Since the temperature change in the experiments stabilized and slowly rebounded at the end of the leakage process, the third phase described in Section 4.1.2 was not studied, so the analysis of the temperature change over time for each sensor in each group of experiments focused on the phase where the temperature plummeted after the leakage occurred.
In this group of experiments, the experimental data in different experimental parameters are reproducible. In this section, the temperature distribution is analyzed from the point of view of comparison with each measurement point of the same measuring rod and comparison with each measurement point of the same height level, using a leak diameter of 5 mm and a leak pressure of 15 MPa as the research object.
4.2.1 Analysis of the temperature change pattern of each measuring rod measurement point
This subsection is analyzed with each measuring rod as an independent individual, analyzing the temperature variation with time for different vertical distances of the measuring points on the same rod and the minimum temperature variation with vertical distance for each measuring point.
The measuring rod No. 3 in the axial region is located directly above the leakage port and is the rod with the largest temperature drop among all the measuring rods. As can be seen from Fig. 7, the maximum temperature difference measured in the experiment reached 42.8℃, and the temperature difference of most measurement points on the No. 3 rod decreased with the increasing distance from the leak, but the largest temperature difference occurred at the 20 cm vertical distance from the leak rather than at the 10 cm nearest to the leak. The reason is that the CO2 jet at the vertical distance of 20 cm from the leak is in the most intense phase change state and absorbs a large amount of heat from the surrounding soil environment, making the temperature difference at this location the largest. At the location below 20 cm, the temperature at the location below 20 cm is slightly higher than that at 20 cm because the CO2 jet energy is larger, so that a large amount of CO2 does not phase change at the lower location. The temperature at the other locations of the measurement point becomes less energy absorbed as the distance increases, so the temperature difference becomes smaller.
The temperature change of each measurement point on the No. 3 pole with time is shown in Fig. 8, the temperature change is "first drop and then rise" phenomenon, and the time point of temperature drop are relatively close, if the time is extended indefinitely, the temperature will be the same as the ambient temperature.
The temperature of the measurement points on the No.3 measuring rod from the vertical distance of 10 cm to 50 cm from the leakage port have a common feature, that is, the temperature curve has a roughly similar trend of ups and downs, there is a "sudden drop - rapid recovery - and then drop" phenomenon, and marked in Fig. 8. The main reasons for this phenomenon are as follows.
1) The soil medium around the leakage port was blown away by the huge impact. After the rupture of the rupture disc, the pressure-bearing device instantly released pressure and produced a CO2 jet. According to the experimental results of the literature [114], the impact force out of 0.25 m from the leakage port was 1500 N or even higher, and under the action of the powerful jet impact, the soil medium near the measurement point of the lower part of the No. 3 measuring rod was instantly blown away to the surrounding, making instantaneous influx of air in the surrounding environment. Because the thermal conductivity of air is much smaller than the thermal conductivity of soil media (air thermal conductivity 0.023 W·(m·K)−1, industrial sand thermal conductivity 0.27 W·(m·K)−1), resulting in the temperature of the air in the region is higher than the soil media, making the temperature curve will appear a short time back up; and then under the action of gravity, the lower temperature of the soil media back down under the action of gravity, so that the temperature again decreased. so that the temperature drops again.
2) Dry ice accumulation. After the leak of the pressure-bearing device, the pressure inside the device is instantaneously reduced, and the violent phase change of CO2 leads to the accumulation of a little dry ice condensed into CO2 at the leak, which hinders the release of CO2 and makes the state of CO2 release unstable, which leads to the incoherent leakage of low-temperature gas and a brief recovery of temperature.
The horizontal distance between the No.2 measuring rod and the leakage port is 2 cm, taking the No. 2 measuring rod as an example to analyze the law of temperature change in the near field, as can be seen from Fig. 9, the temperature change on the No. 2 measuring rod has two phenomena.
1) Closer to the leak of the two vertical distance of the lower measurement points (10 cm at 20 cm) temperature trends are in line with the measurement points in the No. 3 pole "plunge - rapid recovery - and then fall" changes, the reason is the same as the No. 3 pole.
2)The remaining five points of temperature in the value of the existence of a large drop, the location of the lower two points of the temperature river amplitude is smaller.
Analysis of the reasons for the generation of two temperature changes. The initial form of CO2 jet forming is vertical upward, so that it has less ripple for the surrounding measurement points at lower locations; with the upward injection of CO2 gas, turbulence occurs under the action of the pore resistance of porous media, making the gas in the porous media seepage trajectory changes, most of the seepage CO2 acts on the position between 40 cm and 50 cm of the No. 2 measuring rod, resulting in a vertical distance from the leak The temperature drop at 10 cm and 20 cm is much smaller than the temperature drop at 40 cm and 50 cm of the leak.
Since the temperature drop measured at the near measurement point is mainly from the CO2 jet, and the temperature drop measured at the far measurement point is mainly from the seepage of CO2 in the soil medium and heat transfer, the temperature drop rate at the near measurement point is significantly higher than the temperature drop rate at the far measurement point.
The whole process can be divided into three regions according to the trend of curve changes in Fig. 10. The first region is located at the measurement points closer to the leak (at 10 cm and 20 cm), and the temperature difference between them is very small, only about 6℃. This indicates that these two measurement points are not affected by the diffusion of CO2 gas at low temperature, because the initial shape of the CO2 jet is vertical upward, so that these two points are not affected by the CO2 jet and phase change heat absorption, but only by the heat transfer, which makes the temperature of the two places decrease.
The second region is located slightly further away from the measurement points 20 cm, 30 cm, 40 cm and 50 cm from the part of the measurement point. Unlike the case of measurement rod No. 3, the temperature drop in this region increases with the vertical distance and reaches a maximum temperature difference of 15.6℃ at 50 cm. The temperature drop at the measurement point in this distance interval is the most obvious, because the CO2 jet is turbulent under the action of the pore resistance of the soil medium, which makes the gas percolation trajectory in the porous medium change, and most of the percolating CO2 acts on the position between 40 cm and 50 cm of measurement rod No.2. The temperature drop of each measurement point of measurement rod No. 2 is the largest at 40–50 cm.
The third region is located at the top 50 cm, 60 cm and 70 cm of the measuring point part of the No. 2 measuring rod, the temperature drop in this region decreases with the increase of vertical distance, the law is consistent with the characteristics of heat transfer, indicating that this region is not affected by the jet action wave.
Take No. 1 and No. 4 measuring rod as an example to analyze the temperature change law in the middle and far field, as shown in Fig. 11, the temperature change with time at each measuring point on No. 1 and No. 4 measuring rod is roughly the same, only the temperature change at the vertical distance of 40 cm and 50 cm measuring point is slightly different. The reason is that the sandy soil texture is not uniform enough. Although the soil environment laid in the experiment was uniform enough, it was still far from the ideal condition, which led to the difference between measuring rod No.1 and No.4 even though they were in a symmetrical position. And the CO2 jet pattern is not completely symmetrical due to the accumulation of dry ice at the leakage port caused by the CO2 release. A similar description of the morphology of the CO2 jet is given in the literature [114]. Due to the violent phase change in the pressure-bearing device during the rapid CO2 release, the jet pattern of the CO2 release is highly random and unpredictable. This is one of the main reasons why the data from the two measurement rods are not completely symmetrical.
Based on the good agreement between the data of measuring rod 1 and measuring rod 4, the deployment scheme can be simplified. Ignoring the small differences in temperature, it is assumed that the soil medium is homogeneous and that the properties are independent of location and orientation. Therefore, the temperature sensors can be placed in one direction only.
As shown in Fig. 12, by comparing the data of these two measuring rods it can be concluded that the measurement points of the two rods have a good consistency in the temperature difference trend, as well as the characteristics of the temperature in each region. The temperature drop of the measurement point in the top region increases with the increase of the vertical height. On the contrary, the temperature of the three sensors at 0, 10 cm and 20 cm, which are closer to the leak, has almost no change. The remaining measurement points satisfy the rule that the larger the vertical distance, the larger the temperature difference.
Since the initial shape of CO2 jet is vertical upward, the measurement points at the lower part of measuring rod No. 1 and No. 4 are not affected by CO2 jet and phase change heat absorption, but only by heat transfer, so there is almost no temperature change at the measurement points at the lower part of measuring rod No. 1 and No. 4; due to the existence of pore resistance of soil medium, CO2 kinetic energy is gradually dissipated and the movement of CO2 changes from jet to percolation diffusion of porous medium. Due to the existence of the pore resistance of the soil medium, the kinetic energy of CO2 is gradually dissipated, and the movement of CO2 changes from jet flow to percolation diffusion in the pores of the porous medium.
As can be seen from Fig. 13, the closer the measuring rod from 5 to 8 is to the leak, the greater the maximum temperature difference obtained from the measuring point on the rod. At the same time, the temperature difference also tends to increase as the vertical distance from the leak is larger.
Table 4
Time for each measuring point of No. 5 ~ 8 measuring rods to reach the lowest temperature/s
Vertical distance/cm
|
Measuring rod number
|
NO.5
|
NO.6
|
NO.7
|
NO.8
|
0
|
2 496
|
2 004
|
1 920
|
1 640
|
10
|
2 974
|
2 232
|
1 882
|
1 622
|
20
|
2 876
|
3 104
|
2 596
|
1 634
|
30
|
2 778
|
2 718
|
1 290
|
1 524
|
40
|
2 698
|
2 794
|
3 040
|
2 806
|
50
|
1 996
|
1 968
|
2 892
|
2120
|
60
|
78
|
78
|
154
|
2 578
|
70
|
78
|
88
|
76
|
98
|
Combined with Fig. 13 and Table 4, the time required to reach the minimum temperature at the measurement points on different rods was also different, and the time required to reach the minimum temperature was longer as the distance of the measurement rods from the leakage port was farther. After being obstructed by the pore resistance of the porous medium, the jet form changed at a distance of about 40 cm from the leakage port, and the trajectory of CO2 gradually changed from vertical upward to seepage around.
4.2.2 Analysis of temperature change pattern of each horizontal measurement point
The analysis method in this subsection is to take the measurement points located on the same horizontal plane as the object of study and observe the temperature change of each measurement point in relation to the horizontal distance from the leakage point.
The relationship between the maximum temperature difference of each measurement point on the horizontal plane at a vertical distance of 10 cm and 20 cm from the leak and the distance from the leak is shown in Fig. 14. The temperature of the measurement points on these two horizontal planes only changed significantly at the measurement points of measuring rod No. 2 and measuring rod No. 3. That is, at these two heights, only the area within 20 mm from the leakage port has a significant temperature drop.
On the horizontal surface relatively close to the leak, the area of temperature drop is smaller and the lowest temperature is located directly above the leak. From the morphology of the jet based on the CO2 injection, it is known that the closer to the leak, the smaller the radiation area of the jet; the area outside this range is not affected by the jet, and all heat transfer is carried out through heat transfer.
The maximum temperature difference between the vertical distance of 30 cm and 40 cm from the leak is similar to that between 10 cm and 20 cm, and the relationship between the maximum temperature difference and the horizontal distance from the leak is shown in Fig. 15.
The results show that the larger the horizontal distance from the leak port in the same plane, the smaller the maximum temperature difference. As the distance between the measurement point and the leakage port increases, its temperature influence decreases and the temperature drop becomes flatter. The gradient of temperature drop in each horizontal plane is different.
The relationship between the maximum temperature difference and the horizontal distance from the leak at the vertical height of 50 cm to 70 cm is shown in Fig. 16. The temperature change is also similar to the trend of the rest of the horizontal surface, and the overall trend shows that the smaller the horizontal distance between the measurement point and the leak, the greater the temperature difference. At the distance of about 50 mm from the horizontal distance, the order of the temperature difference between the horizontal surfaces changed. The temperature difference of the horizontal surface at 50 cm, which is the smallest vertical distance from the leak, is the smallest. In turn, at 225 mm, the temperature difference of the horizontal plane at 70 cm, which is the farthest from the vertical distance of the leak, is the largest, and as the horizontal distance between the measurement point and the leak increases, the larger the vertical distance, the larger the temperature drop of the plane. The two main reasons for the above phenomenon are as follows.
(1) Combined with the data from the previous planes, it can be seen that the movement of CO2 in the porous medium after it is released from the leak is "funnel" type.
(2) The porosity of the soil medium at the bottom is larger than the porosity of the surface soil due to the gravitational effect of the soil, and thus its pore resistance is larger, so the CO2 percolation in it is more difficult and the maximum temperature difference is smaller.
Figure 17 shows the temperature thermogram of each horizontal surface at the moment of 80 s using Matlab software. From the figure, it can be seen that the highest measuring point on each measuring rod has reached the lowest temperature, and the temperature distribution law of each horizontal surface at this time has the following characteristics.
(1) In each horizontal surface of the measurement point, the measurement point with the largest temperature difference is located directly above the leak, and the lowest temperature in the entire region appears in the vertical distance from the leak 20 cm horizontal surface.
(2) Each horizontal measurement point in the closer to the leak, the more rapid the temperature drop, the temperature drop with the rise of the horizontal surface and decrease.
(3) As the vertical height of the horizontal surface from the leak increases, the range of the low temperature region in the horizontal surface also gradually expands.
It can be seen that after the pipeline leaks in the sandy soil for a period of time, the temperature distribution in the soil medium is funnel-shaped due to the phase change of dense-phase liquid or supercritical CO2, heavy gas diffusion, pore resistance, fluid percolation, and energy transfer. When the vertical distance from the leak is 10–20 cm, the temperature drop is more obvious only in the area directly above the leak, which is caused by the absorption of ambient heat by the phase change of dense-phase liquid or supercritical CO2, and the morphology of the high-pressure gas jet. The high-pressure jet of low-temperature gas can maintain a relatively stable motion trajectory vertically upward at close range under the effect of initial kinetic energy. With the increase of injection height, the kinetic energy of CO2 jet is lost due to the influence of soil pore resistance and gravity, and the motion form of CO2 in soil media is changed from seepage to percolation in soil media pores, and the motion trajectory is changed from vertical upward to diffusion to the surrounding environment. The low temperature gas gradually diffuses to the surroundings, the temperature difference gradually decreases, and the low temperature area gradually expands.