A squealer tip arrangements of a turbine blade

Abstract

A squealer tip (100) arrangement on the top of a turbine blade, comprising: a boundary rim (110) that extends along pressure side (PS) and suction side (SS) of the squealer tip (100) of the turbine blade, wherein the boundary rim (110) includes leading edge (LE) and trailing edge(TE), and forms a cavity (140) with a bottom surface; and at least one ridge (130) is arranged on the surface of the cavity (140) at an angle to the boundary rim (110), wherein each end of the at least one ridge (130) is disconnected from the boundary rim (110) and the at least one ridge (130) is oriented perpendicular or almost perpendicular to a camber line (160). An expander (400) configurated to expand a supercritical carbon dioxide flow, wherein it comprises at least turbine blade (410) provided with the squealer tip (100).

Description

TITLEA squealer tip arrangements of a turbine blade.DESCRIPTIONTECHNICAL FIELD

[0001] The subject-matter disclosed herein relates to a squealer tip of the top of a turbine blade comprising edge ridges disconnected from a boundary rim. A particularly advantageous field of application is the use of the innovative squealer tips of the turbine blades in an expander for an oxy-fuel combustion cycle.BACKGROUND ART

[0002] The squealer tips of a turbine blades, are used to reduce tip leakage flow and improve efficiency. It involves creating a cavity on the blade tip, which helps in controlling the flow between the stator and rotor.

[0003] The design and optimization of gas turbines is a crucial aspect of the energy industry. One issue that has gained significant attention in recent years is tip leakage flow in gas turbines. Tip clearances, which are provided between the turbine blade tip and the stationary casing, allow free rotation of the blade and accommodate mechanical and thermal expansions.

[0004] Turbine efficiency and performance can be improved by minimizing tip leakage losses in the blades. These losses are an unavoidable result of the flow passing through the narrow gap between the blade tip and shroud, mixing with the main flow and causing disruption in the flow pattern.

[0005] By reducing tip leakage losses, a turbine can operate more efficiently, resulting in lower fuel consumption, fewer emissions, and ultimately loweroperating costs. In addition, it can prolong the lifespan of the turbine by reducing wear and tear on the blades and other components. Therefore, minimizing tip leakage losses is essential for optimizing the turbine’s performance and ensuring its long-term sustainability.

[0006] Gas turbines use tip clearances between the turbine blade tip and the stationary casing to prevent rubbing and accommodate expansions. Unfortunately, these clearances create aerodynamic losses and leakage, which reduce turbine efficiency and work output. In high-performance turbines, the tip leakage flow is intense, significantly impacting turbine performance. As a result, developing new or enhanced designs that cool the blade tip and seal the leakage flow is crucial. Proper tip clearance control is vital to optimizing gas turbine performance and output.

[0007] However, this narrow space becomes instrumental in the leakage of hot gases when the pressure difference between the pressure side and the suction side of the flow builds up. This is undesirable as it reduces turbine efficiency and work output. According to some studies, tip leakage loss could account for one-third of the total aerodynamic loss in turbine rotors. Further, leakage flows bring in extra heat, which raises the blade tip metal temperature, thereby increasing the tip thermal load. Over the years, various tip design features have been proposed as solutions, such as tips with winglets, and honeycomb tip

[0008] The following patent documents related to squealer tips of turbine blades are known: US8672629, US2017328229, KR102155797, US9045988B2, US2022 / 090511 Al, EP2780551, and US2019 / 169999A1. However, they differ both technically and functionally from the present invention.

[0009] In particular, the limitations of honeycomb tips are that they aresymmetrical in any direction, which can limit their effectiveness in reducing tip leakage flow. Additionally, they have limited capability to regulate aerodynamic flow compared to other configurations and can be less resistant to mechanical and thermal damage.

[0010] Furthermore, the limitations of a basic squealer tips are that they can cause increased aerodynamic losses initially until they adapt to operating conditions. They require regular maintenance to prevent wear and maintain efficiency, and they may be less effective in dissipating heat, increasing the thermal load on the blade tip.

[0011] However, it is desirable to propose a innovative squealer tip of turbine blade to minimize the tip leakage flow and enhance overall turbine efficiency.SUMMARY

[0012] According to a first aspect, the subject-matter disclosed herein relates to a squealer tip arrangement on the top of a turbine blade, comprising: a boundary rim that extends along pressure side (PS) and suction side (SS) of the squealer tip of the turbine , wherein the boundary rim includes leading edge (LE) and trailing edge(TE), and forms a cavity with a bottom surface; and at least one ridge is arranged on the surface of the cavity at an angle to the boundary rim, wherein each end is disconnected from the boundary rim.

[0013] According to second aspect, the subject-matter disclosed herein relates to an expander configurated to expand a supercritical carbon dioxide flow, wherein it comprises at least turbine blade provided with a squealer tip.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A more complete appreciation of the disclosed embodiments of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:Fig. 1 shows a view of a first embodiment of an innovative squealer tip arrangement;Fig. 1A show a least one ridge of the innovative squealer tip arrangement of Fig.l disconnected into two or more segments.Fig. 2 shows a view of a second embodiment of the innovative squealer tip arrangement of Fig.l;Fig. 3 shows a form of a second embodiment of the innovative squealer tip arrangement of Fig.2;Fig. 3A shows one ridge equipped with a chute of the innovative squealer tip arrangement of Fig.2Fig. 4 shows a view of a third embodiment of the innovative squealer tip arrangement of Fig.l;Fig.4A shows a closed configuration of the one ridge of the innovative squealer tip arrangement of Fig.l;Fig. 5 shows a view of a fourth embodiment of the innovative squealer tip arrangement of Fig.l;Figs. 6-6A show a boundary rim with a tip opening of the innovative squealer tip arrangement of Fig.l;Fig. 6B shows the one ridge provided with a rectangular hole of the innovative squealer tip arrangement of Fig.l;Figs.7-10 show a view of a fifth embodiment of the innovative squealer tip arrangement of Fig.l;Figs. 11 show a view of a squealer tip with a winglet of the innovative squealertip arrangement of Fig.1;Fig. 11 A shows a sectional view of one of a plurality of cooling hole openings of the innovative squealer tip arrangement of Fig.1Fig.1 IB shows a sectional view of the countersink bore of the innovative squealer tip arrangement of Fig.1Fig.12 shows a non-symmetric pattern of the innovative squealer tip arrangement of Fig.1;Fig.13 show a view of a squealer tip with a boundary rim chamfered for pressure side (PS) and / or for suction side (SS);Figs. 13A, 13B, and 13C show a sectional view of the boundary rim of the Fig.13;Fig. 14 shows the behavior of the flow leakage in a baseline squealer tip arrangement of Fig.16, obtained by Computational Fluid Dynamics (CFD) prediction;Fig. 15 shows a view of a gap between a ridge and the surface of a cavity of a squealer tip arrangement;Fig. 16 shows a view of a baseline squealer tip arrangement (a prior art); Fig. 17 shows a sectional view of an expander, for an oxyfuel combustion cycle, according to an embodiment of the disclosure.DETAILED DESCRIPTION OF EMBODIMENTS

[0015] The subject matter disclosed herein relates to an innovative arrangement of a squealer tip on the top of a turbine blade contributes to directing the flow more effectively, reducing turbulence and aerodynamic losses.

[0016] This arrangement includes a boundary rim that extends along the pressure side (PS) and the suction side (SS) of the squealer tip of the turbine blade and includes each end of this ridge disconnected from the boundary rim.

[0017] This boundary rim (110) encompasses the leading edge (LE) and the trailing edge (TE), forming a cavity (140) with a bottom surface. Within this cavity (140), at least one ridge (130) is positioned on the surface at an angle to the boundary rim (110). This configuration enhances the turbine’s efficiency by reducing tip leakage flow. The boundary rim acts as a barrier that helps contain the airflow.

[0018] The disconnection of each end of the ridge from the boundary rim allows for greater flexibility in managing the flow, further improving the overall efficiency of the turbine.

[0019] Moreover, the subject-matter disclosed herein relates to a plurality of embodiments of this innovative arrangement of a squealer tip on the top of a turbine blade contribute to minimize tip leakage flow and enhance overall turbine efficiency. These embodiments may include variations in the geometry of the boundary rim, the number, the shape and the orientation of the ridges. Optimizing these parameters, it is possible to achieve significant improvements in both aerodynamic performance and thermal management, thereby extending the operational lifespan of the turbine and reducing maintenance costs

[0020] In the following description, reference is specifically made to supercritical carbon dioxide (sCO2) expander, as a possible exemplary embodiment of a power-generating turbomachine according to the present disclosure. Those skilled in the art of turbomachinery will nevertheless understand that novel features disclosed herein can be used with advantage also in other power-generating turbomachines, such as expanders using a fluid different than carbon dioxide, or using carbon dioxide in a non-supercritical state, or such as a gas turbine, or in a gas turbine engine, including a compressor section, a combustor, and a turbine section.

[0021] Reference now will be made in detail to embodiments of the disclosure, an example of which is illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. In the following description, similar reference numerals used for the illustration of figures of the embodiments to indicate elements performing the same or similar functions. Moreover, for clarity of illustration, some references may be not repeated in all figures.

[0022] In Figures are schematically show an innovative arrangement of a squealer tip arrangement. The innovative arrangement of a squealer tip is generally indicated with reference numeral 100 in Figure 1.

[0023] Considering Fig. 1 a squealer tip 100 arrangement on the top of a turbine blade, comprises:a boundary rim 110 that extends along pressure side (PS) and suction side (SS) of the squealer tip 100 of the turbine blade, wherein the boundary rim 110 includes leading edge (LE) and trailing edge(TE), and forms a cavity 140 with a bottom surface; andat least one ridge (130) is arranged on the surface of the cavity 140 at an angle to the boundary rim 110.

[0024] Advantageously each end of the at least one ridge 130 is disconnected from the boundary rim 110, and as shown in Fig. lAthe at least one ridge 130 is disconnected into two or more segments 130a, and the at least one ridge 130 is oriented perpendicular or almost perpendicular to a camber line 160.

[0025] Preferably, the at least one ridge 130 is oriented relative to the camber line 160 with an inclination angle a, such that 80° < a < 100°. This angularconfiguration ensures that the ridge is substantially perpendicular to the camber line, which contributes to improved aerodynamic efficiency and structural performance of the blade tip.

[0026] The advantageous orientation of the at least one ridge 130, perpendicular to the camber line 160, and its strategic location at a distance of approximately one leading-edge curvature diameter from the leading edge, were determined through high-fidelity Computational Fluid Dynamics (=CFD) simulations. These analyses revealed that such positioning allows the ridge to intercept and manipulate the vortex structures forming in the squealer tip cavity, particularly near the leading edge. This interaction results in a fluidic sealing effect that significantly reduces tip leakage flow. The CFD results demonstrated a measurable improvement in aerodynamic efficiency compared to conventional squealer tip configurations, validating the functional benefit of this unique geometric arrangement. These findings reveal a novel aerodynamic mechanism that enhances stage performance, by improving fluidic sealing of the system between pressure side and suction side. Preferably, as shown in Fig. 1A the at least one ridge 130 is equipped with at least one cooling hole 133 to allow internal cooling flow of the blades to pass through.

[0027] With non limiting reference to Figures the at least one ridge 130 is a part of additive deposit and / or of the insert assembled from blade root protruding out of the tip cavity 140.

[0028] In particular the turbine blade is manufactured using conventional machining techniques, ensuring structural integrity and dimensional precision of the main body. Preferably the at least one ridge (130), is fabricated by means of Direct Material Deposition (=DMD), an additive manufacturing process that allows for the localized build-up of material with high geometric accuracy. This hybrid approach enables the formation of complex features — such as a disconnected ridge protruding from the cavity surface — which would bedifficult or uneconomical to achieve through subtractive methods alone. The DMD process also allows for material customization, potentially using alloys with enhanced thermal or wear resistance at the tip region.

[0029] In addition, the at least one ridge 130 is angled relative to the surface of the cavity, forming an angle a, with 0<a<90°, and the at least one ridge 130 has a knife-edge cross section profile, and / or a rectangular profile, and / or a trapezoidal profile.

[0030] According to a second embodiment shown in Fig 2 the at least one ridge 130 disconnected from the boundary rim 110 follows a curved path along the surface the said cavity 140 with the concavity facing a leading edge (LE), wherein the leading edge (LE) is the front part that first comes into contact with the flow leakage 190 preferably with a distance within 10 -15 percent of the chord 170 wherein the chord 170 extends from the leading edge (LE) to a trailing edge (TE), cutting the bundle of flow leakage 190. Advantageously wherein the at least one ridge 130 follows a curved path along the surface of said cavity 140 with the intersection point of ridge (130) and chord to be within 15 percent of the chord length, starting from the leading edge.

[0031] According to the second embodiment, as shown in Figs. 3 and 3A, each end of the at least one ridge 130 that follows a curved path, is equipped with a chute 131, wherein each chute 131 is disconnected from the boundary rim 110. The chutes at the ends of the ridges that follow a curved path optimize the direction and speed of the airflow, contributing to better aerodynamic performance. Being disconnected from the boundary rim 110, they can help reduce flow losses

[0032] Fig. 4 shows a third embodiment of the squealer tip 100, wherein the at least one ridge 130 is positioned in the central portion of the cavity 140 in a closed configuration 132 as shown in Fig.4A, preferably circular, anddisconnected from the boundary rim 110. This configuration includes better airflow management, optimizing the direction and speed of the flow, and a reduction in flow losses due to the disconnection from the boundary rim 110, thus contributing to superior aerodynamic performance

[0033] According to fourth embodiment shown in Fig. 5, the at least one ridge 130 is disposed parallel to the pressure side (PS), defining an area below the at least one ridge 130 that is provided with a plurality of holes 133 on the cavity base 140 to form a fluidic film barrier allowing for the creation of a fluidic barrier that enhances aerodynamic efficiency by reducing flow losses and improving surface cooling, contributing to greater durability and performance of the turbine blade. Alternatively, to achieve the same purpose as the previous configuration but in the case where the boundary rim is closed, Fig. 6B shows the squealer tip where at least one ridge 130 is provided with a hole 124, preferably of rectangular shape, in the leading edge (LE) area.

[0034] Advantageously, as shown in Figs. 6 and 7-10, the at least one ridge 130 follows a straight path along the surface of said cavity 140, preferably oriented along a direction perpendicular to the camber line 160, which extends from the leading edge (LE) to a trailing edge (TE), or follows a curved path along the surface of said cavity 140, preferably oriented along a direction parallel to the chord 170, which extends from the leading edge (LE) to a trailing edge (TE).

[0035] In particular, Figs.7-10 show a fifth embodiment of the squealer tip 100 wherein the at least one ridge 130 follows a curved path and straight path along the surface of said cavity.

[0036] As explained above with non-limiting reference to Fig. 7-10 at least one pocket 145 is defined by the at least one ridge 130 of the squealer tip 100 and includes at least one trench 141, preferably with a rectangular shape andpreferably arranged parallel to the at least one ridge 130, as shown in Figs.7 and 7A. As shown in Fig. 8, the at least one pocket 145 includes at least one trench with polygonal shape 142. As shown in Fig. 9, the at least one pocket 145 comprises at least one cooling hole 143.

[0037] Moreover, each pocket 145 defined by the at least one ridge 130 is preferably greater than or equal to one fourth of the length of the chord 170, in particular the chord length of each pocket is one fourth or greater of the chord length.

[0038] Considering Fig. 11 the squealer tip 100 comprises a winglet 150 arranged on the pressure side (PS) of the squealer tip 100, wherein the winglet comprise a plurality of cooling holes opening 151, in particular the Fig.llA shows a sectional view of one of the plurality of cooling hole openings 151.

[0039] Furthermore, the winglet 150 is provided with a countersink bore 152 on the pressure side (PS) to avoid hole closure in case of a rub, in particular the Fig.1 IB shows a sectional view of the countersink bore 152.

[0040] Advantageously the at least one ridge 130 that follows a curved path, is distributed on the surface of the cavity 140 in a non-symmetric pattern, wherein a convex portion of the at least ridge 130 is oriented towards the pressure side (PS) to minimize the tip leakage, as shown in Fig.12.

[0041] As shown in Fig. 13, the internal surface of the boundary rim 110 is chamfered for pressure side (PS) and / or for suction side (SS), in particular, Figs. 13A, 13B, and 13C show a sectional view of the boundary rim 110. Specifically, in Fig. 13 A the boundary rim 110 is chamfered for pressure side (SS), in the Fig. 13B the boundary rim 110 is chamfered for suction side (PS), in Fig.l3C the boundary rim 110 is chamfered for pressure side (PS) and for suction side (SS), to facilitates a formation of large recirculation bubble inside the squealer tip pocket, hence more resistance to the leakage flow frompressure side to suction side.

[0042] Due to mechanical constraints in particular in the expander for an oxyfuel combustion cycle, when it is not possible to use shrouded blades with sealing teeth systems, the use of squealer tip with features to emulate sealing systems like ridge of Fig. 1 are useful.

[0043] Fig. 14 shows the behavior of the flow leakage in the squealer tip 100 of a baseline squealer tip 300 (prior art geometry, see Fig. 16) obtained with a Computational Fluid Dynamics (CFD) prediction, to simulate and predict the behavior of fluid flows. The dense areas (black) indicate regions of intense flow recirculation. Introduction of the at least one ridge 130 creates a partial barrier that influences the airflow. The flow lines illustrate how the air moves around the ridge and through the cavity of the tip 100. The curved ridge helps reduce flow leakage (see Fig. 2), thereby improving the aerodynamic efficiency of the stage.

[0044] Finally, in Fig. 15, an arrangement of a squealer tip 200 on the top of a turbine blade is depicted, comprising: a boundary rim 210 that extends along the pressure side (PS) and suction side (SS) of the squealer tip of the turbine blade. The boundary rim 210 includes the leading edge (LE) and trailing edge (TE), forming a cavity 240 with a bottom surface. At least one ridge 230 is arranged on the surface of the cavity 240 at an angle to the boundary rim 210, with a gap 234 between the ridge and the bottom surface of the cavity 240. Each end of the at least one ridge 230 is connected with the boundary rim 110. This arrangement enhances cooling efficiency.

[0045] Fig. 17 shows an expander 400, where the use of the novel squealer tip arrangement according to the present disclosure may be particularly beneficial. It comprises at least one turbine blade 410 provided with the squealer tip 100.

[0046] Advantageously, the expander 400 is configured to expand a supercritical carbon dioxide flow, specifically designed for a supercritical carbon dioxide thermodynamic cycle, for example, an expander 400 for an oxy-fuel combustion cycle, such as an Allam cycle, or the like.

[0047] In operation, pressurized process fluid, e.g., a mixture of oxygen and carbon dioxide (if the expander 400 is operating in an oxy-fuel cycle), is delivered to the combustor 401, mixed with fuel, and burned to produce hot, pressurized process fluid, i.e., hot and pressurized flue gas, which expands along the expansion flow path formed by the sequentially arranged annular rows of stationary vanes or blades, and annular rows of rotor blades, to generate mechanical power which is made available on the rotor shaft either at the aft end, at the forward end, or both at the aft end and forward end. The mechanical power can be converted into electric power by an electric generator drivingly coupled to the shaft of the rotor and / or used to drive a load, such as a compressor, a pump, or other rotating equipment. A gearbox can be provided along the shaft line, between the expander 400 and the load connected thereto, such that the load can rotate at a speed different from the expander’s rotary speed.

[0048] According to a preferred embodiment, the expander comprises at least four expansion stages, preferably at least six expansion stages, each expansion stage including an annular array of stationary blades and an annular array of rotor blades, the annular array of rotor blades being downstream of the annular array of stationary blades in a forward-to-aft direction; and wherein at least the first and the second expansion stages comprise at least one turbine blade with the squealer tip 100 arrangement. This is particularly beneficial in expanders operating at high pressure, for instance, where the highest available pressure in the thermodynamic cycle is at or above 50 barA, for instance, equal to or higher than 300 barA.

[0049] As will be clear to those skilled in the field of turbomachinery, the construction details of the expander can vary, without thereby departing from the scope of the present disclosure. The novel design of the squealer tip of turbine blade disclosed herein can be advantageously employed also in expanders of different configurations and can be beneficial whenever it may be useful to reduce the process fluid leakage from the pressure side to the suction side of the airfoils of the blades, whose stiffness would make the use of standard shrouded blades unsuitable.

[0050] Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions, and additions may be made to that which is specifically disclosed herein without departing from the scope of the invention as defined in the following claims.

Claims

CLAIMS1. A squealer tip (100) arrangement on the top of a turbine blade, comprising: a boundary rim (110) that extends along pressure side (PS) and suction side (SS) of the squealer tip (100) of the turbine blade, wherein the boundary rim (110) includes leading edge (LE) and trailing edge(TE), and forms a cavity (140) with a bottom surface; and at least one ridge (130) is arranged on the surface of the cavity (140) at an angle to the boundary rim (HO).wherein each end of the at least one ridge (130) is disconnected from the boundary rim (110);wherein the at least one ridge (130) is oriented perpendicular or almost perpendicular to a camber line (160).

2. The squealer tip (100) arrangement of claim 1, wherein the at least one ridge (130) is disconnected into two or more segments (130a).

3. The squealer tip (100) arrangement of claim 1, wherein the at least one ridge (130) follows a curved path along the surface of said cavity (140) with the concavity facing a leading edge (LE), with a distance within 10 -15 percent of the chord (170).

4. The squealer tip (100) of claim 1, wherein each end of the at least one ridge (130) that follows a curved path, is equipped with a chute (131), wherein each chute (131) is disconnected from the boundary rim (110).

5. The squealer tip (100) of claim 1, wherein the at least one ridge (130) is positioned in the central portion of the cavity (140) in a closed configuration (132), preferably circular, and disconnected from the boundary rim (110).

6. The squealer tip (100) of claim 1, wherein the at least one ridge (130)is disposed parallel to the pressure side (PS), defining an area below the at least one ridge (130) that is provided with a plurality of holes (133) on the cavity base (140) to form a fluidic film barrier.

7. The squealer tip (100) of claim 1, wherein the boundary rim (110) is provided with a tip opening (111) at the leading edge (LE) of the turbine blade tip.

8. The squealer tip (100) of claim 1, wherein the at least one ridge (130) is provided with a hole (134), preferably of rectangular shape, in the leading edge (LE) area.

9. The squealer tip (100) of claim 1, wherein the at least one ridge (130) is a part of additive deposit and / or of the insert assembled from blade root protruding out of the tip cavity (140).

10. The squealer tip (100) of claim 1, wherein the at least one ridge (130) follows a straight path along the surface of said cavity (140), preferably oriented along a direction perpendicular to the camber line (160), which extends from the leading edge (LE) to a trailing edge (TE).

11. The squealer tip (100) of claim 1, wherein the at least one ridge (130) follows a curved path along the surface of said cavity (140), preferably oriented along a direction parallel to the chord (170), which extends from the leading edge (LE) to a trailing edge (TE).

12. The squealer tip (100) of claims 10 and 11, wherein the at least one ridge (130) follows a curved path and / or straight path along the surface of said cavity.

13. The squealer tip (100) of claims 1, wherein the at least one ridge (130) is equipped with at least one cooling hole (133) to allow internal cooling flowof the blades to pass through.

14. The squealer tip (100) of claim 1, wherein the at least one ridge (130) is angled relative to the surface of the cavity, forming an angle a, with 0<a<90°.

15. The squealer tip (100) of claim 1, wherein the at least one ridge (130) has a knife-edge cross section profile, and / or a rectangular profile, and / or a trapezoidal profile.

16. The squealer tip (100) of claim 1, wherein at least one pocket (145) is defined by the at least one ridge (130) and includes at least one trench (141), preferably with a rectangular shape and preferably arranged parallel to the at least one ridge (130).

17. The squealer tip (100) of claim 1, wherein the at least one pocket (145) includes at least one pocket with polygonal shape (142).

18. The squealer tip (100) of claim 1, wherein the at least one pocket (145) comprises at least one cooling hole (143).

19. The squealer tip (100) of claim 1, comprising a winglet (150) arranged on the pressure side (PS) of said squealer tip (100), wherein the winglet comprise a plurality of cooling holes opening (151).

20. The squealer tip (100) of claim 19, wherein the winglet (150) is provided with a countersink bore (152) on the pressure side (PS) to avoid hole closure in case of a rub.

21. The squealer tip (100) of claims 1 and 3, wherein the at least one ridge (130) that follows a curved path, is distributed on the surface of said cavity (140) in a non-symmetric pattern, wherein a convex portion of the at least ridge (130) is oriented towards the pressure side to minimize the tip leakage.

22. The squealer tip (100) of claim 16, wherein each pocket (145) defined by the at least one ridge (130) is preferably greater than or equal to one fourth of the length of the chord (170),23. The squealer tip (100) of claim 1, wherein the internal surface of the boundary rim (110) is chamfered for pressure side (PS) and / or for suction side (SS).

24. The squealer tip (100) of claim 1, wherein the turbine blade is a part of an unshrouded turbine, to reduce the flow leakage (190).

25. A squealer tip 200 on the top of a turbine blade is depicted, comprising: a boundary rim 210 that extends along the pressure side (PS) and suction side (SS) of the squealer tip of the turbine blade, wherein the boundary rim (210) includes a leading edge (LE) and a trailing edge (TE), forming a cavity (240) with a bottom surface; and at least one ridge (230) arranged on the surface of the cavity (240) at an angle to the boundary rim (210), with a gap (234) between the at least one ridge (230) and the bottom surface of the cavity (240), and each end of the at least one ridge (230) is connected with the boundary rim (HO).

26. An expander (400) comprising at least turbine blade (410) provided with a squealer tip of any of claims 1 to 25.

27. The expander (400) of claim 26, wherein the expander (400) is configurated to expand a supercritical carbon dioxide flow.

28. The expander (400) of claim 26, comprising at least four expansion stages, preferably at least six expansion stages, each expansion stage including an annular array of stationary blades and an annular array of rotor blades, the annular array of rotor blades being downstream of the annular array of stationary blades in a forward-to-aft; and wherein at least the first and thesecond expansion stages comprise the at least turbine blade with the squealer tip arrangement.

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