1
DESCRIPTION
CENTRIFUGAL COMPRESSOR
Technical Field
[0001]
The present invention relates to a centrifugal compressor
of a turbo charger or the like.
Background Art
[0002]
Conventionally, a centrifugal compressor of a turbo
charger or the like to be used for an internal combustion
engine of a motor vehicle for example, is commonly known.
FIG. 5A is a sectional view showing a relevant section of
a conventional centrifugal compressor. A centrifugal
compressor 10 shown in the drawing compresses fluid such as
gas and air introduced from outside a housing 11, by rotating
an impeller 13 having a large number of blades 12 within the
housing 11. The fluid flow (airflow) formed in this way
travels through an impeller exit (Hereunder, also referred to
as "diffuser entry") 14, being an outer circumferential end of
the impeller 13, then through a diffuser passage 15 and a
scroll 16, and is then delivered to the outside. Reference

2
symbol 17 in the drawing denotes a shaft axis about which the
impeller 13 rotates.
[0003]
The diffuser passage 15 mentioned above is provided in
between the impeller exit 14 and the scroll 16, being a
passage for recovering static pressure by slowing down the
airflow discharged from the impeller 14. The diffuser passage
15 is normally formed from a pair of opposing wall surfaces.
In the description below, one of the pair of the opposing wall
surfaces is called a shroud side wall surface 15a and the
other is called a hub side wall surface 15b.
In a turbo charger of a motor vehicle to be used in
combination with an internal combustion engine, since a wide
compressor operation range is required, a type of diffuser
that does not have vanes (vaneless diffuser) is normally
employed.
[0004]
Incidentally, in recent years, in response to an increase
in flow amount and pressure ratio of the centrifugal
compressor 10, distortion of the airflow flowing into the
diffuser passage 15 is becoming greater. It is thought that a
flow with a large distortion flowing into the diffuser passage
15 results in the occurrence of a phenomenon known as surging,
which determines the operation limit on the small flow amount
side in the diffuser passage 15.

3
In the mechanism that gives rise to surging it is thought
that surging occurs when a backflow area of the airflow
reaches an exit side end section of the diffuser passage 15.
Moreover, since the occurrence of the backflow area mentioned
above is thought to be caused by a flow on the compressor
shroud side within the diffuser passage 15, that is,
distortion of the flow along the shroud side wall surface 15a,
a diffuser structure for reducing such distortion in the flow
is proposed.
[0005]
The distortion in the flow mentioned above refers to the
flow velocity distribution or pressure distribution being in a
non-uniform state. Conventional techniques for making this
uniform employ a structure or method that changes a flow
passage sectional area of the diffuser passage 15, or that
utilizes a circulation passage or the like. Each of such
conventional techniques has a focus on reducing distortion
that occurs on the entry side (impeller exit 14 side) of the
diffuser passage 15.
As a conventional technique for reducing the distortion
mentioned above, for example, provision of a convex section or
a concave section for changing the flow passage sectional area
on the wall surface of the diffuser passage 15 has been
proposed. Such a convex section or concave section regularize
the airflow along the circumferential direction by changing

4
the passage shape in the circumferential direction, thereby
enabling an improvement in compression efficiency. (for
example, refer to Patent Document 1).
Patent Document 1: Japanese Unexamined Patent
Application, Publication No. Hei 10-176699
Disclosure of Invention
[0006]
However, in a small centrifugal compressor such as a
turbo charger for a motor vehicle in particular, it is
difficult to measure the internal flow of the airflow flowing
within the diffuser passage. Therefore, since distortion in
actual internal flow has yet to be sufficiently understood and
furthermore the phenomena up to the point of surging have not
been explained, it is necessary to understand these and
develop an effective surging prevention method to widen the
operation range of the centrifugal compressor.
[0007]
As shown in FIG. 5B, the conventional diffuser passage 15
generally has a shape in which a pair of opposing wall
surfaces, namely a shroud side wall surface 15a and a hub side
wall surface 15b, are in a parallel form, so that an axial
direction flow passage width W of the diffuser passage 15 is
constant in the circumferential direction. Having examined a
flow pattern of up to surging, by carrying out internal flow

5
measurement using a model for the conventional diffuser
passage 15 configured in this way, it was revealed that in the
downstream area of the backflow area (shown with an arrow A in
the drawing) becomes a flow area that reaches the diffuser
exit 18 side end section of the diffuser passage 15, the
airflow separates from the hub side wall surface 15b in the
vicinity of the diffuser exit 18, forming a backflow area
(shown with an arrow B in the drawing). That is to say, it is
thought that it is not a shroud side backflow area A, but a
backflow area B which causes surging to occur.
[0008]
It is thought that such a flow separation from the hub
side wall surface 15b occurring in the vicinity of the
diffuser exit 18 cannot be suppressed by a conventional
technique, as disclosed in Patent Document 1 for example, in
which airflow is regularized along the circumferential
direction by changing the passage shape in the circumferential
direction.
In consideration of the above circumstances, an object of
the present invention is to provide a centrifugal compressor
having a diffuser structure in which airflow is unlikely to
separate from a hub side wall surface on the hub side wall
surface downstream side within the diffuser passage (in the
vicinity of the diffuser exit).
[0009]

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The present invention employs the following means to
solve the above problems.
A centrifugal compressor according to the present
invention is a centrifugal compressor provided with a diffuser
passage for recovering static pressure by slowing down
discharged air from an outer circumferential end of an
impeller that rotates within a housing, wherein a hub side
wall surface of the diffuser passage is provided with an
inclined area that approaches toward a shroud side, in a
position on a downstream side of a portion parallel with a
normal line direction of a section surface of an impeller exit.
[0010]
According to such a centrifugal compressor, since the hub
side wall surface of the diffuser passage is provided with the
inclined area that approaches the shroud side, in the position
on the downstream side of the portion parallel with the normal
line direction of the section surface of the impeller exit,
the radial direction velocity of the low velocity area that
occurs on the hub side wall surface increases in the inclined
area, and the radial direction velocity distribution within
the diffuser passage is made uniform.
The inclined area in this case is a portion of an
inclined plane, a curved surface, or a stepped plane, formed
on the hub side wall surface for example, and must be such
that, in this inclined area, the axial direction flow passage

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width of the diffuser passage narrows down from the upstream
side toward the downstream side by inclining the hub side wall
surface so as to approach toward the opposed shroud side wall
surface as it approaches the downstream side.
[0011]
In the above aspect of the invention, a preferable
position in the diffuser passage length direction for
providing the inclined area, in the diffuser passage in a
range between the diffuser entry taken as a base point (0) and
the diffuser exit (1), is within a range on the downstream
side (exit side) having a ratio of 0.3 to 0.7.
Moreover, in the above aspect of the invention, it is
preferable that the inclined area provided in the diffuser
passage be such that the maximum value of an amount of
projection from the hub side wall surface toward the shroud
side wall surface is set to approximately 1/3 to 1/5 of the
passage width, being equivalent to the size of the measured
backflow area.
Furthermore, in the above aspect of the invention, the
preferable inclination angle is less than or equal to 20
degrees based on the normal line of the impeller exit section
when the inclined area is taken as a plane. However, a more
preferable inclination angle is between 2 degrees and 10
degrees, inclusive, based on the normal line of the impeller
exit section. An excessively large inclination angle is not

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preferable, as the airflow is re-accelerated due to the
reduction in the passage area in this case.
[0012]
According to the present invention described above, since
on the hub side wall surface of the diffuser passage there is
provided the inclined area that approaches the shroud side in
the position on the downstream side of the portion parallel
with the normal line of the impeller exit section, the radial
direction velocity of a low velocity area that occurs on the
hub side wall surface increases in this inclined area.
Therefore, the radial direction velocity distribution within
the diffuser passage is made uniform, and separation of the
airflow is unlikely to occur locally, so that the surge flow
rate can be reduced and the compressor operation range can be
widened.
Moreover, the present invention described above is suited
for widening the operation range of a small centrifugal
compressor provided with a vaneless diffuser, such as a turbo
charger for a motor vehicle, which requires a particularly
wide compressor operation range.
Brief Description of Drawings
[0013]

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[FIG. 1] is a sectional view of a diffuser passage
showing a first embodiment of a centrifugal compressor
according to the present invention.
[FIG. 2] is a sectional view showing a first modified
example of FIG. 1.
[FIG. 3] is a sectional view showing a second modified
example of FIG. 1.
[FIG. 4] is a sectional view of a diffuser passage
showing a second embodiment of a centrifugal compressor
according to the present invention.
[FIG. 5A] is a sectional view showing a relevant section
of the centrifugal compressor.
[FIG. 5B] is a sectional view showing a conventional
structure of a diffuser passage.
Explanation of Reference Signs:
[0014]
10: Centrifugal compressor
11: Housing
12: Impeller
14: Impeller exit (diffuser entry)
15, 30: Diffuser passage
15a, 30a: Shroud side wall surface
15b, 30b: Hub side wall surface
16: Scroll

10
18: Diffuser exit
20: Inclined plane (inclined area)
21: Inclined curved surface (inclined area)
22: Inclined inflected line (inclined area)
33: Shroud side inclined section (inclined area)
Best Mode for Carrying Out the Invention
[0015]
Hereunder an embodiment of a centrifugal compressor
according to the present invention is described, based on the
drawings.
As shown in FIG. 5A, a centrifugal compressor 10 is
provided with a diffuser passage 15 that recovers static
pressure by reducing the velocity of a discharged air from an
outer circumferential end of an impeller 13 that rotates
within a housing 11. The diffuser passage 15 is provided so
as to connect between an impeller exit (diffuser entry) 14 and
a scroll 16, and is formed in between a pair of opposing wall
surfaces that comprise a shroud side wall surface 15a and a
hub side wall surface 15b.
[0016]
FIG 1 is a sectional view of the diffuser passage 15
showing a first embodiment. This diffuser passage 15
introduces from the diffuser entry 14, the discharged air
(indicated with white arrows in the drawing) from the outer

11
circumferential end of the impeller 13, and allows the airflow
guided to the passage between the shroud side wall surface 15a
and the hub side wall surface 15b to flow out from a diffuser
exit 18 into the scroll 16.
In the embodiment shown in the drawing, the hub side wall
surface 15b of the diffuser passage 15 is provided with an
inclined plane 20 that approaches the direction of the shroud
side wall surface 15a in a position on the downstream side in
a portion parallel with a normal line of the section of the
impeller exit. This inclined plane 20 is an inclined area,
formed on the hub side wall surface 15b of the diffuser
passage 15, that becomes closer to the shroud side wall
surface 15a as the distance from the diffuser entry 14
increases, approaching the diffuser exit 18 and narrowing an
axial direction flow passage width W, which is a distance
between the opposing wall surfaces, down to Wa.
[0017]
That is to say, in the diffuser passage 15 of normal line
direction flow passage length L, the inclined area formed by
the inclined wall surface 20 inclines toward the direction of
the shroud side wall surface 15a from the upstream side of a
passage of a flow passage length La parallel with the normal
line direction, and is provided in a downstream portion of
flow passage length Lb. Here, taking 1 as the flow passage
length L, the preferred length of the downstream portion Lb,

12
which becomes the inclined area, is a portion remaining on the
downstream side where the length La of the upstream portion
parallel with normal line direction is 0.3 to 0.7. In other
words, the preferred position for providing the inclined area
in the lengthwise direction of the diffuser passage 15 may be
set so that, where the flow passage length from the diffuser
entry 14 which is taken as a base point (0), to the diffuser
exit 18, which is taken as an end point (1), is taken as L
(L=l), the length La of the upstream portion is set at a ratio
of 0.3 to 0.7, and the length Lb of the downstream portion is
set at a ratio of 0.7 to 0.3. Therefore, since the flow
passage length L is the total length of the length La of the
upstream portion and the length Lb of the downstream portion
(L = La + Lb), the total length L of the upstream portion
length La and the downstream portion length Lb is always "1".
[0018]
Moreover, it is preferable that the inclined area
provided in the diffuser passage 15 be such that a projection
amount of the inclined wall surface from the hub side wall
surface 15b to the shroud side wall surface 15a is set to be
approximately 1/3 to 1/5 of the passage width at the diffuser
exit 18, where the projection amount of the inclined wall
surface 20 reaches its maximum value Wb. That is to say,
since the maximum value Wb of the projection amount is 1/3 to
1/5W (Wb ~ 1/3 to 1/5W), the axial direction flow passage

13
width Wa narrowed by the inclined wall surface 20 is set to be
approximately 2/3 to 4/5 of the axial direction flow passage
width W (Wa ~ 2/3 to 4/5W).
Moreover, as is the case with the inclined wall surface
20 mentioned above, the angle of inclination 0 where the
inclined area is a plane surface, is preferably set less than
or equal to 20 degrees based on the normal line of the
impeller exit section. However, a further preferable angle of
inclination 6 is greater than or equal to 2 degrees and less
than or equal to 10 degrees based on the normal line of the
impeller exit section.
The normal line and normal line direction of the impeller
exit section mentioned above refers to a straight line or
direction that extends radially outward from an axial center
line 17 of rotation of the impeller 13 and passes the impeller
exit section, and it practically approximates the airflow
direction.
[0019]
As described above, the diffuser passage 15 provided with
the inclined area formed by the inclined wall surface 20 is
provided with: an upstream side area where the shroud side
wall surface 15a and the hub side wall surface 15b are both
parallel with the normal line direction and the axial
direction flow passage width W is constant; and an inclined
area on the downstream side where the axial direction flow

14
passage width W to the diffuser exit 18 side is narrowed by
the inclined wall surface 20, in which the hub side wall
surface 15b inclines toward the shroud side wall surface 15a.
Therefore the airflow introduced from the diffuser entry
14 is slowed down by flowing through the diffuser passage 15,
and recovers its static pressure. However, at this time, on
the downstream side closer to the diffuser exit 18, the
airflow in a low velocity area, which occurs in the vicinity
of the wall surface and is thought to cause the airflow to
separate from the hub side wall surface 15b, is guided to the
inclined wall surface 20 so that it gradually flows toward the
shroud side wall surface 15a.
[0020]
The low velocity area in this case is an area where a
velocity component in the radial direction from the diffuser
entry 14 toward the diffuser exit 18 is low. In the example
shown in the drawing, the radial direction corresponds to the
normal line direction.
[0021]
As a result, the airflow in the low velocity area that
has occurred in the vicinity of the wall surface of the hub
side wall surface 15b increases its velocity component in the
radial direction. Therefore, in the diffuser passage 15, the
velocity distribution in the radial direction becomes uniform
and flow separation is unlikely to occur locally.

15
As described above, if flow separation is unlikely to
occur locally in the diffuser passage 15, the surge flow rate
can be reduced, enabling the operation range of the
centrifugal compressor to be widened. In particular, if the
invention is applied in the case where a wide range of
compressor operation is required from a small sized
centrifugal compressor such as a turbo charger provided with a
vaneless diffuser, widening of the range can be easily
achieved.
[0022]
Incidentally, considering the case of providing a similar
inclined area on the shroud side wall surface 15a, the axial
direction flow passage width W becomes narrower in the
opposite direction as it approaches the diffuser exit 18.
However, since there is not a low velocity area which is
thought to be a cause of flow separation, present in close
proximity to the wall surface of the shroud side wall surface
15a in the vicinity of the diffuser exit 18, the airflow
guided to the inclined wall surface 20 is accelerated by
gradually flowing toward the hub side wall surface 15b.
Therefore, a difference between the accelerated velocity on
the shroud side wall surface 15 and the velocity on the hub
side wall surface 15b, in which the low velocity area is
present, increases, and hence non-uniformity of the velocity
distribution in the radial direction increases.

16
[0023]
Next, a first modified example of the inclined area
mentioned above is described, based on FIG. 2. The same
reference symbols are given to parts that are the same as in
the above embodiment, and detailed descriptions thereof are
omitted.
In this first modified example, in place of the inclined
plane 20 in FIG. 1, an inclined curved surface 21 forms an
inclined area. This inclined curved surface 21 is the same as
the inclined plane 20 with respect to the preferred position
along the diffuser passage length direction in which the
inclined area is to be provided (ratio of length Lb), and with
respect to the maximum value Wb of the projection amount from
the hub side wall surface 15b toward the shroud side wall
surface 15a, and the curvature may be appropriately set to
satisfy these conditions. The curved surface in this case may
be either a concave curved surface or convex curved surface
when seen from inside the diffuser passage 15.
[0024]
Since the radial direction velocity component of the low
velocity area airflow that has occurred in the vicinity of the
wall surface of the hub side wall surface 15b also increases
even when the inclined curved surface 21 provided is formed by
such an inclined area, the velocity distribution in the radial

17
direction is made uniform so that flow separation becomes
unlikely to occur locally.
Therefore, the surge flow rate can be reduced, and hence
the operation range of the centrifugal compressor can be
widened and, in particular, if the invention is applied in a
case where a wide range of compressor operation is required of
a small size centrifugal compressor provided with a vaneless
diffuser, the compressor operation range can be easily widened.
[0025]
Next, a second modified example of the inclined area
mentioned above is described, based on FIG. 3. The same
reference symbols are given to parts that are the same as in
the above embodiment, and detailed descriptions thereof are
omitted.
In this second modified example, in place of the inclined
plane 20 in FIG. 1, an inclined inflected line 22 forms an
inclined area. This inclined inflected line 22 is formed from
a plane inclined section 22a and a parallel section 22b on the
diffuser exit 18 side, the parallel section 22b in this case
being parallel to the shroud side wall surface 15a and the hub
side wall surface 15b.
Moreover, this inclined inflected line 22 is the same as
the inclined plane 20 with respect to the preferred position
along the diffuser passage length direction in which the
inclined area is to be provided (ratio of length Lb), and with

18
respect to the maximum value Wb of the projection amount from
the hub side wall surface 15b toward the shroud side wall
surface 15a.
[0026]
Since the radial direction velocity component of the low
velocity area airflow that has occurred in the vicinity of the
wall surface of the hub side wall surface 15b also increases
even when the inclined inflected line 22 provided is formed by
such an inclined area, the velocity distribution in the radial
direction is made uniform so that flow separation becomes
unlikely to occur locally.
Therefore, the surge flow rate can be reduced, and hence
the operation range of the centrifugal compressor can be
widened and, in particular, if the invention is applied in a
case where a wide range of compressor operation is required of
a small size centrifugal compressor provided with a vaneless
diffuser, the compressor operation range can be easily widened.
The inclined inflected line 22 shown in the drawing is a
combination of the inclined section 22a and the parallel
section 22b. However, an inclined section 22a of two or more
steps may be combined, and furthermore, a curved surface may
also be combined.
[0027]
FIG. 4 is a sectional view showing a second embodiment of
the centrifugal compressor according to the present invention.

19
The same reference symbols are given to parts that are the
same as in the first embodiment, and detailed descriptions
thereof are omitted.
In the present embodiment, a diffuser passage 30 is
divided into three areas. Specifically, from the upstream
side, a hub side inclined section 31, a parallel section 32
parallel with the normal line direction of the impeller exit
section, and a shroud side inclined section 33 are integrally
connected. Therefore, compared to the first embodiment shown
in FIG. 1, the hub side inclined section 31 is added on the
most upstream side, and, in the shroud side inclined section
33, the shroud side wall surface 30a and the hub side wall
surface 30b are arranged parallel with each other so as to
incline toward the shroud side at the same inclination angle.
[0028]
When employing this kind of configuration also, in the
shroud side inclined section 33 of the diffuser passage 30,
the hub side wall surface 30b is provided with an inclined
area that approaches the shroud side in a position on the
downstream side of the parallel section 32, which is a portion
parallel with the normal line of the impeller exit section.
That is to say, by having the hub side wall surface 30b of the
shroud side inclined section 33 approach the shroud side, an
inclined area having the same effect as that of the inclined
plane 20 mentioned above is formed.

20
As a result, since radial direction velocity component of
the low velocity area airflow that has occurred in the
vicinity of the wall surface of the hub side wall surface 30b
is increased by the hub side wall surface 30b that serves as
an inclined plane similar to the inclined plane 20, the
velocity distribution in the radial direction is made uniform
so that flow separation becomes unlikely to occur locally in
the diffuser passage 30.
Therefore, the surge flow rate can be reduced, and hence
the operation range of the centrifugal compressor can be
widened and, in particular, if the invention is applied in a
case where a wide range of compressor operation is required of
a small size centrifugal compressor provided with a vaneless
diffuser, the compressor operation range can be easily widened.
[0029]
As described above, on the hub side wall surface 15a of
the diffuser passage 15, since the inclined area that
approaches the shroud side is provided in a position on the
downstream side of the portion parallel with the normal line
direction of the impeller exit section, the radial direction
velocity of the low velocity area that occurs on the hub side
wall surface 15a increases, and the radial direction velocity
distribution within the diffuser passage 15 is made uniform.
Therefore, airflow is unlikely to separate locally from the
wall surface in the vicinity of the diffuser exit 18 of the

21
diffuser passage 15, and hence the surge flow rate can be
reduced and the compressor operation range can be widened.
The present invention is not limited to the above
embodiments and can be appropriately modified without
departing from the scope of the present invention.

22
CLAIMS
1. A centrifugal compressor provided with a diffuser passage
for recovering static pressure by slowing down discharged air
from an outer circumferential end of an impeller that rotates
within a housing,
wherein a hub side wall surface of said diffuser passage
is provided with an inclined area that approaches toward a
shroud side, in a position on a downstream side of a portion
parallel with a normal line direction of a section surface of
an impeller exit.

There is provided a centrifugal compressor having a diffuser structure in which airflow is unlikely to separate from a hub side wall surface on a hub side wall surface downstream side within a diffuser passage. In a centrifugal
compressor provided with a diffuser passage 15 for recovering static pressure by slowing down discharged air from an outer circumferential end of an impeller that rotates within a housing 11, a hub side wall surface 15b of the diffuser
passage 15 is provided with an inclined plane 20 that approaches toward a shroud side, in a position on a downstream side of a portion parallel with a normal line direction of a
section surface of an impeller exit.