While global efforts for environmental protection of the Earth progress, strengthening of regulations regarding flue gas or fuel efficiency in internal combustion engines, such as engines of automobiles is continuing. Since turbochargers can reduce engine emissions compared to natural aspiration by sending compressed air into an engine, turbochargers are devices that are very effective in fuel efficiency improvement and CO2 reduction.
In turbochargers, a turbine is rotationally driven by engine flue gas, thereby rotating a delivery fan of a coaxial centrifugal compressor. The air compressed by the rotation of the delivery fan is raised in pressure by being reduced in speed by a diffuser, and is supplied to the engine through a scroll flow path. In addition, regarding methods for driving turbochargers, not only driving using flue gas, but also various forms, such as driving using an electric motor and driving a prime mover via a transmission, are known.
If flow rate decreases in the centrifugal compressor used for such a turbocharger, surging that is the pulsation of the whole system occurs. In order to expand the operating range of a centrifugal compressor, it is necessary to reduce a critical flow rate at which surging occurs.
In contrast, PTL 1 discloses a technique of forming a recirculation flow path that allows a portion of the air sucked into a delivery fan to circulate therethrough, thereby expanding the operating range. This recirculation flow path is formed in a compressor cover that covers the delivery fan. This recirculation flow path is formed when an inner tube member having fins disposed at intervals in a peripheral direction is externally fitted to a compressor cover body in an axial direction. In this case, when outer peripheral ends of the fins abut against the compressor cover from the inside, the recirculation flow path is partitioned into a plurality of sections in the peripheral direction. A portion of the air sucked into the delivery fan is introduced into the recirculation flow path as a swirling flow, and is straightened by the above fins.