Impeller of centrifugal compressor, centrifugal compressor, and turbocharger

An impeller of a centrifugal compressor is an impeller 5 of a centrifugal compressor, that is, a compressor impeller 5 and includes a hub, at least one airfoil portion erected on a hub surface of the hub, and a first fillet. The at least one airfoil portion has a trailing edge configured such that a distance between the trailing edge and an axis of the centrifugal compressor increases with increasing distance from a back surface of the hub. The first fillet is formed on a radially outer side of an outer peripheral surface of a back plate portion forming a back surface portion of the hub. The first fillet connects the outer peripheral surface of the back plate portion and the trailing edge of the at least one airfoil portion.

TECHNICAL FIELD

The present disclosure relates to an impeller of a centrifugal compressor, the centrifugal compressor, and a turbocharger.

BACKGROUND

For example, a turbocharger is known as a turbo device for improving an output of an engine by utilizing energy of an exhaust gas discharged from the engine. The turbocharger rotary drives a turbine impeller by the exhaust gas discharged from the engine, thereby rotary driving a compressor impeller coaxially connected to the turbine impeller to compress intake air and supplying the compressed intake air to the engine (see, for example, Patent Document 1).

CITATION LIST

Patent Literature

SUMMARY

Technical Problem

In recent years, there has been a demand for a high compression ratio of a compressor, and in order to achieve the high compression ratio, a high peripheral speed of a compressor impeller (impeller) is required.

In order to increase the peripheral speed of the impeller, it is conceivable to change the shape of a trailing edge of an airfoil portion, in addition to increasing a rotation speed of the impeller.

For example, in the centrifugal compressor disclosed in Patent Document 1 described above, a part of a trailing edge of an airfoil is projected radially outward relative to a maximum diameter portion of a hub of an impeller, thereby increasing the peripheral speed at the trailing edge.

However, simply projecting a part of the trailing edge of the airfoil radially outward relative to the maximum diameter portion of the hub of the impeller may lead to an increase in stress due to a centrifugal force acting on the airfoil portion and a decrease in natural frequency of the airfoil portion.

In view of the above, an object of at least one embodiment of the present disclosure is to increase the compression ratio of the centrifugal compressor while ensuring durability of the centrifugal compressor.

Solution to Problem

(1) An impeller of a centrifugal compressor according to at least one embodiment of the present disclosure includes a hub, at least one airfoil portion erected on a hub surface of the hub, the at least one airfoil portion having a trailing edge configured such that a distance between the trailing edge and an axis of the centrifugal compressor increases with increasing distance from a back surface of the hub, and a first fillet which is formed on a radially outer side of an outer peripheral surface of a back plate portion forming a back surface portion of the hub, the first fillet connecting the outer peripheral surface and the trailing edge of the at least one airfoil portion.

(2) A centrifugal compressor according to at least one embodiment of the present disclosure includes the impeller of the centrifugal compressor as defined in the above (1), and a compressor housing for housing the impeller.

(3) A turbocharger according to at least one embodiment of the present disclosure includes the centrifugal compressor as defined in the above (2).

Advantageous Effects

According to at least one embodiment of the present disclosure, it is possible to increase the compression ratio of a centrifugal compressor while ensuring durability of the centrifugal compressor.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described below with reference to the accompanying drawings. It is intended, however, that unless particularly identified, dimensions, materials, shapes, relative positions and the like of components described or shown in the drawings as the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present disclosure.

Further, for instance, an expression of a shape such as a rectangular shape or a tubular shape shall not be construed as only the geometrically strict shape, but also includes a shape with unevenness or chamfered corners within the range in which the same effect can be achieved.

On the other hand, the expressions “comprising”, “including”, “having”, “containing”, and “constituting” one constituent component are not exclusive expressions that exclude the presence of other constituent components.

(Overall Configuration of Turbocharger1)

First, with reference toFIG.1, a turbocharger which is provided with a centrifugal compressor including an impeller according to some embodiments will be described.FIG.1is a schematic cross-sectional view of the turbocharger according to some embodiments. As shown in the figure, a turbocharger1is provided with a centrifugal compressor2including a compressor impeller5. The turbocharger1includes a rotational shaft4, a compressor impeller5(impeller5) disposed on one end portion of the rotational shaft4, a turbine impeller8disposed on another end portion of the rotational shaft4, and a bearing24for rotatably instructing the rotational shaft4. The bearing24is located between the compressor impeller5and the turbine impeller8in the axial direction of the rotational shaft4. Although not particularly limited, the turbocharger1according to some embodiments is a turbocharger mounted on, for example, an automobile engine or the like.

The compressor impeller5includes a hub6, and a plurality of airfoil portions7erected on a hub surface61of the hub6. The turbine impeller8includes a hub11, and a plurality of airfoils9erected on a hub surface11aof the hub11. The rotational shaft4, the compressor impeller5, and the turbine impeller8have a common central axis AX.

Further, the turbocharger1includes a compressor housing10for housing the compressor impeller5, a turbine housing12for surrounding the turbine impeller8, and a bearing housing14located between the compressor housing10and the turbine housing12in the axial direction of the rotational shaft4. The compressor housing10and the bearing housing14, and the turbine housing12and the bearing housing14may be fastened by bolts (not shown), respectively.

The compressor housing10includes an air inlet16opening axially outward in one end portion of the turbocharger1in the axial direction, and forms an annular flow passage18located on the radially outer side of the compressor impeller5.

Moreover, the turbine housing12includes an exhaust gas outlet20opening axially outward in another end portion of the turbocharger1in the axial direction, and forms an annular flow passage22located on the radially outer side of the turbine impeller8.

The turbocharger1having the above-described configuration operates as follows, for example.

Air flows into the compressor impeller5via the air inlet16, and the air is compressed by the compressor impeller5rotating with the rotational shaft4. The thus generated compressed air is temporarily discharged from the turbocharger1via the annular flow passage18formed on the radially outer side of the compressor impeller5and is supplied to, for example, a combustion engine (not shown).

In the combustion engine, fuel is combusted with the above-described compressed air, and a combustion gas is generated by this combustion reaction. The combustion gas flows into the turbine impeller8via the annular flow passage22formed on the radially outer side of the turbine impeller8, as an exhaust gas discharged from the combustion engine. The flow of the above-described inflow exhaust gas applies a rotational force to the turbine impeller8, thereby driving the rotational shaft4. The exhaust gas having finished work in the turbine is discharged from the turbocharger1via the exhaust gas outlet20.

Next, the compressor impeller5(impeller5) according to some embodiments will be described more specifically.

FIG.2is a schematic perspective view of the impeller according to an embodiment.

FIG.3is a schematic view showing a schematic meridional cross-section of the impeller according to an embodiment.

Since the basic configuration of the impeller5according to another embodiment described later is the same as that of the impeller5according to an embodiment, in the following description, the impeller5according to an embodiment and the impeller5according to another embodiment will be described with reference toFIGS.2and3.

As shown inFIGS.2and3, in the impeller5according to some embodiments, each of the plurality of airfoil portions7disposed around the hub6of the impeller5extends between a leading edge26located on a most upstream side and a trailing edge28located on a most downstream side in a flow direction of a fluid flowing into the impeller5, and between a hub side end30and a shroud side end (tip end)32. The hub side end30corresponds to a position of the airfoil portion7connected to the hub6. The shroud side end32is an end located opposite to the hub side end30and is located adjacent to the compressor housing10(seeFIG.1).

In the impeller5according to some embodiments, the hub6includes a back plate of the impeller5, that is, a back plate portion forming a back surface portion of the hub6. In the following description, the back plate portion will also be referred to as a back plate portion67.

In the impeller5according to some embodiments, a surface on a back surface side of the back plate portion67is a back surface63of the hub6. The back plate portion67has an outer peripheral surface65which is a radially outer surface of the back plate portion67. In the impeller5according to some embodiments, each of the plurality of airfoil portions7is inclined so as to tilt toward the side of a pressure surface72. That is, each of the plurality of airfoil portions7is formed to gradually be directed from the side of a suction surface71to the side of the pressure surface72, toward the shroud side end32from the hub side end30.

In the following description, when a rotational direction of the impeller5is illustrated, it is represented by an arrow R.

FIG.4Ais a view schematically showing a part of the impeller on an outer peripheral side when the impeller is viewed from a back surface according to an embodiment.

FIG.4Bis a view schematically showing a part of the impeller on the outer peripheral side when the impeller is viewed from the back surface according to another embodiment.

As described above, each of the plurality of airfoil portions7is inclined so as to tilt toward the side of the pressure surface72. However, inFIGS.4A and4B, for convenience, the airfoil portion7is represented without reflecting the above-described inclination of the airfoil portion7.

FIG.5Ais a schematic meridional cross-sectional view of the impeller according to an embodiment, and shows a case where the suction surface of the airfoil portion is viewed from a first angular position C5awhich is an angular position of the back plate portion inFIG.4A.

FIG.5Bis a schematic meridional cross-sectional view of the impeller according to another embodiment, and shows a case where the suction surface of the airfoil portion is viewed from a first angular position C5bwhich is an angular position of the back plate portion inFIG.4B.

FIG.6Ais a schematic meridional cross-sectional view of the impeller according to an embodiment, and shows a case where the suction surface of the airfoil portion is viewed from a second angular position C6awhich is an angular position of the back plate portion inFIG.4A.

FIG.6Bis a schematic meridional cross-sectional view of the impeller according to another embodiment, and shows a case where the suction surface of the airfoil portion is viewed from a second angular position C6bwhich is an angular position of the back plate portion inFIG.4B.

FIG.7is a schematic meridional cross-sectional view of the impeller according to an embodiment, and is a meridional cross-sectional view at a third angular position C7awhich is an angular position of the back plate portion inFIG.4A. A meridional cross-sectional view at a third angular position C7b, which is an angular position of the back plate portion67inFIG.4B, is the same as the meridional cross-sectional view at the third angular position C7ashown inFIG.4A, and thus in the following description, other embodiments will also be described with reference to the meridional cross-sectional view ofFIG.7.

A difference between the impeller5according to an embodiment shown inFIGS.4A,5A, and6Aand the impeller5according to the another embodiment shown inFIGS.4B,5B, and6B is mainly the presence or absence of an inter-airfoil fillet105described later.

In the impeller5according to some embodiments, as shown inFIGS.4A,4B,5A,5B,6A,6B and7, in order to improve the pressure ratio in the centrifugal compressor2by improving the peripheral speed at the trailing edge28, the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67. More specifically, in the impeller5according to some embodiments, as shown inFIGS.5A,5B,6A,6B and7, each of the airfoil portions7has the trailing edge28configured such that a distance between the trailing edge28and the central axis (axis) AX of the centrifugal compressor2increases with increasing distance from the back surface63of the hub6. In the impeller5according to some embodiments, as shown inFIGS.5A,5B,6A,6B and7, the trailing edge28is formed such that the distance between the trailing edge28and the axis AX (seeFIG.3) is the shortest at a position of the back plate portion67connected to the outer peripheral surface65and the distance between the trailing edge28and the axis AX gradually increases toward a front surface side (a left side in the figure) along the axis AX.

In the following description, with respect to a direction along the axis AX in the impeller5, a direction from the leading edge26to the back surface63will be referred to as an axial back surface side, or simply be referred to as a back surface side, and a direction from the back surface63to the leading edge26will be referred to as an axial front surface side, or simply be referred to as a front surface side.

In the impeller5according to some embodiments, as shown inFIGS.3,4A,4B,5A,5B,6A,6B and7, in order to increase the peripheral speed of the impeller5, the entire trailing edge28projects radially outward from the outer peripheral surface65of the back plate portion67. Not the entire trailing edge28but a part of the trailing edge28may be projected radially outward from the outer peripheral surface65of the back plate portion67.

If the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67as in the impeller5according to some embodiments, the vicinity of the trailing edge28of the airfoil portion7is separated from the hub surface61, which may cause a decrease in natural frequency of the airfoil portion7. Further, if the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67as in the impeller5according to some embodiments, due to a centrifugal force acting on a portion projecting radially outward from the outer peripheral surface65of the back plate portion67, a stress generated in the airfoil portion7increases as compared with a case without the above-described portion.

Thus, the impeller5according to some embodiments includes the following configuration.

That is, as shown inFIGS.4A,4B,5A,5B,6A,6B and7, the impeller5according to some embodiments includes a first fillet110connecting the trailing edge28and the outer peripheral surface65of the back plate portion67. As shown inFIGS.4A,4B,5A,5B,6A,6B and7, the first fillet110according to some embodiments is formed on a radially outer side of the outer peripheral surface65of the back plate portion67forming the back surface portion of the hub6. As shown inFIGS.5A,5B,6A,6B and7, the first fillet110according to some embodiments smoothly connects the trailing edge28and the outer peripheral surface65of the back plate portion67. Thus, in the meridional view, a section where an angle changes suddenly does not occur in a connection portion51(seeFIG.3) between the trailing edge28and the outer peripheral surface65of the back plate portion67.

In the impeller5according to some embodiments, the first fillet110can be formed so as to connect the trailing edge28and the outer peripheral surface65of the back plate portion67within a range excluding a range that overlaps with at least a second fillet82and a third fillet83described later, when the impeller5is viewed from the radially outer side.

The shape of the trailing edge28in a case without the first fillet110is indicated by a double-dotted chain line as a virtual trailing edge28A inFIGS.5A,5B,6A and6B, for example.

In the impeller5according to an embodiment, as shown inFIGS.5A and6A, an end portion of the virtual trailing edge28A on the side of the hub6(back surface side) contacts a front surface side-edge portion of the outer peripheral surface65of the back plate portion67, that is, a radially outer edge portion of the hub surface61. InFIG.6A, a position of the outer peripheral surface65when it is assumed that the first fillet110is not formed is represented by a double-dotted chain line65B.

Further, in the impeller5according to the another embodiment, as shown inFIGS.5B and6B, the end portion of the virtual trailing edge28A on the side of the hub6(back surface side) contacts a front surface side-edge portion of a virtual outer peripheral surface65A which is assumed that the inter-airfoil fillet105described later is not provided.

In the impeller5according to some embodiments, as shown inFIGS.4A,4B,5A,5B,6A,6B and7, since the first fillet110connects the trailing edge28of the airfoil portion7and the outer peripheral surface65of the back plate portion67, it is possible to improve rigidity of the airfoil portion7in the vicinity of the trailing edge28. Thus, it is possible to suppress the decrease in natural frequency of the airfoil portion7while increasing the peripheral speed of the impeller. Further, in the impeller5according to some embodiments, as shown inFIGS.4A,4B,5A,5B,6A,6B and7, since the first fillet110can bear a part of the above-described stress, it is possible to suppress the stress on the airfoil portion7in the vicinity of the trailing edge28. Therefore, with the impeller5according to some embodiments shown inFIGS.4A,4B,5A,5B,6A,6B, and7, it is possible to ensure the durability of the impeller5while increasing the peripheral speed of the impeller.

Further, with the impeller5according to some embodiments shown inFIGS.4A,4B,5A,5B,6A,6B and7, in a case where the impeller5is manufactured by machining, it is possible to mitigate the sudden change in angle from the trailing edge28to the outer peripheral surface65of the back plate portion67when cutting from the trailing edge28to the outer peripheral surface65of the back plate portion67, facilitating processing.

In some embodiments, as shown inFIGS.4A and4B, the impeller5further includes the second fillet82connecting the hub surface61and the suction surface71of the airfoil portion7, and the third fillet83connecting the hub surface61and the pressure surface72of the airfoil portion7. The first fillet110includes a fillet portion on suction surface102connecting the second fillet82and the outer peripheral surface65of the back plate portion67, and a fillet portion on pressure surface103connecting the third fillet83and the outer peripheral surface65of the back plate portion67.

As described above, if the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67, due to the centrifugal force acting on the portion projecting radially outward from the outer peripheral surface65of the back plate portion67, the stress generated in the airfoil portion7increases as compared with the case without the above-described portion. However, in some embodiments, as shown inFIGS.4A and4B, since the fillet portion on suction surface102and the fillet portion on pressure surface103can also bear a part of the stress generated in the airfoil portion7, it is possible to further suppress the stress on the airfoil portion7in the vicinity of the trailing edge28.

Further, in some embodiments, as shown inFIGS.4A and4B, since the first fillet110includes the fillet portion on suction surface102and the fillet portion on pressure surface103, it is possible to further improve rigidity of the airfoil portion7in the vicinity of the trailing edge28. Thus, it is possible to further suppress the decrease in natural frequency of the airfoil portion7.

In some embodiments, as shown inFIGS.4A and4B, a circumferential length of the fillet portion on pressure surface103is greater than a circumferential length of the fillet portion on suction surface102.

In the case where the impeller5is manufactured by machining, if the airfoil portion7is formed to be inclined toward the side of the pressure surface72, it is easy to insert a tool used for cutting between the hub surface61and the suction surface71of the airfoil portion7, but it is difficult to insert the tool between the hub surface61and the pressure surface72of the airfoil portion7. Thus, despite an attempt to reduce the thickness of the second fillet82and the third fillet83as much as possible, the third fillet83is more likely to remain thick than the second fillet82, and the circumferential length of the third fillet83is likely to be greater than that of the second fillet82. Therefore, if the fillet portion on suction surface102and the fillet portion on pressure surface103are respectively formed in accordance with the shapes of the second fillet82and the third fillet83, the circumferential length of the fillet portion on pressure surface103is likely to be greater than the circumferential length of the fillet portion on suction surface102. On the contrary, in order to make the circumferential length of the fillet portion on pressure surface103less than the circumferential length of the fillet portion on suction surface102, it takes time and effort for processing. Therefore, according to some embodiments, processing becomes easy.

In the impeller5according to some embodiments, for example, as shown inFIGS.4A and4B, the airfoil portion7includes a first airfoil portion7A and a second airfoil portion7B adjacent to the first airfoil portion7A at an interval in the circumferential direction on the side of the suction surface71of the first airfoil portion7A. Then, as shown inFIGS.4B and5B, the impeller5according to the another embodiment further includes the inter-airfoil fillet105which connects the fillet portion on suction surface102formed on the side of the suction surface71of the first airfoil portion7A and the fillet portion on pressure surface103formed on the side of the pressure surface72of the second airfoil portion2B, on the outer peripheral side of the back plate portion67.

In a case where the impeller5is formed by cutting work, if the outer circumference is cut while rotating the impeller5around the axis AX, the inter-airfoil fillet105is also formed in the outer peripheral portion of the back plate portion67when the first fillet110is formed. If the inter-airfoil fillet105is not provided, it is necessary to remove the inter-airfoil fillet105by cutting or the like in the case where the inter-airfoil fillet105is formed as described above.

Therefore, with the impeller5according to the another embodiment, the impeller5is processed easily as compared with the case without the inter-airfoil fillet105.

If the impeller5is formed by cutting work as described above, the inter-airfoil fillet105does not project to the side of the hub surface61.

(Regarding Shape of First Fillet110)

The shape of the first fillet110will be described mainly with reference toFIGS.5A,5B, and8.FIG.8is a schematic meridional cross-sectional view for describing another embodiment regarding the shape of the first fillet, and shows a case where the suction surface of the airfoil portion is viewed from the first angular position C5awhich is the angular position of the back plate portion inFIG.4A.FIGS.5A,5B, and8each show the range of the first fillet110by an auxiliary line.

In some embodiments, as shown inFIGS.5A,5B, and8, at least a part of the first fillet110has a curved shape in which a center of curvature C exists on the radially outer side of the outer peripheral surface65, in a meridional cross-section of the impeller5. That is, for example, in the impeller5according to an embodiment, as shown inFIG.5A,5B, the first fillet110forms the curved shape from a first end surface110aon the side of the trailing edge28to a second end surface110bon the side of the outer peripheral surface65in the meridional cross-section of the impeller5. In the embodiment shown inFIG.5A,5B, the first fillet110is formed along one arc AR1centered on the one center of curvature C in the meridional cross-section of the impeller5. However, the curvature may change between the first end surface110aand the second end surface110b.

Further, as in the embodiment shown inFIG.8described later, in the meridional cross-section of the impeller5, if the first fillet110includes a first curved portion111, a second curved portion113, and a straight portion115from the first end surface110ato the second end surface110b, the curvature may be the same or may be different between the first curved portion111and the second curved portion113.

Since at least a part of the first fillet110has the curved shape in which the center of curvature C exists on the radially outer side of the outer peripheral surface65in the meridional cross-section of the impeller5, as compared with a case without the curved shape, a position of a radially outer surface110sof the first fillet110is located on the radial inner side. That is, according to the embodiments shown inFIGS.5A,5B, and8, in the meridional cross-section of the impeller5, for example, as compared with a case where the first end surface110aand the second end surface110bare connected by a plane190indicated by a long dashed double-dotted straight line inFIG.5A, the position of the radially outer surface110sof the first fillet110is located on the radially inner side. Thus, as compared with the case without the curved shape as described above, it is possible to reduce the thickness of the first fillet and to suppress the stress generated by the centrifugal force.

In some embodiments, as shown inFIG.8, at least a part of the first fillet110may have a linear shape in the meridional cross-section of the impeller5.

For example, in the embodiment shown inFIG.8, the first fillet110includes the first curved portion111, the second curved portion113, and the straight portion115. The first curved portion111and the second curved portion113each have the curved shape in which the center of curvature exists on the radially outer side of the outer peripheral surface65, in the meridional cross-section of the impeller5. The straight portion115has the linear shape in the meridional cross-section of the impeller5.

For example, in the embodiment shown inFIG.8, in the first fillet110, in the first curved portion111, the first curved portion111, the straight portion115, and the second curved portion113are disposed in order from the side of the trailing edge28to the side of the outer peripheral surface65.FIG.8shows, by a double-dotted chain line, an assumed case where a virtual arc AR2, in which the center of curvature exists on the radially outer side of the outer peripheral surface65, connects the first curved portion111and the second curved portion113in the meridional cross-section of the impeller5.

Since at least a part of the first fillet110has the linear shape in the meridional cross-section of the impeller5, processing becomes easy when the impeller5is formed by cutting work.

Since the centrifugal compressor2according to some embodiments includes the impeller5according to some embodiments described above, it is possible to increase the compression ratio of the centrifugal compressor2while ensuring the durability of the centrifugal compressor2.

Further, since the turbocharger1according to some embodiments includes the above-described centrifugal compressor2, it is possible to increase the compression ratio of the centrifugal compressor2while ensuring the durability of the centrifugal compressor2.

The present disclosure is not limited to the above-described embodiments, and also includes an embodiment obtained by modifying the above-described embodiments and an embodiment obtained by combining these embodiments as appropriate.

For example, in some embodiments described above, the first fillet110is formed for each of all the airfoil portions7. However, the first fillet110may be formed for at least one airfoil portion7.

Further, in some embodiments described above, the second end surface110bof the first fillet110on the side of the outer peripheral surface65is located on the front surface side relative to the back surface side-edge portion on the outer peripheral surface65of the back plate portion67. However, the second end surface110bof the first fillet110on the side of the outer peripheral surface65may be located in the back surface side-edge portion on the outer peripheral surface65of the back plate portion67.

The contents described in the above embodiments would be understood as follows, for instance.

(1) An impeller5of a centrifugal compressor2according to at least one embodiment of the present disclosure is the impeller5of the centrifugal compressor2, that is, the compressor impeller5and includes a hub6, at least one airfoil portion7erected on a hub surface61of the hub6, and a first fillet110. The at least one airfoil portion7has a trailing edge28configured such that a distance between the trailing edge28and an axis AX of the centrifugal compressor2increases with increasing distance from a back surface63of the hub6. The first fillet110is formed on a radially outer side of an outer peripheral surface65of a back plate portion67forming a back surface portion of the hub6. The first fillet110connects the outer peripheral surface65of the back plate portion67and the trailing edge28of the at least one airfoil portion7.

As described above, in the case where the peripheral speed of the impeller5is to be increased by changing the shape of the trailing edge28of the airfoil portion7, simply projecting a part of the trailing edge28of the airfoil portion7radially outward from the maximum diameter portion of the hub6of the impeller5may lead to the increase in stress due to the centrifugal force acting on the airfoil portion7and the decrease in natural frequency of the airfoil portion7.

That is, if the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67, the vicinity of the trailing edge28of the airfoil portion7is separated from the hub surface61, which may cause the decrease in natural frequency of the airfoil portion7. However, with the above configuration (1), since the first fillet110connects the trailing edge28of the airfoil portion7and the outer peripheral surface65of the back plate portion67, it is possible to improve the rigidity of the airfoil portion7in the vicinity of the trailing edge28. Thus, it is possible to suppress the decrease in natural frequency of the airfoil portion7while increasing the peripheral speed of the impeller5.

Further, if the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67, due to the centrifugal force acting on the portion projecting radially outward from the outer peripheral surface65of the back plate portion67, the stress generated in the airfoil portion7increases as compared with the case without the above-described portion. However, with the above configuration (1), since the first fillet110can bear a part of the above-described stress, it is possible to suppress the stress on the airfoil portion7in the vicinity of the trailing edge28.

Therefore, with the above configuration (1), it is possible to ensure the durability of the impeller5while increasing the peripheral speed of the impeller5.

Further, with the above configuration (1), in the case where the impeller5is manufactured by machining, it is possible to mitigate the sudden change in angle from the trailing edge28to the outer peripheral surface65of the back plate portion67when cutting from the trailing edge28to the outer peripheral surface65of the back plate portion67, facilitating processing.

(2) In some embodiments, in the above configuration (1), the impeller5further includes a second fillet82connecting the hub surface61and a suction surface71of the airfoil portion7, and a third fillet83connecting the hub surface61and a pressure surface72of the airfoil portion7. The first fillet110includes a fillet portion on suction surface102connecting the second fillet82and the outer peripheral surface65of the back plate portion67, and a fillet portion on pressure surface103connecting the third fillet83and the outer peripheral surface65of the back plate portion67.

As described above, if the vicinity of the trailing edge28of the airfoil portion7is projected radially outward from the outer peripheral surface65of the back plate portion67, due to the centrifugal force acting on the portion projecting radially outward from the outer peripheral surface65of the back plate portion67, the stress generated in the airfoil portion7increases as compared with the case without the above-described portion. However, with the above configuration (2), since the fillet portion on suction surface102and the fillet portion on pressure surface103can also bear a part of the above-described stress, it is possible to further suppress the stress on the airfoil portion7in the vicinity of the trailing edge28.

Further, with the above configuration (2), since the first fillet110includes the fillet portion on suction surface102and the fillet portion on pressure surface103, it is possible to further improve the rigidity of the airfoil portion7in the vicinity of the trailing edge28. Thus, it is possible to further suppress the decrease in natural frequency of the airfoil portion7.

(3) In some embodiments, in the above configuration (2), a circumferential length of the fillet portion on pressure surface103is greater than a circumferential length of the fillet portion on suction surface102.

In the case where the impeller5is manufactured by machining, if the airfoil portion7is formed to be inclined toward the side of the pressure surface72, it is easy to insert the tool used for cutting between the hub surface61and the suction71surface of the airfoil portion7, but it is difficult to insert the tool between the hub surface61and the pressure surface72of the airfoil portion7. Thus, despite the attempt to reduce the thickness of the second fillet82and the third fillet83as much as possible, the third fillet83is more likely to remain thick than the second fillet82, and the circumferential length of the third fillet83is likely to be greater than that of the second fillet82. Therefore, if the fillet portion on suction surface102and the fillet portion on pressure surface103are respectively formed in accordance with the shapes of the second fillet82and the third fillet83, it is likely to obtain the above configuration (3). Conversely, on the contrary to the above configuration (3), in order to make the circumferential length of the fillet portion on pressure surface103less than the circumferential length of the fillet portion on suction surface102, it takes time and effort for processing. Therefore, with the above configuration (3), processing becomes easy.

(4) In some embodiments, in the above configuration (2) or (3), the airfoil portion7includes a first airfoil portion7A and a second airfoil portion7B adjacent to the first airfoil portion7A at an interval in a circumferential direction on a side of the suction surface71of the first airfoil portion7A. The impeller5further includes an inter-airfoil fillet105which connects the fillet portion on suction surface102formed on the side of the suction surface71of the first airfoil portion7A and the fillet portion on pressure surface103formed on a side of the pressure surface72of the second airfoil portion2B, on an outer peripheral side of the back plate portion67.

In a case where the impeller5is formed by cutting work, if the outer circumference is cut while rotating the impeller5around the axis AX, the inter-airfoil fillet105is also formed in the outer peripheral portion of the back plate portion67when the first fillet110is formed. If the inter-airfoil fillet105is not provided, it is necessary to remove the inter-airfoil fillet105by cutting or the like in the case where the inter-airfoil fillet105is formed as described above.

Therefore, with the above configuration (4), the impeller5is processed easily as compared with the case without the inter-airfoil fillet105.

(5) In some embodiments, in any one of the above configurations (1) to (4), at least a part of the first fillet110has a linear shape in a meridional cross-section of the impeller5.

With the above configuration (5) having the above-described linear shape, processing becomes easy when the impeller5is formed by cutting work.

(6) In some embodiments, in any one of the above configurations (1) to (5), at least a part of the first fillet110has a curved shape in which a center of curvature exists on a radially outer side of the outer peripheral surface65, in a meridional cross-section of the impeller5.

With the above configuration (6), having the above-described curved shape, as compared with the case without the above-described curved shape, the position of the radially outer surface110sof the first fillet110is located on the radial inner side. Thus, as compared with the case without the above-described curved shape, it is possible to reduce the thickness of the first fillet110and to suppress the stress generated by the centrifugal force.

(7) A centrifugal compressor2according to at least one embodiment of the present disclosure includes the impeller5of the centrifugal compressor2according to any one of the above configurations (1) to (6), and a compressor housing10for housing the impeller.

With the above configuration (7), including the impeller5of the centrifugal compressor2according to any one of the above configurations (1) to (6), it is possible to increase the compression ratio of the centrifugal compressor2while ensuring the durability of the centrifugal compressor2.

(8) A turbocharger1according to at least one embodiment of the present disclosure includes the centrifugal compressor2according to the above configuration (7).

With the above configuration (8), including the centrifugal compressor2according to the above configuration (7), it is possible to increase the compression ratio of the centrifugal compressor2while ensuring the durability of the centrifugal compressor2.

REFERENCE SIGNS LIST