Four-step inversion method of thermophysical properties based on analytical model and unsteady numerical model

Abstract

This invention discloses a four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models, comprising the following steps: 1) Using water temperature observation data after the 10th hour of the thermal response test, the thermal conductivity of the soil and rock is calculated using the linear slope method based on a linear heat source analytical model; 2) Using water temperature observation data after the 10th hour of the thermal response test and the obtained thermal conductivity of the soil and rock, assuming the thermal conductivity and volumetric specific heat of the backfill material in the borehole, the volumetric specific heat of the soil and rock is inverted based on a one-dimensional unsteady-state numerical model in the entire space; 3) Using water temperature observation data from the first two hours of the thermal response test, the obtained thermal conductivity and volumetric specific heat of the soil and rock, the thermal conductivity and volumetric specific heat of the backfill material in the borehole are simultaneously inverted based on a one-dimensional unsteady-state numerical model in the entire space; 4) Using water temperature observation data after the 10th hour of the thermal response test, the thermal conductivity of the soil and rock, the thermal conductivity and volumetric specific heat of the backfill material are obtained, and the volumetric specific heat of the soil and rock is inverted based on a one-dimensional unsteady-state numerical model in the entire space.

Description

Technical Field

[0001] This invention relates to the field of ground source heat pump heat exchange technology, and in particular to a four-step inversion method for thermophysical parameters based on a linear heat source analytical model of a vertically buried U-tube heat exchanger and a one-dimensional global unsteady-state numerical model for thermal response tests. Background Technology

[0002] Ground source heat pumps are a type of air conditioning system that utilizes shallow, renewable geothermal energy and offers advantages such as high efficiency, energy saving, and environmental friendliness.

[0003] Underground heat exchangers are a key component that distinguishes ground source heat pump air conditioning systems from conventional air conditioning systems. Vertically buried U-tube underground heat exchangers are the most commonly used type, and their design directly impacts the cost of ground source heat pump projects and their long-term operational safety and reliability. Scientific design of underground heat exchangers requires accurate thermal parameters both inside and outside the heat exchanger orifice. These parameters are typically obtained by acquiring water temperature through constant heat flow thermal response tests and then inverting the results using a specific mathematical model.

[0004] Current methods generally employ a simplified one-dimensional line heat source analytical model for inversion, with inversion parameters including three parameters: borehole thermal resistance, soil thermal conductivity, and volumetric specific heat. The borehole thermal resistance is derived from the assumption of steady-state heat transfer within the borehole in the line heat source model; it is not a thermophysical property parameter but rather a comprehensive parameter reflecting factors such as pipe spacing, PE pipe thermal conductivity, and backfill thermal conductivity. This steady-state heat transfer assumption leads to inaccuracies in the calculations of the line heat source analytical model for the first 10 hours, and inversion typically uses observation data from 10 hours onwards. Currently, thermal response tests generally only measure water temperature. Using only water temperature data is insufficient for inverting the borehole thermal resistance, soil thermal conductivity, and volumetric specific heat using the line heat source analytical model. The method described in the "Technical Specification for Ground Source Heat Pump System Engineering" GB50366-2005 (2009 edition) involves supplementing the borehole thermal resistance calculation formula and then inverting the soil thermal conductivity and volumetric specific heat based on the line heat source model. In this method, the calculation of the thermal resistance inside the borehole is unrelated to the observed data and requires the input of the thermal conductivity of the backfill material. However, the thermal conductivity of the backfill material is often not measured and is mostly estimated based on experience, which introduces uncertainty. The method in the "Technical Specification for Geotechnical Thermal Response Test of Buried Pipe Ground Source Heat Pump" (T / CECS 730-2020) is to first estimate the volumetric specific heat of the soil and rock by consulting data, and then inverse the thermal conductivity of the soil and rock and the thermal resistance inside the borehole based on a linear heat source model. In this method, the volumetric specific heat of the soil and rock obtained from geological data has great uncertainty, and relevant data may not be available. Patent CN 118797936 A provides a stepwise inversion method for the thermal conductivity and thermal diffusivity of the soil and rock, as well as the thermal conductivity and thermal diffusivity of the backfill material. The model used is an analytical model. The disadvantage is that it still uses the concept of steady-state thermal resistance of the U-shaped tube, that is, it does not consider the unsteady-state heat transfer characteristics of the pipe wall. Therefore, it is expected that the data in the initial stage of the simulated thermal response test (within 10 hours) will have a large deviation. Summary of the Invention

[0005] The purpose of this invention is to provide a four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models to address the above-mentioned shortcomings. This method solves the problem that the existing technology uses the concept of steady-state thermal resistance of U-tubes, does not consider the unsteady-state heat transfer characteristics of the tube wall, and has large deviations in the data of the initial stage of the simulated thermal response test.

[0006] This scheme employs a four-step inversion method for thermophysical parameters based on a thermal response test using an analytical model of the linear heat source of a vertically buried U-tube heat exchanger and a one-dimensional global unsteady-state numerical model. It assumes a constant heat flow thermal response test, and that the borehole depth, borehole diameter, pipe spacing, and inner and outer pipe diameters are determined during heat exchanger installation. Furthermore, it assumes that the thermal conductivity and volumetric specific heat of the PE pipes are available from the manufacturer, and the circulating water properties can be obtained from professional manuals. Test data includes initial ground temperature, heating amount, inlet and outlet water temperatures, and flow rate, with sampling intervals of 30-60 seconds. Based on these observations, four parameters are inverted, including the thermal conductivity λ of the soil and rock. s and specific heat ρ s c s The thermal conductivity λ of the backfill material b and specific heat ρ b c b .

[0007] Basic principle: The essence of thermophysical parameter inversion in thermal response tests is to find the minimum error in calculating water temperature: assuming several / groups of thermophysical parameters, using the measured inlet temperature as input, a mathematical model is used to simulate the outlet temperature, and then the root mean square deviation (RMS) of the simulated and measured outlet temperatures is calculated. The thermophysical parameter corresponding to the minimum SMS deviation is then identified as the parameter of the heat exchanger. The innovation of this invention lies in the combined application of a linear heat source analytical model and a full-space one-dimensional unsteady-state numerical model. Specifically, the linear heat source analytical model is used to invert the thermal conductivity of the soil and rock, and then the full-space one-dimensional unsteady-state numerical model is applied to obtain the thermal conductivity of the backfill material, the volumetric specific heat, and the volumetric specific heat of the soil and rock. This invention is achieved through the following scheme: The four-step inversion method for thermal properties based on analytical and unsteady-state numerical models includes the following steps: Includes the following steps: Step S100: Using the water temperature observation data after the 10th hour of the thermal response test, the thermal conductivity of the soil and rock is calculated using the linear slope method based on the linear heat source analytical model. Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained thermal conductivity of the soil and rock, assuming the thermal conductivity and volumetric specific heat of the backfill material in the hole, the volumetric specific heat of the soil and rock is inverted based on the one-dimensional unsteady numerical model of the whole space. Step S300: Using the water temperature observation data 2 hours before the thermal response test, the obtained thermal conductivity and volumetric specific heat of the soil and rock, and based on the one-dimensional unsteady-state numerical model in the whole space, the thermal conductivity and volumetric specific heat of the backfill material in the hole are inverted. Step S400: Using the water temperature observation data after the 10th hour of the thermal response test, the obtained thermal conductivity of the soil and rock, thermal conductivity of the backfill material, and volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on the one-dimensional unsteady-state numerical model in the whole space. Step S100 involves using water temperature observation data after the 10th hour of the thermal response test to calculate the thermal conductivity of the soil and rock using the linear slope method based on a linear heat source analytical model; specifically, it includes the following steps: Step S101, based on the inlet temperature recorded in the thermal response test... outlet temperature Observed values ​​were used to calculate the total heat exchange Q and the heat exchange per unit depth. And calculate the average temperature. :

[0008]

[0009] ; Step S102: Based on the analytical model of the vertically buried U-tube heat exchanger's line heat source, fit the average inlet and outlet temperatures of the heat exchanger after 10 hours. logarithm with time The linear relationship is: ; Step S103: Calculate the thermal conductivity of the soil and rock based on the slope of the straight line.

[0010] Where k is the slope of the line, m is the intercept of the line, Q is the total heat capacity of the heat exchanger, and L is the borehole depth. G represents heat exchanged per unit depth, and G represents flow rate. For specific heat of fluid, For the inlet temperature, For the outlet temperature, is the thermal conductivity of the soil and rock.

[0011] Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained soil-rock conductivity coefficients, assuming the thermal conductivity and volumetric specific heat of the backfill material in the borehole, the volumetric specific heat of the soil-rock is inverted based on a one-dimensional unsteady-state numerical model of the entire space; specifically including the following steps: Step S201: Assume a set of in-hole backfill materials with thermal conductivity and volumetric specific heat value; Step S202: Assuming the lower limit of the volumetric specific heat of soil and rock, and varying the interval, a set of volumetric specific heat values ​​for soil and rock is obtained. ; Step S203: Receive the thermal conductivity of the soil and rock from step S103. ; Step S204, for each soil volumetric specific heat With imported water temperature The "Water Temperature and Ground Temperature Calculation Program" is invoked as input to simulate the corresponding outlet water temperature; Step S205: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock:

[0012] in This represents the simulated outlet temperature at a certain moment. This represents the observed outlet temperature of the thermal response test at a certain moment; Step S206: Take the volumetric specific heat of the soil and rock corresponding to the minimum standard deviation of the outlet water temperature as the preliminary value.

[0013] Step S300 involves using water temperature observation data from two hours prior to the thermal response test, along with the obtained thermal conductivity and volumetric specific heat of the soil and rock, to simultaneously invert the thermal conductivity and volumetric specific heat of the backfill material within the borehole based on a one-dimensional unsteady-state numerical model. This specifically includes the following steps: Step S301: Assume the upper and lower limits of the thermal conductivity of the backfill material in the hole, as well as the variation interval; Step S302: Assume the lower limit and upper limit of the volumetric specific heat of the backfill soil in the borehole, as well as the interval of variation. Step S303: Receive the thermal conductivity of the soil and rock from step S103 and the preliminary value of the specific heat of the soil and rock from step S206. Step S304: For each group of backfill material, the thermal conductivity and volumetric specific heat are calculated using the inlet water temperature as input. The "Water Temperature and Ground Temperature Calculation Program" is then called to simulate the corresponding outlet water temperature. Step S305: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to the thermal conductivity and volumetric specific heat of each group of backfill material. Step S306: Take the thermal conductivity and volumetric specific heat of the backfill material corresponding to the minimum standard deviation of the outlet water temperature as the estimated value.

[0014] Step S400: Using the water temperature observation data after 10 hours of the thermal response test, the obtained thermal conductivity of the soil and rock, the thermal conductivity of the backfill material, and the volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on a one-dimensional unsteady-state numerical model in full space; specifically, the following steps are included: Step S401: Receive the thermal conductivity and volumetric specific heat value of the backfill material in the hole from step S300. Step S402: Receive the thermal conductivity of the soil and rock from step S100; Step S403: Assume the lower limit and upper limit of the volumetric specific heat of rock and soil, and the variation interval, to obtain a set of volumetric specific heat values ​​of rock and soil. Step S404: For each volumetric specific heat of rock and soil, the "Water Temperature and Ground Temperature Calculation Program" is called with the inlet water temperature as input to simulate the corresponding outlet water temperature. Step S405: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock. Step S406: Take the volumetric specific heat of the soil and rock corresponding to the minimum root mean square deviation of the outlet water temperature as the final estimated value.

[0015] In step S201, the assumed thermal conductivity of the in-hole backfill is... and specific heat value ; In step S202, the lower limit of the assumed volumetric specific heat of the soil is... The upper limit is The interval of change is A set of specific heat values ​​of rock and soil were obtained. .

[0016] In step S301, the upper limit of the assumed thermal conductivity of the in-hole backfill is... 3.0, lower limit is 0.1, with a variation interval of .

[0017] In step S302, the lower limit of the assumed volumetric specific heat of the backfill soil in the borehole is... The upper limit is The interval of change is .

[0018] The specific method based on the unsteady numerical model includes the following steps: Step S1: Design a mesh generation method to establish a radial one-dimensional Nr node unsteady diffusion numerical model of the soil and rock region outside the pore backfill of the underground heat exchanger. Step S2: Design a mesh generation method, establish a one-dimensional 2Nf node unsteady convection-diffusion numerical model of fluid along the flow direction, and establish a one-dimensional two-node unsteady heat conduction numerical model of pipe wall radial direction. Step S3: Design a hybrid numerical scheme that combines implicit and explicit representations to achieve coupled calculation of unsteady heat transfer between fluid + PE pipe and backfill + soil. Step S4: Design a two-fold iterative algorithm for the numerical model in step S2 to solve the fluid temperature inside the PE pipe and the inner wall temperature at each time step. Step S5: Design an algorithm based on the implicit and explicit hybrid numerical format to realize the sequential simulation of "receiving inlet temperature → calculating fluid friction temperature and pipe wall temperature → calculating heat transfer on the outer wall of the pipe → calculating the temperature of the solid region outside the pipe".

[0019] Step S1 specifically includes the following steps: The backfill material inside the borehole and the soil and rock outside the borehole are divided into the following radial directions: Two units are fixedly installed for the backfill material inside the hole, and after the third unit ( The borehole is a geotechnical unit. The first unit at the borehole center is a solid cylinder of height L, and the remaining units are annular cylinders. The center coordinates of the second unit of the backfill material are set at the center of the PE pipe. The interface between the second unit and the first unit is set in... At this location, the interface between the second and third units is set at the hole wall r= The geotechnical unit interface is located at the center of the two nodes. For a single hole, the radius of the far boundary can be 10m or 20m. For a group of holes, the radius of the far boundary can be 1 / 2 of the hole spacing. The boundary conditions are set as follows: the upper, lower and far boundaries of the heat exchanger calculation area are considered as adiabatic boundaries. Internal heat source: The internal heat source is only set in the second unit of the backfill. According to the innovative idea of ​​"spatial separation and physical coupling", the single / double U-shaped vertical buried pipe is abstracted as 2 / 4 of the linear heat sources without shape and volume. The heat generation of the internal heat source of the backfill unit 2 is equal to the heat of the outer wall of the pipe.

[0020] Step S2 specifically includes the following steps: The inlet and outlet branches of the U-shaped tube are respectively set along the flow direction. There are 1 node, and 1 group of U-shaped tubes. The fluid node, the first Units are evenly distributed, j=1 and The unit is a control body of 0; The PE pipe wall is divided into sections corresponding to fluid nodes in the flow direction. Each section is insulated from the others; each section of PE pipe has two nodes fixed radially on its wall, one on the inner wall and one on the outer wall; the heat from the outer wall of the pipe is used as the internal heat source for the backfill unit 2; the temperature of the outer wall node is set to be equal to the temperature of the backfill unit 2.

[0021] In summary, due to the adoption of the above technical solution, the beneficial effects of this solution are: (1) More inversion parameters than traditional methods: Traditional methods invert three parameters: thermal resistance inside the borehole, thermal conductivity of soil and rock, and volumetric specific heat. This method inverts four parameters: thermal conductivity of backfill material inside the borehole, volumetric specific heat, thermal conductivity of soil and rock, and volumetric specific heat.

[0022] (2) Lower uncertainty in parameter inversion: Traditional methods either use formulas and parameters unrelated to observation to estimate the thermal resistance inside the borehole, or estimate the volumetric specific heat of the soil and rock by consulting data, both of which involve uncertainty. This method is based entirely on the in-situ observation of water temperature, flow rate, and heat to invert the thermal conductivity and volumetric specific heat of the backfill material inside the borehole. There are a total of 4 parameters, including the thermal conductivity and volumetric specific heat of the soil and rock, which reduces uncertainty. This method uses an exhaustive method to invert the thermophysical parameters, avoiding local extrema.

[0023] (3) Short-time step calculation is more accurate: The traditional method uses the assumption of steady-state heat transfer inside the hole, and the calculation of short-time step unsteady-state response is inaccurate. This method uses a full-space unsteady-state numerical model, which can accurately simulate the short-time step unsteady-state response. Figure 6 The results were verified using thermal response experimental data with a sampling interval of 30 seconds. The same data was input for calculation. The results showed that the outlet temperature calculated by the internationally renowned software trnsys type557a was generally higher than expected in the first 10 hours, with a maximum deviation of 2.6℃ and a root mean square deviation of 0.45℃. The new model of this invention was consistent with the measured values ​​throughout the process, with an initial maximum deviation of only 0.5℃ and a root mean square deviation of 0.05℃.

[0024] (4) More efficient use of observation data: The model based on the steady-state heat transfer assumption inside the hole cannot accurately calculate the water temperature change in the first 10 hours, so the inversion uses data after 10 hours. This method uses a full-space unsteady-state numerical model, which can accurately simulate short-time step response, so it can make full use of the observation data in the first 2 hours of thermal response for inversion. Attached Figure Description

[0025] Figure 1 A schematic diagram of the grid for one-dimensional thermal conductivity numerical calculation of boreholes and soil / rock. Figure 2 A schematic diagram of the numerical calculation grid for one-dimensional axial convection diffusion of fluid and one-dimensional radial heat conduction of pipe wall; Figure 3 This is a schematic diagram illustrating the calculation of the thermal conductivity of soil and rock based on the slope of the logarithmic line of average water temperature versus time using an analytical model with a linear heat source. Figure 4 This is a schematic diagram illustrating the root mean square error of outlet water temperature calculated based on a global one-dimensional numerical model under different volumetric specific heats of soil and rock. Figure 5 To calculate the root mean square error of outlet water temperature under different thermal conductivity and volumetric specific heat of backfill material based on a global one-dimensional unsteady-state numerical model; Figure 6 A schematic diagram of short-time step calculation verification of a one-dimensional implicit-explicit hybrid numerical model of a U-tube heat exchanger; Detailed Implementation All features disclosed in this specification, or all steps in all disclosed methods or processes, may be combined in any way, except for mutually exclusive features and / or steps.

[0026] Any feature disclosed in this specification (including any appended claims and abstract) may be replaced by other equivalent or similar features, unless specifically stated otherwise. That is, unless specifically stated otherwise, each feature is merely one example of a series of equivalent or similar features.

[0027] In the description of this invention, it should be understood that the terms "upper," "lower," "left," "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a predetermined orientation, or be constructed and operated in a predetermined orientation. Therefore, they should not be construed as limitations on this invention.

[0028] Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature.

[0029] Example 1 like Figures 1-6 As shown, the present invention provides a technical solution: The four-step inversion method for thermal properties based on analytical and unsteady-state numerical models includes the following steps: Step S100: Using the water temperature observation data after the 10th hour of the thermal response test, the thermal conductivity of the soil and rock is calculated using the linear slope method based on the linear heat source analytical model. Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained thermal conductivity of the soil and rock, assuming the thermal conductivity and volumetric specific heat of the backfill material in the hole, the volumetric specific heat of the soil and rock is inverted based on the one-dimensional unsteady numerical model of the whole space. Step S300: Using the water temperature observation data 2 hours before the thermal response test, the obtained thermal conductivity and volumetric specific heat of the soil and rock, and based on the one-dimensional unsteady-state numerical model in the whole space, the thermal conductivity and volumetric specific heat of the backfill material in the hole are inverted. Step S400: Using the water temperature observation data after the 10th hour of the thermal response test, the obtained thermal conductivity of the soil and rock, thermal conductivity of the backfill material, and volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on the one-dimensional unsteady numerical model in the whole space. Step S100 involves using water temperature observation data after the 10th hour of the thermal response test to calculate the thermal conductivity of the soil and rock using the linear slope method based on a linear heat source analytical model; specifically, it includes the following steps: Step S101, based on the inlet temperature recorded in the thermal response test... outlet temperature Observed values ​​were used to calculate the total heat exchange Q and the heat exchange per unit depth. And calculate the average temperature. :

[0030]

[0031]

[0032] Step S102: Based on the analytical model of the vertically buried U-tube heat exchanger's line heat source, fit the average inlet and outlet temperatures of the heat exchanger after 10 hours. logarithm with time The linear relationship is: (like Figure 3 (As shown) Step S103: Calculate the thermal conductivity of the soil and rock based on the slope of the straight line.

[0033] Where k is the slope of the line, m is the intercept of the line, Q is the total heat capacity of the heat exchanger, and L is the borehole depth. G represents heat exchanged per unit depth, and G represents flow rate. For specific heat of fluid, For the inlet temperature, For the outlet temperature, is the thermal conductivity of the soil and rock.

[0034] Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained soil-rock conductivity coefficients, assuming the thermal conductivity and volumetric specific heat of the backfill material in the borehole, the volumetric specific heat of the soil-rock is inverted based on a one-dimensional unsteady-state numerical model of the entire space; specifically including the following steps: Step S201, assuming a set of in-hole backfill material thermal conductivity and volumetric specific heat value, for example... , ; Step S202, assuming the lower limit of the volumetric specific heat of soil and rock (e.g.) ) and upper limits (e.g.) ), variation interval (e.g. A set of specific heat values ​​of rock and soil volume were obtained. ; Step S203: Receive the thermal conductivity of the soil and rock from step S103. ; Step S204, for each soil volumetric specific heat With imported water temperature The "Water Temperature and Ground Temperature Calculation Program" is invoked as input to simulate the corresponding outlet water temperature; Step S205: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock:

[0035] in This represents the simulated outlet temperature at a certain moment. This represents the observed outlet temperature of the thermal response test at a certain moment; Step S206: Take the volumetric specific heat of the soil and rock corresponding to the minimum root mean square deviation of the outlet water temperature as the preliminary value (e.g., Figure 4 (As shown).

[0036] Step S300 involves using water temperature observation data from two hours prior to the thermal response test, along with the obtained thermal conductivity and volumetric specific heat of the soil and rock, to simultaneously invert the thermal conductivity and volumetric specific heat of the backfill material within the borehole based on a one-dimensional unsteady-state numerical model. This specifically includes the following steps: Step S301, assuming the upper limit of the thermal conductivity of the backfill material inside the hole (e.g., 3.0), lower limit (e.g.) 0.1), and the interval of change (e.g., ).

[0037] Step S302, assuming the lower limit of the volumetric specific heat of the backfill soil in the borehole, for example... and upper limits, for example and the interval of change, for example .

[0038] Step S303: Receive the thermal conductivity of the soil and rock from step S103 and the preliminary value of the specific heat of the soil and rock from step S206.

[0039] Step S304: For each group of backfill material, the thermal conductivity and volumetric specific heat are calculated using the inlet water temperature as input, and the "water temperature and ground temperature calculation program" is called to simulate the corresponding outlet water temperature.

[0040] Step S305: Calculate the root mean square difference between the simulated and measured values ​​of the outlet water temperature corresponding to the thermal conductivity and volumetric specific heat of each group of backfill material.

[0041] Step S306: Take the thermal conductivity and volumetric specific heat of the backfill material corresponding to the minimum root mean square deviation of the outlet water temperature as estimated values ​​(e.g., Figure 5 (As shown).

[0042] Step S400: Using the water temperature observation data after 10 hours of the thermal response test, the obtained thermal conductivity of the soil and rock, the thermal conductivity of the backfill material, and the volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on a one-dimensional unsteady-state numerical model in full space; specifically, the following steps are included: Step S401: Receive the thermal conductivity and volumetric specific heat value of the backfill material in the hole from step S300. Step S402: Receive the thermal conductivity of the soil and rock from step S100; Step S403: Assume the lower limit and upper limit of the volumetric specific heat of rock and soil, and the variation interval, to obtain a set of volumetric specific heat values ​​of rock and soil. Step S404: For each volumetric specific heat of rock and soil, the "Water Temperature and Ground Temperature Calculation Program" is called with the inlet water temperature as input to simulate the corresponding outlet water temperature. Step S405: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock. Step S406: Take the volumetric specific heat of the soil and rock corresponding to the minimum root mean square deviation of the outlet water temperature as the final estimated value (e.g., Figure 3 (As shown).

[0043] The core of this scheme is: calculating the thermal conductivity of soil and rock based on the linear slope method of the linear heat source model → initially estimating the volumetric specific heat of soil and rock based on the one-dimensional unsteady-state model of the whole space using the single variable exhaustive method → ​​estimating the thermal conductivity and volumetric specific heat of backfill material based on the one-dimensional unsteady-state model of the whole space using the bivariate exhaustive method → ​​re-estimating the volumetric specific heat of soil and rock based on the one-dimensional unsteady-state model of the whole space using the single variable exhaustive method. exist Figure 6 Input data: sampling interval 30s, original ground temperature 19℃, flow rate 1.2m³ / h, electric heater power 6kW, hole depth 120m, hole diameter 0.17m, pipe spacing 0.1m, pipe outer diameter 0.032m, pipe inner diameter 0.025m, pipe thermal conductivity 0.4W / (m·K), pipe volumetric specific heat 2MJ / (m³·K), backfill thermal conductivity 1.87W / (m·K), backfill volumetric specific heat 0.3MJ / (m³·K), soil thermal conductivity 1.94W / (m·K), soil volumetric specific heat 0.7MJ / (m³·K).

[0044] Example 2 This invention provides a technical solution: The hybrid scheme algorithm for borehole heat exchangers based on the concept of spatial separation and physical coupling, namely the unsteady-state numerical model-based method in Example 1, includes the following steps: Step S1: Design a mesh generation method to establish a radial one-dimensional Nr node unsteady diffusion (heat conduction) numerical model (Model 1) of the soil and rock region outside the pore backfill of the underground heat exchanger. Step S2: Design a mesh generation method to establish a one-dimensional 2Nf node unsteady convection-diffusion numerical model of the fluid along the flow direction (Model 2.1), and establish a one-dimensional two-node unsteady heat conduction numerical model of the pipe wall radial direction (Model 2.2). Step S3: Design a hybrid numerical scheme that combines explicit and implicit calculations to achieve coupled calculation of unsteady heat transfer between (fluid + pipe wall) and (backfill + soil). Step S4: Design a two-fold iterative algorithm for Model 2 to solve the fluid temperature inside the PE pipe and the pipe wall temperature simultaneously at each time step. Step S5: Design an algorithm based on the implicit and explicit hybrid numerical format to realize the sequential simulation of "receiving inlet temperature → calculating fluid friction temperature and pipe wall temperature → calculating heat transfer on the outer wall of the pipe → calculating the temperature of the solid region outside the pipe".

[0045] The core of this scheme is "spatial separation and physical coupling", which means that the U-shaped tube perforated heat exchanger is spatially separated into two large independent regions (backfill + soil) and (fluid + tube wall). The two large regions are modeled separately, while ensuring the coordinated coupling of temperature and heat parameters at the interface between the two regions.

[0046] Step S1 specifically includes the following steps: Design a mesh generation method to establish a radial one-dimensional N-dimensional grid for the backfill material inside the borehole and the soil and rock region outside the borehole of a vertically buried U-shaped tube heat exchanger. r Node numerical computation model (referred to as Model 1:) The backfill material inside the borehole and the soil and rock outside the borehole are divided into the following radial directions: Two units are fixedly installed for the backfill material inside the hole, and after the third unit ( The borehole is a geotechnical unit. The first unit at the borehole center is a solid cylinder of height L, and the remaining units are annular cylinders. The center coordinates of the second unit of the backfill material are set at the center of the PE pipe. The interface between the second unit and the first unit is set in... At this location, the interface between the second and third units is set at the hole wall r= The interface between the geotechnical units is located at the center of the two nodes. For a single borehole, the radius of the far boundary can be 10m or 20m; for a group of boreholes, the radius of the far boundary can be half the distance between boreholes.

[0047] The boundary conditions are set as follows: the upper, lower, and far boundaries of the heat exchanger calculation region are considered as adiabatic boundaries.

[0048] Internal heat source: The internal heat source is only set in the second unit of the backfill. Based on the innovative idea of ​​"spatial separation and physical coupling", the single / double U-shaped vertical buried pipe is abstracted as 2 / 4 of the linear heat sources without shape and volume; the heat generation of the internal heat source of the backfill unit 2 is equal to the heat of the outer wall of the pipe.

[0049] The impact of Unit 2 on the pipeline: The average temperature of Unit 2 is used as the outer wall temperature of the pipeline.

[0050] Step S2 specifically includes the following steps: A special mesh generation method is designed to establish Model 2, which includes a one-dimensional 2N fluid axis. f Numerical model of nodes (Model 2.1) and one-dimensional two-node numerical model of the pipe wall radial direction (Model 2.2): The inlet and outlet branches of the U-shaped tube are respectively set along the flow direction. There are 1 node, and 1 group of U-shaped tubes. The fluid node, the first Units are evenly distributed, j=1 and The unit is a control body of 0; The PE pipe wall is divided into sections corresponding to fluid nodes in the flow direction. Each PE pipe segment is insulated from the others. Two nodes are fixed radially on each segment's wall: one on the inner wall and one on the outer wall. The heat from the outer wall serves as the internal heat source for the backfill unit 2; the temperature of the outer wall nodes is set equal to the temperature of the backfill unit 2.

[0051] Step S3 specifically includes the following steps: Designing a special hybrid numerical scheme to achieve unsteady heat transfer coupling calculations between (fluid + PE pipe) and (backfill + soil): Numerical Model 1 (Backfill + Soil): All nodes of backfill material inside the borehole and soil outside the borehole ( An explicit numerical discretization equation for unsteady thermal conduction was established using the finite volume method to calculate the radial temperature of the backfill material and soil. When establishing the numerical equation for the temperature of the second unit of the in-hole backfill material, single / double U-shaped vertical buried pipes were simplified to equivalent to 2 / 4 of a line heat source, disregarding their shape and volume, and their internal heat source was set to equal the heat released / absorbed by the outer wall of the U-shaped pipe. An explicit scheme was used to establish the nodal temperature calculation formula to improve the overall solution speed.

[0052] The formulas for calculating the temperature at each node are as follows: (1) The temperature calculation formula for borehole center unit i=1 is:

[0053]

[0054]

[0055]

[0056]

[0057]

[0058]

[0059] in This represents the current borehole center temperature. The borehole center temperature at the previous moment. The temperature of the second element node at the previous time step. The specific heat of the backfill material is the volumetric heat. The thermal conductivity of the backfill material. For the volume of unit 1, The radius of the location of unit 2. Let L be the radius of the interface between element 1 and element 2, and L be the drilling depth. The diameter of the borehole. For pipe spacing, This refers to the outer diameter of the PE pipe.

[0060] (2) The formula for calculating the temperature of the i=2th unit inside the hole is:

[0061]

[0062]

[0063]

[0064]

[0065]

[0066]

[0067] in This represents the current temperature of the second drilling unit. This represents the temperature of the second drilling unit at the previous moment. The temperature of the third unit node at the previous time step. For the volume of unit 2, The radius of the interface between element 2 and element 3 is the radius of the hole wall. The total heat released / absorbed by the outer wall of the PE pipe. The equivalent thermal conductivity at the borehole wall is obtained by averaging the thermal conductivity of the backfill material and the thermal conductivity of the soil and rock.

[0068] (3) The temperature calculation formula for the i=3rd unit is:

[0069]

[0070]

[0071]

[0072]

[0073]

[0074] in The temperature of the third unit at the current moment. This refers to the temperature of the third unit at the previous moment. The temperature of the 4th unit node at the previous moment. For the volume of unit 3, Let be the radius of the interface between element 3 and element 4. The equivalent thermal conductivity at the borehole wall is obtained by averaging the thermal conductivity of the backfill material and the thermal conductivity of the soil and rock. The thermal conductivity of the soil and rock is... It is the volumetric specific heat of rock and soil.

[0075] (4) No. The temperature calculation formula for each unit is:

[0076]

[0077]

[0078]

[0079]

[0080]

[0081] in Let i be the temperature of the i-th unit at the current moment. Let i be the temperature of the i-th unit at the previous time step. The temperature of the (i+1)th cell node at the previous time step. Let i be the volume of element i. Let be the radius of the interface position between element i and element i+1.

[0082] (5) No. The temperature calculation formula for each unit is:

[0083]

[0084]

[0085]

[0086]

[0087]

[0088] in Let i be the temperature of the i-th unit at the current moment. Let i be the temperature of the i-th unit at the previous time step. Let i be the volume of element i. Let be the radius at the position of element node i.

[0089] 2. (Fluid + PE pipe) Numerical Model 2: The temperature of the fluid node inside the PE pipe of the vertically buried U-shaped underground heat exchanger is solved by establishing a discrete equation using the fully implicit QUICK scheme, and the temperature of node 1 on the inner wall of the PE pipe is solved by establishing a discrete equation using the implicit-explicit hybrid scheme.

[0090] The inlet temperature of the inlet branch pipe is known, and the outlet node j=Nf is the outflow boundary. The inlet temperature of the outlet branch pipe is equal to the outlet temperature of the inlet branch, and its outlet node j=1 is the outflow boundary.

[0091] Model coupling method: When establishing the numerical equations for the fluid and inner wall temperature in the PE pipe, the fluid and inner wall temperatures are taken at the same moment, while the outer wall temperature is set to be equal to the temperature of the backfill unit 2 in the hole at the previous moment. The inner wall nodes and fluid nodes have an implicit relationship, which is solved using simultaneous iterative solutions to ensure convergence. The inner wall nodes and outer wall nodes have an explicit relationship, realizing the separate solution of the PE pipe and the external region, thus improving the overall calculation speed.

[0092] Heat exchange on the inner wall of the tube Based on Newton's law of cooling, the current temperature of the inner wall nodes is used. With the corresponding fluid node temperature Calculations are performed. The convective heat transfer coefficient of the pipe is calculated using the classical Dittus correlation in heat transfer. (Pipe outer wall...) Heat absorption / release is measured using the current temperature of the inner wall nodes. and the temperature of the outer wall node at the previous moment calculate.

[0093] The numerical discrete equations for the fluid and the temperature at the joints on the inner wall of the PE pipe, as well as the heat calculation formulas, are as follows: (1) The inlet branch node j=1, (Known); (2) The second fluid node j=2 at the inlet branch has the following numerical discretization equation:

[0094]

[0095]

[0096]

[0097]

[0098]

[0099]

[0100]

[0101] in Let J be the temperature of the j-th fluid element at the current moment. Let j be the temperature of the j-th fluid element at the previous moment. The temperature of the upstream fluid unit at the current moment. The temperature of the downstream fluid unit at the current moment. The temperature of the PE pipe inner wall corresponding to fluid element j at the current moment is... Let j be the length of the fluid element. The distance between unit node j and upstream node j-1, Let P be the distance between node j and downstream node j+1, P be the perimeter of the pipe cross-section, A be the cross-sectional area of ​​the pipe, and u be the current flow velocity in the pipe. Specific heat of fluid volume Let h be the thermal conductivity of the fluid and h be the heat transfer coefficient of the inner wall surface of the pipe. The calculation is performed using the classical Dittus correlation in heat transfer.

[0102]

[0103]

[0104] in Where is the fluid viscosity, Nu is the Nusselt number, Pr is the Prandtl number, and Re is the Reynolds number. For fluid density, The specific heat of the fluid is n, which is 0.4 when the fluid is heated and 0.3 when the fluid is cooled.

[0105] (3) Import support node The numerical discrete equation is:

[0106]

[0107]

[0108]

[0109]

[0110]

[0111]

[0112]

[0113]

[0114] in Let J be the temperature of the j-th fluid element at the current moment. Let j be the temperature of the j-th fluid element at the previous moment. The temperature of the upstream fluid unit at the current moment. The temperature of the downstream fluid unit at the current moment. The temperature of the PE pipe inner wall corresponding to fluid element j at the current moment is... Let j be the length of the fluid element. The distance between unit node j and upstream node j-1, Let be the distance between element node j and downstream node j+1, h be the heat transfer coefficient of the inner wall of the pipe, P be the perimeter of the inner wall of the pipe cross-section, A be the cross-sectional area of ​​the pipe, and u be the current flow velocity inside the pipe. Specific heat of fluid volume is the thermal conductivity of the fluid.

[0115] (4) Import support The temperature of a node is obtained by fitting the temperature values ​​of the three upstream nodes to a quadratic curve and extrapolating it.

[0116] (5) Export support Each node has a temperature equal to the outlet temperature of the inlet branch.

[0117] (6) Export support The numerical discrete equation for the first node is the same as that for the second node of the inlet support.

[0118] (7) Export support The numerical discrete equations of the nodes and the import support are related to the first node. The equations for each node are the same.

[0119] (8) Export support The temperature of a node is obtained by fitting the temperature values ​​of the three upstream nodes to a quadratic curve and extrapolating it.

[0120] (9) With fluid nodes The temperature discretization equation for the corresponding PE pipe inner wall node is as follows:

[0121]

[0122]

[0123]

[0124]

[0125] in Let J be the temperature of the inner wall of the j-th fluid element at the current moment. Let be the pipe wall temperature corresponding to the j-th fluid unit at the previous moment. Let J be the temperature of the j-th fluid element at the current moment. The temperature of the outer wall of the pipe corresponding to the j-th fluid unit at the previous moment is... , Let be the length of fluid element j, h be the heat transfer coefficient of the inner wall surface of the pipe, P be the perimeter of the inner wall of the pipe cross-section, and A be the cross-sectional area of ​​the pipe. For the volumetric specific heat of PE, The thermal conductivity of PE is... Let be the volume of the inner wall unit of the PE pipe corresponding to the j-th fluid unit. The relationship between the inner wall nodes and the fluid nodes is implicit, while the relationship with the outer wall nodes is explicit.

[0126] (10) The heat exchange capacity of the inner wall of each axial unit of the PE pipe is:

[0127] (11) The heat exchange of the outer wall of each axial unit of the PE pipe is:

[0128] (12) The total heat exchange capacity provided by the temperature difference between the inlet and outlet of the U-shaped tube is:

[0129] Where G is the mass flow rate. For the inlet temperature, This refers to the outlet temperature.

[0130] Step S4 specifically includes the following steps: designing a special "double iteration" algorithm to simultaneously solve for the fluid temperature inside the PE pipe and the pipe wall temperature at each time step; based on the above numerical model, the solution steps include: Step S41: Receive drilling depth (L) and PE pipe outer diameter (d) o ) and inner diameter (d) i and mesh parameters; Step S42: Receive the thermal conductivity (λ) of the PE pipe. p ) and volumetric specific heat (ρ p c p ), fluid thermophysical parameters (including density (ρ) f ), specific heat (c f Viscosity (μ) f )wait); Step S43: Receive the inlet temperature (T) of the U-tube. in ), flow rate (F), initial temperature of soil and rock ( ), initial fluid temperature ), initial temperature of the inner wall of the pipe ( ) and other parameters; Step S44: Receive the convective heat transfer coefficient h of the tube wall; Step S45: Assume the inner wall temperature of the PE pipe ; Step S46: Calculate the coefficients of the discrete equations for the fluid element using the QUICK scheme; Step S47: Solve for the fluid element temperature using relaxation iteration (relaxation factor = 0.5). ; Step S48: Based on fluid element temperature And the temperature of the outer wall of the tube at the previous moment. Calculate the inner wall temperature of the PE pipe ; Step S49: Based on step S45 ; and in step eight Update the PE pipe inner wall temperature with a relaxation factor of 0.5. ; Step S410: Calculate the temperature error of the inner wall of the PE pipe. ; Step S411: If the maximum error If the precision is greater than the set precision eps=0.001, use Alternative Proceed to step S46; otherwise, end the iteration and proceed to step S412. Step S412: Determine the fluid temperature at the current moment. Inner wall temperature Calculate the heat exchange of the inner wall of the pipe ; Step S413: Determine the current inner wall temperature The temperature of the outer wall at the previous moment Calculate the heat released / absorbed by the outer wall of the tube. ; Step S414: The fluid inlet temperature Tin and outlet temperature... Calculate the total heat of the heat exchanger .

[0131] Steps S45 to S411 constitute the first external iteration, which is used to solve for the inner wall temperature of the pipe. Step S47 constitutes the second internal iteration, which is used to solve for the fluid temperature. The two iterations bring the fluid temperature and the inner wall temperature of the pipe closer together.

[0132] Step S5 specifically includes the following steps: Design a special algorithm to realize the sequential simulation of "receiving inlet temperature → calculating fluid friction temperature and pipe wall temperature → calculating heat transfer on the outer wall of the pipe → calculating the temperature of the solid region outside the pipe". The basis for the sequential algorithm is that the inner wall nodes and outer wall nodes of the PE pipe are explicitly related. The steps include: Step S51: Receive the borehole diameter (D) b ), the spacing between the U-shaped tubes inside the hole (D) p ), Drilling depth (L), Drilling spacing (D), PE pipe outer diameter (d) o ) and inner diameter (d) i ), mesh parameters; Step S52: Receive the thermal conductivity (λ) of the soil and rock. s ) and volumetric specific heat (ρ s c s ), thermal conductivity of borehole backfill (λ) b ) and volumetric specific heat (ρ b c b ), thermal conductivity of PE pipe (λ) p ) and volumetric specific heat (ρ p c p ), fluid thermophysical parameters (including density (ρ) f ), specific heat (c f Viscosity (μ) f )wait); Step S53: Receive the current temperature (T) at the inlet of the U-tube. in ), flow rate (F), initial temperature of soil and rock ( ), initial temperature of pipe wall ( ), initial fluid temperature ) and other parameters; Step S54: Calculate the tube wall convective heat transfer coefficient at the current moment using the Dittus convective heat transfer correlation in the tube; Step S55: Apply the "two-iteration algorithm" to solve for the fluid friction temperature distribution T at the current moment. f and the temperature of the inner wall of the PE pipe ( ), and calculate the total heat capacity Q of the heat exchanger. f Heat Q of the inner wall of PE pipe fp and the heat Q of the outer wall pb ; Step S56: Based on the heat exchange rate Q of the PE pipe outer wall in step S55 pb and the initial temperature of the soil and rock ( The temperature T of the solid region outside the pipe at the current moment is calculated using the "backfill + explicit numerical model of soil and rock". Step S57: Utilize the current soil and rock temperature T and the current fluid temperature T f and the current pipe wall temperature ( ), respectively update the initial temperature of the soil and rock ( ), initial fluid temperature ), initial temperature of pipe wall ( Proceed to step S53 to begin the calculation of the next time step, until all time steps have been calculated.

[0133] The core of this plan is: A special method for numerical calculation mesh generation of the in-hole backfill material and the external soil and rock area was adopted. Figure 2 The backfill material and soil in the borehole are divided into the following categories along the radial direction: The borehole backfill material consists of two fixed units, followed by soil and rock units. The first unit at the borehole center is a solid cylinder of height L, while the remaining units are annular cylinders. The center coordinates of the second unit of the borehole backfill material are set at the center of the PE pipe (r= The interface between the second unit and the first unit is set in... The interface between the second and third units is set at the hole wall, r= .

[0134] A special U-shaped pipe fluid and pipe wall numerical calculation mesh generation method was also adopted. Figure 3 ): The inlet and outlet branches of the U-shaped tube are respectively set along the flow direction. There are several nodes. The PE pipe wall is divided into several sections in the flow direction. Each section of the PE pipe is insulated from the others. Two nodes are fixed radially on each section of the PE pipe wall: one node on the inner wall and one node on the outer wall.

[0135] It also employs a special numerical scheme that combines fluid, pipe wall, backfill material, and soil / rock sedimentation. (1) One-dimensional backfill material + soil numerical model 1: Backfill material inside the borehole and all joints of soil and rock outside the borehole ( The finite volume method was used to establish an explicit numerical discrete equation for unsteady heat conduction, enabling the calculation of radial temperature of backfill material and soil. When establishing the numerical calculation equation for the temperature of the second unit of the in-hole backfill material, the innovative concept of "spatial separation and physical coupling" was proposed. That is, single / double U-shaped vertical buried pipes are equivalently simplified to 2 / 4 of a line heat source, without considering the influence of their shape and volume. The internal heat source is set to be equal to the heat released / absorbed by the outer wall of the U-shaped pipe; and it is assumed that the temperature of the second unit of the in-hole backfill material is equal to the temperature of the outer wall of the pipe.

[0136] (2) Numerical model of one-dimensional fluid + PE pipe wall: A one-dimensional unsteady convection-diffusion implicit numerical model is established using the QUICK scheme for the fluid nodes. The inlet temperature of the inlet branch pipe is known, and the outlet node j=Nf is the outflow boundary. The inlet temperature of the outlet branch pipe is equal to the outlet temperature of the inlet branch, and its outlet node j=1 is the outflow boundary. For node 1 on the inner wall of the PE pipe, a mixed implicit-explicit scheme numerical discretization equation for unsteady thermal conduction is established using the finite volume method. It is assumed that the outer wall temperature of the pipe is equal to the temperature of the second element of the backfill material inside the hole.

[0137] (3) Model coupling method: When establishing the numerical equations for the fluid and inner wall temperature in the PE pipe, the fluid and inner wall temperatures are taken at the same moment, while the outer wall temperature is set to be equal to the temperature of the backfill element 2 in the hole at the previous moment. The inner wall nodes and fluid nodes have an implicit relationship, which is solved using a simultaneous iterative method to ensure convergence. The inner wall nodes and outer wall nodes have an explicit relationship, realizing the separate solution of the PE pipe and the external region, thus improving the overall calculation speed.

[0138] Heat exchange on the inner wall of the tube Based on Newton's law of cooling, the current temperature of the inner wall nodes is used. With the corresponding fluid node temperature Calculations are performed. The convective heat transfer coefficient of the pipe is calculated using the Dittus correlation. (Pipe outer wall...) Heat absorption / release is measured using the current temperature of the inner wall nodes. and the temperature of the outer wall node at the previous moment calculate.

[0139] A special "double iteration" algorithm for solving fluid and pipe wall temperatures was also adopted: the first external iteration solves the pipe inner wall temperature with a relaxation factor of 0.5; the second internal iteration solves the fluid temperature with a relaxation factor of 0.5.

[0140] It also employs a special sequential simulation algorithm: "receive inlet temperature → calculate fluid friction temperature and pipe wall temperature → calculate heat transfer on the outer wall of the pipe → calculate temperature of the solid region outside the pipe".

[0141] Example 3 This solution can also provide a system, which includes a main program for inverting thermal response test parameters, a numerical calculation mesh generation program for U-tube heat exchangers, a (fluid + pipe wall) - (backfill + soil) coupling program, and a system based on... Figure 1 The numerical grid and "backfill + soil numerical model 1" were used to design a "backfill + soil temperature calculation program" based on Figure 2 Based on the numerical grid and the "fluid + PE pipe wall numerical model 2", a "fluid + PE pipe wall temperature calculation program" was designed.

[0142] The main program for inverting thermal response test parameters has the following functions: (1) Receive the structural parameters of the U-tube heat exchanger, including depth, borehole diameter, tube spacing, tube inner diameter, tube outer diameter, hole spacing or single hole far boundary diameter; (2) Receive the thermal conductivity and volumetric specific heat of the PE pipe; (3) Receive fluid physical property parameters; (4) Receive the initial temperature of the fluid, the initial temperature of the pipe wall, the initial temperature of the backfill, and the initial temperature of the soil and rock; (5) Receive the number of simulation steps and the time step; (6) Receive the measured inlet and outlet temperatures; (7) Receive the measured flow rate; (8) Call the “Numerical Calculation Mesh Generation Program for U-tube Heat Exchangers”; (9) Based on the measured values ​​after 10 hours, calculate the average temperature of the fluid, the logarithm of time, the heat, and the slope of the straight line, and estimate the thermal conductivity of the soil and rock. (10) Based on the thermal conductivity of soil and rock, assume the thermal conductivity and volumetric specific heat of a group of backfill materials, set the volumetric specific heat value of N1 group of soil and rock, input the measured inlet temperature after 10 hours, call the “(fluid + pipe wall) - (backfill material + soil and rock) coupling program” to simulate the outlet temperature; calculate the mean square deviation between the simulated value and the measured value of the outlet temperature; find the minimum mean square deviation and the corresponding preliminary value of the volumetric specific heat of soil and rock.

[0143] (11) Based on the thermal conductivity of the soil and rock and the preliminary value of thermal conductivity, set the thermal conductivity of the N2 group backfill and the volumetric specific heat of the N3 group backfill, input the measured inlet temperature of the first 2 hours, call the “(fluid + pipe wall) - (backfill + soil) coupling program” to simulate the outlet temperature; calculate the mean square deviation between the simulated value and the measured value of the outlet temperature; find the minimum mean square deviation and the corresponding thermal conductivity and volumetric specific heat of the backfill.

[0144] (12) Based on the thermal conductivity of soil and rock, the thermal conductivity of backfill and the volumetric specific heat inversion value, set the volumetric specific heat of soil and rock in group N4, input the measured inlet temperature after 10 hours, call the “(fluid + pipe wall) - (backfill + soil and rock) coupling program” to simulate the outlet temperature; calculate the mean square deviation between the simulated value and the measured value of the outlet temperature; find the minimum mean square deviation and the corresponding final value of the volumetric specific heat of soil and rock.

[0145] based on Figure 1 , Figure 2 Design a "Numerical Calculation Mesh Generation Program for U-tube Heat Exchangers", whose main functions are: (1) Receive heat exchanger structural parameters, including depth, borehole diameter, tube spacing, tube inner diameter, tube outer diameter, hole spacing or single hole far boundary diameter; (2) Set the total number of radial nodes, and calculate the coordinates of each node in the backfill and soil area, as well as the boundary coordinates of each unit.

[0146] (3) Set the total number of vertical nodes in the pipeline, and calculate the coordinates of the fluid nodes and the upstream and downstream coordinates of each unit.

[0147] The main function of the "(fluid + pipe wall) - (backfill + soil) coupling program" is as follows: (1) Receive heat exchanger structural parameters, including depth, borehole diameter, tube spacing, tube inner diameter, tube outer diameter, hole spacing or single hole far boundary diameter; (2) The thermal conductivity and volumetric specific heat of the receiving backfill material, and the thermal conductivity and volumetric specific heat of the soil and rock; (3) Receive fluid physical property parameters and pipe wall physical property parameters; (4) Receive the initial temperature of the fluid, the initial temperature of the pipe wall, the initial temperature of the backfill, and the initial temperature of the soil and rock; (5) Receive the number of simulation steps and the time step; (6) Receive the measured inlet temperature; (7) Receive the measured flow rate; (8) Implement a time-step loop, and implement the following functions at each time step: 1) Call the "Fluid + PE Pipe Wall Temperature Calculation Program"; 2) Call the "Backfill Material + Soil Temperature Calculation Program"; based on Figure 1 Based on the numerical grid and the "backfill + soil numerical model 1", a "backfill + soil temperature calculation program" was designed, whose main functions are: (1) Receive the radial node coordinates and unit boundary coordinates of "backfill + soil"; (2) The thermal conductivity and volumetric specific heat of the receiving backfill material, and the thermal conductivity and volumetric specific heat of the soil and rock; (3) Receive the initial temperature of the backfill material and the soil-rock joint; (4) Reception time step; (5) Receive the total heat released / absorbed by the outer wall of the U-shaped tube; (6) Calculate the explicit equation coefficients for each node, where the total heat released / absorbed by the outer wall of the U-shaped tube is applied to the second unit; (7) Calculate the temperature of each node at the current time; (8) Output the current temperature of each node.

[0148] based on Figure 2 Based on the numerical grid and the "Fluid + PE Pipe Wall Numerical Model 2", a "Fluid + PE Pipe Wall Temperature Calculation Program" was designed, whose main functions are: (1) Receive vertical fluid node coordinates, upstream and downstream boundary coordinates, and pipe diameter; (2) Receive the temperature of the fluid, pipe wall, and backfill at the previous moment; (3) Receive fluid property parameters and PE pipe property parameters; (4) Receive traffic; (5) Reception time step; (6) Calculate the convective heat transfer coefficient of the inner surface; (7) Design an external iterative program to calculate the pipe wall temperature. Its functions include: 1) Calculate the coefficients of the fully implicit QUICK scheme equations for fluid nodes; 2) Iteratively solve for the fluid node temperature; 3) Calculate the equation coefficients of the nodes on the inner wall of the PE pipe; 4) Iteratively solve for the node temperature on the inner wall of the PE pipe; (8) Calculate the heat transfer on the inner and outer walls of the PE pipe; (9) Calculate the total heat based on the inlet and outlet temperatures of the U-shaped tube; (10) Output fluid node temperature, pipe wall temperature, heat of inner pipe wall, heat of outer pipe wall and total heat.

[0149] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models, characterized in that, Includes the following steps: Step S100: Using the water temperature observation data after the 10th hour of the thermal response test, the thermal conductivity of the soil and rock is calculated using the linear slope method based on the linear heat source analytical model. Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained thermal conductivity of the soil and rock, assuming the thermal conductivity and volumetric specific heat of the backfill material in the hole, the volumetric specific heat of the soil and rock is inverted based on the one-dimensional unsteady numerical model of the whole space. Step S300: Using the water temperature observation data 2 hours before the thermal response test, the obtained thermal conductivity and volumetric specific heat of the soil and rock, and based on the one-dimensional unsteady-state numerical model in the whole space, the thermal conductivity and volumetric specific heat of the backfill material in the hole are inverted. Step S400: Using the water temperature observation data after the 10th hour of the thermal response test, the obtained thermal conductivity of the soil and rock, thermal conductivity of the backfill material, and volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on the one-dimensional unsteady-state numerical model in the whole space.

2. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 1, characterized in that: Step S100 involves using water temperature observation data after the 10th hour of the thermal response test to calculate the thermal conductivity of the soil and rock using the linear slope method based on a linear heat source analytical model; specifically, it includes the following steps: Step S101, based on the inlet temperature recorded in the thermal response test... outlet temperature Observed values ​​were used to calculate the total heat exchange Q and the heat exchange per unit depth. And calculate the average temperature. : ； Step S102: Based on the analytical model of the vertically buried U-tube heat exchanger's line heat source, fit the average inlet and outlet temperatures of the heat exchanger after 10 hours. logarithm with time The linear relationship is: ； Step S103: Calculate the thermal conductivity of the soil and rock based on the slope of the straight line. Where k is the slope of the line, m is the intercept of the line, Q is the total heat capacity of the heat exchanger, and L is the borehole depth. G represents heat exchanged per unit depth, and G represents flow rate. For specific heat of fluid, For the inlet temperature, For the outlet temperature, is the thermal conductivity of the soil and rock.

3. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 1, characterized in that: Step S200: Using the water temperature observation data after the 10th hour of the thermal response test and the obtained soil-rock conductivity coefficients, assuming the thermal conductivity and volumetric specific heat of the backfill material in the borehole, the volumetric specific heat of the soil-rock is inverted based on a one-dimensional unsteady-state numerical model of the entire space; specifically including the following steps: Step S201: Assume a set of in-hole backfill materials with thermal conductivity and volumetric specific heat value; Step S202: Assuming the lower limit of the volumetric specific heat of soil and rock, and varying the interval, a set of volumetric specific heat values ​​for soil and rock is obtained. ; Step S203: Receive the thermal conductivity of the soil and rock from step S103. ; Step S204, for each soil volumetric specific heat With imported water temperature The "Water Temperature and Ground Temperature Calculation Program" is invoked as input to simulate the corresponding outlet water temperature; Step S205: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock: in This represents the simulated outlet temperature at a certain moment. This represents the observed outlet temperature of the thermal response test at a certain moment; Step S206: Take the volumetric specific heat of the soil and rock corresponding to the minimum standard deviation of the outlet water temperature as the preliminary value.

4. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 1, characterized in that: Step S300 involves using water temperature observation data from two hours prior to the thermal response test, along with the obtained thermal conductivity and volumetric specific heat of the soil and rock, to simultaneously invert the thermal conductivity and volumetric specific heat of the backfill material within the borehole based on a one-dimensional unsteady-state numerical model. This specifically includes the following steps: Step S301: Assume the upper and lower limits of the thermal conductivity of the backfill material in the hole, as well as the variation interval; Step S302: Assume the lower limit and upper limit of the volumetric specific heat of the backfill soil in the borehole, as well as the interval of variation. Step S303: Receive the thermal conductivity of the soil and rock from step S103 and the preliminary value of the specific heat of the soil and rock from step S206. Step S304: For each group of backfill material, the thermal conductivity and volumetric specific heat are calculated using the inlet water temperature as input. The "Water Temperature and Ground Temperature Calculation Program" is then called to simulate the corresponding outlet water temperature. Step S305: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to the thermal conductivity and volumetric specific heat of each group of backfill material. Step S306: Take the thermal conductivity and volumetric specific heat of the backfill material corresponding to the minimum standard deviation of the outlet water temperature as the estimated value.

5. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 1, characterized in that: Step S400: Using the water temperature observation data after 10 hours of the thermal response test, the obtained thermal conductivity of the soil and rock, the thermal conductivity of the backfill material, and the volumetric specific heat, the volumetric specific heat of the soil and rock is inverted again based on a one-dimensional unsteady-state numerical model in full space; specifically, the following steps are included: Step S401: Receive the thermal conductivity and volumetric specific heat value of the backfill material in the hole from step S300. Step S402: Receive the thermal conductivity of the soil and rock from step S100; Step S403: Assume the lower limit and upper limit of the volumetric specific heat of rock and soil, and the variation interval, to obtain a set of volumetric specific heat values ​​of rock and soil. Step S404: For each specific heat of rock and soil volume, the "Water Temperature and Ground Temperature Calculation Program" is called with the inlet water temperature as input to simulate the corresponding outlet water temperature; Step S405: Calculate the root mean square error between the simulated and measured values ​​of the outlet water temperature corresponding to each volumetric specific heat of the soil and rock. Step S406: Take the volumetric specific heat of the soil and rock corresponding to the minimum root mean square deviation of the outlet water temperature as the final estimated value.

6. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 3, characterized in that: In step S201, the assumed thermal conductivity of the in-hole backfill is... and specific heat value ; In step S202, the lower limit of the assumed volumetric specific heat of the soil is... The upper limit is The interval of change is A set of specific heat values ​​of rock and soil were obtained. .

7. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 4, characterized in that: In step S301, the upper limit of the assumed thermal conductivity of the in-hole backfill is... 3.0, lower limit is 0.1, with a variation interval of ; In step S302, the lower limit of the assumed volumetric specific heat of the backfill soil in the borehole is... The upper limit is The interval of change is .

8. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to any one of claims 1 to 7, characterized in that: The specific method based on the unsteady numerical model includes the following steps: Step S1: Design a mesh generation method to establish a radial one-dimensional Nr node unsteady diffusion numerical model of the soil and rock region outside the pore backfill of the underground heat exchanger. Step S2: Design a mesh generation method to establish a one-dimensional 2N mesh along the flow direction. f A numerical model of unsteady convection and diffusion at the nodes was developed, and a one-dimensional two-node radial heat conduction numerical model of the pipe wall was established. Step S3: Design a hybrid numerical scheme that combines implicit and explicit representations to achieve coupled calculation of unsteady heat transfer between fluid + PE pipe and backfill + soil. Step S4: Design a two-fold iterative algorithm for the numerical model in step S2 to solve the fluid temperature inside the PE pipe and the inner wall temperature at each time step. Step S5: Design an algorithm based on the implicit and explicit hybrid numerical format to realize the sequential simulation of "receiving inlet temperature → calculating fluid friction temperature and pipe wall temperature → calculating heat transfer on the outer wall of the pipe → calculating the temperature of the solid region outside the pipe".

9. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 8, characterized in that: Step S1 specifically includes the following steps: The backfill material inside the borehole and the soil and rock outside the borehole are divided into the following radial directions: Two units are fixedly installed for the backfill material inside the hole, and after the third unit ( The borehole is a geotechnical unit. The first unit at the borehole center is a solid cylinder of height L, and the remaining units are annular cylinders. The center coordinates of the second unit of the backfill material are set at the center of the PE pipe. The interface between the second unit and the first unit is set in... At this location, the interface between the second and third units is set at the hole wall r= The geotechnical unit interface is located at the center of the two nodes. For a single hole, the radius of the far boundary can be 10m or 20m. For a group of holes, the radius of the far boundary can be 1 / 2 of the hole spacing. The boundary conditions are set as follows: the upper, lower and far boundaries of the heat exchanger calculation area are considered as adiabatic boundaries. Internal heat source: The internal heat source is only set in the second unit of the backfill. According to the innovative idea of ​​"spatial separation and physical coupling", the single / double U-shaped vertical buried pipe is abstracted as 2 / 4 of the linear heat sources without shape and volume. The heat generation of the internal heat source of the backfill unit 2 is equal to the heat of the outer wall of the pipe.

10. The four-step inversion method for thermal properties based on analytical models and unsteady-state numerical models according to claim 9, characterized in that: Step S2 specifically includes the following steps: The inlet and outlet branches of the U-shaped tube are respectively set along the flow direction. There are 1 node, and 1 group of U-shaped tubes. The fluid node, the first Units are evenly distributed, j=1 and The unit is a control body of 0; The PE pipe wall is divided into sections corresponding to fluid nodes in the flow direction. Each section is insulated from the others; each section of PE pipe has two nodes fixed radially on its wall, one on the inner wall and one on the outer wall; the heat from the outer wall of the pipe is used as the internal heat source for the backfill unit 2; the temperature of the outer wall node is set to be equal to the temperature of the backfill unit 2.

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