A method for calculating effective thermal conductivity of cracked coal

Abstract

The present invention discloses a method for calculating the effective thermal conductivity of a fractured coal body. The method is based on the calculation of a digital twin coal body, CT scanning of a real fractured coal body, CT reconstruction and multiphase grid division, and then a conjugate heat transfer model is established based on the flow and heat transfer module of COMSOL software. The thermal physical property parameters of the coal skeleton measured by the laser flash measurement method are substituted, and the fracture fluid domain of the measured material is set to obtain a three-dimensional model of the coal skeleton and fractures. The three-dimensional temperature field of the coal body is obtained through the model, and the effective thermal conductivity is calculated. This method can calculate the thermal conductivity of fractured coal samples in all directions and the thermal conductivity in different fracture fluid environments, and can statistically obtain a thermal conductivity cloud map of a vertically specified cross section. This calculation method comprehensively uses experimental tests and steady-state simulations to truly reflect the internal structure of the measured coal body. Compared with traditional experimental measurements, the calculation speed is improved and more results can be obtained. The results are accurate and reliable.

Description

Technical Field

[0001] The present invention relates to the field of thermal property testing, and in particular to a method for calculating the thermal conductivity of a fractured coal body by combining digital core simulation research technology and experimental testing, including the thermal conductivity under the conditions of fracture fluid flow heat transfer and matrix heat conduction mutual feedback. Background Art

[0002] Coal seam gas is a major source of dangerous disasters in coal mines and a non-renewable energy source. In view of the high pressure, strong adsorption, and low permeability of my country's coal seams, it is necessary to take measures to increase the permeability and fracture of the coal body to improve the gas extraction efficiency. Coal body heat injection mining technology has been applied to the field of gas extraction, using high temperature environment to cause gas thermal desorption and solid coal matrix rupture to produce cracks, thereby increasing the porosity and permeability of the coal body. Determining the effective thermal conductivity of the coal body is an important indicator for evaluating the efficiency of heat injection and the influence range of heat injection. Therefore, determining the effective thermal conductivity of the coal body has important theoretical significance and application value.

[0003] At present, the main way to determine the effective thermal conductivity of massive coal is experimental measurement, which is theoretically divided into steady-state method and non-steady-state method. Coal is a porous medium with a pore-crack system, in which the solid phase is also called a solid skeleton, and the space without a solid skeleton is a pore-crack system. After the coal body is damaged, the cracks develop significantly. The traditional method cannot characterize the detailed differences in the three-dimensional space of the crack development during the heat transfer process, and the traditional method is difficult to test the anisotropy and heterogeneity of the thermal conductivity of multi-crack coal bodies, and cannot reflect the influence of the fracture body on the thermal conductivity.

[0004] Digital core technology can accurately characterize the pore and fracture structure of the coal body, reconstruct the CT image of the internal structure of the core and convert it into a high-precision grid structure that can be recognized by currently common simulation and computing software.

[0005] In the fracture reconstruction method of fractured coal body, such as CN 112085825 B, the fracture reconstruction of fractured coal body is realized, but the reconstruction of the coal body skeleton and the entire fracture area cannot be realized, and it cannot be used for heat transfer finite element simulation including fracture convection heat transfer and coal skeleton heat conduction; in the current CT reconstruction simulation heat transfer method, such as CN 111179296 A, only the rock skeleton is modeled, and the mesh of the fracture is not added. In the rock skeleton modeling, when the object is a fractured coal body (especially the coal body after fracture), it has many fractures, and it is necessary to calculate the heat transfer in the fracture, and the composition of the fluid in the fracture will also affect the heat transfer. It is necessary to generate a mesh model of the fracture and the coal body skeleton, and simulate the thermal conductivity by combining the convection heat transfer in the fracture and the heat conduction of the coal body skeleton. Moreover, this method is only a simulation means, and does not consider the heat conduction of the fluid in the rock fracture, the forced or natural convection heat transfer between the solid and fluid phases, and cannot be applied to real coal sample testing.

[0006] Therefore, designing a method that can measure the effective thermal conductivity of fracture-damaged coal is of great significance to coal heat injection mining technology. Summary of the invention

[0007] In view of the above-mentioned technical deficiencies, the purpose of the present invention is to provide a method for calculating the effective thermal conductivity of a coal body containing fractures, which is a new method for calculating the anisotropic thermal conductivity of a coal body containing fractures based on digital core simulation technology and laser method. The coal body skeleton and the entire fracture area are reconstructed based on CT images, and modeling is performed using numerical simulation software. The model is substituted into a laser thermal conductivity meter to measure the thermal conductivity of the coal skeleton. The method can measure the thermal conductivity of fractured coal samples in all directions, the thermal conductivity in different fracture fluid environments, and can statistically obtain a thermal conductivity cloud map of a vertically specified cross-section. The method comprehensively uses experimental tests and steady-state simulations to truly reflect the internal structure of the measured coal body. Compared with traditional experimental measurements, the measurement speed is improved and more results can be obtained, and the results are accurate and reliable.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for calculating the effective thermal conductivity of a coal body containing fractures, comprising the following steps:

[0010] S1: Process the coal sample into two sizes for CT scanning experiment and laser thermal conductivity test experiment respectively;

[0011] S2: After performing CT scanning on the real coal body with fractures in step S1, the AVIZO reconstruction software is used to comprehensively apply the interactive threshold segmentation and interactive top-hat methods to the CT slice image, and the coal skeleton structure is extracted for three-dimensional reconstruction, and the output is a .stl file; the STL file is imported into the STL file mesh repair software for geometric repair, and after removing self-intersections and some small features, the repaired STL file is imported into the NASA meshing software to re-divide the non-structural surface mesh and repair the mesh with poor quality through human-computer interaction, and the surface mesh division of the fractures and coal skeleton is performed using the ANSYS platform, and a surface mesh cube shell is generated at a distance of 1-3m from the surface, so that the connected fractures in the coal body form a solid domain, and an external flow domain is formed outside the matrix, and a multiphase mesh model is built;

[0012] S3: A conjugate heat transfer model is established based on the creeping flow and solid and fluid heat transfer modules of COMSOL software. The thermal physical properties of the coal skeleton measured by the laser flash measurement method in step S1 are substituted into the model. The multiphase grid model obtained in step S2 is subjected to heat conduction simulation. The fracture fluid domain of the measured material is set to obtain a three-dimensional model of the coal skeleton and fractures. The three-dimensional temperature field of the coal body is obtained through the numerical model, and the thermal conductivity of the coal body is calculated based on Fourier's law.

[0013] Preferably, in step S3, process modeling and calculation are implemented by Comsol finite element software, and the steps of establishing a conjugate heat transfer model are as follows:

[0014] S31: The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows:

[0015]

[0016]

[0017] Among them, ρ and μ are the density and viscosity of the fluid, u is the fluid velocity, T is the temperature, and F is the external force acting on the fluid;

[0018] S32: The energy conservation equation for the heat transfer module is:

[0019]

[0020]

[0021] Where q is the conduction heat flux, C p is the heat capacity of the solid at constant pressure;

[0022] S33: The inlet and outlet boundaries are respectively set at the two end planes of the coal matrix in the same direction, which are the inlet of the constant inward heat flux and the isothermal boundary outlet; the heat flux of the inlet boundary and the temperature of the outlet are set, the thermal conductivity and constant pressure heat capacity of the coal matrix are set as the laser thermal conductivity test experiment, and the parameters of the fluid material are set as the corresponding parameters of the selected fluid domain material;

[0023] S34: The temperature of the corresponding boundary obtained by numerical simulation is used to calculate the thermal conductivity of the coal based on Fourier's law. The thermal conductivity calculation formula of the coal is:

[0024]

[0025] where λ eff is the effective thermal conductivity of fractured coal, q is the heat flux at the inlet boundary, α is the ratio of the heat input surface area to the heat conduction cross-sectional area, T in and T out are the inlet and outlet temperatures respectively, and L is the length of the coal body through which heat is conducted.

[0026] Preferably, in step S1, the laser thermal conductivity test experimental method is a laser flash test method, and the thermal conductivity and heat capacity of the coal body obtained by the test are used as the thermophysical property parameters for simulation substitution.

[0027] Preferably, in step S2, the STL file mesh repair software is Geomagic Wrap software.

[0028] Preferably, the heat conduction simulation in step S3 includes heat conduction simulation of fluid in coal pores, forced or natural convection heat transfer between solid and fluid phases, and heat conduction between coal skeletons.

[0029] Preferably, in order to reduce the error, steps S2 and S3 are repeated to measure the CT models at multiple different positions, and the average value is taken to obtain the effective thermal conductivity of the sample.

[0030] Preferably, an internal cross section of the sample is selected for thermal conductivity calculation, and the cross section is cut into 16 CT models covering the cross section. Steps S2 and S3 are repeated to calculate the thermal conductivity of CT models at multiple different positions, and the results are plotted as a contour cloud map to obtain a thermal conductivity cloud map on the plane.

[0031] Preferably, the side length of the internal cross section of the sample is selected according to the required accuracy and measured size to ensure that the side length pixels of each intercepted CT reconstructed model are greater than 200.

[0032] The beneficial effects of the present invention are:

[0033] 1. This calculation method realizes the reconstruction of the entire area of ​​coal skeleton and cracks, and performs mesh repair, generates corresponding mesh models of cracks and coal skeleton, and performs heat transfer finite element simulation including crack convection heat transfer and coal skeleton heat conduction;

[0034] 2. This calculation method combines the digital core simulation research method and the steady-state simulation experiment to calculate the thermal conductivity of the coal body with fractures, characterizing the detailed differences in the three-dimensional space of the fracture development during the heat transfer process. It includes the thermal conductivity of the fracture fluid flow heat transfer and the matrix heat conduction mutual feedback. Compared with the traditional test method, the result has higher accuracy and efficiency, and can realize the calculation of the thermal conductivity of the coal body in all directions and filled with different fracture fluids.

[0035] 3. Test results show that the sampling range and number of samples can be controlled when calculating the thermal conductivity of fractured coal bodies. The use of statistical methods to process the thermal conductivity results can increase the accuracy of the calculation.

[0036] 4. This calculation method can draw a cloud diagram of the thermal conductivity coefficient of a specific cross section of the coal body, which can significantly reflect the heat transfer characteristics of the coal body, such as heterogeneity and anisotropy. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1A flow chart for calculating the thermal conductivity of a fractured coal body by combining the CT simulation method and experimental test provided in Example 1;

[0039] Figure 2 Schematic diagram of the construction steps of the fractured coal body mesh model of Example 1, wherein (a) is a CT model, (b) is an STL model, and (c) is a mesh model;

[0040] Figure 3 The thermal conductivity cloud diagram of the vertically specified cross section of Example 1;

[0041] Figure 4 This is the three-dimensional temperature field of the coal body containing fractures in Example 1. DETAILED DESCRIPTION

[0042] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0043] Example 1

[0044] like Figures 1 to 4 As shown, a method for calculating the effective thermal conductivity of a fractured coal body comprises the following steps:

[0045] S1: Coal samples were made into two sizes for CT scanning experiments and laser thermal conductivity test experiments: (1) The size of the coal sample used for laser thermal conductivity test was processed into a square sample with a thickness of 3 mm and a cross-sectional side length of 10 ± 0.1 mm. (2) Cylindrical coal sample for CT scanning test.

[0046] S2: Perform CT scanning, set the voltage and current to 80KV and 88uA respectively, adjust the CT imaging quality control parameters on this basis, and use the continuous scanning method to cut the coal sample into 1024 vertical layers, with an image resolution of 50.568um.

[0047] S3: Use the laser thermal conductivity meter (NETZSCH LFA 467) to test the thermal conductivity and heat capacity of the thin-sheet coal body as the thermal physical parameters for simulation. The thickness of the test coal sample is 3mm, and there are no cracks inside. It can be considered that the thermal physical parameters of the crack-free coal body skeleton are measured. The measurement results are as follows:

[0048]

[0049] S4: AVIZO reconstruction software was used to apply interactive threshold segmentation and interactive top-hat methods to CT slice images, and the coal skeleton structure was extracted for three-dimensional reconstruction. The size of the subsequent calculation model was limited by computer memory, so a 200×200×200 pixel model with good CT scan quality was cut and output as an .stl file; the STL file was imported into GeomagicWrap software for geometric repair, and after removing self-intersections and some small features, the repaired STL file was imported into NASA meshing software to re-divide the non-structural surface mesh and repair the poor quality mesh through human-computer interaction. The ANSYS platform was used to mesh the cracks and coal skeleton, and a face mesh cube shell was generated 1 mm away from the surface, so that the cracks in the coal body were connected to form a solid domain, and an external flow domain was formed outside the matrix. Figure 2 The construction steps of the fractured coal mesh model are shown in the figure, where (a) is the CT model, (b) is the STL model, and (c) is the mesh model. In the S4 step, the surface mesh file (STL file) generated after AVIZO reconstruction contains many meshes of poor quality, which need to be repaired before finite element calculation, and a small number of meshes cannot be automatically repaired, so human-computer interaction is required to repair individual meshes, and finally the fractured coal mesh model is obtained.

[0050] S5: The obtained grid model is used to simulate the heat conduction of fluid in the fracture, the convection heat transfer between the solid and fluid phases, and the heat conduction between the coal skeletons. The anisotropic thermal conductivity of the fractured coal is obtained by changing the position of the heat source, and the effective thermal conductivity of the coal with different fracture fluids is calculated by changing the fluid material. The process modeling and calculation are realized by Comsol finite element software.

[0051] The modeling process is as follows:

[0052] S51: When the model is used for natural convection, the viscous force is dominant, the inertia term is ignored, and the natural convection caused by gravity is considered. The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows:

[0053]

[0054]

[0055] Among them, ρ and μ are the fluid density and viscosity, u and p are the fluid velocity and pressure, T is the temperature, and F is the external force acting on the fluid.

[0056] S52: The energy conservation equation for the heat transfer module is:

[0057]

[0058]

[0059] Where q is the conduction heat flux, C p is the heat capacity of a solid at constant pressure.

[0060] S53: Set the inlet and outlet boundaries at the two end planes of the coal matrix in the same direction, which are the inlet of constant inward heat flux and the outlet of isothermal boundary. The heat flux of the inlet boundary is set to 2500W / m 2 , the outlet temperature is set to 313.15K, the thermal conductivity and constant pressure heat capacity of the coal matrix are set to the measured data of the laser flash measurement method, and the parameters of the fluid material are set to the corresponding parameters of the selected fluid domain material. The temperature of the corresponding boundary obtained by numerical simulation is used to calculate the thermal conductivity of the coal based on Fourier's law:

[0061]

[0062] where λ eff is the effective thermal conductivity of fractured coal, q is the heat flux at the inlet boundary, α is the ratio of the heat input surface area to the heat conduction cross-sectional area, T in and T out are the inlet and outlet temperatures respectively, and L is the length of the coal body through which heat is conducted.

[0063] S6: To reduce the error, repeat steps 4-6 to measure the CT model at 5 different positions. The results are shown in the table below. The average value is taken to obtain the effective thermal conductivity of the sample, which is 0.498W / (m·K):

[0064] Sample 1 2 3 4 5 average value Thermal conductivity 0.515 0.500 0.509 0.523 0.442 0.498

[0065] S7: Calculate the thermal conductivity of the cross section with a side length of 30 mm inside the sample, cut it into 16 CT models covering the cross section, repeat steps 4-6 to calculate the thermal conductivity of multiple CT models at different positions. Draw the results as a contour cloud map to obtain the thermal conductivity cloud map on the plane, such as Figure 4 shown.

[0066] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for calculating the effective thermal conductivity of a coal body containing fractures, characterized in that: The following steps are involved: S1: Process the coal sample into two sizes for CT scanning experiment and laser thermal conductivity test experiment respectively; S2: After performing CT scanning on the real fractured coal body in step S1, AVIZO reconstruction software is used to comprehensively apply interactive threshold segmentation and interactive top-hat method to the CT slice image, extract its coal skeleton structure for three-dimensional reconstruction, and output it as a .stl file; The STL file was imported into the STL file mesh repair software for geometric repair. After removing self-intersections and some small features, the repaired STL file was imported into the NASA meshing software to re-divide the non-structural surface mesh and repair the poor quality mesh through human-computer interaction. The surface meshing of the cracks and coal skeleton was performed using the ANSYS platform. A surface mesh cube shell was generated at a distance of 1-3m from the surface, so that the connected cracks in the coal body formed a solid domain, and an external flow domain was formed outside the matrix, and a multiphase mesh model was built. S3: A conjugate heat transfer model is established based on the creeping flow and solid and fluid heat transfer modules of COMSOL software, and the thermal physical property parameters of the coal skeleton measured by the laser flash measurement method in step S1 are substituted. The multiphase grid model obtained in step S2 is subjected to heat conduction simulation, and the fracture fluid domain of the measured material is set to obtain a three-dimensional model of the coal skeleton and fractures; The three-dimensional temperature field of the coal is obtained through the numerical model, and the thermal conductivity of the coal is calculated based on Fourier's law; In step S3, process modeling and calculation are implemented by Comsol finite element software, and the steps of establishing the conjugate heat transfer model are as follows: S31: The creeping flow equation and continuity equation derived from the Navier-Stoks steady-state equation are as follows: Among them, ρ and μ are the density and viscosity of the fluid, u and p are the velocity and pressure of the fluid, T is the temperature, and F is the external force acting on the fluid; S32: The energy conservation equation for the heat transfer module is: Where q is the conduction heat flux, C p is the heat capacity of the solid at constant pressure; S33: The inlet and outlet boundaries are respectively set at the two end planes of the coal matrix in the same direction, which are the inlet of the constant inward heat flux and the isothermal boundary outlet; the heat flux of the inlet boundary and the temperature of the outlet are set, the thermal conductivity and constant pressure heat capacity of the coal matrix are set as the laser thermal conductivity test experiment, and the parameters of the fluid material are set as the corresponding parameters of the selected fluid domain material; S34: The temperature of the corresponding boundary obtained by numerical simulation is used to calculate the thermal conductivity of the coal based on Fourier's law. The thermal conductivity calculation formula of the coal is: where λ eff is the effective thermal conductivity of fractured coal, q is the heat flux at the inlet boundary, α is the ratio of the heat input surface area to the heat conduction cross-sectional area, T in and T out are the inlet and outlet temperatures, respectively, and L is the length of heat conduction through the coal body; To reduce the error, steps S2 and S3 are repeated to measure the CT models at multiple different positions, and the average value is taken to obtain the effective thermal conductivity of the sample.

2. A method for calculating the effective thermal conductivity of a fractured coal body according to claim 1, characterized in that: In step S1, the laser thermal conductivity test experimental method is a laser flash test method, and the thermal conductivity and heat capacity of the coal body are obtained by the test as the thermophysical property parameters for simulation substitution.

3. A method for calculating the effective thermal conductivity of a fractured coal body according to claim 1, characterized in that: In step S2, the STL file mesh repair software is Geomagic Wrap software.

4. A method for calculating the effective thermal conductivity of a fractured coal body according to claim 1, characterized in that: The heat conduction simulation in step S3 includes heat conduction simulation of fluid in coal pores, forced or natural convection heat transfer between solid and fluid phases, and heat conduction between coal skeletons.

5. The method for calculating the effective thermal conductivity of a fractured coal body according to claim 1, characterized in that: The internal section of the sample is selected for thermal conductivity calculation, and it is cut into 16 CT models covering the section. Steps S2 and S3 are repeated to calculate the thermal conductivity of multiple CT models at different positions. The results are plotted as a contour cloud map to obtain the thermal conductivity cloud map on the plane.

6. A method for calculating the effective thermal conductivity of a fractured coal body according to claim 5, characterized in that: The side length of the internal cross section of the sample is selected according to the required accuracy and measured size to ensure that the side length pixels of each intercepted CT reconstructed model are greater than 200.

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