A Deviation Design Method for Medium and Deep Coaxial Sleeve Heat Exchangers

Abstract

The present invention provides a deviation design method for a medium-deep coaxial casing heat exchanger, comprising the following steps: Step 1: Establish a transient heat transfer model of the medium-deep coaxial casing heat exchanger and the rock and soil mass, and obtain the temperature distribution of the rock and soil mass; Step 2: Determine the maximum thermal influence radius R of the medium-deep rock and soil mass max ; Step 3: Design the deviation angle θ of the medium-deep coaxial casing heat exchanger d ; The deviation design method of the medium-deep coaxial casing heat exchanger, a design method for a new installation form of the medium-deep coaxial casing heat exchanger - the deviation installation form, reasonably designs the installation form and angle of the heat exchanger on the premise of maximizing the avoidance of thermal interference between adjacent heat exchangers, reduces the wellhead distance between adjacent heat exchangers, and is of great significance for reducing the project land area and reducing the ground pipeline heat loss

Description

Technical Field

[0001] The present invention belongs to the technical field of heating, ventilation and air conditioning, and particularly relates to a deviation design method for a medium-deep coaxial casing heat exchanger. Background Art

[0002] The medium-deep coaxial casing heat exchanger mainly extracts geothermal energy located 2000 - 3000m below the ground surface for building heating. Its heat exchange has the characteristic of "extracting heat without extracting water", which plays a positive role in protecting groundwater and conforms to the concept of ecological civilization construction in China.

[0003] The medium-deep buried tube heat exchanger of the coaxial casing type is composed of an annular cavity and an inner tube, as Figure 1 shown. During drilling, backfill materials are generally filled to reinforce the heat exchanger. The cross-section is as Figure 2 shown. The components from the center outwards are inner tube fluid, inner tube wall, annular cavity fluid, outer tube wall, backfill material, and drilling wall. During the heat exchange process, the fluid at a lower temperature flows into the annular cavity between the inner tube and the outer tube, and exchanges heat with the medium-deep rock and soil mass with a temperature gradient until the heat exchange ends at the bottom of the heat exchanger. In the inner tube, the fluid flows upward and is transported to the ground surface.

[0004] In recent years, the medium-deep coaxial casing heat exchanger has received great attention in the heating industry due to its large heat exchange capacity. With its continuous promotion, it has been applied in some heating projects with a relatively large scale, but generally, multiple medium-deep coaxial casing heat exchangers are required to jointly heat the building. Similar to the traditional shallow buried tube heat exchanger, when multiple heat exchangers operate jointly, the thermal interference problem between adjacent heat exchangers needs to be considered. From the research experience of the thermal influence radius of the shallow buried tube heat exchanger, a distance of 2 - 6m between adjacent heat exchangers can effectively avoid thermal interference. However, for the medium-deep coaxial casing heat exchanger, since there are differences in heat exchange characteristics and reservoir temperature characteristics compared with the shallow buried tube heat exchanger, this value (2 - 6m) is not referenceable. According to the research of scholars in recent years, it can be found that during the long-term heat exchange process of the medium-deep coaxial casing heat exchanger, the thermal influence radius of the rock and soil can reach 50m, that is, the distance between adjacent heat exchangers should be at least 100m to ensure that there is no thermal interference between the two. This conclusion has certain reference value for the layout arrangement of multiple medium-deep coaxial casing heat exchangers. Summary of the Invention

[0005] The purpose of the present invention is to provide a deviation design method for a medium-deep coaxial casing heat exchanger, which mainly combines numerical model development, determination of the maximum thermal influence radius of the rock and soil mass, and determination of the deviation angle for the installation design of the heat exchanger.

[0006] A deviation design method for a medium-deep coaxial casing heat exchanger includes the following steps:

[0007] Step 1: Establish a transient heat transfer model of the medium-deep coaxial casing heat exchanger and the geotechnical body to obtain the temperature distribution;

[0008] Step 2: Determine the maximum thermal influence radius R of the medium-deep geotechnical body max ;

[0009] Step 3: Design the deflection angle θ of the medium-deep coaxial casing heat exchanger d .

[0010] Furthermore, for Step 1: The specific process of establishing a transient heat transfer model of the medium-deep coaxial casing heat exchanger and the geotechnical body to obtain the temperature distribution is as follows:

[0011] A. Establish the heat transfer control equations of the heat exchanger, including the energy control equation of the inner tube fluid and the energy control equation of the annulus fluid:

[0012] The energy control equation of the inner tube fluid is:

[0013]

[0014] In the formula, T fr is the temperature of the inner tube fluid, °C; V fr is the flow velocity of the inner tube fluid, m·s -1 ; k ff is the comprehensive heat transfer coefficient between the inner tube fluid and the annulus fluid, W·m -1 ·K -1 ; T fan is the temperature of the annulus fluid, °C; ρ f is the fluid density, kg / m 3 ; c pf is the specific heat capacity of the fluid, A r is the cross-sectional area of the inner tube, m 2 ;

[0015] The energy control equation of the annulus fluid is:

[0016]

[0017] In the formula, V fan is the flow velocity of the annulus fluid, m·s -1 ; k fg is the comprehensive heat transfer coefficient between the annulus fluid and the backfill material, W·m -1 ·K -1 ; T g is the temperature of the backfill material, °C; A an is the cross-sectional area of the annulus, m 2 ;

[0018] B. Establish the heat transfer control equation of the backfill material:

[0019] The energy control equation of the backfill material is as follows:

[0020]

[0021] In the formula, ρ g is the density of the backfill material, kg·m 3 ; A g is the cross-sectional area of the backfill material, m2; c pg is the specific heat capacity of the backfill material, J·kg -1 ·K -1 ; k gb is the comprehensive heat transfer coefficient between the backfill material and the drilling wall, W·m -1 ·K -1 ; T bw is the temperature of the drilling wall, °C;

[0022] C. Establish the energy control equation of the rock and soil mass:

[0023] The energy control equation of the rock and soil mass is as follows:

[0024]

[0025] In the formula, ρ s is the density of the rock and soil mass, kg·m 3 ; c ps is the specific heat capacity of the rock and soil mass, J·kg -1 ·K -1 ; T s is the temperature of the rock and soil mass, °C; λ s is the thermal conductivity of the rock and soil mass, W·m -1 ·K -1 ;

[0026] D. Set the initial conditions and boundary conditions. The initial condition is that at the same depth, the temperatures of the fluid, the backfill material, and the drilling wall are the same and are consistent with the temperature distribution of the rock and soil mass in the undisturbed state; the boundary conditions include: ① The surface temperature is set as the first kind of boundary condition, and the temperature can be selected as the local average annual temperature or the temperature of the constant temperature layer; ② The far radial boundary of the rock and soil mass is set as the adiabatic condition; ③ The lower surface of the rock and soil mass is set as the fixed heat flux boundary, and the heat flux density is the product of the thermal conductivity of the rock and soil mass and the geothermal gradient; ④ The boundary condition between the rock and soil mass and the heat exchanger is set as the equal heat flux density;

[0027] E. Solve the transient heat transfer model to obtain the temperature distribution.

[0028] Furthermore, the specific process of the second step: determining the maximum thermal influence radius R max of the medium-deep rock and soil mass is as follows:

[0029] a. Taking the whole - life cycle operation as the research object, the total heat - exchange duration of the medium - deep coaxial casing heat exchanger is set to 30 years. Among them, according to the relevant regulations of heating in northern China, the heat - extraction time is set to 4 months per year, and the operation stops for the remaining 8 months.

[0030] b. According to formulas (6) and (9), after the heat exchanger operates for 30 years, the temperature difference between the geotechnical body at different depths \(z_s\) and different radial distances \(r_s\) and the temperature of the geotechnical body in the undisturbed state can be calculated.

[0031] The temperature distribution of the medium - deep geotechnical body in the undisturbed state can be expressed by the following formula:

[0032] T s0 (z s ,r)=T sur +G×z s (13)

[0034] In the formula, T s0 (z s ,r) is the temperature distribution of the geotechnical body in the undisturbed state, °C; T sur is the surface temperature, °C; G is the geothermal gradient, °C·m - 1; z s is the depth of the geotechnical body, m

[0035] The change in the temperature of the geotechnical body can be calculated by the following formula:

[0036] θ s (z s ,r s ,t)=T s (z s ,r s ,t)-T s0 (z s ,r) (14)

[0037] In the formula, θ s is the change value of the geotechnical body temperature, °C; T s (z s ,r s ,t) is the temperature distribution of the geotechnical body at different depths z s 、different radial distances r s at time t, °C;

[0038] c. Taking the radial distance r s where the difference is 0.1 °C as the thermal influence radius of the geotechnical body, the thermal influence radii of the geotechnical body at different depths z s can be obtained, and the maximum value among them is the maximum thermal influence radius R max of the geotechnical body.

[0039] Further, in Step 3: Design the deviation angle θ of the deep coaxial casing heat exchanger d ;

[0040] First, a new concept - the heat influence radius angle is proposed, which is calculated from the length of the heat exchanger and the maximum heat influence radius of the deep geotechnical body. The positional relationship between the two is shown in Figure 4. The conventional installation method of the heat exchanger is to install it perpendicular to the ground surface. When deviating the heat exchanger, keep its position on the ground surface unchanged and deviate it based on the ground surface position (Figure 4(a)). The relative positions of adjacent heat exchangers after deviation are shown in Figure 4(b). When the translation distance at the bottom position of the heat exchanger is the maximum heat influence radius of the deep geotechnical body, the formed included angle is the heat influence radius angle, which is calculated by Equation (10):

[0041] (1) Calculate the heat influence radius angle

[0042]

[0043] In the formula, θ is the heat influence radius angle, °; R max is the maximum heat influence radius of the geotechnical body, m; H is the length of the heat exchanger, m;

[0044] (2) Set the deviation angle. It is advisable that the deviation angle is greater than 4 times the heat influence radius angle:

[0045] θ d ≥4θ (16)

[0046] In the formula, θ d is the deviation angle of the deep coaxial casing heat exchanger, °.

[0047] The advantages of the present invention are as follows: The present invention provides a deviation design method for the deep coaxial casing heat exchanger, a design method for a new installation form of the deep coaxial casing heat exchanger - the deviation installation form. On the premise of avoiding heat interference between adjacent heat exchangers to the greatest extent, the installation form and angle of the heat exchanger are reasonably designed to reduce the wellhead distance between adjacent heat exchangers, which is of great significance for reducing the project land area and reducing the heat loss of the ground pipeline.

[0048] The following will describe the present invention in detail with reference to the accompanying drawings and embodiments. Description of the Drawings

[0049] Figure 1 is the deep coaxial casing heat exchanger.

[0050] Figure 2 is the cross-sectional schematic diagram of the deep casing heat exchanger.

[0051] Figure 3 is the discrete schematic diagram.

[0052] Figure 4 is a schematic diagram of the deviation of the medium-deep buried tube heat exchanger.

[0053] Figure 5 It is the distribution map of the thermal influence radius of the rock and soil mass under the case parameters.

[0054] Figure 6 It is the distribution of the thermal influence radius of the rock and soil mass at different thermal influence radius angles Specific implementation manner

[0055] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined purpose, the specific implementation manner, structural features and their effects of the present invention will be described in detail below in conjunction with the drawings and embodiments.

[0056] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0057] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "aligned", "overlapped", "bottom", "inner", "outer", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation to the present invention.

[0058] The terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features; in the description of the present invention, unless otherwise stated, the meaning of "plurality" is two or more.

[0059] Embodiment 1

[0060] This embodiment provides a deviation design method for a medium-deep coaxial casing heat exchanger, including the following steps:

[0061] Step 1: Establish a transient heat transfer model of the medium-deep coaxial casing heat exchanger and the rock and soil mass, and obtain the temperature distribution of the rock and soil mass;

[0062] Step 2: Determine the maximum thermal influence radius R of the medium-deep rock and soil mass max ;

[0063] Step 3: Design the deviation angle θ of the middle and deep coaxial casing heat exchanger d .

[0064] Furthermore, Step 1: Establish a transient heat transfer model of the middle and deep coaxial casing heat exchanger and the geotechnical body to obtain the specific process of temperature distribution as follows:

[0065] A. Establish the heat transfer control equations of the heat exchanger, including the energy control equation of the inner tube fluid and the energy control equation of the annulus fluid:

[0066] The energy control equation of the inner tube fluid is:

[0067]

[0068] In the formula, T fr is the temperature of the inner tube fluid, °C; V fr is the flow velocity of the inner tube fluid, m·s -1 ; k ff is the comprehensive heat transfer coefficient between the inner tube fluid and the annulus fluid, W·m -1 ·K -1 ; T fan is the temperature of the annulus fluid, °C; ρ f is the fluid density, kg / m 3 ; c pf is the specific heat capacity of the fluid, A r is the cross-sectional area of the inner tube, m 2 ;

[0069] The energy control equation of the annulus fluid is:

[0070]

[0071] In the formula, V fan is the flow velocity of the annulus fluid, m·s -1 ; k fg is the comprehensive heat transfer coefficient between the annulus fluid and the backfill material, W·m -1 ·K -1 ; T g is the temperature of the backfill material, °C; A an is the cross-sectional area of the annulus, m 2 ;

[0072] B. Establish the heat transfer control equation of the backfill material:

[0073] The energy control equation of the backfill material is as follows:

[0074]

[0075] In the formula, ρ g is the density of the backfill material, kg·m3; A gis the cross-sectional area of the backfill material, m 2 ; c pg is the specific heat capacity of the backfill material, J·kg -1 ·K -1 ; k gb is the overall heat transfer coefficient between the backfill material and the drilling wall, W·m -1 ·K -1 ; T bw is the temperature of the drilling wall, °C;

[0076] C. Establish the energy control equation of the rock and soil mass:

[0077] The energy control equation of the rock and soil mass is as follows:

[0078]

[0079] In the formula, ρ s is the density of the rock and soil mass, kg·m 3 ; c ps is the specific heat capacity of the rock and soil mass, J·kg -1 ·K -1 ; T s is the temperature of the rock and soil mass, °C; λ s is the thermal conductivity of the rock and soil mass, W·m -1 ·K -1 ;

[0080] D. Set the initial conditions and boundary conditions. The initial condition is that at the same depth, the temperatures of the fluid, backfill material, and drilling wall are the same and consistent with the temperature distribution of the rock and soil mass in the undisturbed state; the boundary conditions include: ① The surface temperature is set as the first-kind boundary condition, and the temperature can be selected as the local average annual temperature or the temperature of the constant temperature layer; ② The far radial boundary of the rock and soil mass is set as the adiabatic condition; ③ The lower surface of the rock and soil mass is set as the fixed heat flux boundary, and the heat flux density is the product of the thermal conductivity of the rock and soil mass and the geothermal gradient; ④ The boundary condition between the rock and soil mass and the heat exchanger is set as the equal heat flux density;

[0081] E. Solve the transient heat transfer model to obtain the temperature distribution.

[0082] First, discretize the above control equation based on the finite volume method. The discretization principle diagram is as Figure 3 shown: The longitudinal step size in the rock and soil mass region is uniform, while the radial step size increases in a logarithmic distribution along the radius direction, realizing a distribution form where the grid in the rock and soil mass region gradually becomes sparse along the radial direction. To satisfy the heat transfer analysis of the rock and soil mass near the drilling wall, local refinement is required. In summary, the radial step size of the grid in the rock and soil mass region is expressed by the following formula:

[0083]

[0084] Where: u is the number of radial grids in the local refined area; n s ——The number of radial grids in the rock mass area; Y——density index. Determine u and n by verifying grid independence s and the value of Y.

[0085] When the initial conditions and boundary conditions are assigned to the discrete control volume, the temperature distribution of the deep rock mass in the undisturbed state can be expressed as follows:

[0086] T s0 (z s ,r)=T sur +G×z s (twenty two)

[0087] Where, T s0 (z s ,r) is the temperature distribution of the rock mass in the undisturbed state, ℃; T sur is the surface temperature, °C; G is the geothermal gradient, °C·m -1 ;z s is the depth of rock and soil mass, m.

[0088] The bottom boundary of the rock and soil area is set at a vertical distance of 200m below the bottom of the heat exchanger, and the temperature of the deep rock and soil is affected by the earth's heat flow, so the bottom boundary meets the second type of boundary conditions:

[0089]

[0090] Where H is the length of the heat exchanger, m; q bot is the earth heat flux density, W·m -2 , which is calculated by multiplying the geothermal gradient by the thermal conductivity of the rock and soil.

[0091] The radial far boundary of the rock mass area still satisfies the second type of boundary conditions, and it is considered that the rock mass at the far boundary does not exchange heat with the rock mass outside the boundary, so it is an adiabatic boundary:

[0092]

[0093] Finally, the above control equations are combined with the initial and boundary conditions and solved using the Tri-diagonal matrix algorithm to calculate the temperature distribution of the fluid, backfill material, and rock and soil at different times.

[0094] Furthermore, the step 2: determining the maximum heat affected radius R of the medium-deep rock and soil body max The specific process is:

[0095] a. Taking the whole - life cycle operation as the research object, the total heat - exchange duration of the medium - deep coaxial casing heat exchanger is set to 30 years. Among them, according to the relevant regulations of heating in northern China, the heat - extraction time is set to 4 months per year, and the operation stops for the remaining 8 months.

[0096] b. According to formulas (6) and (9), after the heat exchanger operates for 30 years, the temperature difference between the geotechnical body at different depths z s and different radial distances r s and the temperature of the geotechnical body in the undisturbed state can be calculated.

[0097] The temperature distribution of the medium - deep geotechnical body in the undisturbed state can be expressed by the following formula:

[0098] T s0 (z s ,r) = T sur + G×z s (25)

[0099] In the formula, T s0 (z s , r) is the temperature distribution of the geotechnical body in the undisturbed state, °C; T sur is the surface temperature, °C; G is the geothermal gradient, °C·m -1 ; z s is the depth of the geotechnical body, m

[0100] The change in the geotechnical body temperature can be calculated by the following formula:

[0101] θ s (z s ,r s ,t) = T s (z s ,r s ,t) - T s0 (z s ,r) (26)

[0102] In the formula, θ s is the change value of the geotechnical body temperature, °C; T s (z s , r s , t) is the geotechnical body temperature distribution at different depths z s and different radial distances r s at time t, °C;

[0103] c. Taking the radial distance r s at the point where the difference is 0.1 °C as the thermal influence radius of the geotechnical body, the thermal influence radii of the geotechnical body at different depths z s can be obtained, and the maximum value among them is the maximum thermal influence radius R max of the geotechnical body.

[0104] Further, in Step 3: Design the deviation angle θ of the deep coaxial casing heat exchanger d ;

[0105] First, a new concept - the heat influence radius angle is proposed, which is calculated from the length of the heat exchanger and the maximum heat influence radius of the deep geotechnical body. The positional relationship between the two is shown in Figure 4. The conventional installation method of the heat exchanger is to install it perpendicular to the ground surface. When deviating the heat exchanger, keep its position on the ground surface unchanged and deviate it with the ground surface position as the base point (Figure 4(a)). The relative positions of adjacent heat exchangers after deviation are shown in Figure 4(b). When the translation distance at the bottom position of the heat exchanger is the maximum heat influence radius of the deep geotechnical body, the formed included angle is the heat influence radius angle, which is calculated by Equation (10):

[0106] (1) Calculate the heat influence radius angle

[0107]

[0108] In the formula, θ is the heat influence radius angle, °; R max is the maximum heat influence radius of the geotechnical body, m; H is the length of the heat exchanger, m;

[0109] (2) Set the deviation angle. It is advisable that the deviation angle is greater than 4 times the heat influence radius angle:

[0110] θ d ≥4θ (28)

[0111] In the formula, θ d is the deviation angle of the deep coaxial casing heat exchanger, °.

[0112] In summary, for the deviation design method of the deep coaxial casing heat exchanger, it is a design method for the new installation form - the deviation installation form of the deep coaxial casing heat exchanger. On the premise of maximizing the avoidance of thermal interference between adjacent heat exchangers, reasonably design the installation form and angle of the heat exchanger, reduce the wellhead distance between adjacent heat exchangers, which is of great significance for reducing the project land area and reducing the heat loss of the ground pipeline.

[0113] Example 2

[0114] The deep coaxial casing heat exchanger is mainly used for building heating and has been promoted to a certain extent in Shaanxi region in recent years. Therefore, a case in Shaanxi region is used to illustrate the specific application and advantages of the present invention.

[0115] The well structure of the deep coaxial casing heat exchanger, the geothermal geological conditions of the application area, and the operation conditions are specifically shown in Table 1.

[0116] Table 1 Case parameters

[0117]

[0118]

[0119] First, substitute the case parameters into the transient numerical heat transfer model in Step 1, calculate the temperature distribution of the rock and soil mass after 30 years of heat transfer, and according to Formulas (6) and (9), obtain the temperature difference between the rock and soil mass at different depths z s and different radial distances r s from the temperature of the rock and soil mass in the undisturbed state. Take the radial distance r s where the difference is 0.1 °C as the thermal influence radius of the rock and soil mass, and the thermal influence radii of the rock and soil mass at different depths z s can be obtained. As shown in Figure 5 , the blue line is the thermal influence radius of the rock and soil mass at different depths.

[0120] It can be seen that the thermal influence radius of the medium-deep rock and soil mass is not uniform in the depth direction. As the depth increases, the thermal influence radius increases accordingly. The position of the maximum thermal influence radius is close to the depth where the bottom of the heat exchanger is located. Under the case parameters, the maximum thermal influence radius exceeds 70 m. If arranged by the conventional vertical installation method, the spacing between adjacent medium-deep coaxial casing heat exchangers should be at least greater than 2 times 70 m, that is, an installation distance of 140 m, which greatly increases the difficulty of system pipeline connection, insulation, and machine room layout, and puts forward requirements for the available area on the ground.

[0121] Based on the deviated design method of a medium-deep coaxial casing heat exchanger proposed by the present invention, the heat exchanger under the case parameters is arranged with a deviation. According to the above introduction and analysis, the length of the heat exchanger is 2500 m, and the maximum thermal influence radius of the rock and soil mass is 70 m. The thermal influence radius angle can be calculated by Formula (10). Figure 6 The distribution of the thermal influence radius of the medium-deep rock and soil mass under different thermal influence radius angles is given.

[0122] At Figure 6Among them, the position where the heat exchanger is located is at the abscissa of 0 m. 0° represents that the heat exchanger is vertically installed, and θ, 2θ, 3θ, 4θ, and 5θ are installed at 1 times, 2 times, 3 times, 4 times, and 5 times the thermal influence radius angle respectively. On the right side of the position where the heat exchanger is located, that is, the area with a positive abscissa is the area where thermal interference may occur in the rock and soil mass. Through directional drilling, it can be found that the overall thermal influence radius of the rock and soil mass moves to the left side of the heat exchanger. When the directional drilling angle is 1 times the thermal influence radius angle, the thermal influence radius of the rock and soil mass at the bottom of the heat exchanger is close to 0 m. However, it can be found that thermal interference still occurs in most of the rock and soil mass areas, so it is necessary to continue to increase the directional drilling angle of the heat exchanger. When the directional drilling angle is 4 times the thermal influence radius angle, there is no thermal interference in more than 80% of the longitudinal depth of the heat exchanger. And it is known from existing research that the main heat extraction area of the medium-deep coaxial casing heat exchanger is the lower part of the heat exchanger, and the heat exchange between the upper part and the rock and soil mass is very small.

[0123] In summary, 4 times the thermal influence radius angle can be used as the minimum directional drilling angle of the medium-deep coaxial casing heat exchanger. Conversely, a directional drilling angle greater than 4 times the thermal influence radius angle can effectively avoid thermal interference between adjacent heat exchangers. To improve the universality of the present invention, the directional drilling angles of the heat exchanger under different geothermal characteristic parameters are analyzed. The research finds that when the directional drilling angle is 4 times the thermal influence radius angle, the part of the longitudinal depth of the heat exchanger that completely avoids thermal interference exceeds 80% of the total length, further determining the feasibility of the directional drilling angle not less than 4 times the thermal influence radius angle proposed by the present invention.

[0124] The above content is a further detailed description of the present invention in combination with specific preferred embodiments, and it cannot be determined that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention belongs, without departing from the concept of the present invention, several simple deductions or substitutions can still be made, which should all be regarded as belonging to the protection scope of the present invention.

Claims

1. A deviation design method for medium-deep coaxial casing heat exchangers, characterized in that, it includes the following steps: Step 1: Establish a transient heat transfer model of the medium-deep coaxial casing heat exchanger and the geotechnical body, and obtain the geotechnical body temperature distribution; The specific process is: A. Establish the heat transfer control equations of the heat exchanger, including the energy control equation of the inner tube fluid and the energy control equation of the annulus fluid: The energy control equation of the inner tube fluid is: Where, T fr is the temperature of the fluid in the inner tube, in °C; V fr is the flow velocity of the fluid in the inner tube, in m·s -1 ; k ff is the overall heat transfer coefficient between the fluid in the inner tube and the fluid in the annulus, in W·m -1 ·K -1 ; T fan is the temperature of the fluid in the annulus, in °C; ρ f is the fluid density, in kg / m 3 ; c pf is the specific heat capacity of the fluid, A r is the cross-sectional area of the inner tube, in m 2 ; The energy control equation of the annulus fluid is: Where, V fan is the fluid flow velocity in the annular cavity, m·s -1 ; k fg is the comprehensive heat transfer coefficient between the fluid in the annular cavity and the backfill material, W·m -1 ·K -1 ; T g is the temperature of the backfill material, °C; A an is the cross-sectional area of the annular cavity, m 2 ; B. Establish the heat transfer control equation of the backfill material: The energy control equation of the backfill material is as follows: where ρ g is the density of the backfill material, kg·m3; A g is the cross-sectional area of the backfill material, m2; c pg is the specific heat capacity of the backfill material, J·kg-1·K-1; k gb is the overall heat transfer coefficient between the backfill material and the wellbore wall, W·m-1·K-1; T bw is the wellbore wall temperature, °C; C. Establish the energy control equation of the geotechnical body: The energy control equation of the geotechnical body is as follows: where ρ s is the density of the rock and soil mass, kg·m3; c p s is the specific heat capacity of the rock and soil mass, J·kg-1·K-1; T s is the temperature of the rock and soil mass, °C; λ s is the thermal conductivity of the rock and soil mass, W·m-1·K-1; D. Set the initial conditions and boundary conditions. The initial conditions are that at the same depth, the temperatures of the fluid, the backfill material, and the drilling wall are the same and consistent with the temperature distribution of the geotechnical body in the undisturbed state; the boundary conditions include: ① The surface temperature is set as the first kind of boundary condition, and the temperature can be selected as the local average annual temperature or the constant temperature layer temperature; ② The far radial boundary of the geotechnical body is set as the adiabatic condition; ③ The lower surface of the geotechnical body is set as the fixed heat flux boundary, and the heat flux density is the product of the geotechnical body thermal conductivity and the geothermal gradient; ④ The boundary condition between the geotechnical body and the heat exchanger is set as the equal heat flux density; E. Solve the transient heat transfer model to obtain the temperature distribution; Step 2: Determine the maximum thermal influence radius R of the medium and deep rock and soil masses max ; Step 3: Design the deviation angle θ of the deep coaxial casing heat exchanger d ; The specific process is: (1) Calculate the thermal influence radius angle where θ is the thermal influence radius angle, °; R max is the maximum thermal influence radius of the rock and soil mass, m; H is the length of the heat exchanger, m; (2) Set the deviation angle, and it is appropriate that the deviation angle is greater than 4 times the thermal influence radius angle: θ d ≥4θ (6) where θ d is the deviation angle of the coaxial casing heat exchanger in the middle and deep layers, °.

2. A deviation design method for a medium-deep coaxial casing heat exchanger according to claim 1, characterized in that: Step 2: Determine the maximum thermal influence radius R of the medium-deep geotechnical body max The specific process is as follows: a. Taking the full-life cycle operation as the research object, set the total heat exchange duration of the medium-deep coaxial casing heat exchanger to 30 years; among them, the heat extraction time is set to 4 months per year, and the remaining 8 months are out of operation; b. According to formulas (6) and (9), after 30 years of heat exchange in the heat exchanger, the temperature difference between the temperature of the rock and soil mass at different depths z s , different radial distances r s and the temperature of the rock and soil mass in the undisturbed state can be calculated; The temperature distribution of the medium-deep geotechnical body in the undisturbed state can be expressed by the following formula: T s0 (z s ,r) = T sur + G×z s (7) where T s0 (z s , r) is the temperature distribution of the rock and soil mass in the undisturbed state, °C; T sur is the surface temperature, °C; G is the geothermal gradient, °C·m -1 ; z s is the depth of the rock and soil mass, m The change in the geotechnical body temperature can be calculated by the following formula: θ s (z s ,r s ,t) = T s (z s ,r s ,t) - T s0 (z s ,r) (8) where θ s is the change value of the temperature of the rock and soil mass, °C; T s (z s , r s , t) is the temperature distribution of the rock and soil mass at different depths z s and different radial distances r s at time t, °C; c. The radial distance r at a temperature difference of 0.1 °C s is taken as the thermal influence radius of the rock and soil mass. The thermal influence radii of the rock and soil mass at different depths z s can be obtained, and the maximum value among them is the maximum thermal influence radius R of the rock and soil mass max .

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