JP4505523B2 - Axial diffuser for centrifugal compressor - Google Patents

Description

  The present invention relates to an axial diffuser for a centrifugal compressor, and more particularly to an axial diffuser for a centrifugal compressor used in a gas turbine engine, a jet engine, or the like.

  An axial diffuser is attached to a centrifugal compressor used in a gas turbine engine or a jet engine. The axial diffuser of the centrifugal compressor has a plurality of axial diffuser passages at predetermined intervals around the outer circumference in the radial direction of the impeller, and each of the plurality of axial diffuser passages is separated from the impeller. The kinetic energy of the fluid flowing through the diffuser flow path is converted into pressure energy while turning the radial flow of the fluid into an axial flow substantially parallel to the central axis of the impeller (for example, Patent Document 1).

Further, as a diffuser attached to a centrifugal compressor, there is a radial diffuser that converts a fluid from an impeller into a pressure energy by reducing a flow velocity in a radial flow (for example, Patent Document 2).
Special table 2003-512569 gazette JP 2002-98093 A

  If the diffuser tube that defines the flow path of the axial diffuser is a straight pipe, no centrifugal force is generated, and no drift occurs in the fluid flow. However, the actual shape of the diffuser tube is a curved tube that changes the flow direction of the fluid from the tangential direction of the outer diameter of the impeller to the direction of the rotation axis. .

  On the other hand, the conventional axial diffuser does not have a shape that balances the centrifugal force in the longitudinal direction of the channel cross section in the diffuser channel having an elliptical channel cross section.

  As a result, in the conventional axial diffuser, a pressure difference occurs after the first bend, and a weak flow (low momentum flow in the boundary layer) accumulates on the negative pressure side (impeller rotation upstream side) in the diffuser tube, and the diffuser exit The flow velocity becomes slower on the negative pressure side in the vicinity. On the other hand, on the positive pressure side, the flow velocity increases conversely, and at the diffuser outlet, the flow becomes uneven with a large speed difference between the negative pressure side and the positive pressure side.

  Furthermore, when the flow on the negative pressure side cannot resist the centrifugal force (inertial force) generated in the bent portion, the fluid flow is separated from the negative pressure side wall surface, and a vortex is generated. As a result, the kinetic energy of the fluid flow is lost as heat, and at the same time, the effective flow path area is reduced due to the blockage of the flow path due to the vortex, and the speed cannot be reduced as designed.

  For this reason, in the conventional axial diffuser, the static pressure recovery rate Cp = (diffuser outlet static pressure−diffuser inlet static pressure) / (diffuser inlet total pressure−diffuser inlet static pressure) is poor, reducing the effective efficiency of the centrifugal compressor. Cause.

  The problem to be solved by the present invention is that, in an axial diffuser of a centrifugal compressor, the separation of fluid flow on the negative pressure side of the flow path is suppressed, the velocity distribution at the outlet of the diffuser is made uniform, and a high static pressure recovery rate is achieved. It is intended to obtain Cp and improve the effective efficiency of the centrifugal compressor.

  An axial diffuser of a centrifugal compressor according to the present invention has a plurality of axial diffuser passages provided at predetermined intervals in the circumferential direction on the outer periphery of an impeller of the centrifugal compressor, and the plurality of axial diffusers. In each of the flow paths, the kinetic energy of the fluid flowing through the axial diffuser flow path is converted into pressure energy while turning the radial flow of the fluid from the impeller into an axial flow substantially parallel to the central axis of the impeller. An axial diffuser of the centrifugal compressor, wherein each axial diffuser flow path has an upstream end communicating with the outlet of the impeller and a downstream end extending substantially parallel to the central axis of the impeller, And having a cross-sectional area that gradually increases from the upstream end toward the downstream end, Said to direct the negative pressure side in the cross section of the flow path axial diffuser - it is characterized by defining the cross-sectional shape of The flow path.

Therefore, axial diffuser of a centrifugal compressor according to the present invention, the axial diffuser flow path is not a cross-sectional shape having a major axis substantially over the entire region ranging from the inlet to the outlet, the substantially over the whole area The long axis is substantially orthogonal to the direction from the center of the cross-sectional shape toward the central axis of the impeller. Preferably, the cross-sectional shape has a major axis and a minor axis that are orthogonal to each other, and the minor axis faces a direction that passes through the central axis of the impeller. The cross-sectional shape may be substantially elliptical or oval.

  In the axial diffuser of the centrifugal compressor according to the present invention, preferably, each of the axial diffuser flow paths is constituted by an individual tube member, and the upstream end of the diffuser opens toward a tangential direction of the outer periphery of the impeller.

  According to the axial diffuser of the centrifugal compressor of the present invention, the flow velocity is made uniform over the whole of the negative pressure side and the positive pressure side in the cross-sectional shape of each part between the upstream end of the diffuser and the downstream end of the diffuser. The flow velocity distribution at the downstream end of the diffuser can be made highly uniform. For this purpose, the cross-sectional shape has a long axis, and the long axis is substantially orthogonal to the direction from the center of the cross-sectional shape toward the central axis of the impeller, or the cross-sectional shapes are orthogonal to each other. It is good to have a long axis and a short axis, and the short axis has faced the direction which passes the impeller radial line which passes along the center axis line of an impeller. In this manner, the distance from the impeller central axis is the same on the negative pressure side (upstream rotation side of the impeller) and the positive pressure side (downstream rotation side of the impeller) in the cross section of each part of the diffuser flow path.

  As a result, the pressure difference due to the centrifugal force component in the cross-section of each part of the axial diffuser flow path is reduced, and the flow of compressed air flowing through the axial diffuser flow path is biased, that is, the uneven distribution is significantly reduced. In addition, in the diffuser outlet portion, it is possible to avoid an uneven speed distribution with a slow speed at the negative pressure side portion. Moreover, separation of the fluid flow that tends to occur on the negative pressure side of the flow path can be prevented by reducing the pressure difference due to the centrifugal force component in each cross section of the flow path.

  Accordingly, the velocity distribution at the diffuser outlet is made uniform, and at the same time, the separation of the fluid flow is suppressed, the static pressure recovery rate Cp is increased, and the effective efficiency of the centrifugal compressor is improved.

  Embodiments of an axial diffuser of a centrifugal compressor according to the present invention will be described below with reference to FIGS.

  In these drawings, reference numeral 10 denotes a cylindrical outer housing of a centrifugal compressor. An impeller 11 is provided in the outer housing 10 so as to be rotatable around the impeller central axis A. 1 to 3 are views of the centrifugal compressor as viewed from the back side (compressed air outlet side), and the impeller 11 rotates in the clockwise direction as viewed in these drawings.

  An annular radial diffuser member 12 is disposed concentrically with the impeller 11 on the outer side in the radial direction of the impeller 11. The radial diffuser member 12 is fixedly attached to the outer housing 10, and a plurality of radial diffuser flow paths 13 are formed around the circumference of the impeller 11 at a predetermined interval. Each of the radial diffuser flow paths 13 extends linearly in the tangential direction of the outer periphery of the impeller, and the passage cross-sectional area increases as the distance from the outlet end of the impeller 11 increases.

  Thereby, the radial diffuser member 12 reduces the flow velocity in the radial flow of the fluid (compressed air) from the impeller 11 in each radial diffuser flow path 13, and converts the kinetic energy of the fluid into pressure energy.

  If a detailed explanation of the radial diffuser is necessary, refer to Japanese Patent Application Laid-Open No. 2002-98093. As the radial diffuser member 12 of the present embodiment, a radial diffuser disclosed in JP-A-2002-98093 can be used.

  An axial diffuser 30 is provided on the radially outer side of the radial diffuser member 12. The axial diffuser 30 has a plurality of diffuser tubes 31 at predetermined intervals around the circumference of the impeller 11.

  The number of the diffuser tubes 31 is the same as the number of the radial diffuser flow paths 13 of the radial diffuser member 12, and the base end portion is fixed to the outlet end portion of the radial diffuser member 12 by welding or the like. The diffuser tubes 31 each define an axial diffuser flow path 32. In other words, each of the plurality of axial diffuser flow paths 32 is configured by an individual tube member.

  The diffuser inlet portion 32A of the axial diffuser flow path 32 opens toward the tangential direction of the outer periphery of the impeller, and each of the axial diffuser flow paths 32 is located at the outlet portion of the corresponding radial diffuser flow path 13 at the diffuser inlet portion 32A. It is connected smoothly and communicates with the radial diffuser flow path 13.

  In other words, the axial diffuser 30 has a plurality of axial diffuser flow paths 32 at a constant predetermined interval around the outer circumference in the radial direction of the impeller 11. The axial diffuser flow path 32 has a three-dimensionally bent three-dimensional shape. In each of the plurality of axial diffuser flow paths 32, the radial flow of fluid (compressed air) from the impeller 11 is centered on the impeller 11. The kinetic energy of the fluid flowing through the axial diffuser flow path 32 is converted into pressure energy while turning to an axial flow substantially parallel to the axis A.

  That is, the diffuser tube 31 is formed of a curved pipe, and the diffuser inlet portion 32A is open toward the tangential direction of the outer periphery (outer diameter) of the impeller, whereas the diffuser outlet portion 32B is the central axis of the impeller 11 Opening in the direction of A.

  The axial diffuser flow path 32 has an elliptical or oval cross-sectional shape as shown by reference numerals a, b, c... In FIGS. 2 and 3, and has a diffuser outlet portion from the diffuser inlet portion 32A. It is formed in a channel shape that gradually increases the channel cross-sectional area toward 32B. Thus, the cross-sectional shape has a major axis corresponding to a large diameter and a minor axis corresponding to a small diameter. Then, over almost the entire area from the diffuser inlet portion 32A to the diffuser outlet portion 32B, the long axes La, Lb, Lc,. Are substantially orthogonal to the impeller radial direction lines Ra, Rb, Rc... Directed from the center of the cross section toward the central axis of the impeller. The impeller radial direction line is given as a line orthogonal to the central axis A of the impeller 11 on the downstream side of the flow path, but forms an angle other than 90 degrees with respect to the central axis A of the impeller 11 on the upstream side of the flow path. In this embodiment, since the major axis and the minor axis are orthogonal to each other, the minor axes Sa, Sb, Sc... Are the central axis of the impeller 11 in each cross section of the axial diffuser flow path 32. It faces the direction that matches the impeller radius lines Ra, Rb, Rc.

  The major axis and the minor axis are preferably orthogonal to each other, but may be at an angle other than 90 degrees. In particular, when the cross-sectional shape is other than an ellipse or an ellipse, the long axis can be defined as the longest diameter line.

  As described above, the major axis of the elliptical or oval cross-sectional shape in each of the portions a, b, c... Over almost the entire area of the axial diffuser flow path 32 from the diffuser inlet portion 32A to the diffuser outlet portion 32B. La, Lb, Lc,... Are substantially perpendicular to the direction of the impeller radial direction lines Ra, Rb, Rc,... From the center of the cross section toward the central axis of the impeller, or their short axes Sa, Sb, When viewed from the axial direction, Sc... Faces the direction that matches the impeller radial lines Ra, Rb, Rc... Passing through the central axis A of the impeller 11, thereby causing each part of the diffuser flow path 32 to be aligned. The separation distance from the impeller center axis A is equal between the flow path negative pressure side (impeller rotation upstream side) M and the flow path positive pressure side (impeller rotation downstream side) N in the cross section of a, b, c. Shi It made.

  Thereby, the pressure difference due to the centrifugal force component in the cross section of each part a, b, c... Of the axial diffuser flow path 32 is reduced, and the flow of compressed air flowing through the axial diffuser flow path 32 is biased. That is, the occurrence of partial distribution is significantly reduced. As a result, in the diffuser outlet portion 32B, it is possible to avoid a non-uniform velocity distribution having a slow velocity at the negative pressure side portion. Thereby, a high static pressure recovery rate Cp is obtained, and the effective efficiency of the centrifugal compressor is improved. Furthermore, by reducing the pressure difference due to the centrifugal force component in each cross section of the axial diffuser flow path 32, it is possible to prevent separation of the fluid flow that tends to occur on the negative pressure side of the flow path.

  FIG. 4 shows the Mach number distribution at the diffuser outlet portion of the axial diffuser according to this embodiment, and FIG. 5 shows the Mach number distribution at the diffuser outlet portion of the conventional axial diffuser.

  In FIG. 5 (conventional example), a region B indicated by halftone dots is a region having a low flow velocity. This decrease in flow rate is considered to be due to the following reason. That is, in the bending portion of the axial diffuser flow path, the flow of fluid drifts to the flow path positive pressure side N due to centrifugal force (inertial force), and the flow of fluid on the flow path negative pressure side (rotation upstream side of the impeller) M If the centrifugal force cannot be fully resisted, a phenomenon occurs in which the fluid flow is separated from the negative pressure wall surface. In addition, the low momentum flow in the boundary layer on the flow path positive pressure side N may move to the flow path negative pressure side M due to a pressure difference.

  On the other hand, since the present embodiment aims to eliminate the pressure difference in the longitudinal direction of the cross section of the axial diffuser flow path, the flow velocity decrease region B is generated on the flow path negative pressure side M as shown in FIG. Not. In the present embodiment, ideally, a flat Mach number distribution characteristic should be obtained over the entire region from the flow path negative pressure side M to the flow path positive pressure side N, but the flow of low momentum in the boundary layer is pressure. Because of this balance, a phenomenon of gathering in the central portion of the diffuser flow path occurs on the diffuser outlet side, so that the velocity distribution is slightly low in the central portion of the flow path negative pressure side M to the flow path positive pressure side N.

  FIG. 6 shows Mach number distribution characteristics along the major axis direction at several points on the flow path of the axial diffuser according to the present embodiment, and FIG. 7 shows similar Mach number distribution characteristics in the conventional axial diffuser. As shown in FIG. 8, several points on the flow path from the upstream side to the downstream side are respectively represented as points st0, st1, st2, st3, st4, st5, and the point st5 is the downstream end of the diffuser flow path, that is, It shall represent the exit end. In the conventional axial diffuser, the increase in the speed on the negative pressure side is small at st3, and the speed decreases as it goes downstream from points st4 and st5. In particular, the speed decrease at the point st5, that is, at the outlet end is remarkable. As a result, the uniformity of the Mach number distribution along the long axis direction at the point st5 is lost. On the other hand, in the axial diffuser according to the present embodiment, the increase in the speed on the negative pressure side at the point st3 is remarkable and the speed is higher than that on the positive pressure side, and then gradually decelerates to the points st4 and st5. Then, by maintaining the suction side speed at the outlet end relatively high, the Mach number distribution along the major axis direction becomes substantially flat, and a highly uniform flow can be obtained.

FIG. 9 shows the distribution along the flow direction of the flow velocity at a position 5% away from the flow path negative pressure side M in the center direction, in the case of this embodiment by a solid line, and in the case of the prior art by a broken line. Show. In the graph of FIG. 9, the flow direction distance is the meridional length (L) connecting the center positions of the design cross sections, and the area of the cross section at the radial diffuser inlet position (FIG. 3) is represented by an equivalent circle. the diameter divided by the (D1), further indicated more to normalize. In the prior art, the degree of deceleration from the position in the flow direction st3 is large, and this tendency continues to the downstream end, and the speed decrease at the outlet is significant. In contrast, the present embodiment has a feature that the speed at the st3 position is larger than that of the conventional type, and further, the flow proceeds at the st4 position and the st5 position at a reduced speed. Therefore, the speed reduction at the exit is greatly suppressed. As a result, the suction-side speed at the outlet end can be maintained higher than that of the conventional type, leading to the realization of a flat speed distribution in the major axis direction as shown in FIG.

As a result, according to the present embodiment, a high static pressure recovery rate Cp can be obtained as compared with the prior art. FIG. 10 compares the change in the static pressure recovery rate Cp along the flow direction of this embodiment with that of the prior art, and the static pressure recovery rate Cp of this embodiment is at the downstream end, that is, at the outlet portion. It is shown to be slightly larger than that of the prior art. The flow direction distance in FIG. 10 is the same as that in FIG. 9, but in this case, it is not normalized.

1 is a perspective view showing one embodiment of an axial diffuser of a centrifugal compressor according to the present invention. It is a perspective view which shows the diffuser tube of one Embodiment of the axial diffuser of the centrifugal compressor by this invention. It is explanatory drawing which shows the axial diffuser flow path of one Embodiment of the axial diffuser of the centrifugal compressor by this invention. It is an equal distribution map which shows Mach number distribution in the diffuser exit part of the axial diffuser of this embodiment. It is an equal distribution map which shows the Mach number distribution in the diffuser exit part of the conventional axial diffuser. It is a graph which shows the Mach number distribution characteristic along the major axis direction in several points on the flow path of the axial diffuser by this embodiment. It is a graph which shows the Mach number distribution characteristic along the major axis direction in several points on the flow path of the conventional axial diffuser. It is a perspective view which shows one of the tube members which comprise an axial diffuser with the position of several points along a flow path. It is a graph which shows distribution along the flow direction of the flow velocity in the position 5% away in the center direction from the flow-path negative pressure side about this embodiment and the conventional axial diffuser. It is a graph which shows the change of the static pressure recovery rate Cp along a flow direction about this embodiment and the conventional axial diffuser.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Outer housing 11 Impeller 12 Radial diffuser member 13 Radial diffuser flow path 30 Axial diffuser 31 Diffuser tube 32 Axial diffuser flow path 32A Diffuser inlet part 32B Diffuser outlet part

Claims (5)

A plurality of axial diffuser passages provided at predetermined intervals in the circumferential direction on an outer periphery of an impeller of the centrifugal compressor, and each of the plurality of axial diffuser passages includes a flow of fluid from the impeller; An axial diffuser of a centrifugal compressor that converts a kinetic energy of a fluid flowing through the axial diffuser flow path into a pressure energy while turning a radial flow into an axial flow substantially parallel to the central axis of the impeller,
Each axial diffuser flow path has an upstream end communicating with the outlet portion of the impeller and a downstream end extending substantially parallel to the central axis of the impeller, and gradually increases from the upstream end toward the downstream end. Having a cross-sectional area
The axial diffuser flow path, substantially over the entire region ranging from the inlet to the outlet has a cross section shape having a long axis, the long axis of the impeller from the center of the cross-sectional shape over almost the entire An axial diffuser of a centrifugal compressor characterized by being substantially orthogonal to the direction toward the central axis.   The transverse cross-sectional shape of the axial diffuser channel has a minor axis perpendicular to the major axis, and the minor axis faces a direction passing through the central axis of the impeller. An axial diffuser of the centrifugal compressor described. The cross-sectional shape is generally oval or axial diffuser of a centrifugal compressor according to claim 1 or 2 you characterized by forming an oval. The axial diffuser of the centrifugal compressor according to any one of claims 1 to 3, wherein the diffuser upstream end opens toward a tangential direction of an outer periphery of the impeller. The axial diffuser of the centrifugal compressor according to any one of claims 1 to 4, wherein each of the axial diffuser flow paths is configured by an individual tube member.

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