A numerical direct calculation method for flame length of hydrogen-enriched natural gas jet fire based on numerical simulation

Abstract

The application provides a numerical simulation-based direct calculation method for hydrogen-doped natural gas jet fire length. The method can quickly calculate the numerical value of the hydrogen-doped natural gas jet fire length, and mainly includes four steps: step one, establishing a hydrogen-doped natural gas jet fire length numerical simulation model; step two, solving the jet fire length numerical simulation model and post-processing the simulation results; step three, analyzing the correlation between the hydrogen-doped natural gas jet fire length influencing factors and the fire length; and step four, based on the numerical simulation results, selecting the related factors, and using the linear regression method to fit the relationship between the parameter change and the fire length change. The step flow of the method is shown in the abstract drawing.

Description

Technical Field

[0001] This invention patent belongs to the field of large-size jet fire, especially the direct calculation method of jet fire flame length under the condition of natural gas with hydrogen. Background Technology

[0002] Hydrogen, as one of the most promising energy sources under the background of new energy development, boasts advantages such as abundant reserves, cleanliness, environmental friendliness, and excellent combustion characteristics. Natural gas, on the other hand, is one of the world's most important energy sources, widely used in various application scenarios and aspects of national economic production and daily life. Therefore, mixing hydrogen and natural gas not only offers significant economic benefits and development prospects in transportation, but also shows promising application potential in the combustion characteristics of internal combustion engines and micro-power units. In terms of combustion behavior, adding hydrogen to natural gas for combustion can effectively reduce smoke and improve combustion stability.

[0003] Mixing hydrogen and natural gas inevitably leads to other issues, the most significant being its impact on combustion characteristics. Different mixing ratios of the two gases, varying pressures, different ambient wind conditions, and different nozzle diameters will all result in variations in the flame length of the jet flame. The hydrogen blending ratio, pressure, wind speed, and nozzle size are also crucial factors influencing the flame length.

[0004] Currently, the main methods for studying jet fire flame models are experiments and numerical simulations. With the development of computational fluid dynamics, CFD (Computational Fluid Dynamics) software is increasingly used to study jet fires, overcoming some limitations of traditional classical theories and avoiding the problems of high cost and operational difficulty in jet fire experiments.

[0005] The above analysis shows that we need a numerical calculation method applicable to hydrogen-blended natural gas injection fires to ensure that the injection fires can be predicted quickly and accurately under different conditions, and that the numerical results are approximately equal to those obtained from computer simulations.

[0006] This patent employs numerical simulation, using CFD to model the jet fire of hydrogen-blended natural gas. Fluent software numerical simulation predicts the flow field by solving the Navier-Stokes equations. Based on this, a non-premixed combustion model of hydrogen and natural gas is established, which can effectively simulate the temperature field of the jet fire. Fitting the simulation results yields a functional relationship, which can accurately predict the jet fire length under different conditions. Summary of the Invention

[0007] This invention proposes a direct calculation method for the length of jet fire flames based on numerical simulation, in order to achieve rapid prediction.

[0008] To address the aforementioned technical problems, this invention provides a method for directly calculating the flame length of a hydrogen-blended natural gas jet fire based on numerical simulation. First, the Fluent simulation software is used to solve for the numerical simulation model of the hydrogen-blended natural gas jet fire. Second, based on the numerical simulation results, a linear regression method is used to fit the functional relationship between the jet fire flame length and its influencing factors. The specific scheme is as follows: (1) Establish a numerical simulation model of the flame length of hydrogen-doped natural gas jet fire. A two-dimensional geometric plane model with a length of 60 m and a width of 50 m is established, with the injection nozzle located on the left boundary at a distance of 30 m from the bottom edge, as the geometric model of the jet fire flame area. Three different geometric models are established based on three different injection nozzle sizes (1 mm, 2 mm, and 3 mm). The geometric model was meshed. The pressure near the nozzle was relatively high, and the mesh required high precision. In order to better simulate the shape of the flame, after the boundary layer was meshed, a fine mesh was made near the nozzle with a mesh size of 1 mm × 1 mm, while a coarse mesh was made in the area far from the nozzle with a mesh number of 306,650. The parameters and boundary conditions for the numerical simulation model of the flame length of hydrogen-blended natural gas injection fire were set. The k-ω viscosity model was used for the viscosity model, and the combustion model of the hydrogen-blended natural gas was a non-premixed combustion model. The hydrogen blending ratio was a mole fraction. Three different non-premixed gases were generated based on different hydrogen blending ratios, with three values ​​set: 10%, 20%, and 24%. Different boundary conditions were set for each, and two different values ​​for the injection nozzle pressure (30 MPa and 40 MPa). Then, the Bernoulli equation for compressible gases and the ideal gas law were applied. The pressure inlet is converted to a velocity inlet, and the model outlet is set to a pressure outlet. The ambient wind speed is set with three values ​​for two scenarios: no wind (0 m / s) and windy conditions (1 m / s, 2 m / s). The pressure-velocity coupling method uses the Coupled method, and the pressure discretization uses the Second Order method. The setup is then completed with mixed initialization. (2) Solve the numerical simulation model of the jet fire flame length and perform post-processing on the simulation results. Use the Fluent solver to solve the numerical simulation model of the jet fire with hydrogen-doped natural gas. Monitor the outlet pressure during the solution process, and perform post-processing after the calculation converges. Call the temperature field values ​​of the jet fire, take the geometric center line perpendicular to the nozzle as the flame axis, create a path with the flame axis as the standard, extract the temperature values ​​of all points on the path and the corresponding distance values ​​between each point and the nozzle, remove points with temperature values ​​less than 1000K, and process the remaining points. Sort the points by distance, and calculate the difference between the distance of the point farthest from the nozzle and the distance of the point closest to the nozzle to obtain the flame length under this condition. If the flame deviates due to external wind, and the straight line is not perpendicular to the nozzle, extract the line with the largest difference as an approximation of the flame axis, and calculate the difference using the above method to obtain the flame length value. (3) Correlation analysis between factors affecting the flame length of hydrogen-blended natural gas injection fire and flame length. Based on the numerical simulation results, the correlation coefficients between the factors affecting the flame length of hydrogen-blended natural gas injection fire and the flame length were calculated to analyze their correlation. The numerical simulation results show that: the hydrogen doping ratio is negatively correlated with the flame length, the injection nozzle pressure is positively correlated with the flame length, the injection nozzle size is linearly positively correlated with the flame length, and the wind speed is positively correlated with the flame length; (4) Based on the numerical simulation results, relevant influencing factors were selected, and the linear regression method was used to fit the functional relationship between the flame length of the jet fire and these influencing factors. The functional relationship between the flame length of the jet fire and the hydrogen ratio of the hydrogen-blended natural gas, the injection nozzle pressure, the injection nozzle size, and the wind speed was obtained. The specific calculation formula is as follows: L =-6.5025 α 1.02 + 0.0196 P 1.19 + 5.4108 β + 0.4005 v 0.95 (1); In the formula, L Flame length, in meters (m). α The hydrogen doping ratio; P The pressure at the injection nozzle is expressed in MPa. β The nozzle size is in mm. v The ambient wind speed is expressed in m / s.

[0009] Numerical simulation requires a certain level of computer precision and involves a large amount of computation and is time-consuming. Unlike current numerical calculation methods, this invention provides a direct calculation method for the flame length of hydrogen-blended natural gas jet fire based on numerical simulation. This method not only ensures the accuracy of the calculated values ​​but also improves the computational efficiency and minimizes the cost of numerical calculation. Attached Figure Description

[0010] Figure 1 is a flowchart of the direct calculation method for the flame length of hydrogen-doped natural gas jet fire; Figure 2 is a comparison between the numerical simulation results and the fitted values ​​of the flame length of the hydrogen-doped natural gas jet fire. Detailed Implementation Plan

[0011] The present invention will be further described below with reference to examples and accompanying drawings, so that the advantages, features and application steps of the present invention can be easily understood and accepted by those skilled in the art, thereby making a clearer definition of the scope of protection of the present invention.

[0012] A direct calculation method for the flame length of a hydrogen-doped natural gas injection fire based on numerical simulation mainly includes the following steps: Step 1: Establish a numerical simulation model of the flame length of the hydrogen-blended natural gas jet fire. A two-dimensional planar geometric model, 60 m long and 50 m wide, is established as the geometric model of the jet fire flame region. The nozzle is located on the left boundary, 30 m from the bottom edge. Based on the numerical values ​​of three different nozzle sizes (1 mm, ... Three different geometric models were established (2mm, 3mm). Mesh generation was performed on these models. Due to the higher pressure near the nozzle and the need for higher flame precision, a fine mesh (1mm × 1mm) was applied near the nozzle after boundary layer meshing to better simulate the flame morphology, while a coarser mesh was used further away. The geometric models were then imported into Fluent for numerical simulation. A k-ω viscosity model was used. A non-premixed combustion model was employed for hydrogen and natural gas, with the natural gas set as pure methane. Three different non-premixed gases were generated based on different hydrogen doping ratios, with the hydrogen doping ratio expressed as a mole fraction (10%, 20%, 24%). The pressure-velocity coupling method was the Coupled method, and the pressure discretization method was the Second Order method.

[0013] Step 2: Solve the numerical simulation model of the jet fire flame length and post-process the simulation results. Use the Fluent solver to solve the numerical simulation model of the jet fire with hydrogen-doped natural gas. Process the simulation results and record the calculated flame temperature values ​​for each operating condition. For each operating condition, use the geometric center line perpendicular to the nozzle as the flame axis. Create a standard path using the geometric center line as the flame axis, extract the temperature values ​​of all points on the path, and the corresponding distances from each point to the nozzle. Given that relevant research indicates the temperature threshold at the flame edge is approximately 1000K, remove points with temperatures less than 1000K. Then, sort the remaining points by distance, and calculate the difference between the distance to the farthest point and the distance to the nearest point to obtain the jet fire flame length for that operating condition. In cases where external wind causes the jet fire to deviate, the straight line is not perpendicular to the nozzle; the line with the largest difference is approximated as the flame axis. The error of this differential method is much less than 10%, and the obtained value is used as the reference jet fire flame length value.

[0014] Step 3: Correlation analysis between factors influencing the flame length of hydrogen-blended natural gas injection fire and the flame length. Based on the numerical simulation results, the correlation coefficients between the factors influencing the hydrogen-blended natural gas injection fire and the flame length are calculated to analyze their correlation. The ratio of the covariance of each influencing factor and the flame length to the product of the independent and dependent variables is calculated. A value greater than 0 indicates a positive correlation; greater than 0.3 and less than 0.7 indicates a linear positive correlation; greater than 0.7 indicates a strong linear positive correlation; and less than 0 indicates a negative correlation.

[0015] Step four: Based on the numerical simulation results and the judgment principles of the above correlation analysis, four correlated influencing factors are selected, resulting in four independent variables: hydrogen blending ratio of hydrogen-blended natural gas, injection port pressure, The nozzle size and wind speed were used as the four independent and dependent variables. A linear regression method was applied to fit the functional relationship between the jet fire flame length and these influencing factors, as shown in Formula 2.

[0016] L =-6.5025 α 1.02 + 0.0196 P 1.19 + 5.4108 β + 0.4005 v 0.95 (2) In the formula, L Flame length, in meters (m). α The hydrogen doping ratio; P The pressure at the injection nozzle is expressed in MPa. β The nozzle size is in mm. v The ambient wind speed is expressed in m / s.

[0017] The following section describes the calculation and evaluation of the jet flame length using the method described in this invention, based on an experimental condition: The first step, following the method described in Step 1, is to establish a numerical simulation model of the flame length of a hydrogen-blended natural gas injection fire. The region is 60m long and 50m wide, with the injection nozzle located 30m from the bottom on the left boundary, and measuring 1mm in size. This yields the geometric model for this condition. Boundary conditions are set as follows: hydrogen blending ratio of the hydrogen-blended natural gas is 10%; injection nozzle pressure is 30MPa; wind speed is 0m / s. The second step is to solve the simulation model of the jet fire flame length under this working condition according to the method described in step two, and process the simulation results to obtain the flame length value of the hydrogen-blended natural gas jet fire. The third step, following the method described in step three, is to calculate the correlation coefficients between the influencing factors of hydrogen-blended natural gas injection fire and the flame length. The correlation coefficient for hydrogen blending ratio is -10%, for injection nozzle pressure it is 4%, for injection nozzle size it is 99%, and for wind speed it is 8%. From these correlation coefficients, it can be seen that: hydrogen blending ratio is negatively correlated with flame length; injection nozzle pressure is positively correlated with flame length; injection nozzle size is strongly linearly positively correlated with flame length; and wind speed is positively correlated with flame length. Fourth, following the method described in step four, these four variables are correlated with the flame length of the hydrogen-blended natural gas injection fire. Using linear regression, a functional relationship is obtained between the flame length of the hydrogen-blended natural gas injection fire and the hydrogen blending ratio, injection nozzle pressure, injection nozzle size, and wind speed. This method can directly predict the numerical value of the flame length of the hydrogen-blended natural gas injection fire.

Claims

1. A method for direct numerical calculation of flame length in hydrogen-doped natural gas injection fires based on numerical simulation, characterized in that: This calculation method comprehensively considers the effects of hydrogen blending ratio, injection nozzle pressure, injection nozzle size, and ambient wind speed on the flame length of the jet fire, and obtains the relationship between the flame length and the above four main influencing factors. It is applicable to the calculation of the flame length of jet fires made from hydrogen-blended natural gas. The method mainly includes the following steps: Step 1: Establish a numerical simulation model for the flame length of a hydrogen-blended natural gas jet fire. A two-dimensional planar geometric model, 60 m long and 50 m wide, is established as the geometric model of the jet fire flame combustion region. The injection nozzle is located on the left boundary, 30 m from the bottom edge. Three different geometric models are established based on three different nozzle sizes: 1 mm, 2 mm, and 3 mm. The geometric model is then meshed. Due to the higher pressure near the nozzle, a high mesh precision is required. A precise mesh (1 mm × 1 mm) is used near the nozzle, while a coarser mesh is used further away. Ten boundary layers are set, resulting in a total of 306,650 meshes. The parameters and boundary conditions of the numerical simulation model for the flame length of the hydrogen-blended natural gas jet fire are set. The viscosity model uses a k-ω viscosity model, and the combustion model of the hydrogen-blended natural gas uses a non-premixed combustion model. The hydrogen doping ratio is a mole fraction ratio. Three different non-premixed gases are generated based on different hydrogen doping ratios, with three values ​​set: 10%, 20%, etc. 24%; different boundary conditions were set, with two different values ​​for the injection nozzle pressure: 30MPa and 40MPa. The pressure inlet was converted into a velocity inlet, and the model outlet was set as a pressure outlet. The ambient wind speed was set to three values: 0m / s in the windless state and 1m / s and 2m / s in the windy state. The pressure-velocity coupling method adopted the Coupled method, and the pressure discretization adopted the Second Order method, followed by hybrid initialization. Step 2: Solve the numerical simulation model of the jet fire flame length and post-process the simulation results; use the Fluent solver to solve the numerical simulation model of the hydrogen-blended natural gas jet fire flame length, monitor the outlet pressure during the solution process, and post-process the simulation results after convergence; call the temperature field values ​​of the jet fire, take the geometric center line perpendicular to the nozzle as the flame axis, create a standard path with the geometric center line as the flame axis, extract the temperature values ​​of all points on the path and the corresponding distance values ​​between each point and the nozzle, remove points with temperature values ​​less than 1000K, sort the remaining points by distance, and calculate the difference between the distance of the farthest point from the nozzle and the distance of the nearest point to obtain the jet fire flame length under this condition; for cases where the jet fire is deflected due to external wind, the straight line with the largest difference is approximated as the flame axis of the jet fire, and the jet fire flame length value is obtained using the above method; Step 3: Correlation analysis of factors affecting flame length and flame length in hydrogen-blended natural gas injection fire; based on The numerical simulation results were analyzed by calculating the correlation coefficient between the influencing factors of hydrogen-blended natural gas injection fire and the flame length. The numerical simulation results showed that the hydrogen blending ratio was negatively correlated with the flame length, the injection nozzle pressure was positively correlated with the flame length, the injection nozzle size was linearly positively correlated with the flame length, and the ambient wind speed was positively correlated with the flame length. Step four: Based on the numerical simulation results, relevant influencing factors are selected, and the linear regression method is used to fit the functional relationship between the flame length of the jet fire and the influencing factors. The functional relationship between the flame length of the jet fire and the hydrogen blending ratio of the hydrogen-blended natural gas, the injection nozzle pressure, the injection nozzle size, and the wind speed is obtained. The specific calculation formula is as follows: L =-6.5025 α 1.02 + 0.0196 P 1.19 + 5.4108 β + 0.4005 v 0.95； In the formula, L Flame length, in meters (m). α The hydrogen doping ratio; P The pressure at the injection nozzle is expressed in MPa. β The nozzle size is in mm. v The ambient wind speed is expressed in m / s.

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