A Numerical Heat Transfer Model for Thermal Infiltration Coupled with Medium-Deep Casing-Type Buried Pipes and Its Solution Method

Abstract

This invention belongs to the technical field of medium-deep ground source heat pump systems, specifically relating to a numerical heat transfer model and solution method for a medium-deep casing-type buried pipe with thermal infiltration coupling. The invention establishes the governing equations for the thermal infiltration coupling between the medium-deep casing-type buried pipe and the soil / rock; establishes equations for the fluid within the external annulus of the medium-deep casing-type buried pipe; establishes equations for the fluid within the inner pipe of the medium-deep casing-type buried pipe; derives the formulas for the convective heat transfer coefficient and the Nusselt number; and obtains the heat transfer model based on the above equations and formulas. This invention provides a numerical heat transfer model that considers complex factors such as soil / rock stratification, geothermal gradients, and groundwater seepage, enabling it to reveal the influence of groundwater seepage and other factors on the heat extraction characteristics of buried pipes. While ensuring computational accuracy, this invention proposes a method for rapidly solving the model, significantly improving the computational efficiency of the numerical heat transfer model and providing guidance for engineering practice.

Description

Technical Field

[0001] This invention belongs to the technical field of medium-deep ground source heat pump systems, specifically relating to a medium-deep casing-type buried pipe thermal infiltration coupled numerical heat transfer model and its solution method. Background Technology

[0002] In recent years, the development of medium-deep geothermal pipe heating systems and the exploitation of deep, high-grade geothermal energy have become a hot topic of interest in the industry. Therefore, conducting relevant research on medium-deep geothermal pipe heating systems is particularly important.

[0003] Medium-deep casing-type buried pipes are the core component of the system, and their heat extraction characteristics determine the system's heating performance. To investigate the heat extraction performance of medium-deep casing-type buried pipes, existing technologies disclose an analytical model that treats the fluid within the borehole as steady-state heat transfer. Existing technologies also disclose a model that treats the entire heat transfer process as heat transfer along the rock-soil side and heat transfer within the borehole, with the borehole wall as the boundary, and treats the fluid as steady-state heat transfer. Existing technologies have also proposed a semi-analytical model.

[0004] However, analytical or semi-analytical models assume that fluid heat transfer within the borehole is steady-state, while in reality, fluid temperature changes dynamically over time. Therefore, this assumption reduces the model's accuracy and applicability, and analytical or semi-analytical models struggle to handle complex soil and rock stratification, geothermal gradients, and groundwater seepage. Compared to analytical or semi-analytical models, numerical heat transfer models can handle complex boundary conditions and have been applied in commercial and open-source software. However, simulating heat transfer in medium-deep casing-type buried pipes using commercial or open-source software presents challenges such as difficult physical modeling and a large number of meshes, severely limiting our understanding of the long-term heat extraction characteristics of buried pipes. Therefore, there is an urgent need for a highly applicable and fast-responding numerical heat transfer model. A careful search of publicly available literature and patents revealed no literature or patent proposing a numerical heat transfer model for medium-deep casing-type buried pipes that considers groundwater seepage, nor a fast solution method. Summary of the Invention

[0005] This invention provides a numerical heat transfer model and solution method for medium-deep casing-type buried pipes with thermal infiltration coupling. The numerical heat transfer model considers complex factors such as soil stratification, geothermal gradient, and groundwater seepage, and can be used to reveal the influence of factors such as groundwater seepage on the heat extraction characteristics of buried pipes. While ensuring calculation accuracy, this invention proposes a method for rapid solution of the model, which greatly improves the calculation efficiency of the numerical heat transfer model and can be used to guide engineering practice.

[0006] This invention is achieved through the following technical solution:

[0007] A numerical heat transfer model for coupled thermal infiltration in medium-deep casing-type buried pipes, wherein the establishment of the heat transfer model specifically involves...

[0008] Establish the coupling control equations between medium-deep casing-type buried pipes and soil thermal permeability;

[0009] Establish the equations for the fluid within the external annulus of medium-deep casing-type buried pipes;

[0010] Establish equations for fluid flow inside medium-deep casing-type buried pipes;

[0011] The formulas for the convective heat transfer coefficient and the Nusselt number are obtained.

[0012] The heat transfer model is obtained based on the above equations and formulas.

[0013] Furthermore, the establishment of the control equation for the coupling of medium-deep casing-type buried pipes with soil and rock thermal infiltration is specifically as follows: the soil and rock are porous media, composed of two parts: solid soil and rock and groundwater flow, and their control equation is:

[0014]

[0015] In the formula, It is a vector operator; T is the temperature of the porous medium, °C; t is the time, s; ε is the porosity of the porous medium, %; (ρc p ) t The volumetric specific heat capacity of the porous medium, J / (m³). 3 ·K); λ is the thermal conductivity of the porous medium, W / (m·K); q is the intensity of the internal heat source of the porous medium, W / m 3 .

[0016] Furthermore, the equation for establishing the fluid within the external annular cavity of the medium-deep casing-type buried pipe is specifically as follows:

[0017]

[0018] In the formula, C1 is the heat capacity per unit length of the outer tube, J / (m·K); T f1 and T f2 T represents the fluid temperatures in the outer and inner pipes, respectively, in °C. b R0 is the temperature of the borehole wall, in °C; R1 and R2 are the thermal resistances between the outer tube and the borehole and between the fluids in the inner and outer tubes, respectively, in m·K / W; C f is the specific heat capacity of the circulating working fluid, J / (kg·K); M is the mass flow rate of the circulating working fluid, kg / s.

[0019] Furthermore, the equation for establishing the fluid flow inside the medium-deep casing-type buried pipe is specifically as follows:

[0020]

[0021] In the formula, C2 is the heat capacity per unit length of the inner tube, J / (m·K). C1, C2, R1, and R2 can be calculated using the following formula:

[0022]

[0023]

[0024]

[0025] In the formula, d1 and d2 represent the outer and inner diameters of the medium-deep casing-type buried pipe, respectively, in meters (m); ρ f c f ρ1c1, ρ2c2 and ρ g c g The specific heat at constant volume for the circulating working fluid, outer pipe, inner pipe, and backfill material are respectively expressed in J / (m³). 3 ·K); λ p1 , λ p2 and λ g λ represents the thermal conductivity of the outer pipe, inner pipe, and backfill material, respectively, in W / (m·K); h is the convective heat transfer coefficient, in W / (m²·K). 2 ·K).

[0026] Furthermore, the formula for the convective heat transfer coefficient is specifically as follows:

[0027]

[0028] In the formula, λ f Where is the thermal conductivity of the fluid, W / (m·K); D is the characteristic length, m; Nu is the Nusselt number. The specific formula for the Nusselt number is as follows:

[0029]

[0030] In the formula, Re and Pr are the Reynolds number and Prandtl number, respectively, and f is the Darcy drag number, which can be calculated by the following formula:

[0031] f = (1.82·lgRe - 1.64) -2 (10)

[0032]

[0033] In the formula, u f d is the flow rate of the circulating working fluid, in m / s; e υ is the equivalent diameter, in meters; υ is the kinematic viscosity, in meters. 2 / s.

[0034] A solution method for a medium-deep casing-type buried pipe thermal infiltration coupled numerical heat transfer model, wherein the solution method is to solve the above-mentioned medium-deep casing-type buried pipe thermal infiltration coupled numerical heat transfer model, and the solution method requires to clarify the initial and boundary conditions of the governing equations.

[0035] Based on the initial and boundary conditions and the finite difference method to discretize the control equations, the nodal equations are established using the infinite element thermal balance method and the alternating time step method.

[0036] After establishing the equations for each node, the TDMA algorithm and Gaussian elimination method were used to solve the node equations for the soil and rock part and the node equations for the fluid in the medium-deep casing buried pipe, respectively.

[0037] The accuracy of the deep-casing buried pipe thermal infiltration coupling numerical heat transfer model was verified using engineering measured data.

[0038] Furthermore,

[0039] The specific solution method is as follows:

[0040] S101, Input the basic parameters used to solve the model, including the dimensions of the medium-deep casing-type buried pipe and the physical properties along the route;

[0041] S102, distinguish between the heating stage and the shutdown stage of medium-deep casing-type buried pipes, and create global variables thermal resistance R1 and R2, heat capacity C1 and C2;

[0042] S103, the soil and rock inside and outside the borehole are divided into grids with the borehole as the boundary;

[0043] S104 is used to initialize the computational domain based on the surface soil and rock temperature and geothermal gradient.

[0044] S105, determine whether there is water in the soil layer. If yes, proceed to S106; otherwise, proceed to S107.

[0045] S106, invoke the geotechnical control equations that take into account groundwater seepage;

[0046] S107, invoke the simple heat conduction control equation for soil and rock;

[0047] S108, determine whether it is the heating season. If yes, proceed to S109; otherwise, proceed to S112.

[0048] S109, invoke the fluid control equations for the heating phase;

[0049] S110, input the thermal resistance of the heat extraction stage into the control equation of S109. and heat capacity and

[0050] S111, after creating the nodal equations for the soil and fluids in the heat extraction stage, proceed to S115;

[0051] S112, invoke the fluid control equations for the shutdown phase;

[0052] S113, input the thermal resistance of the heat extraction stage into the control equation of S112. and heat capacity and

[0053] S114, after creating the nodal equations for the soil and fluid during the shutdown phase, proceed to S115;

[0054] S115, Create variables to distinguish between solving radial and longitudinal node equations;

[0055] S116, respectively call the TDMA algorithm and the Gauss-Seidel iterative algorithm to solve the nodal equations of soil and fluid;

[0056] S117, solve the nodal equations of soil and fluid alternately in the longitudinal and radial directions;

[0057] S118, determine whether the temperature fields of the soil and fluid converge. If not, return to S103; if so, proceed to S119.

[0058] S119 outputs the soil and rock temperature field, the fluid temperature field, and the borehole wall temperature.

[0059] Furthermore, the initial and boundary conditions of the governing equations are specifically as follows: the initial temperature distribution of the soil temperature field can be calculated by the following formula:

[0060]

[0061] In the formula, T z The temperature of the rock and soil at different depths is ℃; T s denoted as the average temperature of the surface soil and rock, in °C; GG represents the geothermal gradient, in °C / m; and z represents the depth of the soil and rock, in m.

[0062] First-type boundary conditions are set at the radial and ground-level soil-rock boundaries, and third-type boundary conditions are set at the surface, as shown in the following equation:

[0063]

[0064] In the formula, h a The surface convective heat transfer coefficient is W / (m²). 2 ·K); T a T represents the average surface air temperature, in °C. s The surface temperature of the soil and rock is ℃;

[0065] The fluid inlet / outlet and bottom boundary are shown in the following formula:

[0066]

[0067] T f1 =T f2,z=H (15)

[0068] Furthermore, the method of establishing nodal equations using the micro-element thermal balance method and the alternating time step method specifically involves transforming the five-unknown equations into three-unknown equations. After discretizing the governing equations of the porous medium, the nodal equations are as follows:

[0069]

[0070] In the formula,

[0071]

[0072]

[0073] The beneficial effects of this invention are:

[0074] The proposed numerical heat transfer model for medium-deep casing-type buried pipes with thermal infiltration coupling abandons the assumption of existing analytical models that treat fluid as steady-state heat transfer, and can handle complex conditions such as soil and rock stratification, geothermal gradient and groundwater seepage. This not only improves the calculation accuracy of the model, but also broadens its applicability.

[0075] This invention establishes nodal equations using the alternating time step method, transforming a five-unknown equation into a three-unknown equation. By combining the TDMA algorithm and Gaussian elimination, it achieves rapid solution of the model. Under the condition that the calculation error is within acceptable limits, the computational efficiency is improved by several orders of magnitude compared with commercial software, providing theoretical guidance for engineering practice. Attached Figure Description

[0076] Figure 1 This is a schematic diagram of the structure of the present invention.

[0077] Figure 2 This is a flowchart of the solution process for the model of this invention.

[0078] Figure 3 This is a comparison chart of the model calculation data and the measured data of this invention. Detailed Implementation

[0079] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0080] A numerical heat transfer model for coupled thermal infiltration in medium-deep casing-type buried pipes, wherein the establishment of the heat transfer model specifically involves...

[0081] Establish the coupling control equations between medium-deep casing-type buried pipes and soil thermal permeability;

[0082] Establish the equations for the fluid within the external annulus of medium-deep casing-type buried pipes;

[0083] Establish equations for fluid flow inside medium-deep casing-type buried pipes;

[0084] The formulas for the convective heat transfer coefficient and the Nusselt number are obtained.

[0085] The heat transfer model is obtained based on the above equations and formulas.

[0086] Furthermore, the establishment of the control equation for the coupling of medium-deep casing-type buried pipes with soil and rock thermal infiltration is specifically as follows: the soil and rock are porous media, composed of two parts: solid soil and rock and groundwater flow, and their control equation is:

[0087]

[0088] In the formula, It is a vector operator; T is the temperature of the porous medium, °C; t is the time, s; ε is the porosity of the porous medium, %; (ρc p ) t The volumetric specific heat capacity of the porous medium, J / (m³). 3 ·K); λ is the thermal conductivity of the porous medium, W / (m·K); q is the intensity of the internal heat source of the porous medium, W / m 3 .

[0089] Furthermore, the equation for establishing the fluid within the external annular cavity of the medium-deep casing-type buried pipe is specifically as follows:

[0090]

[0091] In the formula, C1 is the heat capacity per unit length of the outer tube, J / (m·K); T f1 and T f2 T represents the fluid temperatures in the outer and inner pipes, respectively, in °C. b R0 is the temperature of the borehole wall, in °C; R1 and R2 are the thermal resistances between the outer tube and the borehole and between the fluids in the inner and outer tubes, respectively, in m·K / W; C f is the specific heat capacity of the circulating working fluid, J / (kg·K); M is the mass flow rate of the circulating working fluid, kg / s.

[0092] Furthermore, the equation for establishing the fluid flow inside the medium-deep casing-type buried pipe is specifically as follows:

[0093]

[0094] In the formula, C2 is the heat capacity per unit length of the inner tube, J / (m·K). C1, C2, R1, and R2 can be calculated using the following formula:

[0095]

[0096]

[0097]

[0098]

[0099] In the formula, d1 and d2 represent the outer and inner diameters of the medium-deep casing-type buried pipe, respectively, in meters (m); ρ f c f ρ1c1, ρ2c2 and ρ g c g The specific heat at constant volume for the circulating working fluid, outer pipe, inner pipe, and backfill material are respectively expressed in J / (m³). 3 ·K); λ p1 , λ p2 and λ g λ represents the thermal conductivity of the outer pipe, inner pipe, and backfill material, respectively, in W / (m·K); h is the convective heat transfer coefficient, in W / (m²·K). 2 ·K).

[0100] Furthermore, the formula for the convective heat transfer coefficient is specifically as follows:

[0101]

[0102] In the formula, λ f Where is the thermal conductivity of the fluid, W / (m·K); D is the characteristic length, m; Nu is the Nusselt number. The specific formula for the Nusselt number is as follows:

[0103]

[0104] In the formula, Re and Pr are the Reynolds number and Prandtl number, respectively, and f is the Darcy drag number, which can be calculated by the following formula:

[0105] f = (1.82·lgRe - 1.64) -2 (10)

[0106]

[0107] In the formula, u f d is the flow rate of the circulating working fluid, in m / s; e υ is the equivalent diameter, in meters; υ is the kinematic viscosity, in meters. 2 / s.

[0108] A solution method for a medium-deep casing-type buried pipe thermal infiltration coupled numerical heat transfer model, wherein the solution method is to solve the above-mentioned medium-deep casing-type buried pipe thermal infiltration coupled numerical heat transfer model, and the solution method requires to clarify the initial and boundary conditions of the governing equations.

[0109] Based on the initial and boundary conditions and the discrete control equations using the finite difference method, the mesh generation form is as follows: Figure 1 As shown, the nodal equations are established using the infinitesimal element thermal equilibrium method and the alternating time step method;

[0110] The grid division specifically includes the following steps.

[0111] S101, Input the basic parameters used to solve the model, including the dimensions of the medium-deep casing-type buried pipe and the physical properties along the route, etc.

[0112] S102, distinguish between the heating stage and the shutdown stage of medium-deep casing-type buried pipes, and create global variables thermal resistance R1 and R2, heat capacity C1 and C2;

[0113] S103, the soil and rock inside and outside the borehole are divided into grids with the borehole as the boundary;

[0114] S104 is used to initialize the computational domain based on the surface soil and rock temperature and geothermal gradient.

[0115] S105, determine whether there is water in the soil layer. If yes, proceed to S106; otherwise, proceed to S107.

[0116] S106, invoke the geotechnical control equations that take into account groundwater seepage;

[0117] S107, invoke the simple heat conduction control equation for soil and rock;

[0118] S108, determine whether it is the heating season. If yes, proceed to S109; otherwise, proceed to S112.

[0119] S109, invoke the fluid control equations for the heating phase;

[0120] S110, input the thermal resistance of the heat extraction stage into the control equation of S109. and heat capacity and

[0121] S111, after creating the nodal equations for the soil and fluids in the heat extraction stage, proceed to S115;

[0122] S112, invoke the fluid control equations for the shutdown phase;

[0123] S113, input the thermal resistance of the heat extraction stage into the control equation of S112. and heat capacity and

[0124] S114, after creating the nodal equations for the soil and fluid during the shutdown phase, proceed to S115;

[0125] S115, Create variables to distinguish between solving radial and longitudinal node equations;

[0126] S116, respectively call the TDMA algorithm and the Gauss-Seidel iterative algorithm to solve the nodal equations of soil and fluid;

[0127] S117, solve the nodal equations of soil and fluid alternately in the longitudinal and radial directions;

[0128] S118, determine whether the temperature fields of the soil and fluid converge. If not, return to S103; if so, proceed to S119.

[0129] S119 outputs the temperature field of soil and rock, the temperature field of fluid, and the temperature of borehole wall, etc.

[0130] After establishing the equations for each node, the TDMA algorithm and Gaussian elimination method were used to solve the node equations for the soil and rock part and the node equations for the fluid in the medium-deep casing buried pipe, respectively.

[0131] The accuracy of the deep-casing buried pipe thermal infiltration coupling numerical heat transfer model was verified using engineering measured data.

[0132] Furthermore, the initial and boundary conditions of the governing equations are specifically as follows: the initial temperature distribution of the soil temperature field can be calculated by the following formula:

[0133]

[0134] In the formula, T z The temperature of the rock and soil at different depths is ℃; T s denoted as the average temperature of the surface soil and rock, in °C; GG represents the geothermal gradient, in °C / m; and z represents the depth of the soil and rock, in m.

[0135] First-type boundary conditions are set at the radial and ground-level soil-rock boundaries, and third-type boundary conditions are set at the surface, as shown in the following equation:

[0136]

[0137] In the formula, h a The surface convective heat transfer coefficient is W / (m²). 2 ·K); T a T represents the average surface air temperature, in °C. s The surface temperature of the soil and rock is ℃;

[0138] The fluid inlet / outlet and bottom boundary are shown in the following formula:

[0139]

[0140] T f1 =T f2 ,z=H (15)

[0141] Furthermore, the method of establishing nodal equations using the micro-element thermal balance method and the alternating time step method specifically involves transforming the five-unknown equations into three-unknown equations. After discretizing the governing equations of the porous medium, the nodal equations are as follows:

[0142]

[0143] In the formula,

[0144]

[0145]

[0146] Next, basic parameters such as soil and rock thermal properties and the dimensions of medium-deep casing-type buried pipes are imported into the model. The computational domain is initialized based on the average temperature and geothermal gradient of the surface soil and rock. Different governing equations are invoked according to different operating stages of the system and whether groundwater is present in the soil and rock, as shown in the attached figure. Figure 1 The grid division shown is used to discretize the upper governing equations using the finite difference method. The nodal equations are established using the alternating time step method and the infinitesimal element thermal equilibrium method. The nodal equations for the soil and rock sections and the fluid within the medium-deep casing type buried pipe are solved based on the TDMA algorithm and Gaussian elimination method. Furthermore, the results are output by determining whether the fluid temperature distribution along the path and the soil and rock temperature field converge.

[0147] Finally, experimental data were used to verify the accuracy of the model, and the comparison results are attached. Figure 3 As shown. From the appendix Figure 3 As can be seen, when the system runs for more than 400 hours, the maximum relative error of the model is less than 6.5%, indicating that the model's computational accuracy is within an acceptable range. Furthermore, on a standard computer (AMD Ryzen 95900X12-Core Processor), solving for a medium-deep casing-type underground pipeline with a burial depth of 2000m takes less than 60 seconds per year, representing a computational efficiency improvement of several orders of magnitude compared to commercial software. Therefore, related software can be developed based on the model proposed in this invention to guide engineering practice.

Claims

1. A method for establishing a numerical heat transfer model for thermal infiltration coupling of medium-deep casing-type buried pipes, characterized in that, The establishment of the heat transfer model is specifically as follows: Establish the coupling control equations between medium-deep casing-type buried pipes and soil thermal permeability; Establish the equations for the fluid within the external annulus of medium-deep casing-type buried pipes; Establish equations for fluid flow inside medium-deep casing-type buried pipes; The formulas for the convective heat transfer coefficient and the Nusselt number are obtained. The heat transfer model is obtained based on the above equations and formulas; Specifically, the control equations for the coupling of medium-deep casing-type buried pipes with soil thermal infiltration are established as follows: the soil is a porous medium composed of two parts: solid soil and groundwater flow. The control equations are as follows: In the formula, It is a vector operator; T The temperature of the porous medium is ℃; t For time, s; The volumetric specific heat capacity of the porous medium, J / (m³). 3 K); The thermal conductivity of the porous medium is W / (m). K); q The intensity of the internal heat source in the porous medium, W / m 3 ; The equation for establishing the fluid within the external annular cavity of the medium-deep casing-type buried pipe is specifically as follows: In the formula, C 1 represents the heat capacity per unit length of the outer tube, in J / (m²). K); and These are the fluid temperatures in the outer and inner pipes, respectively, in °C. The borehole wall temperature is given in °C. R 1 and R 2 represents the thermal resistance between the outer tube and the borehole, and the thermal resistance between the fluids in the inner and outer tubes, respectively (m). K) / W; C f Specific heat capacity of the circulating working fluid, J / (kg) K); M The mass flow rate of the circulating working fluid is kg / s; The specific equation for establishing the fluid flow inside the medium-deep casing-type buried pipe is as follows: In the formula, C 2 represents the heat capacity per unit length of the inner tube, in J / (m²). K), where C 1 、C 2 、R 1 and R 2 can be calculated using the following formula: In the formula, d 1 and d 2 represents the outer and inner diameters of the medium-deep casing-type buried pipe, respectively, in meters (m). , , and The specific heat at constant volume for the circulating working fluid, outer pipe, inner pipe, and backfill material are respectively expressed in J / (m³). 3 K); , and The thermal conductivity of the outer pipe, inner pipe, and backfill material are respectively, in W / (m²). K); h The convective heat transfer coefficient is W / (m²). 2 ·K).

2. The method for establishing a numerical heat transfer model for thermal infiltration coupling of a medium-deep casing-type buried pipe according to claim 1, characterized in that, The formula for the convective heat transfer coefficient is as follows: In the formula, The thermal conductivity of the fluid is W / (m). K); D The characteristic length is m; For Nusel number The specific formula for the Nusselt number is as follows: In the formula, R e and These are the Reynolds number and the Prandtl number, respectively. f for Darcy The drag coefficient can be calculated using the following formula: In the formula, The velocity of the circulating working fluid is denoted as m / s. The equivalent diameter is in meters (m). For kinematic viscosity, m 2 / s.

3. A solution method for a numerical heat transfer model involving thermal infiltration coupling in medium-deep casing-type buried pipes, characterized in that... The solution method is to solve the deep casing buried pipe thermal infiltration coupled numerical heat transfer model in claims 1-2. The solution method requires to clarify the initial and boundary conditions of the control equations. Based on the initial and boundary conditions and the finite difference method to discretize the control equations, the nodal equations are established using the infinite element thermal balance method and the alternating time step method. After establishing the equations for each node, the TDMA algorithm and Gaussian elimination method were used to solve the node equations for the soil and rock part and the node equations for the fluid in the medium-deep casing buried pipe, respectively. The accuracy of the deep-casing buried pipe thermal infiltration coupling numerical heat transfer model was verified using engineering measured data.

4. The solution method for the numerical heat transfer model of a medium-deep casing-type buried pipe with thermal infiltration coupling according to claim 3, characterized in that, The specific solution method is as follows: S101, Input the basic parameters used to solve the model, including the dimensions of the medium-deep casing-type buried pipe and the physical properties along the route; S102, distinguishing between the heating phase and shutdown phase of medium-deep casing-type buried pipes, and creating a global variable thermal resistance. and heat capacity and ; S103, the soil and rock inside and outside the borehole are divided into grids with the borehole as the boundary; S104 is used to initialize the computational domain based on the surface soil and rock temperature and geothermal gradient. S105, determine whether there is water in the soil layer. If yes, proceed to S106; otherwise, proceed to S107. S106, invoke the geotechnical control equations that take into account groundwater seepage; S107, invoke the simple heat conduction control equation for soil and rock; S108, determine whether it is the heating season. If yes, proceed to S109; otherwise, proceed to S112. S109, invoke the fluid control equations for the heating phase; S110, input the thermal resistance of the heat extraction stage into the control equation of S109. and heat capacity and ; S111, after creating the nodal equations for the soil and fluids in the heat extraction stage, proceed to S115; S112, invoke the fluid control equations for the shutdown phase; S113, input the thermal resistance of the heat extraction stage into the control equation of S112. and heat capacity and S114, after creating the nodal equations for the soil and fluid during the shutdown phase, proceed to S115; S115, Create variables to distinguish between solving radial and longitudinal node equations; S116, respectively call the TDMA algorithm and the Gauss-Seidel iterative algorithm to solve the nodal equations of soil and fluid; S117, solve the nodal equations of soil and fluid alternately in the longitudinal and radial directions; S118, determine whether the temperature fields of the soil and fluid converge. If not, return to S103; if so, proceed to S119. S119 outputs the soil and rock temperature field, the fluid temperature field, and the borehole wall temperature.

5. The solution method for the numerical heat transfer model of a medium-deep casing-type buried pipe with thermal infiltration coupling according to claim 3, characterized in that, The initial and boundary conditions of the governing equations are as follows: the initial temperature distribution of the soil temperature field can be calculated by the following formula: In the formula, The temperature of the rock and soil at different depths is given in °C. The average temperature of the surface soil and rock is ℃; GG The geothermal gradient is expressed in °C / m. z The depth of the soil / rock, in meters (m). First-type boundary conditions are set at the radial and ground-level soil-rock boundaries, and third-type boundary conditions are set at the surface, as shown in the following equation: In the formula, h a The surface convective heat transfer coefficient is W / (m²). 2 ·K); T a The average temperature of the air at the Earth's surface is ℃; T s The surface temperature of the soil and rock is ℃; The fluid inlet / outlet and bottom boundary are shown in the following formula: 。 6. The solution method for the numerical heat transfer model of a medium-deep casing-type buried pipe with thermal infiltration coupling according to claim 3, characterized in that, Specifically, the nodal equations established using the micro-element thermal equilibrium method and the alternating time step method are as follows: the five-unknown equations are transformed into three unknown equations, and the nodal equations after discretizing the governing equations of the porous medium are shown in the following formula: 。

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