ANNULAR SPACE CONTOURING

Description

[0001] The present invention relates to a blade grid section, a blade grid and a system comprising the aforementioned blade grid.

[0002] Turbomachinery (such as gas and steam turbines) typically features a flow channel for the passage of a fluid. This flow channel, also known as an "annular space," is bounded radially inwards by the shaft of a rotor and radially outwards by a casing. Unless otherwise stated, the terms "radial," "axial," and "circumferential direction," as well as related terms, are to be understood in this document as referring to the rotational axis of the rotor.

[0003] In the annular space of a turbomachine, blade arrays are arranged (also commonly referred to as "blade ring"). These arrays comprise guide or rotor blades, arranged circumferentially at essentially regular intervals, and associated platforms, also called "deck plates," which typically have an upstream or front edge and a downstream or rear edge. These platform edges define the platform surface in the axial direction; in this document, the "platform surface" refers to the surface of the platform facing the annular space.

[0004] The edge of the platform that the (axial) main flow, which flows through the annular space of the turbomachine during operation, first passes is referred to in this document as the "upstream" platform edge or "front" platform edge; the opposite edge is accordingly referred to as the "downstream" platform edge or "back" platform edge. The terms "downstream" and "upstream," or "front" and "back," refer accordingly to the axial main flow direction and solely to the axial position, i.e., regardless of any possible displacement in the circumferential or radial direction. In particular, a point in this document is to be understood as lying "downstream of the front edges" (or as "downstream of any other point") if, compared to a direct connection of the front edges (with each other) at the platform surface (or...compared to the other point) is arranged axially in / with the main flow direction (i.e. following it); the same applies to the designation "upstream" (with the opposite direction).

[0005] The section of the platform surface bounded axially by the direct (i.e., circumferentially without axial deviations) connections of the leading and trailing edges of adjacent blades to the platform surface, and circumferentially by the pressure side of one blade and the suction side of the other, is referred to in this document as the "blade gap." The width of the blade gap in the circumferential direction, particularly at the leading edges, is called the "pitch spacing" (of the blade grid or a blade grid section or of the blades). It can be measured, in particular, as the circumferential distance between the leading edges of adjacent blades in the area of ​​the platform surface. The distance (measured solely) in the axial direction (the direction of the intended axial main flow) between the leading and trailing edges of the blades is called the (axial) "grid width."

[0006] The pressure side of one blade and the suction side of an adjacent blade each define a so-called blade channel in the circumferential direction. In the radial direction, this blade channel is bounded within the turbomachine by so-called sidewalls. These are formed, firstly, by the platforms, and secondly, by sections radially opposite these platforms: In the case of rotor blades, such an opposite sidewall is typically a radially outer section (especially of the casing, particularly in the case of blades without a shroud, or of a blade shroud); in the case of guide vanes, it is typically a radially inner section (especially of a rotor hub) or a radially outer section.

[0007] A fluid flow guided through a flow channel is regularly influenced by the surfaces of the side walls. Flow layers close to these surfaces are deflected more strongly due to their lower velocity than flow layers further away from the side walls. This creates a secondary flow superimposed on the axial main flow, leading in particular to eddies and pressure losses.

[0008] To reduce secondary currents, contours in the form of raised areas and / or depressions are often incorporated into the side walls.

[0009] Numerous such so-called "sidewall contours" are known from the prior art. For example, the patents or patent applications of the applicant EP 2 487 329 B1, EP 2 787 172 A2, and EP 2 696 029 B1 may be mentioned.

[0010] From EP 2 423 444 A2, a sidewall contour is known in which a protrusion is arranged on the pressure side of a first blade and a depression extends parallel to the suction side of an adjacent blade. The protrusion and the depression form a curved channel.

[0011] Sidewall contours with concave sections are known from documents US2017074101A1, US10287901B2 and EP3090126B1.

[0012] One object of an embodiment of the present invention is to provide a technique with which secondary currents in the annular space of a turbomachine can be advantageously further reduced or the technique can be improved.

[0013] The problem is solved by a blade grid section according to claim 1, a blade grid according to claim 9, and a system according to claim 10. Advantageous embodiments are disclosed in the dependent claims, the description, and the figures. Advantageous embodiments of the invention are the subject of the dependent claims.

[0014] According to one embodiment of the present invention, a blade grid section for a blade grid of a turbomachine is provided, wherein the blade grid section comprises a platform with a platform surface and an upstream platform edge, as well as at least two blades, in particular a first and a second blade, which, by means of their leading and trailing edges on the platform surface, define a blade intermediate strip with axial grid width. In one embodiment, the platform surface of the blade intermediate strip, in particular the platform surface, has a trough, in particular exactly one trough, with a minimum, in particular exactly one global minimum, with respect to a radial position.In particular, the trough is configured in one embodiment such that its bottom is a global minimum, and in another embodiment, the radial position of the trough decreases in the circumferential direction from the suction side of one (first) of the at least two blades to the pressure side of the adjacent other (second) of the at least two blades, relative to a reference surface, towards the bottom of the trough, and increases from there in the circumferential direction, up to a maximum of the radial position of the reference surface. In one embodiment, the bottom of the trough, or the global minimum, corresponds to the greatest distance of the platform surface or contour from a reference surface.

[0015] In one embodiment of the invention, the radial position of the trough decreases in the axial direction in the flow direction with respect to a reference surface towards the bottom of the trough and increases from there in the axial direction, at most up to (and including) the radial position of the reference surface.

[0016] In other words, the radial position of the platform surface, starting from the bottom of the trough, approaches the radial position of the (contoured) platform surface in the circumferential and / or axial direction, especially in any combination of these directions.

[0017] In one embodiment of the invention, the platform surface of the blade intermediate strip, in particular the platform surface, has a trough, in particular exactly one trough, with a maximum whose radial position corresponds at most to a radial position of an uncontoured reference platform surface.

[0018] In one embodiment of the invention, the radial position of the (contoured) platform surface of the blade intermediate strip, in particular the (entire) platform surface, reaches at most a radial position of the reference surface; in particular, the platform surface of the blade intermediate strip, in particular the platform surface, is not intersected by the reference surface.

[0019] In other words, the maximum of the trough and / or, in particular, the platform surface of the blade intermediate strip, especially the (entire) platform surface, in one embodiment corresponds at most to the radial position, in particular the radial level, of a theoretical (preferably imaginary) uncontoured platform surface of the blade intermediate strip and / or an uncontoured platform surface. In particular, it is not raised above this radial position or higher than the radial position of the uncontoured reference (platform) surface. A "reference platform surface" is preferably understood here to be a platform surface that has no contouring, is in particular circumferentially symmetrical, and furthermore, in particular, corresponds to the geometric dimensions of the (contoured) platform surface, apart from the contouring.It has correspondingly identical geometric dimensions, apart from the contouring, and is preferably referred to herein as the "reference surface".

[0020] In one embodiment, the blade grid section, in particular the platform surface and / or the blade intermediate strip of the blade grid section, has no elevation.

[0021] In this context, a "protrusion" preferably refers to a local feature (such as a bump or projection) on the platform surface, in which the platform surface extends radially in the same direction as the blades project from the platform, particularly in comparison to the surface of an uncontoured platform. Thus, a protrusion on a platform that bounds the annular space radially outwards extends radially inwards, while a protrusion on a platform that bounds the annular space radially inwards extends radially outwards.

[0022] In this context, a "trough" preferably refers to a local deformation of the platform surface in the opposite direction, particularly in the opposite direction, of a protrusion (such as a depression, indentation, or niche), especially in which the platform surface extends radially in the opposite direction to the direction in which the blades project from the platform. Thus, a trough on a platform that bounds the annular space radially outward extends radially outward, while a trough on a platform that bounds the annular space radially inward extends radially inward.

[0023] The terms "elevation" and "trough" (as well as terms like "height," "depth," or similar) are based here on an orientation or coordinate system in which the bucket blades and an elevation extend "upwards" from the platform surface. Accordingly, a trough extends in the opposite direction, "downwards." A description using only one direction is therefore not limited to that direction alone, but can also refer to, or does refer to, the other direction.

[0024] The highest and lowest points of a rise or trough are defined as the points where these extend furthest in the respective direction. The highest and lowest points of a rise or trough can each form a section of a surface, a curve, or be singular points.

[0025] A blade grid section can be either a single piece or a composite structure in one embodiment. In one embodiment, the platform can be a single piece or comprise or have two or more parts, each with one of the blades projecting from it, or the platform can be designed as a separate component that is or can be arranged between the blades. Accordingly, in one embodiment, a platform is configured to abut a blade on each side in the circumferential direction and, together with the blades (none, one, or both of which may be integrally formed on the platform), to form a blade grid section according to one of the embodiments disclosed in this document. In one embodiment, the platform can be configured to have its upstream platform edge (at least substantially) abut a further (separate) element (e.g.,to be used in the hub or housing or other blade grid) in the turbomachine.

[0026] In one embodiment, the upstream platform edge is designed to form a section of a gap wall through which cooling fluid is or can be introduced into the annular space of the turbomachine.

[0027] In one embodiment, the trough, in particular the exactly one trough, has at least one local minimum and / or at least one saddle point and furthermore has one, in particular exactly one, global minimum or bottom.

[0028] In one embodiment, the reference surface is designed as a conical surface, which is arranged coaxially with the platform. In this embodiment, the conical surface has a linear generatrix. The conical surface intersects the leading and trailing edges, or their respective extensions, in particular at each (radially lowest) hub position, by ± 5%, and in particular by ± 2%, of the chord length at 50% of the radial extent of the airfoil. In the case of a fillet, it intersects the hub-side fillet boundary at those positions that lie upstream of the leading edge or downstream of the trailing edge, in particular in the extension of the skeletal line, by ± 5%, and in particular by ± 2%, of the chord length at 50% of the radial extent of the airfoil. In a further embodiment, a maximum of the trough with respect to a radial position locally reaches at most a circumferentially symmetrical level, which is formed by the reference surface.In other words, a maximum of the trough is not elevated above the aforementioned conical surface area with respect to a radial position. "Reference surface" can preferably also be understood as the "zero level" or "zero reference level".

[0029] In one embodiment, the reference surface is designed and arranged coaxially with the platform surface, in particular coaxially with the axis of rotation of the turbomachine, such that it contains the radial hub positions of the leading edge and the trailing edge of the airfoil at most plus 5% and / or at most minus 5% of the chord length of the airfoil at 50% of its radial extent.In other words, in one embodiment the reference surface is designed such that it contains the radial hub position of the leading edge of the airfoil, in particular the airfoils, at most plus 5% or at most minus 5% of the chord length of the airfoil at 50% of the radial extent of the airfoil, and the radial hub position of the trailing edge of the airfoil, in particular the airfoils, at most plus 5% or at most minus 5% of the chord length of the airfoil at 50% of the radial extent of the airfoil, wherein the reference surface is (in particular) designed as a conical surface.In one embodiment, the reference surface is configured such that it includes the radial hub position of the leading edge of the airfoil, in particular the airfoils, plus at least 1%, in particular at least 2.5%, or minus at least 1%, in particular minus at least 2.5%, and / or plus at most 15% or minus at most 15% of the chord length of the airfoil at 50% of the radial extent of the airfoil, and the radial hub position of the trailing edge of the airfoil, in particular the airfoils, plus at least 1%, in particular at least 2.5%, or minus at least 1%, in particular at least 2.5%, and / or plus at most 15% and or minus at most 15% of the chord length of the airfoil at 50% of the radial extent of the airfoil, wherein the reference surface is (in particular) configured as a conical surface.In this way, a maximum of the trough and / or, in particular, a maximum level with respect to a radial position of the trough, especially of exactly one trough, of the platform surface contouring can be defined in such a way that a secondary flow is advantageously influenced.

[0030] The term "hub position" is preferably understood as follows: For a platform that bounds the annular space radially outwards, a hub position is preferably understood as the position furthest radially from the axis of rotation, in particular at the leading and / or trailing edge of the blade(s). For a platform that bounds the annular space radially inwards, a hub position is preferably understood as the position furthest from the axis of rotation, in particular at the leading and / or trailing edge of the blade(s). Preferably, the radial "hub position" can be understood as 0% of the radial height.

[0031] In one embodiment, the minimum of the trough is formed at an axial position of at least 40% and / or at most 60%, in each case with respect to the axial grid width, and in particular at an axial position of at least 45% and / or at most 55%. Advantageously, this allows the secondary flow to be influenced particularly favorably in one embodiment.

[0032] The minimum of the trough is designed to be spaced at a maximum of 10% of the circumferential distance from the suction side of an airfoil blade of the blade grid section. In other words, the circumferential distance of the lowest point of the trough from the suction side of an airfoil blade is at most 10%, preferably at most 9%, at most 8%, at most 7%, at most 6%, or at most 5% of the pitch of the blade grid section. In particular, the lowest point of the trough can lie on a boundary line between the suction side of the airfoil blade and the platform surface (so that the aforementioned circumferentially measured distance is zero in each case). This has a beneficial effect on the secondary flow.

[0033] In one embodiment, the trough, in particular the edge of the trough, begins upstream of the leading edge of the airfoil(s), in particular at an axial position that is spaced upstream of the leading edge of the airfoil by at least 1%, in particular at least 2.5% and / or at most 25%, in particular at most 10%, of the chord length of the airfoil at 50% of its radial extent.

[0034] As a result of the flow caused by the respective curvatures of the pressure side of the first and the suction side of the second blade, channel vortices can be reduced particularly effectively in a design with the aforementioned trough dimensions, with at least one minimum, in particular exactly one global minimum.

[0035] In one embodiment, the most upstream point of a trough boundary is arranged upstream of the leading edges of the blades, in particular at least 1% and / or at most 5% of the axial grid width in front of the leading edge of the blades, and may in particular have an axial position substantially in common with the leading edges of the blades, or in one embodiment is arranged at least 1% and / or at most 5% of the axial grid width downstream of the leading edges of the blades.

[0036] In one embodiment, in particular at least 90%, in particular at least 95% of the platform surface of the blade intermediate strip is recessed relative to the reference surface, in particular at least 90%, in particular at least 95%, of the platform surface of the blade intermediate strip is designed as a trough.

[0037] In one embodiment, the platform edge has a waviness, particularly in the circumferential direction. In another embodiment, the waviness can be configured such that a wave from a trough to a crest corresponds to at least one percent of the distance between the platform edge and the leading edge of an airfoil and / or at most 25% of this distance. In another embodiment, the waviness, particularly the radial difference between a trough and a crest, has at least 1% and / or at most 15% of the axial chord length of the airfoil at 50% of its radial extent. In another embodiment, the trough and crest are arranged in the same area spanned by the platform surface; in other words, the transition from trough to crest involves an axial position, but not a radial position, of the platform edge.In one embodiment, the waviness is formed in the radial direction, in particular the troughs and crests are formed circumferentially symmetrically along an axial position at the platform edge, especially the upstream platform edge. In other words, in this embodiment, an axial position of the platform edge is at least substantially constant, and a radial position of the platform edge is formed circumferentially symmetrically in a wave-like manner, in particular such that in one embodiment the crests do not (at least substantially) intersect a conical surface, in particular a conical surface as described herein; in other words, a radial position of the crests does not extend beyond the radial position of the conical surface, in particular corresponds at most to a radial position of the conical surface.In one embodiment, the waviness is formed in both the radial and axial positions of the platform edge; this corresponds in particular to a mixture, especially a combination, of the previously described embodiments of platform edge waviness. In another embodiment, the waviness, in particular the radial difference and / or the axial difference between a trough and a crest, exhibits at least 1% and / or at most 15% of the axial chord length of the airfoil at 50% of its radial extent. Advantageously, this allows the flow over the airfoils to be optimized in one embodiment, in particular by reducing vortices. Furthermore, noise emissions can be improved, in particular reduced, in another embodiment.

[0038] According to one embodiment, a blade channel extends through a blade grid section described herein, in particular a blade grid section according to one of the embodiments disclosed in this document, and is thus bounded by such a blade grid section and a side wall opposite its platform (facing the platform surface). In the circumferential direction, according to one embodiment, a blade channel described herein is bounded by the pressure side of one of the blades of the blade grid section and by the suction side of the (adjacent) other blade opposite it.

[0039] According to one embodiment, a blade channel for a turbomachine is provided, which is bounded by a blade grid section as described herein, as well as by a side wall opposite the platform of the blade grid section.

[0040] In one embodiment of the invention, a blade grid is provided. According to one embodiment, the blade grid comprises at least one blade grid section described herein, in particular according to one of the embodiments disclosed in this document.

[0041] In one embodiment of the invention, a system is provided. According to one embodiment, the system comprises at least one blade grid, in particular a blade grid with at least one blade grid section as described herein.

[0042] According to one embodiment, the platform surface of the at least one blade grid section of the system corresponds to at least a part of a radially inner platform wall of a guide vane cluster, in particular a guide vane cluster of the system.

[0043] According to one embodiment, the system comprises a turbine and / or a compressor; in particular, the system can be a turbomachine, especially a turbine and / or a compressor, and further, in particular, a low-pressure turbine of an aircraft engine.

[0044] According to one embodiment, a system designed as a turbomachine comprises one or more blade grids as described herein.

[0045] In one embodiment, the geometry of the platform surface can influence, and in particular improve, the static pressure field on the platform surface and on the blades, preferably in the edge region. Advantageously, in one embodiment, a reduction of secondary flow, especially of vortices in the blade channel, can be enabled or is achieved through the geometry of the platform surface. In this way, losses can be reduced and / or the flow into a potentially downstream blade array can be improved.

[0046] In one embodiment, the platform surface of the blade intermediate strip, in particular the platform surface, can have a maximum, in particular a zero-height area, i.e., a surface section that is, in particular completely, at zero level. In one embodiment, this section comprises at most 10% of the blade intermediate strip or includes at most 10% of the platform surface between the blades and is in particular arranged such that the distance of the maximum, in particular the zero-height area, from the pressure side of the blade is preferably at most 40%, at most 20%, at most 10%, at most 5%, or at most 2.5% of a pitch spacing of the blade cascade section.

[0047] In one embodiment, the blade grid section, the blade grid, the flow channel, or the platform is specifically part of a low-pressure turbine or is designed to be installed or used in a low-pressure turbine. In one embodiment, the blades can be either guide vanes or rotor blades. Furthermore, in one embodiment, the platform is designed to define a blade channel through the blade grid section, either radially inwards or radially outwards.

[0048] In one embodiment, any fillet present is not part of the annular contour, and in particular not part of the trough of the platform surface. In another embodiment, the hub position of any fillet present is arranged radially below the fillet, in particular at a radial position that is radially below or lower than the fillet, or wherein the hub position has at most a radial position that coincides with the lowest radial position of the fillet, in particular in an extension of the leading edge and / or the trailing edge of the airfoil. A "fillet" as used herein is preferably to be understood as the root radius of the airfoil. In case of doubt, a fillet in one embodiment is to be understood as part of the airfoil. Accordingly, in one embodiment, a hub position of the airfoil (comprising the fillet) is determined.

[0049] The embodiments of the invention described herein can be combined (according to the invention) wherever this is technically sensible and feasible.

[0050] Further advantageous embodiments of the present invention will become apparent from the dependent claims and the following description of preferred embodiments. The following is shown, in part schematically: Fig. 1 shows a blade cage section in top view according to an embodiment of the present invention; and Fig. 2 shows a blade cage section in a section in a frontal view according to an embodiment.

[0051] Fig. 1Figure 1 schematically shows a blade grid section in plan view (with radial viewing direction) in a developed representation. It comprises blades 20, 30, each having a pressure side and a suction side, as well as a platform 10 with a platform surface, an upstream platform edge 10a (relative to the intended main flow direction X), and a downstream platform edge 10b. The platform can be formed in one piece or, for example, in two parts (not shown); in particular, it can comprise two parts, each with one of the blades 20, 30 projecting from it.

[0052] The blades define an intermediate blade section as the surface section located circumferentially U between the pressure side of the first blade 20 and the suction side of the second blade 30, and bounded axially X on the upstream side by a connection of the leading edges 23, 33 of the blades 20, 30, and on the downstream side by a connection of the respective trailing edges 24, 34. These connections run on the platform surface purely circumferentially U (i.e., without deviation in the axial direction), and their spacing corresponds to the axial grid width g of the blade grid section. The platform surface has a contour in the intermediate blade section 11, which is realized by a trough 15, specifically exactly one.

[0053] A pitch distance T is defined as the distance between the leading edges 23, 33 on the platform surface, and a pitch t as the distance from the suction side of one blade to the pressure side of the other blade at an axial position. The distance t' of the minimum 14 from the suction side (particularly with respect to a pitch) corresponds in the illustration to at most 10% of the pitch t. The blade cascade section 1 can be part of a blade cascade 110, or a blade cascade 110 can comprise at least one blade cascade section 1 designed for a system 100.

[0054] Trough 15, as not proportionate and schematic in Figure 1 The axial position shown is at least 40% and at most 60% with respect to the axial grid width g, as exemplified by g / 2 for 50% of the axial grid width g.

[0055] 15' represents the beginning of a trough or the edge of the trough, which begins in front of the leading edge 23, 33 of the airfoils 20, 30 and, in the version shown here, extends beyond the axial position of the trailing edge of the airfoils 20, 30 in the direction of flow.

[0056] The waviness of the upstream platform edge 10a is not shown. This would correspond, for example, to a wavy line 10a.

[0057] In Figure 2A section of a blade grid 1 is schematically shown in a cross-section at the axial position of the (global) minimum of the trough 15 in a frontal view, i.e., from the main flow direction X. The blade grid section can be part of a blade grid 110 designed for a system 100. The circumferential direction U is also shown for orientation. The blades 20, 30 are also shown in cross-section at this axial position. A simplified representation of the connection between the blades and the platform has been omitted, and the blades 20, 30 and the platform 10 are shown as separate components in the cross-section. The minimum 15 is spaced t' from the suction side of the blade 30 by less than 10% of the pitch t.The trough 15 has no elevation compared to a conical surface 12 (shown here in a simplified manner), and is therefore arranged "below" the conical surface 12 with respect to a radial position. Furthermore, in . Fig. 2 It is indicated that the trough has a maximum that corresponds at most to the radial position of an uncontoured platform surface, in particular at most to the radial position of a conical lateral surface 12. This applies, for example, to the trough 15 as shown in Fig. 2 The side of the trough 15 shown on the left in the image is on the pressure side of the blade 20. Furthermore, the platform surface of the blade intermediate strip 11 has no elevation.

[0058] Although exemplary versions were explained in the preceding description, it should be noted that a large number of variations are possible.

[0059] Furthermore, it should be noted that the exemplary embodiments are merely examples and are not intended to restrict the scope of protection, applications, or structure in any way. Rather, the preceding description provides the skilled person with a guideline for implementing at least one exemplary embodiment, whereby various modifications, particularly with regard to the function and arrangement of the described components, can be made without departing from the scope of protection as defined in the claims. Reference symbol list

[0060] 1 Bucket grate section 10 Platform 10a Upstream platform edge 10 Downstream platform edge 11 Bucket intermediate strip 12 Cone shell surface as reference surface 14 (global) minimum 15 Trough 15' (other) design of a trough 20, 30 Blade 23, 33 Leading edges 24, 34 Trailing edges 100 System 110 Bucket grate gaxial grid width gaxial grid width Tpitch spacing tpitch t'distance from suction side Ucircumferential direction Xintended axial main flow direction

Claims

1. Blade row portion (1) for a blade row of a turbomachine, the blade row portion comprising a platform (10) with a platform surface and an upstream-flow-side platform edge (10a) and comprising at least two airfoils (20, 30) which are adjacent in the circumferential direction and which by means of their leading and trailing edges (23, 33, 24, 34) define, on the platform surface, a blade intermediate strip (11) with an axial row width (g), the platform surface of the blade intermediate strip (11) having a trough (15) with a bottom, the trough (15) being designed in such a way that the bottom of the trough (15) is a global minimum (14) of the trough (15) and a radial position of the trough (15) decreases, with respect to a reference surface, down to the bottom of the trough (15) in the circumferential direction (U) from a suction side of one of the at least two airfoils (20, 30) to the pressure side of the adjacent other of the at least two airfoils (20, 30) and increases from said bottom in the circumferential direction (U), at most up to a radial position of the reference surface, and a radial position of the trough (15) decreasing, with respect to the reference surface, down to the bottom of the trough (15) downstream in the axial direction and increasing from said bottom downstream in the axial direction, at most up to a radial position of the reference surface, and the platform surface of the blade intermediate strip (11) assuming at most a radial position of the reference surface, the reference surface corresponding to an uncontoured platform surface, characterized in that in the case of a pitch (t) of the blade intermediate strip in the circumferential direction, the global minimum is spaced apart from a suction side by at most 10% (t').

2. Blade row portion (1) according to the preceding claim, characterized in that the reference surface (12) is designed as a conical surface which is arranged coaxially with the platform, and a maximum of the platform surface of the blade intermediate strip (11), in particular of the trough (15), locally reaches at most a circumferentially symmetric level with respect to a radial position, which level is formed by the reference surface (12).

3. Blade row portion (1) according to either of the preceding claims, characterized in that the reference surface (12) is designed in such a way that it contains the radial hub positions of the leading edge (23, 33) and the trailing edge (24, 34) of the airfoil plus at most 5% or minus at most 5% of the chord length of the airfoil (20, 30) at 50% of its radial extent.

4. Blade row portion (1) according to any of the preceding claims, characterized in that the global minimum is formed at an axial position of at least 40% and / or at most 60%, in each case with reference to the axial row width (g).

5. Blade row portion (1) according to any of the preceding claims, characterized in that the trough (15) begins upstream of the leading edge (23, 33) of the airfoil (20, 30), in particular at an axial position that is spaced apart, upstream, from the leading edge (23, 33) of the airfoil (20, 30) by at most 5% of the chord length of the airfoil at 50% of its radial extent.

6. Blade row portion (1) according to any of the preceding claims, characterized in that the platform surface of the blade intermediate strip (11) has no elevation, in particular in that a radial position of the platform surface of the blade intermediate strip reaches at most a radial position of the reference surface.

7. Blade row portion (1) according to any of the preceding claims, characterized in that a total of at least 90%, in particular at least 95%, of the platform surface of the blade intermediate strip is recessed relative to the reference surface, in particular is designed as a trough.

8. Blade row portion (1) according to any of the preceding claims, characterized in that the platform edge (10a, 10b) has a wavy shape, in particular in the circumferential direction (U).

9. Blade row (110) for a turbomachine, comprising at least one blade row portion (1) according to any of the preceding claims.

10. System (100), the system (100) having a blade row (110) according to the preceding claim, characterized in that the platform surface of the at least one blade row portion (1) corresponds to at least a part of a radially inner platform wall of a guide vane cluster of the system (100).

11. System (100) according to the preceding claim, characterized in that the system (100) has a turbine and / or a compressor, in particular is a turbine and / or a compressor, in particular a low-pressure turbine of an aircraft engine, comprising at least one blade row (110).