Experimental and numerical analyses of mild surge in a transonic centrifugal compressor with a vaneless diffuser

Surge is a well-known destructive flow instability observed in systems that include compressors. When a system encounters the surge, flow rate and pressure fluctuate drastically, significantly reducing performance. Moreover, the surge causes an unstable operation and structural damage to the system. Therefore, a deeper understanding of the surge is essential for achieving better performance and higher reliability in compression systems. This study aimed to elucidate the unsteady flow phenomena during the mild surge cycle, using experimental and numerical analyses. Specifically, we focused on a transonic centrifugal compressor with a vaneless diffuser used in vehicle turbochargers. In the experiment, compressor performance and surge characteristics were investigated using unsteady measurements of pressure and velocity in a test piping system where the compressor was installed. Additionally, a large-scale detached eddy simulation (DES) was conducted on the supercomputer FUGAKU using a computational grid of 900 million cells for the entire piping system. In the simulation, a throttle valve was represented as an outflow boundary condition to control the flow rate. By adjusting the throttle valve parameter to gradually reduce the flow rate, the simulation successfully captured a mild surge characterized by large oscillations in flow rate and pressure, without reverse flow throughout the system. Comparison of time variations in wall static pressure upstream and downstream of the compressor between the experimental and numerical analyses demonstrated that the dominant low-frequency mode excited by the mild surge was accurately reproduced by the DES. Furthermore, detailed flow structures were visualized at key phases during one surge cycle. During the mild surge, a recirculation near the impeller blade tip periodically enlarged and shrank with flow rate fluctuations. The recirculation was initially generated by a spiral-type tip leakage vortex breakdown and drastically evolved by blade stalls near the impeller tip as the flow rate decreased. In the process of the flow rate reduction, rotating stall cells in the vaneless diffuser emerged around pressure rise peak and rotated at 15-20 percent of the impeller rotational speed.