Substantiation of the various turbulence models use and of the need to account for roughness when calculating the viscous flow in low-flow centrifugal compressor stages using the computational gas dynamics.

The work considers the issue of choosing a turbulence model and taking into account surface roughness when creating a design model of low-flow centrifugal compressor stages for calculations using computational gas dynamics methods. The research object is a low-flow stage with a conditional flow coefficient Φ=0.008 and with a relative width at the output of the impeller b2/D2=0.0133. The issue of modeling friction losses in low-flow stages is of principal importance and is directly related to the turbulence model choice and taking into account surface roughness. It is shown that the choice of low-Reynolds turbulence models in the case of unloaded and continuous low-flow stages can be made from among the main common models (Spalart-Allmaras, SST, k-ω) based on the cost-effective calculations, the rate of convergence, the stability of the solution and the relevance of the obtained results. On the other hand, the use of low- Reynolds turbulence models is not relevant due to the inability to correctly introduce roughness corrections. For wall- mounted models, the quality of the grid model and compliance with the dimensionless distance to the wall y+ in the entire design area are particularly important. If y+=25...50 is provided on all calculated area friction surfaces, the grid independence of the solution for the entire gas dynamic characteristic is ensured at the optimal operating mode. y+ cannot enter the transition region 4...15 between the viscous sublayer and the logarithmic velocity profile region. The two main issues of roughness accounting are the conversion of natural roughness to equivalent roughness used in calculations, and the quality requirements of the grid model. The influence of roughness on the low-flow stage characteristics and the disk friction and leaks coefficients is estimated, and the independence of the full head of the stage from the surface roughness is revealed. A separate analysis of the impeller roughness and fixed elements influence was performed. [ABSTRACT FROM AUTHOR]