Numerical investigation of blade-integrated self-recirculating flow control in a centrifugal compressor using the lattice Boltzmann method

Proton exchange membrane fuel cell (PEMFC) systems are promising for future aviation, in which an efficient hydrogen recirculation system is essential for stable operation and high hydrogen utilization. Centrifugal hydrogen recirculation compressors are attractive for this purpose but face challenges in pressure-rise capability and surge margin due to the hydrogen-rich yet continuously varying gas composition. Recent experiments have shown that a blade-integrated self-recirculating flow-control method can provide the desired improvements while maintaining nearly unchanged efficiency. In the present study, the unsteady flow characteristics associated with this method are investigated numerically using the lattice Boltzmann method. The newly introduced hub-side recirculation accounts for approximately three quarters of the original shroud-side leakage flow rate and constitutes the primary contributor to the increased pressure rise, as additional work is imparted to this flow within the blade. The recirculating flow also exhibits periodic fluctuations at frequencies matching the dominant pressure oscillations of the prototype, thereby resulting in a pronounced attenuation of pressure pulsations in the modified configuration. Moreover, certain pressure spectra in the prototype are found to excite instabilities of the suction-surface separated shear layer, promoting roll-up into large-scale vortices, enhanced unsteadiness, and enlarged separation. The self-recirculation flow acts as an effective damper that suppresses these perturbation sources, thereby mitigating the associated mixing and separation losses. The present findings reveal a coherent physical mechanism by which recirculating-flow fluctuations attenuate self-excited pressure oscillations that promote flow separation, offering new insights into passive flow control in turbomachinery and highlighting its potential for broader engineering applications.