Meshing Method for Complex Spiral Structures

Abstract

The invention provides a meshing method with a complex spiral structure. The meshing method with a complex spiral structure is a method for arranging multiple fuel rods inside a spirally wound wire rod bundle assembly, and adjacent fuel rods are fixed by wires wound according to a certain pitch. The meshing method with a complex spiral structure replaces line contact with step contact, reduces the narrow gap geometry area, and effectively improves the mesh quality in this area. The method of rotating and stretching a two-dimensional mesh into a three-dimensional mesh effectively avoids the difficulty of directly generating high-quality meshes for a three-dimensional spiral geometry model. The structured mesh + unstructured mesh method is used to perform structured meshing on local complex areas, and the mesh quality and quantity are guaranteed. Unstructured meshing is performed on other simple areas, making the meshing process simpler. The use of structured meshing for complex areas such as solid-liquid contact areas and spiral and rod contact areas can effectively control the number of meshes in each direction.

Description

Technical Field

[0001] The invention relates to the field of reactor thermal hydraulic technology, and in particular to a grid division method with a spiral complex structure. Background Art

[0002] CFD simulation analysis is an important method to obtain flow field information in rod bundle assemblies. Before performing CFD numerical simulation, it is necessary to first establish a geometric model and divide the grid according to the research objectives. CFD analysis of rod bundle channels often requires huge computational complexity. A certain number of high-quality grids are essential for subsequent CFD calculations. However, the existing technology has major problems with the grid division of spiral rod bundle geometric models: ① The line contact between the spiral structure and the rod forms a narrow slit structure. This structure will cause the grid aspect ratio to be large, resulting in poor grid quality. ② The spiral structure is a special three-dimensional structure. If the grid is generated directly for this structure, the grid distortion will be very large, and the grid will be difficult to fit the actual geometric model, thus failing to correctly reflect the actual flow and heat transfer conditions.

[0003] At present, according to the form and combination of grids, grids can be divided into structured grids and unstructured grids. When dividing the grid of the geometric model with helical rod bundles, both grid methods can be used, but both have certain shortcomings.

[0004] When using structured grid to mesh the helical rod bundle, taking ICEM software as an example, it is necessary to divide the geometric model into many hexahedral blocks. The scale of these blocks is between the size of the geometric model and the actual grid size. Hexahedral and neatly arranged grids form blocks, and multiple different blocks are used to fit the actual geometric model, and finally the entire geometric model is divided into structured grids. However, due to the three-dimensional complexity of the helical rod bundle geometric model, if a structured grid is to be realized, many blocks are required. It is particularly difficult and complicated to fit these blocks to the actual geometry, and it takes a lot of time and energy. In addition, in the helical rod bundle geometric model, there is a narrow gap area formed by the line contact between the spiral and the rod surface. If hexahedral grids are used in these areas, the grid aspect ratio or distortion rate becomes very large, resulting in particularly poor grid quality. In response to these problems, the current solution is to divide the entire computational domain into many areas from the axial direction. When the helical deflection angle of each area is small, it can be divided into blocks and then meshed. This method requires decomposing the computational domain boundary into many parts, and operating and setting each part will make the operation more cumbersome.

[0005] When meshing the helical rod bundle using unstructured meshes, taking Meshing software as an example, the geometric model is directly composed of tetrahedral meshes or tetrahedral meshes + hexahedral meshes. The meshes divided by this method can fit the actual geometric model well, but the number of meshes cannot be effectively controlled, especially in the narrow gap area between the spiral and the rod surface, the number of meshes will increase rapidly and the mesh quality will be poor. To address these problems, the current solution is to mesh the entire rod solid and fluid domain, and use surface contact or non-contact for the narrow gaps of local line contact. However, the biggest problem with this processing method is that the number of meshes is huge, and more CFD computing resources have to be used, which consumes more computer cache and more computing time.

[0006] Therefore, it is of great significance to develop meshing methods with spiral complex structures. Summary of the invention

[0007] The purpose of the present invention is to provide a grid division method with a spiral complex structure to solve the problems existing in the prior art.

[0008] The technical solution adopted to achieve the purpose of the present invention is as follows: a grid division method with a spiral complex structure, a rod bundle assembly with a spiral winding wire includes a plurality of rod bundles arranged in a flow channel. There is a gap between adjacent rod bundles. A single rod bundle includes a fuel rod and a winding wire. The winding wire is wound on the outer wall of the fuel rod with a fixed pitch. The grid division method with a spiral complex structure includes the following steps:

[0009] 1) Establish a three-dimensional geometric model of a rod bundle assembly with spiral windings. The three-dimensional geometric model includes a solid region, a fluid region A and a fluid region B. Among them, a solid region is formed in the fuel rod and the winding. The fluid region A is a near-rod fluid domain containing winding features. The cross-section of the fluid region A is circular. The fluid region A is a twisted structure around the axial direction. The fluid region B is a mainstream fluid domain that does not contain winding features. The fluid region B is the portion of the flow channel excluding the solid region and the fluid region A. The fluid region B has multiple hollow regions.

[0010] 2) Obtain any cross section of the three-dimensional geometric model. Identify the boundary line formed by a single fuel rod and the corresponding winding area in the cross section. Simplify the line contact between the winding and the fuel rod in the boundary line into a step contact to form a solid area boundary line. The area within the solid area boundary line is regarded as the two-dimensional solid area of ​​a single rod bundle.

[0011] A solid region circumscribed circle is constructed according to the solid region boundary line. A separation circle is constructed outside the solid region circumscribed circle. The separation circle is concentric with the circumscribed circle. The diameter of the separation circle is greater than the diameter of the circumscribed circle by at least two boundary layer grid heights. The area between the solid region boundary line and the separation circle is used as the two-dimensional fluid area A of a single rod bundle.

[0012] The two-dimensional solid region and the two-dimensional fluid region A of all rod bundles in the cross section are removed to obtain the two-dimensional fluid region B.

[0013] 3) Use fluid dynamics software to construct a two-dimensional structured grid of the two-dimensional solid region of a single rod bundle to obtain a two-dimensional grid of the solid domain. Rotate and stretch the two-dimensional grid to form a three-dimensional grid of the solid domain.

[0014] 4) Using fluid dynamics software to construct a two-dimensional structured grid of the two-dimensional fluid region A of a single rod bundle, and obtaining a two-dimensional grid of the fluid domain. The two-dimensional grid of the fluid domain is rotated and stretched to form a structured grid of the fluid domain A.

[0015] 5) The generated solid domain three-dimensional grid of the single rod bundle and the structured grid of the fluid domain A are spliced ​​to form Part A.

[0016] 6) Use 3D modeling software to construct a geometric model of the fluid region B. Perform meshing on the geometric model of the fluid region B, and the fluid region B is a 3D unstructured mesh.

[0017] 7) After Part A is replicated in an array, it is spliced ​​with the three-dimensional unstructured grid of fluid area B to form an overall computational domain grid.

[0018] Furthermore, in step 3) and step 4), the two-dimensional structured grids of the solid domain and the fluid region A are constructed using ANSYS ICEM CFD software.

[0019] Furthermore, in step 4), during rotational stretching, the two-dimensional grid of the fluid domain is rotationally stretched along the axis of the fuel rod.

[0020] Furthermore, in step 6), the geometric model of the fluid region B is meshed using Meshing software.

[0021] Furthermore, the total height of the rod bundle assembly with spiral winding wire is 600 mm. The gap between adjacent fuel rods is 10.5 mm. The diameter of the winding wire is 1.2 mm. The pitch of the winding wire is 200 mm.

[0022] The technical effects of the present invention are unquestionable:

[0023] A. Step contact is used to replace line contact, which reduces the narrow gap geometry area and effectively improves the mesh quality in this area;

[0024] B. The method of rotating and stretching a two-dimensional grid into a three-dimensional grid effectively avoids the difficulty of directly generating high-quality grids from a three-dimensional spiral geometry model;

[0025] C. Use the structured grid + unstructured grid method to divide the local complex area into structured grids, and the grid quality and quantity are guaranteed; and use unstructured grids to divide other simple areas, making the grid division process more concise;

[0026] D. For complex areas such as solid-liquid contact area and spiral-rod contact area, structured grid division can effectively control the number of grids in each direction. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a rod bundle geometry with spiral winding wire;

[0028] Figure 2 Two cases of rod bundles with spiral winding wire;

[0029] Figure 3 It is a simplified treatment of the contact mode between the helix and the rod bundle;

[0030] Figure 4 Re-decomposition of the geometric model;

[0031] Figure 5 Structured meshing for solid domains;

[0032] Figure 6 Structured meshing for the fluid domain;

[0033] Figure 7 Unstructured meshing for the fluid domain;

[0034] Figure 8 Merge graphs for all meshes. DETAILED DESCRIPTION

[0035] The present invention is further described below in conjunction with the embodiments, but it should not be understood that the above subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various substitutions and changes are made according to the common technical knowledge and customary means in the art, which should all be included in the protection scope of the present invention.

[0036] Embodiment 1:

[0037] This embodiment provides a meshing method with a spiral complex structure. The rod bundle assembly with a spiral winding wire includes a plurality of rod bundles arranged in a flow channel. There is a gap between adjacent rod bundles. A single rod bundle includes a fuel rod 1 and a winding wire 2. The winding wire 2 is wound on the outer wall of the fuel rod 1 with a fixed pitch. The meshing method with a spiral complex structure includes the following steps:

[0038] 1) Establish a three-dimensional geometric model of a rod bundle assembly with spiral windings. The three-dimensional geometric model includes a solid region 3, a fluid region A4 and a fluid region B5. Among them, a solid region 3 is formed in the fuel rod 1 and the winding 2. The fluid region A4 is a near-rod fluid domain containing winding features. The cross-section of the fluid region A4 is circular. The fluid region A4 is a twisted structure around the axial direction. The fluid region B5 is a mainstream fluid domain that does not contain winding features. The fluid region B5 is the portion of the flow channel excluding the solid region 3 and the fluid region A4. The fluid region B5 has multiple hollow regions.

[0039] 2) Obtain any cross section of the three-dimensional geometric model. Identify the boundary line formed by the area where a single fuel rod 1 and the corresponding winding wire 2 are located in the cross section. Simplify the line contact between the winding wire 2 and the fuel rod 1 in the boundary line into a step contact to form a solid area boundary line. The area within the solid area boundary line is regarded as the two-dimensional solid area of ​​the single rod bundle.

[0040] A solid region circumscribed circle is constructed according to the solid region boundary line. A separation circle is constructed outside the solid region circumscribed circle. The separation circle is concentric with the circumscribed circle. The diameter of the separation circle is greater than the diameter of the circumscribed circle by at least two boundary layer grid heights. The area between the solid region boundary line and the separation circle is used as the two-dimensional fluid area A of a single rod bundle.

[0041] The two-dimensional solid region and the two-dimensional fluid region A of all rod bundles in the cross section are removed to obtain the two-dimensional fluid region B.

[0042] 3) A two-dimensional structured grid of the two-dimensional solid region of a single rod bundle is constructed using fluid dynamics software to obtain a two-dimensional grid 6 of the solid domain. The two-dimensional grid 6 is rotated and stretched to form a three-dimensional grid 7 of the solid domain.

[0043] 4) A two-dimensional structured grid of the two-dimensional fluid region A of a single rod bundle is constructed using fluid dynamics software to obtain a fluid domain two-dimensional grid 8. The fluid domain two-dimensional grid 8 is rotated and stretched along the central axis of the fuel rod 1 to form a structured grid 9 of the fluid domain A.

[0044] See also Figure 2 , Figure 2 a is the case of no interference in the axial spiral, and there is no contact between the projections of the winding wires 2 of adjacent rod bundles. Figure 2 b is a situation where there is interference in the axial spiral, and there is contact between the projections of the winding wires 2 of adjacent rod bundles. The two-dimensional grid eccentric stretching method of this patent can be applied to the two structures in the figure. In step 4), the fluid domain two-dimensional grid 8 is rotationally stretched along the axis of the fuel rod 1. The center of the separating circle in the cross section does not coincide with the center of the fuel rod, but there is a certain distance. Rotational stretching according to the axis is eccentric rotational stretching, and the structured grid 9 of the fluid domain A will be torsional around the axial direction.

[0045] 5) The generated solid domain three-dimensional grid 7 of the single rod bundle and the structured grid 9 of the fluid domain A are spliced ​​to form Part A.

[0046] 6) Use 3D modeling software to construct a geometric model of the fluid region B5. Perform meshing on the geometric model of the fluid region B5, and the fluid region B is a 3D unstructured mesh 11.

[0047] 7) After Part A is replicated in an array, it is spliced ​​with the three-dimensional unstructured grid 11 of the fluid region B to form an overall computational domain grid.

[0048] It is worth noting that there are narrow gaps in the solid region 3 and the fluid region 4. If an unstructured grid is used, the grid quality will be poor and the number of grids will be large. Therefore, a structured grid is used to control the shape and number of grids at the narrow gaps, thereby controlling the quality and quantity of the overall grid. For this spiral structure, the method of rotating and stretching the grid is less labor-intensive. The fluid domain 5 uses an unstructured grid because the fluid domain 5 cannot be formed by axial stretching. If a three-dimensional structured grid is used for direct division, it will be more complicated, so an unstructured grid division method is used.

[0049] Embodiment 2:

[0050] The main steps of this embodiment are the same as those of embodiment 1, wherein in step 3) and step 4), ansys icem cfd software is used to construct a two-dimensional structured grid of the solid domain 3 and the fluid region A4.

[0051] Embodiment 3:

[0052] The main steps of this embodiment are the same as those of Embodiment 1, wherein, in step 6), the geometric model of the fluid region B5 is meshed using Meshing software.

[0053] Embodiment 4:

[0054] The main steps of this embodiment are the same as those of embodiment 1, wherein Figures 1 to 8 , this embodiment takes a 2×2 spirally wound rod bundle assembly as an example, and further describes the embodiment 1 in detail. In this embodiment, the total height of the spirally wound rod bundle assembly is 600 mm. The gap between adjacent fuel rods 1 is 10.5 mm. The diameter of the winding wire 2 is 1.2 mm. The pitch of the winding wire is 200 mm. In order to meet the topological requirements of the geometric structure and ensure the quality of the grid, refer to Figure 3 , the line contact between the wire 2 and the fuel rod 1 is simplified to step contact. The effect of such geometric treatment on the flow and heat transfer characteristics can be ignored. Figure 5 , the two-dimensional grid 6 is rotated and stretched to form a three-dimensional grid 7 in the solid domain. Figure 6, the two-dimensional grid 8 is rotated and stretched to form a three-dimensional grid 9 in the solid domain. For the axial helical interference-free model, direct rotation stretching is performed. For the axial helical interference model, the rotation stretching should be eccentric. See Figure 7 , a geometric model of the fluid region B5 is constructed using a 3D modeling software, and then the geometric model is directly meshed using the Fluent Meshing software to form a 3D unstructured mesh of the fluid region B5 11. Figure 8 , the three-dimensional structured grid 7 of the solid domain, the structured grid 9 of the fluid domain A, and the unstructured grid 11 of the fluid domain B are merged together to form an overall computational domain grid.

[0055] This embodiment solves the meshing problem of the geometric model with a helical rod bundle, making the meshing more efficient. Meanwhile, the number of meshes obtained by the meshing can be controlled and the quality is better.

Claims

1. A meshing method with a spiral complex structure, characterized in that: A rod bundle assembly with spiral winding wires comprises a plurality of rod bundles arranged in a flow channel; there are gaps between adjacent rod bundles; a single rod bundle comprises a fuel rod (1) and a winding wire (2); the winding wire (2) is wound on the outer wall of the fuel rod (1) with a fixed pitch; a grid division method with a spiral complex structure comprises the following steps: 1) Establishing a three-dimensional geometric model of a rod bundle assembly with spiral windings; the three-dimensional geometric model comprises a solid region (3), a fluid region A (4) and a fluid region B (5); wherein a solid region (3) is formed inside a fuel rod (1) and a winding (2); the fluid region A (4) is a near-rod fluid domain containing winding features; the cross section of the fluid region A (4) is circular; the fluid region A (4) is a twisted structure around the axial direction; the fluid region B (5) is a mainstream fluid domain not containing winding features; the fluid region B (5) is a portion of a flow channel excluding the solid region (3) and the fluid region A (4); and the fluid region B (5) has a plurality of hollow regions; 2) obtaining any cross section of the three-dimensional geometric model; identifying a boundary line formed by a single fuel rod (1) and a region where a corresponding winding wire (2) is located in the cross section; simplifying the line contact between the winding wire (2) and the fuel rod (1) in the boundary line into a step contact to form a solid region boundary line; and using the region within the solid region boundary line as a two-dimensional solid region of a single rod bundle; Constructing a solid region circumscribed circle according to the solid region boundary line; constructing a separation circle outside the solid region circumscribed circle; the separation circle is concentric with the circumscribed circle; the diameter of the separation circle is greater than the diameter of the circumscribed circle by at least two boundary layer grid heights; and taking the area between the solid region boundary line and the separation circle as the two-dimensional fluid area A of the single rod bundle; Removing the two-dimensional solid region and the two-dimensional fluid region A of all rod bundles in the cross section, and obtaining the two-dimensional fluid region B; 3) constructing a two-dimensional structured grid of a two-dimensional solid region of a single rod bundle using fluid dynamics software to obtain a two-dimensional grid of the solid domain (6); rotating and stretching the two-dimensional grid (6) to form a three-dimensional grid of the solid domain (7); 4) constructing a two-dimensional structured grid of the two-dimensional fluid region A of the single rod bundle using fluid dynamics software to obtain a two-dimensional grid of the fluid domain (8); rotating and stretching the two-dimensional grid of the fluid domain (8) to form a structured grid of the fluid domain A (9); 5) splicing the generated solid domain three-dimensional grid (7) of the single rod bundle and the structured grid (9) of the fluid domain A to form Part A; 6) constructing a geometric model of the fluid region B (5) using a three-dimensional modeling software; meshing the geometric model of the fluid region B (5), and forming a three-dimensional unstructured mesh of the fluid region B (11); 7) After Part A is replicated in an array, it is spliced ​​with the three-dimensional unstructured grid (11) of the fluid region B to form an overall computational domain grid.

2. The meshing method with a spiral complex structure according to claim 1, characterized in that: In step 3) and step 4), ANSYS ICEM CFD software is used to construct the two-dimensional structured grids of the solid domain (3) and the fluid region A (4).

3. The meshing method with a spiral complex structure according to claim 1, characterized in that: In step 4), during rotational stretching, the two-dimensional grid (8) of the fluid domain is rotationally stretched along the axis of the fuel rod (1).

4. The meshing method with a spiral complex structure according to claim 1, characterized in that: In step 6), the geometric model of the fluid region B (5) is meshed using Meshing software.

5. The meshing method with a spiral complex structure according to claim 1, characterized in that: The total height of the rod bundle assembly with spiral winding wire is 600 mm; the gap between adjacent fuel rods (1) is 10.5 mm; the diameter of the winding wire (2) is 1.2 mm; and the pitch of the winding wire is 200 mm.

Images