###### Abstract
Since modern computer architecture is characterized by multiple storage levels, reasonable grid numbering has a significant impact on the efficient utilization of computational performance. One of the key technologies in CFD preprocessing is the meshing technique. Its theoretical basis is becoming more and more mature. In recent years, the research field of finite element meshing has shifted from two-dimensional plane problems to three-dimensional entities, and the research focus has changed from triangular (tetrahedral) meshes to quadrilateral (hexahedral) meshes, focusing on the fully automatic generation of meshes, mesh adaption and other research. Mesh partitioning can be abstracted as graph partitioning. The relationship of cells and neighbors can be abstracted as the relationship of vertices and edges. The large-scale physical model is divided into multiple blocks by coarse-grained and fine-grained lattice partitioning, which facilitates parallel computation [15]. It is a challenge to achieve high quality meshing as quickly as possible and under the condition that the load balance is satisfied. The efficiency and cell quality of meshing need to be further improved. Another key technique in preprocessing is the grid numbering technique. The effect of grid numbering techniques on the computational efficiency of solving sparse linear systems is a classical topic with many research results, examples of which can be found in the work of [PERSON] et al. [16], [PERSON] and [PERSON] [17], [PERSON] [18], [PERSON] and [PERSON] [19], and [PERSON] et al. are reflected in the work of [PERSON] [20]. Because the heuristic renumbering algorithm consumes a lot of time during preprocessing. Therefore, heuristic algorithms may be more appropriate when computational performance is not a critical issue [21; 22]. In addition, this paper studies how to accelerate the computational performance of general-purpose CFD software, so heuristic algorithms are not considered for the time being. In general, meshing and grid numbering techniques are a traditional approach to performance optimization, and there are many existing methods to choose from. However, these techniques still lack quantitative analysis for large-scale thermal-hydraulic cases. In particular, the optimization effectiveness for general-purpose CFD software executed on supercomputers is still lacking. Therefore, this paper focuses on the effectiveness of the FVM-based computational framework for use in unstructured grids, integratingtypical meshing methods and mesh numbering techniques. Ultimately, a suitable mesh renumbering method is selected to enhance the parallel scale and computational speed of numerical simulation. The main innovative points of this paper are as follows: (1) A general framework for further integration of parallel meshing methods and renumbering schemes in general-purpose CFD software for unstructured meshes; and (2) improved runtime speed and parallel scale for real-world numerical simulations of large-scale complex cases. The case is a thermal hydraulic analysis of a 3 deg3 fuel rod bundle with 39.5 million grid volumes. The structure of this paper is as follows: Section 2 describes the generic CFD software used in this paper and aims to demonstrate the advantages of the software between data structure and parallel communication. Section 3 shows the meshing strategy and the integrated RCM renumbering algorithm. Section 4 demonstrates the correctness of the massively parallel numerical simulations before and after renumbering and the huge improvement of the renumbering algorithm on the numerical simulation efficiency. Conclusions are included in Section 5.