For an engine used in automobiles and the like, an exhaust turbocharger is widely known.  In an exhaust turbocharger, a turbine is rotated by exhaust-gas energy of the engine and a centrifugal compressor directly coupled to the turbine via a rotation shaft compresses intake air and supplies the engine with the intake air to improve the output of the engine. 
In this case, depending on the rotation speed of various types of impeller wheel, there is a lower limit in the flow rate at which the pressure can be normally increased.  If the flow rate is at the lower limit or below, vibration of the intake air occurs at an impeller upstream edge of the impeller wheel and the pressure may no longer increase. 
The above phenomenon is referred to as surge. 
On the other hand, there is also a limit in the maximum intake-air flow rate depending on the rotation speed of the impeller wheel, which is referred to as a choke phenomenon. 
To compare the operation characteristics of a centrifugal compressor of such type, it is known to draw a graph as schematically illustrated in FIG.  10, showing a comparison chart of performance characteristics, where x-axis is the intake-air flow rate and y-axis is the pressure ratio. 
With regard to the surge phenomenon, it is possible to improve the limit at which the surge phenomenon occurs by taking out a part of the intake air from the flow path at the downstream side of the impeller upstream edge of the impeller wheel to bypass the impeller wheel, returning the intake air to an intake channel at the upstream side of the impeller upstream edge, and increasing the apparent intake-air flow rate at the impeller upstream edge. 
FIG.  10 is a comparison diagram illustrating a normal operation range surrounded by a surge line representing the minimum flow rate and a choking line representing the maximum flow rate, for each of a case where a normal compressor is further equipped with a recirculation flow channel, and a case where it is equipped with both of a recirculation flow channel and intake-flow guide vanes. 
The effect to improve the surge phenomenon is most remarkable in the case where both of the recirculation flow channel and the intake-flow guide vanes are provided. 
Accordingly, for a centrifugal compressor, it is desirable to achieve a wide flow-rate range between a choke flow rate and a surge flow rate, in which stable operation is enabled. 
The disclosure of Patent Document 1 is to achieve such an object. 
According to Patent Document 1, the centrifugal compressor includes guide vanes for generating a swirl flow in intake air at an upstream side of an impeller wheel, a swirl-flow generating unit for applying the swirl flow of the intake air to the impeller wheel, and a recirculation flow channel disposed on a housing of the centrifugal compressor.  The recirculation flow channel recirculates a part of the intake air sucked into the impeller wheel to an intake channel at the upstream of the swirl-flow generating unit. 
Such a technique will be described now with reference to FIG.  11. 
An impeller wheel 101 of a centrifugal compressor 100 includes a plurality of vanes 104 that is rotatable inside a housing 102.  The housing 102 has an inner wall disposed adjacently to radially-outer edges 104a of the vanes 104. 
The intake port of the centrifugal compressor 100 includes an outer annular wall 107 forming an intake-air suction inlet 108 and an inner annular wall 109 extending inside the outer annular wall 107 to form an inducer 110. 
A circulation gas channel 111 is formed between the outer annular wall 107 and the inner annular wall 109. 
The circulation gas channel 111 communicates with a housing surface 105 via a downstream opening 113.  The vanes 104 pass through the vicinity of the housing surface 105. 
An upstream opening connects the circulation gas channel 111 and the inducer 110, i.e., the intake-air suction inlet 108. 
Guide vanes 114 are disposed inside the inducer 110 of the upstream opening. 
The guide vanes 114 generate a preceding swirl in the intake air flowing through the inducer 110. 
With the above configuration, if the flow rate of the intake air flowing through the compressor is small, the direction of the intake air flowing through the circulation gas channel 111 reverses.  Thus, the intake air flows through the downstream opening 113 from the impeller wheel 101 and through the circulation gas channel 111 in the upstream direction to be reintroduced into the intake-air suction inlet 108, thereby recirculating in the compressor. 
In this way, the performance of the compressor is stabilized, and the surge margin and the choke flow rate of the compressor are both improved. 
Further, Patent Document 1 discloses accommodating an intake-air guide vane device in the space inside the inner annular wall 109. 
The intake-air guide vane device includes a plurality of guide vanes 114 extending in a radial direction between a nose cone 115 at the center and the inner annular wall 109. 
The guide vanes 114 induce a preceding swirl so that the intake air flows in a direction that promotes the rotation of the impeller wheel 101.  The preceding swirl improves the surge margin (surge limit) of the centrifugal compressor. ( See the case with both of the recirculation channel and guide vanes in FIG.  10). 
Further, according to Patent Document 2 (FIG.  4 in particular) a recirculation channel (cavity) extending in a direction of the flow path of the intake channel and along the circumferential direction is formed on a housing that surrounds the outer periphery of an intake channel. 
The recirculation channel includes an air suction inlet that has an opening at an intermediate position of an impeller wheel, and an intake-air outlet that has an opening in the intake channel at the upstream side of the impeller wheel to open toward the center of the rotational axis of the impeller wheel. 
In the housing between the leading edge (long blade) of the impeller wheel of the intake channel and the intake-air outlet, a plurality of inlet guide vanes are arranged at intervals in the circumferential direction. 
The inlet guide vanes are disposed on the outer side, in the radial direction, of the outer circumferential edge of the leading edge of the impeller wheel, and inclined from the rotational axis. 
The inclining direction of the inlet guide vanes is set so as to swirl the intake air having flowed through the intake channel in a direction opposite to the rotational direction of the impeller wheel. 
If the flow rate of air at the inlet of the impeller wheel is small, incidence (difference between a relative flow angle and a vane angle) of the leading edge of the impeller increases, which may bring about separation of the air flow in the vicinity of the leading edge of the vanes and eventually surging of the centrifugal compressor. 
In view of this, a swirl in the opposite direction to the rotational direction of the impeller wheel is applied to the flow of the intake air around the housing of the leading edge of the impeller so as to suppress generation of separation of air flow in the vicinity of the leading edge of the vanes, thereby improving the surge margin and widening the operation range of the centrifugal compressor.