas-phase reaction, as well as the pyrolysis [36]. Therefore, to examine the effects of an ignition process on burning characteristics, including the CO and soot yields according to an ignition method, both the spark ignition and auto ignition methods were considered in this study. Figure 2 shows the images of the cone heater and the schematics of two exhaust hood heights (h) considered in this study. The exhaust flow rate and hood height (vertical height from the specimen surface to the bottom of the hood) based on ISO 5660-1 are 24 L/s and 0.25 m, respectively. As mentioned in the introduction, the changes in MLR with respect to the external heat flux induce changes in the flame height, thus significantly affecting CO  
Figure 1: Schematic of a cone calorimeter and photograph of test section. and soot yields accordingly. As an additional parameter that may affect the flame height, the changes in the exhaust flow rate may change the entrainment of the surrounding air, which can also significantly affect the flame height due to flame stretch. Therefore, under the condition where external heat flux is fixed at 50 kW/m\({}^{2}\), five different conditions of exhaust flow rate within the range between 24 and 86 L/s were applied.