EP3551889B1 - Return channel of a multistage compressor or expander with twisted vanes - Google Patents

Description

The invention relates to a return stage for throughflow by means of a process fluid along a flow direction of a radial turbomachine, in particular a radial turbo compressor return stage, the return stage extending in a ring around an axis, the return stage being defined radially inward by an inner boundary contour and radially outward by an outer boundary contour, wherein, along a first flow direction, the return stage extends radially outward in a first section, wherein the return stage extends in a second section along the first flow direction, describing an arcuate deflection, from radially outside to radially inward, the return stage extending along the first flow direction in a third section extending from radially outward to radially inward, the return stage extending along the first flow direction in a fourth section describing an arcuate deflection extending from radially inside to axially, with at least one guide vane stage comprising guide vanes extending at least along part of the third section and segmenting the return stage in the circumferential direction into flow channels, with a profile center line of a profile cross section of the guide vanes of the guide vane stage on the part of the inner boundary contour and an inner track on the side the outer boundary contour defines an outer track. In addition, the invention relates to a radial turbo machine, in particular a radial turbo compressor with at least one such return stage.

Radial turbo machines are known as either radial turbo compressors or radial turbo expanders. Unless otherwise stated, the following statements refer to the version as a compressor. The invention is basically just as applicable for expanders as it is for compressors, a radial turbo expander compared to a radial turbo compressor essentially provides a reverse flow direction of the process fluid.

With a radial turbo expander, the energy stored thermodynamically in the process fluid is converted into technical work by means of a drive of the impeller while a process fluid is relaxed and deflected.
In the case of radial turbo compressors, this process is reversed; these convert or store technical work into flow work that is stored thermodynamically in the process fluid. For this purpose, the impellers of the compressor usually suck in a process fluid axially to an axis of rotation or at an angle to the axis of rotation with an axial speed component and accelerate and compress this process fluid by means of the respective impeller - which is also referred to as an impeller - which controls the flow direction of the process fluid deflects radial direction. In the case of a multistage radial turbo compressor, the impeller is followed downstream by a return stage if at least one further impeller is provided downstream.

In the terminology of this invention, a multistage radial turbo machine means that several running wheels are arranged to be rotatable about the same axis of rotation. Here, an impeller is to be equated with a stage of the radial turbomachine. The multistage requires that, in the case of the compressor, the process fluid flowing radially out of the impellers must be guided back in the direction of the axis of rotation and can flow into the subsequent impeller of the downstream stage with an axial speed component. The flow guidance that enables this return of the process fluid is therefore called the "return stage". In the case of the expander, the component can be of identical design and the flow is only flowed through in the opposite direction.

In addition to the return of the process fluid in the direction of the axis of rotation and the deflection of the flow direction of the process fluid In the axial direction, guide vanes are also regularly provided in the return stages, which at least partially or completely neutralize a swirl imposed in the flow from the upstream impeller or even imprint a swirl in the opposite direction for entry into the next downstream stage.

The usual design of a return stage provides that this overall component is supported and aligned by means of a so-called intermediate floor by means of suitable supports, usually in a housing or some other support device. Furthermore, the return stage comprises a so-called vane bottom, which is attached to the intermediate bottom with the guide vanes already explained, forming a return channel. The process fluid flows through the return channel to the next impeller inlet. In this structure, the guide vanes have two functions. On the one hand, the guide vanes have the aerodynamic function of imposing a counter-swirl on the process fluid to the extent that at least the swirl from the upstream stage is largely compensated for, and on the other hand, the guide vanes have the mechanical task of fastening the vane bottom to the intermediate bottom in such a way that, despite the dynamic load, a a secure hold is guaranteed.

In the scriptures DE102014203251A1 , DE 34 303 07 A1 and EP 592 803 B1 each of the return stages of a multi-stage turbo compressor are shown. An aerodynamic consideration of feedback stages contains the US 2010/0272564 A1 and the WO2014072288A1 .

The conventional recycle stages of the prior art have various disadvantages which the invention seeks to avoid. The return stages, which are geometrically rather simple, are for the most part less aerodynamically adapted to the fluidic task, so that the complex three-dimensional flow situation is at least partially disregarded, in particular regarding the blade height differences remain unnoticed and accordingly disproportionately large flow losses occur, which reduce the efficiency. Other solutions, especially the return stage after WO2014072288A1 provide a completely three-dimensionally designed blading of the return stage, which is very difficult to implement in terms of production engineering and requires a complex individual design, so that in any case a better degree of efficiency results than with the simple geometry. In addition, there are major problems in the assembly of the return stage, since the blading, due to the three-dimensional design, is often not able to allow conventional fastening elements to extend between the blade base and the intermediate base through the guide vanes. At this point, expensive special solutions may have to be used so that such a concept ultimately has no chance on the market.

The invention has therefore set itself the task of combining the properties of simplified production, optimized aerodynamics and simple assembly.

In order to achieve the object, a recirculation stage or a radial turbo machine according to claims 1 and 8 is proposed according to the invention. The respective back-referenced subclaims contain advantageous developments of the invention.

Basically, the recirculation stage of a radial turbo machine is used to divert the process fluid from an upstream impeller from the radially outward flow direction to the radially inward direction and to feed it axially to the following downstream impeller. The terms axial, radial, tangential, circumferential direction and the like are here or in this document each related to the central axis around which the return stage extends in an annular manner. In the case of a radial turbo machine, this axis is also the axis of rotation of a rotor or the shaft with the running wheels.

The guide vane stage located in the return stage comprises guide vanes which segment the annular shape of the return stage into individual channels in the circumferential direction. In principle, these guide vanes can also have interruptions (split), but according to the invention are preferably designed to be uninterrupted along the first flow direction. The guide vanes have profiles which - if unwound accordingly - can also be represented in two dimensions. A two-dimensional representation is possible, for example, if the annular channel of the return stage is cut along a central surface extending in the circumferential direction. This cut surface of a single guide vane can be developed into a plane, for a two-dimensional representation. A profile center line of the stacked profiles of the guide vanes can be generated by means of centers of inscribed circles in the profile.

In this way, a profile center line running coordinate can be defined along the first flow direction along a mean height of the respective guide vane. The length of the guide vane along this coordinate is expediently normalized to a total length 1.

The height direction of the guide vane is defined in the present case as the direction which is oriented perpendicular to the flow direction - in particular to the first flow direction - and perpendicular to the circumferential direction.

The profile center line of the guide vane immediately adjacent to the outer boundary contour of the annular channel of the return stage is referred to here as the outer track of the guide vane and the profile center line of the profile cross-section of the guide vane located directly on the inner boundary contour is referred to as the inner track of the guide vane. In this context, the outer boundary contour of the return stage can also be referred to as the boundary contour on the cover disk side, because an impeller provided with a cover disk has this cover disk on the side of the outer boundary contour.

The hub-side flow contour of the impeller is located opposite on the inner boundary contour of the return stage, so that the inner boundary contour of the return stage can also be referred to as the hub-side boundary contour. Due to the complex geometry of the return stage, the inner limit contour cannot always be viewed as being located radially further inward than the outer limit contour for the same positions along a central flow line through the return stage, so that such alternative designations are useful for better understanding.

The circumferential position angle determines the respective position in the circumferential direction of the components referred to - here essentially reference points or lines of the guide vanes, e.g. Points on profile center lines of certain profile cross-sections. The positive direction of the circumferential position angle is selected here against the direction of rotation of the shaft or the rotor. The vertex of this angle coincides with the central axis. For the person skilled in the art, the recirculation stage is always associated with a fluidic task, so that it is fundamentally not expedient to detach the terminology of the recirculation stage from the direction of rotation of the turbo machine.

• in dem ersten Profilabschnitt (PS1): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\mathrm{und}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime \ne 0,$$
  
• in dem zweiten Profilabschnitt (PS2) : OTR (L) =θ_ITR (L) und $$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime =0,$$
  
• in dem dritten Profilabschnitt (PS3): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\mathrm{und}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime \ne 0.$$
  

According to the invention, the three profile sections of the guide vanes of the guide vane stage differ from one another due to the focus of their functions. The first and third profile sections are closely related to an arcuate deflection of the process fluid, the second profile section not having the arcuate deflection as a fluidic task. All three profile sections are related to either a deceleration or an acceleration of the process fluid, so that demanding superimposed aerodynamic processes also take place. In addition, the second profile section is particularly preferred to serve to lead through at least one fastening element for the intermediate base on the blade base. The invention supports these conditions special invoice. The invention advantageously homogenizes the flow over the height extension of the guide vanes in that the following applies to values of L in the profile sections:

• in the first profile section (PS1): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\mathrm{and}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\right)\prime \ne 0,$$
  
• in the second profile section (PS2): OTR (L) = θ _ITR (L) and $$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\right)\prime =0,$$
  
• in the third profile section (PS3): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\mathrm{and}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\right)\prime \ne 0.$$
  

wobei in dem dritten Profilabschnitt (PS3) gilt: $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)<0.$$

A further education is particularly useful in which the following applies in the first profile section: $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)>0,$$

where in the third profile section (PS3) applies: $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)<0.$$

$$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime =0\mathrm{für}\mathrm{genau}\mathrm{ein}\mathrm{L}∍\mathrm{PS}2.$$

An advantageous development of the invention provides that: $$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\right)\prime =0\mathrm{For}\mathrm{exactly}a\mathrm{L.}∍\mathrm{PS}1,$$

$$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L.}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L.}\right)\right)\prime =0\mathrm{For}\mathrm{exactly}a\mathrm{L.}∍\mathrm{PS}2.$$

The middle, second profile section advantageously extends from at most L = 0.4 to at least L = 0.6.

To fasten the intermediate base to the vane base, it makes sense if at least some of the guide vanes in the second profile section have a recess extending from a point on the inner track to a point on the outer track for the implementation of a fastening element between the inner boundary contour and the outer boundary contour. This recess is preferably closed towards the lateral blade profile surfaces. The recess particularly preferably has a straight central extension axis and can in particular be designed as a bore.

The efficiency of the return stage can be further optimized if the guide vanes each have a leading edge are each arranged in the second section, preferably in a region of the arcuate deflection of the second section between 0 ° -90 ° of a first deflection angle to the central axis.

In the case of the curved deflections in the return stage, the deflection angle is the angle difference of a projection of the respective flow direction, in particular the first flow direction, of the return stage in an axial-radial plane at the beginning to the exit of the deflecting section under consideration.

A further improvement in aerodynamics results from the fact that the guide vanes are each arranged with a trailing edge in the fourth section, preferably in an area of the arcuate deflection of the fourth section between 0 ° -60 ° second deflection angle to the axis.

A radial turbomachine according to the invention comprises a return stage of the type already described, the axis around which the return stage extends in the form of a ring being identical to the axis of rotation of a rotor or a shaft which carries running wheels. The return stage leads the flow along the first flow direction from an impeller to an impeller located downstream.

The invention particularly expediently enables the ratio of an intermediate diameter to an outlet diameter to be smaller than 1.5, in particular smaller than 1.4, the outlet diameter being the outlet diameter of the impeller located upstream of the return stage and the intermediate diameter being the diameter of the transition cross section of the return stage from the first section to the second section.

Figur 1

ein axialer Längsschnitt durch den Ausschnitt eines Gehäuses einer Radialturbomaschine mit einer Rückführstufe und Laufrädern,

Figur 2

zeigt eine Darstellung eines Querschnitts gemäß dem in Figur 1 ausgewiesenen Schnitt II-II,

Figur 3

zeigt eine dreidimensionale Widergabe der Leitschaufelstufe einer erfindungsgemäßen Rückführstufe zusammen mit einem Zwischenboden und

Figur 4

zeigt den Umfangspositionswinkeldifferenzverlauf zwischen der äußeren Spur und der inneren Spur der Profilmittellinie einzelner Leitschaufeln der Leitschaufelstufe der Rückführstufe aufgetragen über die auf 1 (dimensionslos) normierte Profillängenlaufkoordinate entlang der ersten Strömungsrichtung.

The invention is explained in more detail below using a special embodiment with reference to drawings. They show schematically:

Figure 1

an axial longitudinal section through the section of a housing of a radial turbomachine with a return stage and impellers,

Figure 2

FIG. 11 shows a representation of a cross section according to FIG Figure 1 designated section II-II,

Figure 3

shows a three-dimensional representation of the guide vane stage of a return stage according to the invention together with an intermediate floor and

Figure 4

shows the circumferential position angle difference profile between the outer track and the inner track of the profile center line of individual guide vanes of the guide vane stage of the return stage plotted over the profile length coordinate normalized to 1 (dimensionless) along the first flow direction.

Figure 1 shows a feedback stage RC of a radial turbo machine RTM, which is designed as a radial turbo compressor CO.

The components explained here by way of example for a radial turbo compressor CO can also be implemented according to the invention, identical in construction, as radial turbo expanders, with a process fluid PF flowing through these components in a radial turbo compressor CO in a first flow direction FD1 and in a radial turbo expander in an opposite, second flow direction FD2. The descriptions in this document always refer to the first flow direction FD1, unless otherwise stated.

Figure 1 shows parts of two successive stages through which flow occurs, a first stage ST1 and a second stage ST2 of a radial turbo machine RTM or radial turbo compressor CO shown in detail, with a return stage RTC between the two stages ST1, ST2 being shown completely schematically. The two stages ST1, ST2 are shown here with impellers arranged to be rotatable about the axis of rotation X, a first impeller IP1 and a second impeller IP2.

In the illustration in FIG. 1, a process fluid PF first flows through the first impeller IP1, flowing in axially and flowing out radially along a first flow direction FD1. An oppositely oriented second flow direction FD2 is also given as an example, as would be the case with a radial expander. Downstream of the first impeller IP1, the process fluid PF, flowing radially outward, reaches a radially outwardly directed first section SG1 and is decelerated there, passes downstream into an approximately 180 ° deflection of a second section SG2 and then into a radially inward one Return of a third section SG3 of the return stage RTC. Downstream of the third section SG3, the process fluid PF reaches the second impeller IP2 in a fourth section SG4, flowing radially inward, deflected in an axially flowing direction, in order to be accelerated radially outward there again.

The return stage RTC comprises a blade bottom RR, guide blades VNS and an intermediate bottom DGP. The intermediate floor DGP is supported by at least one support SUP in a support device - here in a housing CAS - and positioned there. The support SUP and the supporting section of the housing CAS are designed as a tongue and groove connection in a form-fitting manner.

In a manner not shown in more detail, the return stage has RTC or the blade bottom RR and the intermediate bottom DGP has a joint that runs in a common plane essentially along the X axis. This parting line is conveniently located in the same parting line as a parting line, not shown, of the CAS housing.

In principle, it is also conceivable that the rotor is designed to be divisible between two impellers or the impellers are designed to be axially displaceable in relation to one another for the purpose of assembly, so that the return stages RTC can be designed undivided and gradually assembled together with the impellers IP1, IP2 of the rotor before merging with a surrounding housing takes place. The housing CAS can in any case be designed to be divided horizontally or vertically.

The conventional design of the return stage RTC, which is in the Figure 1 is shown, provides that the blade base RR, the guide vanes VNS and the intermediate base DGP are attached to one another. In the present case, this is done using SCR screws, which are shown in simplified form by means of dash-dotted lines. So that the screws SCR on the one hand adequately fasten the blade bottom RR to the intermediate bottom DGP and must therefore have a minimum thickness, on the other hand a sufficiently large through-hole must be provided in the guide vanes VNS so that the profile of the guide vanes VNS must be sufficiently strong.

• einen ersten Profilabschnitt PS1,
• einen zweiten Profilabschnitt PS2,
• einen dritten Profilabschnitt PS3.

The guide vanes are divided into three successive profile sections PS along the first flow direction FD1:

• a first profile section PS1,
• a second profile section PS2,
• a third profile section PS3.

Figure 2 shows schematically a cross section through a radial turbine machine RTM according to the invention, as shown in FIG Figure 1 is shown with II-II. The one mounted on the SH shaft The first impeller IP1 is mounted so that it can rotate about the axis X along the direction of rotation ROT. The directions radially, horizontally and vertically are drawn as an example. The circumferential position angle 8 runs positively against the direction of rotation ROT. The first impeller IP1 has moving blades IPB of a moving blade stage, shown as an example. The trailing edge TEI is entered for a rotor blade IPB. The return stage RTC extends downstream of the first impeller IP1. The return stage RTC has a guide vane stage VST with guide vanes VNS, one of which is shown as an example. The guide vane VNS shown schematically is only shown with its leading edge LER. Overall shows the Figure 2 the relationship between the direction of rotation ROT of the shaft SH or the running wheels IP1, IP2 and the circumferential position angle θ.

Figure 3 shows three-dimensional parts of the return stage RTC, namely the guide vane stage VST with the guide vanes VNS and their three-dimensional design.

The Figure 4 shows the course of the difference between the circumferential position angle of the outer track to the inner track plotted over the profile center line coordinate L, which is specified normalized to an entire length l. A first alternative ALT1 provides that the difference is initially positive and then drops to 0 at approx. 0.3L and there is constant until it drops into negative at approx. 0.65LΔθ. A second alternative ALT2 provides that the circumferential position angle difference Δθ is initially positive in the area of the leading edge LER, then drops into the negative, has a local minimum there and rises again up to a difference of 0 at around 0.3L. There, Δθ remains constant up to about 0.65L and then rises to the positive, up to a local maximum, and then falls back to the negative. In both cases, in a first profile section PS1, the circumferential position angle difference (except for a point of intersection with the 0-axis) is not equal to 0, as is the case in the third profile section PS3. In the second profile section PS2 in the middle of the respective guide vane VNS results in a circumferential position angle difference of 0 constant.

Claims (9)

1. Return stage (RTC) for throughflow by means of a process fluid along a throughflow direction of a radial turbomachine (RTM), in particular radial turbocompressor return stage (RCC), wherein the return stage (RTC) extends about an axis (X) in a ring-shaped manner,
   wherein the return stage (RTC) is defined radially inwardly by an inner delimiting contour (IDC) and radially outwardly by an outer delimiting contour (ODC),
   wherein the return stage (RTC) extends radially outwardly in a first section (SG1) along a first throughflow direction (FD1), wherein the return stage (RTC) extends, in a manner describing an arcuate deflection, radially inwardly from radially outside along the first throughflow direction (FD1) in a second section (SG2),
   wherein the return stage (RTC) extends radially inwardly from radially outside along the first throughflow direction (FD1) in a third section (SG3),
   wherein the return stage (RTC) extends, in a manner describing an arcuate deflection, axially from radially inside along the first throughflow direction (FD1) in a fourth section (SG4), wherein at least one guide vane stage (VST) comprising guide vanes (VNS) extends at least along a part of the third section (SG3) and, in the circumferential direction, segments the return stage into flow channels,
   wherein in each case a profile midline (PML) of a profile cross section (PRC) of the guide vanes (VNS) of the guide vane stage (VST) defines an inner track (ITR) on the side of the inner delimiting contour (IDC) and an outer track (OTR) on the side of the outer delimiting contour (ODC),
   wherein the progressions of the inner track (ITR) and outer track (OTR) are able to be defined as: $$\mathrm{\theta }\left(\mathrm{L}\right)={\mathrm{F}}_{\mathrm{\theta }}\left(\mathrm{L}\right)$$

   

   $$\mathrm{R}\left(\mathrm{L}\right)={\mathrm{F}}_{\mathrm{R}}\left(\mathrm{L}\right),$$

   

   with

   θ: circumferential position angle in a direction of rotation of the radial turbomachine (RTM), with the vertex on an axis (X),

   L: profile midline path coordinate along the first throughflow direction (FD1) along a mid-height of the respective guide vane (VNS), normalized to a total length of 1,

   F_θ(L): functional relationship between circumferential position angle θ and position L on the profile midline,

   R: radius of the position of the inner track (ITR) or outer track (OTR),

   wherein the guide vanes (VNS) have three successive profile sections (PS) along the first throughflow direction (FD1):

   a first profile section (PS1),

   a second profile section (PS2),

   a third profile section (PS3),

   characterized
   in that, in each case for values of L in the profile sections, it holds that:

   in the first profile section (PS1): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\mathrm{and}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime \ne 0,$$

   

   in the second profile section (PS2): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)={\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\mathrm{and}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime =0,$$

   

   in the third profile section (PS3): $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)\ne {\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\mathrm{und}\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime \ne 0.$$

   
2. Return stage (RTC) according to Claim 1, wherein, in the first profile section (PS1), it holds that: $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)>0,$$

   

   wherein, in the third profile section (PS3), it holds that: $${\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)<0.$$

   
3. Return stage (RTC) according to Claim 1 or 2, wherein it holds that: $$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime =0\mathrm{for}\mathrm{exactly}\mathrm{one}\mathrm{L}\ni \mathrm{PS}1,$$

   

   $$\left({\mathrm{\theta }}_{\mathrm{OTR}}\left(\mathrm{L}\right)-{\mathrm{\theta }}_{\mathrm{ITR}}\left(\mathrm{L}\right)\right)\prime =0\mathrm{for}\mathrm{exactly}\mathrm{one}\mathrm{L}\ni \mathrm{PS}3.$$

   
4. Return stage (RTC) according to Claim 1, 2 or 3,
   wherein the second profile section (PS2) extends from at most L=0.4 to at least L=0.6.
5. Return stage (RTC) according to Claim 1, 2, 3 or 4, wherein at least some of the guide vanes (VNS) have a cutout in the second profile section (PS2), extending from a point of the inner track to a point of the outer track, for the leadthrough of a fastening element between the inner delimiting contour (IDC) and the outer delimiting contour (ODC).
6. Return stage (RTC) according to Claim 1, 2, 3, 4 or 5, wherein the guide vanes (VNS) are in each case arranged with an inlet edge (LER) in each case in the second section (SG2), preferably in a region of the arcuate deflection of the second section (SG2) between 0° and 90° of a first deflection angle (BA1) .
7. Return stage (RTC) according to Claim 1, 2, 3, 4, 5 or 6, wherein the guide vanes (VNS) are in each case arranged with an outlet edge (VTE) in each case in the fourth section (SG4), preferably in a region of the arcuate deflection of the fourth section (SG4) between 0° and 60° of a second deflection angle (BA2) .
8. Radial turbomachine (RTM), in particular radial turbocompressor, having at least one return stage (RTC) according to Claim 1, 2, 3, 4, 5, 6 or 7, wherein the radial turbomachine (RTM) has a rotor (ROT) which is mounted in a manner rotatable about the axis (X) and which comprises at least two impellers (IP1, IP2), wherein the return stage (RTC) guides the flow from one impeller (IP1, IP2) to a downstream impeller (IP1, IP2) along the first throughflow direction (FD1).
9. Radial turbomachine (RTM) according to Claim 8,
   wherein the impellers (IP1, IP2) have an outlet diameter (D2) upstream of the return stage (RTC), wherein the transition cross section of the return stage from the first section (SG1) to the second section (SG2) is arranged at an intermediate diameter (DRR), wherein it holds that:
   DRR/D2 < 1.5, in particular DRR/D2 < 1.4,
   with:

   D2: outlet diameter of impellers (IP1, IP2)

   DRR: intermediate diameter of transition cross section of the return stage from the first section (SG1) to the second section (SG2).

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