Shallow multi-U-shaped buried pipe heat exchanger and method for determining the number of U-shaped buried pipes

Abstract

The present invention belongs to the technical field of geothermal energy utilization equipment, and specifically relates to a shallow multi-U-type buried pipe heat exchanger and a method for determining the number of U-type buried pipes. The shallow multi-U-type buried pipe heat exchanger comprises a plurality of U-type buried pipes, each of which is used to fill a circulating fluid, and the plurality of U-type buried pipes are distributed in an inner circle and an outer circle, and the U-type buried pipes in the outer circle are distributed in a circular array, and the U-type buried pipes in the inner circle are also distributed symmetrically, and a borehole backfill material layer wraps the plurality of U-type buried pipes inside, and the surrounding rock and soil layer wraps the outside of the borehole backfill material layer. The heat exchanger of the present invention has maximum heat exchange efficiency and system stability.

Description

Technical Field

[0001] The invention belongs to the technical field of geothermal energy utilization equipment, and specifically relates to a shallow multi-U-shaped underground pipe heat exchanger and a method for determining the number of U-shaped underground pipes. Background Art

[0002] As an important component, building energy consumption has an increasing carbon emission in the whole process year by year. It is necessary to accelerate energy transformation, utilize renewable energy and realize low-carbon economic development. Figure 1 The U-shaped buried pipe ground source heat pump system in the prior art is composed of a heat pump unit and a U-shaped buried pipe heat exchanger. The underground rock and soil are used as the heat source or heat sink, and the circulating fluid in the U-shaped buried pipe is used to exchange heat with the surrounding rock and soil. The U-shaped buried pipe is usually a high-density polyethylene pipe, and the circulating fluid is usually water or antifreeze. At present, the research on U-shaped buried pipe ground source heat pumps is mostly focused on U-shaped buried pipe heat exchangers within a depth of 200m underground, which belongs to the shallow U-shaped buried pipe heat exchange technology. The single-hole U-shaped buried pipe heat exchanger has more research on vertical single U-shaped and double U-shaped, and the drilling utilization rate is low, which will reduce the heat exchange capacity of the U-shaped buried pipe. Therefore, it is necessary to develop a shallow multi-U-shaped buried pipe heat exchanger to meet the effects of high heat exchange efficiency and good stability. Summary of the invention

[0003] In order to solve the above technical problems, the present invention provides a shallow multi-U buried pipe heat exchanger to achieve the effects of high heat exchange efficiency and good stability.

[0004] The object of the present invention is to provide a shallow multi-U underground pipe heat exchanger, comprising a plurality of U-shaped underground pipes, wherein the U-shaped underground pipes comprise two vertical portions and an elbow connected to the two vertical portions, each of the U-shaped underground pipes is used to fill a circulating fluid, and the plurality of U-shaped underground pipes are distributed in an inner circle and an outer circle, wherein the U-shaped underground pipes in the outer circle are distributed in a ring array, and the U-shaped underground pipes in the inner circle are distributed symmetrically;

[0005] The borehole backfill material layer wraps the plurality of U-shaped buried pipes inside, and the surrounding rock and soil layer wraps the outside of the borehole backfill material layer.

[0006] Preferably, the outer circle has 6 U-shaped buried pipes, and two opposite U-shaped buried pipes are arranged in parallel, the inner circle has 2 U-shaped buried pipes, and the two U-shaped buried pipes in the inner circle are arranged in parallel with any two opposite U-shaped buried pipes in the outer circle, the four parallel U-shaped buried pipes are distributed at equal intervals, the center of the inlet of the U-shaped buried pipe is located on the same straight line, and the center of the outlet is also located on the same straight line.

[0007] The outer walls of the borehole backfill material layer and the surrounding rock and soil layer are both cylindrical, and the outer circle composed of 6 U-shaped buried pipes is coaxially arranged with the borehole backfill material layer and the surrounding rock and soil layer. The outer diameter of the U-shaped buried pipe is 32mm, the wall thickness of the U-shaped buried pipe is 3mm, the distance between the inlet center and the outlet center of the U-shaped buried pipe is 64mm, the diameter of the borehole backfill material layer is 360mm, the diameter of the surrounding rock and soil layer is 20m, the distance between the outlet centers of the inner circle of U-shaped buried pipes is 64mm, the U-shaped buried pipes all include vertical parts and elbows, the height of the vertical part is 120m, the bottoms of all U-shaped buried pipes are located in the same horizontal plane, the heights of the borehole backfill material layer and the surrounding rock and soil layer are both 123m, and the angle formed by the line connecting the midpoints of the symmetry axes of the two U-shaped buried pipes in the outer circle and the center of the circle with the center of the circle as the center of the cross section of the surrounding rock and soil layer and the center of the circle is 60°.

[0008] The present invention provides a method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground pipe heat exchanger, and the method for determining the number is as follows:

[0009] According to the requirements of borehole heat exchange, a heat transfer model of shallow multi-U-shaped buried pipe heat exchanger is established. Then, based on the heat transfer model, a three-dimensional full-scale numerical model of heat exchange inside and outside the shallow multi-U-shaped buried pipe coupling is established.

[0010] According to the drilling numerical simulation conditions and the three-dimensional full-size numerical model, the heat transfer efficiency under different numbers of U-shaped buried pipes is tested, and the number of U-shaped buried pipes with the maximum heat transfer efficiency is selected.

[0011] The borehole is a borehole required for installing a shallow multi-U-shaped buried pipe heat exchanger.

[0012] The heat transfer model includes a model of heat transfer inside the borehole and a model of heat transfer outside the borehole; the heat transfer inside the borehole includes convective heat transfer of the circulating fluid, heat conduction of the U-shaped buried pipe wall and heat conduction of the backfill material in the borehole.

[0013] The heat transfer in the borehole is calculated based on the flow time, thermal conductivity, time-averaged temperature and specific heat capacity at constant pressure of the circulating fluid.

[0014] Based on the heat transfer model and CFD software, a three-dimensional full-scale numerical model of heat exchange inside and outside the shallow multi-U-shaped buried pipe coupling pipe is established. Among them, CFD software is called computational fluid dynamics software in Chinese.

[0015] When establishing a three-dimensional full-scale numerical model, the boundary conditions for the operation of the CFD software are set as follows: transient calculation boundary conditions, the circulating fluid inlet is the velocity inlet, and the constant temperature and flow rate are set; the circulating fluid outlet is the free outflow; the upper surface of the three-dimensional full-scale numerical model is adiabatic; the lower surface of the three-dimensional full-scale numerical model is constant temperature, and the temperature is equal to the initial ground temperature of the surrounding rock and soil layer; the outer surface of the three-dimensional full-scale numerical model is set with a symmetry plane; the U-shaped buried pipe wall, borehole wall and rock and soil interface are set to coupled heat transfer.

[0016] The three-dimensional full-scale numerical model is established based on the following parameters: the outer diameter of a single U-shaped underground pipe, the wall thickness of the U-shaped underground pipe, the distance between the inlet and outlet centers of a single U-shaped underground pipe, the diameter of the borehole backfill material layer, the diameter of the surrounding rock and soil layer, the distance between the outlet centers of the inner circle U-shaped underground pipe, the height of the vertical part of the U-shaped underground pipe, the height of the borehole backfill material layer, the height of the surrounding rock and soil layer; and the angle formed by the midpoints of the symmetry axes of the two U-shaped underground pipes in the outer circle and the line connecting the center of the circle when the center of the cross section of the surrounding rock and soil layer is taken as the center of the circle. Among them, the outer diameter of a single U-shaped underground pipe includes the inlet diameter of a single U-shaped underground pipe and the outlet diameter of a single U-shaped underground pipe.

[0017] Preferably, numerical simulation working conditions are designed, the number of U-shaped buried pipes for several shallow multi-U-shaped buried pipe heat exchangers is proposed, a three-dimensional full-size numerical model is established using CFD software, and the heat exchange performance of the U-shaped buried pipes is monitored. The number of U-shaped buried pipes when the heat exchange performance is optimal is the number of U-shaped buried pipes required for the shallow multi-U-shaped buried pipe heat exchanger.

[0018] The heat exchange efficiency is calculated based on the temperature difference between the circulating fluid inlet and the circulating fluid outlet of each U-shaped buried heat exchange pipe, the total heat exchange amount and the rock and soil temperature change of the surrounding rock and soil layer.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention provides a shallow multi-U-shaped underground heat exchanger, and the number of U-shaped underground pipes of the heat exchanger is determined by the following steps: first, a heat transfer model is established according to the drilling heat exchange requirements. The heat transfer model is the key to calculating the heat exchange efficiency and is also a key model for determining the number of U-shaped underground pipes; then, different numbers of U-shaped underground pipes are determined, and a three-dimensional full-scale numerical model of the required shallow U-shaped underground coupling pipe internal and external heat exchange is established using CFD software; based on the three-dimensional full-scale numerical model and CFD software, the boundary conditions for the operation of the CFD software are set to monitor the heat exchange outlet water temperature, total heat exchange and geotechnical temperature changes of different numbers of U-shaped underground pipes; the temperature difference of the circulating fluid inlet and outlet, the total heat exchange of a single hole and the geotechnical temperature rise of the surrounding rock and soil layers under various working conditions are compared to calculate the heat exchange efficiency, and the number of U-shaped underground pipes of the shallow multi-U-shaped underground heat exchanger with the highest heat exchange efficiency is obtained, and then the structural size of the shallow multi-U-shaped underground heat exchanger described in the present invention is obtained. It should be noted that "temperature rise" means temperature rise.

[0021] The shallow multi-U-shaped buried pipe heat exchanger of the present invention has a determined structural dimension, good heat exchange efficiency and stability, and a small relative error, and is significantly improved over the heat exchanger structure of the prior art.

[0022] The three-dimensional full-size numerical model of the U-shaped buried pipe established in this application based on CFD software has verified its reliability and enhanced the accuracy of the U-shaped buried pipe heat exchange data. Compared with the existing single-hole U-shaped buried pipe heat exchanger, it can greatly improve the drilling utilization rate, improve the heat exchange performance of the U-shaped buried pipe, and has good stability.

[0023] The shallow multi-U-shaped underground heat exchanger of the present invention improves the heat exchange capacity of a single borehole. The present invention determines that the total heat extraction of the 8U-type underground heat exchanger is 60.70GJ and the total heat storage is 81.65GJ. Compared with a single U-shaped underground pipe, the total heat extraction is increased by 24.45GJ, an increase of 67.45%, and the total heat storage is increased by 34.43GJ, an increase of 72.91%; compared with two U-shaped underground pipes, the total heat extraction is increased by 17.10GJ, an increase of 39.22%, and the total heat storage is increased by 24.29GJ, an increase of 42.35%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of a U-shaped buried pipe ground source heat pump system in the prior art, wherein the dotted line + arrow represents the flow direction of heating water supply, the heating return water is represented by a solid line + arrow, and the heat pump unit and the U-shaped buried pipe are circled in a rectangular frame.

[0025] Figure 2 Schematic diagram of the heat exchange process of the U-shaped buried pipe.

[0026] Figure 3This is a schematic diagram of the three-dimensional structure of a shallow multi-U-shaped buried pipe heat exchanger.

[0027] Figure 4 for Figure 3 Top view of the .

[0028] Figure 5 for Figure 4 Enlarged view of part A.

[0029] Figure 6 for Figure 3 Longitudinal section of .

[0030] Figure 7 The inlet and outlet temperature differences of the circulating fluids in 5 types of U-shaped underground pipes.

[0031] Figure 8 The total heat exchange rate per hole of 5 types of U-shaped buried pipes.

[0032] Figure 3 to Figure 6 Explanation of the reference numerals: 1, circulating fluid; 2, U-shaped buried pipe; 3, drilling backfill material layer; 4, surrounding rock and soil layer. DETAILED DESCRIPTION

[0033] In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention is further described below in conjunction with specific embodiments and drawings.

[0034] In the description of the present invention, unless otherwise specified, all reagents used are commercially available and all methods used are conventional techniques in the art.

[0035] In the description of the present invention, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0036] The terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features; in the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0037] The purpose of the present invention is to solve the problems of low heat exchange efficiency and poor stability of shallow U-shaped buried pipes. Based on the principle of enhanced heat exchange, the present invention provides a shallow vertical multi-U-shaped buried pipe heat exchanger. The use of this type of heat exchanger can improve the utilization rate of the borehole and achieve the maximum increase in the heat exchange capacity of a single hole of the U-shaped buried pipe.

[0038] To achieve the above object, the present invention provides the following embodiments.

[0039] Example 1

[0040] A shallow multi-U underground heat exchanger, see Figure 3 to Figure 6 , including a plurality of U-shaped buried pipes 2, a borehole backfill material layer 3 and a surrounding rock layer 4, wherein a circulating fluid 1 is filled in each U-shaped buried pipe 2, and the plurality of U-shaped buried pipes 2 are distributed in an inner circle and an outer circle, and the inner circle is located at the center of the outer circle, see Figure 4 The outer circle has 6 U-shaped buried pipes 2, which are distributed in a circular array, and the two opposite U-shaped buried pipes 2 are arranged in parallel; the inner circle has 2 U-shaped buried pipes 2, and the two U-shaped buried pipes 2 in the inner circle are arranged in parallel with any two opposite U-shaped buried pipes 2 in the outer circle. The four parallel U-shaped buried pipes 2 are distributed at equal intervals, and the inlet centers of the four U-shaped buried pipes 2 are located on the same straight line, and the outlet centers of the four U-shaped buried pipes 2 are also located on the same straight line. The bored backfill material layer 3 wraps the multiple U-shaped buried pipes 2 inside, and the bored backfill material layer 3 has a cylindrical outer wall. The gaps between the outer walls of the U-shaped buried pipes 2 are also filled with the bored backfill material. The surrounding rock and soil layer 4 is wrapped outside the bored backfill material layer 3, and the surrounding rock and soil layer 4 has a cylindrical outer wall. The outer circle composed of 6 U-shaped buried pipes is coaxially arranged with the bored backfill material layer 3 and the surrounding rock and soil layer 4. See. Figure 5 , the height of the borehole backfill material layer 3 and the surrounding rock and soil layer 4 are the same. Figure 2 .

[0041] More specifically, the U-shaped buried pipe 2 is a PE pipe. The backfill material for the borehole is a backfill material of the prior art, such as the backfill materials described in CN100398482C, CN100453497C, and CN100395209C. The backfill material for the borehole can also be a material newly developed by a person skilled in the art. The surrounding rock and soil layer 4 is the original soil near the borehole, such as the original rock and soil.

[0042] For more detailed information on the structure and dimensions of shallow multi-U-shaped underground heat exchangers, see Figures 4 to 6 , Figures 4 to 6 The letters and three-dimensional full-size values ​​are shown in Table 1.

[0043] Table 1 Information on the three-dimensional full-size values

[0044]

[0045] It should be noted that φ in Table 1 in ,φ out The values ​​are equal, and their corresponding values ​​are also the outer diameter of the U-shaped pipe, and the wall thickness of the U-shaped buried pipe is also 3mm. Table 1 is also the specific dimensions of the three-dimensional full-scale numerical model.

[0046] Example 2

[0047] A shallow multi-U-shaped underground heat exchanger design method can calculate the optimal number of U-shaped underground pipes so that the multi-U-shaped underground heat exchanger can utilize geothermal energy with maximum efficiency and has high heat exchange efficiency and high stability. The design method includes the following steps:

[0048] S1, according to the drilling heat exchange requirements, a three-dimensional full-scale numerical model of shallow multi-U-shaped buried pipe coupling heat exchange inside and outside the pipe is established, as follows:

[0049] A heat transfer model of shallow multi-U-shaped buried pipe heat exchanger is established. The heat transfer model is divided into a model of heat transfer inside the borehole and a model of heat transfer outside the borehole. The heat transfer inside the borehole includes the convection heat transfer of the circulating fluid, the heat conduction of the U-shaped buried pipe wall, and the heat conduction of the backfill material in the borehole. Among them, the convection heat transfer continuity equation of the circulating fluid is expressed by the following formula:

[0050]

[0051] ①In is the differential operator, is the time-averaged velocity of the circulating fluid, in m / s.

[0052] The momentum equation for convective heat transfer of the circulating fluid is expressed as follows:

[0053]

[0054] ②, τ is the circulating fluid flow time, unit is s; ρ f is the circulating fluid density, in kg / m 3 ; is the average pressure of the circulating fluid, in Pa; ν is the molecular kinematic viscosity of the circulating fluid, in m 2 / s; ν t is the eddy viscosity coefficient of the circulating fluid, m 2 / s; is the time-averaged velocity of the circulating fluid, in m / s.

[0055] The energy equation for convective heat transfer of the circulating fluid is expressed as follows:

[0056]

[0057] ③In the f is the circulating fluid density, in kg / m 3 ; T f is the average temperature of the circulating fluid, in °C; (c p ) f is the specific heat capacity of the circulating fluid at constant pressure, in kJ / (kg·℃); τ is the flow time of the circulating fluid, in s; λ f is the thermal conductivity of the circulating fluid, in W / (m·℃), is the time-averaged velocity of the circulating fluid, in m / s.

[0058] The heat conduction equation of the U-shaped buried pipe wall can be expressed as follows:

[0059]

[0060] Among them, ρ p is the density of U-shaped buried pipe, unit: kg / m 3 ; τ is the circulating fluid flow time, unit s; λ p is the thermal conductivity of the U-shaped buried pipe, in W / (m·℃); T p is the average temperature of the U-shaped buried pipe, in °C; (c p ) p is the specific heat capacity of the U-shaped buried pipe at constant pressure, in kJ / (kg·℃);

[0061] The heat conduction equation of backfill in the borehole can be expressed as follows:

[0062]

[0063] Among them, ρ b The density of the borehole backfill material, in kg / m 3 ; τ is the circulating fluid flow time, unit s; λ b is the thermal conductivity of the drilling backfill material, unit: W / (m·℃); T b is the average temperature of the backfill material during drilling, unit: s; (c p ) b is the specific heat capacity of the borehole backfill material at constant pressure, in kJ / (kg·℃);

[0064] The heat transfer equation outside the borehole can be expressed as follows:

[0065]

[0066] Among them, ρ s is the soil density of the rock layer around the U-shaped buried pipe, in kg / m 3 ; τ is the circulating fluid flow time, unit s; λ s is the thermal conductivity of the rock and soil layer around the U-shaped buried pipe, in W / (m·℃); Ts is the average temperature of the rock and soil layer around the U-shaped buried pipe, unit s; (c p ) s is the specific heat capacity of the rock and soil layer around the U-shaped buried pipe at constant pressure, in kJ / (kg·℃);

[0067] The number of U-shaped buried pipes for several shallow multi-U-shaped buried pipe heat exchangers is proposed, and a three-dimensional full-scale numerical model of heat transfer inside and outside the shallow multi-U-shaped buried pipe coupling is established based on formulas ①~⑥ and CFD software;

[0068] After the three-dimensional full-scale numerical model is constructed, meshing, model solution setting, and boundary condition setting are performed;

[0069] Among them, the grid division adopts the wall function coarse grid, the borehole backfill material layer adopts the tetrahedron unstructured grid, the circulating fluid, U-shaped buried pipe and surrounding rock and soil layer adopt the hexahedron structured grid, and the grid at the elbow of the U-shaped buried pipe is encrypted;

[0070] The three-dimensional full-scale numerical model solution setting adopts a double-precision solver, the standard k-ε model is selected as the turbulence model, the flow field is solved by the semi-empirical method of standard wall function, the second-order upwind discretization format and the SIMPLE pressure correction method are used.

[0071] The boundary conditions for CFD software operation are set as follows: transient calculation boundary conditions, the circulating fluid inlet is velocity-inlet, constant temperature and flow rate are set, the circulating fluid outlet is free outflow, the upper surface of the three-dimensional full-size numerical model is adiabatic, Heat flux = 0, the lower surface of the three-dimensional full-size numerical model is constant temperature, the temperature is equal to the initial ground temperature of the rock and soil, the outer surface of the three-dimensional full-size numerical model is set as symmetry, and the U-shaped buried pipe wall, borehole wall and rock and soil interface are set as coupled heat transfer.

[0072] S2, design the drilling numerical simulation working conditions, change the number of U-shaped buried pipes and monitor the circulating fluid inlet and outlet temperatures and the average rock and soil temperature under different numbers of U-shaped buried pipes, calculate the temperature difference between the circulating fluid inlet and outlet, the total heat transfer and the rock and soil temperature change of the surrounding rock and soil layers, calculate the heat transfer efficiency based on the temperature difference between the circulating fluid inlet and outlet, the total heat transfer and the rock and soil temperature change of the surrounding rock and soil layers, and evaluate the heat transfer performance of the U-shaped buried pipes. Among them, the circulating fluid inlet refers to the inlet of the underground part of the entire heat exchanger, such as Figure 1 The location B where the heating water supply pipeline intersects with the ground is also the total inlet of all U-shaped buried pipes; the circulating fluid outlet refers to the total outlet of all U-shaped buried pipes, such as Figure 1The heating return pipe intersects the ground at position C shown. Note that the circulating fluid inlet and outlet here are different from the inlet and outlet of a single U-shaped underground pipe, and are distinguished by markings in Table 1.

[0073] The method for evaluating the heat exchange performance of U-shaped buried pipes is as follows:

[0074] During the numerical simulation, the fluid inlet and circulating fluid outlet temperatures of the U-shaped buried pipe and the average rock and soil temperature were monitored, and the inlet and outlet temperature difference of the circulating fluid was calculated using the following formula:

[0075] ΔT=T in -T out ⑦

[0076] Where ΔT is the temperature difference between the inlet and outlet of the circulating fluid, in °C; T in T is the circulating fluid inlet water temperature, unit: °C; out is the outlet water temperature of the circulating fluid, in °C.

[0077] The total heat transfer is calculated using the following formula:

[0078] Q=n(c p ) f ρ f uA(T in -T out ⑧

[0079] Where Q is the total heat exchange capacity, unit is W; n is the number of U-shaped buried pipes, unit is "piece"; (c p ) f is the specific heat capacity of the circulating fluid at constant pressure, in kJ / (kg·℃); ρ f is the circulating fluid density, in kg / m 3 ; u is the circulating fluid inlet velocity, unit is m / s; A is the circulating fluid inlet cross-sectional area, m 2 ; T in is the circulating fluid inlet water temperature, temperature ℃; T out is the outlet water temperature of the circulating fluid, in °C.

[0080] The following formula is used to calculate the temperature change of the surrounding rock and soil layers:

[0081]

[0082] Among them, the subscript x is the axial distance, that is, the heat affected radius, in meters; the subscript z is the depth of the U-shaped buried pipe, in meters; the subscript t is a certain time, in hours / days; Δt x,z,t Indicates the temperature change of the rock and soil layer around the U-shaped buried pipe, unit: °C; t RA,z,0It represents the radial boundary temperature at time t = 0, that is, the initial temperature of the surrounding rock and soil layer, in °C; t x*,z,t It represents the temperature at depth x*radius at time t, in degrees Celsius.

[0083] S3, selecting the number of U-shaped underground pipes when the heat exchange efficiency is maximum, and determining a shallow multi-U-shaped underground pipe heat exchanger.

[0084] Example 3

[0085] A stability verification of a shallow multi-U-shaped buried heat exchanger, including:

[0086] S11. According to the requirements of borehole heat exchange, a three-dimensional full-scale numerical model of shallow multi-U-shaped buried pipe coupling heat exchange inside and outside the pipe is established, and the operating parameters and boundary conditions are set;

[0087] Among them, the number of multi-U-shaped buried pipes to be analyzed is 5, 6, 7, 8, and 9, respectively. For the convenience of comparison and distinction, these numbers are recorded as 5U, 6U, 7U, 8U, and 9U respectively; the size of the PE material U-shaped buried pipe is 32mm in outer diameter, 3mm in wall thickness, 360mm in diameter of the borehole backfill material layer, the number of 2U inner circle U-shaped buried pipes remains unchanged, and the number of outer circle U-shaped buried pipes is 3, 4, 5, 6, and 7 respectively, and these outer circles are evenly distributed in the outer circle with the center of the borehole as the center and a radius of 128mm. The surrounding rock and soil layers and the borehole backfill material layers are distributed with the center of the borehole as the central axis. The diameter of the surrounding rock and soil layers is 20m, the borehole depth is 120m, and the rock and soil depth of the surrounding rock and soil layers is 123m. CFD software is used to establish a three-dimensional full-scale numerical model of heat exchange inside and outside the shallow 5U, 6U, 7U, 8U, and 9U buried pipe coupling pipes. The construction of the three-dimensional full-size numerical model adopts the method of Example 2.

[0088] After the three-dimensional full-scale numerical model is constructed, meshing, model solution setting, and boundary condition setting are performed;

[0089] Among them, the grid division adopts the wall function coarse grid, the borehole backfill material layer adopts the tetrahedron unstructured grid, the circulating fluid, U-shaped buried pipe and surrounding rock and soil layer adopt the hexahedron structured grid, and the grid at the elbow of the U-shaped buried pipe is encrypted;

[0090] The three-dimensional full-scale numerical model solution setting adopts a double-precision solver, the standard k-ε model is selected as the turbulence model, the flow field is solved by the semi-empirical method of standard wall function, the second-order upwind discretization format and the SIMPLE pressure correction method are used.

[0091] The boundary conditions are set as follows: transient calculation boundary conditions, the circulating fluid inlet is velocity-inlet, and the constant temperature and flow rate are set. The circulating fluid outlet is free outflow. The upper surface of the three-dimensional full-scale numerical model is insulated, and Heat flux = 0. The lower surface of the three-dimensional full-scale numerical model is constant temperature, and the temperature is equal to the initial ground temperature of the surrounding rock and soil layer. The outer surface of the three-dimensional full-scale numerical model is set with symmetry surface Symmetry, and the U-shaped buried pipe wall, borehole wall and rock-soil interface are set to coupled heat transfer Coupled.

[0092] The CFD software operation parameters are: the initial ground temperature of the surrounding rock and soil layer is 17.392℃, the circulating fluid is circulating water, and the densities of circulating water, U-shaped buried pipe, borehole backfill material and surrounding rock and soil layer are 998.2kg / m 3 , 950kg / m 3 、1780kg / m 3 and 1780kg / m 3 The volume specific heat is 4182 J / (kg·℃), 2300 J / (kg·℃), 1392.21 J / (kg·℃) and 1392.21 J / (kg·℃), respectively. The thermal conductivity is 0.6 W / (m·℃), 0.42 W / (m·℃), 2.1175 W / (m·℃) and 1.9589 W / (m·℃), respectively. The borehole diameter is 360 mm, and the circulation flow rate of the circulating fluid in a single borehole is 11.24 m 3 / h, the numerical simulation condition uses a constant inlet water temperature of 5℃ to extract heat for 122 days, and then a constant inlet water temperature of 25℃ to store heat for 243 days. Among them, the circulating water is the circulating fluid, and the U-shaped buried pipe is a PE pipe.

[0093] The three-dimensional full-scale numerical model was verified for mesh independence and time step stability. An appropriate number of meshes was selected and the time step was 3600s / step.

[0094] The size information of the three-dimensional full-scale numerical model constructed in this embodiment is shown in Table 1, and the structure of 8U is shown in Table 1. Figure 3 to Figure 6 The difference between the three-dimensional full-size numerical model structures of 5U, 6U, 7U, 9U and 8U is that the number of outer circle U-shaped buried pipes is different, but the outer circle U-shaped buried pipes are all distributed in a ring array.

[0095] S21. By monitoring the inlet and outlet temperatures of each U-shaped buried heat exchanger and the average temperature of the rock and soil at each radius of the borehole backfill material layer and the surrounding rock and soil layer, the inlet and outlet temperature difference of the circulating fluid, the total heat exchange and the rock and soil temperature change of the surrounding rock and soil layer are calculated, and the heat exchange performance of the U-shaped buried heat exchanger is comprehensively evaluated, as follows:

[0096] During the numerical simulation, the temperatures of the circulating fluid inlet and outlet of the U-shaped buried pipe and the average temperature of the surrounding rock and soil layers are monitored, and the outlet temperature difference of the circulating fluid is calculated using formula ⑦.

[0097] Figure 7 is the inlet and outlet temperature difference of the circulating fluid for 5 different numbers of U-type buried pipes. It can be calculated that after 122 days of heat extraction, the inlet and outlet temperature differences of different numbers of U-type buried pipes are: 5U is 0.36℃, 6U and 7U are both 0.37℃, 8U and 9U are both 0.38℃; after 243 days of heat storage, the inlet and outlet temperature differences of different numbers of U-type buried pipes are: 5U is 0.22℃, 6U~9U are all 0.23℃, that is, from the inlet and outlet temperature difference, in the same borehole, the more U-type buried pipes there are, the greater the inlet and outlet temperature difference is, and the stronger the heat exchange capacity is. The heat extraction / storage temperature difference of 8U and 9U is the same, indicating that the heat exchange capacity is the same.

[0098] The total heat transfer capacity is calculated using formula ⑧.

[0099] Figure 8 It is the total heat exchange of a single hole for 5 kinds of U-shaped buried pipe numbers. It can be calculated that the total heat intake of the heat exchanger corresponding to 5U, 6U, 7U, 8U and 9U are 57.79GJ, 59.36GJ, 60.15GJ, 60.70GJ, 58.00GJ, and the total heat storage are 77.50GJ, 79.77GJ, 80.85GJ, 81.65GJ, 78.05GJ, respectively. That is, the total heat exchange increases first and then decreases with the increase of the number of U-shaped buried pipes in the borehole, and reaches the maximum value at 8U, and the drilling utilization rate reaches the highest.

[0100] Formula ⑨ is used to calculate the change in rock and soil temperature of the surrounding rock and soil layers.

[0101] It can be calculated that after one year of operation, compared with the initial ground temperature of 17.392°C of the surrounding rock and soil layer, the temperature rise of the rock and soil layer surrounding the heat exchangers corresponding to 5U, 6U, 7U, 8U and 9U are 1.20%, 1.24%, 1.26%, 1.28% and 1.28% respectively.

[0102] Comprehensive comparison shows that 8U has a large temperature difference between the inlet and outlet of the circulating fluid, the largest total heat exchange, a large temperature rise in the surrounding rock and soil layers, the highest borehole utilization rate, and the best heat exchange performance.

[0103] S31. The maximum relative error of the circulating fluid outlet water temperature and the inlet and outlet temperature difference was determined by field tests and numerical simulations. The maximum relative error of the circulating fluid outlet water temperature was less than 2.41%, and the maximum relative error of the inlet and outlet temperature difference was less than 5.85%, which verified that the numerical model of the 8U buried pipe heat exchanger has good reliability.

[0104] In winter, the circulating fluid with a lower temperature flows in from the circulating fluid inlet of the U-shaped underground pipe, flows downward along the U-shaped underground pipe and absorbs the heat of the surrounding rock and soil layers, and flows out from the circulating fluid outlet straight pipe, and the circulating fluid temperature rises; in summer, the circulating fluid with a higher temperature flows in from the circulating fluid inlet of the U-shaped underground pipe, flows downward along the U-shaped underground pipe and releases heat to the surrounding rock and soil layers, and flows out from the circulating fluid outlet straight pipe, and the circulating fluid temperature decreases;

[0105] In this implementation, the U-shaped buried pipe size φ in =φ out , the outer diameter φ is 32mm, the wall thickness of the U-shaped buried pipe is 3mm, D in,out =64mm, D=64mm, the outer circle U-shaped buried pipes are evenly distributed. Taking the center of the cross section of the surrounding rock and soil layer as the center of the circle, the angle formed by the line connecting the midpoints of the symmetry axes of the two outer circle U-shaped buried pipes and the center of the circle is 60°, and the diameter of the drilled backfill material layer is φ ht =360mm, the diameter of the surrounding rock and soil layer is φ tu =20m; more specific parameters are the same as those in Table 1. Figure 6 As shown in the middle section, the depth of the straight section of the U-shaped buried pipe is h = 120m, and the depth of the surrounding rock and soil layer is H = 123m.

[0106] This embodiment provides an optimized operating condition of a shallow multi-U-shaped buried pipe heat exchanger. By comprehensively considering the inlet and outlet temperature difference of the circulating fluid, the total heat exchange and the rock and soil temperature rise of the surrounding rock and soil layers, the number of U-shaped buried pipes of the multi-U-shaped buried pipe heat exchanger is optimized, and the 8U-shaped buried pipe heat exchanger in the present invention is obtained, which improves the heat exchange performance and provides a design reference for actual engineering applications.

[0107] The present invention also carries out the following effect verification:

[0108] First, the number of U-shaped buried pipes in the borehole was optimized. Two U-shaped buried pipes in the inner circle and 3 to 7 U-shaped buried pipes in the outer circle were selected for numerical simulation of constant flow in the borehole. The number of U-shaped buried pipes in the outer circle was 3, 4, 5, 6, and 7, respectively, and the circulating fluid flow rate was 11.24m 3 / h.

[0109] The diameter of the borehole backfill material layer is 360mm, the diameter of the surrounding rock and soil layer is 20m, the depth of the U-shaped buried pipe is 120m, the initial geothermal temperature of the surrounding rock and soil layer is 17.392℃, and the operation condition is 5℃ heat extraction for 122 days and 25℃ heat storage for 243 days. The outlet water temperature of the circulating fluid, the inlet and outlet temperature difference, the total heat exchange and the rock and soil temperature changes of the surrounding rock and soil layers are monitored.

[0110] The outlet temperature of the circulating fluid is the most important parameter in the system operation process, which directly affects the operating performance of the heat pump system. According to the research results, after 122 days of heat extraction under the five working conditions, the average outlet temperatures of the circulating fluids of the heat exchangers with different numbers of U-shaped underground pipes are as follows: 5U is 5.36℃, 6U is 5.37℃, 7U is 5.37℃, 8U is 5.38℃, 9U is 5.38℃; the inlet and outlet temperature differences are as follows: 5U is 0.36℃, 6U is 0.37℃, 7U is 0.37℃, 8U is 0.38℃, 9U is 0.38℃. After 243 days of heat extraction under the five working conditions, the average outlet temperatures of the circulating fluids in the heat exchangers with different numbers of U-shaped underground tubes are as follows: 5U is 24.78°C, 6U is 24.77°C, 7U is 24.77°C, 8U is 24.77°C, and 9U is 24.77°C. The inlet and outlet temperature differences are 0.22°C for 5U, 0.23°C for 6U, 0.23°C for 7U, 0.23°C for 8U, and 0.23°C for 9U.

[0111] That is, no matter whether it is heat extraction or heat storage, for multiple U-shaped underground pipes, the more U-shaped underground pipes there are, the greater the inlet and outlet temperature difference is. When the number of U-shaped underground pipes reaches a certain number, the inlet and outlet temperature difference gradually tends to be stable. Therefore, in a limited space, when the flow rate of the circulating fluid in the borehole is the same, the more U-shaped underground pipes there are, the more advantageous the inlet and outlet temperature difference is.

[0112] Second, the single-hole heat exchange of different numbers of U-shaped buried pipes after the system has been running for one year is analyzed. After a long period of operation, after 122 days of heat extraction with five different numbers of U-shaped buried pipes, the total heat extraction of single holes with different numbers of U-shaped buried pipes are: 5U is 57.79GJ, 6U is 59.36GJ, 7U is 60.15GJ, 8U is 60.70GJ, and 9U is 58.00GJ; after 243 days of heat storage, the total heat storage of single holes are 77.50GJ for 5U, 79.77GJ for 6U, 80.85GJ for 7U, 81.65GJ for 8U, and 78.05GJ for 9U.

[0113] That is, in the same well, the total heat transfer of the U-shaped buried pipe increases first and then decreases with the increase in the number of U-shaped buried pipes. When the number of U-shaped buried pipes is 8U, the drilling utilization rate is significantly higher than that of other numbers of U-shaped buried pipes. Therefore, from the perspective of heat transfer performance, the 8U buried pipe is the best.

[0114] The average temperature of the surrounding rock and soil layer first decreased and then increased. The initial temperature of the surrounding rock and soil layer was 17.392°C. After the system was put into operation for one year, the average temperature of the surrounding rock and soil layer of the heat exchanger corresponding to different numbers of U-type buried pipes was: 5U is 17.601°C, 6U is 17.608°C, 7U is 17.612°C, 8U is 17.614°C, 9U is 17.615°C; the temperature rise of the surrounding rock and soil layer of the heat exchanger corresponding to different numbers of U-type buried pipes was: 5U is 1.20%, 6U is 1.24%, 7U is 1.26%, 8U is 1.28%, and 9U is 1.28%.

[0115] In summary, the inlet and outlet temperature difference of the circulating fluid of the 8U buried pipe is large, the total heat exchange is the largest, the temperature rise of the surrounding rock and soil layer is large, and the heat exchange capacity is optimal.

[0116] Third, heat exchange efficiency comparison: Note that in the following heat exchange efficiency comparison test, the total U-type buried pipe ground source heat pump system is Figure 1 The structure shown is just that the U-shaped buried pipe structure was adjusted in different tests.

[0117] The number of U-shaped buried pipes in the borehole is optimized. The U-shaped buried pipes in the borehole are divided into two circles, the inner circle and the outer circle. The number of the two U-shaped buried pipes in the inner circle remains unchanged, and the number of the U-shaped buried pipes in the outer circle is changed. The initial ground temperature of the surrounding rock and soil layer is 17.392℃, the diameter of the borehole backfill material layer is 360mm, and the single hole circulation flow in the borehole is 11.24m 3 / h, first inlet water temperature is 5℃ to extract heat for 122 days, then inlet water temperature is 25℃ to store heat for 243 days. The number of U-shaped buried pipes are 5U, 6U, 7U, 8U and 9U. After 122 days of heat extraction, the inlet and outlet temperature differences of different numbers of U-shaped buried pipes are as follows: 5U is 0.36℃, 6U and 7U are 0.37℃, 8U and 9U are 0.38℃. After 243 days of heat storage, the inlet and outlet temperature differences of different numbers of U-shaped buried pipes are as follows: 5U is 0.22℃, 6U, 7U, 8U and 9U are 0.23℃. That is, from the inlet and outlet temperature difference, in the same borehole, the more U-shaped buried pipes there are, the greater the inlet and outlet temperature difference is, and the stronger the heat exchange capacity is. The heat extraction / heat storage temperature difference of 8U and 9U is the same, indicating that the heat exchange capacity is the same. Compared with the heat extraction / heat storage of 5U of 57.79GJ / 77.50GJ, the heat extraction of 6U, 7U, 8U and 9U increased by 1.57GJ, 2.36GJ, 2.91GJ and 0.21GJ, respectively, which is 2.72%, 4.0% and 1.2% higher than that of 5U, respectively. .08%, 5.04%, and 0.36%, and the heat storage increased by 2.27GJ, 3.35GJ, 4.15GJ, and 0.55GJ, respectively, which were 2.93%, 4.32%, 5.35%, and 0.71% higher than those of 5U, respectively. That is, the total heat exchange first increased and then decreased with the increase of the number of U-shaped buried pipes in the borehole, and reached the maximum value at 8U, and the drilling utilization rate reached the highest; after one year of operation, the rock and soil temperature rise of the surrounding rock and soil layers of the heat exchangers corresponding to different numbers of U-shaped buried pipes were as follows: 5U temperature rise 1.20%, 6U temperature rise 1.24%, 7U temperature rise 1.26%, 8U temperature rise 1.28%, 9U temperature rise 1.28%; Comprehensive comparison, 8U has a large inlet and outlet temperature difference of the circulating fluid, the largest total heat exchange, a large rock and soil temperature rise of the surrounding rock and soil layer, the highest drilling utilization rate, and the best heat exchange performance. It should be noted that, in this text description, 5U, 6U, 7U, 8U and 9U respectively mean that the number of U-shaped buried pipes in the system is 5, 6, 7, 8 or 9.

[0118] Our research found that the inlet and outlet temperature difference of different U-shaped buried pipe numbers in the borehole gradually increases with the increase of the number of U-shaped buried pipes, and gradually tends to be stable; the single-hole heat exchange capacity of different U-shaped buried pipe numbers in the borehole first increases and then decreases with the increase of the number of U-shaped buried pipes. At 8U, the single-hole heat extraction / heat storage both reach the maximum value, and the borehole utilization rate is the highest; at the end of the operating condition, the rock and soil temperature rise of the surrounding rock and soil layers with different U-shaped buried pipe numbers in the borehole increases with the increase of the number of U-shaped buried pipes, and then tends to be stable. At 8U and 9U, the rock and soil temperature rise of the surrounding rock and soil layers is 1.28%.

[0119] In summary, the shallow multi-U-shaped buried pipe heat exchanger provided by the patent application is composed of a circulating fluid in a U-shaped buried pipe, a U-shaped buried pipe, a backfill material layer in a borehole and surrounding rock and soil layers.

[0120] The present invention provides a shallow 8U-type buried pipe heat exchanger, which mainly optimizes the number of U-type buried pipes.

[0121] The working conditions of the present invention are that the initial ground temperature of the surrounding rock and soil layer is 17.392℃, the single hole circulation flow rate is 11.24m 3 / h, first take out heat at 5℃ for 122 days and then store heat at 25℃ for 243 days.

[0122] The present invention optimizes the number of U-shaped buried pipes in the borehole, the number of the two U-shaped buried pipes in the inner circle remains unchanged, the center distance between the branches of each U-shaped buried pipe in the inner circle is 64 mm, and the center distance between the two U-shaped buried pipes is also 64 mm, that is, the optimization Figure 4 Based on the number of winning bids 2, a backfill diameter of 360 mm was selected, and the simulated inlet and outlet temperature differences, total heat exchange and geotechnical temperature changes of the surrounding rock and soil layers were compared when the number of outer rings was 3U, 4U, 5U, 6U and 7U respectively. The heat exchange capacity of the inner ring 2U and the outer ring 6U was stronger, and the number of U-shaped buried pipes was 8U.

[0123] The present application provides an optimized operating condition of a shallow multi-U-shaped buried pipe heat exchanger. By comprehensively considering the inlet and outlet temperature difference of the circulating fluid, the total heat exchange rate and the rock and soil temperature rise of the surrounding rock and soil layers, the number of U-shaped buried pipes of the multi-U-shaped buried pipe heat exchanger is optimized, and the 8U-shaped buried pipe heat exchanger in the present invention is obtained, which improves the heat exchange performance and provides a design reference for actual engineering applications.

[0124] It should be noted that the component connection relationships not specifically mentioned in the present invention are assumed to adopt the existing technology. Since they do not involve the invention point and are widely used in the existing technology, the structural connection relationships are not described in detail.

[0125] It should be noted that when the present invention involves a numerical range, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, in order to avoid redundancy, the present invention describes a preferred embodiment. Although the preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they know the basic creative concept. Therefore, the attached claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0126] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A shallow multi-U-shaped underground heat exchanger, characterized in that: It comprises a plurality of U-shaped buried pipes, each of which is used to fill a circulating fluid, and the plurality of U-shaped buried pipes are distributed in an inner circle and an outer circle, the U-shaped buried pipes in the outer circle are distributed in a ring array, and the U-shaped buried pipes in the inner circle are distributed symmetrically; The borehole backfill material layer wraps the plurality of U-shaped buried pipes inside, and the surrounding rock and soil layer wraps the outside of the borehole backfill material layer; The outer ring has 6 U-shaped underground pipes, and two opposite U-shaped underground pipes are arranged in parallel, and the inner ring has 2 U-shaped underground pipes, and the two U-shaped underground pipes in the inner ring are arranged in parallel with any two opposite U-shaped underground pipes in the outer ring; The outer walls of the borehole backfill material layer and the surrounding rock and soil layer are both cylindrical, and the outer circle composed of 6 U-shaped buried pipes, the borehole backfill material layer and the surrounding rock and soil layer are coaxially arranged; the outer diameter of the U-shaped buried pipe is 32mm, the wall thickness of the U-shaped buried pipe is 3mm, the distance between the inlet center and the outlet center of the U-shaped buried pipe is 64mm, the diameter of the borehole backfill material layer is 360mm, the diameter of the surrounding rock and soil layer is 20m, the distance between the outlet centers of the two U-shaped buried pipes in the inner circle is 64mm, all the U-shaped buried pipes include vertical parts and elbows, the height of the vertical part is 120m, the height of the borehole backfill material layer and the surrounding rock and soil layer are both 123m, and the angle formed by the line connecting the midpoint of the symmetry axis of the two U-shaped buried pipes in the outer circle and the center of the circle with the center of the circle of the cross section of the surrounding rock and soil layer as the center of the circle is 60°.

2. A method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground pipe heat exchanger, characterized in that: The U-shaped underground pipe is the U-shaped underground pipe according to claim 1; The quantity determination method is as follows: According to the requirements of borehole heat exchange, a heat transfer model of shallow multi-U-shaped buried pipe heat exchanger is established. Then, based on the heat transfer model, a three-dimensional full-scale numerical model of heat exchange inside and outside the shallow multi-U-shaped buried pipe coupling is established. According to the drilling numerical simulation conditions and the three-dimensional full-scale numerical model, the heat transfer efficiency under different numbers of U-shaped buried pipes is tested, and the number of U-shaped buried pipes with the maximum heat transfer efficiency is selected; The heat transfer model is divided into a model for heat transfer inside the borehole and a model for heat transfer outside the borehole. The heat transfer inside the borehole includes the convective heat transfer of the circulating fluid, the heat conduction of the U-shaped buried pipe wall, and the heat conduction of the backfill material in the borehole. The continuity equation of the convective heat transfer of the circulating fluid is expressed by the following formula: In ①, ▽ is a differential operator, is the time-averaged velocity of the circulating fluid, in m / s; The momentum equation for convective heat transfer of the circulating fluid is expressed as follows: ②, τ is the circulating fluid flow time, unit is s; ρ f is the density of the circulating fluid, in kg / m 3 ; is the average pressure of the circulating fluid, in Pa; ν is the molecular kinematic viscosity of the circulating fluid, in m 2 / s; ν t is the eddy viscosity coefficient of the circulating fluid, m 2 / s; is the time-averaged velocity of the circulating fluid, in m / s; The energy equation for convective heat transfer of the circulating fluid is expressed as follows: ③In the f is the density of the circulating fluid, in kg / m 3 ; T f is the average temperature of the circulating fluid, in °C; (c p ) f is the specific heat capacity of the circulating fluid at constant pressure, in kJ / (kg·℃); τ is the flow time of the circulating fluid, in s; λ f is the thermal conductivity of the circulating fluid, in W / (m·℃), is the time-averaged velocity of the circulating fluid, in m / s; The heat conduction equation of the U-shaped buried pipe wall is expressed as follows: Among them, ρ p is the density of U-shaped buried pipe, unit: kg / m 3 ; τ is the circulating fluid flow time, unit s; λ p is the thermal conductivity of the U-shaped buried pipe, unit: W / (m·℃); T p is the average temperature of the U-shaped buried pipe, in °C; (c p ) p is the specific heat capacity of the U-shaped buried pipe at constant pressure, in kJ / (kg·℃); The heat conduction equation for backfill in the borehole is expressed as follows: Among them, ρ b The density of the borehole backfill material, in kg / m 3 ; τ is the circulating fluid flow time, unit s; λ b is the thermal conductivity of the drilling backfill material, unit: W / (m·℃); T b is the average temperature of the backfill material during drilling, unit: s; (c p ) b is the specific heat capacity of the borehole backfill material at constant pressure, in kJ / (kg·℃); The heat transfer equation outside the borehole is expressed as follows: Among them, ρ s is the soil density of the rock layer around the U-shaped buried pipe, in kg / m 3 ; τ is the circulating fluid flow time, unit s; λ s is the thermal conductivity of the rock and soil layer around the U-shaped buried pipe, in W / (m·℃); T s is the average temperature of the rock and soil layer around the U-shaped buried pipe, unit s; (c p ) s It is the specific heat capacity of the rock and soil at constant pressure around the U-shaped buried pipe, in units of kJ / (kg·℃).

3. The method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground heat exchanger according to claim 2, characterized in that: Based on the heat transfer model and CFD software, a three-dimensional full-scale numerical model of heat exchange inside and outside the shallow multi-U-shaped buried pipe coupling pipe is established.

4. The method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground heat exchanger according to claim 3 is characterized in that: When establishing a three-dimensional full-scale numerical model, the boundary conditions for the operation of the CFD software are set as follows: transient calculation boundary conditions, the circulating fluid inlet is the velocity inlet, and the constant temperature and flow rate are set; the circulating fluid outlet is the free outflow; the upper surface of the three-dimensional full-scale numerical model is adiabatic; the lower surface of the three-dimensional full-scale numerical model is constant temperature, and the temperature is equal to the initial ground temperature of the surrounding rock and soil layer; the outer surface of the three-dimensional full-scale numerical model is set with a symmetry plane; the U-shaped buried pipe wall, borehole wall and rock and soil interface are set to coupled heat transfer.

5. The method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground heat exchanger according to claim 4, characterized in that: The three-dimensional full-size numerical model is established based on the following parameters: the outer diameter of a single U-shaped buried pipe, the wall thickness of the U-shaped buried pipe, the distance between the inlet center and the outlet center of a single U-shaped buried pipe, the diameter of the borehole backfill material layer, the diameter of the surrounding rock and soil layer, the distance between the outlet centers of the inner circle of U-shaped buried pipes, the height of the vertical part of the U-shaped buried pipe, the height of the borehole backfill material layer, and the height of the surrounding rock and soil layer; and when the center of the circle of the cross section of the surrounding rock and soil layer is taken as the center of the circle, the angle formed by the midpoints of the symmetry axes of the two U-shaped buried pipes in the outer circle and the line connecting the center of the circle.

6. The method for determining the number of U-shaped underground pipes of a shallow multi-U-shaped underground heat exchanger according to claim 2, characterized in that: The heat exchange efficiency is calculated based on the temperature difference between the circulating fluid inlet and the circulating fluid outlet of each U-shaped buried heat exchange pipe, the total heat exchange amount and the rock and soil temperature change of the surrounding rock and soil layer.

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