# Detection of High-Temperature Gas Leaks in Pipelines Using Schlieren Visualization
## 3 Experimental Results and Discussion
the leaking air surpassed 40 \({}^{\circ}\)C, faint rising flows became noticeable. Due to the low flow velocity, the flow tended to deviate to the left or right, depending on the surrounding air currents. Transparent shadows that were visually distinguishable were observed only when the leaking air temperature reached 70 \({}^{\circ}\)C or higher. While the leakage flow in the PTFE pipe exhibited turbulent characteristics at all temperatures, the flow depicted in Figure 6 can be classified as laminar, regardless of the leakage temperature. Despite the leakage hole being six times larger, the flow rate was maintained constant, reducing the leakage velocity and Reynolds number to 1/36 and 1/6, respectively. Consequently, the low flow velocity made the leakage highly susceptible to surrounding air currents. This led to a flow pattern distinctly different from that observed in the PTFE experiment. As in the preceding experiment, an area for observing pixel brightness values was established at the bottom-center section. Although the area was set to the exact dimensions as in the PTFE experiment, most of the leakage flow was observed to be biased, approximately 15 mm to the left. Consequently, the observation area was shifted by this distance from the center to the left. The observation area (red dotted region) and the calculated \(I_{E}\) for this area are illustrated in Figure 7. In all leakage temperature conditions, the calculated values were below 0.1, which is lower than those observed in the PTFE tube experiment. This can be attributed to the significantly reduced flow velocity resulting from the larger size of the leakage gap in comparison to the 2 mm hole observed in the PTFE tube experiment. Additionally, direct leaks result in high turbulence intensity, which causes more complex shadow patterns due to the presence of local lateral and vertical flow components. In contrast, the low-velocity laminar flow observed in this experiment resulted in less distinct shadow differentiation between pixels. Figure 6: Schlieren imaging of leakage in pipe covered by insulation and aluminum jacket with a gap. Experimental errors are presumed to stem primarily from the blower's flow rate and ambient air current. The blower's flow rate was adjusted by rotating the knob without further verification, potentially introducing an error of about 10%. Conversely, the leakage temperature was measured using a thermocouple with an error margin of 0.2 \({}^{\circ}\)C, resulting in relatively precise control. These errors are reflected in the experimental results as deviations in \(I_{E}\). The standard deviation of \(I_{E}\) used to quantify leakage in both pipes was less than 10% of the mean value. Analyzing 20 images for each leakage temperature case, we found that the deviation in the calculated \(I_{E}\) across all images matched this standard deviation level. The \(I_{E}\) deviation was nearly 10% in the high-turbulence PTFE pipe experiment. In contrast, it was under 4% in the aluminum pipe experiment. The leak experiment involving a pipe covered with insulation and an aluminum jacket may present challenges in determining the presence of a leak through Schlieren flow visualization, in contrast to the previous PTFE tube experiment. This is because the shadows generated by the flow emerging from the gaps in the aluminum jacket are similar to those caused by natural convection on the insulation surface in the PTFE tube experiment when the air temperature exceeds 90 \({}^{\circ}\)C. Suppose the aluminum jacket surface is subjected to direct sunlight. In that case, it may prove challenging to distinguish between the rising flow of hot, leaking air and natural convection. Most leaks occur through narrow cracks rather than circular drilled holes. Therefore, the leakage flow patterns observed in this study are expected to differ from those of actual pipe leaks. However, the most significant factor affecting the leakage pattern in Schlieren visualization is the leakage temperature and velocity.