CN114357838B - Simulation method of coaxial sleeve type buried pipe heat exchanger with variable-flow pipe diameter - Google Patents
Simulation method of coaxial sleeve type buried pipe heat exchanger with variable-flow pipe diameter Download PDFInfo
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Abstract
The invention discloses a simulation method for a middle-deep coaxial sleeve type buried pipe heat exchanger capable of crossing seasons, changing flow and changing pipe diameters, which can rapidly calculate the temperature distribution conditions of fluid, the pipe wall of an inner pipe of the heat exchanger and soil under different working environments. Firstly, assuming that the temperature of fluid is unchanged, solving a soil and inner pipe wall temperature field by taking the fluid as a third type boundary condition; calculating the heat flow of the soil transferred to the fluid according to the wall surface temperature of the inner pipe and the outer pipe and the fluid temperature, and the heat flow of the fluid between the inner pipe and the inner pipe transferred through the wall of the inner pipe; adding the heat flow as a source term into a one-dimensional fluid flow heat transfer equation, and solving a fluid temperature field; advancing one time layer, calculating soil temperature field again according to the fluid temperature field, and repeating the steps until the preset time. The method can accurately and efficiently simulate the working process of the long-period coaxial sleeve type buried heat exchanger, and has the characteristics of wide application range, high calculation efficiency, good accuracy and the like.
Description
Technical Field
The invention belongs to the technical field of efficient utilization of geothermal resources, relates to a numerical heat transfer calculation method, and particularly relates to a simulation method of a coaxial sleeve type buried pipe heat exchanger with variable flow and pipe diameters across seasons.
Background
Geothermal energy is a clean energy which is paid attention to in recent years, and can be combined with a ground source heat pump technology to provide services such as refrigeration, heating and the like for buildings. Compared with other clean energy sources, the geothermal energy has the advantages of rich resources, wide distribution, recycling, high stability and the like. Thus, exploitation of geothermal resources is of increasing interest to research institutions and businesses.
Because of limited urban land resources, a medium-deep buried pipe heat exchanger is derived in order to relieve the restriction of insufficient buried pipe. Compared with common single-U-shaped and double-U-shaped shallow buried pipe heat exchangers, the middle-deep buried pipe heat exchangers are mostly coaxial sleeve-type, and the exploitation of geothermal resources is realized by heating circulating water at a higher temperature deep in the ground; meanwhile, a series of problems such as water level drop, ground subsidence, shortened heat storage life, environmental pollution and the like caused by underground water exploitation are avoided. The middle-deep buried pipe heat exchanger can adopt a coaxial pipe with variable pipe diameter, and the pipe diameter of an outer pipe is increased in a deep stratum, so that the detention time of circulating water in the deep stratum is prolonged, and geothermal resources are reasonably utilized by adopting a variable flow rate method according to the user requirements of heating in winter and hot water supply in summer.
The existing geothermal resource exploitation process still has the problems of imperfect technology, high cost and the like, and in order to solve the problems, the acquisition of the evolution rule of the temperature field in the whole geothermal exploitation period is one of basic works.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention better analyzes the heat transfer performance of the deep buried pipe heat exchanger and improves the environmental adaptability, and aims to provide the simulation method of the coaxial sleeve type buried pipe heat exchanger capable of crossing the seasonal variable flow and diameter, which can realize numerical simulation of the middle-deep coaxial sleeve type buried pipe heat exchanger with the seasonal variable flow and diameter, accurately and efficiently calculate the distribution conditions of temperature fields of stratum, inner pipe wall of the heat exchanger and fluid under different working conditions, provide technical support for exploitation of geothermal resources and reduce the actual exploitation cost.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows:
a simulation method of a coaxial sleeve type buried pipe heat exchanger with variable flow and pipe diameter across seasons comprises the following steps:
step 1, establishing a physical model comprising three areas of soil, a heat exchanger inner pipe wall and fluid in the heat exchanger along the radial direction;
step 2, simplifying a physical model, dividing grids according to structural characteristics of the physical model, adopting non-uniform structured grids in the radial direction and adopting uniform structured grids in the axial direction;
step 3, importing physical parameters of soil, the inner tube wall of the heat exchanger and fluid in the heat exchanger through Excel, wherein geometric parameters of the heat exchanger and flow change conditions along with time;
step 4, establishing a heat conduction equation of the soil and the inner pipe wall of the heat exchanger and an energy equation of fluid flow in the heat exchanger, and performing discretization treatment to obtain a discrete equation of the soil area, the inner pipe wall area of the heat exchanger and the fluid area in the heat exchanger;
step 5, setting a soil area boundary condition;
step 6, assuming that the temperature of fluid in the heat exchanger is unchanged, solving a heat convection coefficient of the outer wall surface of the outer tube of the heat exchanger according to an initial soil temperature field, obtaining a coefficient of a discrete equation, updating the soil temperature field according to the equation coefficient, and outputting the soil temperature field after iteration until the temperature of the outer wall surface of the outer tube of the heat exchanger is unchanged;
step 7, setting boundary conditions of the pipe wall area of the inner pipe of the heat exchanger;
step 8, keeping the temperature of fluid in the heat exchanger unchanged, respectively solving the heat convection coefficients of the inner wall surface and the outer wall surface of the inner tube of the heat exchanger according to the temperature distribution condition of the inner wall surface of the inner tube of the initial heat exchanger, obtaining the coefficient of a discrete equation, updating the temperature field of the inner tube wall of the heat exchanger by the coefficient of the equation, and outputting the internal temperature field of the inner tube wall of the heat exchanger when iterating until the temperature of the inner wall surface of the inner tube is unchanged;
step 9, setting boundary conditions of the fluid region;
step 10, calculating heat flow q between the inner fluid and the outer fluid according to the temperature of the inner fluid and the temperature of the inner wall surface of the inner tube 1 Calculating soil heat flow q according to fluid temperature between the inner pipe and the outer wall surface temperature of the outer pipe f Solving a fluid temperature field;
and step 11, outputting the obtained result after reaching the target time.
Compared with the prior art, the invention can accurately and efficiently simulate the complete life cycle process of the geothermal heat exploitation of the buried pipe heat exchanger under different working conditions, and adjust the inflow according to the seasonal demand, thereby realizing the prediction of the heat exchange effect of the heat exchanger and meeting the actual demands of engineering.
Drawings
Fig. 1 is a schematic flow chart of a fluid-solid coupling heat transfer numerical simulation method provided by an embodiment of the invention.
Fig. 2 is an overall schematic diagram of a physical model according to an embodiment of the present invention.
FIG. 3 shows the variation of outlet water temperature with time during long-period operation according to an embodiment of the present invention.
FIG. 4 is a schematic view showing the distribution of the temperature of the fluid between the inner tube and the outer tube according to the embodiment of the present invention.
Fig. 5 is a graph showing a soil temperature distribution according to an embodiment of the present invention.
FIG. 6 shows a local soil temperature distribution of 1500-1600m according to an embodiment of the present invention.
Detailed Description
Embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples.
The invention relates to a simulation method of a coaxial sleeve type buried pipe heat exchanger with variable flow and pipe diameter across seasons, which can be used for heating and supplying hot water, the running period of the coaxial sleeve type buried pipe heat exchanger is generally more than one year, and heating is carried out in winter and hot water is supplied in summer according to different requirements of users in each season; reasonable utilization of geothermal resources is realized by adopting a variable flow mode, namely, the water inflow in winter is large and the water inflow in summer is small; the pipe diameter of the outer pipe at the upper part of the heat exchanger is smaller than that of the outer pipe at the lower part, namely, the residence time of fluid in the deep stratum is prolonged by increasing the pipe diameter of the outer pipe at the deep stratum.
As shown in fig. 1, taking a fluid-solid coupling heat transfer numerical simulation method as an example, the simulation method of the present invention specifically includes the following steps:
and 1, establishing a physical model comprising three areas of soil, a heat exchanger inner pipe wall and fluid in the heat exchanger along the radial direction.
In this step, the coaxial sleeve type buried heat exchanger is actually a three-dimensional model. The low-temperature water flows in from one pipe orifice of the coaxial pipe, and flows out from the other pipe orifice after fully exchanging heat with the stratum. There are two flow modes: an inner inlet and an outer outlet (inflow from the inner tube and outflow from the outer tube), and an outer inlet and an outer outlet (inflow from the outer tube and outflow from the inner tube). The fluid exchanges heat with the pipe wall in a forced convection mode in the flowing process, so that the fluid obtains the heat of the stratum to raise the temperature.
And step 2, simplifying a physical model, dividing grids according to structural characteristics of the physical model, adopting non-uniform structured grids in the radial direction and adopting uniform structured grids in the axial direction according to an efficient empirical formula.
In the present invention, for example, in a physical model including three areas of soil, an inner tube wall of the heat exchanger, and fluid in the heat exchanger in a radial direction, an inner diameter of the outer tube is 0.25m when it is 0 to 500m underground, and an inner diameter of the outer tube is 0.30m when it is 500 to 2500m underground, wherein since the wall thickness of the outer tube is 0.01m, a radial calculation range is small compared with the soil, the wall thickness of the outer tube of the heat exchanger is ignored.
In order to simplify the model, the inner tube and the outer tube are cylindrical in all calculation areas and transfer heat under the wrapping of surrounding soil, and the area affected by the soil is a symmetrical area, so that the three-dimensional fluid-solid coupling heat transfer problem can be simplified into a two-dimensional plane problem under a cylindrical coordinate system. The soil on the section of the pipeline with the same depth heats the fluid uniformly in the circumferential direction and the radial width of the soil is far smaller than the axial length, so that the temperature of the fluid on the same section can be considered to be uniform, and the problem of flow heat transfer in the pipe of the fluid is simplified into a one-dimensional problem.
To reduce the influence of irrelevant factors, the model is further simplified, and the basic assumption is as follows: (1) The soil around the heat exchange tube is regarded as a uniform medium, the influence of underground water on heat exchange is ignored, and the heat conduction of the underground soil is regarded as pure heat conduction. (2) neglecting the thickness of the outer tube wall. (3) surface air temperature is considered constant.
In a specific implementation, the temperature varies unevenly in the horizontal direction, the grid is suitably encrypted in the region where the temperature varies drastically (near the borehole wall surface), while the grid is suitably sparse at the radially outer boundary sufficiently far from the borehole with small temperature variation.
More specifically, referring to fig. 2, in a radial soil area taking the outer wall surface of the outer tube of the heat exchanger as a starting position, the grid density at the position close to the outer tube wall is high, the grid density at the position far away from the outer tube wall is low, the intersection of grid lines is a node, and the radial grid division basis is as follows:
wherein r is j Is the position of the j-th node in the radial grid; n is the number of total nodes in the radial direction; a is the adjustment parameter of the grid change,r is the soil radial calculation range and is an intermediate variable;
in the axial direction, the grids are uniformly distributed, and the axial grid division basis is as follows:
wherein z is the position of the jth grid in the axial grids; depth is the calculated region axial depth; n (N) z Is the number of total nodes in the axial direction.
And 3, importing physical parameters of soil, the inner tube wall of the heat exchanger and fluid in the heat exchanger through Excel, wherein geometric parameters of the heat exchanger and the flow change along with time.
The invention adopts the Excel table form to input soil and fluid parameters so as to adapt to different working conditions. Wherein the physical parameters comprise soil density, soil heat capacity, soil heat conductivity coefficient, different soil layer depths and initial temperatures, heat exchanger inner tube wall density, heat exchanger inner tube wall heat capacity, heat exchanger inner tube wall heat conductivity coefficient, fluid density, fluid heat capacity, fluid heat conductivity coefficient and fluid inlet temperature; the geometric parameters of the heat exchanger comprise the pipe wall thickness of the inner pipe and the outer pipe of the heat exchanger and the pipe diameter of the inner pipe and the outer pipe; the time-varying flow rate refers to the different inlet flows used in different seasons.
The parameters selected in this embodiment include: soil layer number, inner diameter D of inner tube 0 (m), shallow stratum outer tube inner diameter D 1 (m), deep-stratum outer tube inner diameter D 2 (m), radial computing range R (m), axial computing depth L (m), depth L of bottom of different soil layers center (m) initial temperatures T of different soil layers 0 (. Degree.C.) coefficient of thermal conductivity of soil lambda (W/m.K), soil density ρ (kg/m) 3 ) Heat capacity c of soil p (J/Kg.K), volume flow Q (m 3 And/h), the coefficient of thermal conductivity of the fluid lambda (W/mK), the density of the fluid rho (kg/m) 3 ) Fluid heat capacity c p (J/Kg.K), fluid inlet temperature T inter Thermal conductivity coefficient lambda (W/m.K) of inner tube wall, density rho (kg/m) of inner tube wall 3 ) Heat capacity c of inner pipe wall p (J/Kg.K), the thickness h (m) of the inner and outer tube walls.
In order to be more fit with the actual working condition, the inlet flow is set to be variable along with time, and flow control is realized by adopting an Excel table input mode.
Step 4, establishing a heat conduction equation of the soil and the inner pipe wall of the heat exchanger and an energy equation of fluid flow in the heat exchanger, and performing discretization treatment to obtain a discrete equation of the soil area, the inner pipe wall area of the heat exchanger and the fluid area in the heat exchanger;
specifically, the equation of heat conduction of soil is as follows:
wherein ρ is soil density, c p The specific heat capacity of the soil is T, the temperature is T, the time is T, r is a radial coordinate, lambda is the thermal conductivity of the soil, and z is an axial coordinate;
thus, the soil region discrete equation is:
the expression can be as follows:
a P T P ＝a E T E +a W T W +a S T S +a N T N +b
wherein r is n 、r s Lambda is the radial coordinates of two nodes adjacent to the P node e 、λ w 、λ s 、λ n For the thermal conductivity of four adjacent nodes at the P node, deltar is the radial space step at the P node, deltaz is the axial space step at the P node, (ρc) P For the volumetric specific heat capacity, Δt is the time step, (δz) e 、(δz) w For the axial spatial step size (δr) of the P node at two axially adjacent nodes s 、(δr) n For the radial space step at two radially adjacent nodes of the P node,represents the n+1 th of each nodeThe temperature of the layer,/->Representing the temperature of the nth layer of the P node;
a P 、a E 、a W 、a N b is the coefficient:
the heat conduction equation of the inner tube wall region of the heat exchanger is as follows:
thus, the discrete equation for the tube wall area of the inner tube of the heat exchanger is:
the expression can be as follows:
a′ P T P ＝a′ E T E +a′ W T W +a′ S T S +a′ N T N +b′
wherein a' P 、a′ E 、a′ W 、a′ S 、a′ N B' is a coefficient:
the energy equation for fluid flow in the heat exchanger is as follows:
wherein u is the fluid flow rate, q is the heat flow obtained from the outside by the fluid region;
for heat exchanger inner tube fluid:
in the formula, h pipein Is the convection heat exchange coefficient between the fluid in the inner tube and the inner wall surface of the inner tube of the heat exchanger, T pipein T is the temperature of the inner wall surface of the inner tube fin For the inner tube fluid temperature, D In Is the inner diameter of the inner tube;
for fluid between the inner tube and the outer tube of the heat exchanger:
in the formula, h soil Is the convection heat exchange coefficient between the fluid between the inner tube and the outer tube and the inner wall surface of the outer tube, T soil0 T is the temperature of the boundary in the soil region fout For fluid temperature between inner and outer tubes, D Out Is the inner diameter of the outer tube, h pipeout Is the convective heat transfer coefficient delta between the fluid in the annular tube and the inner wall surface of the inner tube of the heat exchanger pipe T is the thickness of the inner tube wall of the heat exchanger pipeout The temperature of the outer wall surface of the inner tube;
thus, the discrete equation for the fluid region in the heat exchanger is:
the expression can be as follows:
a′ P ′T P ＝a′ S ′T S +a′ N ′T N +b″
wherein a' P ′、a′ S ′、a′ N 'b' is a coefficient:
step 5, setting a soil area boundary condition;
specifically, the upper boundary of the soil is in contact with air, which is a third type of boundary condition, at which:
h air is the convection heat exchange coefficient of the ground surface and the air, T air Is the temperature of the air above the ground surface;
the lower boundary is an adiabatic boundary, at which point:
a E ＝0
the left boundary of the soil (i.e. the inner boundary of the soil) exchanges heat with the fluid, and the third type of boundary condition is that:
the right boundary of the soil (i.e., the outer boundary of the soil) is an adiabatic boundary, at which point:
and 6, assuming that the temperature of fluid in the heat exchanger is unchanged, solving the convective heat transfer coefficient of the outer wall surface of the outer tube of the heat exchanger according to the initial soil temperature field, obtaining the coefficient of a discrete equation, updating the soil temperature field according to the equation coefficient, and outputting the soil temperature field after iteration until the temperature of the outer wall surface of the outer tube of the heat exchanger is unchanged.
In the step, the specific implementation process is as follows:
adopting convection boundary conditions for the upper boundary of the soil, specifically:
where λ is the thermal conductivity of the soil near the ground at that moment in time; d is the boundary cell and adjacent cell centerA distance therebetween; t (T) Soil1 Is the temperature of the upper boundary cell; t (T) Soilp Is the temperature of the cell adjacent to the upper boundary; h is a air Is the heat transfer coefficient of the soil surface; t (T) air Is the air temperature;
the boundary conditions of the convective heat transfer of the soil and the fluid between the inner pipe and the outer pipe are set as follows:
wherein T is Soil0 Is the temperature of the boundary within the soil region; t (T) Soilp ' is the temperature of the cell adjacent to the inner boundary of the soil; t (T) fout Is the temperature of the fluid between the inner tube and the outer tube; h is a soil The heat exchange coefficient is the convection heat exchange coefficient between the fluid between the inner tube and the outer tube and the inner wall surface of the outer tube of the heat exchanger; h is a soil The solution formula of (2) is:
lambda in f Is the heat conductivity coefficient of the fluid, D Out Is the inner diameter of the outer tube, D In Is the inner diameter of the inner tube, delta Pipe Re is the Reynolds number of the fluid flowing between the inner tube and the outer tube for the thickness of the inner tube wall of the heat exchanger, pr f Prayer number for fluid to flow between the inner and outer tubes, and Gr is the glazingfr number for fluid to flow between the inner and outer tubes.
And solving a soil temperature field according to a heat conduction equation and a discrete equation of the soil, updating the heat convection coefficient of the outer wall surface of the outer tube of the heat exchanger according to the soil temperature field, and then updating the soil temperature field to obtain a new soil temperature field. Comparing the temperature of the outer wall surface of the outer tube with the temperature of the upper layer, and when the formula (7) is satisfied:
the temperature of the outer wall surface of the outer tube is kept unchanged, iteration is completed, and the inner boundary parameters and the soil temperature field are updated to obtain the soil temperature distribution conditionIn the condition, T w1 ' is the temperature of the outer wall surface of the outer tube which is newly output, T w1 The temperature of the outer wall surface of the outer tube which is output last time.
And 7, setting boundary conditions of the pipe wall area of the inner pipe of the heat exchanger.
The inner side of the inner tube is contacted with the fluid, and the third type of boundary conditions are adopted; the outer side of the inner tube is in fluid contact with the inner tube and the outer tube, and is a third type of boundary condition; the upper boundary is in contact with the surface air, and is a third type of boundary condition; the lower boundary is an adiabatic boundary.
Specifically, the right boundary of the inner tube wall (i.e., the outer side of the inner tube is in fluid contact with the inner and outer tubes) is a third type of boundary condition, where:
the left boundary of the inner tube wall area and the soil area of the heat exchanger are both in a third type of boundary condition, the upper boundary is in a third type of boundary condition, and the lower boundary is in an adiabatic boundary, so that the boundary conditions of the inner tube wall area, the left boundary, the upper boundary and the lower boundary are the same, and the boundary conditions of the inner tube wall area, the left boundary, the upper boundary and the lower boundary are the same in the areas except for the right boundary:
a′ p ＝a p ，a′ E ＝a E ，a′ N ＝a N ，a′ S ＝a S ，a′ W ＝a W ，b′＝b
and 8, keeping the temperature of the fluid in the heat exchanger unchanged, respectively solving the heat convection coefficients of the inner wall surface and the outer wall surface of the inner tube of the heat exchanger according to the temperature distribution condition of the inner wall surface of the inner tube of the initial heat exchanger, obtaining the coefficient of a discrete equation, updating the temperature field of the inner tube wall of the heat exchanger by the coefficient of the equation, and outputting the internal temperature field of the inner tube wall of the heat exchanger when the temperature of the inner wall surface of the inner tube is unchanged.
In the step, the specific implementation process is as follows:
the upper boundary of the inner pipe adopts convection boundary conditions, specifically:
wherein lambda is the thermal conductivity of the inner tube wall; d is the distance between the boundary cell and the adjacent cell center; t (T) Pipe1 Is the temperature of the upper boundary cell; t (T) Pipep Is the temperature of the cell adjacent to the upper boundary; h is a air Is the heat transfer coefficient of the soil surface; t (T) air Is the air temperature.
The boundary conditions of fluid convection heat exchange between the inner pipe wall and the inner pipe and the outer pipe are as follows:
wherein T is pipeout Is the temperature of the outer boundary of the inner pipe wall; t (T) pipep ' is the temperature of the cell adjacent to the outer boundary of the inner tube; h is a pipeout The heat convection coefficient between the fluid between the inner pipe and the outer pipe and the inner wall surface of the inner pipe of the heat exchanger;
the boundary conditions of the convection heat exchange of the inner wall surface of the inner tube and the fluid of the inner tube are as follows:
wherein T is Pipein Is the temperature of the inner boundary of the inner tube; t (T) Pipep "is the temperature of the cell adjacent to the inner boundary of the inner tube; h is a pipein Is the convection heat exchange coefficient of the inner wall surface of the inner tube; t (T) fin Is the inner tube fluid temperature.
Solving the temperature field of the inner pipe wall according to the heat conduction equation and the discrete equation of the inner pipe wall, updating the heat convection coefficient of the inner wall surface and the outer wall surface of the inner pipe of the heat exchanger, and then updating the temperature field of the inner pipe wall, thereby obtaining a new temperature field of the inner pipe wall. Updating the temperature of the inner wall surface and the outer wall surface of the inner tube, the convection heat exchange coefficient and the node coefficients every three times of iteration, judging whether convergence is carried out every ten times of iteration, and when the temperature of the inner wall surface of the inner tube of the heat exchanger meets the formula (11):
the temperature of the inner wall surface of the inner tube is kept unchanged, iteration is completed, the temperature distribution condition of the tube wall of the inner tube of the heat exchanger is obtained, and the soil temperature distribution condition is output, wherein T is w2 ' is the temperature of the inner wall surface of the inner tube which is newly output, T w2 The temperature of the inner wall surface of the inner tube which is output last time.
And 9, setting the boundary condition of the fluid area.
The upper boundary of the fluid region is an isothermal boundary condition; the lower boundary is an adiabatic boundary. Specifically:
when fluid flows in from the outer tube and flows out from the inner tube, the coefficient of a downstream fluid control equation is calculated:
the upper boundary is an isothermal boundary, at which:
a″ S ＝0；
the lower boundary is an adiabatic boundary:
a″ N ＝0；
the calculated upstream fluid control equation coefficients are the same as the downstream fluid control equation.
Step 10, calculating heat flow q between the inner fluid and the outer fluid according to the temperature of the inner fluid and the temperature of the inner wall surface of the inner tube 1 Calculating soil heat flow q according to fluid temperature between the inner pipe and the outer wall surface temperature of the outer pipe f Solving a fluid temperature field;
specifically, the heat flow obtained from the soil by the fluid at the soil node is calculated first:
q′＝h soil (T soil0 -T fout ) (12)
because the fluid and the soil meshing are not consistent in the axial direction, the obtained heat flow is interpolated to the fluid nodes:
when z soilj And z fouti Is two adjacent nodes and z soilj ＞z fouti In the time-course of which the first and second contact surfaces,
wherein z is soilj Is the position at the j node of the soil; z fouti The position of the soil at the inode; q fouti Heat absorbed by the fluid between the inner tube and the outer tube at the i-node; q soilj Heat flow of soil at the j node; q soilj-1 Is the heat flow of the soil at the j-1 node.
Obtaining the heat flow q absorbed by the fluid in the inner tube 1 。
The heat flow q absorbed by the fluid between the inner tube and the outer tube on the section of the position i is obtained by the same method 2 。
Solving fluid flow energy equation a' by TDMA algorithm P T P ＝a″ S T S +a″ N T N +b″。
When i=1, let:
when i+.1, let:
iterative solution T i-1 ＝α i-1 T i +β i-1 Obtaining the temperature distribution of the fluid in the inner tube and between the inner tube and the outer tube.
Step 11, outputting the obtained result after reaching the target time;
the software outputs simulation results at intervals, including soil temperature field distribution, inner pipe wall temperature field distribution, inner pipe fluid temperature distribution and outlet fluid temperature.
The embodiment of the invention is specifically described in further detail below, taking a middle-deep coaxial sleeve type buried heat exchanger with a cross-seasonal variable flow pipe diameter as an example, and the data table of the constructed physical model is shown in table 1.
Table 1 simulation modeling data sheet
| Simulation object | Numerical value |
| Inner diameter of inner tube | 0.15，m |
| Shallow stratum outer pipe inner diameter (0-500 m) | 0.25，m |
| Deep stratum outer pipe inner diameter (500-2500 m) | 0.30，m |
| Radial calculation range | 10，m |
| Coefficient of thermal conductivity of soil | 2.5，W/(m·K) |
| Depth of bottom soil of borehole | 2500，m |
| Soil density | 2600，kg/m 3 |
| Heat capacity of soil | 878，J/(kg·K) |
| Coefficient of thermal conductivity of fluid | 0.62，W/(m·K) |
| Density of fluid | 1000，kg/m 3 |
| Fluid heat capacity | 4200，J/(kg·K) |
| Fluid inlet temperature | 12，℃ |
| Winter water inflow | 40，m 3 /h |
| Flow rate of water inlet in summer | 20，m 3 /h |
| Coefficient of thermal conductivity of inner pipe wall | 0.4，W/(m·K) |
| Density of inner tube wall | 7801，kg/m 3 |
| Heat capacity of inner pipe wall | 282，J/(kg·K) |
| Thickness of inner and outer tube wall | 0.01，m |
And (3) iteratively solving a soil area discrete equation (namely the formula (1)) and changing boundary conditions according to the updated value of the outer wall surface temperature of the outer tube until the outer wall surface temperature of the outer tube is unchanged.
The discrete equation (namely the formula (2)) of the inner tube wall area of the heat exchanger is iterated, and the boundary condition is changed according to the updated value of the inner tube wall temperature distribution until the inner tube wall surface temperature is unchanged.
The invention selects the operation mode of outer pipe in and inner pipe out, the operation time is 360 days, and the water inflow is changed at 120 days, data is output every 24 hours, and the change condition of outlet water temperature along with time and the temperature distribution condition of soil and inner pipe wall area are obtained when long period operation is performed.
The outlet water temperature is changed with time when the heat exchanger is operated for a long period as shown in fig. 3. As can be seen from the graph, under the working condition, the outlet water temperature is rapidly increased to the peak value in the initial operation stage of the heat exchanger, and is reduced to a certain extent and gradually stabilized along with the increase of time, and the inlet flow of the heat exchanger is 40m at the 120 th day due to the end of the heating season 3 The ratio of/h becomes 20m 3 And/h, the outlet water temperature is increased.
As shown in fig. 4, the temperature distribution of the fluid in the coaxial tube. Under the working condition, the temperature of the fluid between the inner pipe and the outer pipe is increased from 12 ℃ to 26.72 ℃ under the heating of soil and reflux fluid, and the temperature of the fluid between the inner pipe and the outer pipe is reduced from 26.72 ℃ to 21.10 ℃ under the cooling of the fluid between the inner pipe and the outer pipe with lower temperature.
The soil and local soil temperature distribution is shown in fig. 5 and 6. From the figure, it is clear that at a radial range of ten meters, the soil temperature field is not affected by the circulating water.
It is noted that what is not described in detail in the embodiments of the present invention belongs to the prior art known to those skilled in the art.
In summary, the method provided by the embodiment of the invention aims at the numerical simulation method of the middle-deep coaxial sleeve type buried pipe heat exchanger capable of changing the flow rate and the pipe diameter in a seasonal manner, and can be used for rapidly calculating the temperature distribution conditions of fluid, inner pipe wall and soil in different working environments. According to the structural characteristics and the heat transfer characteristics of the medium-deep coaxial sleeve type buried pipe heat exchanger, a proper simplified physical model is established; firstly, assuming that the temperature of fluid is unchanged, solving a soil and inner pipe wall temperature field by taking the fluid as a third type boundary condition; calculating the heat flow of the soil transferred to the fluid according to the wall surface temperature of the inner pipe and the outer pipe and the fluid temperature, and the heat flow of the fluid of the inner pipe and the outer pipe passing through the wall surface; adding the heat flow as a source term into a one-dimensional fluid flow heat transfer equation, and solving a fluid temperature field; advancing one time layer, calculating soil temperature field again according to the fluid temperature field, and repeating the steps until the preset time. The method can accurately and efficiently simulate the working process of the long-period coaxial sleeve type buried heat exchanger, and has the characteristics of wide application range, high calculation efficiency, good accuracy and the like.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions easily contemplated by those skilled in the art within the scope of the present invention should be included in the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.
Claims (9)
1. The simulation method of the coaxial sleeve type buried pipe heat exchanger capable of changing flow and pipe diameter in a cross-season mode is characterized by comprising the following steps of:
step 1, establishing a physical model comprising three areas of soil, a heat exchanger inner pipe wall and fluid in the heat exchanger along the radial direction;
step 2, simplifying a physical model, dividing grids according to structural characteristics of the physical model, adopting non-uniform structured grids in the radial direction and adopting uniform structured grids in the axial direction;
step 3, importing physical parameters of soil, the inner tube wall of the heat exchanger and fluid in the heat exchanger through Excel, wherein geometric parameters of the heat exchanger and flow change conditions along with time;
step 4, establishing a heat conduction equation of the soil and the inner pipe wall of the heat exchanger and an energy equation of fluid flow in the heat exchanger, and performing discretization treatment to obtain a discrete equation of the soil area, the inner pipe wall area of the heat exchanger and the fluid area in the heat exchanger;
step 5, setting a soil area boundary condition;
step 6, assuming that the temperature of fluid in the heat exchanger is unchanged, solving a heat convection coefficient of the outer wall surface of the outer tube of the heat exchanger according to an initial soil temperature field, obtaining a coefficient of a discrete equation, updating the soil temperature field according to the equation coefficient, and outputting the soil temperature field after iteration until the temperature of the outer wall surface of the outer tube of the heat exchanger is unchanged;
step 7, setting boundary conditions of the pipe wall area of the inner pipe of the heat exchanger;
step 8, keeping the temperature of fluid in the heat exchanger unchanged, respectively solving the heat convection coefficients of the inner wall surface and the outer wall surface of the inner tube of the heat exchanger according to the temperature distribution condition of the inner wall surface of the inner tube of the initial heat exchanger, obtaining the coefficient of a discrete equation, updating the temperature field of the inner tube wall of the heat exchanger by the coefficient of the equation, and outputting the internal temperature field of the inner tube wall of the heat exchanger when iterating until the temperature of the inner wall surface of the inner tube is unchanged;
step 9, setting boundary conditions of the fluid region;
step 10, calculating heat flow q between the inner fluid and the outer fluid according to the temperature of the inner fluid and the temperature of the inner wall surface of the inner tube 1 Calculating soil heat flow q according to fluid temperature between the inner pipe and the outer wall surface temperature of the outer pipe f Solving a fluid temperature field;
step 11, outputting the obtained result after reaching the target time;
and 4, the heat conduction equation of the soil is as follows:
wherein ρ is soil density, c p The specific heat capacity of the soil is T, the temperature is T, the time is T, r is a radial coordinate, lambda is the thermal conductivity of the soil, and z is an axial coordinate;
the discrete equation for the soil region is:
namely:
a P T P ＝a E T E +a W T W +a S T S +a N T N +b
wherein r is n 、r s Lambda is the radial coordinates of two nodes adjacent to the P node e 、λ w 、λ s 、λ n For the thermal conductivity of four adjacent nodes at the P node, deltar is the radial space step at the P node, deltaz is the axial space step at the P node, (ρc) P For the volumetric specific heat capacity, Δt is the time step, (δz) e 、(δz) w For the axial spatial step size (δr) of the P node at two axially adjacent nodes s 、(δr) n For the radial space step at two radially adjacent nodes of the P node,represents the temperature of the n+1-th layer of each node, ">Representing the temperature of the nth layer of the P node;
a P 、a E 、a W 、a N b is the coefficient:
the heat conduction equation of the inner tube wall region of the heat exchanger is as follows:
the discrete equation of the tube wall area of the inner tube of the heat exchanger is as follows:
namely:
a′ P T P ＝a′ E T E +a′ W T W +a′ S T S +a′ N T N +b′
wherein a' P 、a′ E 、a′ W 、a′ S 、a′ N B' is a coefficient:
the energy equation for fluid flow in the heat exchanger is as follows:
wherein u is the fluid flow rate, q is the heat flow obtained from the outside by the fluid region;
for heat exchanger inner tube fluid:
in the formula, h pipein Is the convection between the fluid in the inner tube and the inner wall surface of the inner tube of the heat exchangerHeat exchange coefficient T pipein T is the temperature of the inner wall surface of the inner tube fin For the inner tube fluid temperature, D In Is the inner diameter of the inner tube;
for fluid between the inner tube and the outer tube of the heat exchanger:
in the formula, h soil Is the convection heat exchange coefficient between the fluid between the inner tube and the outer tube and the inner wall surface of the outer tube, T soil0 T is the temperature of the boundary in the soil region fout For fluid temperature between inner and outer tubes, D Out Is the inner diameter of the outer tube, h pipeout Is the convective heat transfer coefficient delta between the fluid in the annular tube and the inner wall surface of the inner tube of the heat exchanger pipe T is the thickness of the inner tube wall of the heat exchanger pipeout The temperature of the outer wall surface of the inner tube;
the discrete equation for the fluid region in the heat exchanger is:
a′ P ′T P ＝a′ S ′T S +a′ N ′T N +b″
wherein a' P ′、a′ S ′、a′ N 'b' is a coefficient:
in the step 5, the upper boundary of the soil is contacted with air, which is a third type of boundary condition, and at this time:
a W ＝0，
h air is the convection heat exchange coefficient of the ground surface and the air, T air Is the temperature of the air above the ground surface;
the lower boundary is an adiabatic boundary, at which point:
a E ＝0
the left boundary of the soil exchanges heat with the fluid, which is a third type of boundary condition, and at the moment:
a S ＝0，
the right soil boundary is an adiabatic boundary at which:
a N ＝0，
2. the simulation method of the coaxial sleeve type buried pipe heat exchanger with the pipe diameter capable of changing flow in a cross-season mode according to claim 1, wherein the operation period of the coaxial sleeve type buried pipe heat exchanger is more than one year, heating is performed in winter and hot water is supplied in summer according to different requirements of users in each season; reasonable utilization of geothermal resources is realized by adopting a variable flow mode, namely, the water inflow in winter is large and the water inflow in summer is small; the pipe diameter of the outer pipe at the upper part of the heat exchanger is smaller than that of the outer pipe at the lower part, namely, the residence time of fluid in the deep stratum is prolonged by increasing the pipe diameter of the outer pipe at the deep stratum; in the step 2, in a physical model comprising three areas of soil, an inner pipe wall of the heat exchanger and fluid in the heat exchanger along the radial direction, the inner diameter of the outer pipe is 0.25m when the underground is 0-500 m, and the inner diameter of the outer pipe is 0.30m when the underground is 500-2500 m.
3. The simulation method of the coaxial sleeve type buried pipe heat exchanger capable of changing flow and pipe diameter in a cross-season mode according to claim 1, wherein in the step 2, in a radial soil area taking the outer wall surface of an outer pipe of the heat exchanger as a starting position, the grid density at the position close to the outer pipe wall is high, the grid density at the position far away from the outer pipe wall is low, the intersection of grid lines is a node, and the radial grid division basis is as follows:
wherein r is j Is the position of the j-th node in the radial grid; n is the number of total nodes in the radial direction; a is the adjustment parameter of the grid change,r is the soil radial calculation range and is an intermediate variable;
in the axial direction, the grids are uniformly distributed, and the axial grid division basis is as follows:
wherein z is the position of the jth grid in the axial grids; depth is the calculated region axial depth; n (N) z Is the number of total nodes in the axial direction.
4. The simulation method of the coaxial sleeve type buried pipe heat exchanger with the pipe diameter capable of changing the flow rate in a cross-season mode according to claim 1, wherein the physical parameters comprise soil density, soil heat capacity, soil heat conductivity coefficient, different soil layer depths and initial temperatures, heat exchanger inner pipe wall density, heat exchanger inner pipe wall heat capacity, heat exchanger inner pipe wall heat conductivity coefficient, fluid density, fluid heat capacity, fluid heat conductivity coefficient and fluid inlet temperature; the geometric parameters of the heat exchanger comprise the pipe wall thickness of the inner pipe and the outer pipe of the heat exchanger and the pipe diameter of the inner pipe and the outer pipe; the time-varying flow rate refers to the different inlet flows used in different seasons.
5. The simulation method of a coaxial sleeve type buried pipe heat exchanger with variable flow and pipe diameters in a cross-season mode according to claim 1, wherein in the step 6, a heat convection coefficient h between fluid between an inner pipe and an outer pipe and an inner wall surface of the outer pipe of the heat exchanger soil The solution formula of (2) is:
lambda in f Is the heat conductivity coefficient of the fluid, D Out Is the inner diameter of the outer tube, D In Is the inner diameter of the inner tube, delta Pipe Re is the Reynolds number of the fluid flowing between the inner tube and the outer tube for the thickness of the inner tube wall of the heat exchanger, pr f Prayer number for fluid to flow between the inner and outer tubes, and Gr is the glazingfr number for fluid to flow between the inner and outer tubes.
6. The simulation method of the coaxial sleeve type buried pipe heat exchanger capable of changing flow and pipe diameter in a cross-season mode according to claim 1, wherein in the step 6, a soil temperature field is obtained according to a heat conduction equation of soil, and a heat convection coefficient of an outer wall surface of an outer pipe of the heat exchanger is updated according to the soil temperature field, so that a new soil temperature field is obtained, and when the temperature of the outer wall surface of the outer pipe of the heat exchanger meets the following conditions:
at the moment, the temperature of the outer wall surface of the outer tube is kept unchanged, iteration is completed, and the soil temperature distribution condition is obtained, wherein T is w1 ' is the temperature of the outer wall surface of the outer tube which is newly output, T w1 The temperature of the outer wall surface of the outer tube which is output last time.
7. The simulation method of the coaxial sleeve type buried pipe heat exchanger with the variable flow and pipe diameter across seasons according to claim 1, wherein in the step 7, the right boundary of the pipe wall of the inner pipe is a third type of boundary condition, and at this time:
the left boundary of the inner tube wall area and the soil area of the heat exchanger are both in a third type of boundary condition, the upper boundary is in a third type of boundary condition, and the lower boundary is in an adiabatic boundary, so that the boundary conditions of the inner tube wall area, the left boundary, the upper boundary and the lower boundary are the same, and the boundary conditions of the inner tube wall area, the left boundary, the upper boundary and the lower boundary are the same in the areas except for the right boundary:
a′ p ＝a p ，a′ E ＝a E ，a′ N ＝a N ，a′ S ＝a S ，a′ W ＝a W ，b′＝b
step 8, a heat conduction equation of the wall of the inner pipe of the heat exchanger is used for solving a temperature field of the wall of the inner pipe of the heat exchanger, and the heat convection coefficients of the inner wall and the outer wall of the inner pipe of the heat exchanger are updated, so that a new temperature field of the wall of the inner pipe is obtained, the temperature of the inner wall and the outer wall of the inner pipe, the heat convection coefficients and the coefficients of all nodes are updated three times per iteration, whether convergence is judged ten times per iteration, and when the temperature of the inner wall of the inner pipe of the heat exchanger is satisfied:
wherein T is w2 ' is the temperature of the inner wall surface of the inner tube which is newly output, T w2 The temperature of the inner wall surface of the inner tube which is output last time; at this time, the temperature of the inner wall surface of the inner tube is kept unchanged, and iteration is completed, so that the temperature distribution condition of the tube wall of the inner tube of the heat exchanger is obtained.
8. A simulation method of a coaxial sleeve type buried pipe heat exchanger with a variable flow pipe diameter across seasons according to claim 1, wherein in the step 9, when fluid flows in from the outer pipe and flows out from the inner pipe, a coefficient of a downstream fluid control equation is calculated:
the upper boundary is an isothermal boundary, at which:
a″ S ＝0；
the lower boundary is an adiabatic boundary:
a″ N ＝0；
the calculated upstream fluid control equation coefficients are the same as the downstream fluid control equation.
9. The simulation method of the coaxial sleeve type buried pipe heat exchanger with the variable flow and pipe diameter capable of crossing seasons according to claim 1, wherein in the step 10, the fluid flow energy equation a is solved through a TDMA algorithm P T P ＝a S T S +a N T N +b, when i=1, let:
when i+.1, let:
iterative solution T i-1 ＝α i-1 T i +β i-1 Obtaining the temperature distribution of the fluid in the inner tube and between the inner tube and the outer tube.
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