A hazardous gas risk assessment method based on CFD numerical simulation

Abstract

The present invention discloses a method for risk assessment of hazardous gases based on CFD numerical simulation, comprising the following steps: model establishment and calculation domain setting; gas composition and water mist assumption; physical model setting; grid division; boundary conditions and solver parameter setting; CFD simulation calculation and visualization processing. The advantage of the present invention is that the risk assessment of the ammonia emergency emission diffusion range, ammonia concentration and ammonia water mist scattering range is carried out through CFD numerical simulation, thereby providing an effective evaluation basis for whether the emission or leakage of hazardous gases is safe.

Description

Technical Field

[0001] The present invention relates to the technical field of ship environment analysis, and in particular to a dangerous gas risk assessment method based on CFD numerical simulation. Background Art

[0002] The EU has proposed a goal of "reducing greenhouse gas emissions by 55% by the end of 2030 compared to 1990". The 72nd meeting of the IMO Marine Environment Protection Committee (MEPC) adopted the world's first preliminary strategy for greenhouse gas emissions reduction in the shipping industry. Based on carbon emissions in 2008, it proposed a goal of reducing the carbon emission intensity of the shipping industry by 40% by 2030 and by 70% by 2050 (reducing total carbon emissions by 50%), and formulated countermeasures. In particular, from January 1, 2025, the EU will limit the greenhouse gas emission intensity of fuels throughout their life cycle, forcing ships entering EU waters to meet higher emission requirements, forcing shipping companies to adopt alternative fuels such as low-carbon or zero-carbon, and accelerating the pace of application of alternative fuels.

[0003] At present, alternative fuels are mainly low-carbon or zero-carbon fuels such as LNG, methanol, ammonia and hydrogen. Among them, methanol is volatile and prone to combustion and explosion risks; LNG and hydrogen need to be stored at low temperature and high pressure, and emergency emissions or leaks may occur, thus posing a combustion and explosion risk; in addition to emergency emissions or leaks, ammonia's toxicity and corrosiveness are also major safety risks during use.

[0004] Therefore, it is important to ensure safety when using hazardous gases. Due to the large size of ships, complex navigation conditions, and difficult and dangerous test methods, there is currently no effective method to assess the risks of using hazardous gases. Summary of the invention

[0005] The purpose of the present invention is to provide a hazardous gas risk assessment method based on CFD numerical simulation, which performs risk assessment on the ammonia emergency emission diffusion range, ammonia concentration and ammonia water mist scattering range through CFD numerical simulation, thereby providing an effective evaluation basis for whether the hazardous gas emission or leakage is safe.

[0006] The above technical objectives of the present invention are achieved through the following technical solutions:

[0007] A hazardous gas risk assessment method based on CFD numerical simulation, characterized by comprising the following steps:

[0008] Model establishment and calculation domain setting: The model is built according to the overall layout of the ship and the location of the emergency discharge pipe of ammonia fuel. The numerical simulation uses a full-scale ship and the calculation domain size is set according to the length of the ship;

[0009] Assumptions of gas composition and water mist: Assume air as a single-component gas to simulate the diffusion range of emergency-emitted ammonia in the air. The ammonia concentration is captured by the ammonia content in each calculation grid. When low-temperature ammonia is discharged from the liquid ammonia storage tank and encounters humid air, it will form water mist and scatter onto the hull or container. The Lagrange method is used to simulate and visualize the possible locations where the water mist may scatter.

[0010] Physical model settings: The SST k-ω model is used as the turbulence model, implicit unsteady simulation is used, the Lagrange multinomial flow model is used to simulate water mist, the diameter of the water mist droplets is Nukiyama-Tanazawa distribution, the gas uses a multi-component gas model to simulate air and ammonia, the Fluid Film model is used to simulate a layer of liquid film formed when water mist falls on the hull or container, and the water mist-liquid film conversion model determines the thickness of the liquid film formed on the wall by the droplets impacting the wall;

[0011] Meshing: Hexahedral meshes are used to discretize the computational domain, boundary layer meshes are used to simulate the hull boundary layer, full-scale simulations are performed to ensure that the wall Y+>200, and the volume mesh is encrypted for the ammonia diffusion range;

[0012] Boundary conditions and solver parameter settings: Boundary conditions and initial conditions are set according to the ship operation conditions, and the solid wall where the ammonia mist is scattered is set as the Fluid-Film boundary to capture the liquid film thickness on the wall;

[0013] CFD simulation calculation and visualization processing: evaluate the ammonia mist dispersion range. The ammonia mist dispersion range is visualized through the Lagrange method to determine the ammonia mist dispersion area, and monitor the ammonia concentration near the superstructure and other locations that require attention, and conduct risk assessment based on the ammonia toxicity evaluation standard.

[0014] Preferably, the calculation domain size is: -5.5 times the ship's length ≤ x ≤ 4.5 times the ship's length, -3.5 times the ship's length ≤ y ≤ 3.5 times the ship's length, 0 ≤ z ≤ 4.5 times the ship's length.

[0015] Preferably, the water mist hypothesis simulation mainly focuses on the range of water mist scattering, and does not focus on the process of ammonia and humid air combination. The Lagrange method is used to simulate the size of the formed water mist particles and visualize the possible location of the water mist scattering. The water mist is mainly affected by the disturbance of the flow field and gravity, and scatters onto the hull or container. The water mist translation and rotation motion equations are as follows:

[0016]

[0017]

[0018] In the formula, m i 、vi and ω i denote the mass, velocity and angular velocity of the water mist i, respectively. i is the moment of inertia of the water mist i, F g is the gravity of the water mist i, F fluid is the force exerted by air on ice particle i, T ij is the torque on the water mist, including gravity and the torque of air on the water mist.

[0019] Preferably, the diameter of the water mist droplets is calculated using the specific formula of Nukiyama-Tanazawa distribution:

[0020]

[0021] in, It is an average size.

[0022] Preferably, the gas uses a multi-component gas model to simulate air and ammonia, the scalar diffusion rate of Fick's law of multi-component gas diffusion is replaced by a matrix, and the diffusion flux of component i becomes a function of the mass fractions of all components:

[0023]

[0024] Where D i,j represents the multicomponent diffusion coefficient calculated using the Maxwell-Stefan equation, Y j is the mass fraction of component j in the mixture.

[0025] Preferably, the FluidFilm model continuity equation is as follows:

[0026]

[0027] In the formula, ρ f is the film density, is the film velocity, the subscript f represents the fluid film value, and the quantity S u is the mass source / sink per unit area. Contributions to the mass source come from droplet impact, film peeling, mass transfer, or user-defined sources. This formula is used to calculate the film thickness h f , the volume V and the surface A are functions of the film thickness and its spatial distribution. The momentum conservation equation of the FluidFilm model is as follows:

[0028]

[0029] In the formula, is the mass source S u The momentum source, ρ f It's pressure. is the body force vector, is the viscous stress tensor in the membrane,

[0030] The energy conservation equation of the Fluid Film model is as follows:

[0031]

[0032] In the formula, E f is the total film energy, H f is the total enthalpy of the film, is the film heat flux, is the film viscous stress tensor, is the physical force per unit volume, s E is the energy source term per unit film area, with contributions coming from droplet impact, film debonding, evaporation, condensation, or user-defined sources.

[0033] Preferably, the water mist-liquid film conversion model determines the thickness of the liquid film formed on the wall by the droplets impacting the wall, and the equivalent film thickness calculation formula is as follows:

[0034]

[0035] Where V d is the total volume of all droplets on the cell wall, A c The area covered by the water mist.

[0036] Preferably, the boundary condition settings include setting the outlet as a pressure outlet, setting the calculation domain inlet as a velocity inlet, setting the relative pressure of the ammonia fuel emergency discharge pipe outlet, and setting various solver parameters including stop conditions, maximum number of internal iterations and time step.

[0037] In summary, the beneficial effects of the present invention are as follows: the safety risks and costs of conducting experiments on hazardous gas risk assessment are very high, and there is a scale effect in model-scale experiments, and it is also more difficult to conduct real-scale experiments. Therefore, the use of CFD numerical simulation methods to conduct hazardous gas risk assessment has very important practical significance. The risk assessment method of the present invention can provide an effective evaluation basis for whether hazardous gas emissions or leaks are safe. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a flow chart of a hazardous gas risk assessment method based on CFD numerical simulation;

[0039] Figure 2 The schematic diagram of the overall ship model is shown in Figure 1, taking ammonia as an example;

[0040] Figure 3 This is the ammonia mist scattering range and liquid film thickness diagram;

[0041] Figure 4 is the ammonia concentration curve at all monitoring points;

[0042] Figure 5 is the ammonia concentration cloud map;

[0043] in Figure 2 1 is the emergency exhaust pipe of ammonia fuel, 2 is the superstructure, 3 is the ammonia concentration monitoring point, 4 is the hull, and 5 is the container stack. DETAILED DESCRIPTION

[0044] The specific implementation manner of the present invention is further described below in conjunction with the accompanying drawings, and this embodiment does not constitute a limitation of the present invention.

[0045] like Figure 1 As shown, the present invention provides a dangerous gas risk assessment method based on CFD numerical simulation, taking ammonia risk assessment as an example, comprising the following specific steps:

[0046] Model establishment and calculation domain setting: The model is built according to the overall layout of the ship and the location of the ammonia fuel emergency discharge pipe, such as Figure 2 As shown, it includes an ammonia fuel emergency exhaust pipe 1, a superstructure 2, an ammonia concentration monitoring point 3, a hull 4 and a container stack 5. There are a total of 10 ammonia concentration monitoring points, which are arranged in front of the superstructure. The numerical simulation adopts a real-scale ship, and the calculation domain size is set according to the length of the ship. The calculation domain size is: -5.5 times the ship length ≤ x ≤ 4.5 times the ship length, -3.5 times the ship length ≤ y ≤ 3.5 times the ship length, and 0 ≤ z ≤ 4.5 times the ship length.

[0047] Gas composition and water mist assumptions: The calculation domain is a unidirectional flow of multi-component gases, including ammonia and oxygen, nitrogen, etc. in the air. Considering that the components of the air do not react with ammonia, the air is assumed to be a single-component gas to simulate the diffusion range of emergency-emitted ammonia in the air. The ammonia concentration is captured by the ammonia content in each calculation grid.

[0048] When low-temperature ammonia is discharged from the liquid ammonia storage tank and meets humid air, it will form water mist and scatter on the hull or container stack 5. The water mist hypothesis simulation mainly focuses on the range of water mist scattering, and does not focus on the process of ammonia and humid air combination. The Lagrange method is used to simulate the size of the formed water mist particles and visualize the possible location of water mist scattering. The water mist is mainly affected by the disturbance of the flow field and gravity, and scatters on the hull or container. The translation and rotation motion equations of water mist are as follows:

[0049]

[0050]

[0051] In the formula, m i 、vi and ω i denote the mass, velocity and angular velocity of the water mist i, respectively. i is the moment of inertia of the water mist i, F g is the gravity of the water mist i, F fluid is the force exerted by air on the crushed ice particle i, which includes drag and lift, etc., T ij is the torque on the water mist, including gravity and the torque of air on the water mist.

[0052] Physical model settings: The turbulence model uses the SST k-ω model, implicit unsteady simulation is used, the Lagrange multinomial flow model is used to simulate water mist, and the diameter of the water mist droplets uses the Nukiyama-Tanazawa distribution. The specific calculation formula is:

[0053]

[0054] in, It is an average size.

[0055] The multi-component gas model is used to simulate air and ammonia. The scalar diffusion rate of Fick's law of multi-component gas diffusion is replaced by a matrix, and the diffusion flux of component i becomes a function of the mass fractions of all components:

[0056]

[0057] Where D i,j represents the multicomponent diffusion coefficient calculated using the Maxwell-Stefan equation, Y j is the mass fraction of component j in the mixture.

[0058] The Fluid Film model is used to simulate a liquid film formed when water mist falls on the hull or container. The FluidFilm model solves the transport equations for mass, momentum, energy, species, and volume fraction. The model assumes that the film is thin enough to apply the laminar boundary layer approximation. The Fluid Film model continuity equation is as follows:

[0059]

[0060] In the formula, ρ f is the film density, is the film velocity, and the subscript f indicates the fluid film value. u is the mass source / sink per unit area. Contributions to the mass source come from droplet impact, film exfoliation, mass transfer, or user-defined sources and are used to calculate the film thickness h f , the volume V and the surface A are functions of the film thickness and its spatial distribution. The momentum conservation equation of the FluidFilm model is as follows:

[0061]

[0062] In the formula, is the mass source S u The momentum source, ρ f It's pressure. is the body force vector, is the viscous stress tensor in the film. The energy conservation equation of the Fluid Film model is as follows:

[0063]

[0064] In the formula, E f is the total film energy, H f is the total enthalpy of the film, is the film heat flux, is the film viscous stress tensor, is the physical force per unit volume, s E is the energy source term per unit film area, with contributions coming from droplet impact, film debonding, evaporation, condensation, or user-defined sources.

[0065] The water mist-liquid film conversion model determines the thickness of the liquid film formed on the wall by the droplets impacting the wall. The equivalent film thickness calculation formula is as follows:

[0066]

[0067] Where V d is the total volume of all droplets on the cell wall, A c The area covered by the water mist.

[0068] Meshing: Hexahedral meshes are used to discretize the computational domain, and boundary layer meshes are used to simulate the hull boundary layer. The real-scale simulation ensures that the wall Y+>200, so that the boundary layer mesh can be better transitioned to the volume mesh, and the volume mesh is encrypted in the ammonia diffusion range, so as to more accurately simulate the ammonia diffusion.

[0069] Boundary conditions and solver parameter settings: Boundary conditions and initial conditions are set according to the ship's operating conditions. The solid wall where the ammonia mist is scattered is set as the Fluid-Film boundary to capture the liquid film thickness of the wall. Other walls are set as wall boundaries, the outlet is set as the pressure outlet, the calculation domain inlet is set as the velocity inlet, the ammonia fuel emergency discharge pipe is set to a relative pressure, and the solver parameters are set including the stop condition, the maximum number of internal iterations, and the time step.

[0070] CFD simulation calculation and visualization processing: evaluate the ammonia mist dispersion range. The ammonia mist dispersion range is visualized and simulated by Lagrange method to determine the ammonia mist dispersion area, such as Figure 3 As shown, the range of ammonia mist scattering and the thickness of the liquid film can be seen. The ammonia concentration near the superstructure and other locations that need attention are monitored. The ammonia concentration is measured by calculating the mass proportion of ammonia in the air at each ammonia concentration monitoring point 3. The ammonia concentration curves of the 10 ammonia concentration monitoring points 3 in front of the superstructure are shown in Figure 4 As shown, according to the ammonia toxicity evaluation standard of Lloyd's Register "Guidance Notes for Fuel System Risk Assessments, Hazard Identification-Hydrogen and Ammonia", the ammonia concentrations at the 10 monitoring points are all less than 20ppm, indicating that the front of the superstructure is safe. After post-processing, the following is obtained: Figure 5 The ammonia concentration cloud map shown can be used to monitor ammonia diffusion and concentration and determine the location of water mist scattering, thereby determining the hazard zone and conducting risk assessment.

[0071] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art may make various modifications or equivalent substitutions to the present invention within the essence and protection scope of the present invention, and such modifications or equivalent substitutions should also be deemed to fall within the protection scope of the technical solution of the present invention.

Claims

1. A hazardous gas risk assessment method based on CFD numerical simulation, characterized in that: The steps include: Model establishment and calculation domain setting: The model is built according to the overall layout of the ship and the location of the emergency discharge pipe of ammonia fuel. The numerical simulation uses a full-scale ship and the calculation domain size is set according to the length of the ship; Assumptions of gas composition and water mist: Assume air as a single-component gas to simulate the diffusion range of emergency-emitted ammonia in the air. The ammonia concentration is captured by the ammonia content in each calculation grid. When low-temperature ammonia is discharged from the liquid ammonia storage tank and encounters humid air, it will form water mist and scatter onto the hull or container. The Lagrange method is used to simulate and visualize the possible locations where the water mist may scatter. Physical model settings: The SST k-ω model is used as the turbulence model, implicit unsteady simulation is used, the Lagrange multinomial flow model is used to simulate water mist, the diameter of the water mist droplets is Nukiyama-Tanazawa distribution, the gas uses a multi-component gas model to simulate air and ammonia, the Fluid Film model is used to simulate a layer of liquid film formed when water mist falls on the hull or container, and the water mist-liquid film conversion model determines the thickness of the liquid film formed on the wall by the droplets impacting the wall; Meshing: Hexahedral meshes are used to discretize the computational domain, boundary layer meshes are used to simulate the hull boundary layer, full-scale simulations are performed to ensure that the wall Y+>200, and the volume mesh is encrypted for the ammonia diffusion range; Boundary conditions and solver parameter settings: Boundary conditions and initial conditions are set according to the ship operation conditions, and the solid wall where the ammonia mist is scattered is set as the Fluid-Film boundary to capture the liquid film thickness on the wall; CFD simulation calculation and visualization processing: evaluate the ammonia mist dispersion range. The ammonia mist dispersion range is visualized through the Lagrange method to determine the ammonia mist dispersion area, and monitor the ammonia concentration near the superstructure and other locations that require attention, and conduct risk assessment based on the ammonia toxicity evaluation standard.

2. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The size of the calculation domain is: -5.5 times the ship's length ≤ x ≤ 4.5 times the ship's length, -3.5 times the ship's length ≤ y ≤ 3.5 times the ship's length, 0 ≤ z ≤ 4.5 times the ship's length.

3. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The water mist hypothesis simulation focuses on the range of water mist dispersion, not the process of ammonia and humid air combination. The Lagrange method is used to simulate the size of the formed water mist particles and visualize the possible location of water mist dispersion. The water mist is mainly affected by the disturbance of the flow field and gravity, and is scattered on the hull or container. The translation and rotation motion equations of water mist are as follows: In the formula, m i 、v i and ω i Respectively represent the mass, velocity and angular velocity of water mist i, I i is the moment of inertia of the water mist i, F g is the gravity of the water mist i, F fluid is the force exerted by air on ice particle i, T ij is the torque on the water mist, including gravity and the torque of air on the water mist.

4. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The specific calculation formula of the diameter of the water mist droplets using the Nukiyama-Tanazawa distribution is: in, It is an average size.

5. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The gas adopts a multi-component gas model to simulate air and ammonia. The scalar diffusion rate of Fick's law of multi-component gas diffusion is replaced by a matrix, and the diffusion flux of component i becomes a function of the mass fractions of all components: Where D i,j represents the multicomponent diffusion coefficient calculated using the Maxwell-Stefan equation, Y j is the mass fraction of component j in the mixture.

6. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The continuity equation of the Fluid Film model is as follows: In the formula, ρ f is the film density, is the film velocity, the subscript f represents the fluid film value, and the quantity S u is the mass source / sink per unit area. Contributions to the mass source come from droplet impact, film peeling, mass transfer, or user-defined sources. This formula is used to calculate the film thickness h f , the volume V and the surface A are functions of the film thickness and its spatial distribution, The momentum conservation equation of the FluidFilm model is as follows: In the formula, is the mass source S u The momentum source, ρ f It's pressure. is the body force vector, is the viscous stress tensor in the membrane, The energy conservation equation of the Fluid Film model is as follows: In the formula, E f is the total film energy, H f is the total enthalpy of the film, is the film heat flux, is the film viscous stress tensor, is the physical force per unit volume, s E is the energy source term per unit film area, with contributions coming from droplet impact, film debonding, evaporation, condensation, or user-defined sources.

7. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The water mist-liquid film conversion model determines the thickness of the liquid film formed on the wall by the droplets impacting the wall. The equivalent film thickness calculation formula is as follows: Where V d is the total volume of all droplets on the cell wall, A c The area covered by the water mist.

8. The method for risk assessment of hazardous gases based on CFD numerical simulation according to claim 1, characterized in that: The boundary condition settings include setting the outlet as a pressure outlet, setting the computational domain inlet as a velocity inlet, setting the ammonia fuel emergency discharge pipe outlet to a relative pressure, and setting various solver parameters including stop conditions, maximum number of internal iterations, and time step.

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