## 2 Experimental Setup  
Due to the high potential spontaneous ignition of pure NC, some additives, served as humectant or plasticizer, are mixed with it to improve its stability in the process of production, storage, and transportation. Moreover, previous attempts on the measurements of HRR and mass loss rate (MLR) of pure NC met with difficulty due to its deflagration with a combustion duration less than 1 s [25]. Thus, all the NC samples in current study are finished products with annexing agents. Three NC samples will be tested. Two of them are NC with alcohol humectant, i.e., 30 wt % isopropanol (C\({}_{3}\)H\({}_{8}\)O, labeled as NC-I) and 30 wt % ethanol (C\({}_{2}\)H\({}_{6}\)O, labeled as NC-E), exhibiting as the similar white and soft fibers. The other one is NC with 19.5 wt % dibutyl phthalate (DBP, labeled as NC-D), which is a mixture of NC fiber and plasticizer, compressed together to form into a strip structure. All the samples are commercial products for coating. Before the tests, all the samples are stored in a tightly sealed environment to prevent the evaporation of alcohol humectant or other sample contamination. The nitrogen content is a key parameter for evaluating the performance of NC, and the thermal stability of NC decreases with increasing nitrogen content [4]. To eliminate this factor, the nitrogen  
Figure 1: The scene of catastrophic fire and explosion caused by NC in Tianjin Port, China. contents of current NC substrates are selected as consistent as possible, i.e., 12%, 11.86%, and 11.96% for NC-I, NC-E, and NC-D, respectively, all of which fall into the range of 11.5-12.2% classified as H-type according to the Chinese standard \"Specification for Nitrocellulose of Lacquers\" (WJ9028-2005) [29]. Comparative tests on NC-I and NC-E also indicate that the 0.14% difference in nitrogen content could hardly affect the microstructure and thermal behavior of NC [25]. Thus, the effect of nitrogen content on the combustion behavior of NC sample is not the research focus and can be reasonably neglected. It is also worthwhile to note that the chemical stability of NC depends on the removal of all traces of acid in the manufacturing process [4]. The acidity of current NC samples is not larger than 0.04%, and its influence on the combustion process is also unnecessary to be concerned. Additionally, other physical parameters of the additives are presented in Table 1 for further discussion. The three NC samples are burned in two square steel burners with side lengths (\(L\)) of 8 cm and 10 cm, respectively, which are 2 cm in depth and 0.32 cm \(\pm\) 0.01 cm in wall thickness. For NC-I and NC-E with fiber structures, the samples are filled in the burners to a height of 1 cm from the bottom. The initial weight for the burner with \(L\) = 8 cm is 10 g, and correspondingly, to guarantee the consistent bulk density, that for burner with \(L\) = 10 cm is 15.63 g, with \(\pm\)0.2 g variation. For comparison, the NC-D samples with strip structure are also prepared with identical initial weights for the two dimensions. All the information pertaining to experimental configurations is listed in Table 2. The arrangements of the three samples for \(L\) = 8 cm are illustrated in Figure 2. Although efforts have been made to arrange the NC samples into the burner to achieve the similar bulk density, there is randomness in the preparation of NC sample due to their irregular fiber and strip structures. Uneven sample surface shown in Figure 2 would lead to some uncertainty of the tests. Thus, all the experimental data used for analysis in current study are the average values over several repeated tests.