A simulation method for coaxial heat exchanger of deep-seated ground-source heat pump based on affine transformation

Abstract

The present invention discloses a simulation method of a coaxial heat exchanger of a medium-deep geothermal heat pump based on affine transformation, which relates to the technical field of medium-deep geothermal heat pumps. Aiming at the computational difficulties encountered in the simulation process of the coaxial heat exchanger of the medium-deep geothermal heat pump, the method first constructs an accurate numerical model based on the actual physical characteristics of the buried pipe heat exchanger of the medium-deep geothermal heat pump. Through steps one to five, in combination with the model data set, the control equation is optimized by using affine transformation, numerical simulation is performed by using MATLAB and ANSYSFluent software, and the two are combined by using the self-editing function UDF, finally realizing the efficient simulation and optimization design of the underground coaxial heat exchanger. The method optimizes the complex physical process through mathematical models and numerical simulation, simplifies the equations by using affine transformation, and performs multi-level simulation and correction by combining different software tools, finally realizing the accurate prediction and optimization of the performance of the buried pipe heat exchanger. The computational complexity is greatly reduced and the simulation efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of medium-deep geothermal heat pumps, and in particular to a simulation method of a coaxial heat exchanger of a medium-deep geothermal heat pump based on affine transformation. Background Art

[0002] The buried pipe heat exchanger, also known as (Borehole Heat Exchanger, BHE), is an indispensable key component in the ground source heat pump system. It is responsible for efficiently extracting or dissipating heat from the rock and soil, and its performance is directly related to the overall operating efficiency of the ground source heat pump system. In the application of medium-deep ground source heat pump systems, the role of buried pipe heat exchangers is particularly significant. However, due to the significant axial diameter ratio characteristics of the buried pipe heat exchanger, when using ANSYS Fluent software for simulation analysis, it often faces technical difficulties in meshing. This problem makes it difficult to generate high-quality meshes, which in turn increases the difficulty of calculation and limits the accuracy and reliability of the simulation results. Therefore, how to effectively perform simulation analysis of buried pipe heat exchangers is crucial to guiding the design of medium-deep ground source heat pump systems. This is not only related to the optimization of system performance, but also has far-reaching significance for promoting in-depth research and wide application of medium-deep ground source heat pump technology.

[0003] Affine transformation is a unique geometric transformation that strictly preserves the invariance of linearity and distance ratio. Specifically, after the affine transformation, all points originally located on the straight line will remain on the straight line, and the proportional relationship between the line segments will also be maintained. For example, the midpoint of the line segment still maintains its midpoint attribute after the transformation. Therefore, the affine transformation can be regarded as a special subset of the projective transformation, which does not map any object in the affine space to a plane at infinity, and vice versa. However, in the technical field of ground source heat pumps, although the design and optimization of the borehole heat exchanger (BHE) has always been a hot topic of research, no one has proposed combining the theory of affine transformation with the numerical simulation method, resulting in reduced computational efficiency of a single model. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] In view of the shortcomings of the prior art, the present invention provides a simulation method for the coaxial heat exchanger of a medium-deep ground source heat pump based on affine transformation, which combines the theory of affine transformation with the numerical simulation method to optimize the model and improve the calculation speed. Considering the advantages of affine transformation in maintaining geometric characteristics, introducing it into the numerical simulation of buried pipe heat exchangers may not only help to more accurately describe and predict the performance of the heat exchanger, but also open up new ways for the overall optimization of the ground source heat pump system. Therefore, exploring the application of affine transformation in the field of ground source heat pumps has important academic value and practical significance for promoting the further development of this technology.

[0006] (II) Technical solution

[0007] To achieve the above objectives, the present invention is implemented by the following technical solutions: A simulation method for a coaxial heat exchanger of a deep-seated ground-source heat pump based on affine transformation comprises the following steps:

[0008] Step 1: According to the actual situation, for the coaxial sleeve type medium-deep underground heat exchanger, the heat source circulating water flows downward from the outer tube under the drive of the circulating water pump, and exchanges heat with the surrounding soil through the outer tube wall to form the first heat exchange process; at the same time, heat is exchanged with the inner tube water through the inner tube wall to form the second heat exchange process; according to the energy balance and heat transfer principles, the mathematical model of the underground heat exchanger is established using the basic equations of mass conservation, energy conservation and momentum conservation;

[0009] And according to the first heat exchange process and the second heat exchange process, the first outer tube fluid control equation, the first inner tube fluid control equation and the first soil control equation are obtained;

[0010] Step 2: Collect relevant data in the first heat exchange process and the second heat exchange process, establish a model data set, and perform preprocessing and dimensionless processing on the model data set;

[0011] Step 3: Perform affine transformation on the first outer tube fluid control equation and the first inner tube fluid control equation. According to the actual situation to be simulated, determine the transformation coefficient as needed. Substitute the first outer tube fluid control equation and the first inner tube fluid control equation into Regenerate mathematical expressions using the control equations and affine transformations in step one and step three to obtain the second outer tube fluid control equation, the second inner tube fluid control equation, and the second soil control equation;

[0012] Step 4: Simulate the fluid in the pipe using MATLAB software, and simulate the soil using ANSYSFluent software; Step 5: Combine the MATLAB software and ANSYSFluent software inside and outside the pipe through the self-editing function UDF to correct the mathematical model calculation method of the buried pipe heat exchanger.

[0013] Preferably, the model data set includes but is not limited to the following data: soil temperature t g 、Heat source water density ρ f , soil density ρ g , heat source water specific heat capacity C f Specific heat capacity of soil C g , soil thermal conductivity λ g , thermal conductivity of inner tube wall λ m , thermal conductivity of outer tube wall λ o , inner tube inner wall radius r ii 、Inner tube outer wall radius r io , the inner wall radius of the outer casing r oi , the outer wall radius of the outer casing r oo 、Internal tube cross-sectional area F in , outer casing cross-sectional area F o , Downhole coaxial heat exchanger inner tube flow velocity u in , Downhole coaxial heat exchanger outer tube flow velocity u o , the convection heat transfer coefficient of the inner wall of the inner tube h1, the convection heat transfer coefficient of the outer wall of the inner tube h2, the convection heat transfer coefficient of the inner wall of the outer tube h3 and the contact thermal resistance between the outer tube and the backfill material R c .

[0014] Preferably, the step 1 comprises:

[0015] S11, calculate the heat transfer coefficient K per unit length between the inner tube fluid and the outer tube fluid i formula:

[0016]

[0017] Where π is the circumference of a circle and its value is 3.1415926; h1 is the convection heat transfer coefficient of the inner wall of the inner tube, h2 is the convection heat transfer coefficient of the outer wall of the inner tube, and r ii Indicates the inner wall radius of the inner tube, λ in represents the thermal conductivity of the inner tube wall, r io Represents the radius of the inner tube outer wall, r ii Indicates the inner wall radius of the inner tube;

[0018] S12, calculate the heat transfer coefficient K per unit length between the outer tube fluid and the borehole wall o formula:

[0019]

[0020] Where h3 is the convection heat transfer coefficient of the inner wall of the outer tube, λ o represents the thermal conductivity of the outer tube wall, r oi Indicates the inner wall radius of the outer casing, r oo Indicates the outer wall radius of the outer casing; Rc Indicates the contact thermal resistance between the outer tube and the backfill material;

[0021] S13, the fluid flowing from top to bottom in the outer casing exchanges heat with the surrounding soil through the outer casing wall, and exchanges heat with the inner casing water through the inner casing wall. The first outer casing fluid control equation is generated by the following formula:

[0022]

[0023] In the formula, t f，o Indicates the water temperature in the inner tube of the underground coaxial heat exchanger, t f，o Indicates the water temperature of the outer tube of the underground coaxial heat exchanger, t grout represents the backfill material temperature; ρ f Indicates the density of heat source water, C f Indicates the specific heat capacity of heat source water, F o Indicates the cross-sectional area of ​​the outer casing;

[0024] S14, for the inner tube fluid, during the flow from bottom to top in the inner tube, heat is continuously transferred to the outer tube water through the inner tube wall, and the first inner tube fluid control equation is generated by the following formula:

[0025]

[0026] In the formula, τ represents time, and z represents the axial coordinate;

[0027] S15, for backfill materials and soil, only the radial and axial directions, that is, the heat conduction process in the depth direction, are considered internally. Taking soil as an example, the first soil control equation is generated:

[0028]

[0029] In the formula, R c Represents the contact thermal resistance between the outer tube and the backfill material. In all formulas, the Z-axis is taken downward as the positive direction.

[0030] Preferably, the backfill material and the soil are not considered, and the outside of the outer casing of the model is regarded as soil.

[0031] Preferably, in step 3, the entire model is compressed, that is, the inside and outside of the tube are simultaneously acted on, and the formulas inside and outside the tube need to change simultaneously, and the soil outside the tube is substituted into Substitute x′=βx into the control equation of the fluid in the pipe, where β is the same coefficient in the two equations. The only difference is that the control equations used for the fluid in the pipe and the soil outside the pipe are different, resulting in different signs of the independent variables. z and x actually have the same meaning, which is the axial size of the model.

[0032] Preferably, in step three, the second outer tube fluid control equation is generated by the following formula:

[0033]

[0034] The second inner tube flow control equation is generated by the following formula:

[0035]

[0036] In the formula: Substitute x′=βx into the control equation of the fluid in the pipe, where β is the same coefficient in both formulas.

[0037] Preferably, in step three, the second soil control equation is:

[0038]

[0039] In the formula, the formulas inside and outside the pipe need to change at the same time, and substitute the formulas outside the pipe into Substitute x′=βx into the control equation of the fluid in the pipe, where β is the same coefficient in the two equations. The only difference is that the control equations used for the fluid in the pipe and the soil outside the pipe are different, resulting in different signs of the independent variables. z and x actually have the same meaning, which is the axial size of the model.

[0040] Preferably, in step 4, MATLAB software is used to calculate the fluid in the tube using a finite difference method.

[0041] Preferably, in step 4, ANSYS Fluent software is used to simulate the soil outside the pipe. In the control equation of the soil, a β is generated due to the affine transformation. 2 The coefficient of β 2 Added to the soil thermal conductivity λ g At this time, the thermal conductivity of the soil becomes That is, the soil becomes an anisotropic material; in the simulation, the axial dimension of the soil is compressed, and the soil is changed into an anisotropic material for simulation calculation.

[0042] Preferably, in step five, the MATLAB software and ANSYSFluent software are combined through a self-editing function UDF, the heat power obtained from MATLAB is imported into ANSYSFluent, and the soil temperature from ANSYSFluent is imported into MATLAB for cyclic calculation.

[0043] (III) Beneficial effects

[0044] The present invention provides a simulation method for a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation. It has the following beneficial effects:

[0045] (1) The present invention not only provides a more accurate and efficient simulation tool for the design of medium-deep geothermal heat pumps, but also provides strong technical support and theoretical basis for further exploring and optimizing geothermal heat pump technology. This helps to promote the innovation and development of geothermal heat pump technology and improve its performance and efficiency in practical applications. Due to the universality of the affine transformation principle, the present invention is not only suitable for the simulation of coaxial heat exchangers of medium-deep geothermal heat pumps, but may also be extended to the simulation analysis of other similar complex heat exchange systems, and has broad application prospects.

[0046] (2) In response to the computational difficulties encountered in the simulation process of the coaxial heat exchanger of the medium-deep geothermal heat pump, the present invention proposes a simulation method based on affine transformation. This method first constructs an accurate numerical model based on the actual physical characteristics of the buried pipe heat exchanger of the medium-deep geothermal heat pump. Subsequently, by applying the principle of affine transformation, the axial dimension of the model is effectively compressed, thereby greatly reducing the computational complexity and improving the simulation efficiency while maintaining the key geometric characteristics of the model. This innovative method not only provides a more accurate and efficient simulation tool for the design of medium-deep geothermal heat pumps, but also provides strong technical support and theoretical basis for further exploration and optimization of geothermal heat pump technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 The present invention is a schematic flow chart of the steps of a simulation method for a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation. DETAILED DESCRIPTION

[0048] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0049] Example 1

[0050] See also Figure 1 The present invention provides a simulation method for a coaxial heat exchanger of a deep-seated ground-source heat pump based on affine transformation, comprising the following steps:

[0051] Step 1: According to the actual situation, for the coaxial sleeve type medium-deep underground heat exchanger, the heat source circulating water flows downward from the outer tube under the drive of the circulating water pump, and exchanges heat with the surrounding soil through the outer tube wall to form the first heat exchange process; at the same time, heat is exchanged with the inner tube water through the inner tube wall to form the second heat exchange process; according to the energy balance and heat transfer principles, the mathematical model of the underground heat exchanger is established using the basic equations of mass conservation, energy conservation and momentum conservation, which includes the description of the flow and heat transfer process and the setting of related boundary conditions;

[0052] And according to the first heat exchange process and the second heat exchange process, the first outer tube fluid control equation, the first inner tube fluid control equation and the first soil control equation are obtained;

[0053] Step 2: Collect relevant data in the first heat exchange process and the second heat exchange process, establish a model data set, and perform preprocessing and dimensionless processing on the model data set;

[0054] Step 3: Perform affine transformation on the first outer tube fluid control equation and the first inner tube fluid control equation. According to the actual situation to be simulated, determine the transformation coefficient as needed. Substitute the first outer tube fluid control equation and the first inner tube fluid control equation into Regenerate mathematical expressions using the control equations and affine transformations in step one and step three to obtain the second outer tube fluid control equation, the second inner tube fluid control equation, and the second soil control equation;

[0055] Step 4: Simulate the fluid in the pipe using MATLAB software, and simulate the soil using ANSYSFluent software; Step 5: Combine the MATLAB software and ANSYSFluent software inside and outside the pipe through the self-editing function UDF to correct the mathematical model calculation method of the buried pipe heat exchanger.

[0056] In this embodiment, the method first constructs an accurate numerical model based on the actual physical characteristics of the buried pipe heat exchanger of the medium-deep ground source heat pump. Subsequently, by applying the principle of affine transformation, the axial size of the model is effectively compressed, thereby greatly reducing the computational complexity and improving the simulation efficiency while maintaining the key geometric characteristics of the model. This innovative method not only provides a more accurate and efficient simulation tool for the design of medium-deep ground source heat pumps, but also provides strong technical support and theoretical basis for further exploration and optimization of ground source heat pump technology. Through this simulation method based on affine transformation, the heat transfer process of the underground coaxial heat exchanger can be simulated more accurately, the accuracy and reliability of the model can be improved, and strong support can be provided for the design and optimization of the heat exchanger.

[0057] Through the systematic approach of steps one to five, combined with a rich model data set, the control equations are optimized using affine transformation, numerical simulation is performed using MATLAB and ANSYS Fluent software, and the two are combined through the self-editing function UDF, ultimately achieving efficient simulation and optimization design of underground coaxial heat exchangers. The key to this method is to describe and optimize complex physical processes through mathematical models and numerical simulations, simplify equations using affine transformations, improve computational efficiency, and perform multi-level simulations and corrections by combining different software tools, ultimately achieving accurate prediction and optimization of underground pipe heat exchanger performance.

[0058] Example 2: This example is an explanation of Example 1. Specifically, the model data set includes but is not limited to the following data: soil temperature t g 、Heat source water density ρ f , soil density ρ g , heat source water specific heat capacity C f Specific heat capacity of soil C g , soil thermal conductivity λ g , thermal conductivity of inner tube wall λ m , thermal conductivity of outer tube wall λ o , inner tube inner wall radius r ii 、Inner tube outer wall radius r io , the inner wall radius of the outer casing r oi , the outer wall radius of the outer casing r oo 、Internal tube cross-sectional area F in , outer casing cross-sectional area F o , Downhole coaxial heat exchanger inner tube flow velocity u in , Downhole coaxial heat exchanger outer tube flow velocity u o , the convection heat transfer coefficient of the inner wall of the inner tube h1, the convection heat transfer coefficient of the outer wall of the inner tube h2, the convection heat transfer coefficient of the inner wall of the outer tube h3 and the contact thermal resistance between the outer tube and the backfill material R c .

[0059] In this embodiment, the physical characteristics of the heat exchanger can be described more accurately through a detailed model data set, thereby improving the accuracy of the simulation. Using the simulation results, the design parameters of the heat exchanger can be adjusted to optimize its performance. Compared with physical experiments, the simulation method saves a lot of time and cost, and can discover and solve potential problems in the design stage. By optimizing the fluid flow rate and the thermal conductivity of the tube wall, the efficiency of the heat exchanger can be significantly improved. The model data set covers a variety of influencing factors and can comprehensively evaluate the performance of the heat exchanger under different working conditions.

[0060] Embodiment 3: This embodiment is an explanation of Embodiment 2. Specifically, the step 1 includes:

[0061] S11, calculate the heat transfer coefficient K per unit length between the inner tube fluid and the outer tube fluid i formula:

[0062]

[0063] Where π is the circumference of a circle and its value is 3.1415926; h1 is the convection heat transfer coefficient of the inner wall of the inner tube, h2 is the convection heat transfer coefficient of the outer wall of the inner tube, and r ii Indicates the inner wall radius of the inner tube, λ in represents the thermal conductivity of the inner tube wall, r io Represents the radius of the inner tube outer wall, r ii Indicates the inner wall radius of the inner tube;

[0064] S12, calculate the heat transfer coefficient K per unit length between the outer tube fluid and the borehole wall o formula:

[0065]

[0066] Where h3 is the convection heat transfer coefficient of the inner wall of the outer tube, λ o represents the thermal conductivity of the outer tube wall, r oi Indicates the inner wall radius of the outer casing, r oo Indicates the outer wall radius of the outer casing; R c Indicates the contact thermal resistance between the outer tube and the backfill material;

[0067] S13, the fluid flowing from top to bottom in the outer casing exchanges heat with the surrounding soil through the outer casing wall, and exchanges heat with the inner casing water through the inner casing wall. The first outer casing fluid control equation is generated by the following formula:

[0068]

[0069] In the formula, t f，o Indicates the water temperature in the inner tube of the underground coaxial heat exchanger, t f，o Indicates the water temperature of the outer tube of the underground coaxial heat exchanger, t grout represents the backfill material temperature; ρ f Indicates the density of heat source water, C f Indicates the specific heat capacity of heat source water, F o Indicates the cross-sectional area of ​​the outer casing;

[0070] S14, for the inner tube fluid, during the flow from bottom to top in the inner tube, heat is continuously transferred to the outer tube water through the inner tube wall, and the first inner tube fluid control equation is generated by the following formula:

[0071]

[0072] In the formula, τ represents time, and z represents the axial coordinate;

[0073] S15, for backfill materials and soil, only the radial and axial directions, that is, the heat conduction process in the depth direction, are considered internally. Taking soil as an example, the first soil control equation is generated:

[0074]

[0075] In the formula, R c Represents the contact thermal resistance between the outer tube and the backfill material. In all formulas, the Z-axis is taken downward as the positive direction.

[0076] In this embodiment, through the above steps, a detailed mathematical model is established, which can accurately describe the heat transfer process of the underground coaxial heat exchanger. Using these formulas for simulation analysis helps to optimize the design of the heat exchanger, improve heat exchange efficiency, reduce energy consumption, and provide a reliable theoretical basis for engineering applications.

[0077] Example 4: This example is an explanation of Example 1. Specifically, the backfill material and soil are not considered, and the outside of the outer casing of the model is regarded as soil.

[0078] In this embodiment, the model structure is simplified by treating the outside of the outer casing as soil, and there is no need to deal with the interface problem between the backfill material and the soil separately. The number of model parameters and boundary conditions is reduced, making the mathematical model and numerical simulation more concise and easy to implement. It is suitable for large-scale or long-term simulation scenarios and helps to quickly obtain simulation results.

[0079] Example 5. This example is an explanation of Example 1. Specifically, in step 3, the entire model is compressed, that is, the inside and outside of the tube are simultaneously affected, and the formulas inside and outside the tube need to change simultaneously. Substitute the soil outside the tube into Substitute x'=βx into the control equation of the fluid in the tube, where β is the same coefficient in the two equations. The only difference is that the control equations used for the fluid in the tube and the soil outside the tube are different, resulting in different signs of the independent variables. z and x actually have the same meaning, that is, the axial size of the model. In this embodiment, an appropriate affine transformation coefficient β is selected and uniformly applied to the control equations of the fluid in the tube and the soil outside the tube. Since the control equations for the fluid in the tube and the soil outside the tube are different, the form of the transformed equations is also different, but the same coefficient β is used uniformly. Affine transformation provides a unified processing method, so that the control equations for the fluid in the tube and the soil outside the tube can be processed in a consistent manner. By compressing the model, the number of variables and parameters that need to be processed is reduced, and the calculation efficiency is improved. The transformed equation form is more compact, which helps to solve quickly. Through unified transformation processing, possible deviations in the model are reduced and the accuracy of the overall simulation is improved.

[0080] Embodiment 6: This embodiment is an explanation of Embodiment 1. Specifically, in step 3, the second outer tube fluid control equation is generated by the following formula:

[0081]

[0082] The second inner tube flow control equation is generated by the following formula:

[0083]

[0084] In the formula: Substitute x'=βx into the control equation of the fluid in the pipe, where β is the same coefficient in both formulas.

[0085] Specifically, in step 3, the second soil control equation is:

[0086]

[0087] In the formula, the formulas inside and outside the pipe need to change at the same time, and substitute the formulas outside the pipe into Substitute x'=βx into the control equation of the fluid in the pipe, where β is the same coefficient in the two equations. The only difference is that the control equations used for the fluid in the pipe and the soil outside the pipe are different, resulting in different signs of the independent variables. z and x actually have the same meaning, that is, the axial size of the model.

[0088] In this embodiment, after applying the affine transformation, the control equation of the second outer tube fluid, the control equation of the second inner tube fluid and the control equation of the second soil are obtained, and β is a unified transformation coefficient, which is applied to the control equations of the fluid inside the tube and the soil outside the tube at the same time. Since different control equations are used for the fluid inside the tube and the soil outside the tube, although the signs of the independent variables are different (i.e., z and x actually represent the same axial dimension), the effect after the transformation is consistent. The affine transformation simplifies the model structure so that the control equations inside and outside the tube can be processed in a unified manner. The affine transformation helps to smooth and standardize the distribution of variables in the model and reduce unstable factors in numerical calculations.

[0089] Example 7: This example is an explanation of Example 1. Specifically, in step 4, MATLAB software is used to calculate the fluid in the tube using the finite difference method.

[0090] In this embodiment, MATLAB's powerful numerical computing capabilities and efficient matrix operations make the calculation process of the finite difference method fast and efficient. The finite difference method can achieve high-precision numerical calculations through fine time and space discretization. MATLAB provides a variety of precision control options to ensure the accuracy of the calculation results.

[0091] Example 8, this example is an explanation of Example 1. Specifically, in step 4, ANSYS Fluent software is used to simulate the soil outside the pipe. In the control equation of the soil, a β is generated due to the affine transformation. 2 The coefficient of β 2 Added to the soil thermal conductivity λ g At this time, the thermal conductivity of the soil becomes That is, the soil becomes an anisotropic material; in the simulation, the axial dimension of the soil is compressed, and the soil is changed into an anisotropic material for simulation calculation.

[0092] In this embodiment, the soil is regarded as an anisotropic material, and the control of the thermal conductivity of the soil is achieved by adjusting the affine transformation coefficient, which simplifies the description and calculation of the model. The axial dimension of the soil is compressed to reduce the complexity of the simulation calculation and improve the calculation efficiency. By considering the anisotropic properties of the soil, the thermal conductivity behavior of the soil under actual conditions is more accurately reflected. The application of the affine transformation coefficient makes the simulation results closer to the actual situation and improves the accuracy and reliability of the simulation. Example 9, this embodiment is an explanation of Example 1. Specifically, in step five, the MATLAB software and ANSYSFluent software are combined through the self-editing function UDF, the MATLAB thermal power is imported into ANSYSFluent, and the ANSYSFluent soil temperature is imported into MATLAB for cyclic calculation.

[0093] In this embodiment, the heat extraction power calculated by MATLAB is imported into ANSYSFluent, so that the ANSYSFluent simulation process is closer to the actual situation and the heat source input of the external environment is taken into account. The heat source conditions in the actual environment are taken into account to improve the authenticity and accuracy of the simulation. The soil temperature data obtained in the ANSYSFluent simulation process can be transmitted to MATLAB in real time for further temperature analysis and calculation, thus realizing cyclic iteration. Through cyclic calculation, the simulation results can be continuously optimized, so that the simulation results are closer to the actual situation, providing a more reliable basis for engineering design and optimization.

[0094] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A simulation method for a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation, characterized in that: The following steps are included: Step 1. According to the actual situation, for the coaxial sleeve type medium-deep buried pipe heat exchanger, the heat source circulating water flows downward from the outer pipe under the drive of the circulating water pump, and exchanges heat with the surrounding soil through the outer pipe wall to form the first heat exchange process; at the same time, heat is exchanged with the inner pipe water through the inner pipe wall to form the second heat exchange process; according to the energy balance and heat transfer principles, the mathematical model of the buried pipe heat exchanger is established using the basic equations of mass conservation, energy conservation and momentum conservation; and according to the first heat exchange process and the second heat exchange process, the heat transfer coefficient per unit length between the inner pipe fluid and the outer pipe fluid is obtained by analysis. Formula, heat transfer coefficient per unit length between the outer tube fluid and the borehole wall formula, the first outer tube fluid control equation, the first inner tube fluid control equation and the first soil control equation; Step 2: Collect relevant data in the first heat exchange process and the second heat exchange process, establish a model data set, and perform preprocessing and dimensionless processing on the model data set; The model data set includes the following data: soil temperature , heat source water density , soil density Specific heat capacity of heat source water Specific heat capacity of soil , soil thermal conductivity , Thermal conductivity of inner tube wall , Thermal conductivity of outer tube wall , inner tube inner wall radius , the radius of the inner tube outer wall , Outer casing inner wall radius , Outer wall radius of outer casing , inner tube cross-sectional area , Cross-sectional area of ​​outer casing , Downhole coaxial heat exchanger inner tube flow rate , Downhole coaxial heat exchanger outer tube flow rate , Convective heat transfer coefficient of inner tube inner wall , Convective heat transfer coefficient of inner tube outer wall , Convective heat transfer coefficient of the inner wall of the outer tube Contact thermal resistance between outer tube and backfill material ; The step one comprises: S11, calculate the heat transfer coefficient per unit length between the inner tube fluid and the outer tube fluid formula: In the formula, is the value of pi, which is 3.1415926; is the convection heat transfer coefficient of the inner wall of the inner tube, It is expressed as the convection heat transfer coefficient of the inner tube outer wall, represents the inner wall radius of the inner tube, represents the thermal conductivity of the inner tube wall, Indicates the radius of the outer wall of the inner tube; S12, calculate the heat transfer coefficient per unit length between the outer tube fluid and the borehole wall formula: In the formula, is the convection heat transfer coefficient of the inner wall of the outer tube, represents the thermal conductivity of the outer tube wall, Indicates the inner wall radius of the outer casing, Indicates the outer wall radius of the outer casing; Indicates the contact thermal resistance between the outer tube and the backfill material; S13, the fluid flowing from top to bottom in the outer casing exchanges heat with the surrounding soil through the outer casing wall, and exchanges heat with the inner casing water through the inner casing wall. The first outer casing fluid control equation is generated by the following formula: In the formula, Indicates the water temperature in the inner tube of the underground coaxial heat exchanger. Indicates the water temperature of the outer tube of the underground coaxial heat exchanger. Indicates the backfill material temperature; represents the heat source water density, represents the specific heat capacity of the heat source water, represents the cross-sectional area of ​​the outer tube; S14, for the inner tube fluid, during the flow from bottom to top in the inner tube, heat is continuously transferred to the outer tube water through the inner tube wall, and the first inner tube fluid control equation is generated by the following formula: In the formula, Indicates time; S15, for backfill materials and soil, only the radial and axial directions, that is, the heat conduction process in the depth direction, are considered internally, and the first soil control equation is generated: In the formula, It represents the contact thermal resistance between the outer tube and the backfill material; z represents the axial coordinate, and in all formulas, the Z-axis direction is taken as the positive direction; Step 3: Perform affine transformation on the first outer tube fluid control equation and the first inner tube fluid control equation. According to the actual situation to be simulated, determine the transformation coefficient as needed. Substitute the first outer tube fluid control equation and the first inner tube fluid control equation into , using the control equations and affine transformation in step one and step three to regenerate mathematical expressions, obtaining the second outer tube fluid control equation, the second inner tube fluid control equation, and the second soil control equation; In step 3, the entire model is compressed, that is, the inside and outside of the tube are simultaneously affected, and the formulas inside and outside the tube need to change simultaneously. Substitute the soil outside the tube into , substitute into the control equation of the fluid in the tube , in which the two formulas It is the same coefficient. The only difference is that the control equations used for the fluid inside the pipe and the soil outside the pipe are different, which leads to different signs of the independent variables. z and x actually have the same meaning, that is, the axial size of the model. In step 3, the second outer tube fluid control equation is generated by the following formula: , the second inner tube fluid control equation is generated by the following formula: In the formula: Substitute into the control equation of the fluid in the tube , in which the two formulas is the same coefficient; In step 3, the second soil control equation is: In the formula, the formulas inside and outside the pipe need to change at the same time, and substitute the formulas outside the pipe into , substitute into the control equation of the fluid in the tube , in which the two formulas It is the same coefficient. The only difference is that the control equations used for the fluid inside the pipe and the soil outside the pipe are different, which leads to different signs of the independent variables. z and x actually have the same meaning, that is, the axial size of the model. Step 4: Simulate the fluid in the pipe using MATLAB software, and simulate the soil using ANSYS Fluent software; Step 5: Combine the MATLAB software and ANSYS Fluent software inside and outside the pipe through the self-editing function UDF to modify the mathematical model calculation method of the buried pipe heat exchanger.

2. The simulation method of a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation according to claim 1 is characterized by: Backfill material and soil. Backfill material is not considered, and the area outside the outer casing of the model is regarded as soil.

3. The simulation method of a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation according to claim 1 is characterized in that: In the step 4, MATLAB software is used to calculate the fluid in the tube using a finite difference method.

4. The simulation method of a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation according to claim 3 is characterized by: In step 4, ANSYS Fluent software is used to simulate the soil outside the pipe. In the control equation of the soil, the coefficients generated by the affine transformation are , the coefficient Add to soil thermal conductivity At this time, the soil thermal conductivity becomes , that is, the soil becomes an anisotropic material; in the simulation, the axial dimension of the soil is compressed, and the soil is changed into anisotropic material for simulation calculation.

5. The simulation method of a coaxial heat exchanger of a mid-deep ground source heat pump based on affine transformation according to claim 4 is characterized by: In the step 5, the MATLAB software and the ANSYS Fluent software are combined through the self-editing function UDF, the heat power obtained from MATLAB is imported into ANSYS Fluent, and the soil temperature from ANSYS Fluent is imported into MATLAB for cyclic calculation.

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