Gas diffusion layer reconstruction and two-phase flow model combination method considering binder structure

Abstract

The invention discloses a gas diffusion layer reconstruction and two-phase flow model combination method considering a binder structure, which comprises the following steps of: establishing a gas diffusion layer pure fiber structure by utilizing a random reconstruction method, reconstructing the binder structure by adopting a morphological closed operation algorithm, and regulating and controlling the size of a structure operator to obtain a two-phase flow model. The effective control on the structural volume ratio and the porosity of the binder is realized. A two-phase flow model is established based on reconstruction of a gas diffusion layer with a real structure, so that the flow process of liquid water in the gas diffusion layer is solved and calculated. According to the method, the pore structure of the gas diffusion layer of the fuel cell can be reconstructed more accurately, and structural parameter changes of the gas diffusion layer caused by the existence of the binder, including changes of key parameters such as porosity, pore diameter and tortuosity, can be obtained. The characteristics of the two-phase flow process in the gas diffusion layer can be visually, rapidly and accurately obtained, the relation between the structure of the gas diffusion layer and the two-phase flow process is defined, and the structure of the gas diffusion layer is optimized.

Description

Technical Field

[0001] The present invention belongs to the technical field of fuel cells, and particularly relates to a method for reconstructing a gas diffusion layer based on morphological closing operation, aiming to achieve high-precision simulation of the complex two-phase flow process inside the gas diffusion layer. Background Art

[0002] A proton exchange membrane fuel cell is a device that uses hydrogen as fuel and directly converts chemical energy into electrical energy through an electrochemical reaction. Proton exchange membrane fuel cells are widely used in fields such as transportation, portable power sources, backup power sources, and distributed generation. Due to their characteristics such as high efficiency, low emissions, and rapid response, they have become one of the important technologies for future clean energy systems.

[0003] The gas diffusion layer (GDL) is a key component in fuel cells. Its main functions are to transport reaction gases (such as oxygen) and liquid water, and at the same time conduct heat and electrons. Whether the liquid water inside the gas diffusion layer can be discharged smoothly will directly affect the performance of the proton exchange membrane fuel cell, because water flooding will occur if the liquid water is not discharged in time. Through reasonable design and parameter optimization of the fuel cell structure, it is very important to promote the transport of liquid water inside the gas diffusion layer and improve the performance of the proton exchange membrane fuel cell.

[0004] Simulation modeling can predict the liquid water transport process inside the gas diffusion layer at low cost, so as to optimize its structure and parameters. Although the current simulation models can successfully reconstruct the fiber structure of the gas diffusion layer, they ignore the potential impact of the binder structure on the performance of the gas diffusion layer (CN115017741B). In fact, the existence of the binder structure will significantly change key microstructure parameters such as the porosity, pore size distribution, and tortuosity of the gas diffusion layer, and thus affect the liquid water transport process. Therefore, the method proposed in the present invention, which combines the reconstruction of the gas diffusion layer considering the binder structure with the two-phase flow model, can improve the accuracy of the two-phase flow simulation results of the gas diffusion layer and lay a solid foundation for the optimal design of the gas diffusion layer. Summary of the Invention

[0005] In order to effectively solve the technical problem of the influence of the binder structure on the performance of the gas diffusion layer, the purpose of the present invention is to propose a method that comprehensively considers the combination of the reconstruction of the gas diffusion layer and the two-phase flow model considering the binder structure. Through the following steps, the accurate reconstruction of the real pore structure of the gas diffusion layer is realized, and the model simulation accuracy is significantly improved, so as to provide strong support for the optimization of the fuel cell water management process. The present invention adopts the following technical solutions:

[0006] A method for combining the reconstruction of the gas diffusion layer considering the binder structure and the two-phase flow model is proposed. A pure fiber structure of the gas diffusion layer is established by the random reconstruction method. The binder structure is reconstructed by using the morphological closing operation algorithm. By adjusting the size of the structural operator, the volume ratio and porosity of the binder structure can be effectively controlled. Based on the reconstructed real-structure gas diffusion layer, a two-phase flow model is established to solve and calculate the flow process of liquid water inside the gas diffusion layer.

[0007] Based on the random reconstruction method to reconstruct the fiber structure, a rectangular spatial region with length×width×height is selected, and then it is divided into uniform structural grids. Two points A(x1, y1, z1) and B(x2, y2, z2) are randomly selected within the rectangular spatial region, where y1 = y2, thus forming a rectangular region in the x-z plane.

[0008] Then, multiple cylinders are generated in the x-z plane. After the porosity of this layer reaches the set value, the next layer of grids is generated, and they are stacked layer by layer until the thickness requirement is met. Finally, all the grids occupied by the cylindrical fibers are marked.

[0009] Furthermore, based on the morphological closing operation algorithm to reconstruct the binder structure, the following calculations are carried out:

[0010] According to the preset porosity and the binder ratio, the radius d of the structural operator is selected. The structural operator radius is disk-shaped, and the number of grids N included in a structural operator is determined, where the binder ratio = binder volume / (fiber volume + binder volume).

[0011] Dilation operation: Traverse the fiber grids. Taking the center point coordinates (x0, y0, z0) of each grid as the origin of the structural operator, calculate the number of fiber grids n within the operator range. If the surrounding fiber grids cannot form a complete structural operator, i.e., n < N, then fiber grids are added to the vacant positions.

[0012] Erosion operation: Based on the dilated grids, traverse each grid. Taking the coordinates (x0', y0', z0') of this grid as the origin of the operator, calculate the number of fiber grids n' within the operator range. If the surrounding fiber grids cannot form a complete structural operator, i.e., n' < N, then the grid with the origin coordinates (x0', y0', z0') is deleted.

[0013] Furthermore, a two-phase flow model is established:

[0014] The gas diffusion layer structure considering fibers and binders is imported into the two-phase flow model to solve the flow process of liquid water in the gas diffusion layer, including: setting boundary conditions: inputting the continuity equation, momentum equation, and phase transport equation into the two-phase flow model. After the two-phase flow model is established, finally, the gas diffusion layer structure is imported, and the distribution of liquid water in the gas diffusion layer is output.

[0015] The features and advantages of the present invention are as follows:

[0016] The method of combining the reconstruction of the gas diffusion layer considering the binder structure and the two-phase flow model can comprehensively and deeply consider the influence of the binder structure on the microstructure of the gas diffusion layer, and fundamentally improve the accuracy and reliability of the two-phase flow simulation results of the gas diffusion layer. Compared with the current traditional research methods, the method of the present invention helps to intuitively, quickly and accurately obtain the characteristics of the two-phase flow process in the gas diffusion layer, clarifies the relationship between its structure and the two-phase flow process, and optimizes the structure of the gas diffusion layer. Brief Description of the Drawings

[0017] Appendix Figure 1 It is a schematic block diagram of the principle steps of the present invention.

[0018] Appendix Figure 2 It is a schematic diagram of a fiber structure slice in the example calculation. The black part in the figure is carbon fiber.

[0019] Appendix Figure 3 It is a schematic diagram of a fiber and binder structure slice in the example calculation.

[0020] Appendix Figure 4 It is a schematic diagram of the calculation domain in the example calculation.

[0021] Appendix Figure 5 It is the liquid water cloud diagram output in the example calculation.

[0022] Appendix Figure 6 It is a schematic diagram of the comparison result of the liquid water saturation distribution of the method of combining the reconstruction of the gas diffusion layer considering the binder structure and the two-phase flow model and that obtained by the X-ray technique in the example calculation. Detailed Description of the Preferred Embodiments

[0023] The design scheme of the method of the present invention will be described in detail below with reference to the drawings and through examples. It should be noted that this example is narrative rather than restrictive, and does not limit the protection scope of the present invention.

[0024] As shown in the appendix Figure 1 The overall step principle of the present invention is as follows: First, generate a fiber structure based on the random reconstruction method; then select a structure operator and determine the number of grids included in the structure operator; then perform dilation operation and erosion operation to obtain the binder structure; finally, establish a two-phase flow model, import the structure into the model, and output the distribution of liquid water in the gas diffusion layer.

[0025] The specific embodiments of the method of combining the reconstruction of the gas diffusion layer considering the binder structure and the two-phase flow model are as follows:

[0026] 1. Reconstruct the fiber structure based on the random reconstruction method:

[0027] In a rectangular spatial region with length × width × height, it is divided into a uniform structural grid; two points A(x1, y1, z1) and B(x2, y2, z2) are randomly selected within the rectangular spatial region, where y1 = y2, thus forming a rectangular region in the x-z plane.

[0028] (1) Select a rectangular region with length L = 200 μm and width W = 200 μm, and then divide it into a structural grid with a size of 1 μm × 1 μm × 1 μm.

[0029] (2) Use the following formulas to randomly select two points A(x1, y1, z1) and B(x2, y2, z2) in the x-z plane.

[0030] x1 = L·Rand_x+(L + W)(1 - 1)

[0031] x2 = L·Rand_x-(L + W)(1 - 2)

[0032]

[0033] z1 = (L + W)tan(π·Rand_θ)+W·Rand_z(1 - 5)

[0034] z2 = -(L + W)tan(π·Rand_θ)+W·Rand_z(1 - 6)

[0035] Where Rand_x, Rand_z, and Rand_θ are all numerical values generated by random functions for calculating coordinates, ranging from 0 to 1, and i represents the number of carbon fiber layers. Connect A and B into a straight line, and with this straight line as the axis, generate a cylinder with a determined radius D = 4 μm (this radius is the carbon fiber radius).

[0036] (3) Generate multiple cylinders in this plane. When the porosity of this layer reaches the set value of 0.86, generate the next layer of grid, and stack layer by layer until 24 layers are reached, meeting the requirement of a thickness of 192 μm.

[0037] (4) Finally, mark all the grids occupied by the cylindrical fibers.

[0038] 2. Based on the above steps, the fiber structure of the gas diffusion layer is generated, and a pure fiber structure with a porosity of 0.86 and a fiber diameter of 8 μm is obtained. The generated result is sliced horizontally as shown in the appendix Figure 2 As shown, the black part in the figure is the carbon fiber.

[0039] 3. Reconstruct the binder structure based on the morphological closing operation algorithm:

[0040] (1) Select the radius d = 10 μm (10 times the grid size) of the disk-shaped structure operator according to the preset porosity of 0.78 and the binder ratio of 39%, and determine that the number of grids N = 324 included in one structure operator.

[0041] (2) Dilation operation: Traverse the fiber grids, take the coordinates (x0, y0, z0) of the center point of each grid as the origin of the structure operator, and calculate the number of fiber grids n within the operator range (y = y0 and ). If the surrounding fiber grids cannot form a complete structure operator (n < N), then supplement fiber grids to the vacant positions;

[0042] (3) Erosion operation: On the basis of the dilated grids, traverse each grid, take the coordinates (x0', y0', z0') of this grid as the operator origin, and calculate the number of fiber grids n' within the operator range (y = y0 and ). If the surrounding fiber grids cannot form a complete structure operator (n' < N), then delete the fiber grid where the origin coordinates (x0', y0', z0') are located.

[0043] 4. Based on the above steps, obtain a gas diffusion layer structure containing fibers and binders. The porosity of this structure is 0.76 and the binder ratio is 29.4%. The generated horizontal slice diagram of the result is as shown in the appendix Figure 3 . The black part in the figure is the fiber and binder structure, and it can be observed that Figure 3 There are obvious differences from Figure 2 , especially at the positions where the fibers cross.

[0044] 5. Establish a two-phase flow model, import the gas diffusion layer structure considering fibers and binders into the model, and solve the flow process of liquid water in the gas diffusion layer.

[0045] Furthermore, the conservation equations in the two-phase flow model include:

[0046] (1) Continuity equation:

[0047]

[0048] Among them (m / s) is the velocity vector shared by the two fluids in the entire fluid domain, represents the divergence operator.

[0049] (2) Momentum equation:

[0050]

[0051] Among them, ρ (kg / m3) is the density, μ (Pa·s) is the viscosity, α is the phase fraction, and σ (N m -1 ) is the surface tension coefficient. is the position vector, (m / s 2 ) is the gravity vector, P rgh (Pa) is the corrected pressure, which is used to simplify the boundary conditions and is defined as follows:

[0052]

[0053] where P represents the pressure, κ (m-1) is the mean curvature of the free surface, and its definition is as follows:

[0054]

[0055] where is the surface unit normal vector.

[0056] (3) Phase transport equation:

[0057]

[0058] where (m / s) represents the relative velocity between the two phases.

[0059] 6. Based on the above steps, the two-phase flow model is established. The gas diffusion layer structure is imported into it, and the computational domain is as shown in the appendix Figure 4 . Only the fluid domain part is retained in the computational domain, and the solid domain part (i.e., the fiber and binder structure part) is deleted. The hollow part in the figure is the fiber and binder structure. Further, the liquid water distribution is output, and the result is as shown in the appendix Figure 5 . The gray part in the figure is the fiber and binder structure, and the blue part represents liquid water. It can be observed that liquid water invades from the bottom of the gas diffusion layer, then flows through the pores of the fiber and binder structure, and breaks through to the top. Appendix Figure 5 is the result of the numerical simulation calculation. Liquid water is represented by blue. If liquid water, the fiber and binder structure are all represented by black in the calculation result, it is difficult to distinguish the differences among the three.

[0060] 7. Based on the above model, the X-ray scan results, the results of the model without considering the binder, and the results of the method of the present invention under the same conditions are also compared. The pressure drops at the inlet and outlet of the two models are both set to 4 kPa, which is consistent with the experimental settings. The results are as shown in the appendix Figure 6 . In this example, the thickness is normalized. The relative thickness at the bottom inlet is 0, and the relative thickness at the top outlet is 1. Figure 6 . The abscissa in it is the relative thickness, and the ordinate is the local liquid water saturation. In the region where the relative thickness is the same (i.e., the abscissa is the same), the results calculated by the method of the present invention are closer to the experimental data, verifying that the model of the present invention has higher accuracy.

Claims

1. A gas diffusion layer reconstruction method considering the binder structure and a two-phase flow model were combined to establish a pure fiber structure of a gas diffusion layer using a random reconstruction method, which is characterized by: The binder structure is reconstructed using the morphological closing operation algorithm. By controlling the size of the structural operator, effective control of the volume ratio and porosity of the binder structure is achieved. Based on the reconstructed real structure gas diffusion layer, a two-phase flow model is established to solve and calculate the flow process of liquid water inside the gas diffusion layer. The specific steps are as follows: (1) Select a rectangular spatial region with length × width × height, and then divide it into uniform structural grids; (2) Randomly select two points A(x1, y1, z1) and B(x2, y2, z2) within the rectangular spatial region, where y1 = y2, thus forming a rectangular region in the x-z plane, x1 = L·Rand_x+(L + W)(1 - 1) x2 = L·Rand_x-(L + W)(1 - 2) z1 = (L + W)tan(π·Rand_θ)+W·Rand_z(1 - 5) z2 = -(L + W)tan(π·Rand_θ)+W·Rand_z(1 - 6) where L is the length direction of the rectangular region; W is the width direction of the rectangular region, Rand_x, Rand_z, Rand_θ are all numerical values generated by random functions for calculating coordinates, i represents the number of carbon fiber layers. Connect the two points A and B into a straight line, and generate a cylinder with a determined radius D with this line as the axis. This radius is the carbon fiber radius; (3) Generate multiple cylinders in the x-z plane. When the porosity of this layer reaches the set value, generate the next layer of grids, and stack them layer by layer until the thickness requirement is met; (4) Finally, mark all the grids occupied by the cylindrical fibers.

2. According to claim 1, the method for combining gas diffusion layer reconstruction and two-phase flow model considering the binder structure is characterized by: Based on the morphological closing operation algorithm to reconstruct the binder structure, the calculation implementation steps are as follows: (1) Select the structural operator radius d according to the preset porosity and the binder ratio. The structural operator radius is disk-shaped, and determine the number of grids N contained in one structural operator; where the binder ratio = binder volume / (fiber volume + binder volume); (2) Dilation operation: Traverse the fiber grids. Taking the center point coordinates (x0, y0, z0) of each grid as the origin of the structural operator, calculate the number of fiber grids n within the operator range. If the surrounding fiber grids cannot form a complete structural operator n < N, then supplement fiber grids to the vacant positions; (3) Erosion operation: Based on the dilated grids, traverse each grid. Taking the grid coordinates (x0', y0', z0') as the operator origin, calculate the number of fiber grids n' within the operator range. If the surrounding fiber grids cannot form a complete structural operator n' < N, then delete the grid where the origin coordinates (x0', y0', z0') are located.

3. According to claim 1, the method for combining gas diffusion layer reconstruction and two-phase flow model considering binder structure is characterized by: The implementation steps for establishing a two-phase flow model are as follows: Import the gas diffusion layer structure considering fibers and binders into the two-phase flow model, and solve the flow process of liquid water in the gas diffusion layer, including: (1) Set boundary conditions: During the actual operation of the fuel cell, liquid water enters from one side of the gas diffusion layer and exits from the other side. Set the bottom of the two-phase flow model as the liquid water inlet and the top as the liquid water outlet, and set a fixed liquid water flow rate at the inlet; (2) The conservation equations in the two-phase flow model include: (a) Continuity equation: in is the velocity vector shared by the two fluids in the entire fluid domain, ▽ represents the divergence operator, (b) Momentum equation: Where ρ is density, μ is viscosity, α is phase fraction, σ is surface tension coefficient, is the position vector, is the gravity vector, P rgh To correct the pressure, used to simplify the boundary conditions, P rgh The definition is as follows: Where P represents pressure, κ is the average curvature of the free surface, and κ is defined as follows: in, is the surface unit normal vector, (c) Phase transport equation: in, Indicates the relative speed between the two phases.

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