# Flame Evolution Characteristics for Hydrogen/LPG Co-Combustion in a Counter-Burmer
## 2 Experimental Implementation
### Digital Image Processing
#### 2.2.1 Identifying the Flame Front  
A series of measures were implemented to ascertain the fundamental properties of the flame front, including the following steps:  
A. Background Elimination and Noise Attenuation: The extraction of flame information from the images was complicated by background noise. The noise originated from either particles adhering to the image surfaces or those generated by variations in light intensity. Therefore, a process to minimize the noise level was particularly necessary. Enhancing flame image quality through adjusting the backdrop parameters and employing image subtraction to diminish background noise was achieved by MATLAB code.  
B. Binary Image and Thresholding Process: The thresholding method generated a binary image by comparing pixel intensities in the matrix to a predetermined threshold value. Pixels with luminance above the set threshold were replaced with a value of 1 (white), while all remaining pixels were assigned a value of 0 (black). In the past, several approaches have been proposed for determining the appropriate threshold value, as mentioned in [3; 28]. Typically, these methods are based on the mean intensity level or the intensity histogram of the image. Figure 3 shows the effect of different image brightness levels. During the experimental analysis of flame size and threshold values, two images were obtained: a darker image (a) and a brighter image (b). The thresholds ranged from 0.75 to 0.88, which allowed the reduction in light elements associated with the image. In the final step of this process, images were transformed into a binary format using a threshold value of 0.79.  
Figure 3: Frequency histogram and intensity of image; (a) Darker image, and (b) lighter image.  
Figure 2: Schematic of digital image processing algorithm.  
C. Morphological Operations on Binary Images: Morphological techniques were utilized to improve the quality of the binary images, thereby enhancing the shapes of the flame fronts. Several researchers, including [PERSON], [PERSON] et al. [1], and [PERSON] [13], have conducted a range of morphological image processing steps based on shape-based images. The core methods in morphological processing are dilation and erosion, as explained by [PERSON] et al. [3]. Erosion is the process of removing pixels along the edges of objects, while dilation is the addition of pixels to the boundaries of objects in a binary image. In this study, both erosion and dilation were applied. The number of pixels added or removed depended on the size and shape of the structuring element used in the binary image processing. Consequently, it altered the dimensions and configuration of the flame front, although the combination of these processes retained the overall size of the flame. Small speckles in the images were minimized using the opening operation, whereas gaps in the flame were addressed by employing the closing operation, which will be described in more detail later. The morphological closing operation consisted of applying dilation followed by erosion, using the same structuring element for both steps. The closing procedure filled gaps in the flame front and refined the boundary by modifying its dimensions and shape. When the closing operation was performed on the flame, the flame was initially expanded through dilation and then shrunk via erosion, resulting in a smoother boundary, filled holes, and a preserved shape size. On the other hand, the morphological opening operation involved erosion followed by dilation. As a result, small specks in the image could be removed through the opening operation. Any areas of the flame eroded during erosion could be restored through dilation without altering the main structure of the flame. However, the tiny particles eliminated by erosion were not replaced by dilation. Figure 4 illustrates the image processing methods. The images containing the flames were prepared for further quantitative analysis, including the evaluation of flame progression and measurement of the flame disk properties.