Patent Publication Number: US-2022235794-A1

Title: Scroll structure of centrifugal compressor and centrifugal compressor

Description:
TECHNICAL FIELD 
     The present disclosure relates to a scroll structure of a centrifugal compressor and a centrifugal compressor. 
     BACKGROUND 
     A centrifugal compressor used in a compressor section of a vehicle or marine turbocharger provides kinetic energy to a fluid through the rotation of the impeller and obtains a pressure increase due to centrifugal force by discharging the fluid outward in the radial direction. 
     This centrifugal compressor is required to have a high pressure ratio and high efficiency over a wide operating range. 
     The centrifugal compressor has a scroll passage formed in spiral shape. The scroll passage has a passage connecting portion where a winding start portion and a winding end portion of the scroll passage intersect. 
     In such a centrifugal compressor, a recirculation flow from the winding end portion to the winding end portion may occur at the passage connecting portion. When the recirculation flow enters from the winding end portion to the winding start portion, the direction of the flow of fluid is changed at the passage connecting portion, and loss occurs when the fluid separates from the wall surface that forms the scroll passage at the winding start portion. Patent Document 1 discloses a scroll structure of a centrifugal compressor in which the shape of the passage connecting portion is modified to suppress such loss (see Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: JP5479316B 
       
    
     SUMMARY 
     Problems to be Solved 
     For example, in the scroll structure of a centrifugal compressor described in Patent Document 1, the recirculation flow is suppressed by reducing the cross-sectional area of the passage connecting portion to reduce the loss. 
     However, there are other causes of the loss at the passage connecting portion. For example, generally, the passage connecting portion has a tongue portion separating the scroll passage from an outlet passage connected to the downstream side of the scroll passage at the most downstream position of the scroll passage in the passage connecting portion. Further, generally, the passage connecting portion has, at a position upstream from the tongue portion in the scroll passage, a ridge portion protruding from an inner peripheral surface of the scroll passage on the downstream side along the flow of fluid entering the centrifugal compressor in the axial direction of the centrifugal compressor (hereinafter, referred to as the axially downstream side) toward the axially upstream side of the centrifugal compressor. This ridge portion is connected to the tongue portion on the downstream side of the scroll passage. 
     The fluid blown from the diffuser into the scroll passage flows into the scroll passage along the axially downstream inner peripheral surface of the inner peripheral surface of the scroll passage. Further, the fluid blown from the diffuser into the scroll passage has a velocity component that moves outward in the radial direction of the centrifugal compressor. Therefore, in the vicinity of the passage connecting portion, the fluid blown from the diffuser into the scroll passage tries to flow over the ridge portion from the inner side to the outer side in the radial direction of the centrifugal compressor. Such a flow of fluid is toward the upstream side in the axial direction of the centrifugal compressor along the flow of fluid entering the centrifugal compressor (hereinafter referred to as the axially upstream side). 
     Further, the flow of fluid in the scroll passage has a main flow along the circumferential direction from the winding start portion to the winding end portion and a swirling flow swirling in the scroll passage along the main flow. The swirling flow is toward the axially downstream side. 
     Therefore, the fluid flow that tries to flow over the ridge portion and the swirling flow interfere with each other, causing the fluid to separate from the inner peripheral surface of the scroll passage near the tongue portion. Such separation causes loss of the centrifugal compressor. 
     However, Patent Document 1 does not mention the suppression of fluid separation as described above. 
     In view of the above, an object of at least one embodiment of the present invention is to provide a scroll structure of a centrifugal compressor and a centrifugal compressor with high efficiency over a wide operating range. 
     Solution to the Problems 
     (1) A scroll structure of a centrifugal compressor according to at least one embodiment of the present invention is a scroll structure of a centrifugal compressor having a scroll passage formed in spiral shape, comprising: a tongue portion separating the scroll passage from an outlet passage connected to a downstream side of the scroll passage at a most downstream position of the scroll passage in a passage connecting portion where a winding start portion and a winding end portion of the scroll passage intersect; and a ridge portion protruding from an inner peripheral surface of the scroll passage on an axially downstream side of the centrifugal compressor toward an axially upstream side of the centrifugal compressor, wherein a protruding height of the ridge portion protruding toward the axially upstream side gradually increases toward the tongue portion from a starting position that is located upstream from the tongue portion in the scroll passage. The starting position is a position at an angle of 8 degrees or less in a circumferential direction of the centrifugal compressor from the tongue portion toward an upstream side of the scroll passage. 
     As described above, when the fluid flow that tries to flow over the ridge portion and the swirling flow in the scroll passage interfere with each other, the fluid may separate from the inner peripheral surface of the scroll passage near the tongue portion. Therefore, it is desirable to suppress the interference between the fluid flow that tries to flow over the ridge portion and the swirling flow in the scroll passage. 
     Typically, the starting position is at an angle of about 15 degrees in the circumferential direction of the centrifugal compressor from the tongue portion toward the upstream side of the scroll passage. 
     In contrast, in the above configuration (1), the starting position is at an angle of 8 degrees or less in the circumferential direction of the centrifugal compressor from the tongue portion toward the upstream side of the scroll passage. Accordingly, in the above configuration (1), the range where the ridge portion extends in the circumferential direction of the centrifugal compressor is reduced compared to a typical centrifugal compressor. 
     Since the ridge portion is a portion protruding from the axially downstream inner peripheral surface of the inner peripheral surface of the scroll passage toward the axially upstream side, by reducing the range where the ridge portion extends in the circumferential direction of the centrifugal compressor, the interference between the fluid flow that tries to flow over the ridge portion and the swirling flow in the scroll passage can be suppressed. 
     Therefore, with the above configuration (1), the separation of fluid from the inner peripheral surface of the scroll passage can be suppressed, and the loss due to the separation can be suppressed. Therefore, it is possible to increase the efficiency of the centrifugal compressor over a wide operating range. 
     (2) A scroll structure of a centrifugal compressor according to at least one embodiment of the present invention is a scroll structure of a centrifugal compressor having a scroll passage formed in spiral shape, comprising: a tongue portion separating the scroll passage from an outlet passage connected to a downstream side of the scroll passage at a most downstream position of the scroll passage in a passage connecting portion where a winding start portion and a winding end portion of the scroll passage intersect; and a ridge portion protruding from an inner peripheral surface of the scroll passage on an axially downstream side of the centrifugal compressor toward an axially upstream side of the centrifugal compressor, wherein a protruding height of the ridge portion protruding toward the axially upstream side gradually increases toward the tongue portion from a starting position that is located upstream from the tongue portion in the scroll passage. The protruding height in a position at an angle of 4 degrees in a circumferential direction of the centrifugal compressor from the tongue portion toward an upstream side of the scroll passage is 10% or less of a height dimension of the scroll passage at the winding start portion along an axial direction of the centrifugal compressor. 
     The ridge portion of a typical centrifugal compressor extends in the range about 15 degrees in the circumferential direction of the centrifugal compressor, as described above. Further, in a typical centrifugal compressor, the protruding height of the ridge portion at the connecting position with the tongue portion is often more than 50% of the height dimension of the scroll passage at the winding start portion along the axial direction of the centrifugal compressor. Consequently, in the ridge portion of a typical centrifugal compressor, the protruding height of the ridge portion in the position at an angle of 4 degrees in the circumferential direction of the centrifugal compressor from the tongue portion toward the upstream side of the scroll passage is often more than 30% of the height dimension of the scroll passage at the winding start portion along the axial direction of the centrifugal compressor. 
     Therefore, with the above configuration (2), since the protruding height of the ridge portion in the position at an angle of 4 degrees in the circumferential direction of the centrifugal compressor from the tongue portion toward the upstream side of the scroll passage is 10% or less of the height dimension of the scroll passage at the winding start portion along the axial direction of the centrifugal compressor, the protruding height of the ridge portion near the tongue portion can be made smaller than the protruding height of the ridge portion in a typical centrifugal compressor. Therefore, with the above configuration (2), the interference between the fluid flow that tries to flow over the ridge portion and the swirling flow in the scroll passage can be suppressed. 
     Therefore, with the above configuration (2), the separation of fluid from the inner peripheral surface of the scroll passage can be suppressed, and the loss due to the separation can be suppressed. Therefore, it is possible to increase the efficiency of the centrifugal compressor over a wide operating range. 
     (3) A scroll structure of a centrifugal compressor according to at least one embodiment of the present invention is a scroll structure of a centrifugal compressor having a scroll passage formed in spiral shape, comprising: a tongue portion separating the scroll passage from an outlet passage connected to a downstream side of the scroll passage at a most downstream position of the scroll passage in a passage connecting portion where a winding start portion and a winding end portion of the scroll passage intersect; and a ridge portion protruding from an inner peripheral surface of the scroll passage on an axially downstream side of the centrifugal compressor toward an axially upstream side of the centrifugal compressor, wherein a protruding height of the ridge portion protruding toward the axially upstream side gradually increases toward the tongue portion from a starting position that is located upstream from the tongue portion in the scroll passage. The protruding height is 30% or less of a height dimension of the scroll passage at the winding start portion along an axial direction of the centrifugal compressor. 
     As a result of intensive studies by the inventors, it has been found that when the protruding height of the ridge portion is 30% or less of the height dimension of the scroll passage at the winding start portion along the axial direction of the centrifugal compressor, the effect of suppressing the separation of fluid from the inner peripheral surface of the scroll passage is particularly improved. 
     Therefore, with the above configuration (3), the separation of fluid from the inner peripheral surface of the scroll passage can be effectively suppressed, and the loss due to the separation can be effectively suppressed. 
     (4) A scroll structure of a centrifugal compressor according to at least one embodiment of the present invention is a scroll structure of a centrifugal compressor having a scroll passage formed in spiral shape, comprising: a tongue portion separating the scroll passage from an outlet passage connected to a downstream side of the scroll passage at a most downstream position of the scroll passage in a passage connecting portion where a winding start portion and a winding end portion of the scroll passage intersect; and a ridge portion protruding from an inner peripheral surface of the scroll passage on an axially downstream side of the centrifugal compressor toward an axially upstream side of the centrifugal compressor, wherein a protruding height of the ridge portion protruding toward the axially upstream side gradually increases toward the tongue portion from a starting position that is located upstream from the tongue portion in the scroll passage. The curvature radius of a curve connecting an apex of the ridge portion defining the protruding height from the tongue portion to the starting position is located on the axially upstream side, and the curvature radius gradually increases from the tongue portion to the starting position in at least a portion of the apex. 
     In the above configuration (4), the curvature radius of a curve connecting the apex of the ridge portion from the tongue portion to the starting position gradually decreases from the starting position to the tongue portion in at least a portion of the apex defining the protruding height. Accordingly, the amount of decrease in the protruding height when moving a small distance from the tongue portion toward the starting position is greater in the area closer to the connecting position with the tongue portion where the protruding height is the highest. Therefore, when moving from the tongue portion toward the starting position, the protruding height decreases more steeply in the area close to the connecting position with the tongue portion than in the area far from the connecting position with the tongue portion. Therefore, with the above configuration (4), since the protruding height is reduced as a whole, the interference between the fluid flow that tries to flow over the ridge portion and the swirling flow in the scroll passage can be suppressed. As a result, the separation of fluid from the inner peripheral surface of the scroll passage can be suppressed, and the loss due to the separation can be suppressed. 
     (5) In some embodiments, in any one of the above configurations (1) to (4), a flow passage shape of the scroll passage in a cross-section extending in a direction perpendicular to a centerline of the scroll passage is not circular in the cross-section including the tongue portion, and a flow passage shape of the outlet passage in a cross-section extending in a direction perpendicular to a centerline of the outlet passage approaches circular as the outlet passage extends downstream from a connecting position with the scroll passage, and is circular at a position downstream, in the outlet passage, from the connecting position by a distance equal to or greater than a passage height at the winding end portion along an axial direction of the centrifugal compressor. 
     Generally, in a centrifugal compressor, the flow passage shape (hereinafter referred to simply as cross-sectional shape) of the scroll passage in a cross-section extending in the direction perpendicular to the centerline of the scroll passage is not circular in the cross-section including the tongue portion. On the other hand, the flow passage shape (cross-sectional shape) of the outlet passage in a cross-section extending in the direction perpendicular to the extension direction of the passage is typically circular. Therefore, if the cross-sectional shape of the passage changes abruptly from the scroll passage to the outlet passage, loss occurs, resulting in a decrease in the efficiency of the centrifugal compressor. 
     As a result of intensive studies by the inventors, it has been found that when the cross-sectional shape of the passage is made approach circular over a distance equal to or greater than the passage height of the winding end portion along the axial direction of the centrifugal compressor as in the above configuration (5), the loss can be effectively reduced. 
     Therefore, with the above configuration (5), it is possible to effectively suppress the loss occurring in the passage from the scroll passage to the outlet passage, and it is possible to increase the efficiency of the centrifugal compressor over a wide operating range. 
     (6) A centrifugal compressor according to at least one embodiment of the present invention comprises the scroll structure of a centrifugal compressor having any one of the above configurations (1) to (5) to increase the efficiency over a wide operating range. 
     Advantageous Effects 
     According to at least one embodiment of the present invention, it is possible to increase the efficiency of the centrifugal compressor over a wide operating range. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a cross-sectional view of a centrifugal compressor according to some embodiments. 
         FIG. 2  is a schematic cross-sectional view of a casing of a centrifugal compressor according to some embodiments, when cut along a cross-section perpendicular to the axial direction of the rotational shaft of the centrifugal compressor. 
         FIG. 3  is a cross-sectional view taken along line A-A in  FIG. 2 . 
         FIG. 4  is a cross-sectional view taken along line B-B in  FIG. 2 . 
         FIG. 5  is a schematic perspective view of the inside of the scroll passage when viewed from the direction C in  FIG. 2 . 
         FIG. 6  is a schematic diagram showing the flow passage shape of the scroll passage at the winding end portion and the flow passage shape of the outlet passage. 
         FIG. 7  is a graph showing a relationship between the scroll outlet efficiency and the flow rate in the centrifugal compressor according to the above-described embodiments and a conventional centrifugal compressor. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will now be described in detail 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 in the embodiments shall be interpreted as illustrative only and not intended to limit the scope of the present invention. 
     For instance, an expression of relative or absolute arrangement such as “in a direction”, “along a direction”, “parallel”, “orthogonal”, “centered”, “concentric” and “coaxial” shall not be construed as indicating only the arrangement in a strict literal sense, but also includes a state where the arrangement is relatively displaced by a tolerance, or by an angle or a distance whereby it is possible to achieve the same function. 
     For instance, an expression of an equal state such as “same” “equal” and “uniform” shall not be construed as indicating only the state in which the feature is strictly equal, but also includes a state in which there is a tolerance or a difference that can still achieve the same function. 
     Further, for instance, an expression of a shape such as a rectangular shape or a cylindrical 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, an expression such as “comprise”, “include”, “have”, “contain” and “constitute” are not intended to be exclusive of other components. 
       FIG. 1  is a cross-sectional view of a centrifugal compressor  1  according to some embodiments. The centrifugal compressor  1  according to some embodiments is a centrifugal compressor  1  used in a turbocharger. In the centrifugal compressor  1  according to some embodiments, a turbine wheel of a turbine (not shown) and a compressor wheel  8  are connected by a rotational shaft  3 . The compressor wheel  8  has a plurality of compressor blades  7  provided on the surface of a hub  5  to stand. In the compressor wheel  8 , the outer side of the compressor blades  7  is covered with a compressor housing (casing)  9 . In the centrifugal compressor  1  according to some embodiments, a diffuser  11  is formed on the outer peripheral side of the compressor blades  7 , and a scroll passage  13  is further disposed around the diffuser  11  in a spiral shape. 
       FIG. 2  is a schematic cross-sectional view of the casing  9  of the centrifugal compressor  1  according to some embodiments, when cut along a cross-section perpendicular to the direction of the axis X of the rotational shaft  3  of the centrifugal compressor  1 . The casing  9  includes a scroll passage  13  and an outlet passage  15  connected to the downstream side of the scroll passage  13 . The scroll passage  13  has a winding start portion  17  and a winding end portion  19  of the scroll passage. The scroll passage  13  is formed such that the cross-sectional area increases as it progresses clockwise from the winding start portion  17  as shown in  FIG. 2 . 
     In  FIG. 2 , the rotational direction of the compressor wheel  8  is represented by the arrow R. In the centrifugal compressor  1  according to some embodiments, the compressor wheel  8  rotates clockwise in  FIG. 2 . 
     The flow of fluid in the scroll passage  13  has a main flow  91  (see  FIG. 2 ) along the circumferential direction from the winding start portion  17  to the winding end portion  19  and a swirling flow  93  (see  FIG. 5  described later) swirling in the scroll passage  13  along the main flow  91 . 
     In the following description, the direction of the axis X of the rotational shaft  3  of the centrifugal compressor  1  is referred to as the axial direction of the centrifugal compressor  1  or simply the axial direction. Of the axial direction, the upstream side along the flow of fluid entering the centrifugal compressor  1  is referred to as the axially upstream side, and the opposite side is referred to as the axially downstream side. Further, in the following description, the radial direction of the compressor wheel  8  of the centrifugal compressor  1  is referred to as the radial direction of the centrifugal compressor  1  or simply the radial direction. Of the radial direction, the direction toward the axis X of the rotational shaft  3  is referred to as the radially inner side, and the direction away from the axis X of the rotational shaft  3  is referred to as the radially outer side. 
     Further, in the scroll passage  13  and the outlet passage  15 , of the extension direction of the passage, the upstream side of the main flow of fluid is referred to as the upstream side of the scroll passage  13  and the upstream side of the outlet passage  15 , and the downstream side of the main flow of fluid is referred to as the downstream side of the scroll passage  13  and the downstream side of the outlet passage  15 . The upstream side of the scroll passage  13  and the upstream side of the outlet passage  15  is also simply referred to as the upstream side, and the downstream side of the scroll passage  13  and the downstream side of the outlet passage  15  is also simply referred to as the downstream side. In the scroll passage  13 , the extension direction of the scroll passage  13  is almost the same as the circumferential direction of the centrifugal compressor  1 . 
     In the scroll structure  10  of the centrifugal compressor  1  according to some embodiments, the casing  9  has a passage connecting portion  20  where the winding start portion  17  and the winding end portion  19  of the scroll passage  13  intersect. The passage connecting portion  20  has an opening portion  21  formed on the inner peripheral surface  13   a  of the scroll passage  13  at the winding end portion  19  to communicate with the winding start portion  17 . A tongue portion  25  separating the scroll passage  13  from the outlet passage  15  is formed at the most downstream position of the scroll passage  13  in an opening forming portion  23  which surrounds the opening portion  21 . 
       FIG. 3  is a cross-sectional view taken along line A-A in  FIG. 2 . That is,  FIG. 3  is a schematic cross-sectional view of the casing  9  when the casing  9  is cut along a cross-section extending in the direction perpendicular to the extension direction of the winding end portion  19  at the position including the passage connecting portion  20 .  FIG. 3  also shows the inside of the scroll passage  13  at the winding end portion  19  when viewed from the downstream side to the upstream side of the outlet passage  15 . In  FIG. 3 , the diffuser  11  is not depicted. 
       FIG. 4  is a cross-sectional view taken along line B-B in  FIG. 2 . That is,  FIG. 4  is a schematic cross-sectional view of the casing  9  when the casing  9  is cut along a cross-section extending in substantially the same direction as the extension direction of the winding end portion  19  and extending in the axial direction of the centrifugal compressor  1 .  FIG. 4  also shows the inside of the scroll passage  13  at the winding end portion  19  when viewed from the radially outer side of the centrifugal compressor  1 . 
       FIG. 5  is a schematic perspective view of the inside of the scroll passage  13  when viewed from the direction C in  FIG. 2 . 
     In some embodiments, the casing  9  has a ridge portion  50 . In some embodiments, the ridge portion  50  is a portion protruding from the inner peripheral surface  13   a  of the scroll passage  13  on the axially downstream side of the centrifugal compressor toward the axially upstream side of the centrifugal compressor  1 . In some embodiments, the protruding height HR protruding toward the axially upstream side gradually increases toward the tongue portion  25  from a starting position Ps that is located upstream from the tongue portion  25  in the scroll passage  13 . In other words, in some embodiments, the ridge portion  50  begins to protrude at the starting position Ps toward the axially upstream side from the inner peripheral surface  13   a  on the axially downstream side of the scroll passage  13 , and gradually increases its protruding height HR toward the tongue portion  25 . In some embodiments, the ridge portion  50  is connected to the tongue portion  25  on the downstream side of the scroll passage  13 . 
     In some embodiments, the inner peripheral surface  17   a  of the winding start portion  18  on the axially downstream side and the inner peripheral surface  19   a  of the winding end portion  19  on the axially downstream side are in the same position in the axial direction of the centrifugal compressor  1 . 
     In some embodiments, the ridge portion  50  extends along the circumferential direction of the centrifugal compressor  1  from the starting position Ps toward the tongue portion  25 . 
     In the following description, the center of the scroll passage  13 , i.e., the position through which the centerline AX passes is the center of gravity (centroid) of the scroll passage  13  in a virtual cross-section that extends the scroll passage  13  in the radial direction of the centrifugal compressor  1  and in the axis X direction of the rotational shaft  3 . 
     Hereinafter, the connection region  30  according to some embodiments will be described in detail. 
     The fluid blown from the diffuser  11  into the scroll passage  13  flows into the scroll passage  13  along the axially downstream inner peripheral surface  13   b  of the inner peripheral surface  13   a  of the scroll passage  13 . Further, the fluid blown from the diffuser  11  into the scroll passage  13  has a velocity component that moves outward in the radial direction of the centrifugal compressor  1 . Therefore, in the vicinity of the passage connecting portion  20 , the fluid blown from the diffuser  11  into the scroll passage  13  tries to flow over the ridge portion  50  from the inner side to the outer side in the radial direction of the centrifugal compressor  1  as shown by the arrow  97 . Such a flow of fluid is toward the axially upstream side. 
     Further, the flow of fluid in the scroll passage  13  has the main flow  91  and a swirling flow  93  swirling in the scroll passage  13  along the main flow  91 . The swirling flow  93  is toward the axially downstream side. 
     Therefore, the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  interfere with each other, causing the fluid to separate from the inner peripheral surface  13   a  of the scroll passage  13  near the tongue portion  25 . Such separation causes loss of the centrifugal compressor  1 . 
     Therefore, in some embodiments, the shape of the ridge portion  50  is designed as described below to suppress the interference between the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  in the scroll passage  13 . 
     Specifically, in some embodiments, the starting position Ps is at an angle θ of 8 degrees or less in the circumferential direction of the centrifugal compressor  1  from the tongue portion  25  toward the upstream side of the scroll passage  13 . In some embodiments, the starting position Ps is preferably at an angle θ of 4 degrees or less. 
     In a typical centrifugal compressor, the starting position Ps is at an angle θ of about 15 degrees. 
     In contrast, in some embodiments, the starting position Ps is at an angle θ of 8 degrees or less. 
     Accordingly, in some embodiments, the range where the ridge portion  50  extends in the circumferential direction of the centrifugal compressor  1  is reduced compared to a typical centrifugal compressor. 
     Since the ridge portion  50  is a portion protruding from the axially downstream inner peripheral surface  13   b  of the inner peripheral surface  13   a  of the scroll passage  13  toward the axially upstream side, by reducing the range where the ridge portion  50  extends in the circumferential direction of the centrifugal compressor  1 , the interference between the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  in the scroll passage  13  can be suppressed. 
     Therefore, according to some embodiments, the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  can be suppressed, and the loss due to the separation can be suppressed. Therefore, it is possible to increase the efficiency of the centrifugal compressor  1  over a wide operating range. 
       FIG. 7  is a graph showing a relationship between the scroll outlet efficiency and the flow rate in the centrifugal compressor  1  according to the above-described embodiments and a conventional centrifugal compressor. In  FIG. 7 , the graph shown by the solid line is a graph for the centrifugal compressor  1  according to the above-described embodiments, and the graph shown by the dashed line is a graph for a conventional centrifugal compressor. As shown in  FIG. 7 , the scroll outlet efficiency is improved mainly in the large flow rate region by setting the starting position Ps to the position at an angle θ of 8 degrees or less. 
     In some embodiments, the protruding height HR in the position at an angle θ of 4 degrees in the circumferential direction of the centrifugal compressor  1  from the tongue portion  25  toward the upstream side of the scroll passage  13  is 10% or less of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor  1 . 
     The ridge portion  50  of a typical centrifugal compressor extends in the range about 15 degrees in the circumferential direction of the centrifugal compressor, as described above. Further, in a typical centrifugal compressor, the protruding height HR 1  of the ridge portion  50  at the connecting position with the tongue portion  25  is often more than 50% of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor. Consequently, in the ridge portion  50  of a typical centrifugal compressor, the protruding height HR of the ridge portion  50  in the position at an angle θ of 4 degrees in the circumferential direction of the centrifugal compressor from the tongue portion  25  toward the upstream side of the scroll passage  13  is often more than 30% of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor. 
     Therefore, according to some embodiments, since the protruding height HR of the ridge portion  50  in the position at an angle θ of 4 degrees in the circumferential direction of the centrifugal compressor  1  from the tongue portion  25  toward the upstream side of the scroll passage  13  is 10% or less of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor  1 , the protruding height HR of the ridge portion  50  near the tongue portion  25  can be made smaller than the protruding height HR of the ridge portion  50  in a typical centrifugal compressor. Therefore, according to some embodiments, the interference between the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  in the scroll passage  13  can be suppressed. 
     Therefore, according to some embodiments, the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  can be suppressed, and the loss due to the separation can be suppressed. Therefore, it is possible to increase the efficiency of the centrifugal compressor  1  over a wide operating range. 
     In some embodiments, the protruding height HR in the position at an angle θ of 4 degrees in the circumferential direction of the centrifugal compressor  1  from the tongue portion  25  toward the upstream side of the scroll passage  13  is 20% or less of the protruding height HR 1  in the connecting position with the tongue portion  25 . 
     The ridge portion  50  of a typical centrifugal compressor extends in the range about 15 degrees in the circumferential direction of the centrifugal compressor, as described above. Consequently, in the ridge portion  50  of a typical centrifugal compressor, the protruding height HR of the ridge portion  50  in the position at an angle of 4 degrees in the circumferential direction of the centrifugal compressor from the tongue portion  25  toward the upstream side of the scroll passage  13  is often more than 50% of the protruding height HR 1  in the connecting position with the tongue portion  25 . 
     Therefore, according to some embodiments, since the protruding height HR of the ridge portion  50  in the position at an angle θ of 4 degrees in the circumferential direction of the centrifugal compressor  1  from the tongue portion  25  toward the upstream side of the scroll passage  13  is 20% or less of the protruding height HR 1  in the connecting position with the tongue portion  25 , the protruding height HR of the ridge portion  50  near the tongue portion  25  can be made smaller than the protruding height of the ridge portion of a typical centrifugal compressor. Therefore, according to some embodiments, the interference between the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  in the scroll passage  13  can be suppressed. 
     Therefore, according to some embodiments, the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  can be suppressed, and the loss due to the separation can be suppressed. Therefore, it is possible to increase the efficiency of the centrifugal compressor  1  over a wide operating range. 
     The embodiment in which the protruding height HR in the position at an angle θ of 4 degrees is 20% or less of the protruding height HR 1  may be implemented in combination with the embodiment in which the starting position Ps is at an angle θ of 8 degrees or less or other embodiments described later, or may be implemented alone. 
     Further, in some embodiments, the protruding height HR of the ridge portion  50  is 30% or less of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor  1 . 
     As a result of intensive studies by the inventors, it has been found that when the protruding height HR of the ridge portion  50  is 30% or less of the height dimension Ha of the scroll passage  13  at the winding start portion  17  along the axial direction of the centrifugal compressor  1 , the effect of suppressing the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  is particularly improved. 
     Therefore, according to some embodiments, the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  can be effectively suppressed, and the loss due to the separation can be effectively suppressed. 
     The embodiment in which the protruding height HR of the ridge portion  50  is 30% or less of the height dimension Ha may be implemented in combination with the embodiment in which the starting position Ps is at an angle θ of 8 degrees or less or the embodiment in which the protruding height HR in the position at an angle θ of 4 degrees is 20% or less of the protruding height HR 1 , or may be implemented alone. Further, the embodiment in which the protruding height HR of the ridge portion  50  is 30% or less of the height dimension Ha may be implemented in combination with the other embodiments described later. 
     In some embodiments, the curvature radius r (see  FIG. 4 ) of a curve connecting the apex  51  of the ridge portion  50  defining the protruding height HR from the tongue portion  25  to the starting position Ps is located on the axially upstream side. 
     The curvature radius r gradually increases from the tongue portion  25  to the starting position Ps in at least a portion of the apex  51 . 
     In other words, in some embodiments, the curvature radius r of a curve connecting the apex  51  from the tongue portion  25  to the starting position Ps gradually decreases from the starting position Ps to the tongue portion  25  in at least a portion of the apex  51 . Accordingly, the amount of decrease (dHR) in the protruding height HR when moving a small distance dX from the tongue portion  25  toward the starting position Ps is greater in the area closer to the connecting position with the tongue portion  25  where the protruding height HR is the highest. 
     Therefore, when moving from the tongue portion  25  toward the starting position Ps, the protruding height HR decreases more steeply in the area close to the connecting position with the tongue portion  25  than in the area far from the connecting position with the tongue portion  25 . Therefore, according to some embodiments, since the protruding height HR is reduced as a whole, the interference between the fluid flow that tries to flow over the ridge portion  50  as shown by the arrow  97  and the swirling flow  93  in the scroll passage  13  can be suppressed. As a result, the separation of fluid from the inner peripheral surface  13   a  of the scroll passage  13  can be suppressed, and the loss due to the separation can be suppressed. 
     The embodiment in which the curvature radius r gradually increases from the tongue portion  25  toward the starting position Ps may be implemented in combination with at least one of the above-described embodiments, or may be implemented alone. Further, the embodiment in which the curvature radius r gradually increases from the tongue portion  25  toward the starting position Ps may be implemented in combination with the other embodiments described later. 
       FIG. 6  is a schematic diagram showing the flow passage shape of the scroll passage  13  at the winding end portion  19  and the flow passage shape of the outlet passage  15 , each viewed from the downstream side of the outlet passage  15 . 
     In some embodiments, for example as shown in  FIGS. 5 and 6 , the flow passage shape  13 X of the scroll passage  13  in a cross-section extending in the direction perpendicular to the centerline AX of the scroll passage  13  is not circular in the cross-section including the tongue portion  25 . 
     Further, in some embodiments, the flow passage shape  15 X of the outlet passage  15  in a cross-section extending in the direction perpendicular to the centerline AX of the outlet passage  15  approaches circular as the outlet passage  15  extends downstream from a connecting position  15   a  (see  FIG. 2 ) with the scroll passage  13 , and the flow passage shape  15 X is circular at a position downstream, in the outlet passage, from the connecting position  15   a  by a distance equal to or greater than the passage height Hb (see  FIG. 4 ) at the winding end portion  19  along the axial direction of the centrifugal compressor  1 . 
     Generally, in a centrifugal compressor, the flow passage shape (hereinafter referred to simply as cross-sectional shape)  13 X of the scroll passage  13  in a cross-section extending in the direction perpendicular to the centerline AX of the scroll passage  13  is not circular in the cross-section including the tongue portion  25 . On the other hand, the flow passage shape (cross-sectional shape)  15 X of the outlet passage  15  in a cross-section extending in the direction perpendicular to the extension direction of the passage is typically circular. Therefore, if the cross-sectional shape of the passage changes abruptly from the scroll passage  13  to the outlet passage  15 , loss occurs, resulting in a decrease in the efficiency of the centrifugal compressor  1 . 
     As a result of intensive studies by the inventors, it has been found that when the cross-sectional shape of the passage is made approach circular over a distance equal to or greater than the passage height Hb of the winding end portion  19  along the axial direction of the centrifugal compressor  1  as described above, the loss can be effectively reduced. 
     Therefore, according to some embodiments, it is possible to effectively suppress the loss occurring in the passage from the scroll passage  13  to the outlet passage  15 , and it is possible to increase the efficiency of the centrifugal compressor  1  over a wide operating range. 
     The embodiment in which the cross-sectional shape of the passage approaches circular over a distance equal to or greater than the passage height Hb may be implemented together with at least any one of the above-described embodiments. 
     The present invention is not limited to the embodiments described above, but includes modifications to the embodiments described above, and embodiments composed of combinations of those embodiments. 
     REFERENCE SIGNS LIST 
     
         
           1  Centrifugal compressor 
           9  Compressor housing (Casing) 
           10  Scroll structure 
           13  Scroll passage 
           15  Outlet passage 
           17  Winding start portion 
           19  Winding end portion 
           20  Passage connecting portion 
           25  Tongue portion 
           30  Connection region 
           50  Ridge portion