CN107590336B - Numerical simulation method for influence of gas pipeline leakage on internal flow field - Google Patents
Numerical simulation method for influence of gas pipeline leakage on internal flow field Download PDFInfo
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Abstract
The invention discloses a numerical simulation method for influence of gas pipeline leakage on an internal flow field, which relates to the technical field of gas; the simulation method comprises the following steps: establishing simulation software; modeling and simulation analysis of gas pipeline leakage; analyzing the result of the leakage numerical value of the gas pipeline; the invention analyzes the change of the flow field in the pipeline before and after the leakage of the gas pipeline; calculating fluid mechanics knowledge according to CFD, simulating gas pipeline leakage by Fluent, and analyzing and obtaining the characteristics of the internal flow field of the gas pipeline before and after the gas pipeline leakage according to the fluid mechanics theory; analyzing and obtaining the characteristics of the internal flow field of the gas pipeline before and after leakage of the gas pipeline according to the fluid mechanics theory; calculating fluid mechanics knowledge according to CFD (computational fluid dynamics), simulating gas pipeline leakage under different leakage conditions, wherein the larger the inlet pressure is, the more easily a leakage accident occurs; the larger the leak hole diameter is, the larger the proportion of the high concentration region around the leak hole is.
Description
The technical field is as follows:
the invention relates to a numerical simulation method for influence of gas pipeline leakage on an internal flow field, and belongs to the technical field of gas.
Background art:
at present, gas pipeline transportation is the most convenient and economic transportation mode. However, when the gas pipeline leaks due to soil corrosion, pipeline aging or other damage, if the position of the leakage point cannot be detected in time, the emergency time will be affected, so that it is very important to accurately locate the leakage point, prevent leakage, etc. by exploring the leakage diffusion rule of the gas pipeline. And after a leakage accident occurs, if parameters of a leakage source can be determined in the shortest time, so that an emergency evacuation area is further divided, a basis can be provided for a decision maker to start emergency response, and property loss and casualties are reduced.
Nowadays, with the rapid development of economy in China, industries such as petrochemical industry and the like are increasingly strong, the scale of production and manufacturing of dangerous chemicals is increasingly large, and the transportation and the use of gas pipelines are also increasingly large, so that a great number of gas leakage accidents occur. The leakage of the gas pipeline is fearful because once the leakage causes great harm, a large area of personnel and property are threatened. Accidents caused by gas pipeline leakage all over the country in 2016 only can reach more than 70, and among many accidents, the specific information of an accident center cannot be accurately determined due to untimely rescue, so that the delay and the carelessness of rescue are caused. Therefore, research and analysis of gas pipeline leakage accidents are necessary.
At present, China is in a starting stage in research subjects related to dangerous article leakage accidents, related fields are in a theoretical research stage, and no theoretical basis is determined for property withdrawal and transfer distances of personnel once dangerous article leakage accidents occur. At present, the statistical data in the ERG2000 which is currently adopted in China and is jointly compiled in the United states and Canada has no specific transfer distance, so that the transfer of personnel and property is always larger in error according to the experience of field disposal personnel after an accident occurs. Not only is the leakage difficult to find, but also people can be evacuated only by feeling when a plurality of gas pipeline accidents happen, so that great loss is caused to people, a large amount of manpower and material resources are consumed, and even the life cost is paid.
In order to safely operate a gas pipeline, abundant gas pipeline science theories and advanced technical methods are needed. The safety protection of the gas pipeline in China is mainly realized in a patrol mode, and the leakage position is positioned in a ground detector detection or punching mode after leakage occurs, so that the time consumption is long, and accurate positioning cannot be well realized. Therefore, in order to reduce the loss of accidents and reduce the time for inspecting the leakage points, it is necessary to investigate the leakage process of the gas line so as to prevent the pipeline leakage accidents.
The current research situation analysis at home and abroad:
after the dangerous source leaks, the parameter information of the dangerous source is determined quickly and accurately, and the parameter of the leaking source is determined in the shortest time under some sudden conditions or limited information conditions, so that the emergency evacuation area is further divided, and a basis is provided for a decision maker to start emergency response, so that emergency response decision can be supported by researching the source parameter of the dangerous source leaking suddenly, and property loss and casualties are reduced. Therefore, the method can cover the scientific problems contained in the source intensity inverse calculation of the leakage accident, and meanwhile, a scheme is provided for solving the leakage under the condition of really realizing the sudden leakage accident through the problem research of the key problems under the leakage accident.
In recent years, the leakage accidents of typical urban gas pipelines at home and abroad are striking with one digit, casualties are serious, and resources are greatly wasted. In China, in 1 month in 2017, in a period of less than one month, 42 gas accidents happen only in China, so that 9 people are injured and 60 people die, personal injury and property loss are caused to residents, and economic loss and resource waste are caused to the society. The reasons for pipeline accidents and protective measures should be taken seriously and carefully to avoid the disasters.
In the experimental research method for pipeline leakage, foreign scholars have developed system research as early as the end of the 20 th century, and the main research on the gas leakage process is the calculation of the gas leakage rate. The research development of the leakage model is early, and Levenspiel provides a calculation model of leakage amount when the storage tank generates pipeline full-section fracture and pore leakage in engineering fluid flow and heat transfer; the Nigeria scholars Olorunmaiye and English scholars Catlin respectively calculate the gas leakage rate when the high-pressure gas transmission pipeline is completely broken by using a characteristic value method and a fluid mechanics method.
In recent years, relevant researchers in China also make relevant experimental researches to measure the leakage strength of small holes and large holes of gas on a test bed and measure the leakage strength under different hole diameters. The method is characterized in that a data acquisition system and a Gas Clam underground Gas online monitor are used for carrying out experimental exploration on small hole leakage caused by corrosion and the like of a middle-low pressure Gas pipeline in a city, a full-scale experimental system is used for analyzing the distribution and diffusion rule of natural Gas in soil, the Gas leakage concentration is shown to be changed along with time in an S-shaped curve, and good data support is provided for future experiments and simulation researches. However, these experimental studies are also similar experiments simulated in laboratories, and are not gas leakage simulation experiments in the true sense, and because gas (gas mainly refers to natural gas, and the main component is methane) has the characteristics of flammability and explosiveness, real experiments have great danger. Instead, most studies have been conducted based on numerical simulations.
The problem of gas line leakage is not well studied in the world at present. Previous studies have typically performed simulation analysis using only hypothetical conditions to create a mathematical model or by adding only simple influencing factors. The domestic analysis of numerical simulation and diffusion characteristics after natural gas pipeline leakage is yet to be deeply improved.
The invention content is as follows:
aiming at the problems, the invention aims to provide a numerical simulation method for the influence of the leakage of a gas pipeline on an internal flow field.
The invention relates to a numerical simulation method for influence of gas pipeline leakage on an internal flow field, which comprises the following steps:
the method comprises the following steps: establishing simulation software:
after modeling is completed, grid division is carried out; the simulation by FLUENT software can be divided into the following 3 steps: (1) and a pretreatment stage, which is completed by Gambit; (2) the solving stage is completed by fluent; (3) and a post-treatment stage, which is completed by using a tecplot;
the method is characterized in that a preprocessing stage is completed by adopting preprocessor Gambit software, wherein an unstructured grid generating program is used for generating grids for a relatively complex geometric structure, the grids can be generated under a two-dimensional geometric body and a three-dimensional geometric body, but the number of the manually generated grids is smaller than that of the grids generated in the software, and the consumption of an internal memory is also smaller; the processing part of the analog calculation, namely the solving stage, adopts Fluent software, and the operation comprises the steps of selecting a solving equation, setting fluid materials and physical properties, setting boundary parameters and solving control parameters, solving a discrete equation and visualizing a result; for the post-processing stage, performing post-processing on the grids without local encryption by adopting a technicot, and performing processing on the grids with local encryption by adopting a self-contained post-processing function in Fluent;
step two: modeling and simulation analysis of gas pipeline leakage:
2.1, FLUENT numerical simulation calculation and operation:
adopting Fluent to carry out analog simulation, firstly establishing a physical model in Gambit, then dividing grids, setting boundary conditions for selection, and outputting a Mesh file to carry out the next step; secondly, selecting a solving equation and a required model in Fluent, further setting boundary conditions and control parameters, starting calculation, and performing the next operation on the obtained result; thirdly, importing the file saved after the Fluent solution is finished into the Tecplot for post-processing;
2.1.1 FLUENT solver and settings:
fluent provides two types of solvers, one is a pressure-based solver, and the other is a density-based solver; a pressure-based solver is adopted;
2.1.2 FLUENT runtime Environment settings:
opening the setting of the running environment of Fluent software, and seeing two options of calculating reference pressure and gravity which need to be set; in Fluent, the pressure is a relative pressure value with respect to an operating reference pressure, that is, a relative pressure; the reference pressure is atmospheric environmental pressure and is set as standard atmospheric pressure 101325Pa, and a default point (0, 0, 0) is selected at the position of the reference pressure for research;
the above setting is realized in Fluent, then transient simulation is selected in General, and the change of gravity in the leakage process has a large influence on the leakage process, so that the influence of gravity needs to be considered; as the gas leakage speed is high, the Gravity is started and the Gravity acceleration is set to be-9.8 m/s of the Y-axis direction2；
2.1.3 FLUENT calculation model selection:
when the operating environment is set, after the solver format is selected, selecting a calculation model, wherein a turbulence model is adopted, then starting an energy equation, a k-epsilon equation and a component transport equation in Models, selecting a component transmission model, and defining components as methane and air; the Fluent is directly selected because the Fluent is attached with a material database; of course, the new material can be customized or the parameters and properties of the existing material can be modified according to the requirements;
2.1.4 FLUENT initial condition and boundary condition settings:
initial conditions for the simulation calculation were set as: before the gas pipeline is in a non-leakage state, the concentration and the speed are zero, the flow field is filled with air and is kept in a stable state, and after the gas pipeline is in the non-leakage state, the pressure inlet is 0.4Mpa, the inner diameter is neglected, and the gas flows in at the speed of 1 m/s;
2.1.5, FLUENT solving parameter setting:
adopting Fluent to carry out analog simulation, and carrying out establishment of a physical model, division of grids and selection of boundary conditions in Gambit in the first step; secondly, selecting a solving equation and a required model from Fluent, further setting boundary conditions and control parameters, and starting calculation; finally, the file saved after the Fluent is solved is imported into the Tecplot for post-processing, the image is directly displayed or played and watched frame by frame, and the specific data contained in the file saved by the Fluent can also be called in the file; initializing initial conditions and boundary conditions after the setting is finished, and then setting step length for analog calculation;
2.2, building a straight pipe and T pipe model and outputting pictures:
2 modeling models are adopted, wherein one model is a concept modeling; secondly, carrying out parametric modeling, and actually operating a modeling process by using the two methods through a straight pipe, a T-shaped pipe and a complex pipe; when a numerical method is adopted to solve the control equation, the control equation is dispersed in a space region by a desired method, and then a discrete equation set is obtained by solution; to discretize the governing equation in the spatial domain, a grid must be used;
2.2.1, building an internal model of the straight pipe pipeline and outputting pictures:
the model graph before the straight pipe is leaked is established firstly, because the model after the leakage and the model before the leakage have the same basic parameters except that one leakage point parameter is added, for the straight pipe and the T pipe under the simple conditions, only the model before the leakage is established, and in the complex pipe, the model after the leakage is specifically established so as to output the picture for the data analysis of the next step.
Firstly establishing a straight pipe pipeline model for a straight pipe pipeline; the straight pipe model is built by a DM module of ANSYS, and a straight pipe two-dimensional model graph is drawn according to the built straight pipe pipeline model size by utilizing Gambit software; then, carrying out grid division in an attempt of a workbench;
solving the setting in fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation; as can be seen from the pressure diagram, the pressure at the inlet of the straight pipe is the maximum, the pressure in the straight pipe is gradually reduced along with the flow, and the pressure reaches the minimum when reaching the outlet; dividing grids by comparing with experimental results and the requirement of Gambit on grid quality; after the grid division is finished, a boundary condition type and a calculation area type are specified, a specific boundary condition is set in the Fluent, and the Mesh file can be output and stored after the work is finished;
2.2.2, building an internal model of the T pipe pipeline and outputting pictures:
solving the setting in Fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation, wherein the pressure cloud picture can show that the pressure is obviously changed at the T-shaped connection part of the T-shaped row pipe and is obviously increased compared with the pressure at the stable flowing part;
2.3, establishing a complex pipeline model before and after leakage and outputting pictures:
2.3.1, establishing a model before leakage in the complex pipeline:
the complex pipe model is built by a DM module of an ANSYS, and a straight pipe two-dimensional model graph, specifically a complex pipeline two-dimensional model graph before leakage, is drawn by using the size of Gambit software; in an attempt of workbench, grid division is performed;
3.3.2, internal simulation after leakage of the complex pipeline: establishing a two-dimensional model after leakage of a specific complex pipeline; then establishing a specific complex pipeline post-leakage two-dimensional model diagram; in the workbench's attempt, meshing is performed.
Analyzing the numerical value result of the leakage of the gas pipeline:
3.1, analyzing the CFD numerical simulation result:
on the basis of the second step, analyzing the influence of the leakage factor according to data obtained by the fluent software simulation calculation, and obtaining a result;
3.2, analyzing numerical simulation results:
(1) and internal simulation results after the straight pipeline leaks:
applying ANSYS14.0 to make a scale-down residual error map; performing later-stage processing to obtain a time-dependent change diagram of the concentration of methane in the straight pipe; the straight pipe leakage graph clearly shows that the concentration of methane changes obviously at the leakage point along with the change of time, and the leakage point is gradually stabilized at 125 seconds;
(2) and internal simulation results after T pipeline leakage:
the T-shaped row pipe model is built by a DM module of an ANSYS, in a workbench attempt, grid division is carried out, and an ANSYS14.0 is used for making a proportional reduction residual error map; performing post-processing to obtain a time-dependent change chart of the methane concentration in the pipeline of the T-shaped row of pipes; the leakage graph of the T-shaped tube shows that the concentration of methane changes obviously at the leakage point along with the change of time, and the leakage point is gradually stabilized at 125 seconds;
(3) and internal simulation results after the complex pipeline leaks:
since the model map has already been established, it is analyzed directly on the basis of the second step; solving the setting in fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation;
comprehensive analysis, the internal flow field analysis of the pipeline before the leakage of the T-shaped pipe and after the leakage of the complex pipe obtains a basic conclusion: before leakage, the pressure vector and the velocity vector in the pipeline are maximum at an inlet, gradually become smaller along with the flow of fuel gas, and reach minimum at an outlet; after the leakage occurs, the internal pressure vector and the velocity vector of the leakage device are greatly changed, when the leakage device is at a leakage point, the pressure and the velocity can be obviously changed, and the change of the methane concentration at the leakage point is gradually stable along with the lapse of time.
3.3, pipeline leakage influence factors:
3.3.1 Effect of changes in line pressure on leakage:
in practical engineering, the larger the pressure of the pipeline is, the more easily leakage accidents occur, and the temperature and the speed in the pipeline are increased correspondingly.
3.3.2 Effect of leak Aperture variation on leakage:
when the pipeline pressure is constant and the aperture of the leakage hole is small, the mass flow of the gas leaked from the leakage hole to the soil environment in unit time is reduced, so that the diffusion range in the soil is small in the same time, and the dangerous area of leakage is small. On the contrary, the larger the leakage aperture is, the larger the proportion of the high concentration region around the leakage aperture is.
Preferably, the turbulence model is a standard k-epsilon turbulence model.
Compared with the prior art, the invention has the beneficial effects that: analyzing the change of the flow field inside the pipeline before and after the leakage of the gas pipeline; performing a geometric modeling process of the pipeline on the problem of leakage of the gas pipeline, performing grid division, performing post-processing by using Tecplot after the Fluent is used for solving, analyzing the internal flow field of the gas pipeline by using the obtained picture, and performing detailed analysis on the influence factors of the leakage; the analysis is comprehensive, and can carry out later maintenance.
Description of the drawings:
for ease of illustration, the invention is described in detail by the following detailed description and the accompanying drawings.
FIG. 1 is a schematic diagram of a straight pipe model according to the present invention;
FIG. 2 is a two-dimensional model of a straight pipe according to the present invention;
FIG. 3 is a straight tube mesh division diagram in accordance with the present invention;
FIG. 4 is a diagram of a T-row pipe model according to the present invention;
FIG. 5 is a diagram of a model before leakage of a complex pipe according to the present invention;
FIG. 6 is a two-dimensional model of a complex pipe of the present invention before leakage;
FIG. 7 is a grid-partitioned diagram of a complex pipeline before leakage in accordance with the present invention;
FIG. 8 is a diagram of a complex pipe model after leakage in accordance with the present invention;
FIG. 9 is a diagram of a two-dimensional model of a complex pipe of the present invention after a leak;
FIG. 10 is a grid-partitioned plot of a complex pipe of the present invention after a leak;
FIG. 11 is a simulation of a straight tube of the present invention after leaking;
FIG. 12 is a simulation diagram of leakage of T-row tube in the present invention.
The specific implementation mode is as follows:
in order that the objects, aspects and advantages of the invention will become more apparent, the invention will be described by way of example only, and in connection with the accompanying drawings. It is to be understood that such description is merely illustrative and not intended to limit the scope of the present invention. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present invention.
As shown in fig. 1 to 12, the following technical solutions are adopted in the present embodiment:
firstly, basic theory of fluid mechanics and simulation software:
1.1, basic theory of hydrodynamics:
in the study of fluid mechanics problems, there are three most basic conservation laws, including conservation of mass, momentum and energy. The invention applies several basic fluid mechanics equations, and fluid flow applies turbulent flow state, and at the same time uses turbulent flow transport equation to make calculation.
The inside of the gas pipeline is all combustible gas, and due to the compressibility of the gas, the flow of the gas in the pipeline is unstable. After a given inlet pressure, as the pressure of the gas decreases as it flows in the gas pipeline, the density of the gas inside the pipeline also gradually decreases, and its speed gradually decreases, and the change in the gas density is negligible only in the low-pressure pipeline. Thus, to find the 4 parameters affecting the leakage flow of the natural gas pipeline, which are a function of both the coordinates (z.y.x) and the time T, the pressure p, the density ρ, the flow velocity u, and the temperature T, respectively, involve several basic fluid mechanics equations described below.
1.1.1, continuity equation:
since any flow problem must satisfy the law of mass conservation motion, the law can be expressed as the increase in mass in a fluid infinitesimal per unit time is equal to the net mass flowing into the infinitesimal in the same time interval. According to the certain rule, a mass conservation equation, which is often called a continuity equation, can be obtained.
The continuity equation is a concrete representation of the law of conservation of mass in fluid mechanics, and any flow problem must satisfy the law of conservation of mass, i.e. it represents that the increased fluid mass and the decreased fluid mass are the same in a series of ways:
where rho- -density of gas, kg/m3；
t- - -time, s;
ui-velocities (u, v, w) in three directions (x, y, z), m/s.
The continuity equation for a particular flow is expressed below.
Flow of incompressible gas:
steady gas flow:
1.2.1, momentum equation:
the momentum equations (navier-stokes equations), also known as N-S equations, are used to study any form of motion. Since the flow of gas is in accordance with newton's second law, in accordance with the equation of motion, the momentum equation can be expressed as: the rate of change of the momentum of the fluid in the body with respect to time is equal to the sum of the various forces acting on the body from the outside. This equation derives the conservation of momentum equation in the i direction based on newton's second law.
Where μ — the dynamic viscosity of the fluid, pas;
p-absolute pressure, Pa;
g-acceleration of gravity, m/s2。
For a natural gas pipeline then can be written:
wherein D- -inner diameter of the pipe, m;
p- - -pressure of flowing gas in the gas pipe, Pa;
f- - -mass force, N;
g- -acceleration of gravity, m2/s；
Theta-angle of inclination, rad, between the pipe and the horizontal;
λ - - - -coefficient of on-way resistance.
1.1.3 energy equation:
since the flow of gas in the pipeline is inherently accompanied by the flow of energy, an energy equation is involved. The energy equation law is the fundamental law that a flow system containing heat exchange must satisfy. The law of energy equations can be expressed as: the increasing rate of the energy in the infinitesimal body is equal to the net heat flow entering the infinitesimal body plus the work done by physical force and surface force on the infinitesimal body, and then according to the first law of thermodynamics, i.e. the law of conservation of energy, the energy equation of the gas flow can be obtained:
wherein H- - -heat given off per unit mass of gas, J/kg;
e- -internal energy of gas, J/kg;
z- - -pipe position height, m;
h- -enthalpy of gas, J/kg.
1.1.4, ideal gas equation of state:
when the gas in question is an ideal gas, the volume of the gas molecules is ignored, and they are considered as particles; not counting molecular potential energy, the collision between molecules and the wall is completely elastic, no kinetic energy loss is caused, and the gas state equation is as follows:
p＝ρRT (1-7)
in fact, when the gas is in a standard environment condition, the gas state equation is as follows:
p＝ZρRT (1-8)
wherein Z- -the compressibility factor,
p- -absolute pressure, Pa;
v- -specific volume of gas, m3·kg-1；
R- -general gas constant, J.kg-3·K-1；
T- -thermodynamic temperature, K.
When the gas pressure meets P.PC-1<<2 or T.TC-1>2 and p.pc at the same time-1When < l, the gas is determined to be an ideal gas. The critical pressure Pc of the gas (methane) is 4.59MPa, the critical humidity TC is-82.6 ℃, and the simulated gas pipeline in the experiment is a gas (methane) conveying pipeline in the city, the conveying pressure of the gas (methane) conveying pipeline is generally between 0.2 and 0.4MPa and is far less than the critical pressure, so the gas (methane) conveying pipeline can be regarded as ideal gas.
(1) Ideal non-compressible gas density equation:
in incompressible gas, the density of the gas depends only on the operating pressure, whereas in Fluent settings the operating pressure is typically set to standard atmospheric pressure, so that in the incompressible model the density of the gas is a density value at standard atmospheric pressure.
(2) Ideal compressible gas density equation:
whether the flowing gas has compressibility can be measured in terms of Mach number M, as shown by the equation:
M≡u/c (1-10)
wherein U- -gas flow velocity, m/s-1；
C- -sonic velocity m/s-1。
When the mach number is much less than 1(M < 0.1), the compressibility of the gas can be neglected, while the density of the gas can also be considered unchanged with pressure, but when M is close to 1, the effect of compressibility on the density of the gas needs to be considered.
For a compressible gas, the density solution is shown by the formula:
wherein: p-gauge pressure per infinitesimal, Pa.
It can be seen that the effect of gauge pressure on each of the microelements on density is added to the method of compressible gas density calculation so that the density varies with the gauge pressure on the microelements.
1.2, turbulence equation:
for a turbulence equation, a standard k-epsilon double-stroke model is selected for solving, and the double-stroke model is helpful for solving the problems of end flow fluid distance change and end flow time accumulation in the transmission process and is suitable for a gas port flowing in a pipeline. The standard k-epsilon equation is formed by mutually borrowing and deriving experimental phenomena and empirical formulas, is a semi-empirical equation, and is widely applied to the field of fluid flow due to accurate calculation and wide applicable turbulence range.
Compared with other bi-equation models, the standard k-epsilon bi-symmetric model in the time average model saves resources and ensures the calculation accuracy to the maximum extent, so that the model is a preferred model for simple turbulent motion in the simulation calculation. In the k-epsilon model, k is turbulence energy, epsilon is dissipation ratio of the turbulence energy, the k and epsilon respectively reflect characteristic speed and characteristic length scale, the turbulence viscosity coefficient is determined mainly by solving two additional equations, Boussinesq is used for supposition simplification, and the contact turbulence stress is solved:
k equation:
the equation of ε:
wherein C isε＝0.09，Cε1＝1.44，Cε2＝1.92，CDThe turbulent prandtl number for the turbulence energy k and the turbulence energy dissipation rate epsilon is 0.8: sigmak＝1.0，σε1.3, and upsilont＝CμK2/ε。
K- -turbulent kinetic energy, J;
ε - -turbulent dissipation ratio.
1.3, computational fluid dynamics:
computational fluid dynamics (cfd) is a product of the combination of modern hydrodynamics and computer science, and the most common research methods in hydrodynamics, namely computational hydrodynamics, theoretical hydrodynamics and experimental hydrodynamics, are the main basis and theoretical guidance for the research of hydrodynamics. Theoretical analysis can provide needed basis and data for experimental research, experiments can provide needed data and calculation results for numerical research, numerical simulation is one of quantitative research experiments, and the characteristics determine the advantages of computational fluid dynamics in research work.
CFD electronic computer is used as tool, and various discretized mathematical methods are applied to carry out numerical experiment, computer simulation and research analysis on hydromechanics problems so as to solve various practical problems.
The basic principle of CFD is to solve a nonlinear differential equation set by a numerical method, and the equation set is obtained by simultaneous mass, momentum, energy and self-defined scalar equations, thereby showing the flow field form and realizing the improvement and amplification of process devices.
1.4, establishing simulation software:
fluent is one of the more common CFD software packages in the world at the forefront today and is also one of the most common software in fluid modeling today.
When the modeling is completed, meshing is performed. The grid division of Fluent has many advantages, it can provide flexible grid characteristics, can support multiple grids, and makes the user freely choose to use unstructured and structured grids to divide very complicated geometric problems, and can optimize the grids according to the obtained calculation results in the solving process by using the grid adaptive characteristics provided by Fluent.
The Fluent software is easy for beginners to use, can allow users to define various boundary conditions, such as flow inlet and outlet boundary conditions, wall boundary conditions and the like, can adopt component inputs of various local cartesian and cylindrical coordinate systems, and all the boundary conditions can change along with space and time, including axial symmetry, periodic change and the like. It can be used in energy and space industry. The method has various mathematical physical models, analysis methods and discrete modes, thereby meeting the requirements of calculation precision, reliability, stability and the like in the research field. The Fluent has a wide application range, is suitable for a plurality of fields such as fluid flow, chemical reaction and the like, for example, the flow of compressible and incompressible fluid is solved by simulation; or steady state and transient flow.
The simulation by FLUENT software can be divided into the following 3 steps:
1. a pretreatment stage, which is completed by Gambit;
2. a solving stage, which is completed by fluent;
3. the post-treatment phase is completed with a tecplot.
The method is implemented by adopting a preprocessor Gambit software in a preprocessing stage, wherein an unstructured grid generating program contained in the preprocessor can effectively generate grids for a relatively complex geometric structure, the grids can be generated under a two-dimensional geometric body and a three-dimensional geometric body, but the number of the manually generated grids is smaller than that of the grids generated in the software, and the consumption of the memory is also smaller. And the processing part of the analog calculation, namely the solving stage, adopts Fluent software, and the operation comprises the steps of selecting a solving equation, setting fluid materials and physical properties, setting boundary parameters and solving control parameters, solving a discrete equation, visualizing a result and the like. For the post-processing stage, the grids without local encryption can be post-processed by adopting the tecplot, and the grids with local encryption can be processed by adopting the self-contained post-processing function in Fluent.
TABLE 1-1 CFD software simulation
II, secondly: modeling and simulation analysis of gas pipeline leakage:
2.1, FLUENT numerical simulation calculation and operation:
and (3) performing analog simulation by using Fluent, establishing a physical model in Gambit in the first step, dividing grids, setting boundary conditions for selection, and outputting a Mesh file to perform the next step. And in the second step, a solution equation and a required model are selected from Fluent, boundary conditions and control parameters are further set, and calculation is started, so that the simulation result is almost decisive, and the obtained result can be subjected to the next operation. Thirdly, importing the file saved after the Fluent solution is finished into the Tecplot for post-processing;
2.1.1 FLUENT solver and settings:
fluent provides two types of solvers, one being a pressure-based solver and the other being a density-based solver. The two solvers have the same solving object, and the solved control equations used by the solvers are the equations in the step one, and are continuity equations, momentum equations, energy equations and turbulence equations describing conservation of mass, conservation of momentum and conservation of energy. Although both solvers work well for most flow solutions, for certain flow situations, selecting one of the two solvers may be more accurate in the solution results; a pressure-based solver is used.
2.1.2 FLUENT runtime Environment settings:
in the setting of the running environment of the Fluent software, two options of setting the calculation reference pressure and the gravity can be seen. In Fluent, the pressure is a relative pressure value with respect to an operation reference pressure, that is, a relative pressure. The reference pressure was atmospheric ambient pressure, set to a standard atmospheric pressure of 101325Pa, and the reference pressure position was investigated with a default point (0, 0, 0) selected.
The above settings are implemented in Fluent, and then Transient simulation (Transient) is selected in General, and the change of gravity during the leakage process has a large influence on the leakage process, so that the influence of gravity (without considering buoyancy) needs to be considered. As the gas leakage speed is high, the Gravity is started and the Gravity acceleration is set to be-9.8 m/s of the Y-axis direction2。
2.1.3 FLUENT calculation model selection:
when the operating environment is set, and after the solver format is selected, a calculation model is selected, and the problems to be considered are as follows: the calculation model considers whether heat transfer is considered (the invention does not consider heat transfer), whether the fluid flow state is laminar or turbulent (turbulent flow is used), whether it is multiphase flow (no), whether there is a phase change (no), whether there is a chemical composition change and a chemical reaction (here it does not consider that the leak is chemically changed), whether radiation (no) is considered, etc. As the invention adopts a turbulence model, the standard k-epsilon turbulence model which is most widely applied is selected, a standard wall function can be adopted, the other indexes can be default indexes, then an Energy equation (Energy), a k-epsilon equation and a component Transport equation (specials Transport) are started in Models, a component Transport model (specials Transport) is selected, and components are defined as methane and air (methane-air). Since the Fluent is accompanied by a material database, the Fluent can be directly selected. Of course, the new material can be customized or the parameters and properties of the existing material can be modified according to the requirements;
2.1.4 FLUENT initial condition and boundary condition settings:
the initial condition is the flow condition of the flow problem at each point on the flow field at the initial moment. I.e. the state of each point in the flow field in the internal model of the gas pipeline at the starting moment. The initial conditions of the simulation calculation of the invention are set as follows: before the gas pipeline is in the non-leakage state, the concentration and the speed are zero, the flow field is filled with air and is kept in a stable state, and after the gas pipeline is in the non-leakage state, the pressure inlet is 0.4Mpa, the inner diameter is neglected, and the gas flows in at the speed of 1 m/s.
Because the pressure exists in the natural gas pipeline, the outside is normal atmospheric pressure, the pressure difference exists between the natural gas pipeline and the natural gas pipeline, the boundary condition of the pressure outlet is selected for the boundary above and the boundary right, and the pressure value is standard atmospheric pressure.
The boundary conditions are divided into a plurality of types and vary according to the problem. And the flow state of the fluid in the flow field at any time can meet the set boundary condition.
The wall surface boundary condition wall is selected for the pipe wall. If no setting is specified, the fluent automatically defaults all interfaces except the inlet and the outlet to be wall, the invention adopts the default value, and the specific setting of each boundary condition is shown in the following table 2-1.
TABLE 2-1 boundary conditions settings
2.1.5, FLUENT solving parameter setting:
the method adopts Fluent to carry out analog simulation, and carries out establishment of a physical model, division of grids and selection of boundary conditions in Gambit in the first step; secondly, selecting a solving equation and a required model from Fluent, further setting boundary conditions and control parameters, and starting calculation; and finally, importing the file saved after the Fluent is solved into the Tecplot for post-processing, directly displaying the image or playing and watching the image frame by frame, wherein the specific data contained in the file saved by the Fluent can also be called. The pipe diameter D is 25mm (the pipe wall thickness is not considered), the leakage hole is an ideal circle, the diameter D of the hole opening is 10mm, the central pressure of the pipeline at the leakage position is 0.4MPa, and the environment and gas temperature is 300K. And calculating by using a small hole leakage model, and determining by using a transient simulation mode. After the setting is finished, the initial condition and the boundary condition can be initialized, then the step length is set for simulation calculation, and the simulation of the straight pipe, the T-shaped pipe and the complex pipe is subjected to 500-step convergence.
2.2, building a straight pipe and T pipe model and outputting pictures:
two modeling models are used, one is Conceptual Modeling (CM), which is mainly used to describe the conceptual structure of a unit. Using the conceptual data model, database designers can devote significant effort to understanding and describing the real world at the beginning of the design phase, while deferring some technical issues related to DBMS to the design phase. The CM concept modeling function adopted by the invention is effective supplement and improvement of two-dimensional sketch and three-dimensional geometric modeling; the method is effective supplement and improvement of two-dimensional sketch and three-dimensional geometric modeling; the second is Parametric Modeling (PM), which is a model that is built and analyzed numerically but not by parameters (variables), and a new model can be built and analyzed by simply changing the parameter values in the model. The parametric modeling parameters can be not only geometric parameters but also attribute parameters such as temperature and materials. In a parameterized geometric modeling system, the range of action of the design parameters is the geometric model. The adjustment of the model size is realized by parameter modification and functional relation setting, so that a large amount of modeling working time is saved. The invention uses the two methods and actually operates the modeling process through a straight pipe, a T-shaped pipe and a complex pipe. When the numerical method is adopted to solve the control equation, the control equation is dispersed in a space region by a desired method, and then the obtained discrete equation set is solved. To discretize the governing equation in the spatial domain, a grid must be used. Various methods of discretizing the regions to generate a mesh have been developed, collectively referred to as mesh generation techniques. When different numerical solutions are adopted for different problems, the required grid forms are different, but the grid generation method is basically consistent. Currently, grids fall into two broad categories, structured grids and unstructured grids. Briefly, the structural grid is relatively regular in space, e.g. for a quadrilateral area, the grid is often arranged in rows and columns, and the row and column lines are relatively distinct. Without the row and column lines being apparent in spatial distribution for the unstructured grid.
2.2.1, building an internal model of the straight pipe pipeline and outputting pictures:
firstly, establishing a model diagram before straight pipe leakage, wherein the model after leakage and the model before leakage have the same basic parameters except that one leakage point parameter is added, and for straight pipes and T pipes, the model before leakage is only established under the simple conditions so as to lay a foundation for establishing a model of a next complex pipe; in a complex pipe, a model before and after leakage is specifically established so as to output a picture for data analysis of the next step.
Straight pipe pipelines are the most common situation in gas pipelines, so a straight pipe pipeline model is established first. Selecting a fuel gas (the main component is methane) pipeline with the length of a straight pipe being 1000mm and the diameter being 25mm as a research object, neglecting the inner diameter of the straight pipe, flowing in at the speed of 1m/s, and establishing a model before the straight pipe is not leaked, wherein the concrete straight pipe simulation is as shown in the following figure 1;
the straight pipe model is built by a DM module of an ANSYS, and a straight pipe two-dimensional model graph is drawn by Gambit software according to the size shown in the figure 1, wherein the straight pipe two-dimensional model graph is specifically shown in the following figure 2;
then, in an attempt of workbench, meshing is carried out, and as a result, straight pipe meshing is shown in FIG. 3;
solving the setting in fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation; as can be seen from the pressure diagram, the pressure at the inlet of the straight pipe is the maximum, and the pressure in the straight pipe is gradually reduced along with the flow and reaches the minimum when reaching the outlet.
And dividing the grids by comparing with the experimental result and the requirement of Gambit on the grid quality. After the grid division is finished, the boundary condition type and the calculation area type are specified, the specific boundary condition is set in the Fluent, and the Mesh file can be output and stored after the work is finished.
The specific method and operation of the pipeline simulation after the leakage is the same as that before the leakage, so that a model after the leakage is not established, and the data can be directly drawn and analyzed in the next step.
2.2.2, building an internal model of the T pipe pipeline and outputting pictures:
the T-shaped pipe mainly researches the condition of the joint of the gas pipelines, in the research, the gas pipelines with the long pipe length of 1000mm, the short pipe length of 500mm and the diameter of 25mm (the main component is methane) of the T-shaped pipe are selected as research objects, the inner diameter of the T-shaped pipe is ignored and flows in at the speed of 1m/s, and a specific T-shaped pipe model is shown in figure 4;
and (3) solving and setting in Fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation, wherein the pressure is obviously changed at the T-shaped connection part of the T-shaped row pipe and is obviously increased compared with the pressure at the stable flowing part.
As can be seen from the velocity diagram, at the T-shaped connection part of the T-shaped pipe, the velocity is obviously changed, and the velocity is obviously increased compared with the steady flow.
2.3, establishing a complex pipeline model before and after leakage and outputting pictures:
2.3.1 establishing a model before leakage in the complex pipeline:
the complex pipe is the simplification of the actual gas pipeline, has the foundation of straight pipe and T-shaped row pipe, and can establish a model of the complex pipeline. Selecting a natural gas pipeline as a complex pipe research object, wherein a natural gas real pipeline is shown in the figure, the shape of the complex pipe pipeline is shown in the figure (the size is marked as below), a fuel gas pipeline with the diameter of 25mm (the main component is methane) is used as the research object, the inner diameter of a T-shaped pipe is neglected, the fuel gas flows in at the speed of 1m/s, and a model before the leakage of the specific complex pipeline is shown in figure 5;
the complex pipe model is built by a DM module of an ANSYS, a straight pipe two-dimensional model graph is drawn by Gambit software according to the size shown in the figure 5, and the specific complex pipe two-dimensional model graph before leakage is shown in the figure 6;
in the workbench's attempt, meshing is performed, and for planar and axisymmetric flow problems, only a face mesh needs to be generated. For three-dimensional problems, the surface grid can be divided firstly and then expanded to a body, and because the two-dimensional gas pipeline simulation is established, only the surface grid is involved. The specific complex pipeline pre-leakage meshing is shown in fig. 7;
3.3.2, internal simulation after leakage of the complex pipeline:
the two-dimensional model diagram after the leakage of the specific complex pipeline is shown in FIG. 8; the two-dimensional model diagram after the leakage of the specific complex pipeline is shown in FIG. 9; in the workbench attempt, meshing is performed, and the result is shown in fig. 10.
Thirdly, analyzing the numerical result of the leakage of the gas pipeline:
3.1, analyzing the CFD numerical simulation result:
because the gas flow is unstable at the moment when the gas pipeline starts to leak, the unstable state is used for carrying out simulation analysis on the gas leakage so as to observe the change rule of the internal leakage of the natural gas pipeline along with the time. The analysis of the invention uses the pipeline pressure P as 0.4MPa, the pipe diameter D as 25mm and the leakage aperture D as 10mm as examples to analyze the methane concentration distribution rule in a flow stable state in detail. From the simulated diagrams of the straight pipe, the T-shaped pipe and the complex pipe, it can be seen that after the gas leaks, the concentration of methane near the leakage port in the pipe is very high, but with the increase of time, because the density difference exists between the gas and the air, the leaking speed is slowly stable, the speed and the concentration decay are very fast, and finally, the speed and the concentration gradually tend to be stable. The method is mainly based on the second step, and analyzes the influence of leakage factors (two aspects of pipeline pressure and pipeline diameter) according to data obtained by fluent software simulation calculation, and obtains a result.
3.2 analysis of numerical simulation results:
the change of the internal pressure and speed of the gas pipeline after leakage is analyzed in detail by taking the pipeline pressure P as 0.4MPa, the pipe diameter D as 25mm and the leakage hole diameter D as 10mm as models.
The simulation results are led into Tecplot for post-processing, so that the mole fraction isoline distribution of methane at different moments can be obtained, and the volume occupied by the same mole of gas is the same under the same temperature and pressure, so that the mole fraction of methane is equal to the mass fraction of methane.
(1) Internal simulation results after the straight pipeline leaks:
a fuel gas pipeline (the main component is methane) with the straight pipe length of 1000mm, the diameter of 25mm and the leakage aperture of 10mm is selected as a research object, the inner diameter of the straight pipe is neglected, the fuel gas flows in at the speed of 1m/s, and the simulation after the straight pipe leaks is shown in figure 11:
applying ANSYS14.0 to make a scale-down residual error map; performing later-stage processing to obtain a time-dependent change diagram of the concentration of methane in the straight pipe; the straight pipe leakage graph clearly shows that the concentration of methane changes obviously at the leakage point along with the change of time, and the leakage point is gradually stabilized at 125 seconds;
(2) internal simulation results after T pipeline leakage:
as shown in fig. 12, a fuel gas (mainly comprising methane) pipeline with a long pipe 1000mm long, a short pipe 500mm long, a diameter 25mm short and a leakage aperture of 10mm is selected as a research object, the inner diameter of a T-shaped row pipe is neglected, the fuel gas flows in at a speed of 1m/s, and the specific T-shaped row pipe leakage simulation is as shown in fig. 12;
the T-shaped row pipe model is built by a DM module of an ANSYS, in a workbench attempt, grid division is carried out, and an ANSYS14.0 is used for making a proportional reduction residual error map; performing post-processing to obtain a time-dependent change chart of the methane concentration in the pipeline of the T-shaped row of pipes; the leakage graph of the T-shaped tube shows that the concentration of methane changes obviously at the leakage point along with the change of time, and the leakage point is gradually stabilized at the 125 th second.
(3) Internal simulation results after complex pipeline leakage:
since the model map has already been built, it is analyzed directly on the basis of the second step.
Applying ANSYS14.0 to make a scaled-down residual error map, and carrying out 576 steps in total; solving the setting in fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation;
and (3) comprehensively analyzing, wherein a basic conclusion is obtained by analyzing the internal flow fields of the pipeline before the leakage of the straight pipe and the T-shaped pipe and after the leakage of the complex pipe: before leakage, the pressure vector and the velocity vector in the pipeline are maximum at an inlet, gradually become smaller along with the flow of fuel gas, and reach minimum at an outlet; after the leakage occurs, the internal pressure vector and the velocity vector of the leakage device are greatly changed, when the leakage device is at a leakage point, the pressure and the velocity can be obviously changed, and the change of the methane concentration at the leakage point is gradually stable along with the lapse of time.
3.3, pipeline leakage influence factors:
the factors influencing natural gas leakage are many and are described in the following table 3-1 (specifically, the research is mainly carried out from two aspects of pipeline pressure and leakage port size, the gas in the pipeline is assumed to be methane, and the actual fluent simulation option is selected to be methane-air);
TABLE 3-1 influence of Natural gas line leakage
3.3.1 Effect of changes in line pressure on leakage:
the gas pipelines are different in environment, distance for conveying natural gas and laying mode, so that the design pressure of the pipelines is different. The long-distance pipeline has high pressure, but the pressure generally does not exceed 3.6MPa, while the natural gas transportation in cities is performed in a low-pressure mode because the transportation pressure is generally between 0.2 and 0.4MPa in consideration of the reasons of dense buildings, complex personnel activities and the like, and the transportation of civil natural gas entering residential buildings and the like is performed. Therefore, the pipeline pressure is one of important indexes of natural gas pipeline transportation, and therefore, the study of the relation between natural gas leakage diffusion and the pipeline pressure is particularly important for safe construction and quick inspection of a pipeline network. The simulation is mainly aimed at the gas pipeline leakage condition in cities, so that the gas pipeline leakage condition in the pipeline pressure range of about 0.5MPa is mainly researched. When a natural gas pipeline with the diameter of 25mm is broken (namely d is 25mm), simulating the pressure and the speed cloud chart under the condition of leakage under different pressures (the invention selects gas pipelines with pipeline inlet pressures of 2kpa, 0.3Mpa and 0.4Mpa and 3 different inlet pressures to simulate (just researches on simulated complex pipes), and simultaneously uniformly sets other parameters of the gas pipeline with the diameter of 25mm and the diameter of a leakage hole of 10 mm).
When the pipeline inlet pressure is 2 kpa: initially the concentration in the inlet was maximal and gradually increased with time, and by 450 steps (125s) the concentration in the tube reached a maximum. The temperature becomes lower at the pipe connection and the temperature in the rest of the pipe is quite even.
In practical engineering, the larger the pressure of the pipeline is, the more easily leakage accidents occur, and the temperature and the speed in the pipeline are increased correspondingly. Therefore, the pipes with high pressure rating are strictly managed, and the maintenance work for the pipes is usually strengthened.
3.3.2 Effect of leak Aperture variation on leakage:
when the pipeline pressure is constant and the aperture of the leakage hole is small, the mass flow of the gas leaked from the leakage hole to the soil environment in unit time is reduced, so that the diffusion range in the soil is small in the same time, and the dangerous area of leakage is small. On the contrary, the larger the leakage aperture is, the larger the proportion of the high concentration region around the leakage aperture is. Thus, when the pipe has a large leakage hole, its leakage state is dangerous.
Therefore, it can be concluded that in practical engineering, it is a relatively intuitive and effective measure to prevent pipeline accidents by enhancing the management and protection of the gas pipeline and minimizing the leakage aperture.
Generally speaking, various factors influencing the leakage of the natural gas pipeline are analyzed through numerical simulation of front and back leakage of an internal flow field of the gas pipeline according to a simulation result diagram, so that a conclusion is drawn, the pipeline leakage accident is prevented, and a theoretical basis is provided for investigation after the accident occurs.
The specific implementation mode has the following characteristics:
(1) according to CFD computational fluid mechanics knowledge, fuel gas pipeline leakage is simulated by Fluent, and leakage concentrations of a straight pipe, a T-shaped pipe and a complex pipe are shown by pictures, so that a theoretical basis is provided for later research.
(2) According to the fluid mechanics theory, the characteristics of the internal flow field of the gas pipeline before and after the gas pipeline leaks are obtained through analysis. According to the fluid mechanics theory, the characteristics of the internal flow field of the gas pipeline before and after the gas pipeline leaks are obtained through analysis. As can be seen from the pressure diagram, the pressure at the inlet of the straight pipe is the maximum, the pressure in the straight pipe is gradually reduced along with the flow, and the pressure reaches the minimum when reaching the outlet; at the T-shaped connection part of the T-shaped row pipe, the pressure is obviously changed, and the pressure is obviously increased compared with the pressure at the stable flowing part. As the gas leakage progresses, the proportion of the high concentration region around the leakage hole increases.
(3) Calculating fluid mechanics knowledge according to CFD (computational fluid dynamics), simulating gas pipeline leakage under different leakage conditions, wherein the larger the inlet pressure is, the more easily a leakage accident occurs; the larger the leak hole diameter is, the larger the proportion of the high concentration region around the leak hole is.
It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. Any reference sign in a claim should not be construed as limiting the claim concerned.
Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.
Claims (1)
1. The numerical simulation method for the influence of the leakage of the gas pipeline on the internal flow field is characterized by comprising the following steps of: the simulation method comprises the following steps:
step 1, establishing a model: in the preprocessing stage, preprocessing is completed by adopting preprocessing Gambit software, wherein an unstructured grid generating program contained in the preprocessing Gambit software is used for generating grids for a geometric structure;
1.1, building models of straight pipes and T-shaped pipes and dividing grids:
adopting a parametric modeling method, and establishing straight pipe, T-shaped pipe and complex pipeline models;
1.1.1, building an internal model of the straight pipe pipeline and dividing grids:
firstly, establishing a model diagram before straight pipe leakage, and drawing a straight pipe two-dimensional model diagram according to the size of the established straight pipe pipeline model by using Gambit software; and carrying out grid division;
1.1.2, building an internal model of the T-shaped pipe and dividing a grid:
solving the setting in Fluent, and making a pressure cloud picture and a speed cloud picture before leakage after calculation, wherein the pressure cloud picture shows that the pressure is obviously changed at the T-shaped connection part of the T-shaped pipe and is obviously increased compared with the pressure at the stable flowing part;
1.2, establishing and meshing complex pipeline models before and after leakage:
the complex pipeline model utilizes Gambit software to draw a complex pipeline two-dimensional model graph according to the size; and carrying out grid division;
step 2, simulation analysis of gas pipeline leakage:
2.1, Fluent numerical simulation calculation and operation:
adopting Fluent to carry out analog simulation, selecting a solving equation and a required model in Fluent, further setting boundary conditions and control parameters, starting calculation, and obtaining a result which can be used for the next operation; importing the file saved after the Fluent solution is finished into the Tecplot for post-processing;
2.2, Fluent solver and setting:
fluent adopts a pressure-based solver;
2.3, setting a Fluent running environment:
opening the setting of the running environment of Fluent software, and setting two options of calculating reference pressure and gravity; the reference pressure is atmospheric environment pressure and is set to be standard atmospheric pressure 101325Pa, and default points (0, 0 and 0) are selected at the reference pressure position;
transient simulation is selected in General, Gravity is started, and the Gravity acceleration is set to be-9.8 m/s in the Y-axis direction2；
2.4 Fluent calculation model selection:
after the solver format is selected and the operating environment is set, a calculation model is selected, which adopts a turbulence model, then an energy equation is started in Models,
an equation and a component transport equation, and defining components in the component transport equation as methane and air; selecting a material database; or customizing a new material, or modifying the parameters and properties of an existing material;
2.5, setting initial conditions and boundary conditions of Fluent:
initial conditions for the simulation calculation were set as: the pressure inlet is 0.4Mpa, the inner diameter is neglected, and the water flows in at the speed of 1 m/s;
2.6, setting the Fluent solving parameters:
initializing initial conditions and boundary conditions after the setting is finished, and then setting step length for analog calculation;
step 3, analyzing the numerical result of the leakage of the gas pipeline:
3.1, analyzing the CFD numerical simulation result:
on the basis of the step 2, analyzing the influence of the leakage factors according to data obtained by the Fluent software simulation calculation, and obtaining a result;
3.2, analyzing numerical simulation results:
3.2.1, internal simulation result after the straight pipe leaks:
applying ANSYS14.0 to make a scale-down residual error map; performing later-stage processing to obtain a time-dependent change diagram of the concentration of methane in the straight pipe;
3.2.2, internal simulation result after T-shaped pipe leakage:
the T-shaped pipe model uses ANSYS14.0 to make a proportional reduction residual error map; performing later-stage treatment to make a time-dependent change chart of the methane concentration in the T-shaped pipe;
3.2.3, internal simulation result after the leakage of the complex pipeline:
the analysis of the internal flow field of the pipeline after the leakage of the complex pipeline leads to a basic conclusion: before leakage, the pressure vector and the velocity vector in the pipeline are maximum at an inlet, gradually become smaller along with the flow of fuel gas, and reach minimum at an outlet; when leakage occurs, the internal pressure vector and the velocity vector of the leakage pipe change, when the leakage pipe is at a leakage point, the pressure and the velocity can obviously change, and the change of the methane concentration at the leakage point is gradually stable along with the lapse of time;
3.3, pipeline leakage influence factors:
3.3.1 analysis of the effects of changes in line pressure on leaks:
the pipeline pressure is positively correlated with the leakage amount, the pressure at the inlet of the straight pipe is maximum, the pressure in the straight pipe is gradually reduced along with the flow, and the pressure reaches the minimum when reaching the outlet; at the T-shaped connection part of the T-shaped pipe, the pressure is obviously changed, the pressure is obviously increased compared with the pressure at the stable flowing part, the proportion of a high-concentration area around a leakage hole is increased along with the progress of leakage of the fuel gas, the leakage accident is more likely to happen when the pipeline pressure is higher, and the temperature and the speed in the pipeline are increased at the moment;
3.3.2 Effect of leak Aperture variation on leakage:
when the pipeline pressure is constant and the aperture of the leakage hole is small, the mass flow of the fuel gas leaked from the leakage hole to the soil environment in unit time is reduced, so that the diffusion range in the soil is small in the same time, and the leakage danger area is small; on the contrary, the larger the leakage aperture is, the larger the proportion of the high concentration region around the leakage aperture is.
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| CN114811448B (en) * | 2022-04-13 | 2023-09-22 | 中南大学 | Method for pipeline leakage detection, leakage flow velocity estimation and leakage positioning under flowing condition |
| CN116644689B (en) * | 2023-07-24 | 2023-11-03 | 北京工业大学 | Method and system for strong and rapid back calculation of atmospheric pollution source of local scale under complex underlying surface |
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| CN117828795B (en) * | 2024-01-05 | 2024-08-27 | 西南石油大学 | Calculation method for gas small hole leakage equivalent diameter of gas transmission pipe network in region |
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