Impact of Vaneless Diffuser Diameter Ratio on Complex Internal Flow and Aerodynamic Performance of Centrifugal Axial Compressors

This paper conducts a numerical investigation into the impact of the vaneless diffuser diameter ratio (Dr) on the complex internal flow patterns and aerodynamic performance of a centrifugal-axial natural gas compressor. The study primarily focuses on the spatiotemporal evolution of the internal flow and the mechanisms of flow instability within the impeller and diffuser under different Dr conditions. Under stable operating conditions, compared to the case at Dr=۱.۱۵, the polytropic efficiency of the centrifugal-axial compressor exhibits a significant improvement of ۹.۹۲ percentage points as Dr increases. However, when the Dr increases to ۱.۷, the internal flow of the compressor approaches a critical state. Significant flow separation and backflow within the impeller are clearly evidenced by the turbulent kinetic energy distribution inside the impeller and the pressure distribution at different spanwise of the vaneless diffuser. The analysis of the unsteady simulations centers on the critical Dr of ۱.۷. The evolution of vortices within the impeller flow field is analyzed using the Q-criterion. The results show that the vortices are mainly concentrated in the center region of the impeller passage. By examining the processes of vortex generation, development, merging, and breakdown, this study reveals the underlying mechanism of flow deterioration under the critical operating condition. At the critical condition, pressure fluctuations at the vaneless diffuser inlet are analyzed via Fast Fourier Transform (FFT). Furthermore, the total pressure distribution within the diffuser is examined at three characteristic peak instants, elucidating the influence of the diameter ratio on vaneless diffuser instability. Collectively, these findings provide a theoretical foundation and novel insights for optimizing the aerodynamic design of centrifugal-axial compressors used in natural gas transmission pipelines. In addition to theoretical contributions, this work offers significant practical benefits, enhancing the economic efficiency and operational stability of pipeline systems. Optimizing compressor selection based on these results can reduce pipeline pressure losses and guide optimal pipeline layout configurations.

کلیدواژه ها:

Centrifugal axial compressors ، Pressure pulsation ، Spatiotemporal evolution ، Diameter ratio ، Flow instability