A casing slot structure for cooling blade tips

Abstract

This invention relates to the field of gas turbine cooling technology, specifically to a casing slot structure for cooling blade tips, comprising: an inclined channel disposed on the turbine casing and communicating with the blade tip clearance inlet; the inlet of the inclined channel is located on the turbine casing near the upstream portion, and the outlet is located on the turbine casing near the downstream portion, forming an axial tilt angle; the inclined channel guides the cooling airflow into the blade tip clearance; a step is disposed at the outlet of the inclined channel, and the step is formed by the turbine casing near the upstream inner sidewall being higher than the turbine casing near the downstream inner sidewall; this application protects the cooling airflow in the casing slot structure from the entrainment effect of high-temperature mainstream combustion gas through the step, shielding the cooling airflow from the direct impact of high-temperature mainstream combustion gas on the cooling airflow; the inclined channel allows the cooling airflow to enter the blade tip region at a high axial velocity, reducing the mixing between the cooling airflow and the high-temperature mainstream combustion gas in the blade channel; enhancing the coverage effect of the cooling airflow on the blade tip wall, and improving the cooling efficiency of the turbine blade tip.

Description

Technical Field

[0001] This invention relates to the field of gas turbine cooling technology, and more specifically to a casing slot structure for cooling blade tips. Background Technology

[0002] Gas turbines are characterized by their compact structure, high power, rapid start-up, and environmental friendliness, enabling quick start-up and shutdown to meet the peak-shaving needs of the power grid. In recent years, to further improve the output power and thermal efficiency of gas turbines, the temperature parameters at the turbine inlet have gradually increased. The turbine inlet temperature of a certain H-class gas turbine has reached nearly 1500℃, exceeding the withstand limits of the first-stage stationary and moving blade materials. To prevent blade failure, on the one hand, it is necessary to develop high-temperature alloy materials that can withstand higher temperatures; on the other hand, it is necessary to design the blade structure. High-pressure cooling gas is introduced from the compressor outlet of the gas turbine, entering the blades, end walls, and casing through channels. The high-pressure cooling gas flows into the blade channel from the cooling slots and cooling holes on various turbine components, forming a cold gas film on the blade surface, isolating the blades from the high-temperature combustion gas and protecting the blade materials from the impact of the high-temperature combustion gas.

[0003] like Figure 2 As shown, the high-temperature mainstream combustion gas 19 from the combustion chamber outlet 1 enters the turbine's first-stage blade passage. To prevent the ablation of the wall materials of the turbine casing 4, the upstream end wall 10 of the stationary blade, and the upstream end wall 12 of the moving blade, a first casing cooling hole 2, a second casing cooling hole 3, a casing cooling slot 9 (located between the turbine stationary blade 7 and the turbine casing 4), and an upstream rim seal 11 of the moving blade are sequentially provided downstream of the combustion chamber outlet 1, and a high-pressure cooling airflow is introduced; a cold gas film is formed near the turbine casing 4 and the end wall surface to protect them from the effects of the high-temperature combustion gas. During the operation of the gas turbine, to avoid friction between the turbine moving blade 8 and the turbine casing 4, a radial clearance exists between the turbine moving blade 8 and the turbine casing 4, called the moving blade tip clearance, or blade tip clearance 6.

[0004] like Figure 1 and Figure 2As shown, the pressure side 15 and suction side 16 are located on both sides of the blade tip gap 6. Under the action of the pressure gradient between the pressure side 15 and suction side 16, the high-temperature mainstream combustion gas 19 in the blade passage forms a blade tip gap leakage flow 14 in the blade tip gap 6. To reduce the leakage of the blade tip gap leakage flow 14 and reduce the work loss of the blade, a groove 18 is provided at the blade tip. The structure of the groove 18 is similar to a labyrinth seal tooth to change the flow pattern of the blade tip gap leakage flow 14. Although a scraping vortex 13 and a pressure side angular vortex 17 are formed in the groove 18 to block the further acceleration of the blade tip gap leakage flow 14, the scraping vortex 13 and the pressure side angular vortex 17 rotate in opposite directions, forming a pair of opposing vortices. This causes the high-temperature combustion gas on the pressure side 15 to impact the blade tip wall, which is not conducive to the convective heat transfer of the blade tip wall. In the specific implementation of the cooling structure, an existing casing slot structure 24 is provided upstream of the leading edge of the moving blade tip. This existing casing slot structure 24 can utilize the cooling airflow from the high-pressure section of the compressor to cool the tip clearance 6. However, the existing casing slot structure 24 is perpendicular to the main flow channel, and the cooling airflow is easily drawn into the blade channel by the high-temperature main flow gas 19, preventing the formation of an effective cooling film near the moving blade tip wall and thus failing to provide cooling protection for the wall material. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is to overcome the defect that the existing casing slot structure cannot form an effective cooling film near the blade tip wall, and thus cannot provide cooling protection for the wall material.

[0006] To overcome the above-mentioned defects, the present invention provides a casing slot structure for cooling blade tips, comprising:

[0007] An inclined channel is provided on the turbine casing, and the inclined channel is connected to the inlet of the blade tip clearance; the inlet of the inclined channel is located on the turbine casing near the upstream part, and the outlet of the inclined channel is located on the turbine casing near the downstream part, thereby forming an axial tilt angle; the inclined channel is adapted to guide the cooling airflow into the blade tip clearance.

[0008] A step is provided at the exit of the inclined channel, and the step is formed by the inner sidewall of the turbine casing near the upstream end being higher than the inner sidewall of the turbine casing near the downstream end.

[0009] Optionally, the axial tilt angle of the inclined channel is set such that the axial velocity of the high-temperature mainstream combustion gas and the axial velocity of the cooling gas flow are equal.

[0010] Optionally, a pre-rotation air inlet is provided on the casing slot structure. The pre-rotation air inlet is inclined to form a circumferential tilt angle. The angle value of the circumferential tilt angle is suitable to make the cooling airflow have a circumferential velocity.

[0011] Optionally, the circumferential tilt angle of the pre-swirl inlet is such that the circumferential velocity of the cooling airflow is equal to the circumferential velocity of the turbine blade rotation.

[0012] Optionally, an axial gap is provided between the step and the blade tip on the turbine blade; a radial gap is provided between the downstream portion of the turbine casing and the blade tip near the leading edge on the turbine blade.

[0013] Optionally, the size of the radial clearance is the same as the height of the step.

[0014] Optionally, the height of the step is 1% of the height of the turbine blade.

[0015] Optionally, the axial clearance has the same size as the radial clearance.

[0016] Optionally, the axial tilt angle ranges from 20 to 40 degrees.

[0017] Optionally, the circumferential tilt angle ranges from 32 to 52 degrees.

[0018] The technical solution of the present invention has the following advantages compared with the prior art:

[0019] 1. The casing slot structure for cooling blade tips provided by the present invention includes: an inclined channel disposed on the turbine casing, the inclined channel communicating with the inlet of the blade tip gap; the inlet of the inclined channel is located on the turbine casing near the upstream portion, and the outlet of the inclined channel is located on the turbine casing near the downstream portion, thereby forming an axial tilt angle; the inclined channel is adapted to guide cooling airflow into the blade tip gap; a step disposed at the outlet of the inclined channel, and the step is formed by the inner sidewall of the turbine casing near the upstream portion being higher than the inner sidewall of the turbine casing near the downstream portion; the present application adopts the above technical solution, and the step protects the cooling airflow in the casing slot structure from the influence of high-temperature mainstream combustion gas entrainment by the step, effectively shielding the direct impact of high-temperature mainstream combustion gas on the cooling airflow; the inclined channel allows the cooling airflow to enter the blade tip region at a high axial velocity, thereby reducing the mixing phenomenon between the cooling airflow and the high-temperature mainstream combustion gas in the blade channel; enhancing the coverage effect of the cooling airflow on the blade tip wall, and improving the cooling efficiency of the turbine blade tip. Ultimately, this ensures that the cooling airflow completely adheres to and covers the surface of the turbine blade tip, forming a film cooling protection, enhancing the film coverage effect, reducing the impact of high-temperature mainstream combustion gas intrusion, and preventing the high-temperature mainstream combustion gas from burning the turbine blade tip; thereby extending the service life of the turbine blade and ensuring the safe and stable operation of the gas turbine.

[0020] 2. The axial tilt angle of the inclined channel described in this invention is suitable to make the axial velocity of the high-temperature mainstream gas and the axial velocity of the cooling airflow equal. This application adopts the above technical solution to completely avoid the mixing between the cooling airflow and the high-temperature mainstream gas in the blade channel and the entrainment effect of the high-temperature mainstream gas, so as to obtain the highest cooling efficiency and the best cooling effect.

[0021] 3. The present invention provides a pre-swirl inlet on the casing slot structure. The pre-swirl inlet is inclined to form a circumferential tilt angle. The angle of the circumferential tilt angle is suitable to give the cooling airflow a circumferential velocity. The present application adopts the above technical solution, and by providing a pre-swirl effect for the cooling airflow through the pre-swirl inlet with the circumferential tilt angle, it ensures that the cooling airflow has a circumferential velocity, thereby reducing the relative movement between the cooling airflow and the turbine blade tip. Ultimately, the cooling airflow completely adheres to and covers the turbine blade tip wall surface, reducing the impact of high-temperature mainstream combustion gas intrusion.

[0022] 4. The circumferential tilt angle of the pre-swirl inlet described in this invention is suitable to make the circumferential velocity of the cooling airflow equal to the circumferential velocity of the turbine blade rotation; this application adopts the above technical solution to ensure that the circumferential velocity of the cooling airflow is the same as the circumferential velocity of the turbine blade, which significantly improves the cooling effect of the turbine blade tip wall.

[0023] 5. An axial gap is provided between the step and the blade tip on the turbine blade in this invention; a radial gap is provided between the downstream portion of the turbine casing and the blade tip near the leading edge on the turbine blade; this application adopts the above technical solution to avoid friction between the turbine casing and the turbine blade by setting axial and radial gaps; and protects the turbine blade for safe operation.

[0024] 6. The radial clearance dimension of the present invention is the same as the height dimension of the step; the above technical solution adopted in this application is the result of calculating and comparing the cooling efficiency of the turbine blade tip wall under different step height conditions by using numerical simulation method, so as to obtain the highest cooling efficiency and the best cooling effect.

[0025] 7. The height dimension of the step described in this invention is 1% of the height dimension of the turbine blade; the above technical solution adopted in this application is the result of calculating and comparing the cooling efficiency of the turbine blade tip wall under different step height conditions using numerical simulation methods, so as to obtain the highest cooling efficiency and the best cooling effect.

[0026] 8. The axial clearance dimension of the present invention is the same as the radial clearance dimension; the above technical solution adopted in this application is the result of calculating and comparing the cooling efficiency of the turbine blade tip wall under different step height conditions using numerical simulation method, so as to obtain the highest cooling efficiency and the best cooling effect.

[0027] 9. The axial tilt angle of the present invention is in the range of 20-40 degrees. Although the smaller the axial tilt angle, the higher the cooling efficiency of the turbine blade tip wall is, the smaller the axial tilt angle will also cause the strength problem of the turbine casing. Therefore, the axial tilt angle is limited to maximize the cooling efficiency of the turbine blade tip wall and obtain the best cooling effect.

[0028] 10. The circumferential tilt angle of the present invention is in the range of 32-52 degrees; the present application adopts the above technical solution to ensure that the circumferential velocity of the cooling airflow is equal to the circumferential velocity of the turbine blade tip as much as possible, so as to obtain the highest cooling efficiency and the best cooling effect. Attached Figure Description

[0029] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the morphology of tip clearance leakage flow in the prior art;

[0031] Figure 2 This is a schematic diagram of the meridional cross-sectional structure of a turbine translating blade and a turbine blade in the prior art;

[0032] Figure 3 This is a schematic diagram of the meridional cross-sectional structure of the turbine blade and turbine rotor blade provided in the embodiments of the present invention.

[0033] Figure 4 for Figure 3 A magnified schematic diagram of the local structure at point C;

[0034] Figure 5 This is a three-dimensional structural diagram of the casing slot structure at the leading edge of the turbine blade provided in an embodiment of the present invention.

[0035] Figure 6 for Figure 5 A magnified schematic diagram of the local structure at point D;

[0036] Figure 7 This is a schematic diagram of the cooling airflow at the casing slot structure provided in the embodiment of the present invention;

[0037] Figure 8This is a schematic diagram comparing the flow direction of the cooling airflow provided in the embodiments of the present invention with that of the cooling airflow in the prior art in the Y-Z plane;

[0038] Figure 9 This is a schematic diagram comparing the flow direction of the cooling airflow provided in the embodiment of the present invention with that of the cooling airflow in the prior art in the X-Y plane.

[0039] Explanation of reference numerals in the attached figures:

[0040] 1. Combustion chamber outlet; 2. First casing cooling hole; 3. Second casing cooling hole; 4. Turbine casing; 5. Casing slot structure; 6. Blade tip clearance; 7. Turbine stationary blade; 8. Turbine moving blade; 9. Casing cooling slot; 10. Upstream end wall of stationary blade; 11. Upstream rim seal of moving blade; 12. Upstream end wall of moving blade; 13. Scraping vortex; 14. Blade tip clearance leakage flow; 15. Pressure side; 16. Suction side; 17. Pressure side angular vortex; 18. Groove; 19. High-temperature mainstream combustion gas; 20. Cooling airflow; 21. Step; 22. Pre-swirling inlet; 23. Step vortex; 24. Existing casing slot structure; 25. Axial clearance; 26. Axial tilt angle; 27. Radial clearance; 28. Circumferential tilt angle. Detailed Implementation

[0041] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0043] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0044] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0045] like Figures 3 to 9 One specific embodiment of the casing slot structure 5 for the cooling blade tip shown includes: an inclined channel disposed on the turbine casing 4 and a step 21 disposed at the outlet of the inclined channel.

[0046] like Figure 3 and Figure 4 As shown, the inclined channel communicates with the inlet of the blade tip clearance 6; the inlet of the inclined channel is located on the upstream portion of the turbine casing 4, and the outlet of the inclined channel is located on the downstream portion of the turbine casing 4, thus forming an axial tilt angle 26, which is the angle between the axis of the inclined channel and the axis of the turbine casing 4; the inclined channel is adapted to guide the cooling airflow 20 into the blade tip clearance 6. Figure 2As shown, the inner wall of the existing turbine casing 4 near the upstream end and the inner wall of the existing turbine casing 4 near the downstream end are equal (i.e., the radial radii of the stationary blade casing and the moving blade casing are equal). The cooling airflow 20 is unobstructed after leaving the existing casing slot structure 24, making it easy to mix with the high-temperature mainstream combustion gas 19, thus weakening the cooling effect of the cooling airflow 20. In the existing casing slot structure 24, the slots are perpendicular to the mainstream combustion gas passage, and the cooling airflow 20 has no axial velocity, making it easier to mix with the high-temperature mainstream combustion gas 19, weakening the cooling effect. The circumferential velocity of the cooling airflow 20 differs significantly from the circumferential velocity of the rotating turbine blade 8 tip, making it impossible to effectively cover the blade tip wall of the turbine blade 8. Introducing a step 21 into the casing slot structure 5 can effectively shield the direct impact of the high-temperature mainstream combustion gas 19 on the cooling airflow 20, reducing the mixing phenomenon between the cooling airflow 20 and the high-temperature mainstream combustion gas 19. The step 21 is formed by the inner wall of the turbine casing 4 near the upstream end being higher than the inner wall of the turbine casing 4 near the downstream end. Further, the axial tilt angle 26 of the inclined channel is suitable to make the axial velocity of the high-temperature mainstream combustion gas 19 equal to the axial velocity of the cooling airflow 20. An axial gap 25 is provided between the step 21 and the blade tip on the turbine blade 8; a radial gap 27 is provided between the downstream portion of the turbine casing 4 and the blade tip near the leading edge of the turbine blade 8. Specifically, the size of the radial gap 27 is the same as the height of the step 21. When the blade tip gap 6 is uniformly sized everywhere, the blade tip gap 6 and the radial gap 27 are the same size. The height of the step 21 is 1% of the blade height of the turbine blade 8. The size of the axial gap 25 is the same as the size of the radial gap 27. The axial tilt angle 26 ranges from 20 to 40 degrees. More specifically, the axial tilt angle 26 is 30 degrees.

[0047] like Figure 5 , Figure 6 , Figure 8 and Figure 9 As shown, the circumferential direction is defined as the X-axis, the radial direction as the Y-axis, and the axial direction as the Z-axis. A pre-rotating air inlet 22 is provided on the casing slot structure 5. The pre-rotating air inlet 22 is inclined, forming a circumferential tilt angle 28. The angle of the circumferential tilt angle 28 is suitable to give the cooling airflow 20 a circumferential velocity. The circumferential tilt angle 28 of the pre-rotating air inlet 22 is related to the rotational speed and radius of rotation of the turbine blade 8. Furthermore, the value of the circumferential tilt angle 28 of the pre-rotating air inlet 22 is suitable to make the circumferential velocity of the cooling airflow 20 equal to the circumferential velocity of the turbine blade 8. Specifically, the angle of the circumferential tilt angle 28 ranges from 32 to 52 degrees. More specifically, the angle of the circumferential tilt angle 28 is 42 degrees.

[0048] like Figure 7As shown, the casing slot structure 5 of the cooling blade tip of the present invention guides the high-temperature mainstream gas 19 to form a stepped vortex 23 in the axial gap 25 between the step 21 and the blade tip. The stepped vortex 23 blocks the flow tendency of the high-temperature mainstream gas 19 to the blade tip gap 6 of the turbine blade 8 and avoids the direct impact of the high-temperature mainstream gas 19 on the blade tip wall.

[0049] like Figure 8 and Figure 9 As shown, in the Y-Z plane, the flow direction of the cooling airflow 20 under the existing casing slot structure 24 is perpendicular to the high-temperature mainstream combustion gas 19, and there is no axial velocity; the cooling airflow 20 under the casing slot structure 5 of the present invention has velocity components in both the radial and axial directions. In the X-Y plane, the flow direction of the cooling airflow 20 under the existing casing slot structure 24 is perpendicular to the high-temperature mainstream combustion gas 19, and there is no circumferential velocity; the cooling airflow 20 under the casing slot structure 5 of the present invention has velocity components in both the radial and circumferential directions.

[0050] The main analysis process of the casing slot structure 5 for cooling blade tips described in this application is briefly described as follows: First, the axial tilt angle 26 of the casing slot structure 5 is changed. The smaller the tilt angle, the higher the cooling efficiency of the turbine blade tip wall. However, an excessively small tilt angle can also cause strength problems for the turbine casing 4. After comprehensive consideration, the tilt angle 26 is equal to 30° to maximize the cooling efficiency of the turbine blade tip wall. Second, numerical simulation is used to calculate and compare the cooling efficiency of the turbine blade tip wall under different step heights of the step 21. The numerical results show that when the height of the step 21 of the casing slot structure 5 is equal to the height of the blade tip clearance 6 of the turbine blade 8, both being equal to 1% of the height of the turbine blade 8, the cooling efficiency of the turbine blade tip wall is the highest. Finally, the circumferential tilt angle 28 of the casing slot structure 5 was determined. Numerical simulation results show that when the blowing ratio M of the cooling airflow 20 is 1.0 and the rotational speed n of the turbine blade is 3000 r / min, setting the circumferential tilt angle 28 to 42° can ensure that the circumferential velocity of the cooling airflow 20 is equal to the circumferential velocity of the turbine blade tip 8.

[0051] The principle of the casing slot structure 5 for cooling blade tips described in this application is briefly described as follows: Figure 2 As shown, the incident angle of the cooling airflow 20 under the existing casing slot structure 24 is perpendicular to the high-temperature mainstream combustion gas 19, which easily leads to mixing with the high-temperature mainstream combustion gas 19, reducing the cooling effect on the tip wall of the downstream turbine blade 8. Simultaneously, due to the lack of circumferential velocity in the cooling airflow 20, there is a circumferential velocity difference between it and the tip of the rotating turbine blade 8. The cooling airflow 20 is vortexed and drawn to the vicinity of the suction side 16 within the blade channel, failing to provide cooling protection for the tip wall of the downstream turbine blade 8. Figure 3, Figure 4 and Figure 7 As shown, the casing slot structure 5 of the present invention, through the action of the step 21, allows the high-temperature mainstream combustion gas 19 to form a stepped vortex 23 in the axial gap 25 between the step 21 and the tip of the turbine blade 8 after passing through the step 21, thus blocking the intrusion of the high-temperature mainstream combustion gas 19 into the tip gap 6. Numerical simulation results show that the cooling effect of the blade tip wall of the turbine blade 8 is optimal when the height of the step 21 is equal to the height of the tip gap 6 of the turbine blade 8, and both are equal to 1% of the blade height of the turbine blade 8. Figures 4 to 9 As shown, the casing slot structure 5 of the present invention reduces the mixing of the cooling airflow 20 with the high-temperature mainstream combustion gas 19 when the cooling airflow 20 passes through, and the cooling airflow 20 is closer to the tip wall of the turbine blade 8. Numerical simulation results show that the cooling effect of the turbine blade 8 tip wall is optimal when the axial tilt angle 26 of the casing slot structure 5 is 30° and the circumferential tilt angle 28 is 42°. The casing slot structure 5 of the present invention reduces the mixing and entrainment effect of the high-temperature mainstream combustion gas 19 on the cooling airflow 20 to a certain extent, ensuring the coverage effect of the cooling airflow 20 on the tip wall of the turbine blade 8, and improving the cooling efficiency of the turbine blade 8 tip wall.

[0052] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A casing slot structure for cooling blade tips, characterized in that, include: An inclined channel is provided on the turbine casing (4), and the inclined channel is connected to the inlet of the blade tip clearance (6); the inlet of the inclined channel is located on the turbine casing (4) near the upstream part, and the outlet of the inclined channel is located on the turbine casing (4) near the downstream part, thereby forming an axial tilt angle (26); the inclined channel is adapted to guide the cooling airflow (20) into the blade tip clearance (6); A step (21) is provided at the outlet of the inclined channel, and the step (21) is formed by the inner side wall of the turbine casing (4) near the upstream end being higher than the inner side wall of the turbine casing (4) near the downstream end. A pre-rotation air inlet (22) is provided on the casing slot structure. The pre-rotation air inlet (22) is inclined to form a circumferential tilt angle (28). The angle value of the circumferential tilt angle (28) is suitable to make the cooling airflow (20) have a circumferential velocity.

2. The casing slot structure for cooling blade tips according to claim 1, characterized in that, The axial tilt angle (26) of the inclined channel is such that the axial velocity of the high-temperature mainstream gas (19) and the axial velocity of the cooling gas flow (20) are equal.

3. The casing slot structure for cooling blade tips according to claim 1, characterized in that, The value of the circumferential tilt angle (28) of the pre-swirl inlet (22) is suitable to make the circumferential velocity of the cooling airflow (20) equal to the circumferential velocity of the turbine blade (8).

4. The casing slot structure for cooling blade tips according to any one of claims 1-3, characterized in that, An axial gap (25) is provided between the step (21) and the blade tip on the turbine blade (8); a radial gap (27) is provided between the downstream portion of the turbine casing (4) and the blade tip on the turbine blade (8) near the leading edge.

5. The casing slot structure for cooling blade tips according to claim 4, characterized in that, The size of the radial gap (27) is the same as the height of the step (21).

6. The casing slot structure for cooling blade tips according to claim 5, characterized in that, The height dimension of the step (21) is 1% of the blade height dimension of the turbine blade (8).

7. The casing slot structure for cooling blade tips according to claim 6, characterized in that, The axial clearance (25) has the same dimensions as the radial clearance (27).

8. The casing slot structure for cooling blade tips according to claim 1 or 2, characterized in that, The axial tilt angle (26) has a numerical range of 20-40 degrees.

9. The casing slot structure for cooling blade tips according to claim 1 or 3, characterized in that, The circumferential tilt angle (28) has a numerical range of 32-52 degrees.

Images