A variable geological structure test system and test method for energy tunnels

Abstract

The present invention discloses a variable geological structure test system and test method for an energy tunnel, comprising a test box and a temperature-controlled water tank, an insulation water tank, and a nanofluid mixing box distributed around and connected to the test box; the test box comprises an outer box and an energy tunnel module, the outer box being installed with an inner box, the inner box being installed with a lower module, a middle pore module, and an upper module in order from bottom to top, and the energy tunnel module being installed in the middle area of the middle pore module and the upper module; the fluid area between the outer box and the inner box is a insulation water area, the fluid area between the inner box and the middle pore module is a nanofluid area, and the fluid area inside the energy tunnel module is a circulating water area. The present invention can perform multiple working condition tests according to test needs, improve the implementation efficiency of multiple groups of tests, avoid material waste, and ensure the accuracy of test results during the test process.

Description

Technical Field

[0001] The present invention belongs to the technical field of underground engineering model testing, and in particular relates to a variable geological structure testing system and a testing method for energy tunnels. Background Art

[0002] An energy tunnel is a new type of geothermal utilization that combines the main structure of a tunnel or ancillary structures, uses heat exchange tubes buried in the segments or lining, and uses the rock and soil around the tunnel as a low-level heat source or heat sink. At present, existing energy tunnel model test chambers are mostly designed with a single working mode, which cannot adapt to the flexible adjustment of multi-parameter working conditions such as energy structure, soil thickness, porosity, and soil ratio. When the test plan changes, the original test chamber needs to be dismantled as a whole. The soil and concrete structure filled in the chamber are mostly disposable and difficult to recycle after removal, resulting in large material consumption during the test process and significantly increased costs. At the same time, the size of the energy tunnel segments in the existing model test is limited, resulting in cumbersome installation and connection of the heat exchange tubes, which seriously affects the efficiency of the test chamber construction and the continuity of multiple groups of tests, and restricts the implementation of multi-working condition tests of energy tunnels. Summary of the Invention

[0003] The purpose of the present invention is to provide a variable geological structure test system and test method for energy tunnels, which can carry out multi-condition tests according to test needs, improve the implementation efficiency of multiple groups of tests, avoid material waste, and ensure the accuracy of test results during the test process.

[0004] To achieve the above-mentioned object, the present invention provides a variable geological structure test system for energy tunnels, comprising a test box and a temperature-controlled water tank, a heat-insulating water tank, and a nanofluid mixing box distributed around the test box and connected to the test box;

[0005] The test box includes an outer box and an energy tunnel module. The inner box is installed in the outer box. The lower module, the middle pore module, and the upper module are installed in the inner box from bottom to top. The energy tunnel module is installed in the middle area of the middle pore module and the upper module.

[0006] The fluid area between the outer box and the inner box is the insulation water area, the fluid area between the inner box and the middle pore module is the nanofluid area, and the fluid area inside the energy tunnel module is the circulating water area.

[0007] As a further solution of the present invention: the test box is provided with a circulating water inlet valve and a circulating water outlet valve connected to the temperature-controlled water tank, an insulation water inlet valve and an insulation water outlet valve connected to the insulation water tank, and a nanofluid inlet valve and a nanofluid outlet valve connected to the nanofluid mixing box;

[0008] The insulation water outlet valve is connected to the insulation water tank through the insulation water filter and insulation water pump which are connected in sequence;

[0009] The nanofluid inlet valve is connected to the nanofluid mixing box through a nanofluid pump.

[0010] As a further solution of the present invention, the inner box is fixed to the center of the outer box by outer box fixing rods and outer box fixing bolts, the top opening of the outer box is connected to a test box cover, and an exhaust valve is installed on the test box cover.

[0011] As a further solution of the present invention: the inner box includes an inner box bottom plate, an inner box base is connected above the inner box bottom plate, inner box side panels forming a rectangular cavity are connected around the upper periphery of the inner box base, adjacent inner box side panels are connected by inner box fixing bolts, the top end of the inner box side panel protrudes outward and is provided with an inner box flange connected to the outer box fixing rod, the inner side of the inner box side panel is provided with an interlayer guide groove and a trough body for installing the lower module, the middle pore module and the upper module.

[0012] As a further solution of the present invention: the upper module includes an upper module base plate, the upper module side plates are connected to the periphery above the upper module base plate, and the flow channel longitudinal partition is connected in the middle, the adjacent upper module side plates are connected by upper module bolts, the upper module side plates and the top ends of the flow channel longitudinal partitions are jointly connected to the upper module cover plate, and cooperate with the upper module side plates and the upper module base plate to form a "U"-shaped cavity, and the outer side of the upper module side plates is provided with an upper module plug that matches the trough body.

[0013] As a further solution of the present invention: the central pore module includes a grid plate and a longitudinal flow channel partition, the longitudinal flow channel partition is arranged in the middle of the grid plate, a plurality of soil units are connected to the grid plate through soil unit bolts, and a central pore module plug matching the trough body is provided on the outside of the grid plate.

[0014] As a further solution of the present invention: the soil unit includes a soil unit bottom plate, a soil unit side plate connected to the periphery above the soil unit bottom plate, and a soil unit cover plate connected to the top of the soil unit side plate.

[0015] As a further solution of the present invention: the lower module includes a lower module base plate, the lower module side plates are connected to the periphery above the lower module base plate, the adjacent lower module side plates are connected by lower module bolts, the top of the lower module side plates are connected to the lower module cover plate, and the lower module base plate and the lower module side plates are combined to form a rectangular cavity, and the outer side of the lower module side plates is provided with a lower module plug that matches the trough body.

[0016] As a further solution of the present invention: a plurality of energy tunnel modules are provided, adjacent ones of which are fixed together by bolts of energy tunnel modules and connected by inter-loop pipes to form a series heat exchange pipeline;

[0017] The energy tunnel module includes an energy tunnel module side panel, and the two sides of the energy tunnel module side panel are respectively connected to the energy tunnel module front cover plate and the energy tunnel module rear cover plate to form a cavity. The energy tunnel module front cover plate and the energy tunnel module rear cover plate are provided with flow channels, and are divided into multiple circulating water areas by transverse partitions of the flow channels. A circular concrete pipe segment is connected to the middle of the cavity, and a heat exchange pipe is connected inside the concrete pipe segment. The two ends of the heat exchange pipe are fixed by a heat exchange pipe fixing sleeve, and are respectively connected to the flow channels on the energy tunnel module front cover plate and the energy tunnel module rear cover plate.

[0018] To achieve the above object, the present invention further provides a test method for a variable geological structure test system for an energy tunnel, comprising the following steps:

[0019] S1. Install the inner box and fix it in the center of the outer box;

[0020] S2. Determine the thickness of the lower module, the middle pore module, and the upper module;

[0021] S3, filling the lower module, the middle pore module and the upper module with the corresponding soil samples from the in-situ test;

[0022] S4. Place the lower module, the middle pore module, and the upper module into the inner box in sequence, and align the inserts of each module with the corresponding slots of the inner box;

[0023] S5. Determine the size of the concrete segments, prefabricate the energy tunnel modules with the longitudinal heat exchange tubes, and fill them with the corresponding soil samples from the in-situ test;

[0024] S6. Inserting multiple energy tunnel modules into the empty slots formed by the longitudinal partitions of the upper flow channel of the upper module and the middle pore module, and connecting them into a circulation loop;

[0025] S7. Install the temperature-controlled water tank, the heat-insulating water tank, the nanofluid mixing tank, and the supporting water pump, and complete the pipe connections;

[0026] S8, open the insulation water and circulating water pipeline system, start the temperature control water tank and set the heating power to 0;

[0027] S9. Determine the volume fraction of nanoparticles based on the effective thermal conductivity of the soil test and the Maxwell–Garnett equation in the in situ test;

[0028] S10, adding nanoparticles to the nanofluid mixing box according to the calculation results, and opening the nanofluid piping system;

[0029] S11. Set the test temperature through the temperature-controlled water tank and record the return water temperature and flow rate of the circulating water;

[0030] S12. After the test is completed, the circulating water pipeline, the insulation water pipeline and the nanofluid pipeline are emptied in sequence.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1) Through modular design, the present invention can quickly adjust the number of heat exchange tubes in the energy tunnel, the porosity of the porous strata, and the thickness of different strata according to actual needs, overcoming the shortcomings of existing energy tunnel model test systems, such as fixed working conditions and poor adaptability.

[0033] 2) The modules in the test chamber of the present invention are detachable and re-assembled, eliminating the need to dismantle the entire chamber due to test plan adjustments, significantly reducing soil and concrete sample waste and lowering testing costs. Furthermore, the heat exchange tubes in the energy tunnel module are quickly fixed and connected via a fixed sleeve and front and rear cover plate flow channel system, avoiding the cumbersome installation of heat exchange tubes in existing model tests and improving the efficiency of arranging multiple sets of continuous tests.

[0034] 3) This invention introduces nanofluids to compensate for the effective thermal conductivity of the soil and combines the Maxwell–Garnett formula to calculate the volume fraction of nanoparticles, effectively solving the problem of insufficient thermodynamic similarity of soil thermal conductivity when simulating soil pores and ensuring the accuracy of the test data. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0036] Figure 2 It is a schematic structural diagram of the test box of the present invention;

[0037] Figure 3 Schematic diagram of the internal structure of the test box of the present invention

[0038] Figure 4 This is a schematic diagram of the inner box structure of the present invention;

[0039] Figure 5 This is a schematic diagram of the energy tunnel module structure of the present invention;

[0040] Figure 6 Schematic diagram of the internal structure of the energy tunnel module of the present invention

[0041] Figure 7 This is a schematic diagram of the upper module structure of the present invention;

[0042] Figure 8 This is a schematic diagram of the structure of the middle pore module of the present invention;

[0043] Figure 9 This is a schematic diagram of the lower module structure of the present invention;

[0044] Figure 10 It is a schematic diagram of the module structure of the present invention;

[0045] Figure 11 Schematic diagram of the water circulation system of the present invention.

[0046] In the figure: 1, test box, 2, temperature-controlled water tank, 3, insulation water tank, 4, nanofluid mixing box, 5, circulating water inlet valve, 6, circulating water outlet valve, 7, nanofluid inlet valve, 8, nanofluid outlet valve, 9, insulation water inlet valve, 10, insulation water outlet valve, 11, temperature-controlled water tank outlet pipe, 12, temperature-controlled water tank inlet pipe, 13, nanofluid outlet pipe, 14, nanofluid pump, 15, nanofluid inlet pipe, 16, insulation water tank outlet pipe, 17, insulation water Pump, 18, Insulated water tank inlet pipe, 19, Insulated water filter, 20, Test chamber cover, 21, Exhaust valve, 22, Outer box, 23, Outer box fixing rod, 24, Outer box fixing bolts, 25, Inner box, 26, Inner box side panel, 27, Inner box base, 28, Inner box bottom plate, 29, Inner box flange, 30, Tank body, 31, Interlayer guide groove, 32, Inner box fixing bolts, 33, Circulating water inlet pipe, 34, Circulating water outlet pipe, 35, Insulated water inlet pipe, 36, Insulated water outlet pipe, 37, inter-ring pipe, 38, energy tunnel module, 39, energy tunnel module front cover, 40, heat exchange tube fixing sleeve, 41, heat exchange tube, 42, concrete pipe segment, 43, energy tunnel module side plate, 44, energy tunnel module rear cover, 45, energy tunnel module bolt, 46, flow channel, 47, flow channel transverse partition, 48, upper module, 49, upper module cover, 50, upper module side plate, 51, upper module bottom plate, 52, upper Upper module bolts, 53, upper module plug-in, 54, flow channel longitudinal partition, 55, middle pore module, 56, middle pore module plug-in, 57, grid plate, 58, soil unit, 59, soil unit cover, 60, soil unit side plate, 61, soil unit bottom plate, 62, soil unit bolts, 63, lower module, 64, lower module cover, 65, lower module side plate, 66, lower module bottom plate, 67, lower module bolts, 68, lower module plug-in. DETAILED DESCRIPTION

[0047] The present invention will be further described below by way of examples.

[0048] like Figures 1 to 4 and Figure 10 As shown, a variable geological structure test system for energy tunnels includes a test box 1 and a temperature-controlled water tank 2, a heat-insulating water tank 3, and a nanofluid mixing box 4 distributed around and connected to the test box 1;

[0049] The test box 1 includes an outer box 22 and an energy tunnel module 38. The inner box 25 is installed in the outer box 22. The lower module 63, the middle pore module 55, and the upper module 48 are installed in the inner box 25 from bottom to top. The energy tunnel module 38 is installed in the middle area of the middle pore module 55 and the upper module 48.

[0050] The fluid area between the outer box 22 and the inner box 25 is the insulation water area, the fluid area between the inner box 25 and the middle pore module 55 is the nanofluid area, and the fluid area inside the energy tunnel module 38 is the circulating water area.

[0051] Further, such as Figure 1 and Figure 11 As shown, the test box 1 is provided with a circulating water inlet valve 5 and a circulating water outlet valve 6 connected to the temperature-controlled water tank 2, an insulation water inlet valve 9 and an insulation water outlet valve 10 connected to the insulation water tank 3, and a nanofluid inlet valve 7 and a nanofluid outlet valve 8 connected to the nanofluid mixing box 4;

[0052] The insulation water outlet valve 10 is connected to the insulation water tank 3 through the insulation water filter 19 and the insulation water pump 17 which are connected in sequence;

[0053] The nanofluid inlet valve 7 is connected to the nanofluid mixing box 4 via a nanofluid pump 14 .

[0054] Further, such as Figures 2 to 4 As shown, the inner box 25 is fixed to the center of the outer box 22 by the outer box fixing rod 23 and the outer box fixing bolt 24. The top opening of the outer box 22 is connected to the test box cover 20, and the test box cover 20 is installed with an exhaust valve 21.

[0055] Furthermore, the inner box 25 includes an inner box bottom plate 28, an inner box base 27 is connected above the inner box bottom plate 28, and an inner box side plate 26 forming a rectangular cavity is connected around the upper periphery of the inner box base 27. Adjacent inner box side plates 26 are connected by inner box fixing bolts 32. The top end of the inner box side plate 26 protrudes outward and is provided with an inner box flange 29 connected to the outer box fixing rod 23. The inner side of the inner box side plate 26 is provided with an interlayer guide groove 31 and a groove body 30 for installing the lower module 63, the middle pore module 55, and the upper module 48. The groove body 30 includes a square groove and a triangular groove. The square groove is provided above the triangular groove. The interlayer guide groove 31 is located on one side of the groove body 30. The length of the interlayer guide groove 31 spans the lower module 63, the middle pore module 55, and the upper module 48.

[0056] Further, such as Figure 7As shown, the upper module 48 includes an upper module base plate 51, the upper periphery of the upper module base plate 51 is connected to the upper module side plate 50, and the middle is connected to the flow channel longitudinal partition plate 54, and the adjacent upper module side plates 50 are connected by upper module bolts 52. The upper module side plates 50 and the top ends of the flow channel longitudinal partition plates 54 are jointly connected to the upper module cover plate 49, and cooperate with the upper module side plates 50 and the upper module base plate 51 to form a "U"-shaped cavity, and the outer side of the upper module side plate 50 is provided with an upper module plug-in block 53 that matches the trough body 30.

[0057] Further, such as Figure 8 As shown, the middle pore module 55 includes a grid plate 57 and a longitudinal flow channel partition 54. The longitudinal flow channel partition 54 is arranged in the middle of the grid plate 57. A plurality of soil units 58 are connected to the grid plate 57 through soil unit bolts 62. A middle pore module plug 56 matching the trough body 30 is provided on the outside of the grid plate 57.

[0058] Furthermore, the soil unit 58 includes a soil unit bottom plate 61 , a soil unit side plate 60 connected to the periphery of the soil unit bottom plate 61 , and a soil unit cover plate 59 connected to the top of the soil unit side plate 60 .

[0059] Further, such as Figure 9 As shown, the lower module 63 includes a lower module base plate 66, and the lower module side plates 65 are connected to the periphery above the lower module base plate 66. Adjacent lower module side plates 65 are connected by lower module bolts 67. The top of the lower module side plates 65 are connected to the lower module cover plate 64, and cooperate with the lower module base plate 66 and the lower module side plates 65 to form a rectangular cavity. The outer side of the lower module side plates 65 is provided with a lower module plug 68 that matches the trough body 30.

[0060] The lower module 63, the middle pore module 55, and the upper module 48 are matched with the trough body 30 through their respective plug-ins and are installed in the inner box 25 in sequence. The energy tunnel module 38 is inserted into the empty groove formed by the longitudinal partition 54 of the flow channel on the upper module 48 and the middle pore module 55.

[0061] Further, such as Figure 3 、 Figure 5 and Figure 6 As shown, the energy tunnel modules 38 are provided in multiple groups, with six groups shown in the figure. The adjacent energy tunnel modules are connected and fixed by energy tunnel module bolts 45, and connected by inter-ring pipes 37 to form a series heat exchange pipe 41; the water inlet of the first energy tunnel module 38 is connected to the circulating water inlet valve 5 through the circulating water inlet pipe 33, and the water outlet of the last energy tunnel module 38 is connected to the circulating water outlet valve 6 through the circulating water outlet pipe 34, thereby connecting to form a circulation.

[0062] The energy tunnel module 38 includes an energy tunnel module side panel 43, and the two sides of the energy tunnel module side panel 43 are respectively connected to the energy tunnel module front cover plate 39 and the energy tunnel module rear cover plate 44 to form a cavity. The energy tunnel module front cover plate 39 and the energy tunnel module rear cover plate 44 are provided with a flow channel 46, and are divided into multiple circulating water areas by a flow channel transverse partition 47. A circular concrete pipe segment 42 is connected to the middle of the cavity, and a heat exchange tube 41 is connected to the concrete pipe segment 42. The two ends of the heat exchange tube 41 are fixed by a heat exchange tube fixing sleeve 40, and are respectively connected to the flow channel 46 on the energy tunnel module front cover plate 39 and the energy tunnel module rear cover plate 44.

[0063] A test method for a variable geological structure test system for an energy tunnel comprises the following steps:

[0064] S1: Place the outer box 22 in the center of the test system, fix the inner box side panels 26, inner box base 27 and inner box bottom plate 28 into a whole with the inner box fixing bolts 32, and fix the fixed inner box 25 in the center of the outer box 22 with the outer box fixing rods 23 and outer box fixing bolts 24.

[0065] S2: Determine the thickness of the lower stratum based on the similarity ratio between the on-site in-situ test and the model test, and select the lower module side plate 65 of corresponding size based on the thickness.

[0066] The thickness of the central pore stratum is determined based on the similarity ratio between the on-site in-situ test and the model test, and the soil unit side plate 60 of the corresponding size is selected based on the thickness; in order to ensure that the nanoparticles do not settle in the central pore module 55, the length and width of the soil unit 58 need to be calculated through Pe.

[0067] The volume of the central pore module 55 is Vt. The porosity of the corresponding formation, measured through in-situ testing, is η. The spacing between soil cells 58 is d, and the length, width, and height of soil cells 58 are a, b, and c, respectively. The outer shell of soil cell 58 includes a soil cell cover 59, side plates 60, and a bottom plate 61. The thickness of the outer shell is t.

[0068] The number of soil cells 58 can be expressed as: ;

[0069] Calculate the sedimentation velocity according to the Stokes sedimentation velocity formula: ;

[0070] where v s is the sedimentation velocity, ρ p is the nanoparticle density, ρ f is the density of the base liquid, g is the acceleration of gravity, r is the radius of the nanoparticle, μ f is the base fluid dynamic viscosity.

[0071] Calculate the average diffusion velocity: ;

[0072] Where D is the Brownian diffusion coefficient, k B is the Boltzmann constant, T is the lowest temperature during the test, it is the initial temperature of the formation during the summer test condition, and it is the water supply temperature of the temperature-controlled water tank during the winter test condition.

[0073] Calculate the Pe number: ;

[0074] Determine whether the calculated Pe number is less than 0.5. If Pe < 0.5, create the middle porous layer according to the current a, b, c, and d. If Pe ≥ 0.5, reset a, b, c, and d until Pe < 0.5.

[0075] The corresponding grid plate 57, soil unit cover plate 59 and soil unit bottom plate 61 are selected according to the calculation results.

[0076] The thickness of the upper stratum is determined based on the similarity ratio between the on-site in-situ test and the model test, and the upper module side plates 50 of corresponding sizes are selected based on the thickness.

[0077] S3: Soil samples are taken from the lower stratum of the in-situ test, and the cavity between the lower module cover 64 and the lower module bottom plate 66 is filled, and the lower module bolts 67 are used to fix them to form the lower module 63; soil samples are taken from the middle porous stratum of the in-situ test, and the cavity between the soil unit cover 59 and the soil unit bottom plate 61 is filled, and the soil unit bolts 62 are used to fix them, and the completed soil units 58 are placed in the grid plates 57 in turn to form the middle porous module 55; soil samples are taken from the upper stratum of the in-situ test, and the cavity between the upper module cover 49 and the upper module bottom plate 51 is filled, and the upper module bolts 52 are used to fix them to form the upper module 48.

[0078] S4: Place the lower module 63, the middle pore module 55 and the upper module 48 into the inner box 25 in sequence, align the lower module plug 68 on the lower module 63 with the triangular groove in the slot body 30 on the inner box 25, align the middle pore module plug 56 on the middle pore module 55 with the square groove in the slot body 30 on the inner box 25, and align the upper module plug 53 on the upper module 48 with the square groove on the inner box 25. The setting of the square groove and the triangular groove can fix the height of the middle pore module 55. The upper module plug 53 can adopt a square structure or a triangular structure, as long as it can be fixed in position corresponding to the slot body 30.

[0079] S5: Determine the size of the concrete pipe segment 42 based on the on-site tunnel size and the similarity ratio of the model test, prefabricate the concrete pipe segment 42 with the heat exchange tube 41 arranged longitudinally, and determine the size of the energy tunnel module 38. Place the concrete pipe segment 42 with the heat exchange tube 41 into the annular protrusion on the rear cover plate 44 of the energy tunnel module. Fill the cavity between the front cover plate 39 and the rear cover plate 44 of the energy tunnel module according to the in-situ test soil test, and fix it with the energy tunnel module bolts 45. Fix the heat exchange tube fixing sleeve 40 at both ends of each heat exchange tube 41 to form the energy tunnel module 38.

[0080] S6: Place multiple energy tunnel modules 38 in parallel into the empty groove formed by the longitudinal partitions 54 of the upper flow channel of the upper module 48 and the middle pore module 55, use the inter-annular pipes 37 to connect different energy tunnel modules 38 in an interlaced manner to form a series heat exchange loop, connect the water inlet of the first energy tunnel module 38 to the circulating water inlet valve 5 through the circulating water inlet pipe 33, connect the water outlet of the last energy tunnel module 38 to the circulating water outlet valve 6 through the circulating water outlet pipe 34, connect the nanofluid inlet pipe 15 to the nanofluid inlet valve 7, connect the nanofluid outlet pipe 13 to the nanofluid outlet valve 8, connect the insulation water inlet pipe 35 to the insulation water inlet valve 9, and connect the insulation water outlet pipe 36 to the insulation water outlet valve 10. After the above pipeline connections are completed, install the test box cover 20.

[0081] S7: Place the temperature-controlled water tank 2, the thermal insulation water tank 3, the nanofluid mixing box 4, the nanofluid pump 14 and the thermal insulation water pump 17 around the test box 1 in sequence, the circulating water outlet of the temperature-controlled water tank 2 is connected to the circulating water inlet valve 5 in sequence, the circulating water inlet is connected to the circulating water outlet valve 6, the outlet of the thermal insulation water tank 3 is connected to the thermal insulation water tank outlet pipe 16, the thermal insulation water inlet valve 9, the thermal insulation water outlet valve 10, the thermal insulation water filter 19, the thermal insulation water pump 17 and the thermal insulation water tank inlet pipe 18 in sequence, and the liquid outlet of the nanofluid mixing box 4 is connected to the nanofluid outlet pipe 13, the nanofluid pump 14, the nanofluid inlet pipe 15 and the nanofluid inlet valve 7 in sequence.

[0082] S8: Open the insulation water inlet valve 9, the insulation water pump 17 and the insulation water outlet valve 10, so that the insulation water fills the fluid area between the outer box 22 and the inner box 25, open the circulating water inlet valve 5 and the circulating water outlet valve 6, start the temperature-controlled water tank 2, set the heating power to 0, and monitor the return water temperature until the return water temperature reaches the initial temperature in the in-situ test.

[0083] S9: Calculate the volume fraction of nanoparticles in the nanofluid based on the effective thermal conductivity of the soil sample obtained from the in-situ test and the Maxwell–Garnett formula.

[0084] Since the inner box 25 in the present invention mainly affects the soil unit 58 in the middle pore module 55, and has a relatively small impact on the entire stratum where the upper module 48 and the lower module 63 above and below it are located, thermal conductivity compensation is mainly performed on the middle pore module 55.

[0085] The thermal conductivity of the outer shell material of the soil unit 58 is ks, and the thermal conductivity of the base fluid in the nanofluid is kf.

[0086] By adding nanoparticles with a volume fraction of φ into the nanofluid base liquid, a nanofluid with a thermal conductivity of knf is obtained.

[0087] The purpose of using nanofluid in the present invention is to increase the effective thermal conductivity keff of the nanofluid in the soil unit 58 and the middle pore module 55 to the target value by adjusting the type and volume fraction of nanoparticles, so as to reduce the negative impact of the shell of the soil unit 58 on the heat transfer process.

[0088] For the composite system of the soil unit 58 and the nanofluid in the middle pore module 55, the effective thermal conductivity k is calculated using the Maxwell–Garnett formula. eff : ;

[0089] Among them, φ s is the volume fraction of the shell of soil unit 58 in the composite system, and is calculated as follows: ;

[0090] k nf Determined by the volume fraction φ of the nanofluid, the calculation formula is as follows: ;

[0091] Among them, k p is the thermal conductivity of the nanoparticles.

[0092] Therefore, when the effective thermal conductivity k of the corresponding formation in the in-situ test is obtained through thermal response testing, eff Then, the shell volume fraction of soil element 58 is determined by s The thermal conductivity k of the nanofluid is calculated by the implicit equation obtained from the Maxwell–Garnett formula of the composite system. nf :

[0093] ;

[0094] The obtained k nf Substituting into the Maxwell–Garnett formula of nanofluid, the volume fraction φ of nanoparticles in the nanofluid is obtained as: ;

[0095] The number of soil units 58 and the volume fraction of nanoparticles are obtained according to the above calculations. After the soil units 58 are evenly placed in the middle pore module 55, a corresponding amount of nanoparticles is added to the nanofluid mixing box 4 and an experiment is carried out.

[0096] S10: Add nanoparticles to the nanofluid mixing box 4 according to the calculation results, open the nanofluid inlet valve 7, close the nanofluid outlet valve 8, start the nanofluid pump 14, and fill the upper module 48, the middle pore module 55 and the lower module 63 with nanofluid.

[0097] S11: The temperature-controlled water tank 2 is set to the test temperature and the test is started. During the test, the return water temperature and flow rate of the circulating water are recorded by the thermometer and flow meter provided in the temperature-controlled water tank 2.

[0098] S12: After the test is completed, close the circulating water inlet valve 5, drain the circulating water through the temperature-controlled water tank 2, close the insulation water inlet valve 9, and after the insulation water pump 17 has emptied the insulation water, close the insulation water outlet valve 10 and the insulation water pump 17, close the nanofluid inlet valve 7 and the nanofluid pump 14, open the nanofluid outlet valve 8, and at the same time remove the upper inner box fixing bolts 32 to drain the nanofluid in the inner box 25.

Claims

1. A variable geological structure test system for energy tunnels, characterized in that: It comprises a test box (1), a temperature-controlled water tank (2), a heat-insulating water tank (3), and a nanofluid mixing box (4) which are distributed around the test box (1) and connected to the test box (1); The test box (1) includes an outer box (22) and an energy tunnel module (38), wherein an inner box (25) is installed in the outer box (22), and a lower module (63), a middle pore module (55), and an upper module (48) are installed in the inner box (25) from bottom to top, and the energy tunnel module (38) is installed in the middle area of the middle pore module (55) and the upper module (48); The fluid area between the outer box (22) and the inner box (25) is a heat preservation water area, the fluid area between the inner box (25) and the middle pore module (55) is a nanofluid area, and the fluid area inside the energy tunnel module (38) is a circulating water area; The inner box (25) includes an inner box bottom plate (28), an inner box base (27) is connected above the inner box bottom plate (28), an inner box side plate (26) is connected to the periphery of the inner box base (27) to form a rectangular cavity, adjacent inner box side plates (26) are connected by inner box fixing bolts (32), the top of the inner box side plate (26) protrudes outward and is provided with an inner box flange (29) connected to the outer box fixing rod (23), and the inner side of the inner box side plate (26) is provided with an interlayer guide groove (31) and a groove body (30) for installing a lower module (63), a middle pore module (55), and an upper module (48); The upper module (48) includes an upper module base plate (51), an upper module side plate (50) is connected to the upper periphery of the upper module base plate (51), and a flow channel longitudinal partition plate (54) is connected in the middle. Adjacent upper module side plates (50) are connected by upper module bolts (52). The top ends of the upper module side plates (50) and the flow channel longitudinal partition plates (54) are connected to the upper module cover plate (49) and cooperate with the upper module side plates (50) and the upper module base plate (51) to form a "U"-shaped cavity. An upper module plug (53) matching the trough body (30) is provided on the outer side of the upper module side plates (50); The middle pore module (55) includes a grid plate (57) and a flow channel longitudinal partition (54), the flow channel longitudinal partition (54) is arranged in the middle of the grid plate (57), a plurality of soil units (58) are connected to the grid plate (57) via soil unit bolts (62), and a middle pore module plug (56) matching the trough body (30) is provided on the outside of the grid plate (57); The energy tunnel modules (38) are provided in multiple groups, and adjacent groups are connected and fixed by energy tunnel module bolts (45), and connected by inter-ring pipes (37) to form a series heat exchange pipeline; The energy tunnel module (38) includes an energy tunnel module side plate (43), and both sides of the energy tunnel module side plate (43) are connected to the energy tunnel module front cover plate (39) and the energy tunnel module rear cover plate (44) to form a cavity. The energy tunnel module front cover plate (39) and the energy tunnel module rear cover plate (44) are provided with flow channels (46) and are divided into multiple circulating water areas by flow channel transverse partitions (47). A circular concrete pipe segment (42) is connected to the middle of the cavity, and a heat exchange tube (41) is connected to the concrete pipe segment (42). Both ends of the heat exchange tube (41) are fixed by heat exchange tube fixing sleeves (40) and are respectively communicated with the flow channels (46) on the energy tunnel module front cover plate (39) and the energy tunnel module rear cover plate (44).

2. A variable geological structure test system for energy tunnels according to claim 1, characterized in that: The test box (1) is provided with a circulating water inlet valve (5) and a circulating water outlet valve (6) connected to the temperature-controlled water tank (2), a heat preservation water inlet valve (9) and a heat preservation water outlet valve (10) connected to the heat preservation water tank (3), and a nanofluid inlet valve (7) and a nanofluid outlet valve (8) connected to the nanofluid mixing box (4); The insulation water outlet valve (10) is connected to the insulation water tank (3) through the insulation water filter (19) and the insulation water pump (17) which are connected in sequence; The nanofluid inlet valve (7) is connected to the nanofluid mixing box (4) via a nanofluid pump (14).

3. A variable geological structure test system for energy tunnels according to claim 1 or 2, characterized in that: The inner box (25) is fixed to the center of the outer box (22) through the outer box fixing rod (23) and the outer box fixing bolt (24). The top opening of the outer box (22) is connected to a test box cover (20), and an exhaust valve (21) is installed on the test box cover (20).

4. A variable geological structure test system for energy tunnels according to claim 3, characterized in that: The lower module (63) includes a lower module base plate (66), a lower module side plate (65) connected to the periphery of the upper portion of the lower module base plate (66), adjacent lower module side plates (65) are connected by lower module bolts (67), the top of the lower module side plate (65) is connected to a lower module cover plate (64), and cooperates with the lower module base plate (66) and the lower module side plate (65) to form a rectangular cavity, and a lower module plug (68) matching the trough body (30) is provided on the outer side of the lower module side plate (65).

5. The variable geological structure test system for energy tunnel according to claim 3, characterized in that: The soil unit (58) comprises a soil unit bottom plate (61), a soil unit side plate (60) connected to the periphery of the upper portion of the soil unit bottom plate (61), and a soil unit cover plate (59) connected to the top of the soil unit side plate (60).

6. A test method for a variable geological structure test system for an energy tunnel according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1, install the inner box (25) and fix it in the center of the outer box (22); S2, determining the thickness of the lower module (63), the middle pore module (55) and the upper module (48); S3, filling the lower module (63), the middle pore module (55) and the upper module (48) with the corresponding soil samples from the in-situ test; S4, placing the lower module (63), the middle pore module (55), and the upper module (48) into the inner box (25) in sequence, and aligning the inserts of each module with the corresponding slots (30) of the inner box (25); S5, determining the size of the concrete segments (42), prefabricating the energy tunnel module (38) for arranging the longitudinal heat exchange tubes (41), and filling the corresponding soil sample in the in-situ test; S6, inserting a plurality of energy tunnel modules (38) into the empty slot formed by the longitudinal partitions (54) of the upper flow channel of the upper module (48) and the middle pore module (55), and connecting them into a circulation loop; S7, installing the temperature-controlled water tank (2), the heat-insulating water tank (3), the nanofluid mixing tank (4) and the matching water pump, and completing the pipeline connection; S8, start the insulation water and circulating water pipeline system, start the temperature control water tank (2) and set the heating power to 0; S9. Determine the volume fraction of nanoparticles based on the effective thermal conductivity of the soil sample in the in situ test and the Maxwell–Garnett equation; S10, adding nanoparticles to the nanofluid mixing box (4) according to the calculation results, and opening the nanofluid pipeline system; S11, setting the test temperature through the temperature-controlled water tank (2), and recording the return water temperature and flow rate of the circulating water; S12. After the test is completed, the circulating water pipeline, the insulation water pipeline and the nanofluid pipeline are emptied in sequence.

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