# Study on Two-Phase Mixing Inside Flow Focusing/Blurring Nozzle by Gray Distribution Analysis Method
## 3 Gray Distribution Analysis of Experimental Images
### Feasibility Analysis
In case 2, the boundary of the gas-liquid two-phase fluids in the mixing zone is also fuzzy. The gas-liquid fluids mixed, but it is obvious that the gas-liquid flow in the mixing zone is not uniform. The flow pattern in the mixing zone is in a transition state between flow focusing and flow blurring. At this time, there are both liquid drops and distorted two-phase interface in the mixing zone, which makes the light enter the SLR camera through transmission or scattering. Therefore, the pixel points of the image are either bright or dark, which is very nonuniform, resulting in a large range of gray distribution of the image and a smooth gray distribution curve. Figure 4 and its analysis show that the gray distribution of the experimental image is different when the flow pattern of the mixing zone of the flow focusing/blurring nozzle is different, so it is completely feasible to use the gray distribution of the two-phase flow images to identify the flow pattern. In fact, there are many methods to distinguish the flow pattern in the mixing zone, and the two-phase flow in the mixing zone under the flow blurring is the real difficulty in the study of this nozzle. Generally, the mixing intensity and uniformity can be used to characterize the two-phase mixing in the mixing zone under the flow blurring, but there is no suitable instrument to measure these two parameters. The gray distribution of the experimental images can be used to analyze the gas-liquid mixing intensity and uniformity. Figures 5 and 6 show the experimental images with different mixing intensity or uniformity and their gray distribution curves, respectively. Figure 6: The experimental image with different mixing uniformity and its gray distribution. Figure 5a,b show the two-phase flow images in the mixing zone with different mixing intensity. Compared with case 1, the gas-liquid flow in the mixing zone is closer to fog in case 2, and the boundary of the gas-liquid fluid is more fuzzy, which indicates that the gas-liquid mixing intensity in case 2 is stronger. By comparing the gray distribution of case 1 and case 2, it can be found that the average gray scale level of the images is lower when the mixing intensity of two-phase is larger. This is because, when the gas-liquid mixing intensity is relatively high the number of broken droplets is large and the size of the droplets is small, which makes the scattering effect stronger. Therefore, the light entering the image acquisition equipment is weakened and the average gray level of the image is relatively low. Figure 6a,b show the two-phase flow images in the mixing zone with different mixing uniformity. Compared with case 2, the gray scale level of the gas-liquid flow images at different positions in the mixing zone in case 1 is significantly different, which indicates that the uniformity of the gas-liquid fluid mixing in the mixing zone in case 1 is low. The gray distribution of the images indicates that the frequency value of the maximum frequency gray scale level is larger when the gas-liquid mixing is uniform and the gray distribution curve of the image is more concentrated. When the two-phase mixing is nonuniform, the size of the liquid drops at different positions in the mixing zone is different, and the scattering effect is different, resulting in different gray levels of the pixels. When the two-phase mixing is uniform, the scattering effect of the liquid drops at different positions in the mixing zone is the same, and the gray scale level of the pixels is almost the same. Therefore, when the two-phase mixing is uniform, the gray distribution curve of the two-phase flow image is more concentrated. The above research shows that the difference of the flow pattern, the gas-liquid mixing intensity and mixing uniformity in the mixing zone can be described by the gray distribution of the image. Therefore, the influence of flow parameters and nozzle structure on the flow pattern, the gas-liquid mixing intensity and uniformity can be studied by gray distribution of the images.