Figure 2 Flame processing images.
Flame morphology and flame height variations were recorded by digital and high-speed cameras. In each experiment, the two cameras were set at a certain distance from the cup burner, so the entire flame pattern could be recorded. In this study, Otsu’s method was used to process flame images to obtain flame height, and flame height (h) was defined as the vertical distance from the nozzle to the flame tip[19, 20]. First, the original images were converted to gray scale images. each pixel point had its own luminance value in the gray scale image and the threshold value of the image was obtained objectively by using Otsu method. Then based on the luminance value and threshold value, the binary image could be obtained[21]. Finally, the binary image was processed to become the intermittency distribution image by processing program (Figure 2), which showed the probability of the flame occurring.
2.2 Theoretical calculations
In order to explore the detailed path of the fire extinguishing mechanism, Gaussian 16 was used for theoretical calculations, and Gauss view was used to view the optimized geometries of space[22, 23]. On the basis of DFT calculations, B3LYP/6-311++G (d, p) base sets were used for calculations. Reactants, products, transition states (TSs) and intermediates (IMs) were analyzed by DFT calculations, and the above calculation results were verified by using virtual frequencies, and there were no virtual frequencies for reactants, IMs and products, but only TSs had virtual frequencies[24, 25]. Then, the relationship and transformation relationship between the reactants and the products were analyzed using IRC theory. Meanwhile, in order to verify the accuracy of the reaction energy calculation value, single-point energy calculation and zero-point energy correction were performed on all stationing points. A more precise energy value was calculated at the CCSD/aug-cc-pVDZ level, the optimized structures were also employed in a series of single-point energy calculations by using coupled-cluster theory, and the correctness of each reaction path was verified to calculate the zero-point vibrational energy (ZPE) at the CCSD/aug-cc-pVDZ level[26, 27]. The energy barrier is the energy difference between the TSs and the reactants.