CN110954350B - Indoor simulation device and method for heat dissipation of submarine pipeline - Google Patents
Indoor simulation device and method for heat dissipation of submarine pipeline Download PDFInfo
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Abstract
The invention discloses an indoor simulation device and method for heat dissipation of a submarine pipeline, and belongs to the technical field of oil and gas storage and transportation engineering. According to the composition and characteristics of a thermodynamic system of the submarine hot oil pipeline, based on the similarity principle, an indoor simulation experiment system of the submarine hot oil pipeline is designed. By controlling parameters such as temperature and flow of fluid, static and dynamic heat dissipation experiments of the submarine pipeline are carried out. The experimental research is carried out on the conditions of different bottom layer flow velocities by adjusting and simulating the flow velocity and the flow direction of the seawater, so that the influence of the bottom layer water flow velocity on the heat dissipation of the pipeline and the distribution of the soil temperature field is obtained. The influence of the bottom water flow speed and the flow direction on the heat dissipation of a submarine pipeline and the temperature distribution of submarine soil is further researched by measuring and simulating the bottom flow speed of the seawater. Determining a convective heat transfer model according to the bottom layer flow velocity determined by the experiment, and returning the boundary to the convective heat transfer essence; the characterization method and the application range of the seawater/seabed heat exchange boundary are determined, and the stable boundary condition characterization method is provided for establishing a submarine pipeline heat dissipation mathematical model.
Description
Technical Field
The invention belongs to the technical field of oil and gas storage and transportation engineering, and particularly relates to an indoor simulation device and method for heat dissipation of a submarine pipeline.
Background
Submarine pipelines are important ties for connecting offshore oil and gas fields, oil and gas storage equipment and land terminals. In recent years, the construction pace of submarine pipelines in China is remarkably accelerated and the total mileage of the submarine pipelines is continuously increased under the impetus of the development of offshore oil. Subsea hydrocarbon pipeline transportation is a complex multiple coupling problem involving fluid hydraulic thermal coupling, fluid and subsea environment thermal coupling. When a numerical simulation method is adopted to research the heat dissipation problem of a submarine oil and gas pipeline, the problems of mathematical modeling, numerical solution and the like need to be solved in sequence. Compared with a land pipeline, the seabed and bottom layer seawater convection heat exchange is the biggest characteristic of a submarine pipeline. The method for determining the reasonable characterization of the bottom layer seawater/seabed convective heat transfer boundary can not only highlight the heat dissipation characteristics of a submarine pipeline, but also is an important component and necessary step for establishing a submarine pipeline mathematical model. According to the thermal influence area model, when the heat dissipation problem of the buried pipeline is solved, the soil heat dissipation calculation area can be simplified into a rectangular or circular two-dimensional space. Determining boundary conditions is an important step in mathematical modeling. The bottom seawater/seabed boundary is the upper boundary of the thermodynamic affected zone. The heat exchange form of the seabed soil and the bottom layer seawater can be regarded as convective heat exchange caused by the fact that the fluid sweeps across an infinite large plane, the heat exchange strength of the heat exchange is influenced by factors such as the temperature and the flow velocity of the seawater, the heat dissipation rate of the seabed soil to the seawater is determined, and the heat dissipation rate of a pipeline and the temperature field distribution in the seabed soil are indirectly influenced.
Scholars at home and abroad develop a great deal of experimental research aiming at the problem of heat dissipation of submarine pipelines. According to the Bohai sea bottom flow velocity (0.5-1 m/s), scholars such as Wu nationality, break-even soldiers, poplar marked characters and the like in China consider that the scholars have little influence on heat transfer of submarine pipelines. It can be seen from the numerical simulation literature published at home and abroad that, when a submarine pipeline heat dissipation mathematical model is established, Dirichlet boundary processing is adopted in most researches for a bottom seawater/seabed boundary except for individually adopting a fixed convection heat transfer intensity value, and the convection boundary is defined by the temperature of the bottom seawater (as shown in table 1). These studies focused on fluid temperature and seafloor soil temperature field changes, but did not analyze the seawater/seafloor heat exchange boundary in depth.
Table 1 partial research on bottom layer seawater/seabed heat exchange boundary treatment method
| Year of year | Authors refer to | Subject and problem | Bottom seawater/seabed heat exchange boundary treatment method |
| 2008 | Barletta A et al | Subsea pipeline launch process | Constant temperature boundary, using sea water temperature |
| 2008 | Lu T, etc | Pipeline stop conveying in frozen soil area | Constant temperature boundary, using sea water temperature |
| 2009 | Break down soldier | Pipeline stop and restart | Constant temperature boundary, using sea water temperature |
| 2014 | Zhengli et al | Subsea pipeline preheating | Fixed convective heat transfer coefficient: 500W/(m)2·℃) |
| 2014 | Bai Y and the like | Sea pipe steady state heat transfer | Constant temperature boundary, using sea water temperature |
| 2015 | Sund F etc | Sea pipe transient heat transfer | Constant temperature boundary, using sea water temperature |
| 2016 | Chakraborty S et al | Sea pipe steady state heat transfer | Constant temperature boundary, using sea water temperature |
| 2018 | Yuan et al | Subsea pipeline restart | Constant temperature boundary, using sea water temperature |
The boundary condition of the surface soil contacting with the natural environment directly affects the numerical simulation result. Oosterkamp et al compared the influence of surface soil heat exchange boundary conditions such as constant temperature and convection on the temperature field of soil around a buried natural gas pipeline and the heat dissipation rate of the pipeline, showed that there were significant differences in the application effects of different boundary models, and indicated that the simulation accuracy obtained using the constant temperature conditions was the lowest. According to the Chakraborty, through theoretical analysis, corresponding submarine pipeline heat dissipation shape parameter calculation models are respectively established on the basis of a bottom layer seawater/seabed constant temperature boundary and a constant heat flux density boundary, and experimental data show that the calculation accuracy is reduced due to the application of the constant temperature boundary. The sea areas in China are wide and are comprehensively affected by factors such as tidal current and wave-induced underflow, and the bottom layer flow velocity distribution of each sea area is different from the grasped Bohai sea condition. Therefore, it is questionable whether the constant temperature boundary can be applied to all sea areas. The mode of treating the bottom layer seawater/seabed heat exchange boundary by uniformly adopting the seawater temperature only highlights the influence of the seawater temperature, blurs the action of flow velocity, and is not in accordance with the nature of the bottom layer seawater/seabed convection heat exchange. Although the method achieves satisfactory results in simulation research, the application range of the method is unclear, and the method is indistinguishable from other treatment methods, and the essence of the method is that the influence of the flow velocity of the bottom-layer seawater on the heat dissipation of the submarine pipelines and the submarine soil is unclear.
Disclosure of Invention
In order to solve the defects in the prior art, the invention aims to provide an indoor simulation device and method for submarine pipeline heat dissipation, wherein the indoor simulation experimental device for submarine pipeline heat dissipation is constructed according to the similarity principle, the influence of the bottom water flow speed and the flow direction on the submarine pipeline heat dissipation and submarine soil temperature distribution is researched, sufficient experimental data are provided for mathematical modeling, the accuracy of a mathematical model is verified, and the application conditions of different bottom water/seabed heat exchange boundary models are determined.
The invention is realized by the following technical scheme:
the invention discloses an indoor simulation device for heat dissipation of a submarine pipeline, which comprises a sand box, an experiment pipe section, an experiment pipe fluid circulating system, a simulated seawater circulating system, a constant-temperature water circulating system and a data acquisition and control system, wherein the sand box is connected with the experiment pipe section through a pipeline;
the inside of the sand box is a sealed cavity, a water outlet is arranged on the sand box, and a water discharge valve is arranged on the water outlet; a constant-temperature water circulation layer, a sand layer and a simulated seawater layer are sequentially arranged in the sand box from bottom to top, a heat conduction partition plate is arranged between the constant-temperature water circulation layer and the sand layer, and a heat insulation material is arranged on the outer wall of the sand box;
the constant-temperature water circulation layer is connected with a constant-temperature water circulation system; the simulated seawater layer is connected with a simulated seawater circulating system, the simulated seawater circulating system comprises two sets of water inlet and return branches, one set of water inlet and return branches is parallel to the experimental pipe section, and the other set of water inlet and return branches is perpendicular to the experimental pipe section; the experimental pipe section penetrates through the sand layer, and two ends of the experimental pipe section are connected with the experimental pipe fluid circulating system;
the inlet and the outlet of the experiment pipe section are respectively provided with a pressure monitoring device, and a second flowmeter is arranged in the experiment pipe fluid circulating system; the bottom of the simulated seawater layer is provided with a plurality of flow velocity monitoring devices; a plurality of temperature monitoring devices are arranged in different sandy soil buried depths of the experimental pipe section and the sandy soil layer; a first flowmeter is arranged in the simulated seawater circulating system, and a first liquid level meter and a first thermometer are arranged in the simulated seawater layer; a second liquid level meter and a second thermometer are arranged in the constant-temperature water circulation layer;
the pressure monitoring device, the first flowmeter, the second flowmeter, the flow rate monitoring device, the temperature monitoring device, the first liquid level meter, the first thermometer, the second liquid level meter and the second thermometer are respectively connected with the data acquisition and control system.
Preferably, the simulated seawater circulation system comprises a simulated seawater storage device, a first circulating pump and a first valve which are connected through a circulating pipeline, and the simulated seawater storage device is connected with a first temperature control device; the first circulating pump, the first valve and the first temperature control device are respectively connected with the data acquisition and control system.
Preferably, the water inlet and return branch parallel to the experimental pipe section comprises a parallel inlet pipeline and a parallel outlet pipeline which are communicated with the inside of the simulated seawater layer, and the parallel inlet pipeline and the parallel outlet pipeline are arranged in parallel with the experimental pipe section; the water inlet and return branch perpendicular to the experimental pipe section comprises a vertical inlet pipeline and a vertical outlet pipeline which are communicated with the inside of the simulated seawater layer, and the vertical inlet pipeline and the vertical outlet pipeline are arranged perpendicular to the experimental pipe section; valves are arranged on the parallel inlet pipeline, the parallel outlet pipeline, the vertical inlet pipeline and the vertical outlet pipeline.
Further preferably, the water outlet, the parallel outlet pipeline and the vertical outlet pipeline are all provided with filter screens, and the aperture of each filter screen is smaller than the particle size of sand in the sand layer.
Preferably, the experiment pipe fluid circulation system comprises a fluid storage device, a third circulating pump and a second valve which are connected through pipelines, and the fluid storage device is connected with a third temperature control device; and the third circulating pump, the second valve and the third temperature control device are respectively connected with the data acquisition and control system.
Preferably, the constant temperature water circulation system comprises a constant temperature water storage device and a second circulating pump which are connected through pipelines, the constant temperature water storage device is connected with a second temperature control device, and the second circulating pump and the second temperature control device are respectively connected with the data acquisition and control system.
Preferably, a support frame is connected between the heat-conducting partition plate and the inner wall of the sand box.
Preferably, a covering net is arranged between the constant-temperature water circulation layer and the sand layer.
Preferably, the outer wall of the sand box is provided with an insulating layer.
The invention discloses a method for simulating heat dissipation of a submarine pipeline by adopting the indoor simulation device for heat dissipation of the submarine pipeline, which comprises the following steps:
steady state experiment:
starting a constant-temperature water circulation system to keep the constant-temperature water circulation layer at a constant temperature; starting a simulated seawater circulating system, starting a drainage valve, injecting simulated seawater into a sand box until the simulated seawater flows out of the drainage valve, and closing the drainage valve; forming a flow direction parallel or vertical to the experimental pipe section by the simulated seawater layer through two sets of water inlet and return branches in the simulated seawater circulating system, and monitoring flow data through a first flowmeter;
after the flow rate data and the simulated seawater layer temperature data are monitored to be stable by the flow rate monitoring device and the temperature monitoring device, starting a fluid circulating system of the experiment pipe to enable fluid at a given temperature to flow in the experiment pipe section, and monitoring flow data by a second flowmeter;
monitoring temperature data of different sandy soil buried depths in an experiment pipe section and a sandy soil layer through a temperature monitoring device, monitoring pressure data of an inlet and an outlet of the experiment pipe section through a pressure monitoring device, changing the temperature of constant-temperature water in a constant-temperature water circulation layer, simulating the temperature of seawater, the temperature of fluid in the experiment pipe section, simulating the flow velocity of the seawater or the flow velocity of the fluid in the experiment pipe section after the temperature data and the pressure data are stable, and transmitting the monitored data to a data acquisition and control system through the pressure monitoring device, a first flowmeter, a second flowmeter, a flow velocity monitoring device, a temperature monitoring device, a first liquid level meter, a first thermometer, a second liquid level meter and a second thermometer to perform numerical simulation; for the bottom layer seawater/seabed convective heat transfer boundary, adding a function related to the bottom layer flow velocity on the basis of an incompressible fluid external sweep plane heat transfer model, and fitting a bottom layer seawater/seabed convective heat transfer coefficient expression formula and an applicable flow velocity range thereof according to steady-state experiment results and numerical simulation results of different simulated seawater flow velocities;
unsteady state experiment:
on the basis of a steady state experiment, change the temperature of the fluid in the experiment pipeline section, bury the temperature data of depths through different sandy soils of temperature monitoring devices monitoring experiment pipeline section and sand layer, tend to stably again until temperature data, through pressure monitoring device, first flowmeter, second flowmeter, velocity of flow monitoring device, temperature monitoring device, first level gauge, first thermometer, second level gauge and second thermometer data transmission that the monitoring obtained carries out numerical simulation to data acquisition and control system.
Compared with the prior art, the invention has the following beneficial technical effects:
the invention discloses an indoor simulation device for heat dissipation of a submarine pipeline. By controlling parameters such as temperature and flow of fluid, static and dynamic heat dissipation experiments of the submarine pipeline are carried out. The experimental research is carried out on the conditions of different bottom layer flow velocities by adjusting and simulating the flow velocity and the flow direction of the seawater, so that the influence of the bottom layer water flow velocity on the heat dissipation of the pipeline and the distribution of the soil temperature field is obtained. The influence of the bottom layer seawater flow velocity and the flow direction on the heat dissipation of a submarine pipeline and the temperature distribution of submarine soil is researched by measuring and simulating the bottom layer seawater flow velocity. Meanwhile, the simulated seawater can be switched through the valve, so that the relation between the simulated seawater and the laid pipeline in multiple angles such as parallel and perpendicular angles is realized, and multiple simulation purposes are realized. On the other hand, the indoor experiment can provide sufficient experimental data for mathematical modeling, so as to verify the precision of the mathematical model and determine the application conditions of different bottom layer water/seabed heat exchange boundary models.
Furthermore, a support frame is connected between the heat conduction partition plate and the inner wall of the sand box, so that the strength of the device can be improved, and the heat conduction partition plate is prevented from being extruded and deformed by a sand layer to influence the measurement precision.
Furthermore, a covering net is arranged between the constant-temperature water circulation layer and the sand layer, so that sand is prevented from being taken away by water flow.
Furthermore, the outer wall of the sand box is provided with a heat insulation layer, so that heat loss in the device is avoided, and the measurement precision is not affected.
The invention discloses a method for simulating heat dissipation of a submarine pipeline by adopting the indoor simulator for heat dissipation of the submarine pipeline, which is characterized in that a convective heat transfer model is determined according to the bottom flow rate measured by experiments, so that the boundary returns to the convective heat transfer essence; the method combines numerical simulation and physical experiments, determines the influence of different boundary processing methods such as convective heat transfer, simulated seawater temperature, surface soil temperature definition seawater/seabed heat transfer boundary and the like on the simulation precision, determines the characterization method and the application range of the seawater/seabed heat transfer boundary, and provides a stable boundary condition characterization method for establishing a submarine pipeline heat dissipation mathematical model. On the basis of a steady-state experiment, transient experimental research can be carried out by changing the hot water temperature of the fluid circulation system in the pipe and simulating the seawater conditions, a large amount of experimental data are collected through an indoor simulation experiment, the distribution of the temperature field of the sand in the sand box is drawn, the heat dissipation rate of the pipeline is calculated, the influence of the flow velocity of the seawater on the heat dissipation of the pipeline and the distribution of the temperature field is further summarized, and a foundation is laid for further optimizing a seawater/seabed heat exchange model.
Drawings
Fig. 1 is a schematic overall structure diagram of an indoor simulation device for heat dissipation of a submarine pipeline according to the present invention;
FIG. 2 is a technical roadmap for the present invention;
FIG. 3 is a schematic structural diagram of two sets of inlet return water branches of the simulated seawater circulation system of the present invention;
FIG. 4 is a schematic view illustrating the flow direction of simulated seawater in the simulated seawater circulation system of the present invention;
FIG. 5 is a calculated heat transfer area for a buried subsea pipeline;
fig. 6 is a schematic diagram of a mapping relationship between a physical coordinate system and a parameter coordinate system.
In the figure: 1-a first flowmeter, 2-a first valve, 3-a first circulating pump, 4-a simulated seawater storage device, 5-a first temperature control device, 6-a circulating pipeline, 6-1-a parallel inlet pipeline, 6-2-a parallel outlet pipeline, 6-3-a vertical inlet pipeline, 6-4-a vertical outlet pipeline, 7-a sealing cover, 8-a sand box, 9-an experimental pipe section, 10-a heat conducting partition plate, 11-a heat insulating material, 12-a supporting frame, 13-a thermostatic water bath, 14-a drainage valve, 15-a temperature monitoring device, 16-a flow rate monitoring device, 17-a second temperature control device, 18-a thermostatic water storage device, 19-a second circulating pump, 20-a third temperature control device and 21-a fluid storage device, 22-third circulation pump, 23-second valve, 24-second flow meter, 25-pressure monitoring device.
Detailed Description
The invention will now be described in further detail with reference to the following drawings and specific examples, which are intended to be illustrative and not limiting:
fig. 1 is an indoor simulation device for heat dissipation of a submarine pipeline according to the present invention, which includes:
1) experiment pipe fluid circulation system: the device is composed of a fluid storage device 21, a third temperature control device 20, a third circulating pump 22, a second valve 23, a second flowmeter 24, a pressure monitoring device 25 and a connecting pipeline, wherein hot water flow in the pipe replaces hot oil flow.
2) Simulating a seawater circulating system: circulating water is controlled to flow through the surface of sandy soil at an experimental flow and temperature so as to simulate the convective heat exchange between seawater and a seabed. The device comprises a first temperature control device 5, a first circulating pump 3, a first valve 2, a first flowmeter 1 and a circulating pipeline 6. In order to prevent the sandy soil from being taken away by water flow, a steel wire mesh is covered on the surface of the sandy soil.
3) A constant-temperature water circulation system: and a constant-temperature water bath is arranged at the bottom of the heat-preservation sand box and used for simulating a seabed soil constant-temperature layer. The flask is separated from the thermostatic water bath by a heat-conducting partition plate 10 and is supported by a support frame 12. The water bath temperature is controllable and is used for simulating the conditions of different constant temperature layers. The thermostatic waterbath 13 is wrapped with a heat insulating material 11.
4) The data acquisition and control system comprises: a plurality of temperature monitoring devices 15 are arranged at different positions along the experimental pipe section and at different depths of sandy soil, pressure monitoring devices 25 are arranged at the outlet and the inlet of the pipe section, and all detection data are transmitted to a computer and are recorded, analyzed and displayed by the computer.
5) The experimental sand box 8 comprises a simulated seawater layer, a sand layer, an experimental pipe section 9 and a constant temperature water bath 13 from top to bottom respectively; the periphery of the sand box 8 is wrapped by heat insulation materials 11, and the upper part of the sand box is sealed by a sealing cover 7.
6) The true temperature of the simulated seawater and the fluid in the thermostatic water bath is measured by an immersion thermometer.
7) The water depth of the constant-temperature water bath and the depth of the simulated seawater are monitored by a liquid level meter.
8) All the water outlets or outlets are provided with a screen to prevent sand from being carried away.
In order to study the influence of the bottom flow rate of seawater on the heat dissipation of the submarine pipeline, the bottom water flow rate needs to be measured. The invention adds a flow velocity monitoring device 16 which can be a portable electromagnetic flow velocity detector at the bottom of the simulated water, and the flow velocity detection data and the temperature detection data are connected into a data detection system together. Meanwhile, in order to investigate the influence of two flow directions perpendicular to each other, water inlets were provided in both directions parallel and perpendicular to the axial direction of the duct. As shown in fig. 3, the water inlet and return branch parallel to the experimental pipe section 9 comprises a parallel inlet pipeline 6-1 and a parallel outlet pipeline 6-2 communicated with the inside of the simulated seawater layer, and the parallel inlet pipeline 6-1 and the parallel outlet pipeline 6-2 are arranged in parallel with the experimental pipe section 9; the water inlet and return branch vertical to the experimental pipe section 9 comprises a vertical inlet pipeline 6-3 and a vertical outlet pipeline 6-4 which are communicated with the inside of the simulated seawater layer, and the vertical inlet pipeline 6-3 and the vertical outlet pipeline 6-4 are arranged vertically to the experimental pipe section 9; valves are arranged on the parallel inlet pipeline 6-1, the parallel outlet pipeline 6-2, the vertical inlet pipeline 6-3 and the vertical outlet pipeline 6-4, the simulated seawater is connected with an inflow pipe with uniform holes, and the switching of the flow direction is realized by the control of the valves.
On the basis of the experimental system, indoor simulation research is carried out. And carrying out a steady-state orthogonal experiment of heat dissipation of the submarine pipeline by taking the simulated seawater temperature, flow velocity and flow direction as experimental factors. On the basis of a steady-state experiment, the hot water temperature of the fluid circulation system in the pipe is changed, the seawater condition is simulated, and transient experimental research is carried out. Through indoor simulation experiments, a large amount of experimental data are collected, the sand temperature field distribution in the sand box is drawn, the pipeline heat dissipation rate is calculated, the influence of the seawater flow velocity on the pipeline heat dissipation and the temperature field distribution is further summarized, and a foundation is laid for determining a convective heat transfer model in the next step.
(II) numerical simulation
On the basis of experimental data, the convective heat transfer coefficient of the seawater/seabed at the bottom layer is determined based on the fluid skimming flat plate heat transfer theory. The influence of different boundary condition processing methods such as a convection heat transfer boundary, a constant temperature boundary and the like on the mathematical model is contrastively analyzed, a bottom layer seawater/seabed heat transfer boundary representation method and an application range related to the bottom layer seawater flow velocity are established, and a stable boundary condition model is provided for establishing a submarine pipeline heat dissipation mathematical model.
Considering the symmetry, the indoor mock sand box can be simplified to a rectangular thermodynamic calculation area as shown in FIG. 5. Wherein, the left and right boundaries are thermal insulation boundaries, and the lower boundary is a constant temperature layer boundary. The upper boundary of the soil contacting with the bottom water is the content to be solved by the invention.
In essence, the bottom seawater/seabed heat exchange mode is convection heat exchange, the incompressible fluid sweepback plane heat transfer theory is followed, and the convection heat exchange coefficient has a functional relation with Re and Pr. A submarine pipeline heat dissipation mathematical model corresponding to the experimental system is established based on a bottom layer seawater/seabed convective heat transfer boundary, and numerical solution is carried out by applying principles such as computational fluid mechanics, numerical heat transfer, and the like. And (4) integrating the calculation result with the experimental data, fitting the heat exchange coefficient of the bottom layer seawater/seabed, and establishing a corresponding relation related to the bottom layer water flow speed. In the invention, the basic thermophysical property of the simulated seawater is obtained by detecting the thermophysical property, and the bottom layer flow velocity is measured by a flow velocity detection device arranged on the simulated water bottom layer.
Except for the convection boundary, the measured simulated seawater temperature TseaAnd surface soil temperature TsurfAnd also as a means of treating the bottom water/sea bed boundary to be analyzed. Therefore, three submarine pipeline heat dissipation mathematical models corresponding to the experimental system can be obtained, and the upper boundaries of the soil calculation areas are respectively as follows:
therefore, according to different upper boundary representations of the calculation region, a submarine pipeline heat dissipation mathematical model corresponding to the experimental system can be obtained:
and (3) aiming at the same experiment parameters, establishing a submarine pipe heat dissipation mathematical model according to the three seawater/seabed boundary conditions respectively, and carrying out numerical solution. And comparing the numerical calculation result with the experimental data to obtain the influence of different boundary processing modes on the simulation precision. The applicable flow rate range of the three boundary condition characterization methods can be determined by simulating and comparing experimental parameters of different flow directions and flow rates.
The above process route is shown in fig. 2.
The complete implementation process of the design of the invention comprises three links of developing experiments, establishing a numerical simulation basic format and contrasting and representing the heat exchange boundary of the bottom layer seawater/seabed.
1. Experimental procedure
The experimental system was connected as shown in fig. 1 and the tightness of the piping, valves and equipment was checked.
The cooling water in the constant-temperature water storage device 18 is maintained to a given temperature T by the second temperature control device 17d(ii) a And starting a second circulating pump 19 to fill the constant-temperature water bath 13 with constant-temperature water, and forming circulation with the constant-temperature water bath tank through a connecting pipeline.
The simulated seawater in the simulated seawater storage device 4 is maintained to a given temperature T by the first temperature control device 5water. And starting the first circulating pump 3, injecting simulated seawater into the sand box 8, and returning to the simulated seawater storage device 4 through the water return pipeline 6. The flow of the simulated seawater is regulated by a first valve 2, and the specific value is monitored by a first flowmeter 1. And (4) continuously filling water into the sand box 8, and closing the drain valve 14 when the simulated seawater flows out from the drain port of the sand box and the sand box is considered to be completely saturated by the simulated seawater.
And starting the temperature monitoring device 15 and the flow rate monitoring device 16, and respectively recording the temperature and the flow rate change of the simulated seawater in the water injection process. The data to be monitored does not change any more and can be regarded as simulating the formation of an initial temperature field.
The water in the fluid storage means 21 is maintained to a given temperature T by the third temperature control means 20f. The third circulation pump 22 was turned on to allow hot water to flow into the test section 9 and circulate through the connecting line to the fluid reservoir 21. The hot water flow is regulated by a second valve 23, the specific value being monitored by a second flow meter 24. The temperature of different depths of the sand box 8 and the temperature changes of the inlet and the outlet of the experimental pipe section 9 in the hot water circulation process are recorded by the temperature monitoring device 15. The pressure change of the inlet and the outlet of the experimental pipe section 9 is monitored by a pressure monitoring device 25. When the temperature and pressure monitoring data are not obviously changed any more, the stable temperature field of the pipeline heat dissipation can be considered to be formed.
Changing Td、Twater、TfSimulating the flow velocity of seawater and the flow velocity of hot water, repeating the experimental steps, and performing orthogonal steady-state experiment to obtain a large amount of temperaturesVelocity and flow rate monitoring data.
2. Numerical simulation process
The experimental pipe section is divided into space step length delta z according to the axial length. Adopting a fluid flow and heat dissipation model in the finite difference format discrete formula (2):
in the formula, c, rho and beta are respectively the specific heat capacity, density and expansion coefficient of hot water in the pipe and can be obtained by calculation through an empirical formula; f is the friction coefficient and can be calculated by Darcy formula. After the experimental system is stable, the flow velocity V of the fluid in the pipe is a fixed value and can be calculated by a flowmeter:
in steady state experiment, the pressure drop of experiment pipeline along the way
The method can be obtained by approximately dividing the monitoring values of the inlet pressure and the outlet pressure of the experimental pipe section by the length of the experimental pipe section:
in the formula, pin、poutRespectively are the measured values of the inlet pressure and the outlet pressure of the experimental pipe section, and L is the length of the experimental pipe section.
Therefore, a numerical solving format of the fluid temperature in the experimental pipe section can be obtained:
in the formula, q is heat dissipation from the pipeline to the sandy soil, and the heat conduction model of the pipeline and the sandy soil needs to be solved.
Using a quadrilateralThe four-node isoparametric model (the mapping relation between the physical coordinates and the parameter coordinates of the unit is shown in figure 6) establishes the steady-state finite element solving format of the pipeline and sand heat conduction mathematical model (namely in the formula (2))
)。
KT＝P (8)
Wherein K is an overall temperature stiffness matrix; t is the sand and pipe temperature column vector to be solved; p is the total temperature load column vector; kmn、
Total and unit temperature stiffness matrix elements, respectively; pmIs an overall temperature load vector element;
a modifier for a cell temperature stiffness matrix resulting from the third type boundary;
a correction for a cell temperature loading column vector resulting from a third type of boundary condition; j is a Jacobian matrix mapped from the parameter space to the physical space; n is a radical ofm(xi, η) (m ═ i, j, k, l) is a node shape function when the parameter coordinate system is mapped to the physical coordinate system; alpha is a convection heat release coefficient of the fluid in the experimental tube and the tube wall, and can be obtained by calculation according to the flow rate and the temperature of the fluid; s is the length of the edge of the cell that lies at the boundary of the third class.
By solving equations (8) to (13) simultaneously with equation (7), the temperature of the fluid in the experimental pipe and the temperature field distribution of the external sand can be calculated, wherein the fluid is separated from the inlet by a distance of delta z.
3. Bottom layer seawater/seabed heat exchange boundary comparison representation
The bottom seawater/seabed heat exchange boundary is the upper boundary of finite element calculation. Using convection boundaries, respectively
Measuring the simulated seawater temperature TseaAnd surface soil temperature TsurfThe boundary is processed. When the simulated seawater temperature T is adoptedseaOr surface soil temperature TsurfIn the invention, the overall rigidity matrix in the multiplying large number processing formula (8) is adopted to enable the boundary node temperature to be equal to the given temperature TseaOr Tsurf. For example:
suppose node temperature TiIs a given value TseaFor the corresponding main diagonal element k in the coefficient matrixiiMultiplying by ω, right-hand correspondence using ω kiiTseaAnd (4) replacing.
The use of the multiply-large principle yields a non-exact solution. T isiWith a given value TseaThe degree of approximation of (c) is related to the value of ω, and may be 10 in general10～1014Of the order of magnitude.
Using convection boundaries
When the bottom seawater/seabed heat exchange boundary is represented, the overall rigidity matrix needs to be corrected. For nodes located on the bottom sea/seabed boundary:
wherein h is the convective heat transfer coefficient of the bottom layer seawater/seabed.
Adopting numerical simulation means, respectively applying the three modes to treat the bottom layer seawater/seabed convective heat transfer boundary, and substituting into Tf、Tsea、Tsurf、Pin、PoutV and the like, and the temperature T of the outlet of the experimental pipeline can be calculatedoutAnd sand temperature field distribution. And comparing the calculated result with the measured value in the experimental system, and analyzing the average relative error of the result to distinguish the advantages and disadvantages of the three processing modes.
Meanwhile, the h value is adjusted, so that the average relative error in the method is minimum, and the optimal h value can be obtained. Simulating the bottom flow velocity of the seawater, measured V by the flow velocity monitoring device 16sea. A series of h and V can be obtained by applying experimental values under different simulated seawater flow velocitiesseaNumerical values. The method is characterized in that the theory of the heat transfer of an incompressible fluid outer sweep plane is referred, and V is added on the basis of an empirical formulaseaFunction of (2)
Specific forms of h and VseaFitting the experimental values to further determine a final bottom layer seawater/seabed convective heat transfer coefficient expression formula and a corresponding flow velocity range.
(II) experimental study on heat dissipation of pipelines in different flow directions
The invention can realize the heat dissipation simulation experiment for simulating two flow directions, namely parallel and vertical flow directions of seawater and the buried pipeline. The function mainly depends on the switching control of the valve group to enter the simulated seawater flow direction of the sand box. FIG. 3 is a top view of the flask. The water inlet for simulating seawater is arranged in the axial direction and the vertical direction of the pipeline. Assuming that the experimental pipe section is laid from left to right in the sand box (shown by a solid line in fig. 4), when the valves a and c are closed and the valves b and d are opened, the flow direction of the simulated seawater is parallel to the experimental pipe section; when the valves b and d are closed and the valves a and c are opened, the flow direction of the simulated seawater is vertical to the experimental pipe section.
The heat dissipation experimental research under different flow directions is carried out, more sufficient experimental data can be collected, and data basis is provided for the fitting type (16).
(III) unsteady heat dissipation experiment of submarine pipeline
The invention can also be used for testing unsteady heat dissipation of the submarine pipeline. The specific implementation steps are as follows:
1. experimental part
On the basis of a steady-state experiment, the temperature of circulating hot water is changed, and the temperature change processes of the inlet and outlet of the experimental pipe section and different burial depths of the sand box are recorded until the monitoring value is stable again.
2. Numerical simulation
Due to the temperature variation, the fluid temperature and the partial derivatives of the pipe and soil temperature with respect to time in equation (2) are no longer zero, and the time step Δ t for the numerical solution needs to be divided. Thus, the discrete format of the fluid temperature within the tube is:
the finite element model also includes a node temperature versus time derivative column vector
The specific solving format is as follows:
wherein, the overall temperature-changing matrix C is newly added, and the calculation method is as follows:
in the formula, Cij、
Respectively, global and cell temperature change matrix elements.
And (5) solving the equations (20) and (17) simultaneously to obtain the time and space changes of the fluid in the experimental pipeline and the sand box soil temperature in the unsteady state change process. Compared with the measured data, the accuracy of the heat exchange convection boundary of the bottom layer seawater/seabed can be tested, and the unsteady state heat dissipation rule of the submarine pipeline can be researched.
It should be noted that the above description is only one embodiment of the present invention, and all equivalent changes of the system described in the present invention are included in the protection scope of the present invention. Persons skilled in the art to which this invention pertains may substitute similar alternatives for the specific embodiments described, all without departing from the scope of the invention as defined by the claims.
Claims (9)
1. An indoor simulation method for heat dissipation of a submarine pipeline is characterized in that the device comprises a sand box (8), an experimental pipe section (9), an experimental pipe fluid circulating system, a simulated seawater circulating system, a constant-temperature water circulating system and a data acquisition and control system;
a sealed cavity is arranged in the sand box (8), a water outlet is arranged on the sand box (8), and a water discharge valve (14) is arranged on the water outlet; a constant-temperature water circulation layer, a sand layer and a simulated seawater layer are sequentially arranged in the sand box (8) from bottom to top, a heat conduction partition plate (10) is arranged between the constant-temperature water circulation layer and the sand layer, and a heat insulation material (11) is arranged on the outer wall of the sand box (8);
the constant-temperature water circulation layer is connected with a constant-temperature water circulation system; the simulated seawater layer is connected with a simulated seawater circulating system, the simulated seawater circulating system comprises two sets of water inlet and return branches, one set of water inlet and return branches is parallel to the experimental pipe section (9), and the other set of water inlet and return branches is vertical to the experimental pipe section (9); the experimental pipe section (9) penetrates through a sand layer, and two ends of the experimental pipe section (9) are connected with an experimental pipe fluid circulating system;
the inlet and the outlet of the experiment pipe section (9) are respectively provided with a pressure monitoring device (25), and a second flowmeter (24) is arranged in the experiment pipe fluid circulating system; the bottom of the simulated seawater layer is provided with a plurality of flow velocity monitoring devices (16); a plurality of temperature monitoring devices (15) are arranged in the experiment pipe section (9) and at different sandy soil burying depths of the sandy soil layer; a first flowmeter (1) is arranged in the simulated seawater circulating system, and a first liquid level meter and a first thermometer are arranged in the simulated seawater layer; a second liquid level meter and a second thermometer are arranged in the constant-temperature water circulation layer;
the pressure monitoring device (25), the first flowmeter (1), the second flowmeter (24), the flow rate monitoring device (16), the temperature monitoring device (15), the first liquid level meter, the first thermometer, the second liquid level meter and the second thermometer are respectively connected with the data acquisition and control system;
the method comprises the following steps:
steady state experiment:
starting a constant-temperature water circulation system to keep the constant-temperature water circulation layer at a constant temperature; starting a simulated seawater circulating system, starting a drain valve (14), injecting simulated seawater into the sand box (8) until the simulated seawater flows out of the drain valve (14), and closing the drain valve (14); forming a flow direction parallel or vertical to the experimental pipe section (9) by the simulated seawater layers through two sets of water inlet and return branches in the simulated seawater circulating system, and monitoring flow data through the first flowmeter (1);
after monitoring that the flow speed data and the simulated seawater layer temperature data are stable through the flow speed monitoring device (16) and the temperature monitoring device (15), starting a fluid circulating system of the experiment pipe to enable fluid at a given temperature to flow in the experiment pipe section (9), and monitoring flow data through a second flowmeter (24);
monitoring temperature data of different sandy soil buried depths in an experiment pipe section (9) and a sandy soil layer through a temperature monitoring device (15), monitoring pressure data of an inlet and an outlet of the experiment pipe section (9) through a pressure monitoring device (25), changing the temperature of constant-temperature water in a constant-temperature water circulation layer after the temperature data and the pressure data are stable, simulating the temperature of seawater, the temperature of fluid in the experiment pipe section (9), the flow rate of the simulated seawater or the flow rate of the fluid in the experiment pipe section (9), and transmitting the monitored data to a data acquisition and control system through the pressure monitoring device (25), a first flow meter (1), a second flow meter (24), a flow rate monitoring device (16), the temperature monitoring device (15), a first liquid level meter, a first thermometer, a second liquid level meter and a second thermometer for carrying out numerical simulation; for the bottom layer seawater/seabed convective heat transfer boundary, adding a function related to the bottom layer flow velocity on the basis of an incompressible fluid external sweep plane heat transfer model, and fitting a bottom layer seawater/seabed convective heat transfer coefficient expression formula and an applicable flow velocity range thereof according to steady-state experiment results and numerical simulation results of different simulated seawater flow velocities;
unsteady state experiment:
on the basis of a steady state experiment, change the temperature of interior fluid of experiment pipe section (9), bury the temperature data of depths through different sandy soils of temperature monitoring device (15) monitoring experiment pipe section (9) interior and sandy soil layer, tend to stably again until temperature data, through pressure monitoring device (25), first flowmeter (1), second flowmeter (24), velocity of flow monitoring device (16), temperature monitoring device (15), first level gauge, first thermometer, the data transmission that second level gauge and second thermometer obtained will monitor carries out numerical simulation to data acquisition and control system.
2. The indoor simulation method for heat dissipation of the submarine pipeline according to claim 1, wherein the simulated seawater circulation system comprises a simulated seawater storage device (4), a first circulating pump (3) and a first valve (2) which are connected through a circulating pipeline (6), and the simulated seawater storage device (4) is connected with a first temperature control device (5); the first circulating pump (3), the first valve (2) and the first temperature control device (5) are respectively connected with a data acquisition and control system.
3. The indoor simulation method for heat dissipation of the submarine pipeline according to claim 1, wherein the water inlet and return branch parallel to the experimental pipe section (9) comprises a parallel inlet pipeline (6-1) and a parallel outlet pipeline (6-2) communicated with the inside of the simulated seawater layer, and the parallel inlet pipeline (6-1) and the parallel outlet pipeline (6-2) are arranged in parallel to the experimental pipe section (9); the water inlet and return branch vertical to the experimental pipe section (9) comprises a vertical inlet pipeline (6-3) and a vertical outlet pipeline (6-4) communicated with the inside of the simulated seawater layer, and the vertical inlet pipeline (6-3) and the vertical outlet pipeline (6-4) are arranged vertical to the experimental pipe section (9); valves are arranged on the parallel inlet pipeline (6-1), the parallel outlet pipeline (6-2), the vertical inlet pipeline (6-3) and the vertical outlet pipeline (6-4).
4. The indoor simulation method for heat dissipation of a submarine pipeline according to claim 3, where the drainage openings, the parallel outlet pipes (6-2) and the vertical outlet pipes (6-4) are provided with a screen having a pore size smaller than the particle size of the sand in the sand layer.
5. The indoor simulation method for heat dissipation of a submarine pipeline according to claim 1, wherein the laboratory pipe fluid circulation system comprises a fluid storage device (21), a third circulation pump (22) and a second valve (23) which are connected through a pipeline, and the fluid storage device (21) is connected with a third temperature control device (20); the third circulating pump (22), the second valve (23) and the third temperature control device (20) are respectively connected with the data acquisition and control system.
6. The indoor simulation method for heat dissipation of submarine pipelines according to claim 1, wherein the constant-temperature water circulation system comprises a constant-temperature water storage device (18) and a second circulating pump (19) connected by a pipeline, the constant-temperature water storage device (18) is connected with a second temperature control device (17), and the second circulating pump (19) and the second temperature control device (17) are respectively connected with the data acquisition and control system.
7. The method for indoor simulation of submarine pipeline heat dissipation according to claim 1, wherein a support frame (12) is connected between the heat-conducting partition (10) and the inner wall of the sand box (8).
8. The indoor simulation method of submarine pipeline heat dissipation according to claim 1, wherein a cover net is placed between the constant-temperature water circulation layer and the sand layer.
9. The indoor simulation method for heat dissipation of a submarine pipeline according to claim 1, wherein the outer wall of the sand box (8) is provided with an insulating layer.
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