Prediction method and test device for heat transfer performance of buried pipe ground source heat pump heat exchange system in abandoned mines

Abstract

The present invention relates to a prediction method and test device for heat exchange performance in a buried pipe ground source heat pump heat exchange system in an abandoned mine. The method comprises: establishing a mathematical model, establishing a control equation, and determining the initial conditions and boundary conditions of the control equation. The mathematical model can be calculated using the non-isothermal pipe flow module and the solid heat transfer module of the Comsol Multiphysics software to obtain the results of the influence of different factors on the heat exchange performance. At the same time, a test device for heat exchange of a buried pipe ground source heat pump system will also be established to verify the accuracy of the numerical simulation results. Furthermore, an orthogonal simulation test is carried out for each influencing factor, and the extreme difference of the orthogonal test results is used as the judgment standard to compare the influence of each factor on the heat exchange performance, obtain the main control factors of the mine geothermal underground heat exchange system, and realize the prediction of the heat exchange performance.

Description

Technical Field

[0001] The invention relates to a method and a test device for predicting heat exchange performance in a ground-source heat pump heat exchange system with buried pipes in abandoned mines. Background Art

[0002] With the transformation of energy structure, the proportion of fossil energy is decreasing, and the utilization rate of renewable energy is increasing year by year. Geothermal energy, as a renewable and clean energy, has many advantages such as large reserves, small footprint, low geothermal exploration cost, energy saving and environmental protection, and has broad development prospects and potential.

[0003] Due to the over-exploitation of mineral resources in the early days, the number of abandoned mines has increased in recent years. Due to long-term mining, the internal tunnels and mining faces are crisscrossed, providing sufficient space for the development of geothermal energy. In addition, relatively complete exploration work has been carried out during the mining process, which can provide sufficient geological data for geothermal development. As a renewable and clean energy source, mine geothermal energy has many advantages such as large reserves, small footprint, low geothermal exploration cost, energy saving and environmental protection, etc. It has huge market potential and broad development space.

[0004] However, the inventors found that there is little research on the development of mine geothermal energy as an energy source. The current application form is mainly to use ground source heat pumps and air source heat pumps to achieve the purpose of heating living areas and antifreeze at wellheads. In terms of mine geothermal utilization, its form is still concentrated on mine wastewater, gushing water and tunnel gas. At the same time, groundwater source heat pumps are prone to cause problems such as groundwater level drop and recharge difficulties, which is not conducive to the long-term and stable development of mine geothermal energy. A new, stable and reliable mine geothermal mining model is urgently needed.

[0005] Currently, the most widely used and most mature technology is the exploitation model of shallow geothermal energy. According to the different heat sources used, it can be divided into ground source heat pump system and water source heat pump system. Among them, the ground source heat pump system has higher stability and durability due to its closed system.

[0006] However, the current research on ground source heat pump systems is mostly focused on shallow underground pipe design, and there is little research on heat exchange of underground pipes in mines. Since the structure and ground temperature conditions of mines are quite different from those of shallow underground pipe sites, research on the heat exchange performance of ground source heat pump systems in mines is of great significance to the development of geothermal resources in mines. Summary of the invention

[0007] In order to solve the above technical deficiencies, the present invention provides a prediction method and test device for the heat exchange performance of the buried pipe ground source heat pump heat exchange system in an abandoned mine, which provides a fast and effective parameter adjustment object for the design and optimization of the mine geothermal heat exchange system, and is of great significance to the development of mine geothermal resources.

[0008] The present invention is achieved by the following measures:

[0009] A method for predicting heat exchange performance in a ground-source heat pump heat exchange system with buried pipes in abandoned mines, comprising the following steps:

[0010] Step 1, establish a mathematical model of heat exchange efficiency in the ground-source heat pump heat exchange system of the buried pipe in the mine, the parameters of which include the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium involved in the heat conduction outside the U-tube; the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor, and tube diameter of the heat medium involved in the heat convection inside the U-tube;

[0011] Step 2: Use the mathematical model in step 1 to establish a control equation based on heat conduction outside the U-tube and heat convection inside the U-tube, and determine the initial conditions and boundary conditions of the control equation;

[0012] Step 3: The mathematical model in step 1 is expressed by coupling the non-isothermal pipe flow module and the solid heat transfer module of COMSOL Multiphysics to describe the heat exchange process of the underground heat exchange system, and a numerical model of mine geothermal heat exchange is established to calculate the heat exchange performance of the mine ground source heat pump heat exchange system;

[0013] Step 4: Carry out orthogonal simulation tests for each factor calculated by the numerical model, use the extreme value of the orthogonal test results as the evaluation criterion, compare the influence of each factor on the heat transfer performance, obtain the main control factors of the underground heat exchange system of the mine geothermal energy, and realize the prediction of heat transfer performance.

[0014] The heat conduction equation outside the U-tube in step 2 is:

[0015]

[0016] Heat convection equation in a U-tube:

[0017]

[0018] In the formula, ρ m 、c m , λ, h, V, and A1 are the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium respectively; ρ f 、c f , k f ,u,A,e t 、f D ,d h They are the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor and tube diameter of the heat medium; is the Laplace operator; T is the temperature field; P is the water head pressure; q pw is the heat transfer rate per unit volume of the internal heat source; t is the heat exchange time;

[0019] The specific boundary conditions and initial conditions are as follows:

[0020]

[0021] Where, T out , T in [℃] are the outlet water temperature and injection water temperature respectively; ρ[kg / m 3 ] is the density of water; r[m] is the diameter of the U-tube; h[m] is the length of the borehole; q0”[W / m 2 ] is the heat flux density, that is, the heat transfer rate of the medium per unit area and in the direction of the temperature gradient; k [W / (m·K)] is the thermal conductivity of the medium; Q f [W]Heat transfer power.

[0022] This method includes the following reasonable assumptions and simplifications: assuming that the physical properties of the rock mass and the bored cast body are uniform, ignoring the cracks and hydraulic movement inside the rock mass, ignoring the influence of geothermal gradient on the temperature of the geometric model and the influence of temperature change on the thermophysical properties of the solid part of the model; ignoring the existence of contact thermal resistance in the heat exchange process, assuming that the two media are in close contact, and the temperatures of the two media on the contact surface are the same; simplifying the pipeline into a one-dimensional line, using the fluid in the pipeline as a heat source, and treating the fluid heat transfer as one-dimensional turbulent heat transfer. The specific flow process of the fluid in the pipe is ignored, and it is assumed that the temperature and flow rate at each position on each section of the pipeline are the same.

[0023] In step 4, the main controlling factors of the underground heat exchange system of the mine geothermal include water injection rate, original ground temperature, pipe diameter, borehole length, water injection temperature, and formation thermal conductivity.

[0024] A test device for applying the method for predicting heat exchange performance in a ground-source heat pump heat exchange system of an abandoned mine buried pipe as described in claim 1, comprising a water bath insulation box and an insulation box body, wherein a constant-pressure water supply tank is arranged in the water bath insulation box, the inlet of the constant-pressure water supply tank is connected to an air intake pipe, the air intake pipe is connected to an air compressor, the air intake pipe is provided with a pressure control valve, the outlet of the constant-pressure water supply tank is connected to an outlet pipe, the outlet pipe is provided with a pipeline valve, an electric thermal resistor and a flow meter with a regulating valve; the insulation box body is filled with sand and soil for simulating heat exchange rock mass in a mine buried pipe system, and a U-shaped heat exchange tube with an upward opening for simulating an actual heat exchanger is vertically embedded in the insulation box body, the end of the outlet pipe is connected to an opening of one end of the heat exchange tube, the other end of the heat exchange tube is connected to a drain pipe, and the drain pipe is connected to a tail water recovery tank; a temperature probe for detecting the temperature of the sand and soil is arranged in the insulation box body, and the signal of the temperature probe is connected to a data collection system and a computer.

[0025] The outer and inner sides of the side walls of the above-mentioned insulation box are covered with pearl foam cotton, and sealant is filled between the pearl foam cotton and the side walls; the sand in the insulation box is composed of water, cement and sand, and the weight ratio is water: cement: sand = 0.5:1:2; the heat exchange tube in the insulation box adopts a copper tube body with a thickness of 5mm and an inner diameter of 5cm.

[0026] The beneficial effects of the present invention are that it can predict the heat exchange performance of a buried pipe ground source heat pump heat exchange system in a mine with different structures and ground temperature conditions, thereby providing a theoretical basis for the development of geothermal resources in the mine. The prediction method is reliable and highly accurate, and can be verified by a test device, further improving the accuracy of the prediction method. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the process of the present invention;

[0028] Figure 2 A schematic diagram of a heat exchange model in a mine buried pipe ground source heat pump system of the present invention;

[0029] Figure 3 It is a schematic diagram of the test device of the present invention;

[0030] Figure 4 The experimental results of the inlet and outlet temperatures of the physical model experiment and the temperatures of each monitoring point of the experimental subject in the present invention are compared with the numerical simulation results;

[0031] Figure 5 This is a relationship diagram between the original ground temperature and the average temperature difference between the inlet and outlet of the present invention;

[0032] Figure 6 It is a relationship diagram between the thermal conductivity of the formation and the average temperature difference between the inlet and outlet of the present invention;

[0033] Figure 7 A relationship diagram between the borehole diameter and the average temperature difference between the inlet and outlet of the present invention;

[0034] Figure 8 This is a relationship diagram between the branch pipe spacing and the average inlet and outlet temperature difference of the present invention;

[0035] Fig. 9 A relationship diagram between the borehole length and the average temperature difference between the inlet and outlet of the present invention;

[0036] Fig.10 It is the relationship between the water injection temperature and the average inlet and outlet temperature of the present invention.

[0037] In the figure: 1. air compressor, 2. pressure control valve, 3. water bath insulation box, 4. constant pressure water supply tank, 5. pipeline valve, 6. flow meter, 7. electric thermal resistor, 8. insulation box, 9. temperature probe, 10. tail water recovery tank, 11. heat exchange tube, 12. data collection system, 13. computer. DETAILED DESCRIPTION

[0038] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0039] Embodiment 1:

[0040] like Figure 1 , 2 As shown, a method for predicting heat exchange performance in a ground-source heat pump heat exchange system of an abandoned mine buried pipe comprises the following steps:

[0041] Step 1, establish a mathematical model of heat exchange efficiency in the ground-source heat pump heat exchange system of the buried pipe in the mine, the parameters of which include the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium involved in the heat conduction outside the U-tube; the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor, and tube diameter of the heat medium involved in the heat convection inside the U-tube;

[0042] Step 2: Use the mathematical model in step 1 to establish a control equation based on heat conduction outside the U-tube and heat convection inside the U-tube, and determine the initial conditions and boundary conditions of the control equation;

[0043] Step 3: The mathematical model in step 1 is expressed by coupling the non-isothermal pipe flow module and the solid heat transfer module of COMSOL Multiphysics to describe the heat exchange process of the underground heat exchange system, and a numerical model of mine geothermal heat exchange is established to calculate the heat exchange performance of the mine ground source heat pump heat exchange system;

[0044] Step 4: Carry out orthogonal simulation tests for each factor calculated by the numerical model, use the extreme value of the orthogonal test results as the evaluation standard, compare the influence of each factor on the heat exchange performance, obtain the main control factors of the underground heat exchange system of the mine geothermal energy, and realize the prediction of heat exchange performance. In step 4, the main control factors of the underground heat exchange system of the mine geothermal energy include water injection rate, original ground temperature, pipe diameter, borehole length, water injection temperature, and formation thermal conductivity.

[0045] The heat conduction equation outside the U-tube in step 2 is:

[0046]

[0047] Heat convection equation in a U-tube:

[0048]

[0049] In the formula, ρ m 、c m , λ, h, V, and A1 are the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium respectively; ρ f 、c f , k f ,u,A,e t 、f D ,d h They are the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor and tube diameter of the heat medium; is the Laplace operator; T is the temperature field; P is the water head pressure; q pw is the heat transfer rate per unit volume of the internal heat source; t is the heat exchange time;

[0050] The specific boundary conditions and initial conditions are as follows:

[0051]

[0052] Where, T out , T in [℃] are the outlet water temperature and injection water temperature respectively; ρ[kg / m 3 ] is the density of water; r[m] is the diameter of the U-tube; h[m] is the length of the borehole; q0”[W / m 2 ] is the heat flux density, that is, the heat transfer rate of the medium per unit area and in the direction of the temperature gradient; k [W / (m·K)] is the thermal conductivity of the medium; Q f [W]Heat transfer power.

[0053] This method includes the following reasonable assumptions and simplifications: assuming that the physical properties of the rock mass and the bored cast body are uniform, ignoring the cracks and hydraulic movement inside the rock mass, ignoring the influence of geothermal gradient on the temperature of the geometric model and the influence of temperature change on the thermophysical properties of the solid part of the model; ignoring the existence of contact thermal resistance in the heat exchange process, assuming that the two media are in close contact, and the temperatures of the two media on the contact surface are the same; simplifying the pipeline into a one-dimensional line, using the fluid in the pipeline as a heat source, and treating the fluid heat transfer as one-dimensional turbulent heat transfer. The specific flow process of the fluid in the pipe is ignored, and it is assumed that the temperature and flow rate at each position on each section of the pipeline are the same.

[0054] like Figure 3As shown, a test device for predicting the heat exchange performance of the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1 is applied, comprising a water bath insulation box 3 and an insulation box body 8. A constant pressure water supply tank 4 is arranged in the water bath insulation box 3. The inlet of the constant pressure water supply tank 4 is connected to an air intake pipe, and the air intake pipe is connected to an air compressor 1. A pressure control valve 2 is arranged on the air intake pipe. The outlet of the constant pressure water supply tank 4 is connected to a water outlet pipe, and a pipeline valve 5, an electric thermal resistor 7 and a flow meter 6 with a regulating valve are arranged on the water outlet pipe. The insulation box The body 8 is filled with sand and soil for simulating the heat exchange rock mass in the underground pipe system of the mine, and a U-shaped heat exchange tube 11 with an upward opening for simulating the actual heat exchanger is vertically embedded in the insulation box 8, the end of the outlet pipe is connected to one end opening of the heat exchange tube 11, and the other end opening of the heat exchange tube 11 is connected to a drain pipe, and the drain pipe is connected to a tail water recovery tank 10; the insulation box 8 is provided with a temperature probe 9 for detecting the temperature of the sand and soil, and the signal of the temperature probe 9 is connected to a data collection system 12 and a computer 13. Pearl foam cotton is laid on the outer and inner sides of the side wall of the insulation box 8, and sealant is filled between the pearl foam cotton and the side wall; the sand and soil in the insulation box are composed of water, cement and sand, and the weight ratio is water: cement: sand = 0.5: 1: 2; the heat exchange tube 11 in the insulation box adopts a copper tube body with a thickness of 5mm and an inner diameter of 5cm.

[0055] Verification of a prediction method for heat transfer performance of a buried pipe ground source heat pump system in a mine.

[0056] In order to study the heat transfer performance of mine heat exchange system under real conditions, a set of buried pipe micro heat exchange experimental device was established to carry out constant heat transfer experiments and obtain the temperature changes of fluid and surrounding rock to verify the reliability of the numerical model of mine geothermal heat exchange.

[0057] A miniature test bench for underground heat exchangers in mines was established based on refrigeration conditions. In the experiment, the water in the water tank was heated, and a larger external constant temperature water tank was used to keep the former warm in a water bath; a soil box filled with sand was used as the heat exchange environment, and a cast heat exchanger was buried inside; an air compressor 1 was used as the power system to drive the water to flow through the heat exchanger, and the inlet and outlet water temperatures and soil temperature changes were detected by temperature sensors.

[0058] The mine buried pipe heat exchanger micro-experimental platform is mainly composed of an insulation box 8, an air compressor 1, a constant pressure water tank, a water bath insulation box 3, a tail water recovery tank, an insulation pipeline, a constant pressure valve, a flow meter 6, a thermal resistor, a Pt1000 temperature sensor, a data collector and a computer 13, etc.

[0059] The insulation box and heat exchanger are the main parts of the simulation experiment, representing the heat exchange rock mass and the actual heat exchanger in the underground pipe system of the mine, respectively. The test bench box is made of 25mm thick wood boards, and a 1 / 2PVC pipe is used as a tunnel model on the top of the box. In order to prevent the indoor ambient temperature from affecting the internal temperature of the box, 2cm thick pearl foam cotton is laid inside and outside the tank, and the gap between the box and the insulation layer is sealed and bonded. After standing for 24 hours, the internal soil filling and the layout of the heat exchanger and temperature sensing components are completed. A PVC pipe with an inner diameter of 5cm is used as a mold to cast a cement mortar stone body containing a U-shaped tube. In the selection of heat exchange rock and soil, this paper uses sand instead of underground rock. In terms of heat exchanger pipes, since the test bench uses a small pipe diameter, it is difficult to match the corresponding pipe diameter size of the organic material pipes commonly used in engineering on the market. Copper pipes have a great advantage in size and have a good thermal conductivity, which can more clearly reflect the temperature response. Therefore, this test bench selects 5mm copper pipes to make U-shaped pipes. In terms of filling materials, cement mortar with a ratio of water: cement: sand = 0.5:1:2 is selected for pouring.

[0060] The circulation system mainly includes an air compressor 1, a constant pressure water tank 4, a water bath insulation box 3, a flow meter 6 and a circulation pipeline. Since the flow rate required in the experiment is relatively small, ordinary water pumps will produce pulse-like fluctuations that have a greater impact on the test flow rate. Here, an air compressor 1 is selected to cooperate with a pressure control valve 2 to maintain a constant pressure state in the water tank to achieve the purpose of pumping fluid. At the same time, a flow meter 6 with a control valve is connected in the pipeline to ensure stable flow during the experiment. In terms of insulation, in order to prevent the water tank and pipeline from being exposed to the air and causing rapid heat loss, a water bath insulation box 3 and an insulation material casing are set up to insulate both.

[0061] Due to material and condition limitations, it is difficult for the experimental device to achieve the insulation effect during winter heating conditions. Therefore, the experiment is carried out by injecting hot water (362.64K) into the experimental body at normal temperature (299.2K). The buried pipe micro heat exchange experimental device is divided into four parts: high-temperature water supply part, experimental body part, tail water recovery part and data acquisition part. The main principle of the constant heat transfer heat transfer experiment is: the constant temperature water tank is heated to a predetermined temperature by a water bath insulation box 3, and the hot water is pressed to the experimental body for heat exchange and then discharged to the tail water collection box. The inlet and outlet temperatures and the temperature inside the experimental body are collected by the data collector, where the rotor flowmeter 6 controls the water inlet flow rate, and the water injection rate is 0.5L / min. Data is collected once per second, and the experimental time is 120min.

[0062] The micro-experiment heat exchange process was numerically simulated using Comsol Multiphysics, and then compared with the experimental results to verify the applicability of the mathematical model. After the experiment, the original soil density in the experimental body was measured to be 1761kg / m3, and the other thermophysical parameters are shown in Table 1 below.

[0063] Table 1 Calculation model parameter settings

[0064]

[0065] The experimental results of the temperatures at the inlet and outlet and at each monitoring point of the experimental body are compared with the simulation results, such as Figure 4 shown.

[0066] In the initial stage, the simulated value of the outlet water temperature is higher than the experimental value and increases rapidly. This is because the outlet temperature is at room temperature before the experiment begins. In the initial stage, the outlet temperature rises rapidly, and the temperature sensor response time in the experiment is not as fast as the simulated probe response time. But overall, the outlet water temperatures of the two have the same change law, that is, a sharp increase in the early stage and then a slow increase. For the response temperature of the experimental subject, the two have the same change law, and the temperature response time of each monitoring point is not much different. Due to the heterogeneity of the soil, the maximum error of about 3K occurs at the A2 monitoring point. It can be seen that the use of the numerical model of mine geothermal heat exchange can accurately reflect the temperature change law of the rock mass, and the error is relatively small, which proves its applicability in the calculation of mine geothermal heat exchange system.

[0067] After verifying the accuracy of the prediction method proposed in this invention, the numerical model of buried pipe heat exchange in mines will be further used to explore the influence of different factors, including objective factors and human-controlled factors, on the heat transfer performance of geothermal heat exchange systems in mines.

[0068] From the analysis of the heat transfer form of the model, it can be seen that the factors affecting the heat transfer performance of a single tube in a mine geothermal heat exchange system are mainly determined by the material properties and mining methods of the three regions that constitute the underground heat exchange system: (1) surrounding rock control factors: the temperature of the original stratum, the thermal conductivity of the surrounding rock of the stratum, and the specific heat capacity of the stratum; (2) filling medium control factors: borehole diameter, filling medium thermal conductivity; (3) U-tube control factors: U-tube diameter, length, branch spacing, injection temperature, and injection rate, a total of 10 factors. From an engineering perspective, the above factors can be roughly divided into objective factors in the mine and human control factors. According to existing research and common mining geological data, the values ​​of each factor can be taken as shown in Table 2, and five levels are set for each factor to analyze the influence of a single factor on the heat transfer performance of a single tube, where the branch spacing is the ratio of the U-tube distance to the difference between the borehole diameter and the U-tube diameter.

[0069]

[0070] The underground heat exchange system consists of Figure 3 The geometric models are composed of the same form but different sizes, which are divided into U-shaped tubes, borehole filling bodies and formation rock mass. The tunnel structure is taken into account in the rock mass. The equivalent radius of the tunnel is a semicircle of 2m. The rock mass is a cuboid of 10m×10m×(h+10)m. The borehole and the U-shaped tube are set on the central axis of the rock mass along the h direction. The size selection is shown in Table 2. The simulation calculation is carried out for 90 consecutive days. The calculation results are taken once a day. The average temperature difference between the inlet and outlet is used as the evaluation index to explore the influence of different influencing factors on the heat transfer performance.

[0071] The boundary temperature around the surrounding rock is set to be the same as the original ground temperature. The values ​​of the original ground temperature, formation heat capacity, thermal conductivity, thermal conductivity of borehole filling materials, water injection temperature and water injection rate are shown in Table 2. In addition, the remaining thermophysical parameters of the model are selected according to common mining geological parameters. The density of the formation surrounding rock is selected as 2600kg / m3, and the borehole filling medium is set according to ordinary cement mortar, with density and specific heat capacity set to 1800kg / m3 and 1050J / (kg·K), respectively.

[0072] Shallow buried pipes generally use heat transfer, inlet and outlet temperature difference and heat transfer power as indicators to evaluate heat transfer performance. Since the mine geothermal heat exchange system and shallow buried pipes have similar heat transfer mechanisms, the average inlet and outlet temperature difference within 90 days is used to evaluate the heat transfer performance of the heat exchange system.

[0073] Objective factors affecting heat exchange efficiency include:

[0074] Figure 5 This is the relationship between the original ground temperature and the average temperature difference between the inlet and outlet. Figure 5 It can be seen that the original ground temperature increased from 293.15K to 313.15K, and the average temperature difference between the inlet and outlet of level 3 increased from 1.638K to 4.619K. Moreover, for every 5K increase in the original ground temperature, the corresponding increase in the average temperature difference between the inlet and outlet was basically stable at 0.745K, and the growth rate was less than 0.67%. It can be considered that the influence of the original ground temperature on the heat exchange efficiency of the buried pipe in the mine is a linear function relationship, and its influence is related to the heat exchange intensity formed by other factors in the system.

[0075] Figure 6 It is the relationship between the thermal conductivity of the formation and the average temperature difference between the inlet and outlet. Figure 6It can be seen that with the increase of formation thermal conductivity, the heat transfer performance continues to improve, and the corresponding average temperature difference growth rates at the inlet and outlet are 12.35%, 8.21%, 5.86% and 4.47% respectively, and the growth rate gradually decreases. This is because in the heat exchange process, it takes a certain amount of time for the heat in the formation to diffuse to the U-tube, and the heat transfer of the rock with a large thermal conductivity is faster and the heat transfer performance is high; as the heat exchange process continues, the heat in the formation is lost, and the heat supply of the remote formation cannot keep up, resulting in a gradual decrease in the overall heat transfer performance. This phenomenon continues to increase with time and gradually dominates, resulting in a decrease in the improvement of the heat transfer performance caused by the thermal conductivity of the rock mass.

[0076] As the constant pressure heat capacity of the formation continues to increase, the heat exchange performance is also continuously improving. The growth rates of the average temperature difference between the inlet and outlet are 2.24%, 1.755%, 1.450% and 1.189% respectively, and the growth rate has slightly decreased. Constant pressure heat capacity is a physical quantity that characterizes the heat storage capacity. The higher its value, the more heat is stored in the rock mass of equal volume. However, in the heat extraction process, the same water output affects a smaller formation range, and the heat exchange performance is high. As the heat exchange time increases, the heat accumulation in the local range reaches saturation, the heat influence range gradually spreads to the far end, the influence of constant pressure heat capacity begins to weaken, and the overall heat exchange performance continues to decrease.

[0077] The influence of human control factors on heat transfer efficiency includes:

[0078] Figure 7 is the relationship between the borehole diameter and the average temperature difference between the inlet and outlet. Figure 8 It is the relationship between the branch spacing and the average temperature difference between the inlet and outlet. Fig. 9 It is the relationship between the borehole length and the average temperature difference between the inlet and outlet; Fig.10 is the relationship between the injection temperature and the average inlet and outlet temperatures. Figure 7 , Figure 8 , Fig. 9 , Fig.10 It can be seen that the analysis of human control factors can obtain the following results: the influence of the two factors of borehole diameter and water injection temperature on the average temperature difference between the inlet and outlet is roughly negatively correlated, and the borehole diameter shows nonlinear characteristics, and the water injection temperature shows linear characteristics; the average temperature difference between the inlet and outlet is roughly positively correlated with the other factors, and the influence of each factor on the increase in the average temperature difference between the inlet and outlet gradually decreases, indicating that these factors can only become the main factors affecting the heat exchange performance of the mine geothermal heat exchange system within a certain range.

[0079] According to the above analysis, the influence of various factors on the average temperature difference between inlet and outlet is shown in Table 2.

[0080] Table 2 Relationship between various factors and heat transfer performance

[0081]

[0082] In the table, T[K] is the original ground temperature; λ[W / (m·K)] is the thermal conductivity; C[J / (kg·K)] is the formation heat; D[m] is the borehole diameter; λs[W / (m·K)] is the thermal conductivity of the filling material; l / (D-2d) is the branch pipe spacing; d[mm] is the pipe diameter; h[m] is the borehole length; Tin[K] is the water injection temperature; v[m / s] is the water injection rate.

[0083] Furthermore, orthogonal simulation tests were carried out for the above-mentioned factors. The extreme values ​​of the orthogonal test results were used as the evaluation criteria to compare the effects of various factors on the heat transfer performance and obtain the sensitivity of the factors affecting the heat transfer efficiency of the underground heat exchange system of the mine geothermal energy.

[0084] Optionally, orthogonal simulation tests are carried out for the above-mentioned factors, and the extreme values ​​of the orthogonal test results are used as the evaluation criteria to compare the effects of various factors on the heat transfer performance, obtain the main control factors of a single tube in the mine geothermal underground heat exchange system, and realize the prediction of heat transfer performance.

[0085] The L50(511) orthogonal test table was selected for simulation test. According to Table 2, 5 levels were set for each factor, and a set of blank variables was added, totaling 50 sets of simulation tests. The remaining boundary conditions and thermophysical parameters were consistent with the single factor analysis.

[0086] Through calculation, the extreme difference analysis results of the inlet and outlet temperature differences of each factor are shown in the following table.

[0087]

[0088]

[0089] In the table, T[K] is the original ground temperature; λ[W / (m·K)] is the thermal conductivity; C[J / (kg·K)] is the formation heat; D[m] is the borehole diameter; s [W / (m·K)] is the thermal conductivity of the filling material; l / (D-2d) is the branch pipe spacing; d[mm] is the pipe diameter; h[m] is the borehole length; Tin[K] is the water injection temperature; and v[m / s] is the water injection rate.

[0090] Through the results of extreme difference analysis, it is found that the sensitivity of the factors affecting the average temperature difference between the inlet and outlet is, from large to small, water injection rate, original ground temperature, pipe diameter, borehole length, water injection temperature, formation thermal conductivity, filling material thermal conductivity, formation heat capacity, borehole diameter and branch pipe spacing, especially the water injection rate, original ground temperature, pipe diameter, borehole length, water injection temperature and formation thermal conductivity have a greater impact on the heat exchange performance.

[0091] The above is only a preferred implementation of this patent. It should be pointed out that for ordinary technicians in this technical field, several improvements and substitutions can be made without departing from the technical principles of this patent. These improvements and substitutions should also be regarded as the scope of protection of this patent.

Claims

1. A method for predicting heat exchange performance in a ground-source heat pump heat exchange system with buried pipes in abandoned mines, characterized in that: The following steps are involved: Step 1, establish a mathematical model of heat exchange efficiency in the ground-source heat pump heat exchange system of the buried pipe in the mine, the parameters of which include the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium involved in the heat conduction outside the U-tube; the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor, and tube diameter of the heat medium involved in the heat convection inside the U-tube; Step 2: Use the mathematical model in step 1 to establish a control equation based on heat conduction outside the U-tube and heat convection inside the U-tube, and determine the initial conditions and boundary conditions of the control equation; Step 3: The mathematical model in step 1 is expressed by coupling the non-isothermal pipe flow module and the solid heat transfer module of COMSOL Multiphysics to describe the heat exchange process of the underground heat exchange system, and a numerical model of mine geothermal heat exchange is established to calculate the heat exchange performance of the mine ground source heat pump heat exchange system; Step 4: Carry out orthogonal simulation tests for each factor calculated by the numerical model, use the extreme value of the orthogonal test results as the evaluation criterion, compare the influence of each factor on the heat transfer performance, obtain the main control factors of the underground heat exchange system of the mine geothermal energy, and realize the prediction of heat transfer performance.

2. The method for predicting heat exchange performance in the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1, characterized in that: The heat conduction equation outside the U-tube in step 2 is: Heat convection equation in a U-tube: In the formula, ρ m 、c m , λ, h, V, and A1 are the density, specific heat capacity, thermal conductivity, convection heat transfer coefficient, solid volume, and solid contact area of ​​the solid medium respectively; ρ f 、c f , k f ,u,A,e t 、f D ,d h They are the density, specific heat capacity, thermal conductivity, flow rate, cross-sectional area of ​​the tube, surface roughness of the tube, Darcy friction factor and tube diameter of the heat medium; is the Laplace operator; T is the temperature field; P is the water head pressure; q pw is the heat transfer rate per unit volume of the internal heat source; t is the heat exchange time; The specific boundary conditions and initial conditions are as follows: Where, T out , T in [℃] are the outlet water temperature and injection water temperature respectively; ρ[kg / m 3 ] is the density of water; r[m] is the diameter of the U-tube; h[m] is the length of the borehole; q0”[W / m 2 ] is the heat flux density, that is, the heat transfer rate of the medium per unit area and in the direction of the temperature gradient; k [W / (m·K)] is the thermal conductivity of the medium; Q f [W]Heat transfer power.

3. The method for predicting heat exchange performance in the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1, characterized in that: The following reasonable assumptions and simplifications are included: assuming that the internal physical properties of the rock mass and the bored cast body are uniform, ignoring the cracks and hydraulic movement inside the rock mass, ignoring the influence of geothermal gradient on the temperature of the geometric model and the influence of temperature change on the thermophysical properties of the solid part of the model; ignoring the existence of contact thermal resistance in the heat exchange process, assuming that the two media are in close contact, and the temperatures of the two media on the contact surface are the same; simplifying the pipeline into a one-dimensional line, using the fluid in the pipeline as the heat source, and treating the fluid heat transfer as one-dimensional turbulent heat transfer. The specific flow process of the fluid in the pipe is ignored, and it is assumed that the temperature and flow rate at each position on each section of the pipeline are the same.

4. The method for predicting heat exchange performance in the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1, characterized in that: In step 4, the main controlling factors of the underground heat exchange system of the mine geothermal include water injection rate, original ground temperature, pipe diameter, borehole length, water injection temperature, and formation thermal conductivity.

5. A test device using the method for predicting heat exchange performance in the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1, characterized in that: It includes a water bath insulation box and an insulation box body. A constant-pressure water supply tank is arranged in the water bath insulation box. The inlet of the constant-pressure water supply tank is connected to an air inlet pipe, the air inlet pipe is connected to an air compressor, the air inlet pipe is provided with a pressure control valve, the outlet of the constant-pressure water supply tank is connected to an outlet pipe, the outlet pipe is provided with a pipeline valve, an electric thermal resistor and a flow meter with a regulating valve; the insulation box body is filled with sand and soil for simulating the heat exchange rock mass in the underground pipe system of the mine, and a U-shaped heat exchange tube with an upward opening for simulating an actual heat exchanger is vertically embedded in the insulation box body, the end of the outlet pipe is connected to an opening of the heat exchange tube, the other end of the heat exchange tube is connected to a drain pipe, and the drain pipe is connected to a tail water recovery tank; a temperature probe for detecting the temperature of the sand and soil is arranged in the insulation box body, and the signal of the temperature probe is connected to a data collection system and a computer.

6. The test device according to claim 5, which is applied to the method for predicting heat exchange performance in the abandoned mine buried pipe ground source heat pump heat exchange system according to claim 1, is characterized in that: The outer side and inner side of the side wall of the heat preservation box are both covered with pearl foam cotton, and the space between the pearl foam cotton and the side wall is filled with sealant; the sand in the heat preservation box is composed of water, cement and sand, and the weight ratio is water: Cement: sand = 0.5:1:2; the heat exchange tube in the insulation box is made of copper with a thickness of 5mm and an inner diameter of 5cm.

Images