tailieunhanh - Computational Fluid Dynamics 2011 Part 9

Tham khảo tài liệu 'computational fluid dynamics 2011 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 234 Computational Fluid Dynamics Fig. 7. Flow dynamic phenomena around the impeller blade a blade-loading distributions b streamtubes for the overall flow c streamline distribution on the pressure side and d streamline distribution on the suction side Fig. 8. Flow dynamic phenomena around the diffuser blade a blade-loading distributions b velocity vectors near the exit hub of the suction side c streamline distribution on the pressure side and d streamline distribution on the suction side 235 Application of Computational Fluid Dynamics to Practical Design and Performance Analysis of Turbomachinery Ộ Fig. 9. The performance characteristic curves of a mixed-flow pump Cavitation performance characteristics Generally cavitation occurs in the liquid system when the local absolute static pressure in a flowing liquid is reduced to or below the vapour pressure of the liquid thereby forming vapour bubbles. The bubbles suddenly collapse as they are convected to a high-pressure region. The consequent high-pressure impact may lead to hardware damage . local pitting and erosion and emit noise in the form of sharp crackling sounds. Cavitation may also degrade the performance characteristics of hydraulic machinery. Figure 10 demonstrates the cavitation performance characteristic curves of a mixed-flow pump developed by the present optimal design method considering the highest pump efficiency. In order to investigate the flow dynamics of cavitation for the near-design flowrate Ộ in Fig. 10 under the design pump speed the side-view of the cavitating flow along the impeller blade has been photographed through the window of the test facility and compared with the isosurface plots generated by the CFD code for the vapour fraction of the fluid. In this figure the cavitation parameter ơ is defined as the ratio of the net positive suction head NPSH to the pump total head. At case 1 a small