Analysis and Research on Experimental Process of Water Thermal Migration of Freeze–Thaw Cracked Rock Based on Particle Tracking Method and Thermal Imaging Technology

Abstract

In high-altitude and cold regions, external dynamic geological processes, such as glacial melting and other processes are intense, which frequently results in surface dynamic geological processes, such as slope collapse, landslides, debris flows, and ice avalanches along the route. For high and steep slopes in high-altitude regions containing controlled fractures, the key is to grasp the water-heat process and the evolution of the frost heaving force induced by it within the fractures. This can then lead to the exploration of the multi-phase and multi-field damage propagation, and a disaster mechanism within the fractures under repeated freezing and thawing. The visual tracking of the water-heat migration process within the fractured rock mass is helpful in observing the evolution process of frost-heaving force and providing a theoretical basis for the frost-heaving mechanism. First, research on particle tracking, thermal imaging tests, and image processing technology was conducted to demonstrate that particle tracking and thermal imaging techniques can track the freezing front within the rock fractures and the migration of liquid water inside the rock. Then, by selecting fluorescent particles and improving the observation window and using a waterproof insulation cardboard, the development of a visualization device system for the water-heat migration process was achieved, allowing the tracking of the water-heat migration process. The results of the verification test showed that under freezing and thawing conditions, the experimental device could effectively track the temporal and spatial changes of water-heat migration inside and outside the rock fractures and monitor the real-time changes of the freezing front. Reliable experimental results were obtained, which provided a visual record of the water-heat migration and water-ice phase transition within the fractured rock mass during the freezing and thawing process. Combining thermal imaging technology with the real-time recording of the motion rate of fluorescent particles, this experiment described the movement speed of the freezing front and the convection of free water within the fractures in rock water-heat migration, which is of significant importance for the study of the frost-heaving force under the influence of water-heat migration.

1. Introduction

The “the Belt and Road”, the “Land Silk Road New Economic Belt” is gradually being built as the great international passage. In the future, a series of major international channels of the “New Economic Belt of the Land Silk Road” around the international economic cooperation corridors will be built, such as China–Pakistan, the New Eurasian Land Bridge, China, Mongolia and Russia, and China–Central Asia and West Asia. China is gradually strengthening key construction of infrastructure in border areas such as Tibet and Xinjiang. The typical characteristics of the above areas are that they are all located in alpine or permafrost regions.

The crustal uplift activity is strong in these areas. It is extremely complex in terms of geological structure and landform. A large number of high and steep rock slopes are exposed, and their overall stability is extremely poor. At the same time, due to the influence of the climate and environment of the region, which is cold and has high altitude, external dynamic geological processes, such as glacier thawing, in the cold region are strong. It leads to the frequent occurrence of epigenetic dynamic geological processes, such as slope collapse, landslide, debris flow and salivary ice along the line [1,2]. It is particularly important that the geological hazards that are widely developed along the slope are obviously different from those in low-altitude areas, showing typical characteristics of glacial geological hazards in cold and high altitude regions, such as landslide that mostly develops in the moraine debris platform formed after the ice recedes. Debris flow is water containing ice and snow melt. Collapse refers to talc slope and rock falling under the action of freezing weathering. Geological disasters, such as giant collapse and landslide damage, destroy the road surface, bridges, and slope protection facilities along the way, resulting in the tragedy of vehicle damage and human death [3]. It poses a huge threat to the construction and operation of key projects in the later stage of the region.

As stated by Murton [4], an academic of the Royal Society of England, the water-heat migration inside the cracked rock mass is the core reason for its frost heaving and cracking. International cold region engineering expert Krautblatter [5] also pointed out that the problem of frost heaving and cracking of cracked rock must be based on the cracked rock itself, and the evolution mechanism of freeze–thaw damage must be explored from the perspective of hydro-thermal mechanical coupling multi-field effects and multi-space scales. According to the catastrophic nature of high and steep rock slopes in high and cold regions, the key is to comprehensively study the freeze–thaw damage of cracked rock mass. The freeze–thaw damage of cracked rock mass is the fatigue damage caused by the frost heave load of the internal water-filled crack under the combined action of water and heat. It can be seen that the crack is the core factor causing the anisotropy of rocks. How does one clearly and visually record the water and heat transfer and monitor the change process of the frozen front of cracked rock in real time? This is the most core and basic research subject on freeze–thaw damage characteristics of cracked rocks under the combined action of water and heat.

Many scholars have conducted a number of studies on the hydrothermal migration of cracked rock. O. Sass [6] pointed out that there is a considerable amount of unfrozen water and supercooled water in the rock formation below the freezing point, and monitored the migration of water to the frozen area, indicating that the phenomenon of water migration also exists during the freezing process of the rock. The experimental data obtained by J. B. Murton et al. [4] showed that the main cause of damage to cracked rock caused by the expansion of rock cracks is ice segregation. When the temperature decreases, water migrates to and freezes in the frozen area under the action of segregation potential. As the temperature rises, the segregated ice melts and migrates away from the frozen region, resulting in the subsidence of the bedrock surface in the cold region. S. Akagawa and M. Fukuda [7] also believed that segregated freezing is also the fundamental reason for the migration of tuff pore water. Under the action of temperature gradient, the hydraulic head pressure in the frozen area is lower than that of the unfrozen area, and the water will freeze toward the tuff under the pressure gradient. However, the tuff is still regarded as a homogeneous continuous medium in the research. Field tests in the Arctic region have shown that unfrozen water in fine aggregate porous rocks and cracked rocks within a few meters of the surface of the frozen zone will also migrate to the normal frozen zone, resulting in ice segregation and condensation to form an ice-rich zone [8]. However, there are few studies on the water migration process of cracked rocks, and a consensus has not yet been reached on the migration mechanism. The research content mainly focuses on theoretical models and laboratory experiments. The research methods are also divided into two levels: one is treating the rock as a continuous medium and using permafrost-related theories to study the water migration mechanism in low-temperature rocks; the other is treating cracked rocks as a dual-porous medium and mainly studying the moisture migration mechanism in rock fractures. Most studies mainly focus on the first level, and there are few studies on the mechanism of water migration in rock fractures during the freeze–thaw process. For equivalent continuous rocks without macroscopic fractures, the second frost heave theory can better explain the migration process of unfrozen water to the frozen area, but for cracked rocks, the frost heaving damage in the porous medium area is relatively small, and the the water migration, freezing and expansion-driven fracture expansion process determines the strength of the rock. The mechanism of water migration caused by the water–ice phase transition in rock fissures is also very complicated. The introduction of separation pressure is used to explain the process of frost heaving and the cracking of rock and soil. Many scholars also tried to use separation pressure to explain the movement of water in rock fissures. The transport mechanism, the separation pressure on membrane water, was first defined by B. V. Derjaguin and N.V. Churaev [9]. In fact, there is a layer of unfrozen water film between the ice–rock interface in the frozen rock medium, and the existence of the unfrozen water film provides a channel for water migration. Under the temperature gradient, there is a difference in the thickness of the unfrozen water film, which causes the unfrozen water to migrate to the low-temperature freezing area.

B. V. Derjaguin et al. [10] and V. D. Churaev et al. [11] believe that the separation pressure on the unfrozen water film is the cause of frost heave in porous media. When the separation pressure exceeds the pressure that the pore skeleton can bear, the pores will be pulled apart. However, there is no unified explanation for the formation mechanism of the unfrozen water film on the surface of the ice body. There are three main theories recognized by the majority of scholars: pressure melting, frictional heat melting, and the pre-melting characteristics of the ice body itself [12,13,14].

In order to further study the mechanism of frost heaving in rock, this paper proposes an experimental method based on fluorescent particles as moisture tracers to meet the visual tracking requirements of cracked rock in freeze–thaw environments. It uses processed cracked rock samples to eliminate the influence of interfering particles on the experimental data, and adds thermal imaging control devices, image acquisition systems, etc. A series of feasibility tests were carried out to verify the validity of the proposed experimental method and device system, and the rules of water and heat transfer of rock samples were obtained.

2. Water and Heat Transfer Experimental Method

2.1. Key Technologies for Visualization of Water and Heat Transfer Experiments

Matsuoka [15] observed the fracture state of bidirectional freezing in soft porous rocks through indoor freeze–thaw cycle tests to infer the rock hydrothermal migration process. Neaupane [16] used finite elements to simulate the freezing and thawing process, and the developed code takes into account the phase change of pore water during freezing. S. Duca [17] used ultrasonic wave speed measurements to infer the depth of continuous ice in fractured gneiss intact. Hui Liu [18,19], based on the microscopic information contained in these CT images, detected and clearly observed the porosity caused by the volume expansion of water inside the rock and the freezing process of pore water. XU [20] studied the effect of different cycle parameters, the LCO2 temperature on the change of coal pores based on infrared thermal imaging and low-field nuclear magnetic resonance. Hailiang Jia [21] conducted experiments where sandstone samples were subjected to 50 freeze–thaw cycles, and their T2 spectra measured after each cycle via the nuclear magnetic resonance (NMR) method. This allowed the evolution of pore size distribution, porosity (pore volume), permeability (pore connectivity), tortuosity (pore curvature), and pore size uniformity coefficient (pore non-uniformity) to be comprehensively estimated. Compared with this paper, previous experiments on water and heat transfer lack visibility and can not directly observe the change rule of water content. In this experiment, on the one hand, the movement process of water can be recorded in real time by means of the particle-tracking method through the fissure viewing window. On the other hand, real-time temperature changes of rock mass fissures and the migration of the frozen front inside rock mass can be observed by thermography.

2.2. Application of Particle Tracking Technology

It is by particle tracking velocimetry (PTV) that the speed of a single particle is accurately calculated by tracking its motion path. It has been widely used in the field of flow measurement in recent years [22,23,24,25]. As shown in Figure 1, Shuo Huang [26] conducted a PIV-based laboratory experiment to study the influence of the groin on the density-induced flow in an estuary navigation channel. By establishing an experimental bench made of plexiglass and using two CCB cameras to record synchronously, the PIV observations clearly show vertically stratified density currents caused by a salinity gradient.

Figure 2 shows the particle image velocimetry test device. The relevant motion information of particles is obtained by comparing the images collected at t1 and t2 time. At the same time, there is a lack of effective experimental means for the real-time observation of water–ice changes in cracked rocks, such as water and heat transfer, water–ice phase change, and THM system. Figure 3 shows the movement of tracer particles in the process of water and heat transfer of crack water. In this experiment, the particle tracking method is used to add a proper amount of tracer particles in the fissure water, so that the tracer particles are suspended in the fissure water. During the freezing process, the trace and velocity of the tracer particles are recorded to infer the microscopic process of crack water, heat transfer, water–ice phase change and THM. It provides strong data support for the study of the frost heaving force model.

The main technical difficulty of particle-tracking velocimetry is the visualization of the test process under the action of water and heat transfer in cracked rock. Some scholars [27,28,29] use a numerical simulation method to study the influence of stress change on the transmission process in a single crack to avoid this difficulty. However, due to various assumptions of software design, numerical simulation cannot completely simulate the real liquid flow, and the experiment is irreplaceable.

The difficulty of visualization in the experiment of water and heat transfer in rock crack is that the crack shape is deep, narrow and thick. Due to the large thickness of the cracked rock, the particles in the observation window overlap many times and it is difficult for the light source to illuminate the observation area. However, this experiment is based on the use of standardized cracked rock, which has a regular crack shape and a wide crack opening for easy manual operation. The white waterproof background paper inserted in the middle of the crack brightens the background and reduces the observation thickness.

In the particle-tracking velocimetry experiment, the key of the feasibility study is the fluidity, stability and observability of the selected particles because the object of observation is tracer particles. At present, the solid tracer particles commonly used in particle-tracking and velocity measurement experiments are polystyrene powder, aluminum powder, glass ball, synthetic cotton particles, oil, milk, pollen, and self-provided tracer particles of various properties. After consulting various relevant documents [30,31,32,33], several particles were selected according to their density, particle size and refractive index: pollen, glass beads, hollow glass beads and polystyrene fluorescent microspheres. The experimental results are summarized as follows. Pollen as a tracer has the advantages of good reflective effect and good tracking, but because it is difficult to collect pollen and because of the different varieties of pollen, the pollen density reflects large individual differences, which cannot be used in this experiment. Glass beads are often used as tracer particles due to their high refractive index and uniform size, but they do not meet the requirements of this experiment due to their high density and poor tracking. As a tracer particle, hollow glass beads can be uniformly and stably distributed in water for a long time. However, considering the limited experimental environment, the observation field is poor. Finally, it selected 3 nm polystyrene microsphere containing fluorescent dye as the tracer particle. Figure 4 shows the photos taken by a professional CCD camera under natural light and ultraviolet light, respectively. The particle has the characteristics of strong fluidity, good stability and good observation due to its small size, density and being close to water.

In addition, the freezing temperature test results show that adding fluorescent particles into the fissure water will not change the freezing temperature of the rock, and a small amount of fluorescent particles will not change the density of the fissure water. Therefore, it will not change the water and heat transfer process and freezing time by adding polystyrene microspheres to the fissure water.

2.3. Application of Thermal Imaging Technology

It is used for thermal imaging observation of temperature change and water and heat transfer of cracked rock during freezing. On the one hand, the rock crack wall temperature recorded by the particle-tracking experiment results and thermal imaging technology is comprehensively analyzed. On the one hand, it comprehensively analyzes the particle movement track and the temperature change of the rock crack wall during freezing and thawing. Finally, it records the frozen front in the rock in real time and intuitively during the freezing and thawing process. Figure 5 shows that the cracked rock sample and its thermal imaging image. It freezes the thermal imaging camera and cracked rock mass in a low-temperature test chamber for 4 h to create a freezing environment.

2.4. Application of Thermal Imaging Technology

Akkurt [34] determined the velocity field of turbulent flow in a channel with obstacles whose shape was optically complex to enhance heat transfer. Using a ray-tracing-based image correction method, the high-level distortion of the PIV image caused by the heart-shaped depression was eliminated. Based on HSS PIV (high-speed volumetric particle image velocimetry), Čantrak [35] uses invariant maps to calculate measurement results. All states of turbulence anisotropy are thoroughly analyzed by applying the invariant theory on HSS PIV results. In order to track the water and heat transfer process in real time, an image acquisition device is added in this experiment. This image acquisition device is composed of an industrial camera, bracket, ultraviolet light source, fill light, thermal imaging camera and image processing terminal. It adjusts the distance between the sample and the industrial camera, the exposure time of the camera, the aperture and other parameters according to the sample height, and adjusts the brightness of the fill light and the purple light according to the light needs. In addition, it needs to install a filter in front of the industrial camera lens to reduce the interference caused by the reflection of the purple light. Its industrial camera is connected to the PC terminal and automatically collects images at a certain time interval according to the needs of the experiment. After image acquisition, it uses Photoshop, MATLAB, ImageJ and other software to process the image as follows:

• Cropping: It cuts the image according to the experimental requirements to maximize the amount of useful information in the picture frame;
• Adjusting parameters and highlighting of important information: Because it is difficult to optimize camera parameters directly in the process of image acquisition, in order to better obtain information from graphics, Photoshop and MATLAB are used to adjust the image parameters uniformly in the later stage, such as brightness, contrast, etc. This strengthens the parts that need to be highlighted in order to highlight useful information. For example, it makes the contrast between frozen ice and unfrozen water more obvious, and does not change the image structure. It is uniformly adjusted to ensure that the processing process of each image is consistent;
• Particle tracking and recording: It uses ImageJ software to cut the video, and then optimizes the parameters of each frame of image, adjusts the gray threshold, and makes the particles clearer. It uses software to limit the particle size, tracking range and filter the tracked particles. It records the motion path and speed of the selected particles in detail.

2.5. Tracking of Freezing and Thawing Process of Cracked Rock

These two experimental methods of fluorescein tracing technology and image processing technology can reliably track the migration process of water in rock cracks. The experimental material of this sample is granite. First, it immerses the sample in water for 2 days to make it fully saturated. Then water solution of polystyrene microsphere is poured into the rock fissure. It is subject to one-way freezing for 4 h to ensure that it is completely frozen in a low temperature test chamber with a set temperature of −20 °C. Finally, we take out the rock and melt it at normal temperature. The experimental equipment used in the whole process of the experiment includes a thermal imager, an ultraviolet light source and an image acquisition device to automatically collect the images of the sample.

In the freezing experiment, the freezing front movement process of the front and the top of the cracked rock mass is shown in Figure 6 and Figure 7 respectively. The freezing front of the cracked rock mass moves from bottom to top, from outside to inside, and finally moves to the vicinity of the crack. It can be seen from the observation details that the frozen front near the crack first appears at the bottom of the crack, as shown in Figure 6. As shown in Figure 7, after the rock freezes, the crack begins to freeze, and the frozen front migrates from outside to inside.

It is also consistent with the freezing process of crack water taken by a high-speed camera and the freezing sequence of rock mass taken by a thermal imager. It can be seen from Figure 8 that with heat transfer, the crack water migrates rapidly to the upper part of the crack wall and gradually forms ice crystals on both sides of the rock crack wall. The frozen fronts on both sides of the crack meet at the bottom and the upper part of the crack, forming a closed cavity in the middle of the crack. Then, the ice crystals on both sides gradually thicken until they converge, forming a white frozen line in the middle of the crack. Figure 9 is a schematic diagram of the freezing process of the crack water.

It traces fluorescent tracer particles clearly with a professional CCD camera. Combined with the results of thermography and the high-speed camera, the water-heat transfer process of cracked water is comprehensively analyzed. The micro evolution process of the frozen front of crack water is recorded intuitively. The various color-line segments shown in Figure 10 are tracer particle migration trajectories. Data such as velocity, direction and coordinates of all observed particles can be obtained by software output.

In the melting experiment, the frozen front movement processes of the frozen vertical surface and the overlooking surface of the cracked rock are shown in Figure 11 and Figure 12 respectively. The melting of the frozen front first occurs at the upper edge and corner of the rock, and the frozen front of the front elevation of rock decreases in a sector to the middle and lower part until it disappears. The frozen front of the cracked rock moves from the edge to hte interior until the cracked rock decreases evenly, and finally, melts inside the cracked rock. As shown in Figure 11, the bottom of the crack finally melts. Figure 12 shows that the crack ice gradually melting from the outside to the inside after the rock melts.

The melting process of crack ice is shown in Figure 13. The first melting occurs at the junction of the rock crack and the crack ice, and at the junction of ice on both sides, i.e., the white frozen line. Due to the large contact area, the ice at the upper of the crack melts fastest and is “sharpened” on both sides. The middle part of the ice begins to melt at the freezing line and appears as a cavity. The ice layer at the bottom of the crack gradually melts away from the rock wall. As external heat enters the interior of the crack, the crack ice is completely separated from the rock wall and suspended in water, and finally, the crack ice is completely melted. The melting process is shown in Figure 14, which coincides with the frozen front movement pattern recorded by the thermal imager. At the same time, it uses particle tracking to synchronously track the motion field of crack water during the melting process of crack ice. Figure 15 shows the white frozen line of crack ice.

The above experimental results show that the particle-tracking method, and thermal imaging technology can be used to trace water migration in cracked rock during the freeze–thaw process.

3. Experimental Device System

3.1. Conventional Hydrothermal Migration Experimental Device

At present, there are relatively few experimental studies on water and heat transfer during the rock freeze–thaw process at home and abroad. The experimental equipment uses NMR, CT scanning and other methods. It can carry out rock damage research during a routine freeze–thaw experiment, and realize the real-time monitoring of temperature, deformation and other data. It plays an important role in studying freeze-thaw damage of rock.

These experimental devices have certain practicability in the freeze–thaw damage. However, the applicability of this device is poor if the water and heat transfer law of cracked rock is to be studied deeply and the process of water transfer during freeze–thaw process is tracked in real time. On the one hand, the existing experimental device wraps the whole sample with insulating cotton, which isolates the outside temperature during the freezing process, and is, thus, unable to provide a visual environment. On the other hand, because the crack is too small, there are no effective means to observe the freeze–thaw process for the flow field.

3.2. Hydrothermal Migration Measuring Device Based on Particle Tracking and Thermal Imaging Technology

It aims at observing the thermal migration and phase transition of water ice under freezing and thawing conditions. According to the purpose of this experiment, a particle-tracking velocimetry experimental system is built. Figure 16 is a particle-tracking velocimetry experimental system, which consists of a light source, a CCD camera, and a computer. The tracer particles in the measuring area are illuminated by a light source. The CCD camera collects the images of moving particles and converts them into digital signals via the image acquisition card, and stores them in the computer. The computer can acquire, display, analyze and calculate images under the control of relevant software. At the same time, real-time temperature changes are recorded synchronously with a thermal imager.

To facilitate the observation of the dynamic trajectory of tracer particles in the crack, as shown in Figure 17 and Figure 18, it sets one end of the crack as a “particle observation window”, i.e., plexiglass, which is latex-embedded in the crack wall. On the one hand, the elasticity of the latex will not restrain the deformation and cracking. On the other hand, the Plexiglas window facilitates the observation of particle motion. For the sake of simplicity, none of the legends in this paper draw a particle observation window. The crack on the other side of the rock sample is sealed with flexible silicone.

It is worth noting that in order to prevent water from being injected into the cracked rock, the movement field of water flow is changed due to the different rate of water injected, and a suitable amount of tracer particles are mixed evenly with water. Then a small peristaltic pump is used to inject water with tracer particles into the crack. During the observation, the crack window is divided into three equal sections along the direction of the crack, and tracked during the freezing process. It observes the dynamic trajectories and moving speeds of particles in different parts of water and heat transfer at the same time. Therefore, the purpose of observing the whole process of the water–ice phase transition is achieved.

It performs a basic processing of the digital images taken during the experiment. The main content includes image enhancement, image segmentation, image edge detection and image feature description. It mainly uses image enhancement methods, such as gray scale transformation, histogram modification, image filtering and image sharpening. Image segmentation is to divide the images into non-overlapping areas and extract the relevant information of tracer particles. According to the discontinuous distribution of the gray value of particle pixels, a threshold segmentation method is used. For the image recognition of the frozen front of ice water, the edge extraction is divided into two steps: first, ice water is segmented from the image, and then the frozen front is extracted from the ice water. After determining the edges of the image, the basic features of the freezing (area and length) can be calculated.

4. Analysis of Water and Heat-Transfer Process

It carries out a thermographic observation of temperature changes and water-heat transfer of the cracked rock during the freezing process, as well as a preliminary analysis of the results. A $10\times 10\times 10$ cm block made of granite with 5 cm long and 4 mm wide through prefabricated crack is set in the middle of the block. The front of the specimen crack is inlayed with latex in the plexiglass viewing window and the back is sealed with latex. It uses a peristaltic pump to slowly and uniformly pour the polystyrene microsphere aqueous solution with a concentration of 2‰ into the crack. It puts the prepared sample into a low temperature test box set at −20 °C for freezing test.

According to the observation of the water migration in the crack, the freeze–thaw state of the crack at different depths is different. In this paper, the crack is divided into three sections from the port to the bottom for synchronous photography. Through digital image processing, the migration path and migration rate of the crack water can be tracked in real time. As shown in Figure 18, this paper takes the middle section of the crack as an example. The observation of fluorescent particles in the freezing experiment found that the majority of particles within 0.3 mm around the crack wall would be “attracted” by the ice layer and would slow down the movement speed due to the temperature difference. Therefore, in the freezing experiment, the measurement range of moving particles near the crack wall is defined as within 0.3 mm from the crack wall or crack ice, and the measurement range of other is defined as the measurement range of moving particles inside the crack. At the beginning of the freezing experiment, the particle motion velocity in the crack water was large and disordered. Figure 19 shows the real-time migration rate of particles in the crack during freezing. In the first 20 min of the experiment, the range of particle motion speed rapidly narrowed, and the speed decreased sharply. The particle velocity in the middle of the crack was basically the same as that near the crack wall. After 20 min of freezing test, the rate of decline of the particles speed in the middle of the crack and near the crack wall were slowed down. At the same time, the temperature of the crack wall has decreased to 0 °C, and the particle motion velocity near the crack wall was significantly lower than that in the middle of the crack. After 63 min, all particles near the crack wall were frozen, which means that the measurement range in the middle of the crack was less than 0.3 mm. Because the temperature gradient gradually decreased, the freezing speed was very slow. It was not until 133 min later that the crack ice on both sides would merge, and then the crack ice would be completely frozen. As illustrated in Figure 14, it was worth noting that there would be a “frozen line” from the top to the bottom inside the crack when the crack ice was completely frozen. Because the crack water level was falling with the decrease of free water during the freezing process, it was the ice layers in two directions that formed the “ice water ice” interface, but due to the lack of free water connecting the ice layers, it cannot form hydrogen bonds with the ice layers. The ice layers in two directions cannot be completely connected, so a “frozen line” was formed inside the crack.

According to Figure 20, the real-time migration rate change of the particles in the middle of the crack during the melting process, can be seen that in the first 12 min because the sample was directly exposed from the low temperature environment of −20 °C to the normal temperature environment of 30 °C, resulting in a large temperature gradient and rapid melting. However, since the melting had just occurred, the fracture temperature was still very low, there was not much free water, and there was no smooth movement channel, so the particle movement speed was very low within 5 min after the melting. It was noteworthy that the ice layer near the crack wall melted first in the first two minutes compared with the ice layer inside the crack. On the one hand, it is because the specific heat capacity of water is larger than that of rock, and the rock transports heat for the ice layer, which is conducive to the melting of the ice layer. On the other hand, because of the existence of the “freezing line”, the ice layers inside the crack in both directions were exposed to the cold air, which was not conducive to heat transfer, so the ice layers inside the crack melted slowly. However, the free water in the crack gradually increased, and the free water warped the ice layers inside the crack. The temperature of the free water is higher than that of the cold air near the ice layer, thus accelerating the melting process of the ice layers inside the crack. At the same time, the ice-water phase change absorbed a lot of heat, which undoubtedly delayed the melting process of the ice layers near the crack wall. It can be seen in Figure 19 that after 2 min, the melting speed of the ice layer inside the crack completely exceeded that of the ice layer near the crack wall until the sample completely melted.

5. Conclusions

In this paper, an experimental method based on the device system is proposed by combining particle tracking method and thermal imaging technology. A water and heat transfer measurement device system has been established, and validation and research experiments of the device system have been carried out. The following following results and conclusions were obtained:

• It is a hydrothermal migration experimental device system used in this experiment, which combines a particle tracking method with thermal imaging technology. It can visually detect the hydrothermal migration process of cracked rock and crack water in real time. The device system can play an important role in the research of water and heat transfer. It is combined with image processing technology and can meet the requirements of water and heat transfer and other frost heaving related experimental research;
• It is based on the established device system, which intuitively describes the migration process of freezing front and the phase change process of free water in the fracture during the freezing and thawing process of water-bearing cracked rock. For rock crack water, the ice layers grow from the crack walls at both ends to the middle during the freezing process, until the ice layers at both ends squeeze each other. In the process of melting, the contact surface between the rock wall and the ice layer first melts, and then the “frozen line” between the ice layer and the ice layer begins to melt. When the rock wall and the ice layer completely melt, the ice sheet emerges from the water. It is the phase change process of the cracked rock water that can directly affect the stress state of cracked rock, which is of great significance to the study of frost heaving force under the influence of water and heat transfer;
• In this experiment, it is combined with thermal imaging technology to record the movement rate of particles in real time, so as to describe the movement speed of frozen front and the convection of free water in crack during the hydrothermal migration of cracked rock. The proposed experimental methods mainly include the real-time visual tracking of particle motion under freezing and thawing conditions and using image processing to explore more useful information in the image, so as to track the water–ice phase change process and the migration law of free water;
• In the crack of rocks, the micro-hydraulic field exhibits a certain regularity in response to temperature changes. During the freezing process, the flow velocity of the rock crack water decreases sharply, while the velocity tends to stabilize as the temperature gradient weakens. During the melting process, as the rock crack ice gradually melts, the free water in the crack increases gradually, and the water flow field shows a clear temperature orientation. As the temperature of the cracked rock rises to room temperature, the particle motion becomes turbulent.