CN111695306A - Valve transient characteristic simulation method based on three-dimensional water hammer CFD - Google Patents

Abstract

The invention discloses a valve transient characteristic simulation method based on three-dimensional water hammer CFD, comprising: constructing a three-dimensional mathematical model control equation of a pipeline including a ball valve; The simulation model is meshed, and the boundary layer part and the ball valve part are meshed to determine the calculation domain of the three-dimensional mathematical model; the water body compressible state equation, the water hammer wave velocity equation and the ball valve dynamic switch control function are introduced; the initial conditions and boundary conditions, solve the governing equations of the 3D mathematical model, and monitor the transient characteristics of the ball valve. On the basis of the three-dimensional simulation, the method of the present application fully considers the influence of the compressibility of the water body and the viscous bottom layer on the simulation results, and uses the three-dimensional simulation model to accurately and intuitively reproduce the transient process of the dynamic closing of the ball valve in the pipeline, which improves the simulation performance. The accuracy of this method provides a reference for the pipeline transient case example.

Description

A simulation method of valve transient characteristics based on three-dimensional water hammer CFD

technical field

The invention relates to hydraulic numerical calculation of hydropower stations (pumping stations), in particular to a method for simulating transient characteristics of valves based on three-dimensional water hammer CFD.

Background technique

The hydraulic transition process often occurs in various pressurized pipeline systems, including long-distance water transmission projects across basins, hydropower stations and pumping stations, urban water supply systems, and agricultural irrigation systems. Normal or unexpected pump or turbine shutdown or rapid valve opening and closing can often cause abnormal pressure fluctuations or even water hammer, which can rupture pipes and damage other overcurrent devices. Therefore, accurate numerical simulation of water hammer is crucial for the rational design and safe operation of piping systems.

The traditional numerical simulation method is the one-dimensional numerical simulation using the characteristic line method. Although the calculation speed of this simulation method is fast, because the traditional one-dimensional numerical simulation method is too simplified for the actual situation, the steady-state dissipation formula is used to approximate the pipeline. Shear stress, often underestimating the energy dissipation of the pipeline, leads to a lack of simulation accuracy. On the above basis, in order to improve the numerical accuracy of water hammer pressure, a dynamic friction model is added, but one-dimensional simulation is still used, and there is still a large error between the simulation results and the experimental data.

For transient pipeline flow, there have been related papers that have carried out 3D numerical simulations on the basis of experiments, but these papers have not fully considered the influence of the viscous bottom layer, and have not thoroughly studied the transient changes of the ball valve. Because the vortex has three-dimensional characteristics, it is difficult to accurately simulate the vorticity of the tube wall with a one-dimensional method and describe the relationship between dynamic vorticity and energy dissipation. In addition, when the traditional one-dimensional method deals with the dynamic hydraulic loss caused by valve closing in abnormal pipeline fluid flow, it is assumed that the flow state is stable and the energy loss is consistent at the same speed. The state dynamics and unstable flow properties of local eddies and backflow during valve closing are complex three-dimensional flow problems that are difficult to determine with one-dimensional methods or experimental measures.

SUMMARY OF THE INVENTION

Purpose of the invention: The purpose of this application is to provide a valve transient characteristic simulation method based on three-dimensional water hammer CFD, which is used to solve the defect of large simulation error in the existing method.

Technical solution: The present invention provides a valve transient characteristic simulation method based on three-dimensional water hammer CFD, including:

(1) Construct the governing equations of the three-dimensional mathematical model of the pipeline containing the ball valve;

(2) Create a three-dimensional simulation model of the pipeline containing the ball valve according to the working condition example;

(3) The three-dimensional simulation model is meshed, and the boundary layer part and the ball valve part are meshed to determine the calculation domain of the three-dimensional mathematical model;

(4) Introduce the water body compressibility state equation and water hammer wave velocity equation to describe the water body compressibility;

(5) Introduce the dynamic switch control function of the ball valve for the dynamic closing operation of the ball valve;

(6) Determine the initial conditions and boundary conditions according to the working conditions, solve the control equations of the three-dimensional mathematical model, and monitor the transient characteristics of the ball valve.

Further, the three-dimensional mathematical model constructed in step (1) is represented by the following governing equations:

是速度矢量，

和

分别是i、j方向流速矢量，p水体压力，

是剪切力，

是重力加速度，

是体积力。where ρ is the density of the water body, t is the time,

is the velocity vector,

and

are the flow velocity vectors in the i and j directions, respectively, the p water pressure,

is the shear force,

is the gravitational acceleration,

is volume force.

Further, in step (2), the three-dimensional simulation model adopts the structure of "first reservoir-first pipeline-ball valve-second pipeline-second reservoir", wherein the ball valve is cut into independent units.

Further, the compressible state equation of water body is specifically expressed as:

The water hammer velocity equation is specifically expressed as:

Among them, K _f is the volume elastic modulus of the fluid, D is the inner diameter of the pipe, E is the bulk elastic modulus of the pipe, and e is the thickness of the pipe wall.

Further, the dynamic switch control function of the ball valve is specifically expressed as:

Among them, ω is the angular velocity of the ball valve, t is the time, θ is the rotation angle of the ball valve, and t ₁ is the closing time of the ball valve.

Further, in step (4), the compressibility of the water body is described by a liquid model that introduces a compressible source term, and the turbulence model is coupled on the basis of solving the continuity equation, momentum equation and energy equation, so that the entire solution system is closed.

Further, the turbulence model adopts the SST k-ω turbulence model.

Further, the initial conditions include: the auxiliary calculation area and the fluid state in the full flow channel, and the pressure of the water inlet and outlet;

Boundary conditions include: pipeline wall state, dynamic closing of ball valves, and constant pressure at the boundaries of the first and second reservoirs.

The present application also provides a computer-readable storage medium, the storage medium includes computer instructions, and when the computer instructions are executed, the above-mentioned three-dimensional water hammer CFD-based valve transient characteristics simulation method can be implemented.

Beneficial effects: Compared with the prior art, the simulation method disclosed in this application has the following advantages:

(1) The water hammer velocity is introduced into the CFD calculation software by defining the compressibility of the water body, which is closer to reality;

(2) By refining the boundary layer mesh and selecting an appropriate turbulence model to consider the influence of the frictional resistance of the boundary layer, the introduction of the turbulence model can more accurately simulate the three-dimensional transient flow in the liquid, and further simulate the boundary layer pair flow. Impact;

(3) The flow velocity field and pressure field of any section of the whole flow channel at any calculation moment can be reproduced intuitively and dynamically;

(4) The transient change of fluid caused by valve closing in pressurized pipeline is simulated finely.

Description of drawings

Fig. 1 is the flow chart of the valve transient characteristic simulation method of the application;

Fig. 2 is the three-dimensional simulation model in the embodiment of the application;

3 is a schematic diagram of the grid division of the pipeline structure of the application;

Fig. 4 is the comparison diagram of the calculation result of the water hammer pressure fluctuation in the simulation example of the application and the experimental result;

5 is a partial flow field diagram of the ball valve calculated in the simulation example of the application;

FIG. 6 is a partial pressure diagram of the ball valve calculated in the simulation example of the application.

Detailed ways

Below in conjunction with accompanying drawing and embodiment, the present invention is further described:

The present invention provides a method for simulating valve transient characteristics based on three-dimensional water hammer CFD, as shown in Figure 1, including:

S101 constructs the governing equations of the three-dimensional mathematical model of the pipeline containing the ball valve. Specifically, the constructed three-dimensional mathematical model is represented by the following governing equations:

是速度矢量，

和

分别是i、j方向流速矢量，p是水体压力，

是剪切力，

是重力加速度，

是体积力。where ρ is the density of the water body, t is the time,

is the velocity vector,

and

are the flow velocity vectors in the i and j directions, respectively, p is the water pressure,

is the shear force,

is the gravitational acceleration,

is volume force.

S102, according to the working condition instance, create a three-dimensional simulation model of the pipeline including the ball valve. Specifically, in this embodiment, the ANSYS software DesignModeler module can be used to create a three-dimensional model of a pressurized pipeline according to an engineering example, and the ball valve is fully opened at the beginning. As shown in Figure 3, the three-dimensional simulation model adopts the "first reservoir-first pipeline- Ball Valve-Second Pipe-Second Reservoir" structure, where the ball valve is cut as a separate unit. It can be set that the first reservoir 201 and the second reservoir 205 are the upstream reservoir and the downstream reservoir respectively, the first pipeline 202 and the second pipeline 204 are the upstream water pipeline and the downstream water pipeline respectively, and the ball valve 203 is arranged on the upstream water pipeline and the downstream water pipeline. Between the downstream water pipelines, connect the upstream water pipeline and the downstream water pipeline. 206 is the air above the reservoir.

S103 divides the three-dimensional simulation model into a mesh, and performs mesh refinement on the boundary layer part and the ball valve part to determine the calculation domain of the three-dimensional mathematical model. Specifically, in the embodiment of the present application, the created three-dimensional simulation model is poured into the ICEM module of the ANSYS software, each boundary surface is defined, and the grid is divided to determine the calculation domain. The computational domain is the entire 3D simulation model, including upstream and downstream reservoirs, pipelines and valves. The entire computational domain adopts a structured grid, namely a hexahedral grid, which is refined in the pipe wall part and the ball valve part.

S104 introduces the water body compressibility state equation and the water hammer wave velocity equation to describe the water body compressibility. The compressibility of water body is described by a liquid model that introduces a compressible source term, and the turbulence model is coupled on the basis of solving the continuity equation, momentum equation and energy equation, so that the entire solution system is closed. The turbulence model adopts the SST k-ω turbulence model.

Specifically, in the embodiment of the present application, a user-defined function UDF (User Define Functions) can be used to add the water body compressible state equation and the water hammer velocity equation.

The density equation of water body, as the compressible state equation of water body, is specifically expressed as:

The water hammer velocity equation is specifically expressed as:

Among them, K _f is the volume elastic modulus of the fluid, D is the inner diameter of the pipe, E is the bulk elastic modulus of the pipe, and e is the thickness of the pipe wall.

The turbulent flow model can characterize the turbulent flow change law in the calculation, and at the same time have an effect on the viscous bottom layer, and further simulate the influence of the boundary layer on the flow, so as to simulate the three-dimensional transient flow in the liquid more accurately.

S105 introduces the ball valve dynamic switch control function for the ball valve dynamic closing operation. Specifically, in the embodiment of the present application, a user-defined function UDF can be used to add a ball valve dynamic switch control function according to the dynamic closing rule of the ball valve, which is specifically expressed as:

Among them, ω is the angular velocity of the ball valve, t is the time, θ is the rotation angle of the ball valve, and t ₁ is the closing time of the ball valve.

S106 determines initial conditions and boundary conditions according to the working condition instance, solves the control equation of the three-dimensional mathematical model, and monitors the transient characteristics of the ball valve. The initial conditions include: two auxiliary calculation areas established upstream and downstream, the fluid state in the full flow channel, and the pressure at the inlet and outlet; the boundary conditions include: the state of the pipe wall, the dynamic closing of the ball valve, and the constant pressure at the boundary of the first and second reservoirs.

Specifically, in the embodiment of the present application, three-dimensional simulation software can be used to solve the control equation:

Import the divided mesh file into the FLUENT module of the ANSYS software, and set the initial conditions and boundary conditions according to the working case example.

After the above settings, the control equations are solved to obtain the simulation results of the transient characteristics of the ball valve.

The present invention also provides a computer-readable storage medium, the storage medium includes computer instructions, and when the computer instructions are executed, the above-mentioned three-dimensional water hammer CFD-based valve transient characteristic simulation method can be implemented.

Simulation:

The transient simulation method of the ball valve of the present application is verified through an example of working conditions. In the modeling process, some components that will not affect the hydraulic properties in the flow channel are simplified. The simplified simulation system is shown in Figure 2. The main components of the system are: upstream reservoir, upstream water pipeline, ball valve, downstream water pipeline, and downstream reservoir. The simulation object of this example is a water hammer experiment of a certain pressure pipeline. The length of the pipeline is 37.23m, the inner diameter of the pipeline is 22.1mm, and the water hammer wave propagation speed is 1319m/s. There are three working conditions in the experiment. m/s, 0.2m/s and 0.3m/s.

The initial and boundary conditions are set as follows:

Initial conditions

A. Set the auxiliary calculation area and the whole flow channel as a single phase, that is, all liquid water, and load the water compressible state equation UDF;

B. According to the working condition example, add gravity in the auxiliary calculation area and the full flow channel, so that the pressure of the water inlet and outlet is consistent with the actual situation;

Boundary conditions

A. Wall: static, smooth and no slip;

B. Ball valve dynamic closing process: load the ball valve to close the UDF and control its dynamic closing;

C. Upstream and downstream reservoirs: constant pressure boundary.

This working condition example is simulated and calculated by the present invention, and the calculation result is compared with the experimental data. As shown in Figure 4, the curve drawn by the three-dimensional calculation result and the curve drawn by the experimental result have a high degree of coincidence, and the peak values are almost coincident. It can be seen that the simulation method of the present application can effectively simulate the flow rate and pressure changes in the transition process of the pump in the pumped-storage power station.

The post-processing software Tecplot was used to realize the visibility of the velocity field and pressure field of the whole channel during the transition process. Fig. 5 is a velocity field diagram of the ball valve and its connecting pipeline, wherein Fig. 5(a) is a dynamic valve closing condition, and Fig. 5(b) is a static valve closing condition. Fig. 6 is a pressure field diagram of the ball valve and its connecting pipeline, wherein Fig. 6(a) is a dynamic valve closing condition, and Fig. 6(b) is a static valve closing condition. It can be seen from the figure that not only the accuracy of the three-dimensional calculation results can fully meet the requirements, but also the types of output data are diverse, and the output results are more intuitive.

As will be appreciated by those skilled in the art, the embodiments of the present application may be provided as a method, a system, or a computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) having computer-usable program code embodied therein.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the present application. It will be understood that each flow and/or block in the flowchart illustrations and/or block diagrams, and combinations of flows and/or blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to the processor of a general purpose computer, special purpose computer, embedded processor or other programmable data processing device to produce a machine such that the instructions executed by the processor of the computer or other programmable data processing device produce Means for implementing the functions specified in a flow or flow of a flowchart and/or a block or blocks of a block diagram.

These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory result in an article of manufacture comprising instruction means, the instructions The apparatus implements the functions specified in the flow or flow of the flowcharts and/or the block or blocks of the block diagrams.

These computer program instructions can also be loaded on a computer or other programmable data processing device to cause a series of operational steps to be performed on the computer or other programmable device to produce a computer-implemented process such that The instructions provide steps for implementing the functions specified in the flow or blocks of the flowcharts and/or the block or blocks of the block diagrams.

Claims (9)

1. A valve transient characteristic simulation method based on a three-dimensional water hammer CFD is characterized by comprising the following steps:

(1) constructing a control equation of a three-dimensional mathematical model of a pipeline containing a ball valve;

(2) according to the working condition example, a three-dimensional simulation model of the pipeline containing the ball valve is created;

(3) carrying out grid division on the three-dimensional simulation model, and carrying out grid encryption on a boundary layer part and a ball valve part to determine a three-dimensional mathematical model calculation domain;

(4) introducing a water body compressibility state equation and a water hammer wave velocity equation for describing water compressibility;

(5) introducing a ball valve dynamic switch control function for carrying out ball valve dynamic closing operation;

(6) and determining initial conditions and boundary conditions according to the working condition examples, solving a control equation of the three-dimensional mathematical model, and monitoring the transient characteristics of the ball valve.

2. The method of claim 1, wherein the three-dimensional mathematical model constructed in step (1) is represented by the following governing equation:

where ρ is the density of the body of water and t isThe time of day is,

is a vector of the velocity of the beam,

and

flow velocity vectors in the directions of i and j, water body pressure p,

is a shear force which is a force of shearing,

is the acceleration of the force of gravity,

is the volumetric force.

3. The method according to claim 2, wherein in the step (2), the three-dimensional simulation model adopts a structure of 'first reservoir-first pipeline-ball valve-second pipeline-second reservoir', wherein the ball valve is cut into independent units.

4. The method of claim 3, wherein the water body compressibility equation of state is specifically expressed as:

the water hammer wave velocity equation is specifically expressed as:

wherein, K_fThe bulk modulus of fluid, D the inner diameter of the pipe, E the bulk modulus of the pipe, and E the wall thickness.

5. The method according to claim 3, wherein the ball valve dynamic on-off control function is specified as:

wherein, omega is the angular velocity of the ball valve, t is time, theta is the rotation angle of the ball valve, t₁The ball valve closure time.

6. The method of claim 1, wherein in the step (4), the compressibility of the water body is described by adopting a liquid model introducing a compressible source term, and the turbulence model is coupled on the basis of solving a continuity equation, a momentum equation and an energy equation, so that the whole solving system is closed.

7. The method of claim 6, wherein the turbulence model is an SST k- ω turbulence model.

8. The method according to any one of claims 1 to 7, wherein the initial conditions comprise: the fluid state and the pressure of a water inlet and a water outlet in the area and the full flow channel are calculated in an auxiliary mode;

the boundary conditions include: pipeline wall surface state, ball valve dynamic closing, first reservoir and second reservoir boundary constant pressure.

9. A computer readable storage medium comprising computer instructions which, when executed, implement the method of any of claims 1 to 8.

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