Energy pile thermosetting coupling numerical simulation method based on ABAQUS

Abstract

The invention provides an ABAQUS-based energy pile thermosetting coupling numerical simulation method, and relates to the technical field of energy geotechnical engineering.The method comprises the steps that a three-dimensional geometric model of a soil body, a pile body and a heat exchange pipe is constructed; the soil body is partitioned, and the pile body and the heat exchange tube are subjected to Boolean operation to generate an integrated pile body component with the common joint of an interface; assembling to form a three-dimensional solid model and defining material parameters; performing grid discretization by adopting a temperature-displacement coupling unit; establishing a multi-stage analysis step sequence, wherein the multi-stage analysis step sequence comprises two crustal stress balance and temperature-displacement coupling analysis steps; applying load and boundary conditions; and finite element solution is carried out, and temperature field, thermal stress and pile-soil interface mechanical response data are extracted. According to the method, the whole energy pile construction and service process can be truly reflected, accurate coupling calculation of the temperature field and the stress field is achieved, and a reliable basis is provided for energy pile design optimization.

Description

Technical Field

[0001] This invention relates to the field of energy geotechnical engineering technology, and in particular to a numerical simulation method for energy pile thermo-solid coupling based on ABAQUS. Background Technology

[0002] Energy piles are a novel building energy-saving technology that combines ground source heat pump technology with pile foundations. By embedding heat exchange pipes within the pile, heat exchange between the ground and the superstructure is achieved. They can both bear the building load and utilize shallow geothermal energy, significantly saving land area and drilling costs compared to traditional borehole ground source heat pump technology, making them a research hotspot in geotechnical engineering. However, during operation, energy piles undergo thermal expansion or contraction deformation due to temperature changes, leading to thermal stress and deformation in the pile and surrounding soil. More importantly, temperature field changes further drive pore fluid flow and alter pore pressure distribution, forming a significant thermo-hydraulic-mechanical (THM) three-field coupling effect. This multi-field interaction directly affects the mechanical behavior of the pile-soil interface, the pile's bearing capacity, long-term stability, and heat exchange efficiency. Therefore, a precise analytical model based on the THM full coupling theory is needed to comprehensively evaluate the performance of energy piles.

[0003] While research on energy piles has yielded some results, several shortcomings remain. At the theoretical analysis level, traditional methods such as the load transfer method neglect the continuity of the soil and fail to consider the influence of pile resistance on soil displacement. Elastic analysis methods not only struggle to reveal the complex stress-deformation characteristics under thermo-mechanical coupling but also fail to address the coupling effect of temperature gradients and Darcy flow, completely ignoring the contribution of the fluid field to the coupled response. In the field of numerical simulation, existing models have significant limitations: some models remain at a simplified thermo-solid two-dimensional level, failing to reflect real three-dimensional conditions; most models do not consider the changes in fluid properties with temperature and employ absolutely rough or single friction assumptions for pile-soil contact models, making it difficult to accurately reproduce the multi-field coupling behavior of piles and soil; furthermore, existing THM coupling models are mostly based on software such as COMSOL, lacking standardized modeling schemes for ABAQUS, and the advantages of ABAQUS in simulating the constitutive and contact behavior of geotechnical materials have not been fully utilized. These limitations result in significant biases in the prediction of the THM coupling response of energy piles by existing models. In addition, although in-situ tests and laboratory model tests are important means to study the coupling characteristics of energy piles, they have problems such as high cost, long time consumption, difficulty in systematically controlling operating parameters, and difficulty in comprehensively characterizing micro-coupling processes such as dynamic evolution of pore pressure.

[0004] Chinese invention patent CN119918334A discloses a method for calculating the thermo-coupling response of a variable stiffness energy pile group. This method simulates the nonlinear interaction between piles and soil by selecting a load transfer model, calculates the elastic displacement interaction factors of pile-side-soil and pile-end-soil considering pile-pile interactions, determines the pile-side-soil and pile-end-soil interaction models for the variable stiffness energy pile group, and finally calculates the thermo-coupling response of the variable stiffness energy pile group by substituting the interaction model into an algorithm for a constant stiffness energy pile group. This method can calculate the magnitude and distribution of coupling stress, strain, and displacement of variable stiffness energy pile groups with different pile diameters and lengths, and considers the "reinforcement-curtain" effect between variable stiffness energy piles in the group, as well as the group effect. However, this patent uses a semi-analytical calculation method based on the load transfer method, which cannot intuitively reproduce the three-dimensional geometry of the energy piles, the spatial arrangement of the heat exchange pipes, and the layering characteristics of the soil. It is difficult to accurately simulate the complex thermo-solid coupling behavior of the pile-heat exchange pipe interface and the three-dimensional spatial distribution characteristics of the temperature and stress fields in the soil. Summary of the Invention

[0005] In view of this, the present invention provides a numerical simulation method for thermo-solid coupling of energy piles based on ABAQUS. By establishing a three-dimensional solid model including soil, pile and heat exchanger pipe, Boolean operation technology is used to realize the geometric topology fusion of pile and heat exchanger pipe. A multi-stage analysis step sequence is designed to simulate the entire process from the original ground stress equilibrium to pile formation and then to thermal cycling. The mechanical and thermal contact behavior of pile-soil interface is accurately defined, and a three-dimensional refined numerical simulation of temperature field distribution, thermal stress evolution and mechanical response of pile-soil interface under thermal cycling conditions is realized. This overcomes the limitation of existing theoretical calculation methods that cannot intuitively reproduce three-dimensional geometric shape and spatial distribution characteristics.

[0006] The technical solution of this invention is implemented as follows: This invention provides a numerical simulation method for the thermo-structure interaction of energy piles based on ABAQUS, comprising: S1. Use three-dimensional solid modeling to construct three-dimensional geometric models of soil components, pile components and heat exchanger pipe components respectively; S2. Divide the soil components into layered zones according to the soil layer distribution and divide the pile body influence area. Perform Boolean operations on the pile body components and heat exchange pipe components to form a heat exchange pipe area inside the pile body. Generate an integrated pile body component with common interface nodes and cover the original pile body component. Assemble the zoned soil components, integrated pile body components and heat exchange pipe components into a unified coordinate system to form a three-dimensional solid model. S3. Define geometric sets for each component in the 3D solid model and assign material constitutive parameters to them; S4. The three-dimensional solid model is discretized by using temperature-displacement coupled element type to obtain discretized finite element model; S5. Establish a multi-stage analysis step sequence for the discretized finite element model, including the first stage geostress equilibrium analysis step, the second stage geostress equilibrium analysis step, and the temperature-displacement coupling analysis step, and set the output variables and solution control parameters. S6. In the first stage of the geostress balance analysis step, the pile area is set to a failure state and a gravity load is applied. In the second stage of the geostress balance analysis step, the pile area is reactivated and the pile-soil contact relationship is established. The mechanical and thermal behaviors of the contact relationship are defined. In the temperature-displacement coupling analysis step, the temperature gradient constraint of the thermal cycle condition is applied to the heat exchange tube and the surface heat exchange boundary condition is applied. The displacement constraint boundary condition is applied to the integrated solid model to form a complete finite element analysis model. S7. Solve the finite element analysis model using finite element methods to obtain the convergence results. S8. Extract the temperature field distribution data, thermal stress evolution data, and pile-soil interface mechanical response data of the energy pile under thermal cycling conditions from the calculation convergence results.

[0007] Preferably, the three-dimensional solid modeling method used in step S1 includes: the soil component is generated into a three-dimensional solid by cross-sectional stretching based on the total depth of the soil layer; the pile component is generated into a three-dimensional solid by cross-sectional stretching based on the total length of the pile; and the heat exchange pipe component is generated into a three-dimensional solid by sweeping based on the spatial path of the U-shaped pipe. The sweeping method realizes the three-dimensional geometric reconstruction of the heat exchange pipe by defining the topological association between the spatial curve path and the pipe cross-section.

[0008] Preferably, in step S2, when dividing the soil components into layers, a spatial division reference surface is established based on the location of the soil layer interface. The soil components are divided using the spatial division reference surface as a geometric condition to achieve geometric separation of different soil layers. When dividing the pile influence area of ​​the soil components, the pile top projection outline is defined on the top surface of the soil according to the pile geometry. An extended division surface is established along the length of the pile body using the pile top projection outline as a boundary condition to form a geometric division between the pile influence area and the surrounding soil.

[0009] Preferably, in step S2, when performing Boolean operations on the pile body component and the heat exchanger tube component, the pile body component is defined as the basic entity for geometric topology fusion, and the heat exchanger tube component is defined as the tool entity for geometric topology fusion. Through Boolean subtraction operations, a cavity region corresponding to the geometry of the heat exchanger tube is formed inside the pile body, generating an integrated pile body component with shared nodes at the interface and covering the original independent pile body component. In the integrated pile body component, the pile body region and the heat exchanger tube cavity region achieve continuous transmission of displacement field and temperature field at the interface through physical overlap of nodes.

[0010] Preferably, the applicability criterion for Boolean operations is: When the interfacial bond strength Greater than or equal to thermally induced shear stress When the field variables at the interface satisfy the strong continuity condition, Boolean operations are used to implement the shared node topology; When the interfacial bond strength Less than thermal shear stress At this time, relative slippage or debonding may occur at the interface. A contact model is used to simulate the mechanical behavior of the interface. Among them, interfacial bonding strength The calculation formula is: In the formula, This is the design value of the compressive strength of concrete, in MPa. The normal stress at the interface is expressed in MPa. Thermal shear stress The calculation formula is: In the formula, This is the elastic modulus of the pile, expressed in GPa. The coefficient of linear expansion of the pile body is expressed in K. -1 ; This represents the maximum temperature difference, expressed in °C. The Poisson's ratio of the pile body; This is the soil shear modulus, expressed in GPa. This is the shear modulus of the pile, expressed in GPa.

[0011] Preferably, in step S3, the material constitutive parameters assigned to each component include: defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, coefficient of thermal expansion, and yield stress for each soil layer of the soil component to characterize the elastoplastic mechanical behavior and thermal response characteristics of the soil; defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion for the pile region in the integrated pile component to characterize the thermoelastic constitutive relationship of the concrete material; defining fluid density, thermal conductivity, specific heat capacity, dynamic viscosity, equation of state, and coefficient of thermal expansion for the heat exchange tube cavity region in the integrated pile component to characterize the thermophysical properties and temperature dependence of the fluid; and defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion for the heat exchange tube component to characterize the thermoelastic constitutive relationship of the tube material.

[0012] Preferably, in step S4, when discretizing the three-dimensional solid model into a mesh, the soil component adopts a hexahedral element shape and a structured mesh division method to adapt to the regular geometric characteristics of soil stratification; the integrated pile component adopts a tetrahedral element shape and a free mesh division method to adapt to the complex geometric topology caused by the heat exchanger tube cavity; and the heat exchanger tube component adopts a tetrahedral element shape and a free mesh division method to adapt to the spatial curve geometric characteristics of the U-shaped tube.

[0013] Preferably, in step S5, the settings parameters for the multi-stage analysis step sequence include: the analysis time length for both the first-stage and second-stage geostress balance analysis steps is set to 1, the increment solution type is set to automatic time step, the maximum number of increment steps is 10000, the initial increment step size is 0.01, and the minimum increment step size is 1×10⁻⁶. -5 The equation solver type is set to asymmetric matrix solver, and the geometric nonlinear solution option is activated. The temperature-displacement coupled analysis step is set to automatic time step type, maximum increment step size of 10000, initial increment step size of 0.01, and minimum increment step size of 1×10⁻⁶. -5 Define the maximum allowable temperature change within a single load increment step, set the equation solver type to asymmetric matrix solver, and activate the geometric nonlinear solution option.

[0014] Preferably, step S5 further includes enabling an adaptive mesh re-meshing mechanism in the temperature-displacement coupled analysis step, using the temperature gradient as an error indicator variable to achieve dynamic optimization of the mesh topology, and estimating the element error value. The calculation formula is: In the formula, For the accurate temperature gradient field reconstructed based on gradient recovery technology; This is the numerical temperature gradient field obtained by discretization calculation of the current grid. For the unit integration field; When the unit error estimate Greater than the preset threshold Local mesh re-division is triggered at a preset threshold. The calculation formula is: In the formula, The sensitivity coefficient for grid encryption; To solve the domain The maximum temperature gradient value within the range is determined, and mesh remapping is performed in each increment step to track the dynamic evolution of the temperature field.

[0015] Preferably, the pile-soil contact relationship defined in step S6 includes mechanical contact behavior and thermal contact behavior. The mechanical contact behavior includes tangential friction behavior and normal hard contact behavior, and the thermal contact behavior includes the interfacial thermal conductivity coefficient. The pile-soil contact relationship defines the pile surface as the main control surface and the soil surface as the subordinate response surface. The surface heat exchange boundary condition simulates the heat exchange between the soil surface and the external environment by setting the surface convective heat transfer coefficient and the ambient temperature.

[0016] The present invention has the following advantages over the prior art: This invention establishes a three-dimensional solid geometric model including soil, pile, and heat exchanger pipes, and combines Boolean operations to achieve geometric topological fusion of the pile and heat exchanger pipes. It designs a multi-stage analysis step sequence to simulate the entire construction process from the initial ground stress equilibrium to pile formation and then to thermal cycling. Based on the temperature-displacement coupling unit, it realizes bidirectional coupling calculation of temperature field and displacement field, which can intuitively reproduce the three-dimensional geometric shape of energy pile, spatial arrangement of heat exchanger pipes and soil layer characteristics. It accurately obtains the temperature field distribution, thermal stress evolution and pile-soil interface mechanical response data of energy pile under thermal cycling conditions, overcoming the limitation of existing theoretical calculation methods that cannot accurately simulate three-dimensional spatial distribution characteristics. This invention uses Boolean subtraction to embed the heat exchange tube inside the pile body, generating an integrated pile body component with common nodes at the interface. The displacement field and temperature field are continuously transmitted at the interface through physical overlap of nodes, avoiding the need to define complex pile-tube contact units and contact algorithms. This ensures that the field variables at the interface meet the strong continuity condition, reduces the computational cost and convergence problem caused by contact iteration, and improves simulation efficiency and numerical stability. This invention establishes a judgment criterion based on the comparison of interfacial bond strength and thermally induced shear stress. By quantitatively calculating the bond strength of the concrete-heat exchanger tube interface and the interfacial shear stress caused by thermal cycling, a Boolean operation is used to realize the common node topology when the bond strength is sufficient, and a contact model is used to simulate the relative slip or debonding behavior of the interface when the bond strength is insufficient. This criterion provides a clear physical criterion for the selection of pile-tube interface modeling methods and ensures the consistency between the numerical model and the actual mechanical behavior of the interface in engineering. The multi-stage analysis sequence designed in this invention establishes the original geostress field by setting the pile area to a failure state and applying gravity load in the first stage; reactivating the pile area and establishing pile-soil contact relationship to simulate the pile formation process in the second stage; and applying temperature gradient constraints to simulate thermal cycling conditions in the third stage. This phased simulation strategy truly reflects the entire construction and service process of energy piles from the original soil to pile formation and operation, avoiding the initial condition deviation caused by directly applying the stress state after pile formation, and improving the engineering applicability of the simulation results. The adaptive mesh re-partitioning mechanism employed in this invention uses temperature gradient as an error indicator variable. It evaluates the element error by calculating the deviation between the recovered temperature gradient value of the element and the numerical solution. When the element error exceeds a preset threshold, it triggers local mesh re-partitioning. This mechanism can automatically refine the mesh in areas with drastic temperature gradient changes and maintain a sparser mesh in areas with gentle temperature gradient changes. This ensures the accuracy of the temperature field solution while avoiding the waste of computational resources caused by global mesh refinement, thus achieving a balance between computational accuracy and computational efficiency. This invention defines the mechanical contact properties of pile-soil contact relationship, including tangential friction behavior and normal hard contact behavior, and the thermal contact properties of interfacial thermal conductivity coefficient. The mechanical behavior simulates the relative slippage and normal separation phenomena that may occur at the pile-soil interface, while the thermal behavior characterizes the thermal resistance effect of the pile-soil interface. This dual contact behavior definition can realistically reflect the mechanical response and heat transfer characteristics of the pile-soil interface under thermo-mechanical coupling, thus improving the accuracy of pile-soil interaction simulation. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a flowchart of the method of the present invention; Figure 2 This is a schematic diagram of the soil component of the present invention; Figure 3 This is a schematic diagram of the pile components of the present invention; Figure 4 This is a schematic diagram of the heat exchanger component of the present invention; Figure 5 This is a schematic diagram of the assembly of the present invention; Figure 6 This is a schematic diagram of the mesh of each component of the present invention; Figure 7 This is a data curve diagram of the present invention. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] like Figure 1 As shown, this invention provides a numerical simulation method for the thermo-structure coupling of energy piles based on ABAQUS, comprising: S1. Use three-dimensional solid modeling to construct three-dimensional geometric models of soil components, pile components and heat exchanger pipe components respectively; S2. Divide the soil components into layered zones according to the soil layer distribution and divide the pile body influence area. Perform Boolean operations on the pile body components and heat exchange pipe components to form a heat exchange pipe area inside the pile body. Generate an integrated pile body component with common interface nodes and cover the original pile body component. Assemble the zoned soil components, integrated pile body components and heat exchange pipe components into a unified coordinate system to form a three-dimensional solid model. S3. Define geometric sets for each component in the 3D solid model and assign material constitutive parameters; S4. Discretize the 3D solid model using temperature-displacement coupled element type to obtain a discretized finite element model; S5. Establish a multi-stage analysis step sequence for the discretized finite element model, including the first stage geostress equilibrium analysis step, the second stage geostress equilibrium analysis step, and the temperature-displacement coupling analysis step, and set the output variables and solution control parameters. S6. In the first stage of the geostress balance analysis step, the pile area is set to a failure state and a gravity load is applied. In the second stage of the geostress balance analysis step, the pile area is reactivated and the pile-soil contact relationship is established. The mechanical and thermal behaviors of the contact relationship are defined. In the temperature-displacement coupling analysis step, the temperature gradient constraint of the thermal cycle condition is applied to the heat exchange tube and the surface heat exchange boundary condition is applied. The displacement constraint boundary condition is applied to the integrated solid model to form a complete finite element analysis model. S7. Solve the finite element analysis model using finite element methods to obtain the convergence results. S8. Extract the temperature field distribution data, thermal stress evolution data, and pile-soil interface mechanical response data of the energy pile under thermal cycling conditions from the calculation convergence results.

[0021] In one embodiment of the present invention, step S1 uses a three-dimensional solid modeling method to construct three-dimensional geometric models of the soil component, the pile component and the heat exchange pipe component respectively, wherein the heat exchange pipe component is generated in a sweeping manner along the spatial path of the U-shaped pipe to form a three-dimensional geometric model.

[0022] Specifically, this embodiment uses the component module of ABAQUS software to achieve three-dimensional geometric modeling. For soil and pile components, an extrusion modeling method is used. Two-dimensional cross-sections are drawn according to actual geometric dimensions and extruded along the length direction to generate three-dimensional deformable solids. For heat exchanger tube components, a sweep modeling method is used. First, the spatial curve path of the U-shaped heat exchanger tube is defined. This path consists of two vertical straight line segments and a semi-circular arc segment connected at the bottom. Then, the annular cross-section of the pipe is defined. The sweep command is used to sweep the annular cross-section along the spatial curve path to generate the three-dimensional solid of the U-shaped heat exchanger tube. This sweep method achieves accurate reconstruction of the complex spatial geometry of the heat exchanger tube by defining the topological association between the spatial curve path and the pipe cross-section.

[0023] In one embodiment of the present invention, step S2 divides the soil component into layers according to the soil layer distribution using a segmented entity method, and divides the pile influence area using an extended surface method; the pile component and the heat exchange pipe component are geometrically and topologically fused using a Boolean subtraction operation method to form a heat exchange pipe region inside the pile, generating an integrated pile component with common nodes at the interface. In this integrated pile component, the pile region and the heat exchange pipe region achieve continuous transmission of displacement field and temperature field at the interface through physical overlap of nodes.

[0024] Specifically, this embodiment uses the component module of ABAQUS software to implement soil zoning. A horizontal segmentation reference surface is established at the actual burial depth of each soil layer interface using the create reference surface function. The segmentation entity function is used to divide the continuous soil into several independent soil layer regions. A vertical extension segmentation surface is established in the soil according to the cross-sectional shape of the pile body using the extension surface function, dividing the soil into the pile-affected area and the surrounding soil. Subsequently, Boolean operations are performed using the assembly module of ABAQUS software. The pile component and the heat exchanger pipe component are added to the assembly module and adjusted to the same coordinate system, ensuring that the heat exchanger pipe component is located inside the pile component and meets the relative position requirements in actual engineering. A Boolean subtraction operation is performed on the two entity components using the merge and cut entity tool. The pile component is defined as the basic entity for geometric topology fusion, and the heat exchanger pipe component is defined as the tool entity for geometric topology fusion. A cavity region completely corresponding to the geometry of the heat exchanger pipe is formed inside the pile, generating an integrated pile component with shared interface nodes that covers the original independent pile components.

[0025] The core of this Boolean operation is to allow the pile body and heat exchange tube region to be completely shared in terms of geometric topology and mesh nodes. This avoids the numerical discontinuity problem of interface field transmission in non-shared node models, ensures the continuity of the heat conduction path between the heat exchange tube and the pile body and the lossless transmission of thermal stress, which meets the essential requirement of multi-field coupling for cross-medium field coordination.

[0026] In one embodiment of the present invention, the applicability criterion for Boolean operations is as follows: When the interfacial bond strength Greater than or equal to thermally induced shear stress When the field variables at the interface satisfy the strong continuity condition, Boolean operations are used to implement the shared node topology. When the interfacial bond strength Less than thermal shear stress At this time, relative slippage or debonding may occur at the interface. In this case, the contact model method should be used to define the pile-pipe contact relationship in step S6. Among them, interfacial bonding strength The calculation formula is: In the formula, This is the design value of the compressive strength of concrete, in MPa. The normal stress at the interface is expressed in MPa; specifically, the correction for the steel pipe pile is as follows: , The yield strength of the steel; Thermal shear stress The calculation formula is: In the formula, This is the elastic modulus of the pile, expressed in GPa. The coefficient of linear expansion of the pile body is expressed in K. -1 ; This represents the maximum temperature difference, expressed in °C. The Poisson's ratio of the pile body; This is the soil shear modulus, expressed in GPa. This is the shear modulus of the pile, expressed in GPa.

[0027] This criterion provides a clear physical basis for selecting the pile-tube interface modeling method. When the bond strength between the concrete pile and the heat exchange tube is sufficient to resist the interface shear stress caused by thermal cycling, the Boolean operation common node method, which has higher computational efficiency and better numerical stability, can be used, avoiding the definition of complex contact parameters and contact iteration processes. The Boolean cutting modeling method has the following advantages over traditional modeling methods: common nodes ensure the continuity of displacement, temperature, and shear stress at the pile-tube interface without numerical abrupt changes, eliminating the need to set contact parameters and avoiding calculation errors caused by parameter value deviations; and it reduces contact iteration steps, shortening the calculation time under the same working conditions and improving computational efficiency.

[0028] In one embodiment of the present invention, after completing the Boolean operation, the partitioned soil components and the integrated pile components are assembled into a unified coordinate system to form a three-dimensional solid model.

[0029] Specifically, this embodiment uses the assembly module of ABAQUS software and employs tools such as creating solids and translating / rotating solids to add the partitioned soil components and integrated pile components to a unified coordinate system, ensuring that the integrated pile components are located within the pile influence area of ​​the soil components.

[0030] It should be noted that if the heat exchanger tube component exists as an independent entity, it needs to be assembled into a unified coordinate system. If the heat exchanger tube component has little impact on the bearing capacity of the pile foundation, the heat exchanger tube part can be simplified into a fluid cavity and the independent heat exchanger tube component is no longer retained. In this case, the assembly module only contains two components: the partitioned soil component and the integrated pile component. The heat exchanger tube cavity area is already embedded in the integrated pile component.

[0031] In one embodiment of the present invention, step S3 defines geometric sets for each component in the three-dimensional solid model and assigns material constitutive parameters. The soil material is defined with elastoplastic mechanical constitutive parameters and thermal parameters, the pile material is defined with thermoelastic constitutive parameters, and the heat exchange tube cavity material is defined with fluid thermophysical parameters. If the heat exchange tube component exists as an independent entity, the heat exchange tube material is defined with thermoelastic constitutive parameters.

[0032] Specifically, this embodiment uses the component module and property module of ABAQUS software to define geometric sets and assign material values. Geometric sets are created for each soil layer, pile region, and heat exchanger cavity region within the partition. In the property module, the required materials for each component are created through the material manager. Density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, coefficient of thermal expansion, and yield stress are defined for the soil material to characterize the elastoplastic mechanical behavior and thermal response characteristics of the soil. The yield stress adopts the Drucker-Prager or Mohr-Coulomb yield criterion. The plastic properties of the soil are defined; for the pile material, density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion are defined to characterize the thermoelastic constitutive relationship of the concrete material; for the heat exchanger tube cavity material, fluid density, thermal conductivity, specific heat capacity, dynamic viscosity, equation of state, and coefficient of thermal expansion are defined, where dynamic viscosity and equation of state are used to describe the flow characteristics of the fluid, and the coefficient of thermal expansion is used to describe the change of fluid density with temperature; if the heat exchanger tube component exists as an independent entity, then the density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion are defined for the heat exchanger tube material. After completing the material definitions, corresponding sections are created using the section manager and assigned to each geometric set.

[0033] In one embodiment of the present invention, step S4 uses a temperature-displacement coupled element type to discretize the three-dimensional solid model into a mesh. This element type has both temperature and displacement degrees of freedom, and can realize bidirectional coupled calculation of temperature field and displacement field.

[0034] Specifically, this embodiment uses the mesh module of ABAQUS software to discretize the mesh. For the soil component, hexahedral elements and structured meshing techniques are used to adapt to the regular geometric features of the layered soil. For the integrated pile component, tetrahedral elements and free meshing techniques are used to adapt to the complex geometric topology caused by the heat exchanger tube cavity. If the heat exchanger tube component exists as an independent entity, tetrahedral elements and free meshing techniques are used to adapt to the spatial curve geometry of the U-shaped tube. After meshing, the mesh quality needs to be checked to ensure that there are no distorted elements, especially at the interface between the pile region and the heat exchanger tube cavity region in the integrated pile component, where the mesh should maintain good topological continuity.

[0035] In one embodiment of the present invention, step S5 establishes a multi-stage analysis step sequence for the discretized finite element model, including a first-stage geostress balance analysis step, a second-stage geostress balance analysis step, and a temperature-displacement coupling analysis step, and sets output variables and solution control parameters.

[0036] Specifically, this embodiment uses the analysis step module of ABAQUS software to establish a multi-stage analysis step sequence. After the initial analysis step, the first-stage geostress equilibrium analysis step, the second-stage geostress equilibrium analysis step, and the temperature-displacement coupling analysis step are created sequentially. Each analysis step adopts an incremental solution type with automatic time step, and solution control parameters such as the maximum number of incremental steps, the initial incremental step size, and the minimum incremental step size are set. The equation solver type is an asymmetric matrix solver, and the geometric nonlinear solution option is activated. In the temperature-displacement coupling analysis step, the maximum allowable temperature change within a single load incremental step is defined to control the stability of the temperature field solution. Output variables, including key variables such as temperature, displacement, stress, and strain, are set through the field output request manager and the history output request manager.

[0037] The design of this multi-stage analysis sequence can truly reflect the entire construction and service process of the energy pile, from the original soil to the pile formation and then to operation, avoiding the initial condition deviation caused by directly applying the stress state after pile formation.

[0038] In one embodiment of the present invention, step S5 further includes enabling an adaptive mesh re-division mechanism to improve the calculation accuracy in regions with drastic temperature gradient changes.

[0039] Specifically, in the temperature-displacement coupled analysis step of the ABAQUS software, adaptive mesh re-meshing is enabled. This is achieved through the Adaptivity function, which activates the adaptive mesh re-meshing mechanism. The temperature gradient is set as the error indicator variable, and the element error estimate is... The calculation formula is: In the formula, For the accurate temperature gradient field reconstructed based on gradient recovery technology; This is the numerical temperature gradient field obtained by discretization calculation of the current grid. For the unit integration field; When the unit error estimate Greater than the preset threshold Local mesh re-division is triggered at a preset threshold. The calculation formula is: In the formula, This is the sensitivity coefficient for grid encryption, with a value ranging from 0.05 to 0.15. To solve the domain The maximum temperature gradient value within.

[0040] In the software operation, execute Adaptivity→Remeshing Rule→Create, select ErrorIndicator Variables as TEMP GRADIENT, and set Output Frequency to All increments of step to ensure that gradient changes are checked in each increment step. This mechanism can automatically refine the mesh in areas with drastic temperature gradient changes near the heat exchanger tube cavity, while maintaining a sparser mesh in areas far from the heat exchanger tube, achieving a balance between computational accuracy and computational efficiency.

[0041] In one embodiment of the present invention, step S6 sets the pile region to a failure state and applies gravity load in the first stage of the geostress balance analysis step, reactivates the pile region and establishes pile-soil contact relationship in the second stage of the geostress balance analysis step, and applies temperature constraints and boundary conditions in the temperature-displacement coupling analysis step.

[0042] Specifically, this embodiment uses the interaction module and load module of ABAQUS software to implement the above settings. First, a contact interaction type is created in the interaction module, defining the tangential friction behavior, normal hard contact behavior, and interface thermal conductivity coefficient of the pile-soil contact relationship. The tangential behavior simulates the shear friction characteristics of the pile-soil interface by setting the friction coefficient, the normal behavior is defined as hard contact that allows the interface to separate normally but does not allow them to embed into each other, and the thermal behavior characterizes the thermal resistance effect of the pile-soil interface by setting the interface thermal conductivity coefficient. A surface film condition interaction type is created, and the film coefficient is edited to define the convective heat transfer between the soil surface and the external environment.

[0043] In the first stage of the geostress balance analysis, the pile region in the integrated pile component is set to an invalid state, so that this region does not participate in the assembly of the stiffness matrix in the calculation, simulating the original state of the soil before pile formation. In the second stage of the geostress balance analysis, the pile region is reactivated, and the pile-affected region in the soil component is set to an invalid state, so that this region is replaced by the activated pile region, simulating the pile formation process. At the same time, the surface-to-surface contact between the pile and soil is defined, and the pile surface is designated as the main control surface and the soil surface as the subordinate response surface. In the temperature-displacement coupling analysis step, the surface heat exchange conditions of the soil surface are defined, and the ambient temperature is set to realize the simulation of heat exchange between the soil surface and the external environment.

[0044] In one embodiment of the present invention, if a contact model is used to simulate the pile-pipe interface instead of a Boolean operation-based common-node approach, then in the second-stage geostress balance analysis step, surface-to-surface contact needs to be set for the pile-pipe contact surface. The heat exchanger surface is defined as the master control surface, and the pile surface as the subordinate response surface. The contact interaction attribute is set to the contact interaction type created in the previous step. Specifically, in the interaction module, the type is selected as surface-to-surface contact, the pile-pipe contact surface is selected, the pipe surface is defined as the master plane, the pile surface as the subordinate plane, and the contact interaction attribute is set to the contact interaction type created in the previous step. However, in this embodiment, since the pile and heat exchanger have formed an integrated pile component with common nodes through Boolean operations, the field variables at the interface are continuously transmitted through the physical overlap of nodes. Therefore, it is not necessary to define the pile-pipe contact relationship again, avoiding the computational cost and convergence problems caused by contact iteration.

[0045] In one embodiment of the present invention, step S6 further includes applying various loads and boundary conditions in the load module, including creating a gravity load in the first stage of the geostress balance analysis step to establish the original geostress field, setting fixed constraints on the soil boundary surface to simulate the boundary constraint conditions of the soil, setting vertical displacement constraints on the pile top surface to simulate the load of the superstructure borne by the pile top, and setting temperature gradient constraints on the heat exchange tube cavity region in the integrated pile component to simulate the temperature change during the flow of fluid in the heat exchange tube.

[0046] It should be noted that a stable and balanced initial stress field already exists in the real rock and soil mass under natural conditions. However, the initial geometry in the numerical model is not the same as the state before the stress field was formed. If gravity is applied directly to the model without performing geostress balancing, the model will produce an initial deformation that does not conform to reality. Therefore, it is necessary to establish the original geostress field through the first-stage geostress balancing analysis step, and set the pile area to the failure state in this analysis step so that the model reflects the real rock and soil mass state before the pile was formed.

[0047] In one embodiment of the present invention, step S7 performs finite element analysis on the finite element model to obtain the calculation convergence result.

[0048] Specifically, this embodiment uses the job module of ABAQUS software to create and edit analysis jobs, perform data checks to confirm the integrity of the model and the correctness of various settings, and submit the job after confirmation. ABAQUS will generate an inp calculation file and call the solver to perform finite element solution. During the solution process, the running status of the task can be monitored through the job manager. After the calculation is completed, the calculation convergence result is obtained.

[0049] In one embodiment of the present invention, step S8 extracts the temperature field distribution data, thermal stress evolution data and pile-soil interface mechanical response data of the energy pile under thermal cycling conditions from the calculation convergence results.

[0050] Specifically, this embodiment uses the visualization module of ABAQUS software to view contour maps of temperature, displacement, and stress field distributions using the Field Output function, intuitively displaying the multi-field coupled response characteristics of the energy pile and surrounding soil. The path extraction tool is used to extract data along specific paths, such as temperature distribution curves along the pile depth, shear stress distribution curves at the pile-soil interface, and load-settlement curves at the pile top. This data can be used to evaluate the bearing capacity, temperature response characteristics, and long-term stability of the energy pile, providing a quantitative reference for the design optimization and performance evaluation of energy piles in practical engineering.

[0051] like Figures 2-7 As shown, the implementation process of the present invention is further illustrated below through a specific embodiment: This embodiment simulates the thermo-solid coupling response of an energy pile with a diameter of 0.6m and a length of 30m under thermal cycling conditions. The pile is a cast-in-place concrete pile with a U-shaped heat exchange tube embedded inside. The heat exchange tube has an outer diameter of 0.032m and an inner diameter of 0.026m, and the distance between the two branches of the U-shaped tube is 0.3m. The soil consists of two layers: an upper layer of clay at a depth of 15m and a lower layer of rock at a depth of 15m. The soil model has a planar dimension of 10m × 10m and a total depth of 30m.

[0052] In step S1, a three-dimensional geometric model is established using the component module of ABAQUS. The soil component is generated into a rectangular column of 10m×10m×30m by stretching. The pile component is generated into a cylinder with a diameter of 0.6m and a length of 30m by stretching. The heat exchange pipe component is generated into a U-shaped pipe entity by sweeping. The spatial path of the U-shaped pipe consists of two vertical straight segments with a length of 30m and a semi-circular arc segment with a radius of 0.15m. The pipe cross-section is an annular shape with an outer diameter of 0.032m and an inner diameter of 0.026m.

[0053] In step S2, a horizontal reference plane is created at a depth of 15m to divide the soil component into an upper layer of clay and a lower layer of rock. A circular outline with a diameter of 0.6m is drawn at the center of the top surface of the soil and an extended segmentation surface is established at a depth of 30m to form the pile influence area. The pile component and the heat exchange tube component are subjected to Boolean subtraction operation to generate an integrated pile component. Considering that the U-shaped tube component has little impact on the bearing capacity of the pile foundation, the heat exchange tube is simplified to a fluid cavity in this embodiment, and the independent heat exchange tube component is no longer retained. The partitioned soil component and the integrated pile component are assembled into a unified coordinate system.

[0054] In step S3, geometric sets are created for each soil layer, pile area, and heat exchanger tube cavity area; material property parameters are defined as shown in the table below: For fluid materials, a density of 1000 kg / m³ is defined. 3 Thermal conductivity 0.6 W / (m·K), specific heat capacity 4200 J / (kg·K), dynamic viscosity Pa·s and coefficient of thermal expansion K -1 .

[0055] In step S4, the soil component is subjected to hexahedral elements with a mesh size of 1m and structured partitioning technology, and the element type is C3D8T temperature-displacement coupled hexahedral element; the integrated pile component is subjected to tetrahedral elements with a mesh size of 0.1m and free partitioning technology, and the element type is C3D4T temperature-displacement coupled tetrahedral element.

[0056] In step S5, a multi-stage analysis step sequence is established. The first stage, the geostress equilibrium analysis step, has a time length of 1 step. The incremental solution uses an automatic time step, with a maximum increment of 10000 steps, an initial increment step size of 0.01, and a minimum increment step size of [missing value]. The equation solver is an asymmetric matrix, and geometric nonlinearity is enabled. The parameter settings for the second stage geostress equilibrium analysis step are the same as those for the first stage. The time length of the temperature-displacement coupling analysis step is 100 days, and the other parameter settings are the same as those for the first two analysis steps. The maximum allowable temperature change within a single load increment step is defined as 5℃.

[0057] In step S6, the friction coefficient of the pile-soil contact relationship is defined as 0.4, and the interfacial thermal conductivity is defined as 1.0 W / (m²). 2 ·K), the soil surface film coefficient is 10W / (m 2 •K); In the first stage of the ground stress equilibrium analysis, the pile area is set to an invalid state and a gravity load is applied with a gravitational acceleration of 9.8 m / s². 2Fixed constraints are set on the sides and bottom of the soil. In the second stage of the ground stress balance analysis step, the pile area is reactivated, the soil-pile influence area is set to an invalid state, and the pile-soil contact is defined. In the temperature-displacement coupling analysis step, the heat exchange conditions on the soil surface are defined, the ambient temperature is 20℃, the vertical displacement constraint is set on the pile top, and the temperature gradient constraint is set on the heat exchange tube cavity area, with the inlet temperature at 35℃ and the outlet temperature at 30℃, simulating the summer cooling conditions.

[0058] In step S7, an analysis job is created and submitted, and the convergence result is obtained after about 2 hours of calculation.

[0059] In step S8, the temperature field distribution cloud map, the vertical displacement distribution cloud map of the pile body, the distribution curve of shear stress at the pile-soil interface along the pile depth, and the load-settlement curve at the pile top are extracted. The temperature response characteristics, thermal stress evolution law, and bearing capacity changes of the energy pile under thermal cycling conditions are analyzed, providing a quantitative basis for the design optimization and performance evaluation of the energy pile.

[0060] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A numerical simulation method for thermo-structure interaction of energy piles based on ABAQUS, characterized in that, include: S1. Use three-dimensional solid modeling to construct three-dimensional geometric models of soil components, pile components and heat exchanger pipe components respectively; S2. Divide the soil components into layered zones according to the soil layer distribution and divide the pile body influence area. Perform Boolean operations on the pile body components and heat exchange pipe components to form a heat exchange pipe area inside the pile body. Generate an integrated pile body component with common interface nodes and cover the original pile body component. Assemble the zoned soil components, integrated pile body components and heat exchange pipe components into a unified coordinate system to form a three-dimensional solid model. S3. Define geometric sets for each component in the 3D solid model and assign material constitutive parameters to them; S4. The three-dimensional solid model is discretized by using temperature-displacement coupled element type to obtain discretized finite element model; S5. Establish a multi-stage analysis step sequence for the discretized finite element model, including the first stage geostress equilibrium analysis step, the second stage geostress equilibrium analysis step, and the temperature-displacement coupling analysis step, and set the output variables and solution control parameters. S6. In the first stage of the geostress balance analysis step, the pile area is set to a failure state and a gravity load is applied. In the second stage of the geostress balance analysis step, the pile area is reactivated and the pile-soil contact relationship is established. The mechanical and thermal behaviors of the contact relationship are defined. In the temperature-displacement coupling analysis step, the temperature gradient constraint of the thermal cycle condition is applied to the heat exchange tube and the surface heat exchange boundary condition is applied. The displacement constraint boundary condition is applied to the integrated solid model to form a complete finite element analysis model. S7. Solve the finite element analysis model using finite element methods to obtain the convergence results. S8. Extract the temperature field distribution data, thermal stress evolution data, and pile-soil interface mechanical response data of the energy pile under thermal cycling conditions from the calculation convergence results.

2. The numerical simulation method for thermal-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, The three-dimensional solid modeling methods used in step S1 include: the soil component is generated into a three-dimensional solid by cross-sectional stretching based on the total depth of the soil layer; the pile component is generated into a three-dimensional solid by cross-sectional stretching based on the total length of the pile; and the heat exchange pipe component is generated into a three-dimensional solid by sweeping based on the spatial path of the U-shaped pipe. The sweeping method realizes the three-dimensional geometric reconstruction of the heat exchange pipe by defining the topological association between the spatial curve path and the pipe cross-section.

3. The numerical simulation method for energy pile thermo-structure coupling based on ABAQUS according to claim 1, characterized in that, In step S2, when dividing the soil components into layers, a spatial division reference surface is established based on the location of the soil layer interface. The soil components are divided using the spatial division reference surface as a geometric condition to achieve geometric separation of different soil layers. When dividing the pile influence area of ​​the soil components, the pile top projection outline is defined on the top surface of the soil according to the pile geometry. An extended division surface is established along the length of the pile body using the pile top projection outline as a boundary condition to form a geometric division between the pile influence area and the surrounding soil.

4. The numerical simulation method for thermal-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, In step S2, when performing Boolean operations on the pile body component and the heat exchanger tube component, the pile body component is defined as the basic entity for geometric topology fusion, and the heat exchanger tube component is defined as the tool entity for geometric topology fusion. Through Boolean subtraction operations, a cavity region corresponding to the geometry of the heat exchanger tube is formed inside the pile body. An integrated pile body component with common nodes at the interface is generated and covers the original independent pile body component. In the integrated pile body component, the pile body region and the heat exchanger tube cavity region achieve continuous transfer of displacement field and temperature field at the interface through physical overlap of nodes.

5. The numerical simulation method for thermo-structure coupling of energy piles based on ABAQUS according to claim 4, characterized in that, The criteria for judging the applicability of Boolean operations are as follows: When the interfacial bond strength Greater than or equal to thermally induced shear stress When the field variables at the interface satisfy the strong continuity condition, Boolean operations are used to implement the shared node topology; When the interfacial bond strength Less than thermal shear stress At this time, relative slippage or debonding may occur at the interface. A contact model is used to simulate the mechanical behavior of the interface. Among them, interfacial bonding strength The calculation formula is: In the formula, This is the design value of the compressive strength of concrete, in MPa. The normal stress at the interface is expressed in MPa. Thermal shear stress The calculation formula is: In the formula, This is the elastic modulus of the pile, expressed in GPa. The coefficient of linear expansion of the pile body is expressed in K. -1 ; This represents the maximum temperature difference, expressed in °C. The Poisson's ratio of the pile body; This is the soil shear modulus, expressed in GPa. This is the shear modulus of the pile, expressed in GPa.

6. The numerical simulation method for thermo-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, In step S3, the material constitutive parameters assigned to each component include: for each soil layer of the soil component, defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, coefficient of thermal expansion, and yield stress to characterize the elastoplastic mechanical behavior and thermal response characteristics of the soil; for the pile region in the integrated pile component, defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion to characterize the thermoelastic constitutive relationship of the concrete material; for the heat exchange tube cavity region in the integrated pile component, defining fluid density, thermal conductivity, specific heat capacity, dynamic viscosity, equation of state, and coefficient of thermal expansion to characterize the thermophysical properties and temperature dependence of the fluid; and for the heat exchange tube component, defining density, elastic modulus, Poisson's ratio, thermal conductivity, specific heat capacity, and coefficient of thermal expansion to characterize the thermoelastic constitutive relationship of the tube material.

7. The numerical simulation method for thermo-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, In step S4, when the three-dimensional solid model is meshed, the soil components are divided into hexahedral elements and structured meshes to adapt to the regular geometric features of soil stratification. The integrated pile components adopt tetrahedral unit shapes and free mesh generation methods to adapt to the complex geometric topology caused by the heat exchange tube cavity; the heat exchange tube components adopt tetrahedral unit shapes and free mesh generation methods to adapt to the spatial curve geometric characteristics of the U-shaped tube.

8. The numerical simulation method for thermal-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, In step S5, the parameters for setting the multi-stage analysis step sequence include: the analysis time length for both the first-stage and second-stage geostress equilibrium analysis steps is set to 1, the increment solution type is set to automatic time step, the maximum number of increment steps is 10000, the initial increment step size is 0.01, and the minimum increment step size is 1×10. -5 The equation solver type is set to asymmetric matrix solver, and the geometric nonlinear solution option is activated. The temperature-displacement coupled analysis step is set to automatic time step type, maximum increment step size of 10000, initial increment step size of 0.01, and minimum increment step size of 1×10⁻⁶. -5 Define the maximum allowable temperature change within a single load increment step, set the equation solver type to asymmetric matrix solver, and activate the geometric nonlinear solution option.

9. The numerical simulation method for thermo-structure coupling of energy piles based on ABAQUS according to claim 8, characterized in that, Step S5 also includes enabling an adaptive mesh re-meshing mechanism in the temperature-displacement coupled analysis step, using the temperature gradient as an error indicator variable to achieve dynamic optimization of the mesh topology, and estimating the element error value. The calculation formula is: In the formula, For the accurate temperature gradient field reconstructed based on gradient recovery technology; This is the numerical temperature gradient field obtained by discretization calculation of the current grid. For the unit integration field; When the unit error estimate Greater than the preset threshold Local mesh re-division is triggered at a preset threshold. The calculation formula is: In the formula, The sensitivity coefficient for grid encryption; To solve the domain The maximum temperature gradient value within the range is determined, and mesh remapping is performed in each increment step to track the dynamic evolution of the temperature field.

10. The numerical simulation method for thermo-structure coupling of energy piles based on ABAQUS according to claim 1, characterized in that, The pile-soil contact relationship defined in step S6 includes mechanical contact behavior and thermal contact behavior. Mechanical contact behavior includes tangential friction behavior and normal hard contact behavior. Thermal contact behavior includes interfacial thermal conductivity coefficient. The pile surface is defined as the primary control surface and the soil surface as the subordinate response surface. The surface heat exchange boundary condition simulates the heat exchange between the soil surface and the external environment by setting the surface convection heat transfer coefficient and the ambient temperature.

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