###### Abstract  
Numerical simulation of thermal hydraulics of nuclear reactors is widely concerned, but large-scale fluid simulation is still prohibited due to the complexity of components and huge computational effort. Some applications of open source CFD programs still have a large gap in terms of comprehensiveness of physical models, computational accuracy and computational efficiency compared with commercial CFD programs. Therefore, it is necessary to improve the computational performance of in-house CFD software (YHACT, the parallel analysis code of thermohydraulics) to obtain the processing capability of large-scale mesh data and better parallel efficiency. In this paper, we will form a unified framework of meshing and mesh renumbering for solving fluid dynamics problems with unstructured meshes. Meanwhile, the effective Greedy, RCM (reverse Cuthill-Mckee), and CQ (cell quotient) grid renumbering algorithms are integrated into YHACT software. An important judgment metric, named median point average distance (MDMP), is applied as the discriminant of sparse matrix quality to select the renumbering methods with better effect for different physical models. Finally, a parallel test of the turbulence model with 39.5 million grid volumes is performed using a pressurized water reactor engineering case component with 3*3 rod bundles. The computational results before and after renumbering are also compared to verify the robustness of the program. Experiments show that the CFD framework integrated in this paper can correctly perform simulations of the thermal engineering hydraulics of large nuclear reactors. The parallel size of the program reaches a maximum of 3072 processes. The renumbering acceleration effect reaches its maximum at a parallel scale of 1536 processes, 56.72%. It provides a basis for our future implementation of open-source CFD software that supports efficient large-scale parallel simulations. m 202  
There is a violent flow of liquid between the bar bundles and between the bar bundles and the walls, and they are responsible for the transfer of momentum and heat. It is widely believed that the flow pulsations in the gap are closely related to the large-scale vortices. It has been studied that vortexes will form in pairs on both sides of the slit [3; 4]. The flow and heat transfer characteristics of coolant flowing through the core of a nuclear reactor are extremely complex. To analyze the turbulent motion and thermodynamics of large-scale fuel rod bundles [5; 6; 7; 8; 9; 10], large-scale CFD numerical simulations based on FVM (finite volume method) are a widely accepted solution. However, large-scale CFD numerical simulations require an extremely large amount of mesh data. The usual solution is to encrypt the mesh of the physical model. The larger the amount of data, the larger the solution size and the accuracy of the solution are naturally improved. However, this will lead to more and more complex and difficult to solve calculations. Today, the use of large-scale CFD techniques in thermal hydraulic analysis is still very difficult due to high technical requirements and shortage of computational resources [11]. In the CFD, simulation of fine flow field structures in real-world engineering cases is still forbidden [12]. As a result, there is widespread interest in improving computational efficiency by well-developed parallel technology. The CFD solving process involves three parts: pre-processing, numerical solving, and post-processing. The use of appropriate meshing methods and mesh numbering in the preprocessing process will greatly improve the computational performance of fluid dynamics simulations. Taking FVM [13; 14] as an example, after meshing the physical model, assembly of linear systems in parallel from the number of grid cells of each divided component is necessary. Then, solving them iteratively, In addition, the numbering of the grid determines the order of access to the individual processes for parallel computation and affects the cache hit rate.