Return channel with non-constant return channel vane pitch and centrifugal turbine including said return channel

Abstract

The present invention discloses a return passage (15) for a centrifugal turbine (1). The return passage includes a plurality of return passage vanes (15.1) arranged around a return passage axis (A-A). Each return passage vane (15.1) includes a leading edge (15.3) at a first distance from the return passage axis (A-A) and a trailing edge (15.5) at a second distance from the return passage axis, the second distance being less than the first distance. A respective flow passage is defined between each pair of adjacent return passage vanes (15.1). The return passage vanes (15.1) are arranged around the return passage axis (A-A) with a non-constant pitch.

Description

[0001] Specification Technical Field

[0002] The present disclosure relates to radial turbines. More specifically, embodiments of the present disclosure relate to centrifugal turbines, such as centrifugal compressors and / or centrifugal pumps, including one or more novel bladed / vaned return channels. Background Art

[0003] Centrifugal compressors are used in a variety of applications to increase the pressure of a gas. A centrifugal compressor includes stationary components such as a housing, and one or more impellers arranged to rotate within the housing. The mechanical energy delivered to the impeller is transferred to the gas in the form of kinetic energy by the rotating impeller. The gas flow accelerated by the impeller passes through a diffuser circumferentially surrounding each impeller, which collects the gas flow and reduces its velocity, thereby converting the kinetic energy into gas pressure. If the compressor includes multiple impellers, a return channel is arranged between the diffuser of the upstream impeller and the inlet of the downstream impeller to convey the gas from the upstream impeller to the downstream impeller.

[0004] To better guide the gas flow through the diffuser and the return channel and improve pressure recovery, bladed diffusers and bladed return channels have been developed. While improving the compressor efficiency, the bladed / vaned return channels generate pressure pulses that excite the vibration of the blades of the impeller arranged downstream of the return channel. The impeller vibration may cause impeller failure due to high cycle fatigue (HCF). This problem is particularly prominent when the frequency of the vibration excited by the bladed return channel in the impeller arranged downstream thereof approaches or coincides with the critical frequency of the impeller, such that a resonance phenomenon can be generated. Currently, to limit this problem, the number of return channel blades is selected such that the frequency of the vibration induced by the return channel on the downstream impeller does not coincide with the resonance frequency of the impeller.

[0005] An improved return channel design aimed at more effectively reducing the vibration in a compressor impeller would be welcomed in the art. Summary of the Invention

[0006] According to one aspect, a novel bladed / vaned return channel for a centrifugal turbine, specifically a centrifugal compressor, is disclosed herein. The return channel includes a plurality of return channel blades arranged around a return channel axis. Each return channel blade includes a leading edge and a trailing edge. A corresponding flow passage is defined between each pair of adjacent, i.e., consecutive, return channel blades. The return channel blades are arranged around the return channel axis with a non-constant pitch.

[0007] According to another aspect, there is disclosed herein a centrifugal turbine, specifically a centrifugal compressor, which includes a stationary component, such as a housing, and at least two impellers, the at least two impellers being arranged to rotate within the stationary component (i.e., within the housing). A diffuser is arranged downstream of each impeller. Additionally, the novel vane-type return channel as described above is arranged between the first impeller and the second impeller. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] When considered in conjunction with the accompanying drawings, a more complete understanding of the disclosed embodiments of the present invention and many of its attendant advantages will be readily obtained by reference to the following detailed description, which will also become better understood, wherein:

[0009] Figure 1 A schematic cross-sectional view of a portion of the compressor is shown;

[0010] Figure 2 A schematic cross-sectional view of the return channel according to a plane orthogonal to the axis of rotation in one embodiment is shown;

[0011] Figure 3 An isometric view of a portion of the return channel is shown;

[0012] Figure 4 A schematic cross-sectional view of the return channel according to a plane orthogonal to the axis of rotation in another embodiment is shown; and

[0013] Figure 5 and Figure 6 Shows a comparison graph displaying the harmonic analysis of the impeller vibration in the embodiments according to the background art and Figure 2 and Figure 4 the embodiments of. DETAILED DESCRIPTION

[0014] To reduce the vibration of the impeller blades in a centrifugal turbine (specifically a centrifugal compressor), the blades of one, some, or all of the return channels of the turbine are arranged according to a non-constant pitch, i.e., the spacing between at least one pair of return channel blades defining the return channel flow path is different from the spacing between at least another pair of return channel blades defining another return channel flow path. The non-constant pitch has a beneficial effect in reducing the amplitude of the impeller blade vibration, as will be described in detail below.

[0015] Now referring to Figure 1 , a portion of the centrifugal compressor 1 is shown. Figure 1The cross-section is limited to two stages of the centrifugal compressor. The number of compressor stages and thus the number of impellers can vary in different compressors depending on the compressor design and compressor requirements. The novel features of the return channels according to the present disclosure can be embodied in one, some, or preferably all of the return channels provided in the compressor.

[0016] The compressor includes a stationary component 3, such as a housing 3, in which a diaphragm 5 that separates successive compressor stages is arranged. Each compressor stage includes an impeller 7 supported for rotation in the housing 3. The impeller 7 can be shrink-fitted onto a rotating shaft 9. In other embodiments not shown, according to designs known to those skilled in the art of centrifugal compressors, the impeller 7 can be a stacked impeller, which is not disclosed herein. The impeller 7 and the shaft 9 cumulatively form a compressor rotor, which is arranged to rotate in the housing 3 about a rotation axis A-A. The impeller 7 has an impeller hub 7.1 from which a plurality of impeller blades 7.3 project. Each impeller blade 7.3 has a leading edge 7.5 and a trailing edge 7.7. The leading edge 7.5 is arranged along the impeller inlet, and the trailing edge 7.7 is arranged along the impeller outlet. In Figure 1 the illustrated embodiment, the impeller 7 further includes a shroud 7.9. In other embodiments, the impeller 7 can be a semi-open impeller, in which case the shroud 7.9 will be omitted.

[0017] A diffuser 11 is arranged around each impeller outlet. Each diffuser 11 surrounds and is coaxial with the outlet of the impeller 7, i.e., the central axis of the diffuser 11 coincides with the rotation axis A-A of the impeller 7.

[0018] In Figure 1 the embodiment, the diffuser 11 is a so-called vaned / bladed diffuser. Each vaned diffuser is provided with a plurality of diffuser blades 11.1 arranged around the diffuser axis A-A. The purpose of the diffuser blades 11.1 is to redirect the incoming gas flow in a more radial direction, i.e., to reduce the tangential component of the velocity of the gas flow entering the diffuser 11 and to increase the pressure recovery and overall stage efficiency. Each diffuser blade 11.1 includes a leading edge 11.3 and a trailing edge 11.5.

[0019] In other embodiments, the diffuser 11 can be a non-vaned diffuser, i.e., the diffuser blades 11.1 can be omitted.

[0020] A return elbow 13 is provided downstream of each diffuser 11 (except for the diffuser (not shown) after the most downstream impeller). The return elbow 13 produces a 180-degree turn from radially outward to radially inward in the direction of the gas flow leaving the diffuser 11. After the return elbow 13, a return passage 15 is provided which guides the gas flow from the return elbow 13 inwardly to the next impeller 7. The function of the return passage is to deliver the gas flow evenly to each impeller 7 downstream with minimum loss. Each return passage 15 is provided with a plurality of return passage vanes / blades 15.1. Each pair of adjacent return passage vanes 15.1 forms a gas flow passage therebetween. The shape and distribution of the return passage vanes 15.1 will be described in more detail below. As described above, the most downstream diffuser is not provided with a return elbow 13 but is fluidly coupled to a volute (not shown) which collects the gas flow from the last compressor stage. The volute is in turn fluidly coupled to the compressor outlet (not shown).

[0021] Continuing reference Figure 1 , Figure 2 and Figure 3 shows a cross-sectional view and an isometric view of one of the return passages and associated return passage vanes 15.1 in the return passage 15 in one embodiment. A similar configuration may be provided for all or some of the return passages 15 of the compressor 1.

[0022] The return passage vanes 15.1 are circumferentially arranged around a return passage axis coinciding with the rotational axis A-A. Each return passage vane 15.1 includes a leading edge 15.3 and a trailing edge 15.5. The leading edge 15.3 is arranged at a first distance from the axis A-A, and the trailing edge 15.5 is arranged at a second distance from the axis A-A, the second distance being less than the first distance.

[0023] In some embodiments, the return passage vanes 15.1 may have a curved shape with a concave pressure side and a convex suction side, both extending from the leading edge to the trailing edge, as Figure 2 shown. Other simpler shapes may be provided where the suction side and the pressure side of each vane are substantially symmetric with respect to the arc of the vane.

[0024] In Figure 2In an embodiment, all the return channel vanes 15.1 have the same shape. Additionally, all the return channel vanes 15.1 are arranged at the same distance from the central axis A-A of the return channel 15, such that the leading edges 15.3 and trailing edges 15.5 of the return channel vanes 15.1 are respectively arranged on the outer circumference and the inner circumference. However, this is not mandatory and alternative embodiments are possible. For example, the return channel vanes 15.1 can have a variable chord. The chord is the distance between the leading edge and the trailing edge of the vane. Additionally, the trailing edge and / or the leading edge can be arranged at a variable radial distance from the central axis A-A of the return channel 15. That is, there can be at least two return channel vanes 15.1 having corresponding trailing edges 15.5 arranged at two different distances from the central axis A-A, and / or at least two return channel vanes 15.1 can have corresponding leading edges 15.3 arranged at two different distances from the central axis A-A.

[0025] Additionally, the return channel 15 can have a variable profile and / or a variable height in the tangential direction as well as in the flow direction. Further, the return channel vanes 15.1 can also have a variable inclination.

[0026] As Figure 2 shown, the spacing S (i.e., the pitch between two adjacent or consecutive return channel vanes 15.1 between which the corresponding flow passages are formed) is non-constant. The pitch or spacing variation can follow different criteria. Figure 2 An embodiment provides 18 vanes arranged to form four 90° sectors. Two of the sectors include five vanes arranged at 18° to each other, while the other two sectors include four vanes arranged at 22.5°. For Figure 2 each flow passage in, the angle between each pair of adjacent return channel vanes 15.1 is indicated. Thus, in this embodiment, the distribution of the return channel vanes 15.1 is regular, i.e., the distribution pitch repeats in subsequent sectors extending over the complete 360° around the return channel 15.

[0027] In other embodiments, the distribution can be completely random, as for example Figure 4 shown. Here, 18 return channel vanes 15 are arranged such that the angle between consecutive or adjacent return channel vanes 15.1 defining the corresponding flow passages varies randomly, for example from a minimum of 17° to a maximum of 23°. The variable angular spacing corresponds to a variable pitch between pairs of adjacent return channel vanes 15.1.

[0028] The effect of the non-uniform or non-constant distribution of the return channel vanes 15.1 on the vibration of the impeller vanes 7.3 can be understood from Figure 5 and Figure 6 the two figures of, Figure 5 and Figure 6Shows the corresponding harmonic quantities representing the excitation source in three different cases. In both figures, the circumferential order is plotted on the horizontal axis and the amplitude is plotted on the vertical axis.

[0029] More specifically, in Figure 5 , a comparison is shown between the harmonic quantities in a centrifugal compressor of the prior art and the harmonic quantities in a compressor including a distribution pattern (i.e., a regular repetition of two different pitches of 18° and 22.5° respectively) of return channel vanes 15.1 according to Figure 2 . The harmonic quantity is increased substantially by the non-constant pitch, and the excitation amplitude is decreased.

[0030] Figure 4 's embodiment represents a further improvement over the embodiment of Figure 2 , as can be understood from Figure 6 . Figure 6 The diagram shown in Figure 2 shows a comparison of the harmonic quantities in the embodiment of Figure 4 with the harmonic quantities in the embodiment of Figure 4 . According to Figure 2 , the return channel vanes 15.1 are arranged in a completely random manner. Compared with the embodiment of

[0031] , the harmonic quantity is further increased and the maximum excitation amplitude is further decreased.

[0032] As a further improvement, the pitch and chord of the return channel vanes 15.1 can be related to each other to further improve the efficiency of the turbine. More specifically, the pitch and chord can be selected such that the density of the associated flow path defined between two adjacent return channel vanes 15.1 remains substantially constant. The density is the ratio between the chord of the vane (i.e., the distance between the trailing edge and the leading edge of the vane) and the pitch between two consecutive vanes. In this context, "substantially constant" can be understood as a density within the range of + / - 20% around a constant preset density value. According to the embodiments disclosed herein, "substantially constant" can be understood as a density maintained within the range of + / - 10% around a preset constant density value, and preferably within the range of + / - 5% and more preferably within the range of + / - 2% around the preset constant density value.

[0033] The correlation between the chord and the pitch causes the decrease in density caused by the increase in pitch between the return channel vanes 15.1 to be at least partially offset by an increase in chord length.

[0034]

[0035] Where Bi is the chord of one of the two return channel vanes 15.1 that define the ith flow path Pi. More specifically, Bi is the chord of the return channel vane whose suction side faces the ith flow path Pi. In the context of the present invention, the compactness of the return channel flow path is defined as the ratio between the chord of the return channel vane 15.1 whose suction side faces the flow path and the pitch between the two return channel vanes 15.1 (which define the flow path therebetween).

[0036] By making the chord B of the first return channel vane 15.1 of each ith flow path Pi dependent on the pitch or spacing Si between the two return channel vanes that form that path, the effect of the change in compactness caused by the pitch change is balanced by a change in the chord.

[0037] Thus, by utilizing an increase in the chord of the relevant return channel vane 11.1 to balance the decrease in compactness that would be caused by an increase in pitch, the beneficial effect of pitch change in reducing impeller vibration is achieved without any negative impact on compressor operability.

[0038] In a preferred embodiment, the relationship between each return channel vane chord Bi and the pitch or spacing Si of each ith flow path Pi is such that the compactness σ of said flow path Pi remains constant.

[0039] However, a strictly constant compactness value is not mandatory. Beneficial effects in terms of enhanced compressor operability can also be achieved if the compactness remains substantially constant around a preset value. According to the embodiments disclosed herein, "substantially constant" can be understood as a compactness that remains within a range of + / - 10%, and preferably within a range of + / - 5% and more preferably within a range of + / - 2% around a preset constant compactness value.

[0040] For improved vibration reduction, the diffuser vanes 11.1 can also be arranged according to a varying, i.e., non-constant or non-uniform, pitch.

[0041] The above embodiments specifically refer to a centrifugal compressor. However, the novel return channels according to the present disclosure can also be advantageously used in a centrifugal pump having a structure similar to Figure 1 the structure shown.

[0042] Exemplary embodiments have been disclosed above and illustrated in the drawings. Those skilled in the art will understand that various changes, omissions, and additions can be made to the specifically disclosed content herein without departing from the scope of the invention as defined in the following claims.

Claims

1. A return channel for a centrifugal turbine, the return channel including a plurality of return channel vanes arranged around a return channel axis; Each return channel vane includes: a leading edge at a first distance from the return channel axis and a trailing edge at a second distance from the return channel axis, the second distance being less than the first distance; wherein a respective flow passage is defined between each pair of adjacent return channel vanes; characterized in that the return channel vanes are arranged around the return channel axis with a non-constant pitch, wherein the return channel vanes have a variable chord length; and wherein the pitch between each pair of adjacent first and second return channel vanes and the chord of one of the first and second return channel vanes are selected such that the density of each flow passage is maintained within a range about a constant density value.

2. The return channel according to claim 1, wherein the return channel vanes are arranged according to a random pitch.

3. The return channel according to claim 1 or 2, wherein the return channel vanes have a non-constant chord.

4. The return channel according to any one of the preceding claims 1 to 2, wherein the return channel vanes have a variable profile in the tangential direction and / or the flow direction.

5. The return channel according to any one of the preceding claims 1 to 2, wherein the return channel vanes have a variable radial position of the leading edge.

6. The return channel according to any one of the preceding claims 1 to 2, wherein the return channel vanes have a variable radial position of the trailing edge.

7. The return channel according to any one of the preceding claims 1 to 2, wherein the return channel vanes have a variable inclination.

8. The return channel according to any one of the preceding claims 1 to 2, wherein the return channel height is variable in the tangential direction and / or the flow direction.

9. The return channel according to any one of the preceding claims 1 to 2, wherein the range is equal to + / - 20% of the constant density value.

10. The return channel according to any one of the preceding claims 1 to 2, wherein the range is equal to + / - 10% of the constant density value.

11. The return channel according to any one of the preceding claims 1 to 2, wherein the range is equal to + / - 5% of the constant density value.

12. The return channel according to any one of the preceding claims 1 to 2, wherein the range is equal to + / - 2% of the constant density value.

13. A centrifugal turbine, the centrifugal turbine including: A stationary member; At least a first impeller and a second impeller arranged to rotate about a rotation axis; A first diffuser around the first impeller and a second diffuser around the second impeller, the first diffuser and the second diffuser being adapted to convert the velocity of a fluid flow from the respective impeller into pressure; and A return channel arranged between the first diffuser and the second impeller according to any one of the preceding claims 1 to 12.

14. The centrifugal turbine according to claim 13, wherein the diffuser is a vane diffuser, and wherein the diffuser vanes are arranged according to a constant or non-constant pitch.

15. The centrifugal turbine according to claim 13 or 14, wherein the turbine is a centrifugal compressor.

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