CN113704941A - Deep-layer double-pipe heat exchanger heat transfer model calculation method - Google Patents

Deep-layer double-pipe heat exchanger heat transfer model calculation method Download PDF

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CN113704941A
CN113704941A CN202111019426.8A CN202111019426A CN113704941A CN 113704941 A CN113704941 A CN 113704941A CN 202111019426 A CN202111019426 A CN 202111019426A CN 113704941 A CN113704941 A CN 113704941A
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fluid
heat transfer
heat exchanger
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罗怡雯
汤昌福
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Beijing Zhongdi Jinshi Technology Co ltd
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    • G06F30/10—Geometric CAD
    • G06F30/18—Network design, e.g. design based on topological or interconnect aspects of utility systems, piping, heating ventilation air conditioning [HVAC] or cabling
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    • G06—COMPUTING; CALCULATING OR COUNTING
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Abstract

The invention provides a deep sleeve heat exchanger heat transfer model calculation method. The method comprises the following steps: establishing a heat transfer model of a deep sleeve heat exchanger, wherein the deep sleeve heat exchanger sequentially comprises a stratum, a backfill material, an outer sleeve pipe and an inner pipe from outside to inside, and the backfill material is filled between the stratum and the outer sleeve pipe; determining the assumed conditions of a deep-layer double-pipe heat exchanger heat transfer model, wherein the heat transfer model comprises a formation heat transfer model and a fluid flow heat transfer model in a pipe; and solving the underground heat transfer model of the deep sleeve heat exchanger. The method considers the influence of the earth temperature gradient, establishes and solves an underground heat transfer model of the deep sleeve heat exchanger, and deduces a quasi-steady solution of the average temperature of the fluid at the inlet and the outlet of the sleeve heat exchanger and a nominal heat extraction calculation analytical formula. The analytic formula is simpler and more practical, the rapid calculation of the nominal heat of the deep sleeve heat exchanger can be realized, and the method is particularly suitable for engineering technicians.

Description

Deep-layer double-pipe heat exchanger heat transfer model calculation method
Technical Field
The invention relates to the technical field of geothermal energy exploitation, in particular to a deep casing heat exchanger heat transfer model calculation method.
Background
The development of geothermal energy is more and more emphasized in China, and particularly, shallow geothermal energy is applied in a large area through a ground source heat pump technology. In recent years, researchers try to popularize the construction application technology of the medium-deep sleeve heat exchanger in order to solve the problem of clean heating in northern winter in China. The depth of the middle-deep layer sleeve heat exchanger is usually between 1000m and 3000m, water is used as a circulating medium, the water-cooled heat exchanger has the advantages of small occupied area, high ground temperature and no water taking, is particularly suitable for being applied to severe cold areas, has a plurality of demonstration of middle-deep layer sleeve building heating projects in the north of China, and has added provisions aiming at the technology of the middle-deep layer sleeve heat exchanger in newly revised technical regulations of the buried pipe heat exchanger.
In order to scientifically analyze the heat transfer capacity and influence of the double-pipe heat exchanger and guide engineering practice, a great deal of research has been carried out by scholars. Zanchini et al analyzed the effects of thermal shorts, circulation flow, and tubing and heat exchanger geometry on the heat exchange performance of shallow jacket heat exchangers using finite element software COMSOL. Lous and the like establish an inside and outside decoupling three-dimensional unsteady model by utilizing three-dimensional finite element software FEFLOW, and study the influence of rock-soil thermal physical property parameters, deep buried pipe materials, an operation mechanism and other factors on the heat exchange performance of the coaxial sleeve type deep sleeve. Research shows that compared with shallow geothermal resource utilization, the deep system can obtain higher temperature and investment return, and the deep sleeve heat exchanger has more potential in 125-600 kW installed capacity. A two-dimensional numerical model of a deep-layer double-pipe heat exchanger is established by using Matlab software of Holmberg and the like, and a performance curve of the double-pipe heat exchanger within a burial depth range of 300-1000 m is given.
With the popularization of the heating technology of the middle-deep layer sleeve building, the research on the heat transfer aspect of the middle-deep layer sleeve heat exchanger in China is gradually increased. Zhang soldiers and the like establish steady-state heat exchange analytical models of the casing heat exchangers in different fluid circulation modes based on the assumption that the temperature of the drill hole wall is uniform and constant, and analyze the fluid temperature analysis characteristics in the depth direction of the drill hole and the influence factors of the heat exchange capacity of the heat exchanger. The analytical model does not take into account the effects of the geothermal gradient. The hole-bore-shaped dragon and the like adopt open source numerical simulation software OpenGeoSys to study the influence of the ground temperature gradient on the heat exchange characteristic of the deep-layer double-pipe heat exchanger, and the study shows that the influence of the ground temperature gradient on the outlet temperature of the heat exchanger is not considered. The Wangshou and the like simulate a 2605m deep-layer double-pipe heat exchanger system in the Qingdao city by using Fluent software, and test data verifies that effective measures are recommended to be taken by the deep-layer double-pipe heat exchanger to improve the heat insulation performance of the inner pipe. Wang Zhihua et al established a numerical model considering vertical thermophysical property differences, and performed simulation analysis on a certain exemplary engineering project. In addition, the heat exchanger structure disclosed by the geothermal energy double-wall double-sleeve type efficient heat exchanger (201811490173.0) comprises a support, two water tanks, a water inlet mechanism, an observation mechanism, a control mechanism, a connecting mechanism and a conveying mechanism, the structure is complex, the heat is seriously transmitted among all parts, and the heat transmission efficiency is low.
Note that, at present, the simulation of the heat exchange performance of the deep casing heat exchanger is mainly based on a numerical method, such as a finite element method, a finite difference method, and has more mature software FLUENT, COMSOL, OpenGeoSys, and the like. The existing analytic models generally fail to consider the effect of the earth temperature gradient and are only suitable for shallow casing calculation. In addition, compared with an analytical model, the numerical method is complex in operation, long in calculation time and inconvenient for engineering calculation.
Disclosure of Invention
The invention aims to solve the technical problem that the conventional method cannot solve the technical problem of quick calculation of a deep double-pipe heat exchanger heat transfer model considering low-temperature gradient, and provides a deep double-pipe heat exchanger heat transfer model calculation method.
The invention is realized by the following technical scheme: a deep-layer double-pipe heat exchanger heat transfer model calculation method comprises the following steps:
step S1: establishing a heat transfer model of the deep sleeve heat exchanger, wherein the deep sleeve heat exchanger sequentially comprises a stratum, a backfill material, an outer sleeve pipe and an inner pipe from outside to inside, and the backfill material is filled between the stratum and the outer sleeve pipe;
step S2: determining the assumed conditions of the deep-layer casing heat exchanger heat transfer model, wherein the heat transfer model comprises a formation heat transfer model and an in-pipe fluid flow heat transfer model;
step S3: and solving the underground heat transfer model of the deep sleeve heat exchanger.
In a preferred embodiment of the present invention, in the step S1, the backfill material is cement mortar.
In a preferred embodiment of the present invention, the assumed conditions of the deep shell and tube heat exchanger heat transfer model include:
average formation thermal conductivity of λsW/(m.k); volumetric heat capacity of cs,J/(m3K); and the vertical thermophysical property is uniformly distributed, and the initial temperature distribution of the stratum meets the following requirements:
Tg(z)＝T0+gG·H
in the formula, Tg(z) is the formation initial temperature distribution, deg.C; gGIs the ground temperature gradient, DEG C/m; t is0The average temperature of the earth's surface is DEG C; h is the formation depth, m;
the volume flow of the circulating fluid is constant;
steady heat transfer is performed in the drill hole, and the axial vertical direction of the stratum can be ignored; the heat flow of the double pipe heat exchanger is constant.
In a preferred embodiment of the present invention, in step S2, the formation heat transfer model is: steady state heat transfer is performed in the drill hole, and the temperatures of the fluid in the inner pipe and the fluid in the annulus respectively meet the following conditions:
Tf1＝Tf2+qf1·R12
Tf2＝Tb+qf2·R2b
in the formula, Tf1Is the temperature of the fluid in the inner tube, (° c); t isf2Is the temperature of the fluid in the annulus, (° c); t isbBorehole wall temperature, (° c); q. q.sf1The intensity of heat flow of heat exchange between fluid in the inner pipe and the pipe wall of the inner pipe is (W/m); q. q.sf2The intensity of heat flow of heat exchange between the annular fluid and the pipe wall of the outer pipe is (W/m); r12Is the thermal resistance between the inner tube fluid and the annulus fluid, (m.k/W); r2bIs the thermal resistance between the annular fluid and the borehole wall (m.K/W).
In a preferred embodiment of the present invention, in the step S2, the fluid flow heat transfer model in the pipe is: the deep casing pipe is replacedThe fluid flowing heat transfer in the heater is mainly convection heat transfer, and the temperature T of the fluid in the inner pipef1With annular fluid Tf2Respectively satisfy:
Figure BDA0003241264230000031
Figure BDA0003241264230000032
in the formula, cfIs the volumetric heat capacity, J/(m)3.K)；WfFor circulating fluid flow, m3/s；Tf1Is the temperature of the fluid in the inner tube, (° c); t isf2Is the temperature of the fluid in the annulus, (° c); q. q.sf1The intensity of heat flow of heat exchange between the fluid in the inner pipe and the pipe wall of the inner pipe is (W/m); q. q.sf2The intensity of heat flow of heat exchange between the fluid of the outer pipe and the pipe wall of the outer pipe is (W/m).
In a preferred embodiment of the present invention, in the step S3, the solving the deep casing heat exchanger subsurface heat transfer model includes solving a pseudo-steady state solution of the average temperature of the inlet and outlet fluid of the deep casing heat exchanger and a nominal heat extraction calculation analytic formula.
In a preferred embodiment of the present invention, the nominal heat extraction amount calculation formula is:
Figure BDA0003241264230000033
in the formula, QN，QNNominal heat removal, (W); t is0The average temperature of the earth's surface is DEG C; gGTo a ground temperature gradient, K
(ii)/m; h is the formation depth, m; t isfinInlet temperature, deg.C; lambda [ alpha ]sThe average thermal conductivity of the formation, W/(m.K); c. CsIs the average volumetric heat capacity of the formation, J/(m)3.K)；rbIs the borehole radius, m; t is time, s; gamma is euler constant, gamma is 0.577216; wfFor circulating fluid flow, m3/s；R12Is the thermal resistance between the inner tube fluid and the annulus fluid, (m.k/W); r2bIs the thermal resistance between the annular fluid and the borehole wall (m.K/W).
The invention has the beneficial effects that: the method is based on the basic theory of underground heat transfer, establishes an underground heat transfer model of the deep sleeve heat exchanger in consideration of the geothermal gradient effect, and carries out analytic solution. The steady-state solution of the long-term heat transfer of the double-pipe heat exchanger is obtained by an asymptotic analysis method and is used for calculating the nominal heat exchange quantity of the double-pipe heat exchanger, and verification is carried out by comparing with literature. Compared with a numerical solution, the analytic solution is simpler and more practical, the rapid calculation of the heat exchange characteristics of the deep sleeve heat exchanger can be realized, and the method is particularly suitable for engineering technicians.
Drawings
FIG. 1 is a flow chart of a deep shell and tube heat exchanger heat transfer model calculation method of the present invention;
FIG. 2 is a schematic view of a double-tube heat exchanger of the present invention;
FIG. 3 is a comparison of nominal heat extraction calculations.
Wherein, 1-stratum, 2-backfill material, 3-casing outer pipe, 4-inner pipe
Detailed Description
The present invention will be described in further detail with reference to examples, but the embodiments of the present invention are not limited thereto.
Fig. 1 shows a flow of a deep shell and tube heat exchanger heat transfer model calculation method according to the present invention.
As shown in fig. 1, the method for calculating the heat transfer model of the deep casing heat exchanger provided by the invention comprises the following steps:
step S1: establishing a heat transfer model of the deep sleeve heat exchanger;
step S2: determining the assumed conditions of the deep-layer casing heat exchanger heat transfer model, wherein the heat transfer model comprises a formation heat transfer model and an in-pipe fluid flow heat transfer model;
step S3: and solving the underground heat transfer model of the deep sleeve heat exchanger.
Fig. 2 is a schematic diagram of a cased heat exchanger comprising a formation 1, backfill material 2, an outer casing tube 3, and an inner casing tube 4.
The casing heat exchanger can be seen as comprising a plurality of porous medium layers with different thermodynamic and physical properties from outside to inside, the deep casing heat exchanger is installed in a drill hole, and cement mortar is backfilled between the outer wall of the casing outer pipe 3 and the surrounding stratum 1 to ensure the contact and heat transfer between the casing outer pipe 3 and surrounding rocks. Circulating fluid flows into the bottom of the heat exchanger from an inlet of the outer pipe 3 of the casing, rises to the ground through the inner pipe 4 and then flows out, and in the process, the fluid exchanges heat with the surrounding stratum and absorbs heat from the surrounding stratum 1 to heat.
The following assumptions were used in the mathematical modeling:
1) average formation thermal conductivity of λsW/(m.k); volumetric heat capacity of cs，J/(m3K); and the vertical thermophysical property distribution is uniform. Initial temperature distribution of stratum
Tg(z)＝T0+gG·H (1)
In the formula, Tg(z) is the formation initial temperature distribution, deg.C; gGIs the ground temperature gradient, DEG C/m; t is0The average temperature of the earth's surface is DEG C; h is the formation depth, m;
2) the volume flow of the circulating fluid is constant and is Wf，m3S; coefficient of thermal conductivity of lambdafW/(m.k); heat capacity of cf，J/(m3K); viscosity of mufPa/s; inlet temperature TfinDEG C; outlet temperature TfoutDEG C; radius of the bore hole is rbM; the depth is H, m; the heat conductivity coefficient of the backfill material is lambdagW/(m.k); the radius of the inner pipe of the sleeve is riiM; inner pipe external diameter rieM; the heat conductivity coefficient of the inner tube is lambdaiW/(m.k); the inner diameter of the outer tube is raiM; outer diameter of the outer tube is raeM; coefficient of thermal conductivity lambda of the outer tubea，W/(m.K)；
3) Steady state heat transfer is assumed in the drill hole, and the axial vertical direction of the stratum can be ignored; constant heat flow of the double-pipe heat exchanger is QG。
The formation heat transfer model is as follows: steady state heat transfer is carried out in the drill hole, and the temperatures of the fluid in the inner tube and the fluid in the annulus respectively meet the requirements
Tf1＝Tf2+qf1·R12 (2)
Tf2＝Tb+qf2·R2b (3)
In the formula, Tf1Is the temperature of the fluid in the inner tube, (° c); t isf2Is the temperature of the fluid in the annulus, (° c); t isbBorehole wall temperature, (° c); q. q.sf1The intensity of heat flow of heat exchange between fluid in the inner pipe and the pipe wall of the inner pipe is (W/m); q. q.sf2The intensity of heat flow of heat exchange between the annular fluid and the pipe wall of the outer pipe is (W/m); r12Is the thermal resistance between the inner tube fluid and the annulus fluid, (m.k/W); r2bIs the thermal resistance between the annular fluid and the borehole wall (m.K/W).
T in formula (3)bFor borehole wall temperature, the borehole may be considered to be an infinite geodetic centerline heat source, and the borehole wall temperature T may be considered to be the borehole wall temperature T when the formation vertical heat transfer is negligiblebSatisfy the requirement of
Tb-Tg(z)＝qf2·Tu(t,rb) (4)
In the formula, TuFor infinite line heat source function:
Figure BDA0003241264230000051
where Ei is an exponential integration function. The synthesis of formula (3) and (4) can obtain
Figure BDA0003241264230000052
The heat transfer model of fluid flow in the pipe is as follows: the flowing heat transfer of the fluid in the double-pipe heat exchanger is mainly the convection heat transfer, and the temperature T of the fluid in the inner pipef1With annular fluid Tf2Respectively satisfy
Figure BDA0003241264230000053
Figure BDA0003241264230000054
In general terms (2), (3), (6), in-line flow equations (7) and (8) can be further written as
Figure BDA0003241264230000055
Figure BDA0003241264230000061
In the formula, cfIs the volumetric heat capacity, J/(m)3.K)；WfFor circulating fluid flow, m3/s；Tf1Is the temperature of the fluid in the inner tube, (° c); t isf2Is the temperature of the fluid in the annulus, (° c); q. q.sf1The intensity of heat flow of heat exchange between the fluid in the inner pipe and the pipe wall of the inner pipe is (W/m); q. q.sf2The intensity of heat flow of heat exchange between the fluid of the outer pipe and the pipe wall of the outer pipe is (W/m); t isuAs a function of line heat source.
Solving the underground heat transfer model of the deep sleeve heat exchanger, wherein the solution conditions are as follows:
at the inlet and outlet, the fluid temperature satisfies:
z＝0，-cfWf·(Tf1-Tf2)＝QG (11)
the temperature of the fluid at the bottom of the double-pipe heat exchanger is continuous, and the following conditions are met:
z＝H，Tf1-Tf2＝0 (12)
solving the analytic solution, solving under the dimensionless coordinate, and defining the adopted dimensionless quantity as:
length of dimensionless
Figure BDA0003241264230000062
Dimensionless time
Figure BDA0003241264230000063
Dimensionless thermal resistance
RD＝2πλs·R (15)
Dimensionless temperature
Figure BDA0003241264230000064
Dimensionless geothermal gradient
Figure BDA0003241264230000065
And dimensionless parameters
Figure BDA0003241264230000066
Substituting dimensionless quantity definitions into equations (9) - (12) to obtain dimensionless equations
Figure BDA0003241264230000071
Figure BDA0003241264230000072
And boundary conditions
zD＝0，ChD·(TfD1-TfD2)＝1 (21)
zD＝1，TfD1-TfD2＝0 (22)
From equation (19), it can be found
Figure BDA0003241264230000073
Derived from the above formula
Figure BDA0003241264230000074
Substituting (23) and (24) into (20) to obtain
Figure BDA0003241264230000075
In the formula, thermal resistance R2sDIs defined as
RD2s＝RD2b+TuD(tD,rb) (26)
A particular solution of equation (25) is that,
Figure BDA0003241264230000076
the two characteristic roots of equation (25) are each
Figure BDA0003241264230000077
The solution of the non-homogeneous equation (25) can be written as
TfD1＝C·exp(η1zD)+D·exp(η2zD)-ChDR12D·gGD (29)
The coefficients C and D are constants and are determined by boundary conditions. According to (23), the temperature of the outer tube fluid 2 can be obtained as
TfD2＝TfD1+ChDRD12[C·η1exp(η1zD)+D·η2exp(η2zD)]+ChDR12DgGD (30)
Further, an expression of the temperature difference between the fluid in the inner pipe and the fluid in the outer pipe can be obtained
TfD1-TfD2＝-ChDRD12[Cη1exp(η1zD)+Dη2exp(η2zD)]-ChDRD12gGD (31)
The boundary conditions (21) and (22) are respectively substituted into (31), coefficients C and D can be obtained by solving,
Figure BDA0003241264230000081
Figure BDA0003241264230000082
wherein Δ η ═ η1-η2 (34)
Then at the inlet and outlet of the double pipe heat exchanger (z)D0) average temperature T of fluidfmDThe requirements are met,
TfmD＝TfmD,s+TfmD,g (35)
in the above formula, TfmD,sRepresents the heat transfer effect of the formation and meets the requirements
Figure BDA0003241264230000083
And TfmD,gRepresenting the effect of ground temperature gradient and satisfying
Figure BDA0003241264230000084
And (3) solving a quasi-steady state solution, wherein according to a characteristic value eta expression, when the time is very large, the delta eta is between 0 and eta is between 0, and an approximate expression exists:
Figure BDA0003241264230000085
Figure BDA0003241264230000086
thus the stratum action term TfmD,sCan be approximated as：
Figure BDA0003241264230000087
And the geothermal gradient contribution term TfmD,gCan be approximated as:
Figure BDA0003241264230000088
the vertical average value of the average initial formation temperature is obtained. Therefore, when the time is longer, the average temperature of the fluid at the inlet and the outlet of the deep sleeve heat exchanger can be approximate to
Figure BDA0003241264230000091
The formula (42) has a dimensional form of
Figure BDA0003241264230000092
And when the test time is long enough, satisfy
Figure BDA0003241264230000093
Equation (43) can be written as:
Figure BDA0003241264230000094
at this time, the average temperature curve of the inlet and outlet fluid is in a linear relation with logarithmic time.
The fluid inlet temperature and the average temperature are satisfied
Figure BDA0003241264230000095
Substituting formula (45) to obtain nominal heat calculation formula
Figure BDA0003241264230000096
Nominal heat extraction quantity is calculated and compared, a deep-layer sleeve heat exchange project is adopted, the deep drilling depth is 1600m, the ground source heat pump technology is combined to provide heat for the building and provide domestic hot water, and other parameters of the heat exchanger are shown in the table 1. And comparing the calculation result of the formula (47) with the calculation result of the numerical simulation method.
TABLE 1 double-tube Heat exchanger parameters
Figure BDA0003241264230000097
Figure BDA0003241264230000101
Square-bright et al propose to evaluate the heat exchange capacity of a deep casing heat exchanger by using nominal heat extraction. The nominal heat extraction of the deep casing heat exchanger is satisfied, and the inlet fluid temperature of the casing heat exchanger is not lower than 5 ℃ after the deep casing heat exchanger continuously operates for three months (90 days) according to the nominal heat extraction. Square-bright et al calculated the thermal conductivity (lambda) of different formations using a numerical methodsThe nominal heat extraction curves of the deep-layer double-pipe heat exchanger are changed along with the gradient of the earth temperature when the depth of the drilled hole is 1W/(m.K), 2W/(m.K), 3W/(m.K) and the depth of the drilled hole is different (H is 1000m, 2000m and 3000 m).
The method adopts the formula (47) to quickly calculate the nominal heat extraction amount of the deep casing heat exchanger, and compares the nominal heat extraction amount with the result of the square brightness person, as shown in fig. 3, the straight line is the result given by the square brightness person, and the triangle, the circle and the square frame are the calculation results of the formula (47), so that the nominal heat extraction amounts calculated by the two are basically consistent. However, the calculation result of the formula (47) is simpler and more convenient than a numerical simulation method, and the efficiency is greatly improved.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention in any way, and all technical and methodological aspects of the present invention may be embodied in any form or modified form without departing from the scope of the present invention.

Claims (7)

1. A deep-layer double-pipe heat exchanger heat transfer model calculation method is characterized by comprising the following steps:
step S1: establishing a heat transfer model of the deep sleeve heat exchanger, wherein the deep sleeve heat exchanger sequentially comprises a stratum, a backfill material, an outer sleeve pipe and an inner pipe from outside to inside, and the backfill material is filled between the stratum and the outer sleeve pipe;
step S2: determining the assumed conditions of the deep-layer casing heat exchanger heat transfer model, wherein the heat transfer model comprises a formation heat transfer model and an in-pipe fluid flow heat transfer model;
step S3: and solving the underground heat transfer model of the deep sleeve heat exchanger.
2. The deep shell and tube heat exchanger heat transfer model calculation method of claim 1, wherein: in the step S1, the backfill material is cement mortar.
3. The deep shell and tube heat exchanger heat transfer model calculation method of claim 2, wherein: in step S2, the assumed conditions of the deep shell and tube heat exchanger heat transfer model include:
average heat conductivity coefficient of stratum is lambdasVolumetric heat capacity of csAnd the vertical thermophysical property is uniformly distributed, and the initial temperature distribution of the stratum meets the requirement
Tg(z)＝T0+gG·H
In the formula, Tg(z) is the formation initial temperature distribution, K; gGIs the geothermal gradient, K/m; t is0The average temperature of the earth's surface is DEG C; h is the formation depth, m;
the volume flow of the circulating fluid is constant;
steady heat transfer is carried out in the drilled hole, and the axial vertical direction of the stratum can be ignored; the heat flow of the double pipe heat exchanger is constant.
4. The deep shell and tube heat exchanger heat transfer model calculation method of claim 3, wherein: in step S2, the formation heat transfer model is: steady state heat transfer is performed in the drill hole, and the temperature of the fluid in the inner pipe and the temperature of the fluid in the annulus respectively meet the requirement
Tf1＝Tf2+qf1·R12
Tf2＝Tb+qf2·R2b
In the formula, Tf1Is the temperature of the fluid in the inner tube, deg.C; t isf2The temperature of the fluid in the annulus, deg.C; t isbBorehole wall temperature, deg.C; q. q.sf1The intensity of heat flow of heat exchange between fluid in the inner pipe and the pipe wall of the inner pipe is W/m; q. q.sf2The intensity of heat exchange heat flow between the annular fluid and the pipe wall of the outer pipe is W/m; r12Is the thermal resistance between the inner tube fluid and the annulus fluid, m.K/W; r2bIs the thermal resistance between the annular fluid and the borehole wall, m.K/W.
5. The deep-layer double-pipe heat exchanger heat transfer model calculation method according to any one of claims 1 to 4, characterized in that: in step S2, the in-pipe fluid flow heat transfer model is: the flowing heat transfer of the fluid in the deep sleeve heat exchanger is mainly based on convection heat transfer, and the temperature T of the fluid in the inner tubef1With annular fluid Tf2Respectively satisfy
Figure FDA0003241264220000011
Figure FDA0003241264220000021
In the formula, cfIs the volumetric heat capacity, J/(m)3.K)；WfFor circulating fluid flow, m3/s；Tf1Is the temperature of the fluid in the inner tube, deg.C; t isf2The temperature of the fluid in the annulus, deg.C; q. q.sf1Between the fluid in the inner tube and the wall of the inner tubeHeat exchange heat flow intensity, W/m; q. q.sf2The intensity of heat flow of heat exchange between the fluid of the outer pipe and the pipe wall of the outer pipe is W/m.
6. The deep shell and tube heat exchanger heat transfer model calculation method of claim 1, wherein: in step S3, the solving the deep casing heat exchanger subsurface heat transfer model includes solving a pseudo-steady state solution of the average temperature of the fluid at the inlet and the outlet of the deep casing heat exchanger and a nominal heat extraction calculation analytic formula.
7. The deep shell and tube heat exchanger heat transfer model calculation method of claim 6, wherein: the nominal heat extraction amount is calculated by the formula
Figure FDA0003241264220000022
In the formula, QN，QNNominal heat pickup, W; t is0The average temperature of the earth's surface is DEG C; gGIs the geothermal gradient, K/m; h is the formation depth, m; t isfinInlet temperature, deg.C; lambda [ alpha ]sThe average thermal conductivity of the formation, W/(m.K); c. CsIs the average volumetric heat capacity of the formation, J/(m)3.K)；rbIs the borehole radius, m; t is time, s; gamma is euler constant, gamma is 0.577216; wfFor circulating fluid flow, m3/s；R12Is the thermal resistance between the inner tube fluid and the annulus fluid, m.K/W; r2bIs the thermal resistance between the annular fluid and the borehole wall, m.K/W.
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