# Detection of High-Temperature Gas Leaks in Pipelines Using Schlieren Visualization
## 3 Experimental Results and Discussion  
Two experimental setups were devised using a Schlieren optical system to observe and detect leaks in high-temperature gas flow pipelines, such as those found in power plants. The initial configuration entailed a PTFE tube with a direct perforation. The outcomes, documented at varying temperatures of the leaking air, are illustrated in Figure 4. Since the laboratory air temperature was maintained at 14 \({}^{\circ}\)C, the actual temperature difference between the ambient environment and the leaking air ranged from a minimum of 6 \({}^{\circ}\)C to a maximum of 86 \({}^{\circ}\)C. At a leaking air temperature of 20 \({}^{\circ}\)C, the leak was only discernible to a slight degree. However, at temperatures exceeding 30 \({}^{\circ}\)C, the leak could be unambiguously identified. Figure 4: Schlieren imaging of leakage in a PTFE tube depending on the leaking temperature. As the temperature of the leaking air increased, the width of the leak expanded, and the shadows of the leaking flow became more distinct. While the flow patterns remained similar between 30 \({}^{\circ}\)C and 50 \({}^{\circ}\)C, more apparent shadow differentiation was noted at temperatures above 60 \({}^{\circ}\)C. The flow area of the leaking gas approximately doubled when comparing the flow at 60 \({}^{\circ}\)C to that at 100 \({}^{\circ}\)C. Given that the volumetric flow rate of the ambient air entering through the hot air blower was kept constant, the 40 \({}^{\circ}\)C temperature increase corresponds to an approximate 10% increase in volumetric flow rate and leakage velocity. However, as the temperature and velocity of the leaking air increase, the surrounding air is heated and entrained, resulting in a broader flow area and a more extensive leakage pattern. Furthermore, as the temperature of the injected air increases, flows presumably due to natural convection on the insulated surface were observed. This phenomenon can be found in images where the injected air temperature is above 60 \({}^{\circ}\)C, as rising flows to the left and right of the main flow. Despite the PTFE tube being covered with urethane insulation, visualization of natural convection occurs as the surface temperature of the insulation rises above that of the surrounding environment due to the increased temperature of the injected air. To confirm the presence of leaks from the images at varying temperatures, the brightness values of individual pixels were extracted to calculate the \(I_{E}\), as shown in Figure 5. The region where the brightness value was calculated is visible in the experimental image with an injected air temperature of 100 \({}^{\circ}\)C, which is inserted at the top left of Figure 5 (red dotted region). As expected, the \(I_{E}\) value shows a steady increase with the rise in the leaking air temperature. A sudden expansion of the flow width is observed when the leaking air temperature exceeds 60 \({}^{\circ}\)C. These results lead to the conclusion that a leak is present in the pipe if the temperature difference between the hot leaking air and the surrounding air is 16 \({}^{\circ}\)C or more, or if the \(I_{E}\) is 0.1 or higher. The experiment in Figure 6 involved imaging leaks from a perforated pipe surrounded by insulation and a cover jacket with gaps. Similar to the previous PTFE leak experiment, this test was carried out in an environment with ambient air at 14 \({}^{\circ}\)C. In this experiment, the temperature of the air injected into the aluminum pipe was incrementally increased from 20 \({}^{\circ}\)C to 100 \({}^{\circ}\)C in 10 \({}^{\circ}\)C intervals, and Schlieren flow visualization images were captured. The leak gap in this experiment had a larger area compared to the PTFE tube experiment. As a result, the flow velocity was relatively lower, making it more difficult to observe distinct shadows due to the insulation and aluminum jacket. When the temperature of  
Figure 5: Calculation of \(I_{E}\) depending on temperature of leaking air inside the PTFE tube.