FI20215249A1

FI20215249A1 - Turbomachine

Info

Publication number: FI20215249A1
Application number: FI20215249A
Authority: FI
Inventors: Matti Malkamäki
Original assignee: Apugenius Oy
Priority date: 2021-03-08
Filing date: 2021-03-08
Publication date: 2022-09-09
Also published as: WO2022189696A1

Abstract

A turbomachine comprises a radial impeller (101) comprising a first stage (102) and a second stage (103). The first stage is a closed impeller stage and vanes of the second stage are connected to a shroud (104) of the first stage. A first axial channel (105) conducts fluid to the first stage and a second axial channel (107) conducts the fluid to the second stage. The first axial channel is coaxially inside the second axial channel. Mutually adjacent rim portions (B) of the first axial channel and the shroud (104) form a different flow resistance between the second axial channel and the first stage of the radial impeller than between the first axial channel and the second stage of the radial impeller so that momentum of the fluid reduces leakage by acting against a difference between static pressures of the first and second axial channels.

Description

A turbomachine Field of the disclosure The disclosure relates generally to turbomachines such as e.g. turbo-compressors, pumps, and blowers. More particularly, the disclosure relates to a turbomachine that comprises a radial impeller comprising an impeller stage that is a closed impeller stage and another impeller stage whose vanes are connected to a shroud of the first mentioned impeller stage. Background In many turbomachines it is advantageous or even necessary to have more than one turbomachine stage each operating at its own pressure range so that fluid is interacting with impeller vanes of the turbomachine stage. In this document, the word “impeller” is used in conjunction with compressors, blowers, and pumps in which one or more impellers is/are used to increase pressure and flow of fluid. The fluid can be gas or liquid such as e.g. air, steam, or water. In a radial turbo-compressor, fluid is received via an axial channel which directs the fluid to a center area of a radial impeller. The radial impeller is configured to rotate in a casing that directs the fluid from an outer perimeter of the radial impeller to a channel that discharges the fluid. The casing constitutes typically a volute which - 20 surrounds the radial impeller and whose cross-sectional flow area increases

QA S towards an outlet that is typically tangentially directed. The casing may further se constitute a diffuser that converts kinetic energy of the fluid into pressure by 2 gradually slowing down the velocity of the fluid..

I E A radial impeller can have two stages so that a first stage is a closed impeller stage I 25 and vanes of the second stage are connected to a shroud of the first stage. The S second stage can be a closed impeller stage, a semi open impeller stage, or an N open impeller stage. It is also possible that a radial impeller has three or more stages. A radial turbomachine having a two-stage radial impeller of the kind mentioned above comprises a first axial channel having a fluid transfer connection with a center inlet area of the first stage of the radial impeller, a first annular channel having a radial fluid transfer connection with an outer perimeter of the first stage of the radial impeller, a second axial channel having a fluid transfer connection with an annular inlet area of the second stage of radial the impeller where the annular inlet area of the second stage surrounds the center inlet area of the first stage, and a second annular channel having a radial fluid transfer connection with an outer perimeter of the second stage of the radial impeller, wherein the first axial channel is coaxially inside the second axial channel. In many cases, the above-mentioned second impeller stage can be used as a high-pressure stage whereas the above- — mentioned first impeller stage can be used as a low-pressure stage. It is however also possible that the first impeller stage is used as a high-pressure stage whereas the second impeller stage is used as a low-pressure stage. Two-stage radial turbomachines of the kind described above are advantageous especially in small systems that need to be cost effective. The above-described two- stage radial turbomachine can be cost effective because only one rotating shaft is needed. Furthermore, unlike in turbomachines having a two-stage radial impeller implemented as a back-to-back configuration, there is no need to handle the problem how to isolate a shaft from an axial inlet channel e.g. to avoid bearing lubrication oil from mixing with fluid flowing in the axial inlet channel surrounding the shaft. In typical turbo-compressors, an inlet channel is especially sensitive for pressure and/or flow distortions effecting to the efficiency. Thus, a further challenge related to the back-to-back configuration is that an inlet channel of a compressor — stage that is on the same side as the shaft cannot be designed as freely as an inlet S channel of the other compressor stage, and thereby the efficiency of the first g 25 mentioned compressor stage can be lower than that of the other compressor stage.

00 7 The above-described two-stage radial turbomachine where vanes of the second E stage are connected to a shroud of the first stage is however not free from I challenges. One of the challenges is related to a gap between the first axial channel = and the shroud of the first stage of the radial impeller because a pressure difference N 30 between the first and second axial channels may cause unwanted leakage via the above-mentioned gap. It is straightforward to implement two or more turbomachine stages with two or more rotating shafts, but this approach is typically too expensive for small systems that need to be cost effective. Summary The following presents a simplified summary in order to provide a basic understanding of some aspects of various invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying embodiments of the invention. In this document, the word “geometric” when used as a prefix means a geometric concept that is not necessarily a part of any physical object. The geometric concept can be for example a geometric point, a straight or curved geometric line, a geometric plane, a non-planar geometric surface, a geometric space, or any other geometric entity that is zero, one, two, or three dimensional. In accordance with the invention, there is provided a new turbomachine that can be for example a turbo-compressor, a pump, or a blower. A turbomachine according to the invention comprises: - a radial impeller comprising a first stage and a second stage so that the first - 20 stage is a closed impeller stage and vanes of the second stage are connected O to a shroud of the first stage, 3 0 - afirst axial channel having a fluid transfer connection with a center inlet area 7 of the first stage of the radial impeller,

T a o - a second axial channel having a fluid transfer connection with an annular- © 25 shaped inlet area of the second stage of the radial impeller, the annular- O shaped inlet area of the second stage of the radial impeller surrounding the center inlet area of the first stage of the radial impeller,

- a first annular channel having a radial fluid transfer connection with an outer perimeter of the first stage of the radial impeller, and - a second annular channel having a radial fluid transfer connection with an outer perimeter of the second stage of the radial impeller.

The above-mentioned first and second axial channels are tubular, and the first axial channel is coaxially inside the second axial channel. A rim portion of the first axial channel facing towards the radial impeller and a rim portion of a center area of the shroud of the first stage are shaped to cause a different flow resistance between the second axial channel and the first stage of the radial impeller than between the first axial channel and the second stage of the radial impeller. This is implemented so that the above-mentioned rim portions are shaped to make a leakage flow between the second axial channel and the first stage of the radial impeller and a leakage flow between the first axial channel and the second stage of the radial impeller to meander in different ways. Therefore, dynamic pressure caused by momentum of — the fluid is utilized to eliminate or at least reduce leakage caused by a difference between static pressures of the first and second axial channels.

In cases where the turbomachine is a turbo-compressor and the second impeller stage is used as a high-pressure stage and the first impeller stage is used as a low- pressure stage, the above-mentioned rim portions are shaped to cause a greater flow resistance from the second axial channel to the first stage of the radial impeller than from the first axial channel to the second stage of the radial impeller. In cases N where the turbomachine is a turbo-compressor and the first impeller stage is used N as a high-pressure stage and the second impeller stage is used as a low-pressure 3 stage, the above-mentioned rim portions are shaped to cause a greater flow 3 25 resistance from the first axial channel to the second stage of the radial impeller than = from the second axial channel to the first stage of the radial impeller.

o N In a turbomachine according to an exemplifying and non-limiting embodiment, the N difference in the flow resistances is implemented at least partly so that the above- N mentioned rim portions are radially overlapping. In cases where the turbomachine is a turbo-compressor and the second impeller stage is used as a high-pressure stage and the first impeller stage is used as a low-pressure stage, the rim portion of the first axial channel surrounds the rim portion of the center area of the shroud of the first stage.

In cases where the turbomachine is a turbo-compressor and the first impeller stage is used as a high-pressure stage and the second impeller stage is used as a low-pressure stage, the rim portion of the center area of the shroud of the 5 first stage surrounds the rim portion of the first axial channel.

In a turbomachine according to an exemplifying and non-limiting embodiment, the difference in the flow resistances is implemented at least partly so that the rim portion of the first axial channel is conical and/or the rim portion of the center area of the shroud of the first stage is conical.

In cases where the second impeller stage is used as a high-pressure stage and the first impeller stage is used as a low- pressure stage, a cone angle of each conical rim portion opens in a flow direction in the first and second axial channels.

In cases where the first impeller stage is used as a high-pressure stage and the second impeller stage is used as a low-pressure stage, the cone angle of each conical rim portion opens against the flow direction in the first and second axial channels.

In a turbomachine according to an exemplifying and non-limiting embodiment, the difference in the flow resistances is implemented at least partly so that an end surface of the rim portion of the first axial channel is conical and/or an end surface of the rim portion of the center area of the shroud of the first stage is conical.

In cases where the second impeller stage is used as a high-pressure stage and the first impeller stage is used as a low-pressure stage, a cone angle of each conical end surface opens in a flow direction in the first and second axial channels.

In cases S where the first impeller stage is used as a high-pressure stage and the second O impeller stage is used as a low-pressure stage, the cone angle of each conical end 3 25 surface opens against the flow direction in the first and second axial channels. = Exemplifying and non-limiting embodiments are described in accompanied 2 dependent claims. o N Various exemplifying and non-limiting embodiments both as to constructions and to N methods of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific exemplifying and non- limiting embodiments when read in conjunction with the accompanying drawings.

The verbs “to comprise” and “to include” are used in this document as open limitations that neither exclude nor reguire the existence of unrecited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of “a” or ‘an’, i.e. a singular form, throughout this document does not exclude a plurality. Brief description of the figures Exemplifying and non-limiting embodiments and their advantages are explained in greater detail below in the sense of examples and with reference to the accompanying drawings, in which: figures 1a, 1b, and 1c illustrate a turbomachine according to an exemplifying and non-limiting embodiment, figures 2 and 3 illustrate details of turbomachines according to exemplifying and non-limiting embodiments, and figure 4 illustrates a turbomachine according to an exemplifying and non-limiting embodiment. Description of exemplifying and non-limiting embodiments The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description are not exhaustive unless otherwise N 20 explicitly stated.

N 3 Figures 1a, 1b, and 1c illustrate a turbomachine according to an exemplifying and S non-limiting embodiment. In the text below with reference to figures 1a-1c, operation E of the turbomachine as a turbo-compressor is explained. Figure 1a shows a section o taken along a line A-A shown in figure 1b. A geometric section plane is parallel with > 25 the yz-plane of a coordinate system 199. Figure 1b shows the turbomachine when O seen along the negative z-direction of the coordinate system 199. Figure 1c shows a magnification of a part B of figure 1a. The turbomachine comprises a radial impeller 101 comprising a first stage 102 and a second stage 103 so that the first stage is a closed impeller stage and vanes of the second stage are connected to a shroud 104 of the first stage. In this exemplifying case, the second stage 103 of the radial impeller is a closed impeller stage comprising a shroud 114 so that the vanes of the second stage 103 are between the shroud 104 of the first stage and the shroud 114 of the second stage. It is however also possible that the second stage is an open or semi open impeller stage. The radial impeller 101 is connected to a shaft 116 a part of which is shown in figure 1a. The radial impeller 101 is configured to rotate in a casing 117 that constitutes a first annular channel 118 which receives fluid from an outer perimeter of the first stage 102 of the radial impeller 101 and a — second annular channel 119 which receives the fluid from an outer perimeter of the second stage 103 of the radial impeller 101. In this exemplifying case, the first and second annular channels 118 and 119 are volutes for receiving the fluid from the first and second stages 102 and 103 of the radial impeller 101, respectively. The casing 117 may further constitute a diffuser that converts kinetic energy of the fluid — into pressure by gradually slowing down fluid velocity.

In the exemplifying turbomachine illustrated in figures 1a-1c, the second stage 103 of the radial impeller 101 is used as a high-pressure stage and the first stage 102 of the radial impeller 101 is used as a low-pressure stage. The turbomachine comprises a first axial channel 105 that is configured to conduct fluid to a center inlet area of the first stage 102 of the radial impeller 101. The turbomachine comprises a channel 106 that is connected to the first annular channel 118 and configured to discharge the fluid. The turbomachine comprises a second axial — channel 107 that is configured to conduct the fluid discharged via the channel 106 S to an annular-shaped inlet area of the second stage 103 of the radial impeller 101. g 25 The annular-shaped inlet area of the second stage 103 of the radial impeller 3 surrounds the center inlet area of the first stage 102 of the radial impeller. In figure E 1a, flow channels from the channel 106 to the second axial channel 107 are depicted 2 schematically with dashed lines. The turbomachine comprises a channel 108 that is > connected to the second annular channel 119 and configured to discharge the fluid. 3 30 The exemplifying turbomachine illustrated in figures 1a-1c comprises an intercooler 115 between the channel 106 and the second axial channel 107.

The above-mentioned first and second axial channels 105 and 107 are tubular, and the first axial channel 105 is coaxially inside the second axial channel 107. As presented in figure 1c that shows the part B of figure 1a, a rim portion 109 of the first axial channel 105 facing towards the radial impeller and a rim portion 110 of the center area of the shroud 104 of the first stage are shaped to cause a greater flow resistance from the second axial channel 107 to the first stage of the radial impeller than from the first axial channel 105 to the second stage of the radial impeller. As illustrated in figure 1c, the rim portions 109 and 110 are shaped to make a leakage flow from the second axial channel 107 to the first stage of the radial impeller to meander more than a leakage flow from the first axial channel 105 to the second stage of the radial impeller. Thus, unwanted leakage from the second axial channel 107 having a higher pressure to the first axial channel 105 having a lower pressure can be reduced. In figure 1c, a flow path from the second axial channel 107 to the first stage of the radial impeller is depicted with a dash-and-dot line 112 and a flow path from the first axial channel 105 to the second stage of the radial impeller is depicted with a dashed line 113. The difference in static pressures between the second and first axial channels 107 and 105 tends to push fluid from the second axial channel 107 to the first stage of the radial impeller according to the dash-and- dot line 112. On the other hand, the shaping of the rim portion 109 of the first axial channel 105 and the rim portion 110 of the center area of the shroud 104 cause that momentum of fluid tends to push the fluid from the first axial channel 105 to the second stage of the radial impeller according to the dashed line 113. Thus, the momentum of the fluid is arranged to act against the difference in the static N pressures and thereby leakage from the second axial channel 107 to the first stage N 25 of the radial impeller is eliminated or at least reduced. Therefore, dynamic pressure 2 caused by the momentum of the fluid is used for eliminating or at least reducing 7 leakage caused by a difference in static pressures of the first and second axial E channels 105 and 107.

S > There are many different shapes that can be used in the above-mentioned rim 3 30 portions 109 and 110 to achieve the above-discussed difference in the flow resistances so that there is the greater flow resistance from the second axial channel 107 to the first stage of the radial impeller than from the first axial channel 105 to the second stage of the radial impeller. Some exemplifying cases are described below with reference to figures 1a, 1c, 2, and 3.

In the exemplifying turbomachine illustrated in figures 1a-1c, the difference in the flow resistances is implemented so that: i) the rim portions 109 and 110 are radially overlapping so that the rim portion 109 surrounds the rim portion 110, ii) the rim portions 109 and 110 are conical so that cone angles open in a flow direction in the first and second axial channels 105 and 107 i.e. in the negative z-direction of the coordinate system 199, and iii) end surfaces of the rim portions 109 and 110 are conical so that cone angles open in the above-mentioned flow direction.

In the exemplifying turbomachine shown in figure 1a, a rim portion of an outer perimeter of the shroud 104 and a rim portion of a wall 120 between first and second annular channels 118 and 119 are shaped to cause a greater flow resistance from the second stage 103 of the radial impeller to the first annular channel 118 than from the first stage 102 of the radial impeller to the second annular channel 119.

Therefore, dynamic pressure caused by the momentum of the fluid is used for eliminating or at least reducing leakage caused by difference in static pressures. As illustrated in figure 1a, the rim portion of the outer perimeter of the shroud 104 and the rim portion of the wall 120 are shaped to make a leakage flow from the second stage 103 of the radial impeller 101 to the first annular channel 118 to meander more than a leakage flow from the first stage 102 of the radial impeller 101 to the second annular channel 119. In the exemplifying turbomachine illustrated in figure 1a, the rim portion of the outer perimeter of the shroud 104 and the rim portion of S the wall 120 are axially overlapping so that the rim portion of the outer perimeter of O the shroud 104 is first in the direction of flow in the first and second axial channels 2 25 105 and 107. Furthermore, an end surface of the rim portion of the outer perimeter 7 of the shroud 104 is conical and an end surface of the rim portion of the wall 120 is E conical so that cone angles of the conical end surfaces open against the direction S of flow in the first and second axial channels 105 and 107.

O Figures 2 and 3 illustrate details of turbomachines according to exemplifying and non-limiting embodiments. The details shown in figures 2 and 3 correspond to the details shown in figure 1c. In the exemplifying case illustrated in figure 2, the difference in the flow resistances is implemented so that the rim portion 209 of the first axial channel 205 and the rim portion 210 of the shroud 204 of the first stage of the radial impeller are radially overlapping so that the rim portion 209 surrounds the rim portion 210. In figure 2, a flow path from the second axial channel 207 to the first stage of the radial impeller is depicted with a dash-and-dot line 212 and a flow path from the first axial channel 205 to the second stage of the radial impeller is depicted with a dashed line 213. In the exemplifying case illustrated in figure 3, the difference in the flow resistances is implemented so that the end surface of the rim portion 309 of the first axial channel 305 and the end surface of the rim portion 310 of the shroud 304 of the first stage of the radial impeller are conical so that the cone angles open in the flow direction. In figure 3, a flow path from the second axial channel 307 to the first stage of the radial impeller is depicted with a dash-and-dot line 312 and a flow path from the first axial channel 305 to the second stage of the radial impeller is depicted with a dashed line 313.

The above-described principle is applicable also in cases where a radial impeller comprises three or more stages and a turbomachine comprises three or more coaxially nested channels and three or more annular channels.

Figure 4 shows a section view of a turbomachine according to an exemplifying and non-limiting embodiment. The geometric section plane is parallel with the yz-plane of a coordinate system 499. In the text below with reference to figure 4, operation of the turbomachine as a turbo-compressor is explained. The turbomachine comprises a radial impeller 401 comprising a first stage 402 and a second stage 403 so that N the first stage 401 is a closed impeller stage and vanes of the second stage 403 are N connected to a shroud 404 of the first stage. The radial impeller 401 is connected to 3 25 a shaft a part of which is shown in figure 4. In this exemplifying turbomachine, the I first stage 402 of the radial impeller 401 is used as a high-pressure stage and the E second stage 403 of the radial impeller 401 is used as a low-pressure stage. The I radial impeller 401 is configured to rotate in a casing 417 that constitutes a first = annular channel 418 which receives fluid from an outer perimeter of the first stage N 30 402 of the radial impeller 401 and a second annular channel 419 which receives the fluid from an outer perimeter of the second stage 403 of the radial impeller 401. In this exemplifying case, the first and second annular channels 418 and 419 are volutes for receiving the fluid from the first and second stages 402 and 403 of the radial impeller 401, respectively. The casing 417 may further constitute a diffuser that converts kinetic energy of the fluid into pressure by gradually slowing fluid velocity.

The turbomachine comprises a first axial channel 405 that is configured to conduct fluid to a center inlet area of the first stage 402 of the radial impeller 401. The turbomachine comprises a channel 406 that is connected to the first annular channel 418 and configured to discharge the fluid. The turbomachine comprises a second axial channel 407 that is configured to conduct the fluid to an annular-shaped inlet area of the second stage 403 of the radial impeller 401. The annular-shaped inlet area of the second stage 403 of the radial impeller surrounds the center inlet area of the first stage 402 of the radial impeller. The turbomachine comprises a channel 408 that is connected to the second annular channel 419 and configured to discharge the fluid. In this exemplifying turbomachine, the fluid discharged by the channel 408 is supplied to the first axial channel 405. In figure 1a, a flow channel from the channel 408 to the first axial channel 405 is depicted schematically with a dashed line.

The above-mentioned first and second axial channels 405 and 407 are tubular, and the first axial channel 405 is coaxially inside the second axial channel 407. As presented in figure 4, a rim portion of the first axial channel 405 facing towards the radial impeller and a rim portion of the center area of the shroud 404 are shaped to cause a smaller flow resistance from the second axial channel 407 to the first stage N 402 of the radial impeller than from the first axial channel 405 to the second stage N 403 of the radial impeller. These rim portions are shaped to make a leakage flow 2 25 from the second axial channel 407 to the first stage 402 of the radial impeller to 7 meander less than a leakage flow from the first axial channel 405 to the second E stage 403 of the radial impeller. Thus, unwanted leakage from the first axial channel I 407 having a higher pressure to the second axial channel 407 having a lower = pressure can be reduced. The difference in static pressures between the first and N 30 second axial channels 405 and 407 tends to push fluid from the first axial channel 405 to the second stage 403 of the radial impeller. On the other hand, the shaping of the rim portion of the first axial channel 405 and the rim portion of the center area of the shroud 404 cause that momentum of fluid tends to push the fluid from the second axial channel 407 to the first stage 402 of the radial impeller. Thus, the momentum of the fluid is arranged to act against the difference in the static pressures and thereby leakage from the first axial channel 405 to the second stage 403 of the radial impeller is eliminated or at least reduced. Therefore, dynamic pressure caused by the momentum of the fluid is used for eliminating or at least reducing leakage caused by a difference in static pressures of the first and second axial channels 405 and 407. In the exemplifying turbomachine illustrated in figure 4, the difference in the flow resistances is implemented so that: i) the rim portion of the first axial channel 405 and the rim portion of the center area of the shroud 404 are radially overlapping so that the rim portion of the shroud 404 surrounds the rim portion of the first axial channel 405, ii) these rim portions are conical so that cone angles open against a flow direction in the first and second axial channels 405 and 407 i.e. the cone angles open in the positive z-direction of the coordinate system 499, and iii) end surfaces of these rim portions are conical so that cone angles open against the above- mentioned flow direction.

In the exemplifying turbomachine shown in figure 4, a rim portion of an outer perimeter of the shroud 404 and a rim portion of a wall 420 between first and second annular channels 418 and 419 of the casing 417 are shaped to cause a smaller flow resistance from the second stage 403 of the radial impeller to the first annular channel 418 than from the first stage 402 of the radial impeller to the second annular N channel 419. Therefore, dynamic pressure caused by the momentum of the fluid is N used for eliminating or at least reducing leakage caused by difference in static 2 25 pressures. As illustrated in figure 4, the rim portion of the outer perimeter of the 7 shroud 404 and the rim portion of the wall 420 are shaped to make a leakage flow E from the second stage 403 of the radial impeller 401 to the first annular channel 418 I to meander less than a leakage flow from the first stage 402 of the radial impeller = 401 to the second annular channel 419. In the exemplifying turbomachine illustrated N 30 in figure 4, the rim portion of the outer perimeter of the shroud 404 and the rim portion of the wall 420 are axially overlapping so that the rim portion of the shroud 404 is later in the direction of flow in the first and second axial channels 405 and

407. Furthermore, an end surface of the rim portion of the outer perimeter of the shroud 404 is conical and an end surface of the rim portion of the wall 420 is conical so that cone angles of the conical end surfaces open in the direction of flow in the first and second axial channels 405 and 407.

The specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Lists and groups of examples provided in the description given above are not exhaustive unless otherwise explicitly stated.

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Claims

What is claimed is:

1. Aturbomachine comprising: - aradial impeller (101, 401) comprising a first stage (102, 402) and a second stage (103, 403) so that the first stage is a closed impeller stage and vanes of the second stage are connected to a shroud (104, 404) of the first stage, - a first axial channel (105, 205, 305, 405) having a fluid transfer connection with a center inlet area of the first stage of the radial impeller, - a second axial channel (107, 207, 307, 407) having a fluid transfer connection with an annular-shaped inlet area of the second stage of the radial impeller, the annular-shaped inlet area of the second stage of the radial impeller surrounding the center inlet area of the first stage of the radial impeller, - a first annular channel (118, 418) having a radial fluid transfer connection with an outer perimeter of the first stage of the radial impeller, - asecondannular channel (119, 419) having a radial fluid transfer connection with an outer perimeter of the second stage of the radial impeller, wherein the first and second axial channels are tubular, and the first axial channel is coaxially inside the second axial channel, characterized in that a rim portion (109, 209, 309) of the first axial channel facing towards the radial impeller and a rim portion (110, 210, 310) of a center area of the shroud of the first stage are shaped N 20 to cause a different flow resistance between the second axial channel and the first N stage of the radial impeller than between the first axial channel and the second stage 2 of the radial impeller to enable dynamic pressure caused by momentum of fluid to 7 at least reduce leakage caused by a difference between static pressures of the first E and second axial channels. 2 > 25

2. Aturbomachine according to claim 1, wherein the rim portion (109, 209, 309) O of the first axial channel (105) and the rim portion (110, 210, 310) of the center area of the shroud (104) are shaped to cause a greater flow resistance from the second axial channel (107, 207, 307) to the first stage (102) of the radial impeller than from the first axial channel (105, 205, 305) to the second stage (103) of the radial impeller.

3. Aturbomachine according to claim 1, wherein the rim portion of the first axial channel (405) and the rim portion of the center area of the shroud (404) are shaped to cause a smaller flow resistance from the second axial channel (407) to the first stage (402) of the radial impeller than from the first axial channel (405) to the second stage (403) of the radial impeller.

4. A turbomachine according to any one of claims 1-3, wherein the rim portion (109, 209) of the first axial channel (105, 205, 405) and the rim portion (110, 310) of the center area of the shroud (104, 204, 404) are radially overlapping.

5. Aturbomachine according to any one of claims 1-4, wherein the rim portion (109) of the first axial channel (105, 405) is conical.

6. A turbomachine according to any one of claims 1-5, wherein the rim portion (110) of the center area of the shroud (104, 404) is conical.

7. Aturbomachine according to any one of claims 1-6, wherein an end surface of the rim portion (309) of the first axial channel (105, 405) is conical.

8. Aturbomachine according to any one of claims 1-7, wherein an end surface of the rim portion (310) of the center area of the shroud (104, 404) is conical.

9. Aturbomachine according to any one of claims 1-8, wherein a rim portion of N 20 an outer perimeter of the shroud (104, 404) and a rim portion of a wall (120, 420) N between the first and second annular channels (118, 119, 418, 419) are shaped to 2 cause a different flow resistance between the second stage (103, 403) of the radial 7 impeller and the first annular channel than between the first stage (102, 402) of the E radial impeller and the second annular channel to enable dynamic pressure caused I 25 by momentum of the fluid to at least reduce a leakage caused by difference between = static pressures of the first and second annular channels.

N

10. A turbomachine according to claim 9, wherein the rim portion of the outer perimeter of the shroud (104) and the rim portion of the wall (120) between the first and second annular channels (118, 119) are shaped to cause a greater flow resistance from the second stage (103) of the radial impeller to the first annular channel than from the first stage (102) of the radial impeller to the second annular channel.

11. A turbomachine according to claim 9, wherein the rim portion of the outer perimeter of the shroud (404) and the rim portion of the wall (420) between the first and second annular channels (418, 419) are shaped to cause a smaller flow resistance from the second stage (403) of the radial impeller to the first annular channel than from the first stage (402) of the radial impeller to the second annular channel.

12. A turbomachine according to any one of claims 9-11, wherein the rim portion of the outer perimeter of the shroud (104, 404) and the rim portion of the wall (120, 420) between the first and second annular channels (118, 119, 418, 419) are axially overlapping.

13. Aturbomachine according to any one of claims 9-12, wherein an end surface of the rim portion of the outer perimeter of the shroud (104, 404) is conical and an end surface of the rim portion of the wall (120, 420) between the first and second annular channels (118, 119, 418, 419) is conical, cone angles of the conical end surfaces opening in a same axial direction.

14. Aturbomachine according to any one of claims 1-13, wherein the second stage _ (103) of the radial impeller is a closed impeller stage comprising a shroud (114) of O the second stage, the vanes of the second stage being between the shroud of the se first stage and the shroud of the second stage.

S

15. A turbomachine according to any one of claims 1-14, wherein the E 25 —turbomachine comprises an intercooler (115) between the first annular channel I (118) and the second axial channel (107).

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