CN113468743A - Medium-deep buried pipe fluid temperature field analysis method considering groundwater seepage - Google Patents

Medium-deep buried pipe fluid temperature field analysis method considering groundwater seepage Download PDF

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CN113468743A
CN113468743A CN202110735976.3A CN202110735976A CN113468743A CN 113468743 A CN113468743 A CN 113468743A CN 202110735976 A CN202110735976 A CN 202110735976A CN 113468743 A CN113468743 A CN 113468743A
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王昌龙
王鑫
鲁进利
张朋远
孙彦红
方晗
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Anhui University of Technology AHUT
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Abstract

The invention provides a temperature field analysis method of a fluid of a middle-deep buried pipe, which considers groundwater seepage and is used for calculating temperature fields of the fluid of the middle-deep buried pipe and the fluid of an outer pipe along with depth and time. Firstly, assuming that the heat transfer in a drill hole is steady-state heat transfer and approximating the drill hole as a moving linear heat source, further establishing a steady-state heat transfer equation of an inner pipe fluid and an outer pipe fluid, and establishing an equation of the wall surface temperature of the drill hole and the heat flow transferred to rock and soil by the drill hole by adopting a moving linear heat source model and combining a hot flow superposition principle; secondly, dividing the drill hole of the middle-deep buried pipe and rock soil outside the drill hole into a plurality of sections along the axial direction, setting a time step length, then dispersing the equation, and solving by adopting an iterative method, thereby obtaining the temperature field of the fluid inside and outside the middle-deep buried pipe. The method considers the influence of groundwater seepage on the buried pipe in the middle and deep layer, has high enough precision and small calculation amount.

Description

Medium-deep buried pipe fluid temperature field analysis method considering groundwater seepage
Technical Field
The invention belongs to the technical field of ground source heat pumps, and particularly relates to a method for analyzing a temperature field of a fluid in a middle-deep buried pipe in consideration of groundwater seepage, which is used for analyzing the temperature field of a fluid in an inner pipe and a fluid in an outer pipe in the middle-deep buried pipe.
Background
The medium-deep buried pipe can effectively extract the heat energy of medium-deep rock-soil mass, and has the advantages of small floor area, high heat transfer strength, low requirement on annual cold and heat load balance and the like, thereby having good application prospect. The medium-deep buried pipe fluid temperature field analysis method is a theoretical basis of medium-deep buried pipe simulation and design, so that the research on the medium-deep buried pipe fluid temperature field analysis method has important significance. At present, heat transfer models for analyzing the temperature field of the fluid in the middle-deep buried pipe are mainly classified into analytical models and numerical models.
However, the existing analysis models do not consider the influence of groundwater seepage, and the existing research shows that the influence of groundwater seepage on the heat transfer performance of the buried pipe in the middle and deep layer is large, so that the analysis models have large errors when analyzing the condition that the groundwater seepage exists. In addition, the numerical model modeling process is complicated, the calculation time is long, and the numerical model is difficult to be widely applied to actual engineering.
In 12 months in 2017, the book 60 of the geophysical journal, the book 12 th period 4741-4752 of the Yanglong, Chenqianfei, Shaohui ice, Panzhong faih, Liangliu bear and Wangquan impotence disclose an article named deep well heat exchange technical principle and heat exchange quantity evaluation, and aiming at the geothermal geological conditions of typical areas in the north of China, the article respectively adopts a Beier analytical method and a dual continuous medium numerical simulation method (based on an OpenGeoSys simulation platform) to calculate the heat exchange quantity under the short-term (4 months) heat collection situation and the long-term (30 years) heat collection situation. The analytical method and the numerical method both show that the upper limit of the heat exchange power of the linear meter is not more than 150W. Under the condition of intermittent heat collection, namely heat supply for 12 hours every day and 12 hours stopping, the linear meter heat exchange power can be doubled, but the total heat exchange quantity is basically unchanged, and the fluctuation of the water temperature in one day is obviously increased. Sensitivity analysis is carried out on the numerical model, and the fact that under the condition of a certain geothermal gradient, the influence of the well depth on the heat exchange power of the extension meter is small, and the influence of the formation heat conductivity on the heat exchange power is obvious is found. Finally, the main means for improving the heat exchange capacity of the deep well heat exchange technology is to increase the heat convection in the formation around the well, or to increase the contact area between the circulating water and the rock.
In 2 months of 2020, the book 41 of the solar energy journal of < 2 nd > 158 th and 164 th pages, Cai Anhulong, Liujun, Wan Feng and Wangzhihua disclose an article named as 'simulation and stability research on heat exchange performance of deep buried pipe heat exchangers', and the article takes a deep ground source heat pump buried pipe heat exchanger as a research object and carries out numerical simulation and experimental research on heat exchange characteristics of the deep buried pipe heat exchanger. And establishing a deep buried pipe heat exchanger heat transfer model considering the axial geothermal gradient, performing simulation calculation, and verifying the correctness of the model through the test data of the demonstration engineering site. The performance stability of the heat exchange of the deep buried pipe heat exchanger is researched, and the heat exchange performance of the deep buried pipe heat exchanger is basically stable under continuous long-term operation and intermittent long-term operation. When the rock-soil temperature recovery device operates according to different operation-stop ratios, the rock-soil temperature recovery effect is good. Research results show that the deep ground source heat pump has better heat exchange performance and operation stability, and provides a new idea for development and utilization of deep ground heat energy.
In 7 months of 2020, volume 38, page 7, No. 887 and No. 892 of renewable energy, Du sweet, satisfaction, Jiang nationality, Fangliang and Fangzhuan disclosed an article named "modeling and heat extraction analysis of heat transfer of a casing pipe type buried pipe heat exchanger in middle and deep layers" which simulates and calculates nominal heat extraction amount of the casing pipe type buried pipe heat exchanger in middle and deep layers by a numerical simulation method. Simulation results show that the nominal heat extraction of the double pipe type intermediate and deep buried pipe heat exchanger increases with the drilling depth, the ground heat flow, the circulating water flow and the local atmospheric annual average temperature. The geological condition distribution of the surrounding soil layers of the sleeve type middle-deep buried pipe heat exchanger also influences the nominal heat extraction quantity of the middle-deep buried pipe heat exchanger, and the specific expression is that the smaller the heat conductivity coefficient of the shallow soil layer is, the larger the nominal heat extraction quantity of the middle-deep buried pipe heat exchanger is; the higher the heat conductivity coefficient of the deep soil layer is, the higher the nominal heat extraction quantity of the middle-deep buried pipe heat exchanger is. By adjusting relevant parameters of the buried pipe heat exchanger and selecting appropriate optimization measures such as buried pipe burying place, the double-pipe type intermediate-deep buried pipe heat exchanger can achieve considerable nominal heat extraction.
However, the analytical model in the first document, the numerical model in the second document, and the numerical model in the third document ignore the influence of groundwater seepage, and the numerical model in the first document can analyze the influence of groundwater seepage, but the calculation process of the numerical model is very complicated and the calculation amount is large. In summary, the existing methods for analyzing the temperature field of the fluid in the middle-deep buried pipe have the defects that the influence of groundwater seepage cannot be analyzed or the calculated amount is large.
Disclosure of Invention
1. Problems to be solved
Aiming at the defects that the existing method for analyzing the fluid temperature field of the middle-deep buried pipe cannot analyze the influence of underground water seepage or has large calculation amount, the method for analyzing the fluid temperature field of the middle-deep buried pipe considering the underground water seepage is used for calculating the temperature fields of the fluid of the inner pipe and the fluid of the outer pipe in the middle-deep buried pipe along with the change of depth and time, not only considers the influence of the underground water seepage in rock and soil, but also considers the temperature gradient of the ground and the temperature change of the fluid of the inner pipe and the fluid of the outer pipe in the vertical direction, has small required calculation amount, and is convenient to calculate.
2. Technical scheme
In order to solve the problems, the technical scheme adopted by the invention is as follows:
the invention relates to a middle-deep buried pipe fluid temperature field analysis method considering groundwater seepage, which comprises the steps of firstly, assuming that heat transfer in a drill hole is steady-state heat transfer and approximating the drill hole as a moving linear heat source, further establishing a steady-state heat transfer equation of inner pipe fluid and outer pipe fluid, and establishing an equation of the wall surface temperature of the drill hole and heat flow transferred to rock and soil by the drill hole by adopting a moving linear heat source model and combining a heat flow superposition principle; secondly, dividing the drill hole of the middle-deep buried pipe and rock soil outside the drill hole into a plurality of sections along the axial direction, setting a time step length, then dispersing the equation, and solving by adopting an iterative method, thereby obtaining the temperature field of the fluid inside and outside the middle-deep buried pipe.
Specifically, the analysis method of the present invention is as follows:
establishing a steady-state heat transfer equation of the inner pipe fluid and the outer pipe fluid, and assuming that the heat transfer in the drill hole of the middle-deep buried pipe is steady-state heat transfer, the heat transfer equation of the inner pipe fluid is as follows:
Figure BDA0003140164820000031
the heat transfer equation for the outer tube fluid is:
Figure BDA0003140164820000032
wherein V (t) -the volume flow of the inner and outer pipe fluid (water);
(ρc)f-volumetric specific heat capacity of the inner and outer tube fluids;
Ti(z, t) -temperature of the inner tube fluid;
z-axial coordinate (i.e., depth);
t is time;
Ta(z, t) -the temperature of the outer tube fluid;
l is the length of the buried pipe in the middle deep layer;
Ria-thermal resistance between inner and outer pipe fluids:
Figure BDA0003140164820000033
rii-the inner radius of the inner tube;
rio-the outer radius of the inner tube;
kip-the thermal conductivity of the inner tube;
hi-convective heat transfer coefficient of the inner tube fluid:
Figure BDA0003140164820000034
fi-Darcy friction coefficient of the inner tube fluid:
fi＝[0.79ln(Rei)-1.64]-2 (5)
Reireynolds number of inner tube fluid:
Figure BDA0003140164820000035
ν -kinematic viscosity of inner and outer tube fluids;
Pr-Plantt number of inner and outer tube fluids;
kf-thermal conductivity of the inner and outer tube fluids;
ha-convective heat transfer coefficient of the outer tube fluid:
Figure BDA0003140164820000041
rei-the inner radius of the outer tube;
fa-darcy coefficient of friction of the outer tube fluid:
fa＝[0.79ln(Rea)-1.64]-2 (8)
Reareynolds number of outer tube fluid:
Figure BDA0003140164820000042
q (z, t) -heat flow imparted to the rock by the borehole:
Figure BDA0003140164820000043
Rab-outer tube fluidThermal resistance to the borehole wall:
Figure BDA0003140164820000044
kep-the thermal conductivity of the outer tube;
reo-the outer radius of the outer tube;
kg-the thermal conductivity of the backfill;
rb-the borehole radius;
Figure BDA0003140164820000045
-average temperature of the borehole wall:
Figure BDA0003140164820000046
θ -circumferential coordinate;
Tb(theta, z, t) -the temperature of the wall surface of the drilled hole is analyzed and calculated by adopting a moving line heat source model and combining a hot flow superposition principle:
Figure BDA0003140164820000047
tn-the time corresponding to the nth time instant;
T0(z) -initial temperature of the rock-soil;
r-radial coordinate;
g (r, θ, z, t) — the G function of the moving line heat source model:
Figure BDA0003140164820000051
u (z) -groundwater seepage velocity at infinity from the borehole;
ks(z) -thermal conductivity of the rock-soil;
as(z) -thermal diffusivity of the rock-soil;
Figure BDA0003140164820000056
-an integral variable;
simultaneous equations (12), (13), and (14) can be derived:
Figure BDA0003140164820000052
wherein A and B are both intermediate variables:
Figure BDA0003140164820000053
Figure BDA0003140164820000054
and determining the boundary conditions of the top and the bottom of the buried pipe in the middle deep layer as follows:
Figure BDA0003140164820000055
Ti(z,t)|z＝L＝Ta(z,t)|z＝L,(t＞0) (19)
Qin(t) total heat exchange power of the buried pipe at the middle and deep layer;
Tin(t) -the inlet fluid temperature of the buried pipe at the mid-depth layer;
at the initial moment, the temperature of the inner and outer pipe fluids is equal to the initial temperature of the rock soil:
Ti(z,t)|t＝0＝Ta(z,t)|t＝0＝T0(z),(0≤z≤L) (20)
furthermore, the method for solving the heat transfer equation of the buried pipe in the middle deep layer obtained by the method comprises the following steps:
drill for burying pipe in middle and deep layerThe hole and the rock soil outside the drilled hole are divided into M sections along the axial direction, wherein the length of any middle section is L/(M-1), the lengths of the first section and the Mth section are delta z/2, and the axial coordinate of any M section is zm＝(m-1)L/(M-1)；
According to the total time t of the required simulationtolSetting a fixed time step Δ t, the total number of time segments N equals ttol,/Δ t, and tn＝nΔt；
For any time tnDiscretizing equation (1) in the axial direction:
Figure BDA0003140164820000061
wherein C is an intermediate variable:
Figure BDA0003140164820000062
for any time tnDiscretizing equation (2) in the axial direction:
Figure BDA0003140164820000063
for any time tnEquations (10) and (15) are separately discretized:
Figure BDA0003140164820000064
Figure BDA0003140164820000065
simultaneous equations (24) and (25) can be found:
Figure BDA0003140164820000066
wherein the content of the first and second substances,D(zm,tn) Intermediate variables are:
Figure BDA0003140164820000067
thus, D (z)m,tn) Can be represented by equations (24) and (27) and tnCalculating the temperature field of the outer pipe fluid and the average temperature field of the wall surface of the drill hole at all moments before the moment;
for any time tnEquations (18) and (19) may be discretized into the following equations, respectively:
Figure BDA0003140164820000068
Ti(zM,tn)＝Ta(zM,tn) (29)
based on tnSolving the equations (21), (23), (26), (28) and (29) by using an iterative method to calculate tnThe inner tube fluid temperature, the outer tube fluid temperature and the average borehole wall temperature, T, at each timei(z1,tn)、Ti(z2,tn)…Ti(zM,tn)、Ta(z1,tn)、Ta(z2,tn)…Ta(zM,tn)、
Figure BDA0003140164820000069
Therefore, based on the above steps, t can be calculated sequentially1、t2、t3…tNThe temperature distribution at the moment.
3. Advantageous effects
Compared with the prior art, the invention has the beneficial effects that:
(1) according to the method for analyzing the fluid temperature field of the middle-deep buried pipe considering the groundwater seepage, axial segmentation is only carried out on drilling holes of the middle-deep buried pipe and rock soil, and complicated two-dimensional or three-dimensional grid division is not needed, so that the calculated amount is small, and the method is suitable for practical engineering application; meanwhile, the invention considers the change of the rock-soil temperature along with the depth, and also considers the groundwater seepage in the rock-soil by adopting the moving line heat source model, thereby being capable of analyzing the influence of the groundwater seepage, and the calculated precision of the fluid temperature field of the medium-deep buried pipe is higher. Because the influence of underground water seepage is considered, when the underground water seepage speed is zero, the part is directly discarded, and higher calculation accuracy can be ensured.
(2) According to the method for analyzing the fluid temperature field of the middle-deep buried pipe in consideration of the groundwater seepage, the change of parameters such as the rock-soil temperature, the rock-soil thermophysical property and the groundwater seepage speed along with the depth is considered, and the complicated and variable heat transfer process of the middle-deep buried pipe can be simulated conveniently.
Drawings
FIG. 1 is a sectional view of a buried pipe at a mid-deep layer;
FIG. 2 is a comparison of the outlet fluid temperature calculated in example 1 of the present invention with the Fluent software simulation results;
FIG. 3 is a comparison of inlet and outlet fluid temperatures calculated in example 2 of the present invention with the results calculated by the comparative model.
Detailed Description
The invention will now be described with reference to specific examples in which the groundwater seepage velocity in example 1 is greater than zero and the groundwater seepage velocity in example 2 is equal to zero. The object of the invention is a buried pipe in the middle depth, the schematic view of which is sectioned as shown in fig. 1.
Example 1
In this embodiment, for a certain virtual middle-deep buried pipe, the fluid temperature field at different times is calculated by using the analysis method for the fluid temperature field of the middle-deep buried pipe, and the calculation result is compared with the Fluent software simulation result. The parameters of the buried pipe at the middle and deep layer are shown in Table 1.
Given that the length L of the buried pipe in the middle deep layer is 1000M, the borehole and the rock soil are divided into 201 sections along the axial direction, that is, M is 201, and Δ z is L/(M-1) is 5M;
the total time period simulated was 100 hours, i.e., t tol100 hours. Setting the time step Δ t to 2 minutes, the number of time segments N to ttol/Δt＝3000。
Initial time (i.e. t)0Time of day) inner and outer pipe fluid temperatures and rock initial temperatures are functions with respect to depth:
Figure BDA0003140164820000071
based on t0Solving equations (21), (23), (26), (28) and (29) by adopting a Gauss-Seidel iteration method to calculate t1The inner tube fluid temperature, the outer tube fluid temperature and the average borehole wall temperature, T, at each timei(z1,t1)、Ti(z2,t1)…Ti(z201,t1)、Ta(z1,t1)、Ta(z2,t1)…Ta(z201,t1)、
Figure BDA0003140164820000081
Figure BDA0003140164820000082
In the same way, based on t1Temperature field solution at time t2The inner tube fluid temperature, the outer tube fluid temperature and the average borehole wall temperature, T, at each timei(z1,t2)、Ti(z2,t2)…Ti(z201,t2)、Ta(z1,t2)、Ta(z2,t2)…Ta(z201,t2)、
Figure BDA0003140164820000083
Figure BDA0003140164820000084
By analogy, t can be calculated in turn1、t2、t3…t3000And (4) calculating the fluid temperature field by the inner pipe fluid temperature, the outer pipe fluid temperature and the average temperature of the wall surface of the drill hole at each section of time.
TABLE 1 parameters of buried pipe in the middle deep layer of example 1
Parameter(s) Numerical value Unit of
Length L of buried pipe in middle deep layer 1000 m
Inner radius r of the inner tubeii 0.038 m
Outer radius r of the inner tubeio 0.045 m
Inner radius r of the outer tubeei 0.083 m
Outer radius r of the outer tubeeo 0.089 m
Radius of borehole rb 0.1 m
Thermal conductivity k of the inner tubeip 0.05 W/m/K
Thermal conductivity k of the outer tubeep 10.0 W/m/K
Thermal conductivity k of backfillg 0.73 W/m/K
Thermal conductivity k of rock and soils(z) 2.36 W/m/K
Thermal diffusivity of rock and soil as(z) 8.46×10-7 m2/s
Groundwater seepage velocity u (z) at infinity from borehole 1.0×10-6 m/s
Thermal conductivity k of inner and outer pipe fluidsf 0.6 W/m/K
Volume of fluid inside and outside pipeSpecific heat capacity (ρ c)f 4.17×106 J/m3/K
Kinematic viscosity v of inner and outer tube fluids 1.00×10-6 m2/s
Plantt number Pr of inner and outer tube fluids 6.95 -
Volume flow of inner and outer pipe fluid V (t) 1.0×10-2 m3/s
Temperature T of fluid at inlet of buried pipe in middle and deep layerin(t) 15.0 ℃
The calculated outlet fluid temperature (i.e., z) of this example1Inner tube fluid temperature at depth) versus Fluent software simulation results are shown in figure 2. It can be seen from fig. 2 that the outlet fluid temperature calculated by the method of the present invention is well matched with the Fluent software simulation result, and the deviation between the outlet fluid temperature and the Fluent software simulation result is small, wherein the difference between the outlet fluid temperature and the Fluent software simulation result is less than 0.05 ℃ after 100 hours, which indicates that the method of the present embodiment has high calculation accuracy when analyzing the condition with groundwater seepage.
Example 2
In this embodiment, for a virtual middle-deep buried pipe, the fluid temperature field at different times is calculated by using the analysis method for fluid temperature field of middle-deep buried pipe of the present invention, and the calculation result is compared with other model simulation results. The parameters of the buried pipe at the middle and deep layer are shown in Table 2.
Given that the length L of the buried pipe in the middle and deep layers is 2000M, the borehole and the rock soil are divided into 401 segments along the axial direction, i.e., M is 401, and Δ z is L/(M-1) is 5M;
the total time period simulated was 80 hours, i.e. t tol80 hours. Setting the time step Δ t to 2 minutes, the number of time segments N to ttol/Δt＝2400。
Initial time (i.e. t)0Time of day) inner and outer pipe fluid temperatures and rock initial temperatures are functions with respect to depth:
Figure BDA0003140164820000091
the calculation steps are the same as those in embodiment 1, and are not described herein again.
TABLE 2 parameters of buried pipe in the middle deep layer of example 2
Parameter(s) Numerical value Unit of
Length L of buried pipe in middle deep layer 2000 m
Inner radius r of the inner tubeii 0.018 m
Outer radius r of the inner tubeio 0.02 m
Inner radius r of the outer tubeei 0.056 m
Outer radius r of the outer tubeeo 0.057 m
Radius of borehole rb 0.06 m
Thermal conductivity k of the inner tubeip 0.4 W/m/K
Thermal conductivity k of the outer tubeep 0.4 W/m/K
Thermal conductivity k of backfillg 0.73 W/m/K
Thermal conductivity k of rock and soils(z) 3.28 W/m/K
Thermal diffusivity of rock and soil as(z) 1.46×10-6 m2/s
Groundwater seepage velocity u (z) at infinity from borehole 0 m/s
Thermal conductivity k of inner and outer pipe fluidsf 0.59 W/m/K
Volumetric specific heat capacity (ρ c) of inner and outer pipe fluidsf 4.19×106 J/m3/K
Kinematic viscosity v of inner and outer tube fluids 1.14×10-6 m2/s
Plantt number Pr of inner and outer tube fluids 8.09 -
Volume flow of inner and outer pipe fluid V (t) 1.2×10-2 m3/s
Total heat exchange power Q of middle-deep buried pipein(t) -100000 W
The calculated inlet fluid temperature (i.e., z) of this example1Outer tube fluid temperature at depth) and outlet fluid temperature (i.e., z)1The temperature of the inner tube fluid at depth) versus the comparative model calculation results are shown in fig. 3. It can be seen that the calculation result of the present embodiment has a slightly low accuracy in the short-term time range and a high accuracy in the medium-and long-term time range, which indicates that the calculation accuracy of the method of the present embodiment is also high when the groundwater seepage velocity is zero.
The present invention and its embodiments have been described above schematically, without limitation, and what is shown in the drawings is only one of the embodiments of the present invention, and the actual structure is not limited thereto. Therefore, if the person skilled in the art receives the teaching, without departing from the spirit of the invention, the person skilled in the art shall not inventively design the similar structural modes and embodiments to the technical solution, but shall fall within the scope of the invention.

Claims (8)

1. A middle-deep buried pipe fluid temperature field analysis method considering groundwater seepage is characterized in that: firstly, assuming that the heat transfer in a drill hole is steady-state heat transfer and approximating the drill hole as a moving linear heat source, further establishing a steady-state heat transfer equation of an inner pipe fluid and an outer pipe fluid, and establishing an equation of the wall surface temperature of the drill hole and the heat flow transferred to rock and soil by the drill hole by adopting a moving linear heat source model and combining a hot flow superposition principle; secondly, dividing the drill hole of the middle-deep buried pipe and rock soil outside the drill hole into a plurality of sections along the axial direction, setting a time step length, then dispersing the equation, and solving by adopting an iterative method, thereby obtaining the temperature field of the fluid inside and outside the middle-deep buried pipe.
2. A method for analyzing the temperature field of a fluid in a medium-deep buried pipe considering groundwater seepage according to claim 1, wherein the steady state heat transfer equation of the fluid in the inner pipe and the fluid in the outer pipe is established:
the energy equation for the inner tube fluid is as follows:
Figure FDA0003140164810000011
wherein V (t) is the volume flow rate of the inner tube fluid and the outer tube fluid;
(ρc)f-the volumetric specific heat capacity of the inner tube fluid and the outer tube fluid;
Ti(z, t) -temperature of the inner tube fluid;
z-axial coordinate, i.e., depth;
t is time;
Ta(z, t) -the temperature of the outer tube fluid;
Ria-thermal resistance between the inner tube fluid and the outer tube fluid;
l is the length of the buried pipe in the middle deep layer;
the energy equation for the outer tube fluid is as follows:
Figure FDA0003140164810000012
where q (z, t) -the heat flow transmitted by the borehole to the earth.
3. A method of analyzing a temperature field of a fluid in a medium-deep buried pipe considering a seepage of groundwater according to claim 2, wherein: said q (z, t) is calculated according to the following formula,
Figure FDA0003140164810000013
wherein R isab-thermal resistance between the outer tube fluid and the bore hole wall surface;
Figure FDA0003140164810000014
-the average temperature of the borehole wall is calculated according to equation (12),
Figure FDA0003140164810000021
in the formula, theta represents a circumferential coordinate; t isb(θ, z, t) -temperature of the borehole wall.
4. A method of analyzing a temperature field of a fluid in a medium-deep buried pipe considering a seepage of groundwater according to claim 3, wherein: the T isb(theta, z, t) is analyzed by adopting a moving line heat source model and combining a hot flow superposition principle, a calculation formula is as follows,
Figure FDA0003140164810000022
wherein, tn-the time corresponding to the nth time instant;
T0(z) -initial temperature of the rock-soil;
r-radial coordinate;
rb-the borehole radius;
g (r, θ, z, t) -the G function of the moving line heat source model, as in equation (14),
Figure FDA0003140164810000023
wherein u (z) -groundwater seepage velocity at infinity from the borehole;
ks(z) -thermal conductivity of the rock-soil;
as(z) -thermal diffusivity of the rock-soil;
Figure FDA0003140164810000024
-an integral variable;
combining equations (12), (13), and (14) can result in:
Figure FDA0003140164810000025
wherein A and B are both intermediate variables:
Figure FDA0003140164810000026
Figure FDA0003140164810000027
5. a method of analyzing the temperature field of fluids in buried pipes at a medium depth taking groundwater seepage into account as claimed in claim 4, wherein: the boundary conditions at the top and bottom of the buried pipe at the middle deep layer are determined to be,
Figure FDA0003140164810000031
Ti(z,t)|z＝L＝Ta(z,t)|z＝L,(t＞0) (19)
wherein Q isin(t) total heat exchange power of the buried pipe at the middle and deep layer;
Tin(t) -the inlet fluid temperature of the buried pipe at the mid-depth layer;
at the initial moment, the temperature of the inner and outer pipe fluids is equal to the initial temperature of the rock soil:
Ti(z,t)|t＝0＝Ta(z,t)|t＝0＝T0(z),(0≤z≤L) (20)。
6. a method of analyzing a temperature field of a fluid in a medium-deep buried pipe considering groundwater seepage according to claim 5, wherein: dividing the borehole of the middle-deep buried pipe and rock soil outside the borehole into M sections along the axial direction, wherein the length of any section in the middle is L/(M-1), the length of the first section and the length of the Mth section are delta z/2, and the axial coordinate of any mth section is zm(M-1) L/(M-1); according to the required mouldTotal time t to be simulatedtolSetting a fixed time step Δ t, the total number of time segments N equals ttol,/Δ t, and tn＝nΔt。
7. A method for analyzing the temperature field of fluid in a medium-deep buried pipe considering groundwater seepage according to claim 6, wherein the above equation is discretized:
for any time tnDiscretizing equation (1) in the axial direction:
Figure FDA0003140164810000032
wherein C is an intermediate variable:
Figure FDA0003140164810000033
for any time tnDiscretizing equation (2) in the axial direction:
Figure FDA0003140164810000034
for any time tnEquations (10) and (15) are separately discretized:
Figure FDA0003140164810000035
Figure FDA0003140164810000036
simultaneous equations (24) and (25) can be found:
Figure FDA0003140164810000041
wherein D (z)m,tn) Intermediate variables are:
Figure FDA0003140164810000042
for any time tnEquations (18) and (19) may be discretized into the following equations, respectively:
Figure FDA0003140164810000043
Ti(zM,tn)＝Ta(zM,tn) (29)。
8. a method of analyzing a temperature field of a fluid in a medium-deep buried pipe considering a seepage of groundwater according to claim 7, wherein: based on tnSolving the equations (21), (23), (26), (28) and (29) by using an iterative method to calculate tnThe inner tube fluid temperature, the outer tube fluid temperature and the average borehole wall temperature at each interval in time, i.e.
Figure FDA0003140164810000044
Figure FDA0003140164810000045
Based on the above steps, t can be calculated in sequence1、t2、t3…tNThe temperature distribution at the moment.
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