Weak structure rock mass heat conductivity coefficient testing method based on unsteady state linear source heat transfer theory
By combining dry drilling technology and thermal coupling agent with the theory of unsteady line source heat transfer, the problem of accuracy in testing the thermal conductivity of weakly structured rock masses was solved, enabling accurate testing of difficult-to-process and easily fractured rock masses.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHINA UNIV OF GEOSCIENCES (WUHAN)
- Filing Date
- 2026-01-14
- Publication Date
- 2026-04-10
AI Technical Summary
Existing technologies are insufficient to accurately test the thermal conductivity of weakly structured rock masses. Traditional methods result in test results that deviate from the true value due to sample breakage during processing or poor interfacial contact.
Test holes were drilled using dry drilling technology and backfilled with thermal coupling agent. A thermal probe was used for heating, and data fitting was performed based on the unsteady-state line source heat transfer theory. Data segments in the quasi-steady-state heat transfer stage were selected, and the thermal conductivity was calculated.
To maximize the integrity of the rock mass borehole wall, reduce interfacial thermal resistance, ensure the accuracy of test results, and conform to the actual physical properties of the rock mass.
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Figure CN121830775A_ABST
Abstract
Description
Technical Field
[0001] This application belongs to the field of rock mass thermal conductivity testing technology, and more specifically, relates to a method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory. Background Technology
[0002] The thermal conductivity of rock and soil mass characterizes the thermal conductivity of rocks or soil and is an important parameter for adaptability zoning of shallow geothermal energy development and utilization and assessment of geothermal resource potential. Accurate testing of the thermal conductivity of rock and soil mass has profound engineering significance for the optimized design of ground source heat pump systems and the scientific development of geothermal energy.
[0003] However, research on the thermal conductivity of weakly structured rock masses (such as weakly cemented, easily fractured mudstone, sandstone, and conglomerate located in the transition zone between hard rock and soil, with uniaxial compressive strength between 1 and 50 MPa) is relatively scarce. Due to the poor structural stability, difficulty in in-situ sampling, and extreme fragility during machining of such rock masses, it is often difficult to meet the accuracy requirements of traditional laboratory sample preparation and testing. While traditional steady-state methods (such as the hot plate method and the bar method) and transient methods (such as the hot wire method and the probe method) offer advantages such as ease of operation and portability, when dealing with weakly structured rock masses, sample disturbance or poor interfacial contact often leads to test results that significantly deviate from the true value.
[0004] Therefore, how to accurately test the thermal conductivity of weakly structured rock masses is an urgent problem to be solved. Summary of the Invention
[0005] To address the shortcomings of existing technologies, the purpose of this application is to provide a method for testing the thermal conductivity of weakly structured rock masses based on the theory of unsteady-state line source heat transfer, which can accurately test the thermal conductivity of weakly structured rock masses.
[0006] To achieve the above objectives, in a first aspect, this application provides a method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory, comprising the following steps: S10, a test hole is drilled in a weak rock mass using dry drilling technology, and a thermal coupling agent is backfilled into the test hole; S20, insert the thermal probe into the center of the test hole that has been backfilled with thermal coupling agent, so that the thermal probe comes into contact with the rock mass through the thermal coupling agent; S30, the thermal probe is heated with constant power, and data on the temperature change of the thermal probe over time is collected during the heating process; S40: From the collected data on temperature changes over time, select data segments whose heat transfer time meets the requirements of the quasi-steady-state heat transfer stage. S50, based on the selected data segment, according to the theory of unsteady linear heat source, the linear relationship between temperature and the natural logarithm of heat transfer time is fitted to obtain the slope of the fitted straight line, and the thermal conductivity of the weakly structured rock mass is calculated based on the slope and the heating power.
[0007] The thermal conductivity testing method for weakly structured rock masses based on the unsteady-state line source heat transfer theory provided in this application has the following advantages: First, the dry drilling technique can maximize the integrity of the borehole wall in the weakly structured rock mass, preventing macroscopic disintegration. Second, by backfilling the borehole with a thermal coupling agent and allowing the probe to contact the rock mass through the coupling agent, sufficient contact between the probe and the rock mass can be ensured, effectively reducing the interfacial contact thermal resistance caused by the rough and uneven surface of the weakly structured rock mass or the voids in the pre-drilled hole. In addition, by combining data screening and fitting calculation based on the unsteady-state line heat source theory, the testing deviation caused by sample processing breakage or interfacial thermal resistance in traditional methods can be avoided, thereby achieving accurate testing of the thermal conductivity of difficult-to-process and easily broken weakly structured rock masses, and the test results are more consistent with the actual physical properties of the rock mass.
[0008] As a further optimization, in step S40, data segments whose heat transfer time meets the requirements of the quasi-steady-state heat transfer stage are selected; specifically, data segments whose heat transfer time t satisfies the formula... The data segment; where, For the test hole radius, The thermal diffusivity of the rock mass.
[0009] As a further preferred embodiment, in step S50, the thermal conductivity is calculated based on the slope and the heating power, specifically according to the formula... Calculate; where, Thermal conductivity, Let t be the average temperature of the thermal probe, and q be the heating power per unit length of the thermal probe.
[0010] As a further preferred embodiment, in step S50, the heating power per unit length of the thermal probe... According to the formula Calculated; where, The heating voltage of the thermal probe. To heat the resistance of the circuit, This is the effective heating length of the thermal probe.
[0011] As a further preferred embodiment, in step S30, the heating voltage is selected based on the estimated thermal conductivity range of the weakly structured rock mass. When the estimated thermal conductivity is less than 1 W·m - ¹·K - ¹time, Using 6V; when the estimated thermal conductivity is 1 W·m - ¹·K- ¹~4 W·m - ¹·K - ¹When, Using 12V; when the estimated thermal conductivity is 4 W·m - ¹·K - ¹~6 W·m - ¹·K - ¹When, Uses 15V.
[0012] As a further preferred embodiment, in step S10, the thermal coupling agent is bentonite slurry, which is prepared by mixing bentonite powder and pure water at a mass ratio of 3:1.
[0013] As a further preferred embodiment, in step S10, backfilling the test hole with thermal coupling agent specifically involves injecting the prepared viscous, fluid bentonite slurry into the test hole, and performing step S20 while the bentonite slurry is in a fluid state. As a further preferred option, the heating time in step S30 is set to 290 to 310 seconds.
[0014] Secondly, this application provides a thermal conductivity testing system for weakly structured rock masses, used to implement the testing method described in any one of the above-mentioned methods, comprising: The probe module includes a tube body, a heating module located at the center of the tube body, and a temperature sensor located inside the tube body; The data acquisition module is electrically connected to the temperature sensor and is used to acquire and convert temperature signals. The data analysis module is communicatively connected to the data acquisition module. It is used to receive temperature data, perform data filtering, fitting and calculation, and output the thermal conductivity result.
[0015] As a further preferred embodiment, the data analysis module is a computer with data analysis software installed. The data analysis software is written in Python and incorporates the least squares method for data fitting and calculation.
[0016] It is understandable that the beneficial effects of the second aspect mentioned above can be found in the relevant descriptions in the first aspect mentioned above, and will not be repeated here. Attached Figure Description
[0017] Figure 1 This is a flowchart of the method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory provided in this application; Figure 2 This is a natural logarithm-temperature rise curve of the test time for different backfill materials for limestone samples provided in the embodiments of this application; Figure 3This is a structural diagram of the thermal conductivity testing system for weakly structured rock masses provided in the embodiments of this application.
[0018] In all the accompanying drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is a laptop computer; 2 is a 51 microcontroller; 3 is a data acquisition chip; 4 is soil and rock; 5 is a probe; 6 is copper powder; 7 is a sensor; 8 is backfill material; 9 is a nickel-chromium wire; 10 is a voltage regulator module; 11 is a power bank. Detailed Implementation
[0019] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.
[0020] This application's research has revealed that while traditional testing methods such as steady-state methods (e.g., hot plate method, bar method) and transient methods (e.g., hot wire method, probe method) can meet the requirements for testing the thermal conductivity of most soil and rock masses in engineering, they have significant limitations when dealing with weakly structured rock masses such as low-strength, loosely structured mudstone and rough-surfaced sandstone. Processing and Sampling Bottlenecks: Traditional steady-state methods have extremely high requirements for sample geometry and surface flatness. Weakly structured rock masses, due to their poor cementation and high brittleness, are highly susceptible to micro-crack initiation or overall disintegration during drilling, cutting, and grinding into standard specimens, making it impossible to prepare test samples that meet specifications. Forced processing often alters the original rock structure, causing test results to deviate from the true values.
[0021] Interfacial contact thermal resistance interference: Surface roughness is high in weakly structured sandstone and similar materials, resulting in numerous air gaps at the microscopic level when in contact with traditional planar probes. Since air has extremely low thermal conductivity, these interfacial thermal resistances significantly weaken heat transfer efficiency, leading to measured values often being lower than the true thermal conductivity.
[0022] In-situ disruption: Existing transient probe methods often face the problem that the probe cannot be directly inserted into weakly structured rock masses with slightly higher hardness. Even with pre-drilling, the gap (air layer) between the borehole and the rock can severely interfere with heat conduction, leading to a significant reduction in test accuracy.
[0023] Therefore, in order to solve the problem of inaccurate measurement of thermal conductivity of weakly structured rock masses using traditional methods and to improve the shortcomings of existing testing technologies, it is necessary to innovate traditional thermal conductivity testing methods to address the problems that arise during their application.
[0024] Currently, there are some studies related to thermal conductivity testing methods in China. While these studies have innovated thermal conductivity testing methods based on considerations such as operational efficiency and system design optimization, they mostly focus on system design optimization, stress environment simulation, or improvements in calculation algorithms. They have not fundamentally solved the testing challenges posed by the "difficult to process, easily broken, and poor contact" characteristics of weakly structured rock masses. In particular, efficient solutions are still lacking for interfacial thermal resistance caused by surface roughness and for test result distortion due to sampling difficulties.
[0025] This application, based on the design concept of "unsteady-state linear heat source theory," develops a method for testing the thermal conductivity of weakly structured rock masses. Employing the thermal probe method and thermal coupling technology, it provides a more effective approach for accurately testing the thermal conductivity of weakly structured rock masses. Furthermore, this scheme effectively addresses the problems arising from rock surface processing, resulting in test results that better reflect actual conditions. It offers a new approach to the application of thermal conductivity testing methods.
[0026] like Figure 1 As shown, this application provides a method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory, including steps S10 to S50, which are detailed below: Step S10: Use dry drilling technology to drill test holes in the weak rock mass and backfill the test holes with thermal coupling agent.
[0027] In this step, a low-speed, low-pressure dry drilling technique is used in the hole-forming process. By reducing the mechanical impact load, the integrity of the hole wall in the weak rock mass can be maintained to the maximum extent, preventing macroscopic disintegration. At the same time, by injecting a specific thermal coupling agent into the borehole, a medium can be provided for the subsequent probe insertion. Its fluidity and viscosity can fill the gap between the probe and the hole wall.
[0028] Step S20: Insert the thermal probe into the center of the test hole that has been backfilled with thermal coupling agent, so that the thermal probe comes into contact with the rock mass through the thermal coupling agent.
[0029] This step utilizes a flowing thermal coupling agent to ensure full contact between the probe, backfill, and borehole wall, thereby eliminating air gaps between them.
[0030] Step S30: Heat the thermal probe with constant power and collect data on the temperature change of the thermal probe over time during the heating process.
[0031] This step involves heating with constant power and collecting temperature rise data, which provides a basic input for analysis based on the theory of unsteady-state linear heat sources.
[0032] Step S40: From the collected data on temperature changes over time, select data segments whose heat transfer time meets the requirements of the quasi-steady-state heat transfer stage.
[0033] This step, by filtering data after a specific time period, can exclude the initial stage of the experiment where temperature changes are mainly controlled by the backfill material, thus meeting the conditions for quasi-steady-state analysis and improving the accuracy of subsequent calculations.
[0034] Step S50: Based on the selected data segments, according to the theory of unsteady linear heat sources, the linear relationship between temperature and the natural logarithm of heat transfer time is fitted to obtain the slope of the fitted line, and the thermal conductivity of the weakly structured rock mass is calculated based on the slope and the heating power.
[0035] It should be noted that, since the diameter of the test probe is much smaller than its length, when heating the probe with constant power, the heat transfer from the probe to the surrounding soil and rock in the axial direction of the borehole is relatively small compared to the heat transfer in the radial direction and can be ignored. The underground soil and rock and the temperature field are uniform, and the differences in the soil and rock in the axial direction can be ignored. Moreover, the thermal properties of the soil and rock remain stable during the test, and the heat transfer is carried out by simple conduction, which is consistent with the assumption of the line heat source theory. Therefore, the probe heat conduction problem can be simplified to the unsteady heat conduction problem of an infinitely long uniform heating body in an infinitely large medium.
[0036] Based on this, this step applies the logarithmic curve fitting method of the unsteady-state linear heat source theory to simplify the probe heat conduction problem into a model of an infinitely long linear source in an infinitely large medium. The thermal conductivity of the soil and rock can be directly solved by the slope of the linear relationship.
[0037] The working principle of this application for testing the thermal conductivity of weakly structured rock masses is as follows: First, in terms of the drilling process, this invention adopts low-speed, low-pressure dry drilling technology, which reduces mechanical impact load and maximizes the integrity of the hole wall in the weak rock mass, preventing macroscopic disintegration. Secondly, considering the potential micro-damage (micro-fractures) that may occur during the drilling process, this method utilizes the rheological filling effect of bentonite mud coupling agent to fill the micro-fractures in the borehole wall under pressure. This not only eliminates the thermal resistance interference caused by the air layer, but also achieves "performance repair" of the disturbed zone in a thermodynamic sense, constructing an efficient heat conduction pathway from the probe to the original rock mass; Finally, based on the deep thermal diffusion characteristics of the line heat source theory, the initial nonlinear data, which was significantly affected by the borehole wall environment, was discarded during data processing. Instead, the quasi-steady-state temperature rise slope was extracted after the thermal front penetrated the disturbed zone and entered the primary rock mass. This "time-domain filtering" method ensured that the measured parameters reflected the true physical properties of the undisturbed rock mass, thus guaranteeing the objectivity and representativeness of the test results.
[0038] The thermal conductivity testing method for weakly structured rock masses based on the unsteady-state line source heat transfer theory provided in this application has the following advantages: First, the dry drilling technique can maximize the integrity of the borehole wall in the weakly structured rock mass, preventing macroscopic disintegration. Second, by backfilling the borehole with a thermal coupling agent and allowing the probe to contact the rock mass through the coupling agent, sufficient contact between the probe and the rock mass can be ensured, effectively reducing the interfacial contact thermal resistance caused by the rough and uneven surface of the weakly structured rock mass or the voids in the pre-drilled hole. In addition, by combining data screening and fitting calculation based on the unsteady-state line heat source theory, the testing deviation caused by sample processing breakage or interfacial thermal resistance in traditional methods can be avoided, thereby achieving accurate testing of the thermal conductivity of difficult-to-process and easily broken weakly structured rock masses, and the test results are more consistent with the actual physical properties of the rock mass.
[0039] Based on the same inventive concept, this application also provides a system for testing the thermal conductivity of weakly structured rock masses, comprising: The probe module includes a tube body, a heating module located at the center of the tube body, and a temperature sensor located inside the tube body; The data acquisition module is electrically connected to the temperature sensor and is used to acquire and convert temperature signals. The data analysis module communicates with the data acquisition module to receive temperature data, perform data filtering, fitting and calculation, and output the thermal conductivity result.
[0040] It should be noted that the functions of each module provided in this embodiment can be found in the detailed description of the aforementioned test method, and will not be repeated here.
[0041] In one embodiment, the technical solution to achieve the above objective can be as follows: This embodiment provides a method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory, and a portable instrument for testing the thermal conductivity of weakly structured rock masses. The instrument mainly consists of three parts: a probe module, a data acquisition module, and a data analysis module.
[0042] In this embodiment, the probe module mainly consists of a stainless steel tube, a heating module, and a temperature sensor. The probe has an aspect ratio of 50, a total length of 200mm, a working area length of 192mm, an outer diameter of 4mm, an inner diameter of 3.2mm, and is made of stainless steel. A 0.2mm diameter, 35.2Ω / m resistance enameled constantanyl chromium wire is wound around a 0.5mm diameter stainless steel rod and placed in the center of the stainless steel tube to form the heating module. The temperature sensor is a Pt100 platinum resistance temperature sensor with a measurement accuracy of 0.01℃, placed in the middle of the probe's inner wall. The gap in the probe's center is filled with 300-500 mesh copper powder to reduce the probe's thermal resistance. The probe connection uses an SF6 five-pin aviation connector to connect the probe module to the external circuitry, and finally, Kraft K-704N silicone sealant is used to seal the joints.
[0043] The data acquisition module consists of an STC89C52RC microcontroller and a MAX31865 chip. The microcontroller controls the chip to acquire temperature data, receive and convert electrical signals from the temperature sensor, and send them via serial port to the accompanying STC-ISP software for reading and storage.
[0044] The data analysis module uses Python and employs the "least squares method" to calculate thermal conductivity. It is embedded into the data analysis software to read and process temperature data files and ultimately display the calculation results and fitted image data.
[0045] A method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory, and a portable instrument for testing the thermal conductivity of weakly structured rock masses are presented. The instrument mainly consists of a probe module, a data acquisition module, and a data analysis module. Based on a modified heat transfer calculation method of "line heat source + thermal coupling agent + rock-soil heat transfer," it ensures full contact between the probe and the rock mass, reducing contact thermal resistance. This method solves the problems of difficult sampling and processing of weakly structured rock masses, and inaccurate results from traditional testing methods. The equipment is simple to operate, convenient to use, and provides rapid measurements, suitable for both indoor and field experiments.
[0046] Compared with the prior art, the advantages of this embodiment are: 1) Based on the modified heat transfer calculation method of "line heat source + thermal coupling agent + soil heat transfer", the probe is in full contact with the soil, reducing the contact thermal resistance. This can solve the problems of difficult sampling and processing of weak soil and rock and the inaccurate results of traditional testing methods.
[0047] 2) Traditional methods cannot overcome the rough and uneven surface of sandstone, which leads to interfacial thermal resistance problems, resulting in test results that are smaller than the actual values. This instrument can reasonably solve the problems caused by surface processing, and the test results are more consistent with the actual situation.
[0048] This embodiment provides the specific components, preparation method, and backfilling process of the thermal coupling agent as follows: This thermal coupling agent (bentonite slurry) is mainly composed of the following two substances: (bentonite powder: the main component is montmorillonite, with a particle size of 75µm. Liquid medium: pure water.) Core ratio: It is recommended to use a water:bentonite mass ratio of 3:1 for tests on 6mm diameter boreholes and 4mm diameter probes.
[0049] Preparation method: Mix the weighed bentonite powder with purified water. Stir thoroughly to ensure a uniform mixture. The final bentonite slurry should be viscous and fluid. This consistency balances fluidity and density, ensuring the probe can be inserted smoothly without the material becoming too thin and leaking out. After preparation, store in a sealed plastic container to prevent moisture evaporation and excessive thickening.
[0050] Backfilling process: The prepared viscous bentonite slurry is injected into the test borehole of the rock sample (the borehole diameter is 6 mm).
[0051] While the backfill material is still flowing, the test probe (4 mm in diameter) is smoothly inserted into the center of the borehole. The fluidity and viscosity of the backfill material ensure sufficient contact between the probe, the backfill material, and the borehole wall (rock). By eliminating air gaps between the probe and the borehole wall, the accuracy of the thermal conductivity test is ensured.
[0052] The principle for testing the thermal conductivity of weakly structured rock masses provided in this embodiment is as follows: The method for testing the thermal conductivity of weakly structured rock masses is based on a modified version of the unsteady-state line heat source theory, combined with the principle of the thermal probe method. First, an impact drill is used to drill a hole in the weakly structured rock sample. Bentonite mud coupling agent is backfilled into the hole, and the probe is placed at the center of the hole, allowing it to fully contact the rock through the coupling agent. The diameter d of the test probe is much smaller than its length l. When heating the probe with constant power, the heat transfer from the probe to the surrounding rock mass in the axial direction of the borehole is negligible compared to the heat transfer in the radial direction. The underground rock mass and temperature field are uniform, and the axial differences between the rock and soil are ignored. Furthermore, the thermal properties of the rock mass remain stable during the test, and heat transfer occurs through simple conduction, conforming to the assumptions of the line heat source theory. Therefore, the probe heat conduction problem can be simplified to an unsteady-state heat conduction problem of an infinitely long uniform heating body in an infinitely large medium.
[0053] Based on the above assumptions, using the logarithmic curve fitting method of the infinitely long line source model, the temperature analytical expression is: (1) In the formula, (t) represents the average temperature of the centerline heat source at time t, in °C; T0 represents the initial ground temperature of the soil and rock mass, in °C. λ is the borehole radius, m; t is the heat transfer time, s; q is the heating power per unit length, W / m; λ is the thermal conductivity of the rock and soil, W / (m·K). The thermal diffusivity of the rock and soil mass is m. 2 / s.
[0054] When the running time t is long enough, that is 2 / (4 t) is very small, and equation (1) can be approximately expressed as: (2) (3) (4) In the formula, The borehole thermal resistance is expressed as (m·K) / W. U is Euler's constant, taken as 0.5772; U is the heating voltage, V; R is the heating circuit resistance, Ω; L is the effective length of the probe, m.
[0055] As can be seen from equation (3), the average temperature of the centerline heat source is and The functional relationship satisfies a linear fit of the logarithmic function. The change in the average temperature of the centerline heat source is depicted as a curve representing the natural logarithm of time. When the heating voltage U is constant, the thermal conductivity of the soil and rock can be determined simply by finding the slope k of the fitted graph. (5) (6) Although this physical process is unsteady-state heat conduction, mathematically, when the heating time t is sufficiently long, the logarithmic relationship between temperature rise and time will tend towards a stable linear evolution, i.e., entering a quasi-steady-state stage. Since the temperature change in the initial stage of the experiment is mainly controlled by the backfill material rather than the underground soil and rock mass, to ensure the accuracy of the test data analysis, the linear heat source theory proposed by Hellström was used to calculate and correct for time t. Data before this time period was discarded in the data analysis to meet the requirements of a quasi-steady state. (7) Through experimental research, the instrument testing parameters were optimized. The instrument parameters for this embodiment are set as follows: 1. Heating voltage selection: For soils such as sand with a thermal conductivity of less than 1 W·m -1 ·K -1For soil and rock masses, the instrument should be used with a 6V heating voltage for testing; for clay and other materials with a thermal conductivity of 1~4 W·m -1 ·K -1 For soil and rock masses, to ensure the stability of test data and improve the accuracy of test results, the instrument should use a 12V heating voltage; and the thermal conductivity should be between 4 and 6 W·m. -1 ·K -1 For rock and soil masses, in order to enable the rock mass to exhibit good temperature rise changes and improve the accuracy of instrument testing, it is recommended to use a 15V heating voltage.
[0056] 2. Heating time determination: Based on the test results of the thermal conductivity of various types of rock and soil at different times, it can be seen that the test time of the thermal conductivity tester should be set to about 300 seconds.
[0057] 3. Selection of Thermal Coupling Agent: To investigate the influence of backfill material on the thermal conductivity test of the self-developed instrument, four sets of tests were set up according to different backfill materials. Before the test, a 200mm long hole was drilled in the limestone sample using an impact drill to ensure that the test probe could be fully inserted. The heating voltage for all tests was 15V, and the test time was 300s. Each set of tests was repeated three times, and the average value of the test results was taken. The "temperature rise-time natural logarithm" relationship curves of the limestone samples under different backfill materials are shown below. Figure 2 As shown.
[0058] The test data for each group of backfill materials were obtained by linear fitting of the temperature rise curves and calculation of thermal conductivity (see Table 1). Data analysis shows that different thermal coupling agents have a significant impact on the accuracy of the test results. Among them, when bentonite mud with a mass ratio of 3:1 was used as the borehole coupling backfill material, the test value was closest to the standard value, with a relative error of only 0.20%. This indicates that the mud with this ratio has good fluidity and thermal conductivity continuity, which can effectively fill the gap between the probe and the borehole wall and significantly reduce the interfacial thermal resistance. Therefore, when testing rock samples, using bentonite mud with a mass ratio of 3:1 as the borehole coupling backfill material is beneficial to improving the accuracy of the test results.
[0059] Table 1. Test results of thermal conductivity of limestone with different backfill materials
[0060] In practical applications, preliminary classification and characteristic analysis of the soil and rock materials are necessary before testing to estimate the possible range of their thermal conductivity, thereby selecting an appropriate heating voltage for thermal conductivity testing. Furthermore, for unknown soil and rock materials, a lower heating voltage can be used initially, and the voltage adjusted as needed based on temperature rise changes during the test. Based on these parameters, the thermal conductivity tester for weak-structured rock masses can ensure high stability and accuracy when testing different types of soil and rock masses.
[0061] The following is a specific implementation example of this application: like Figure 3 As shown, the thermal conductivity testing instrument for weakly structured rock masses provided in this embodiment includes a laptop computer 1, a 51 microcontroller 2, a data acquisition chip 3, a rock mass 4, a probe 5, copper powder 6, a sensor 7, backfill material 8, a nickel-chromium wire 9, a voltage regulator module 10, and a power supply 11. The probe 5 has an aspect ratio of 50, a total length of 200mm, a working area length of 192mm, an outer diameter of 4mm, an inner diameter of 3.2mm, and is made of stainless steel. A 0.2mm diameter, 35.2Ω / m enameled nickel-chromium wire 9 is wound around a 0.5mm diameter stainless steel rod and placed in the center of a stainless steel tube to form a heating module. The temperature sensor 7 is a Pt100 platinum resistance temperature sensor with a measurement accuracy of 0.01℃, placed in the middle of the inner wall of the probe 5. The gap in the middle of the probe 5 is filled with 300-500 mesh copper powder 6 to reduce the thermal resistance of the probe 5 itself. The probe 5 connector uses an SF6 five-pin aviation plug to connect the probe module to the external circuit, and finally seals the joint with Kraft K-704N silicone sealant. Before the test, a mobile power supply 11 provides a constant voltage to the instrument, and a regulated power supply 9 is connected to ensure a constant heating power of the system. A portable electric drill is used to drill a hole, and bentonite mud, mainly composed of montmorillonite, is used as a thermal coupling agent to backfill the hole to ensure that the probe 5 can be smoothly inserted into the rock sample. After the probe is inserted into the rock and left to stand for 5 minutes, the test begins. During the test, the microcontroller 2 controls the chip 3 to collect temperature data, receive and convert the electrical signals from the temperature sensor 7, and send them to the matching STC-ISP software via serial port for reading and storage. At the same time, the laptop computer 1 uses Python and the "least squares method" as the method for calculating thermal conductivity, embedding it into the data analysis software to read and process the temperature data file, and finally display the calculation results and fitted image data. During the experiment, it should be noted that after a set of experiments is completed, the temperature of the soil and rock sample will rise due to heating. The next set of experiments should be carried out only after the temperature of the test sample has returned to room temperature.
[0062] This instrument, based on a modified heat transfer calculation method combining a "line heat source + thermal coupling agent + soil-rock heat transfer," ensures full contact between the probe and the soil mass, reducing contact thermal resistance. It addresses the challenges of sampling and processing weakly structured soil masses, as well as the inaccuracy of traditional testing methods. The equipment is simple to operate, convenient to use, and provides rapid measurements, making it suitable for both indoor and field experiments. Furthermore, it overcomes the problem of interfacial thermal resistance caused by rough, uneven rock surfaces, resulting in test results that better reflect actual conditions.
[0063] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.
Claims
1. A method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line-source heat transfer theory, characterized in that, Includes the following steps: S10, a test hole is drilled in a weak rock mass using dry drilling technology, and a thermal coupling agent is backfilled into the test hole; S20, insert the thermal probe into the center of the test hole that has been backfilled with thermal coupling agent, so that the thermal probe comes into contact with the rock mass through the thermal coupling agent; S30, the thermal probe is heated with constant power, and data on the temperature change of the thermal probe over time is collected during the heating process; S40: From the collected data on temperature changes over time, select data segments whose heat transfer time meets the requirements of the quasi-steady-state heat transfer stage. S50, based on the selected data segment, according to the theory of unsteady linear heat source, the linear relationship between temperature and the natural logarithm of heat transfer time is fitted to obtain the slope of the fitted straight line, and the thermal conductivity of the weak structure rock mass is calculated based on the slope and the heating power.
2. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 1, characterized in that, In step S40, data segments whose heat transfer time meets the requirements of the quasi-steady-state heat transfer stage are selected. Specifically, data segments whose heat transfer time t satisfies the formula are selected. The data segment; where, For the test hole radius, The thermal diffusivity of the rock mass.
3. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 1, characterized in that, In step S50, the thermal conductivity is calculated based on the slope and heating power, specifically according to the formula... Calculate; where, Thermal conductivity, Let t be the average temperature of the thermal probe, and q be the heating power per unit length of the thermal probe.
4. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 3, characterized in that, In step S50, the heating power per unit length of the thermal probe is... According to the formula Calculated; where, The heating voltage of the thermal probe. To heat the resistance of the circuit, This is the effective heating length of the thermal probe.
5. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 4, characterized in that, In step S30, the heating voltage is selected based on the estimated thermal conductivity range of the weakly structured rock mass. ; When the estimated thermal conductivity is less than 1 W·m - ¹·K - ¹time, Using 6V; when the estimated thermal conductivity is 1 W·m - ¹·K - ¹~4 W·m - ¹·K - ¹When, Using 12V; when the estimated thermal conductivity is 4 W·m - ¹·K - ¹~6 W·m - ¹·K - ¹When, Uses 15V.
6. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 1, characterized in that, In step S10, the thermal coupling agent is bentonite slurry, which is made by mixing bentonite powder and pure water at a mass ratio of 3:
1.
7. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 6, characterized in that, In step S10, backfilling the test hole with thermal coupling agent specifically involves injecting the prepared viscous, fluid bentonite slurry into the test hole, and performing step S20 while the bentonite slurry is in a fluid state.
8. The method for testing the thermal conductivity of weakly structured rock masses based on the unsteady-state line source heat transfer theory as described in claim 1, characterized in that, The heating time in step S30 is set to 290 to 310 seconds.
9. A system for testing the thermal conductivity of weakly structured rock masses, characterized in that, To implement the test method according to any one of claims 1 to 8, comprising: The probe module includes a tube body, a heating module located at the center of the tube body, and a temperature sensor located inside the tube body; The data acquisition module is electrically connected to the temperature sensor and is used to acquire and convert temperature signals. The data analysis module is communicatively connected to the data acquisition module. It is used to receive temperature data, perform data filtering, fitting and calculation, and output the thermal conductivity result.
10. The thermal conductivity testing system for weakly structured rock masses according to claim 9, characterized in that, The data analysis module is a computer with data analysis software installed. The data analysis software is written in Python and incorporates the least squares method for data fitting and calculation.