Unsteady Computational Fluid Dynamics Simulation of Inlet Fins in Centrifugal Compressor

In this paper, it will be shown that small inlet fins mounted in the compressor inlet pipe can improve the compressor stability limit by alleviating the impeller inlet recirculation flow. Performance tests on the hot gas test bench and unsteady Computational Fluid Dynamics (CFD) simulations were conducted to investigate the mechanisms for surge condition improvement. The stability limit was shifted to lower mass flow rates in the low Mach number region by inlet fins in performance test results. As the compressor rotational speed increased, the stability limit shifted to high mass flow rates (kink characteristic) at a certain pressure ratio. The stability limit was unaffected by inlet fins in the high Mach number region after the kink point. Therefore, it was considered that the factors responsible for triggering surge were different near the kink point. It is thought that the instability of the diffuser does not occur after impeller stall in the low speeds region, but that diffuser stall occurs in the high speeds region causing the kink characteristics. Flow patterns extracted from unsteady CFD at the near surge point determined from the performance tests are analyzed to investigate the difference of surge pattern between low and high Mach number region. In low Mach number region, inlet fins suppress the extension of high entropy flow upstream of the impeller. It is assumed that the high entropy region was recirculation flow attributable to impeller stall. In addition, there are differences in the behavior on compressor characteristics with and without inlet fins. The positive slopes of compressor’s pressure ratio to flow rate become gentle by suppressing the development of pre-swirl (better surge characteristic). In the high Mach number region, flow patterns and compressor characteristics are almost the same in both cases. It is considered that the diffuser stall occurred first, and the diffuser stall triggered surge before impeller stall developed toward upstream. Therefore, the inlet fins did not work sufficiently since there was no recirculation flow from impeller.