Protective device for fiber bragg grating rotating wheel strain sensor of water-turbine generator set

Abstract

The utility model discloses a protective device for a fiber bragg grating rotating wheel strain sensor of a water-turbine generator set, which comprises a base and a waterproof cover, and a placing groove is arranged between the base and the waterproof cover for accommodating the strain sensor. The two ends of the waterproof cover are provided with optical cable holes and optical cable ends, and the adjacent optical cable holes are connected through optical cable protection tubes. The base comprises a fixed end and an inner boss, a sealing groove on the top surface of the fixed end is matched with a sealing ring, the connecting end is provided with an outer boss, the fixed end and the connecting end are connected through a fixed bolt and waterproof sealant, a sealing cavity filled with sealant is arranged in the waterproof cover, the threaded connection part of the optical cable protection tube is filled with sealant, and the waterproof cover is provided with a spiral drainage groove. The device has the advantages that the sealant is waterproof, the sensor is dry and the precision is improved; the convex surface end disperses impact force, the service life of the device is prolonged, and the protection effect of the device is improved; fixing bolts, sealants and counterbores are designed to enhance waterproofness; the spiral drainage grooves conform to water flow, and impact is reduced; the optical cable protection tube is connected in series to protect the sensor, the hose end is attached to the rotating wheel, and the tube fixing seat ensures that the optical cable is stable and well matched with the rotating wheel.

Description

Technical Field

[0001] This utility model relates to the technical field of fiber optic grating protection devices, and in particular to a protection device for a fiber optic grating runner strain sensor in a hydro-generator set. Background Technology

[0002] my country's hydropower industry is developing rapidly, and hydro-generators provide a stable and reliable power supply for people's lives and industry. However, as the capacity and size of hydro-generator units continue to increase, the relative strength of the units decreases. It is necessary to use strain sensors on key components of the unit, such as the runner, to monitor strain in real time to ensure the normal operation of the runner. However, the runner is located in a high-pressure water flow, so the strain sensors on the runner will be subjected to the impact of the high-pressure water flow. To improve the service life of the strain sensors, protection is required.

[0003] Chinese Patent Application No. 202420871391.3 discloses a strain gauge protection device, including a protective component and a strain gauge body. The protective component includes a mounting surface, a strain gauge body, a hook and loop fastener surface, a protective box, a mounting groove, a hook and loop fastener surface, a pin, a protective cover, and a sealing cover. The strain gauge body is adhered to the center of the mounting surface, and hook and loop fastener surfaces are adhered to both sides of the mounting surface near the strain gauge body. Although this design provides protection for the strain gauge body, it still has the following drawbacks:

[0004] The design seals and protects the strain gauge body by using a sealing cover. The sealing cover has a sealing strip on the outside. This sealing strip may deform in a continuous high-pressure water flow environment, resulting in poor waterproofing and consequently a decrease in the detection accuracy of the strain gauge. Summary of the Invention

[0005] The purpose of this invention is to overcome the defects and problems of poor waterproofing effect in the existing technology under continuous high-pressure water flow environment, and to provide a protective device for fiber optic grating runner strain sensor of hydro turbine generator set with better waterproofing effect in continuous high-pressure water flow environment.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by this utility model is: a protective device for a fiber optic grating strain sensor of a hydro-generator set, including a base and a waterproof cover mounted on the base. A placement groove for accommodating the strain sensor is provided between the waterproof cover and the base. The waterproof cover is in the shape of a raised semi-ellipsoid. A first optical cable hole and a second optical cable hole are respectively opened at both ends of the waterproof cover. The first optical cable hole and the second optical cable hole are respectively connected to the first optical cable end and the second optical cable end. The first optical cable end and the second optical cable end between adjacent waterproof covers are connected by an optical cable protection tube.

[0007] Preferably, the base includes an elliptical fixed end and an inner boss vertically fixed to the fixed end. The inner boss is cylindrical, the fixed end is in contact with the bottom surface of the waterproof cover, the inner boss is in contact with the inner side wall of the square waterproof cover, and the placement groove is located inside the inner boss.

[0008] Preferably, the top surface of the fixed end is provided with an annular sealing groove, and a sealing ring is provided in the sealing groove. The connecting end cap of the edge of the waterproof cover is installed on the sealing groove.

[0009] Preferably, the width of the sealing groove is greater than the width of the sealing ring.

[0010] Preferably, the edge of the connecting end is fixedly connected with an annular outer protrusion vertically downwards, and the inner sidewall of the outer protrusion fits against the outer sidewall of the fixed end.

[0011] Preferably, the fixed end has fixed holes equidistantly spaced along its edge, and the connecting end has a connecting hole corresponding to the fixed hole. The connecting hole is a countersunk hole, and a fixing bolt is connected between the connecting hole and the fixed hole. The fixing bolt is a countersunk bolt.

[0012] Preferably, a sealing cavity is provided in the sealing space below the waterproof cover, and the sealing cavity is filled with sealant.

[0013] Preferably, the optical cable protection tube is fitted with a tube fixing seat for fixing.

[0014] Preferably, the outer wall of the waterproof cover is provided with spiral drainage grooves, which are semi-circular arc-shaped.

[0015] This utility model provides a protective device for a fiber optic grating strain sensor of a hydro-generator set, which has the following beneficial effects.

[0016] 1. In the protective device for a fiber optic grating strain sensor of a hydro-generator set, the device includes a waterproof cover and a base. The base is an arc-shaped ring and includes a fixed end and an inner boss. The fixed end is welded to the turbine blade. The top of the fixed end is connected to the connecting end of the waterproof cover. The connecting end is connected to the convex end and the outer boss. A sealing ring is provided between the fixed end and the connecting end. The convex end has a sealing cavity filled with sealant. In application, the fixed end is first welded to the turbine, then the strain sensor is placed inside the fixed end. Then, the connecting end is fixed to the fixed end, and the outer boss on the connecting end covers the outside of the fixed end. At this time, the inner boss fits against the inside of the concave end, and the sealant covers the outside of the strain sensor. The advantages of this utility model also include:

[0017] (1): The sealant used is Sikadur-52, which is a dynamic waterproof epoxy structural adhesive. The sealant is filled into the sealing cavity, that is, the sealant covers the outside of the strain sensor, so it can form a waterproof effect against high pressure water flow, keep the area around the strain sensor dry, and improve the detection accuracy of the strain sensor.

[0018] (2): The connection between the outer boss and the base faces the rotating blade, and the connection end fits into the base, which also forms a waterproof effect; the outer boss, the fixed end, and the inner boss cooperate with each other to fix the position of the connection end, thereby preventing the waterproof cover from shifting under the impact of water flow, and ensuring the reliability of the strain sensor in the long-term working environment of strong water flow impact.

[0019] (3): The convex end is a semi-circular spherical shape. When it is impacted by external water flow, it can evenly disperse the impact force of the water flow, thus effectively reducing the impact of the water flow, making the entire protective device structure more robust and reliable, improving the service life of the device, and also improving the protection of the strain sensor.

[0020] Therefore, this invention has a better waterproof effect in continuous high-pressure water flow environment and can improve the measurement effect of strain sensor.

[0021] 2. In the protective device for a fiber optic grating strain sensor on a hydro-generator set, the fixed end has multiple countersunk holes, and the connecting end has multiple connecting holes. A countersunk bolt is inserted into each of the connecting holes and the fixed holes. In application, the connecting holes are first aligned with the fixed holes, then waterproof adhesive is filled into the connecting holes and the fixed holes, and finally, the fixing bolts are screwed into the connecting holes and the fixed holes. The connecting end is then fixed to the fixed end by the fixing bolts. Because the fixing holes are countersunk, they have minimal impact on water flow. Furthermore, the sealing ring is located inside the fixing bolt, between the fixed end and the connecting end, preventing water from entering through the connecting holes. The top of the fixing bolt is at the same height as the top of the connecting hole, further reducing the impact on water flow. Therefore, this invention has good waterproofing performance.

[0022] 3. In the protective device for the fiber optic grating strain sensor of a hydro-generator set, multiple spiral guide grooves are evenly distributed on the convex end. These spiral guide grooves are semi-circular arc-shaped. During application, the spiral guide grooves align with the direction of water flow impact. When water impacts the convex end, the spiral guide grooves smoothly guide the water flow, facilitating the rotor's rotation in accordance with the water flow and reducing hydraulic losses caused by water impact. Therefore, this device will not significantly affect the water flow, meaning it will not affect the rotor's operation. Thus, this invention works well with the rotor.

[0023] 4. In the protective device for a fiber optic grating strain sensor on a hydro-generator set, the convex end is provided with a first optical cable hole and a second optical cable hole. A first connecting end is inserted into the first optical cable hole and connected to an optical cable protection tube. A second connecting end is inserted into the second optical cable hole and connected to the optical cable protection tube. Waterproof colloid is provided at both connection points. The middle part of the optical cable protection tube is a flexible tube end, and a tube fixing seat is provided on the flexible tube end. In application, the optical cable is inserted into the optical cable protection tube to protect the optical cable. The two ends of the optical cable protection tube are respectively connected to the first connecting end and the second connecting end on different convex ends, thus forming a series connection for multiple strain sensors, realizing the series protection of distributed sensors, reducing the difficulty of wiring distribution on the turbine. Waterproof colloid is provided at the connection points to prevent water impact and intrusion. The flexible tube end can conform to the curved surface of the turbine and can also meet the connection of strain sensors in different directions. The tube fixing seat fixes the flexible tube end to the turbine to ensure the stability of the optical cable's working environment. Therefore, this utility model is beneficial to the stable operation of the optical cable. Attached Figure Description

[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0025] Figure 1 This is a schematic diagram of the structure of this utility model.

[0026] Figure 2 yes Figure 1 A schematic diagram of the convex end.

[0027] Figure 3 yes Figure 1 Schematic diagram of the structure of the inner and outer bosses.

[0028] Figure 4 yes Figure 3 Cross-sectional view.

[0029] Figure 5 yes Figure 4 A schematic diagram of the structure of the fixing bolt.

[0030] Figure 6 yes Figure 5 A schematic diagram of the connecting hole in the middle.

[0031] Figure 7 yes Figure 1 A schematic diagram of the structure of the central base.

[0032] Figure 8 yes Figure 7 A schematic diagram of the inner boss.

[0033] Figure 9 yes Figure 8 Cross-sectional view.

[0034] Figure 10 yes Figure 1 A schematic diagram of the structure of the waterproof cover.

[0035] Figure 11 yes Figure 10 A schematic diagram of the middle connection end.

[0036] Figure 12 This is a schematic diagram of the structure of Example 3.

[0037] Figure 13 yes Figure 12 Top view.

[0038] Figure 14 This is a structural schematic diagram of Example 4.

[0039] Figure 15 yes Figure 14 The left view.

[0040] Figure 16 yes Figure 14 A schematic diagram of the structure of the optical fiber cable protection pipe.

[0041] In the diagram: 1. Waterproof cap; 11. Connecting end; 111. Connecting hole; 12. Convex end; 121. Spiral drainage groove; 122. Placement groove; 13. Outer boss; 132. Sealing ring; 14. Sealing cavity; 15. Sealant; 2. Base; 21. Fixing end; 212. Sealing groove; 211. Fixing hole; 22. Inner boss; 3. Fixing bolt; 4. First optical cable hole; 41. First optical cable end; 42. Internal thread end; 5. Second optical cable hole; 51. Second optical cable end; 52. External thread end; 6. Optical cable protection tube; 61. Flexible tube end; 62. Tube fixing seat; Detailed Implementation

[0042] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] Please see Figure 1 — Figure 16 A protective device for a fiber optic grating strain sensor on a hydro-generator set includes a base 2 and a waterproof cover 1 mounted on the base 2. A placement groove 122 for accommodating the strain sensor is provided between the waterproof cover 1 and the base 2. The waterproof cover 1 is in the shape of a raised semi-ellipsoid. A first optical cable hole 4 and a second optical cable hole 5 are respectively opened at both ends of the waterproof cover 1. The first optical cable hole 4 and the second optical cable hole 5 are respectively connected to a first optical cable end 41 and a second optical cable end 52. The first optical cable end 41 and the second optical cable end 52 between adjacent waterproof covers 1 are connected by an optical cable protection tube 6.

[0044] The base 2 includes an elliptical fixed end 21 and an inner boss 22 vertically fixed to the fixed end 21. The inner boss 22 is cylindrical. The fixed end 21 is in contact with the bottom surface of the waterproof cover 1. The inner boss 22 is in contact with the inner side wall of the square waterproof cover 1. The placement groove 122 is located inside the inner boss 22.

[0045] The top surface of the fixed end 21 is provided with an annular sealing groove 212, and a sealing ring 132 is provided in the sealing groove 212. The connecting end 11 of the edge of the waterproof cover 1 is covered on the sealing groove 132.

[0046] The width of the sealing groove 132 is greater than the width of the sealing ring 212.

[0047] The edge of the connecting end 11 is vertically fixedly connected to an annular outer protrusion 13, and the inner sidewall of the outer protrusion 13 is in contact with the outer sidewall of the fixed end 21.

[0048] The fixed end 21 has fixed holes 211 at equal intervals along its edge. The fixed holes 211 are blind holes. The connecting end 11 has a connecting hole 111 corresponding to the fixed holes 211. The connecting hole 111 is a countersunk hole. A fixing bolt 3 is connected between the connecting hole 111 and the fixed hole 211. The fixing bolt 3 is a countersunk bolt.

[0049] A sealing cavity 14 is provided in the sealing space below the waterproof cover 1, and the sealing cavity 14 is filled with sealant 15.

[0050] The optical cable protective sleeve 6 is fitted with a tube fixing seat 62 for fixing.

[0051] The outer side wall of the waterproof cover 1 is uniformly provided with spiral drainage grooves 121, which are semi-circular arcs.

[0052] The supplementary description of this utility model is as follows:

[0053] The strain sensor described in this utility model refers to a fiber optic grating wheel strain sensor, which is fixed on the outer surface of the wheel. When strain occurs on the outer surface of the wheel, the strain causes strain to occur in the fiber optic grating wheel strain sensor, and the reflected light signal in the fiber optic grating wheel strain sensor produces a center wavelength shift. The strain of the fiber optic grating wheel strain sensor is known through this center wavelength shift, thereby indicating that strain has occurred on the outer surface of the wheel.

[0054] Example 1:

[0055] Please see Figure 1 — Figure 16 A protective device for a fiber Bragg grating strain sensor on a hydro-generator set is disclosed. The device includes a waterproof cover 1 and a base 2. The base 2 is elliptical and includes a fixed end 21 and an inner boss 22. The bottom of the fixed end 21 is welded to one end of the runner. The inner side of the fixed end 21 is perpendicularly connected to the bottom of the inner boss 22. The top of the inner boss 22 extends away from the fixed end 21. The top of the fixed end 21 is connected to the bottom of the connecting end 11 of the waterproof cover 1. The inner side of the connecting end 11 is perpendicularly connected to the outer side of the convex end 12. The convex end 12 is semi-circular and spherical. The inner side of the convex end 12 is connected to one side of the inner boss 22. A sealing cavity 14 is formed between the convex end 12 and the part of the runner located inside the fixed end 21. The sealing cavity 14 is filled with sealant 15. The end of the connecting end 11 away from the convex end 12 is perpendicularly connected to one end of the outer boss 13, and one side of the outer boss 13 is connected to the outer side of the fixed end 21. A sealing groove 212 is provided on the end of the fixed end 21 near the inner boss 22. The sealing groove 212 is a rectangular groove, and a sealing ring 132 is inserted into the sealing groove 212. The width of the sealing groove 212 is greater than the width of the sealing ring 132, and the upper surface of the sealing ring 132 is connected to the lower surface of the connecting end 11. Preferably, the height of the inner boss 22 is two millimeters. The base 2 is made of stainless steel, the waterproof cover 1 is made of stainless steel, and the inner surface of the convex end 12 is coated with a waterproof coating.

[0056] In application, first weld one end of the fixed end 21 to the outer surface of the rotating wheel, then fix the strain sensor to the part of the outer surface of the rotating wheel located inside the fixed end 21. Then connect and fix the connecting end 11 to the fixed end 21. At this time, the inner boss 22 is located inside the convex end 12, and the outer boss 13 is located outside the fixed end 21. The sealant 15 covers the outside of the strain sensor to keep the strain sensor in a dry environment. When the rotating wheel rotates under the action of water flow, the outer boss 13 is located outside the fixed end 21, blocking the water flow impact and preventing water from rushing in. The gap between the outer boss 13 and the fixed end 21 faces the rotating wheel and not the water flow, so it also has a certain waterproof effect. 2. The sealant 15 is tightly attached to the convex end 12 to further prevent water from rushing in. The sealant 15 covers the outside of the strain sensor, further enhancing the waterproof effect. Therefore, the groove 122 can maintain a dry environment, which is beneficial to improving the accuracy of the strain sensor during measurement. When water impacts the outer surface of the convex end 12, since the convex end 12 is semi-ellipsoidal, the convex end 12 evenly disperses the impact force of the external water flow to reduce the impact force on the convex end 12. At the same time, the sealant 15 can also absorb vibration energy. The width of the sealing groove 212 is greater than the width of the sealing ring 132, thus providing compression space for the sealing ring 132. The sealing ring 132 is located between the fixed end 21 and the connecting end 11 to improve the waterproof effect.

[0057] Example 2:

[0058] The basic content is the same as in Example 1, except that:

[0059] Please see Figure 1 — Figure 11 The fixed end 21 has a plurality of evenly distributed fixing holes 211, which are countersunk holes. The connecting end 11 has connecting holes 111 corresponding to the fixing holes 211. Fixing bolts 3 are inserted into the connecting holes 111 and the fixing holes 211. The fixing bolts 3 are countersunk bolts, and the top of the fixing bolts 3 is at the same height as the top of the connecting holes 111. There are nine fixing holes 211, nine connecting holes 111, and nine fixing bolts 3. Preferably, the outer surface of the fixing bolts 3 is a coarse-pitch triangular thread, and the inner surface of the fixing holes 211 and the connecting holes 111 are also coarse-pitch triangular threads. Preferably, the sealing ring 132 is located inside the fixing bolts 3.

[0060] In application, first align the connecting hole 111 with the fixing hole 211, and then screw all the fixing bolts 3 into the connecting hole 111 and the fixing hole 211 in sequence to fix the waterproof cover 1 and the base 2. The top of the fixing bolt 3 is at the same height as the top of the connecting hole 111, so the fixing bolt 3 will not affect the water flow. The fixing bolt 3, the fixing hole 211, and the connecting hole 111 are all coarse-pitch triangular threads, which have better self-locking ability in vibration and impact environments, making the connection between the waterproof cover 1 and the base 2 stable, thereby making the working environment of the strain sensor stable. The inside of the convex end 12 is coated with waterproof paint to further prevent corrosion and moisture. The sealing ring 132 is located inside the fixing bolt 3 to block the water flow into the connecting hole 111 and the fixing hole 211.

[0061] Example 3:

[0062] The basic content is the same as in Example 1, except that:

[0063] Please see Figure 1 — Figure 13 Multiple spiral drainage grooves 121 are evenly distributed on the outer surface of the convex end 12. The opening direction of the spiral drainage grooves 121 faces the outside of the convex end 12, and the spiral drainage grooves 121 are semi-circular arc-shaped.

[0064] In application, when the water flow impacts the convex end 12, the spiral guide groove 121 guides the water flow along the spiral guide groove 121, thereby reducing the impact of the water flow on the convex end 12; the direction of the spiral guide groove 121 can be aligned with the impact direction of the water flow of the impeller to minimize the impact of the water flow.

[0065] Example 4:

[0066] The basic content is the same as in Example 1, except that:

[0067] Please see Figure 1 — Figure 16 One end of the convex end 12 is provided with a first optical cable hole 4, which is connected to one end of a first optical cable end 41. The end of the first optical cable end 41 away from the first optical cable hole 4 is an internally threaded end 42. The other end of the convex end 12 is provided with a second optical cable hole 5, which is connected to one end of a second optical cable end 51. The end of the second optical cable end 51 away from the second optical cable hole 5 is an externally threaded end 52. One end of an optical cable protection tube 6 is sleeved on the internally threaded end 42. The end of the optical cable protection tube 6 outside the internally threaded end 42 is filled with sealing glue. The end of the externally threaded end 52 is wrapped with sealing glue. One end of the optical cable protection tube 6 is inserted into the externally threaded end 52. The middle part of the optical cable protection tube 6 is a flexible tube end 61, and a tube fixing seat 62 is provided on the flexible tube end 61.

[0068] In application, the optical cable is placed inside the optical cable protection tube 6 for protection. Then, one end of the optical cable protection tube 6 is connected to the internal thread end 42 on a convex end 12, and the other end of the optical cable protection tube 6 is connected to the external thread end 52 on another convex end 12. The above steps are repeated to achieve series protection of the distributed strain sensor. A sealing colloid is provided at the connection between the optical cable protection tube 6 and the internal thread end 42 and the external thread end 52 to prevent water from entering the internal thread end 42 and the external thread end 52. The flexible hose end 61 is a stainless steel flexible hose that can fit with the curved surface of the wheel. The flexible hose end 61 can be fixed to the surface of the wheel by the tube fixing seat 62 to prevent the flexible hose end 61 from moving under the impact of water flow, thereby avoiding damage to the connection between the optical cable protection tube 6 and the internal thread end 42 and the external thread end 52.

Claims

1. A protection device for a fiber grating runner strain sensor of a hydroelectric generating unit, characterized by: Includes a base (2) and a waterproof cover (1) mounted on the base (2). A placement slot (122) for accommodating a strain sensor is provided between the waterproof cover (1) and the base (2). The waterproof cover (1) is a raised semi-ellipsoidal shape. Adjacent waterproof covers (1) are connected by optical cable protection tubes (6).

2. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 1, characterized in that: The base (2) includes an elliptical fixed end (21) and an inner boss (22) vertically fixed to the fixed end (21). The inner boss (22) is cylindrical. The fixed end (21) is in contact with the bottom surface of the waterproof cover (1). The inner boss (22) is in contact with the inner side wall of the waterproof cover (1). The placement groove (122) is located inside the inner boss (22).

3. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 2, characterized in that: The top surface of the fixed end (21) is provided with an annular sealing groove (212), and a sealing ring (132) is provided in the sealing groove (212). The connecting end (11) of the edge of the waterproof cover (1) is covered on the sealing groove (212).

4. The protection device of a fiber grating runner strain sensor of a hydroelectric generator set according to claim 1, 2 or 3, characterized in that: The waterproof cover (1) has a convex end (12) on the top, and a first optical cable hole (4) is provided at one end of the convex end (12). The first optical cable hole (4) is connected to one end of the first optical cable end (41), and the end of the first optical cable end (41) away from the first optical cable hole (4) is an internal thread end (42).

5. The protection device for the fiber grating runner strain sensor of a hydroelectric generator set according to claim 4, characterized in that: The other end of the convex end (12) is provided with a second optical cable hole (5), which is connected to one end of the second optical cable end (51). The end of the second optical cable end (51) away from the second optical cable hole (5) is an external thread end (52).

6. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 3, characterized in that: The edge of the connecting end (11) is vertically fixedly connected to an annular outer boss (13), and the inner sidewall of the outer boss (13) is in contact with the outer sidewall of the fixed end (21).

7. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 3, characterized in that: The fixed end (21) has fixed holes (211) at equal intervals on its edge. The connecting end (11) has a connecting hole (111) corresponding to the fixed hole (211). The connecting hole (111) is a countersunk hole. A fixing bolt (3) is connected between the connecting hole (111) and the fixed hole (211). The fixing bolt (3) is a countersunk bolt.

8. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 1, characterized in that: A sealing cavity (14) is provided in the sealing space below the waterproof cover (1), and the sealing cavity (14) is filled with sealant (15).

9. The protection device for the FBG strain sensor of the runner of a hydroelectric generator set according to claim 1, characterized in that: The optical cable protection tube (6) is fitted with a tube fixing seat (62) for fixing, and the tube fixing seat (62) is fixed on the wheel blade or the wheel component.

10. The protection device for the FBG runner strain sensor of a hydroelectric generator set according to claim 1, characterized in that: The outer wall of the waterproof cover (1) is uniformly provided with spiral drainage grooves (121), the opening direction of the spiral drainage grooves (121) is towards the outside of the convex end (12), and the spiral drainage grooves (121) are semi-circular arc-shaped.

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