The method adopted in this paper is flow field calculation. The advantage of this method is that it can obtain detailed flow field aerodynamic parameters and conduct in-depth analysis of the flow structure. On the hardware platform of the high-speed server, the CFX calculation software was used to perform three-dimensional numerical simulation of the stall phenomenon of a centrifugal compressor with a vaneless diffuser. The focus is on the introduction of the air pocket model and internal flow field analysis at the exit boundary.
Computational Method Control Equation and Turbulence Model This paper uses the commercial fluid calculation software CFX to solve the three-dimensional compressible Reynolds average conservation Navier2Stokes equation. The turbulence model selects the RNGk2ε two-equation model, and the wall surface is treated by the scalable wall function.
The computational grid is the computational region meshing. Since the traffic is redistributed in the circumferential direction when the stall occurs, the periodic boundary conditions in the calculation of the single flow channel are no longer reasonable. Therefore, the full flow channel mesh is used in the calculation. A structured grid is generated using CFX's Turbogrid pre-processing software. By selecting the existing mesh topology module, the automatic management of the mesh topology, the optimization of the mesh, the attachment and association of the mesh blocks, and the like, improve the quality and efficiency of the mesh generation. The O-shaped grid is encrypted near the blade, and the average y+ is 20. The blade channel and the inlet and outlet extensions adopt an H-shaped grid. In the tip clearance, a separate H-shaped grid is used, which is embedded in the surrounding O-shaped grid, and 8 grid points are arranged in the radial direction of the gap. The resulting mesh has a minimum angle of 15159° and a total of more than 1.6 million mesh nodes. The calculation area is set up with three monitoring points, located in the middle of the vaneless diffuser, with circumferential spacing of 30° and 90°, respectively, as shown in 2.
The calculation method and boundary conditions are solved by the commercial software CFX to calculate the governing equation, and the numerical simulation of the rotating stall is performed. The software uses a multi-grid algebra solver based on additional correction technology. The solution strategy ensures that the equations are fully coupled (momentum equations and continuous equations are solved synchronously), so that the solver has strong functions and efficiency. In the discrete control of differential equations, CFX uses a finite volume method, but the geometric description based on the finite element method keeps the flexibility of the finite element method geometrical and the finite volume method is very important conservation, space The discrete format is a first-order upwind display format. The calculation process is divided into two stages: steady calculation and unsteady calculation. The result of steady calculation is used as the initial flow field of unsteady calculation. In the calculation, the T/S interface is always used at the impeller runner exit and the vaneless diffuser inlet.
The boundary conditions are specifically set as follows: the inlet of the bucket adopts a uniform inflow condition, given the total pressure, the total temperature and the direction angle of the airflow (axial intake); the outlet of the vaneless diffuser is given a static pressure; the adiabatic solid wall is given no slip The condition is set to the absolute static wall condition; the physical time step is set to 1113x10-5s. The rotor speed calculated in this paper is 441198 kr/min1. In the unsteady calculation of this paper, a basic assumption is that the fluid flows out without leaves. After the diffuser enters an air cavity, it is considered that the mutual coupling of the air cavity and the compressor is critical to the development of unstable flow of the compressor. The air cavity has an important influence on the rotational stall frequency of the compressor and the internal flow field of the compressor. . However, due to the limited computing resources, the air cavity cannot be directly modeled and meshed. Therefore, in the three-dimensional numerical simulation calculation of the rotational stall of the unsteady high-speed centrifugal compressor, the boundary condition of the pure pressure outlet in the steady calculation is changed, and the effect of the air cavity is taken into consideration, as shown. The basic assumptions of the air cavity are as follows: Ignore the velocity and acceleration of the fluid in the air cavity; the pressure distribution in the air cavity is uniform; isentropic; the volume of the air cavity is constant, and the speed of sound in the air cavity is constant.
Conclusion This paper uses commercial CFD software CFX to simulate the rotational stall flow of a centrifugal compressor with a vaneless diffuser, and adds a gas cavity model to the numerical simulation. Through the analysis of the calculation results, it is concluded that during the stall period, as the mass flow decreases, the angle of attack of the bucket inlet increases gradually, resulting in flow separation in the impeller flow passage and the formation of separation vortex. The numerical calculation accurately simulates the development of the stall process of the centrifugal compressor.
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