Hydrothermal coupling model test system and method for simulating liquid nitrogen frozen stratum in different seepage environments

Abstract

The invention discloses a hydrothermal coupling model test system and a hydrothermal coupling model test method for simulating a liquid nitrogen frozen stratum in different seepage environments. The system and the method are used for experimental research of a multi-physical field coupling effect of a test object in a large-seepage liquid nitrogen frozen stratum. The model test system comprises a liquid nitrogen freezing system, a flowing water seepage circulation system and a monitoring system, the liquid nitrogen freezing system is used for controlling the temperature of a liquid nitrogen freezing pipe embedded in a sand layer by changing the liquid nitrogen flow, so that the test sand layer is quickly frozen; the flowing water seepage circulation system simulates different underground water seepage speeds through a pressure pump, and the temperature of test water is kept constant through a heating rod. The monitoring system is used for recording the change trend of the temperature field and the seepage field in the liquid nitrogen stratum freezing process. According to the device, the ultra-low temperature effect of the liquid nitrogen is utilized to perform top-speed freezing on the sand layer under the high-flow-speed seepage environment, so that the freezing curtain can be effectively closed, and the evolution law of a large-seepage-flow stratum liquid nitrogen artificial freezing temperature field and a seepage field is obtained by simulating the liquid nitrogen stratum freezing process under different seepage environments; and a basis is provided for manual liquid nitrogen freezing construction of the large-seepage stratum.

Description

Technical Field

[0001] The invention belongs to the field of liquid nitrogen artificial freezing model test, and in particular relates to a water-heat coupling model test system and method for simulating liquid nitrogen freezing of strata under different seepage environments. Background Art

[0002] Liquid nitrogen artificial freezing method plays a pivotal role in subway tunnel communication channels, shield entrances and exits, and disaster relief projects due to its strong support, efficient water stopping, wide construction range, and large freezing depth. Due to its reliability, compatibility, and low impact on the environment, it has become one of the most favorable geotechnical support methods in various mining, civil, and environmental projects.

[0003] However, with the continuous rise of underground projects, the impact of groundwater seepage on artificial stratum freezing construction has become increasingly significant. When the groundwater seepage rate in the stratum is too fast, a large amount of cold around the freezing pipe will be carried to the middle and lower reaches by the seepage, the development of the freezing wall will slow down, the freezing curtain handover time will increase exponentially, and the thickness of the freezing wall will decrease, affecting the freezing effect, delaying the construction period, and even causing major construction accidents, especially in sand and gravel strata with a large permeability coefficient.

[0004] At present, the experiments on the influence of seepage on the construction of artificial freezing method can be roughly divided into two aspects. On the one hand, the influence of seepage field on the construction process of artificial ground freezing under low seepage environment is studied; on the other hand, the change law of artificial freezing temperature field is studied using brine as freezing medium. There are few studies on artificial ground freezing experiments with ultra-low temperature refrigerant liquid nitrogen, especially in large seepage environment.

[0005] Therefore, it is necessary to develop a hydrothermal coupling model test system that simulates large-seepage liquid nitrogen artificial freezing of the formation and propose an effective test method. Summary of the invention

[0006] Purpose of the invention: The technical problem to be solved by the present invention is to provide a hydrothermal coupling model test system for simulating large seepage liquid nitrogen artificial freezing of strata, which can simulate the freezing process of liquid nitrogen strata under different seepage environments to obtain the evolution law of temperature field and seepage field of liquid nitrogen artificial freezing of strata with large seepage flow, that is, the evolution law of freezing temperature field and seepage field of liquid nitrogen strata in the freezing process of multiple groups of seepage velocity environments can be obtained at one time, thereby improving the test efficiency and reliability, and providing a basis for the construction of liquid nitrogen artificial freezing of strata with large seepage flow.

[0007] Technical solution: The present invention simulates a hydrothermal coupling model test system for liquid nitrogen freezing of strata under different seepage environments, and the system includes:

[0008] The model box has a buffer chamber, a sand filling chamber and a water storage chamber connected in sequence; the buffer chamber is connected with a first water inlet, and the water storage chamber is connected with a first water outlet; the sand filling chamber is filled with several layers of different types of sand in sequence from top to bottom;

[0009] The liquid nitrogen freezing device includes a plurality of liquid nitrogen freezing tubes buried in different types of sand layers in the sand filling chamber, so as to adjust the temperature of the liquid nitrogen freezing tubes buried in the plurality of different sand layers and realize extremely fast freezing of the test sand layer;

[0010] A dynamic water seepage circulation device provides a seepage environment for the model box, comprising a circulating water tank and a water pump provided with a second water inlet and a second water outlet, wherein the second water inlet is connected to the first water outlet on the water storage chamber, and the second water outlet is connected to the first water inlet on the buffer chamber;

[0011] A monitoring system is provided to obtain the changing trends of the temperature field and the seepage field during the freezing process of the liquid nitrogen sand layer, which includes distributed optical fiber temperature sensors and stress sensors buried in several different sand layers, and CNC flow meters respectively located at the first water inlet and the second water outlet to detect the seepage velocity.

[0012] Furthermore, the buffer chamber of the system is provided with a buffer stone, and the height of the first water inlet on the buffer chamber is higher than the height of the first water outlet on the water storage chamber.

[0013] Furthermore, the buffer chamber and the sand filling chamber, as well as the water storage chamber of the sand filling chamber, of the system are all connected through a water-permeable baffle.

[0014] Furthermore, the permeable baffle of the system is evenly provided with permeable holes with a radius of 10 mm, and geotextiles are provided on both sides of the permeable baffle.

[0015] Furthermore, a plurality of liquid nitrogen freezing tubes of the system are arranged in the sand filling chamber at equal intervals and horizontally perpendicular to the water flow direction.

[0016] Furthermore, the liquid nitrogen freezing device of the system also includes a pressurized liquid nitrogen tank and a temperature controller arranged on the liquid outlet pipe of the pressurized liquid nitrogen tank. The liquid inlet pipe of the liquid nitrogen freezing pipe is connected to the liquid outlet pipe of the pressurized liquid nitrogen tank, and the gas outlet pipe of the liquid nitrogen freezing pipe is connected to the atmosphere.

[0017] The method for simulating based on the above-mentioned hydrothermal coupling model test system of the present invention comprises the following steps:

[0018] (1) several liquid nitrogen freezing tubes are buried in predetermined holes of the model box; the sand layer is filled in layers, the sand layer is continuously tamped during the filling process, and at the same time, each sensor is laid in the sand layer; after the filling is completed, a thermal insulation film is laid on the surface of the sand layer, and the sensor circuit is led out and connected to the signal processing receiver;

[0019] (2) Start the water seepage circulation device and reach the preset water flow rate, and monitor the flow rate through the digital control flow meter;

[0020] (3) Open the liquid nitrogen tank control valve and monitor the amount of liquid nitrogen introduced through a flow meter;

[0021] (4) adjusting the pumping pressure of the water pump according to the requirements of different sand layers for seepage velocity, maintaining the seepage velocity constant, and monitoring it through a flow meter;

[0022] (5) The monitored temperature and flow rate data are collected by the data collector, and the relationship between multiple physical fields during the construction process of liquid nitrogen stratum freezing method is obtained by combining the influence of different seepage velocities on the liquid nitrogen freezing temperature field;

[0023] Wherein, in step (4), the seepage velocity and water flow velocity of different sand layers satisfy:

[0024]

[0025] Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0026] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0027]

[0028] The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related:

[0029] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0030] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0031]

[0032] Beneficial effects: Compared with the prior art, the significant advantages of the present invention are: the hydrothermal coupling model test system and method for simulating large seepage liquid nitrogen artificial freezing of strata utilizes the ultra-low temperature effect of liquid nitrogen to rapidly freeze the sand layer under a large flow rate seepage environment, so that the freezing curtain can be effectively closed; and by using multiple layers of sand layers with different permeability coefficients, the evolution laws of the freezing temperature field and the seepage field during the freezing of liquid nitrogen strata under multiple groups of seepage rate environments can be obtained through one test, thereby improving the test efficiency and reliability, and providing a basis for the construction of liquid nitrogen artificial freezing of strata with large seepage. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a structural schematic diagram of the hydrothermal coupling model test system of the present invention;

[0034] Figure 2 for Figure 1 Enlarged view of part A in the middle;

[0035] Figure 3 A schematic diagram of the structure of a water-permeable baffle used in the system of the present invention;

[0036] Figure 4 It is a schematic diagram of multiple sand layers in the simulation box of the present invention;

[0037] Figure 5 A schematic diagram of the structure of the circulating water tank used in the system of the present invention;

[0038] Figure 6 This is a schematic diagram of the structure of the liquid nitrogen freezing tube used in the system of the present invention;

[0039] Figure 7 It is a data monitoring point map of the liquid nitrogen freezing tube of the present invention;

[0040] Figure 8 This is a multi-layer distributed optical fiber temperature measurement diagram used in the present invention. DETAILED DESCRIPTION

[0041] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings.

[0042] It should be noted that the present invention can determine the similarity constants of the model test system according to the basic conditions of the on-site construction of the subway tunnel, including: geometric similarity constants, time similarity constants, temperature similarity constants, groundwater seepage velocity similarity constants, etc.

[0043] The hydrothermal coupling model test system of the present invention can use the ultra-low temperature effect of liquid nitrogen to rapidly freeze the sand layer under a high-velocity seepage environment, so that the freezing curtain can be effectively closed. By simulating the freezing process of liquid nitrogen strata under different seepage environments, the evolution law of the temperature field and seepage field of the liquid nitrogen artificial freezing of the high-seepage strata can be obtained. In addition, through a single seepage freezing model test, the coupling effect between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities in multiple groups of different sand layers can be obtained. Specifically, Figure 1 As shown, the water-heat coupling model test system includes a simulation box 1, a dynamic water seepage circulation device, a liquid nitrogen freezing device and a monitoring system.

[0044] The simulation box 1 includes an inner and outer double-layer structure, a channel steel 23 is installed between the double-layer structures, and a heat insulation cotton is installed between the gaps of the channel steel 23. The inner layer of the simulation box 1 is divided into three chambers, namely a buffer chamber 2, a sand-filled chamber 3, and a water storage chamber 4, and the chambers are connected to each other through a permeable baffle 15. The permeable baffle 15 is evenly provided with permeable holes 16 with a radius of 10 mm, and geotextiles 17 are respectively installed on both sides of the permeable baffle 15. Figure 2 and Figure 3 As shown. The buffer chamber 2 is provided with a first water inlet 5, and the buffer chamber 2 is filled with buffer stones to slow down the water flow speed, so that the water flow of the first water inlet 5 can be uniform and gentle, reducing the scouring effect of the water flow on the sand layer filled in the sand filling chamber 3. The test object is filled in the sand filling chamber 3, and the filling height of the sand layer can be freely controlled. The sand filling chamber 3 is filled with different types of sand layers from top to bottom, for example, Figure 4 As shown, a medium-coarse sand layer 24, a fine sand layer 25 and a silt sand layer 26 can be arranged in sequence from top to bottom. A first water outlet 6 is arranged on the water storage chamber 4, and the height of the first water inlet 5 on the buffer chamber 2 is higher than the height of the first water outlet 6 on the water storage chamber 4. Buffer stones can also be arranged in the water storage tank chamber 4 to slow down the water flow speed, filter the sand particles entrained when the water flows through the sand filling chamber 3, and prevent the sand particles from being lost.

[0045] The dynamic water seepage circulation device includes a circulating water tank 10, a water pump 11 and a digital control flow meter 14. Figure 5As shown. A sedimentation chamber 27 and a clean water chamber 28 are provided in the circulating water tank 10, wherein the sedimentation chamber 27 is connected to the second water inlet 8, the clean water chamber 28 is connected to the second water outlet 9, and a water permeable filter plate 29 is provided between the two chambers. The second water inlet 8 is connected to the first water outlet 6 on the water storage chamber 4, and the second water outlet 9 is connected to the first water inlet 5 on the buffer chamber 2. The numerical control flowmeter 14 is located at the mouth of the second water outlet 9 and is used to monitor the flow rate. Specifically, fill the clean water chamber 28 and the sedimentation chamber 27 with municipal water, turn on the control panel, set the temperature of the heating rod 30, for example, to 20°C, to ensure that the test water flow temperature is constant; when the water temperature remains constant, turn on the water pump 11, and the water pump 11 can provide a constant water pressure of 0 to 4 MPa to simulate a constant groundwater seepage rate. The seepage process of the sand layer in the model test system is as follows: the municipal water in the circulating water tank 10 is pumped out from the second water outlet 9, and enters the buffer chamber 2 in the simulation box 1 from the first water inlet 5 through the water pipe. The municipal water is affected by the buffer stone to form a uniform and gentle water flow. After the secondary buffering and filtration of the permeable baffle 15, it enters the sand filling chamber 3. After the water flows through the sand layer at a uniform speed, it is filtered by the permeable baffle 15 and enters the water storage tank chamber 4, and then flows out of the simulation box 1 through the first water outlet 6. The muddy water flowing out of the first water outlet 6 enters the sedimentation chamber 27 through the second water inlet 8. The sand and impurity particles in the muddy water slowly settle to the bottom, and the clean water accumulates on the top. The clean water enters the clean water chamber 28 through the filter hole on the permeable baffle 15, and then is heated to a certain temperature by the heating rod 30. The constant temperature water is pumped to the model box 1 through the second water outlet 9. In this way, the recycling of water resources can be formed, which can save water resources and at the same time, the simulation of groundwater seepage phenomenon can be realized.

[0046] like Figure 6 and Figure 7 As shown, the liquid nitrogen freezing device used in the present system includes a pressurized liquid nitrogen tank 18, a temperature controller 19 and a plurality of liquid nitrogen freezing tubes 7. Among them, the pressurized liquid nitrogen tank 18 can adopt a tank body with a volume of 480L, and the pressure of the pressurized liquid nitrogen tank can be adjusted to control the pressure between 0.05MPa and 0.2MPa. The temperature of the liquid outlet pipe is controlled by the temperature controller 19. For example, the outlet temperature of the liquid outlet pipe 21 can be controlled at about -100°C; a plurality of liquid nitrogen freezing tubes 7 are arranged at equal intervals in the sand-filled chamber 3 and buried in the sand layer, and are arranged horizontally perpendicular to the water flow direction. The liquid inlet pipe 20 of the liquid nitrogen freezing tube 7 is connected to the liquid outlet pipe 21 of the pressurized liquid nitrogen tank 18, and the gas outlet pipe 22 of the liquid nitrogen freezing tube 7 is connected to the atmosphere. In actual application, the liquid nitrogen freezing tube 7 can be Seamless steel pipe, bottom sealed; the liquid inlet pipe can be Seamless steel pipe, the outlet pipe can be seamless steel pipe; wherein, in order to make the nitrogen distribution more sufficient and the temperature distribution of the liquid nitrogen freezing tube 7 more uniform, holes are opened at equal intervals on the body of the liquid inlet pipe 20. The working mechanism of the liquid nitrogen freezing tube 7 is: the liquid inlet pipe 20 is connected to the pressurized liquid nitrogen tank 18 through a stainless steel pipe, wherein the stainless steel pipe exposed to the air is wrapped with polyurethane insulation cotton, the effect of which is to ensure that the temperature of the liquid nitrogen will not be affected by the external environment during the transportation process, open the pressure valve of the pressurized liquid nitrogen tank 18, and control the initial temperature of the liquid nitrogen to be maintained at -100°C by observing the temperature controller 19. The liquid nitrogen enters the liquid inlet pipe 20 through the stainless steel pipe, and fills the entire liquid nitrogen freezing tube 7 evenly and divergently through the openings. Through the heat exchange effect of the sand layer, the coldness of the liquid nitrogen is absorbed, and the form is converted into gaseous state, and discharged into the atmosphere through the outlet pipe 22.

[0047] The monitoring system includes an automated data collector 31, a distributed optical fiber temperature change receiver 32, a distributed optical fiber temperature change sensor 12 and a stress sensor 13 buried in several different sand layers, and a digital control flow meter 14 located on the second water outlet 9 to monitor the seepage velocity, such as Figure 1 and Figure 8 As shown. Among them, the stress sensor 13 and the distributed optical fiber temperature change sensor 12 are arranged in each sand layer of the sand filling chamber 3, receiving several layers and multiple stress monitoring points and temperature monitoring points, and connected to the automatic data collector 31. Specifically, the stress monitoring point is used to monitor the frost heave stress of the sand layer, and the distributed optical fiber temperature change receiver monitors the temperature change of each sand layer through the distributed optical fiber temperature meter acting on the temperature monitoring point.

[0048] The method of simulating liquid nitrogen freezing of strata under different seepage environments based on the above-mentioned hydrothermal coupling model test system comprises the following steps:

[0049] (1) several liquid nitrogen freezing tubes are buried in predetermined holes of the model box; the sand layer is filled in layers, the sand layer is continuously tamped during the filling process, and at the same time, each sensor is laid in the sand layer; after the filling is completed, a thermal insulation film is laid on the surface of the sand layer, and the sensor circuit is led out and connected to the signal processing receiver;

[0050] (2) Start the water seepage circulation device and reach the preset water flow rate, and monitor the flow rate through the digital control flow meter;

[0051] (3) Open the liquid nitrogen tank control valve and monitor the amount of liquid nitrogen introduced through a flow meter;

[0052] (4) adjusting the pumping pressure of the water pump according to the requirements of different sand layers for seepage velocity, maintaining the seepage velocity constant, and monitoring it through a flow meter;

[0053] (5) The monitored temperature and flow rate data are collected by the data collector, and the relationship between multiple physical fields during the construction process of liquid nitrogen stratum freezing method is obtained by combining the influence of different seepage velocities on the liquid nitrogen freezing temperature field;

[0054] Wherein, in step (4), the seepage velocity and water flow velocity of different sand layers satisfy:

[0055] Wherein, in step (4), the seepage velocity and water flow velocity of different sand layers satisfy:

[0056]

[0057] Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0058] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0059]

[0060] The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related:

[0061] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0062] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0063]

[0064] Example 1

[0065] This embodiment uses three layers of sand as an example to perform a simulation method, which includes the following steps:

[0066] (1) Several liquid nitrogen freezing tubes are buried in the predetermined holes of the simulation box; the sand layer in the simulation box is separated into three layers of sand with different physical properties by geotextile insulation cloth, namely, the upper medium-coarse sand layer, the middle fine sand layer and the lower silt sand layer. The permeability coefficient of the sand layer decreases from top to bottom. The thickness of each sand layer is 300 mm, and the compaction method of each sand layer and the position of the buried monitoring element are kept consistent, as follows:

[0067] The filling of each sand layer is carried out by layered compaction method. During the filling process, the sand layer is micro-compacted every 100mm to make the surface of the sand layer uniform and flat, ensuring the high permeability of the sand layer. Since the temperature of liquid nitrogen is extremely low and the latent heat of phase change is released rapidly, the distributed optical fiber thermometer can monitor the temperature change efficiently and quickly. When compacting to the second layer (200mm), distributed optical fiber thermometers are buried at the stress monitoring points and temperature monitoring points, among which the stress monitoring points are arranged equidistantly in the horizontal and vertical directions, and the temperature monitoring points are arranged in a horizontal ring. Subsequently, the last 100mm of each sand layer is filled and compacted. When the sand layer is filled, an insulation film is laid on the surface of the sand layer and left to stand for 12 hours. After the sand layer is completely solid, the sensor line is led out and connected to the automatic data collector.

[0068] (2) Connect the first water inlet with the second water outlet, and the second water inlet with the first water outlet, start the water seepage circulation system and reach the preset seepage rate and water temperature. The specific operation process is as follows: fill the sedimentation chamber and the clean water chamber with municipal water, turn on the control panel, heat the heating rod, and make the water temperature constant at 20°C; turn on the water pump to pump the municipal water out from the second water outlet and flow into the buffer chamber of the simulation box through the first water inlet; at the same time, fill the water storage chamber with municipal water, turn off the water pump, and let the simulation box stand for 24 hours. When the water level in the water storage tank chamber remains stable, it is considered that the sand layer in the sand filling chamber is saturated. Then, according to the test requirements, set the pumping pressure of the water pump through the control panel to provide a stable water head for the test. At the same time, open the second water inlet of the circulating water tank to allow water to flow in the sand layer. When the flow meter reading is stable for 6 hours, calculate the groundwater seepage rate through the flow meter indication.

[0069] (3) The liquid inlet pipe of the liquid nitrogen freezing tube is connected to the pressurized liquid nitrogen tank, and the gas outlet pipe of the liquid nitrogen freezing tube is connected to the atmosphere, wherein the liquid inlet pipe and the gas outlet pipe of the liquid nitrogen freezing tube are welded to the top of the outer tube wall of the liquid nitrogen freezing tube through a flange plate, and the length of the liquid inlet pipe is much longer than the gas outlet pipe. The stainless steel tube exposed to the air is wrapped with polyurethane insulation cotton, and the pressure valve of the pressurized liquid nitrogen tank is opened. By observing the temperature controller, the initial temperature of the liquid nitrogen is controlled to be maintained at -100°C. The liquid nitrogen enters the liquid inlet pipe through the stainless steel tube, and fills the entire liquid nitrogen freezing tube evenly and divergently through the opening. The amount of liquid nitrogen introduced is recorded by a flow meter.

[0070] (4) According to the requirements of different test sand layers for seepage velocity, the pumping pressure of the water pump is adjusted to maintain a constant water flow velocity, which is monitored by a flow meter.

[0071] At this time, in order to achieve different seepage velocities of the three sand layers under the same water flow velocity conditions, the seepage velocity of each sand layer and the water flow velocity meet the following requirements:

[0072] Wherein, in step (4), the seepage velocity and water flow velocity of different sand layers satisfy:

[0073]

[0074] Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer;

[0075] Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is:

[0076]

[0077] The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related:

[0078] v a1 :v a2 :…:v ai =K1:K2:…:K i ;

[0079] Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and the following can be inferred:

[0080]

[0081] Then, the seepage velocity of each layer is obtained through the ratio of permeability coefficients. According to the above formula, the seepage velocity in different sand layers can be obtained. Through a seepage freezing model test, the coupling effect between the liquid nitrogen freezing temperature field and the seepage field under different seepage velocities in multiple groups of different sand layers can be obtained.

[0082] (6) The temperature and stress data collected by the automatic data acquisition device are combined with the influence of different seepage velocities on the liquid nitrogen freezing temperature field to obtain the relationship between the various physical fields during the construction process of the liquid nitrogen stratum freezing method.

Claims

1. A water-heat coupling model test system simulating liquid nitrogen freezing of strata under different seepage environments, characterized in that: The system includes: The model box (1) has a buffer chamber (2), a sand filling chamber (3) and a water storage chamber (4) connected therein in sequence; the buffer chamber (2) is connected therein with a first water inlet (5), and the water storage chamber (4) is connected therein with a first water outlet (6); the sand filling chamber (3) is filled with a plurality of different types of sand layers in sequence from top to bottom; A liquid nitrogen freezing device comprises a plurality of liquid nitrogen freezing tubes (7) buried in different types of sand layers in a sand filling chamber (3), so as to achieve extremely fast freezing of the sand layers by regulating the temperature of the liquid nitrogen freezing tubes (7) buried in the plurality of different sand layers; A dynamic water seepage circulation device provides a seepage environment for a model box (1), comprising a circulating water tank (10) and a water pump (11) provided with a second water inlet (8) and a second water outlet (9), wherein the second water inlet (8) is connected to a first water outlet (6) on a water storage chamber (4), and the second water outlet (9) is connected to a first water inlet (5) on a buffer chamber (2); A monitoring system is provided to obtain the changing trends of the temperature field and the seepage field during the freezing process of the liquid nitrogen sand layer, and comprises a distributed optical fiber temperature change sensor (12) and a stress sensor (13) both buried in a plurality of different sand layers, and a digital control flow meter (14) located on the second water outlet (9) to monitor the seepage velocity.

2. According to claim 1, the hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments is characterized in that: The buffer chamber (2) is provided with a buffer stone, and the height of the first water inlet (5) on the buffer chamber (2) is higher than the height of the first water outlet (6) on the water storage chamber (4).

3. According to claim 1, the hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments is characterized in that: The buffer chamber (2) and the sand filling chamber (3), as well as the water storage chamber (4) of the sand filling chamber (3) are all connected via a water-permeable baffle (15).

4. The hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments according to claim 3 is characterized in that: The water-permeable baffle (15) is evenly provided with a plurality of water-permeable holes (16), and geotextiles (17) are respectively provided on both sides of the water-permeable baffle (15).

5. According to claim 1, the hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments is characterized in that: The plurality of liquid nitrogen freezing tubes (7) are arranged at equal intervals in the sand filling chamber (3) and are arranged horizontally perpendicular to the water flow direction.

6. The hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments according to claim 1, characterized in that: The liquid nitrogen freezing device also includes a pressurized liquid nitrogen tank (18) and a temperature controller (19) arranged on a liquid outlet pipe (18) of the pressurized liquid nitrogen tank.

7. The hydrothermal coupling model test system for simulating liquid nitrogen freezing of strata under different seepage environments according to claim 6 is characterized in that: The liquid inlet pipe (20) of the liquid nitrogen freezing pipe (7) is connected to the liquid outlet pipe (21) of the pressurized liquid nitrogen tank (18), and the gas outlet pipe (22) of the liquid nitrogen freezing pipe (7) is connected to the atmosphere.

8. A method for simulating based on the hydrothermal coupling model test system according to claim 1, characterized in that: The steps include: (1) several liquid nitrogen freezing tubes are buried in predetermined holes of the model box; the sand layer is filled in layers, the sand layer is continuously tamped during the filling process, and at the same time, each sensor is laid in the sand layer; after the filling is completed, a thermal insulation film is laid on the surface of the sand layer, and the sensor circuit is led out and connected to the signal processing receiver; (2) Start the water seepage circulation device and reach the preset water flow rate, and monitor the flow rate through the digital control flow meter; (3) Open the liquid nitrogen tank control valve and monitor the amount of liquid nitrogen introduced through a flow meter; (4) adjusting the pumping pressure of the water pump according to the requirements of different sand layers for seepage velocity, maintaining the seepage velocity constant, and monitoring it through a flow meter; (5) The temperature and flow rate data monitored by the data acquisition device are collected, and the influence of different seepage velocities on the liquid nitrogen freezing temperature field is combined to obtain the relationship between multiple physical fields during the construction process of the liquid nitrogen formation freezing method; Wherein, in step (4), the seepage velocity and water flow velocity of different sand layers satisfy: Where: v ai is the water flow velocity of each sand layer (m / d); v i is the seepage velocity of the i-th sand layer (m / d); K i is the permeability coefficient of the i-th sand layer (m / d); Δh i / Δl i is the hydraulic gradient of the i-th sand layer; n i is the porosity of the i-th sand layer; Based on the permeability coefficient of each sand layer K1, K2, ..., K i There is a difference between their relative hydraulic gradients Δh / Δl, so the total seepage velocity is: The water flow velocity of each sand layer and its permeability coefficient K1, K2, ..., K i Related: v a1 :v a2 :…:v ai =K1:K2:…:K i ; Then, the total seepage velocity can be used to directly measure the seepage flow rate Q and the seepage cross-sectional area A, and then the following can be inferred:

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