Turbine capable of being directly backed up and provided with double-layer blade self-adaptive separation flow control structure

Abstract

The invention belongs to the technical field of ship power equipment, and relates to a direct reversing turbine with a double-layer blade self-adaptive separation flow control structure, which comprises an ahead-stage turbine, a reversing-stage turbine, a plurality of ahead-stage turbine blades and a plurality of reversing-stage turbine blades, and a suppression structure is arranged on each reversing-stage turbine blade. The restraining structure comprises a plurality of bent pipes, a plurality of bent pipe air inlet holes, a plurality of bent pipe air outlet holes and lines. The dimples are formed in the suction surface of the reversing-stage turbine blade in the radial direction and are close to the grains; the dimples are arranged in the radial direction. High-pressure airflow on the top end face of the reversing stage turbine blade can be guided, so that the high-pressure airflow enters from each bent pipe air inlet hole and flows out from the corresponding bent pipe air outlet hole, part of separated flow on the suction surface of the reversing stage turbine blade is taken away, and the breaking and transition processes of vortexes are promoted in cooperation with the grains and the multiple dimples; and separation flow of the suction surface of the reversing-stage turbine blade is inhibited.

Description

Technical Field

[0001] The invention belongs to the technical field of ship power equipment and relates to a directly reversible turbine with a double-layer blade adaptive separation flow control structure. Background Art

[0002] A gas turbine is an internal combustion power machine used in ships. It uses continuously flowing gas to drive the turbine blades to rotate at high speed, converting the chemical energy of the fuel into mechanical energy. In order to achieve the reversing of the ship, a double-duct direct reversing power turbine is usually designed based on a conventional power turbine. The inner duct forward-stage turbine is used for conventional forward operation, and the outer duct reverse-stage turbine is used for reverse operation. At the same time, the first-stage moving blades of the inner duct forward-stage turbine and the outer duct reverse-stage turbine are designed as upper and lower double-layer blades, that is, the lower layer is the forward-stage turbine blades, and the upper layer is the reverse-stage turbine blades. The forward-stage turbine blades and the reverse-stage turbine blades are connected by a connecting ring.

[0003] However, since the strength of the double-layer blades needs to be considered, the reverse-stage turbine blades are subject to weight restrictions and their number is upper bounded, which can easily cause the reverse-stage turbine to become a low-viscosity turbine, resulting in separated flow on the suction surface of the reverse-stage turbine blades. At the same time, the interaction between the separated flow and the gap flow also increases the loss of leakage flow on the top surface of the reverse-stage turbine blades, reducing the efficiency and power of the reverse-stage turbine. Summary of the invention

[0004] The object of the present invention is to provide a directly reversing turbine with a double-blade adaptive separation flow control structure, which can effectively suppress the separation flow on the suction surface of the reversing-stage turbine blades, reduce the leakage flow loss on the top surface of the reversing-stage turbine blades, and improve the efficiency and power of the reversing-stage turbine.

[0005] To achieve the above-mentioned purpose, the specific technical solution of a direct reverse turbine with a double-layer blade adaptive separation flow control structure provided by the present invention is as follows:

[0006] A turbine capable of direct reversing with a double-layer blade adaptive separation flow control structure comprises a forward-stage turbine and a reverse-stage turbine, wherein the first-stage moving blades of the forward-stage turbine and the reverse-stage turbine are upper and lower double-layer moving blades, wherein the lower-layer moving blades are a plurality of forward-stage turbine blades, and the upper-layer moving blades are a plurality of reverse-stage turbine blades, and wherein a suppression structure is arranged on each reverse-stage turbine blade, and the suppression structure comprises:

[0007] A plurality of curved pipes are arranged inside the reverse-stage turbine blade, one end of the plurality of curved pipes passes through the top end surface of the reverse-stage turbine blade and communicates with the outside of the reverse-stage turbine blade, so that the top end surface of the reverse-stage turbine blade forms a plurality of curved pipe air inlet holes along the axial direction, and the other end of the plurality of curved pipes passes through the suction surface of the reverse-stage turbine blade and communicates with the outside of the reverse-stage turbine blade, so that the suction surface of the reverse-stage turbine blade forms a plurality of curved pipe air outlet holes along its radial direction;

[0008] A groove is provided in a radial direction on the suction surface of the reverse stage turbine blade;

[0009] A plurality of dimples are radially disposed on the suction surface of the reverse stage turbine blade and are located close to the grooves;

[0010] The high-pressure airflow on the top surface of the reverse-stage turbine blade is guided through multiple curved pipes, so that the high-pressure airflow enters from the air inlet of each curved pipe and flows out from the air outlet of the corresponding curved pipe, taking away part of the separated flow on the suction surface of the reverse-stage turbine blade, and cooperating with the grooves and multiple dimples to promote the breakup and transition process of the vortex, thereby suppressing the separated flow on the suction surface of the reverse-stage turbine blade.

[0011] The present invention is also characterized in that:

[0012] The lengths of the multiple curved pipes corresponding to the multiple curved pipe outlet holes increase sequentially along the direction from the top end surface to the bottom end surface of the reverse stage turbine blade.

[0013] The positions of the multiple curved pipe air inlet holes are close to the pressure surface of the reverse stage turbine blade.

[0014] The radius of each curved pipe air inlet hole and the radius of each curved pipe air outlet hole are both 1% to 3% of the axial chord length of the reverse-stage turbine blade.

[0015] The distance between two adjacent air inlet holes of the bent pipe is 1.0 to 5.0 times the diameter of the air inlet holes of the bent pipe.

[0016] The axial spacing between two adjacent air outlet holes of the bent pipe is 1.0 to 3.0 times the diameter of the air outlet holes of the bent pipe, and the radial spacing between two adjacent air outlet holes of the bent pipe is 1.0 to 10.0 times the diameter of the air outlet holes of the bent pipe.

[0017] The diameter of each elbow gradually changes from the corresponding air inlet of the elbow to the corresponding air outlet of the elbow.

[0018] The distance between the curved pipe air inlet near the leading edge of the reverse-stage turbine blade and the leading edge of the reverse-stage turbine blade is 5% to 80% of the axial chord length of the reverse-stage turbine blade, and the distance between the curved pipe air inlet near the trailing edge of the reverse-stage turbine blade and the trailing edge of the reverse-stage turbine blade is 20% to 95% of the axial chord length of the reverse-stage turbine blade.

[0019] The suction surface of each reverse stage turbine blade is provided with grooves in the radial direction, and the suction surface of each reverse stage turbine blade is provided with a plurality of dimples at positions close to both sides of the grooves in the axial direction, and the plurality of dimples are arranged in the radial direction.

[0020] The depth of the groove is 1% to 5% of the thickness of the reverse stage turbine blade, and the groove is located at 20% to 60% of the axial direction of the reverse stage turbine blade.

[0021] The depth of each dimple is 1% to 5% of the thickness of the reverse-stage turbine blade, the span distance of each dimple is 1.0 to 5.0 times the depth of the dimple, and the radial spacing between two adjacent dimples is 1.0 to 3.0 times the span distance of each dimple.

[0022] The direct reversing turbine with a double-layer blade adaptive separation flow control structure of the present invention has the following advantages:

[0023] By arranging multiple curved pipes, multiple curved pipe air inlet holes, multiple curved pipe air outlet holes, and the coordination of grooves and multiple dimples on each reversing-stage turbine blade, the high-pressure airflow on the top surface of the reversing-stage turbine blade can be guided so that the high-pressure airflow enters from each curved pipe air inlet hole and flows out from the corresponding curved pipe air outlet hole, taking away part of the separated flow on the suction surface of the reversing-stage turbine blade, and cooperating with the grooves and multiple dimples to promote the breakup and transition process of the vortex, inhibit the separated flow on the suction surface of the reversing-stage turbine blade, and at the same time, it can also weaken the leakage flow on the top surface of the reversing-stage turbine blade, thereby controlling the separated flow and improving the efficiency and power of the reversing-stage turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the overall structure of the present invention;

[0025] Figure 2 It is a schematic diagram of the structure of the reverse stage turbine blade in the present invention;

[0026] Figure 3a It is a schematic diagram of a top view of the structure of a first distribution of multiple elbow air inlet holes in the present invention;

[0027] Figure 3b It is a schematic diagram of the top view of the second distribution of the plurality of elbow air inlet holes in the present invention;

[0028] Figure 3c It is a top view structural schematic diagram of the third distribution of multiple elbow air inlet holes in the present invention;

[0029] Figure 4 Schematic diagram of the top view of the groove structure in the present invention;

[0030] Figure 5 It is a schematic diagram of the main structure of the dimple in the present invention;

[0031] Figure 6 It is a schematic diagram of the pressure distribution of the reverse stage turbine blades in the present invention.

[0032] Reference numerals:

[0033] 1. Forward-stage turbine; 2. Reverse-stage turbine; 3. Forward-stage turbine blades; 4. Reverse-stage turbine blades; 5. Bend pipe; 6. Bend pipe air inlet; 7. Bend pipe air outlet; 8. Dimples; 9. Grooves. DETAILED DESCRIPTION

[0034] The technical solution in the present application will be described clearly and in detail below in conjunction with the accompanying drawings. Among them, in the description of the embodiments of the present application, unless otherwise specified, " / " means or, for example, A / B can mean A or B: "and / or" in the text is only a description of the association relationship of associated objects, indicating that there can be three relationships, such as A and / or B, which can mean: A exists alone, A and B exist at the same time, and B exists alone. In addition, in the description of the embodiments of the present application, "multiple" refers to two or more than two. The following terms "first" and "second" are used for descriptive purposes only and cannot be understood as implying or suggesting relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0035] like Figure 1 , Figure 2As shown, the present invention provides a directly reversible turbine with a double-layer blade adaptive separation flow control structure, including a forward-stage turbine 1 and a reverse-stage turbine 2, the first-stage moving blades of the forward-stage turbine 1 and the reverse-stage turbine 2 are upper and lower double-layer moving blades, the lower moving blades are multiple forward-stage turbine blades 3, and the upper moving blades are multiple reverse-stage turbine blades 4. The top end surfaces of the multiple forward-stage turbine blades 3 are connected to the bottom end surfaces of the multiple reverse-stage turbine blades 4 through a connecting ring, and the multiple reverse-stage turbine blades 4 are arranged along the circumference of the connecting ring. Each reverse-stage turbine blade 4 is provided with a suppression structure, and the suppression structure includes multiple curved pipes 5, grooves 9 and multiple dimples 8. The multiple curved pipes 5 are arranged inside the reverse-stage turbine blade 4, and one end of the multiple curved pipes 5 passes through the top end surface of the reverse-stage turbine blade 4 and is connected to the outside of the reverse-stage turbine blade 4, so that the top end surface of the reverse-stage turbine blade 4 forms multiple curved pipe air inlet holes 6 along the axial direction, and the other end of the multiple curved pipes 5 passes through the suction surface of the reverse-stage turbine blade 4 and is connected to the reverse-stage turbine The blades 4 are externally connected, so that the suction surface of the reversing-stage turbine blade 4 forms a plurality of curved pipe air outlet holes 7 along its radial direction, the grooves 9 are arranged on the suction surface of the reversing-stage turbine blade 4 along the radial direction, and the plurality of dimples 8 are arranged on the suction surface of the reversing-stage turbine blade 4 along the radial direction. Preferably, the plurality of dimples 8 are divided into two groups, the two groups of dimples 8 are arranged on both sides of the grooves 9, respectively, the two groups of dimples 8 are located on both sides of the grooves 9, and each group of dimples 8 has a plurality of dimples. The plurality of dimples 8 are arranged along the radial direction, and the high-pressure airflow on the top surface of the reversing-stage turbine blade 4 is guided by the plurality of curved pipes 5, so that the high-pressure airflow enters from each curved pipe air inlet hole 6 and flows out from the corresponding curved pipe air outlet hole 7, taking away part of the separated flow on the suction surface of the reversing-stage turbine blade 4, and cooperating with the grooves 9 and the plurality of dimples 8 to promote the process of vortex breakage and transition, inhibit the separated flow on the suction surface of the reversing-stage turbine blade 4, and at the same time, can also weaken the leakage flow on the top surface of the reversing-stage turbine blade, control the separated flow, thereby improving the efficiency and power of the reversing-stage turbine.

[0036] like Figure 2 As shown, the lengths of the multiple curved pipes 5 corresponding to the multiple curved pipe outlet holes 7 increase sequentially along the direction from the top end surface to the bottom end surface of the reverse stage turbine blade 4.

[0037] like Figure 3a , Figure 3b , Figure 3c As shown, the plurality of elbow air inlet holes 6 have various distribution forms along the pressure surface to the suction surface of the reverse stage turbine blade 4, such as Figure 3a As shown, the first distribution of the plurality of elbow air inlet holes 6 is close to the suction surface of the reverse stage turbine blade 4, such as Figure 3b As shown, the second distribution of the plurality of elbow air inlet holes 6 is located between the pressure surface and the suction surface of the reverse stage turbine blade 4, preferably, as shown in FIG. Figure 3cAs shown, the third distribution of the plurality of elbow air inlet holes 6 is close to the pressure surface of the reverse stage turbine blade 4, which can reduce the leakage flow on the top surface of the reverse stage turbine blade 4 to the greatest extent.

[0038] like Figure 2 As shown, the radius R1 of each curved pipe air inlet hole 6 and the radius R2 of each curved pipe air outlet hole 7 are both 1% to 3% of the axial chord length of the reverse-stage turbine blade 4. If R1 and R2 are too small, they will not have any effect. If R1 and R2 are too large, they will easily affect the mainstream flow and have a counter-effect. At the same time, they are also limited by the thickness of the blades. Therefore, setting R1 and R2 to 1% to 3% of the axial chord length of the reverse-stage turbine blade 4 can guide the high-pressure airflow to the greatest extent.

[0039] like Figure 3b As shown, the spacing b1 between two adjacent elbow air inlet holes 6 is 1.0 to 5.0 times the diameter of the elbow air inlet hole 6. If b1 is too small, mutual interference is likely to occur. If b1 is too large, the last air inlet hole is likely to be closer to the trailing edge than the air outlet hole, causing reverse flow.

[0040] like Figure 2 As shown, the axial spacing between two adjacent elbow air outlet holes 7 is 1.0 times to 3.0 times the diameter of the elbow air outlet holes 7, and the radial spacing between two adjacent elbow air outlet holes 7 is 1.0 times to 10.0 times the diameter of the elbow air outlet holes 7. The spacing between the two elbow air outlet holes 7 should not be too large, as it is easy to deviate from the separation zone and have a negative impact on the normal mainstream. If the radial spacing between the two elbow air outlet holes 7 is too small, only the separation flow of the blade part of the blade height can be controlled.

[0041] like Figure 2 As shown, the diameter R3 of each elbow 5 gradually changes from the corresponding elbow air inlet 6 to the corresponding elbow air outlet 7. The variable diameter design of each elbow 5 can adjust the air outlet speed according to the pressure difference, which is equivalent to adding a nozzle structure design.

[0042] like Figure 3b As shown, the distance b2 between the curved pipe air inlet hole 6 near the leading edge of the reverse-stage turbine blade 4 and the leading edge of the reverse-stage turbine blade 4 is 5% to 80% of the axial chord length of the reverse-stage turbine blade 4, and the distance between the curved pipe air inlet hole 6 near the trailing edge of the reverse-stage turbine blade 4 and the trailing edge of the reverse-stage turbine blade 4 is 20% to 95% of the axial chord length of the reverse-stage turbine blade 4, so that the leading edge and the trailing edge of the reverse-stage turbine blade 4 have sufficient thickness, and the strength of the leading edge and the trailing edge of the reverse-stage turbine blade 4 cannot be weakened for the curved pipe air inlet hole 6.

[0043] like Figure 4 , Figure 5As shown, the depth H of the groove 9 is 1% to 5% of the thickness of the reversing-stage turbine blade 4, the groove 9 is located at 20% to 60% of the axial direction of the reversing-stage turbine blade 4, the depth of each dimple 8 is 1% to 5% of the thickness of the reversing-stage turbine blade 4, the spanwise distance D of each dimple 8 is 1.0 to 5.0 times the depth of the dimple 8, the radial spacing S between two adjacent dimples 8 is 1.0 to 3.0 times the spanwise distance of each dimple 8, the depth of the groove 9 and the dimple 8 is considered not to affect the blade strength and thickness, the groove 9 is located in the diversion area, the spacing of the dimples 8 can be just staggered with the bend pipe outlet, and the two have a superimposed effect.

[0044] like Figure 6 As shown, the pressure on the top end surface of the reversing stage turbine blade 4 is greater than the pressure on the suction surface of the reversing stage turbine blade 4, which can indicate that by using the bend 5, the bend air inlet 6 and the bend air outlet 7 of the present invention, the high-pressure airflow can enter the bend 5 from the bend air inlet 6 and flow out from the bend air outlet 7. At the same time, it can be seen that the pressure of the top end surface of the reversing stage turbine blade 4 close to the leading edge is greater than the pressure far from the leading edge. The closer the distance between the bend air inlet 6 and the leading edge of the reversing stage turbine blade 4 is, the greater the pressure difference generated is, and the flow in the bend 5 will be smoother. Therefore, the bend air inlet 6 is closer to the leading edge of the reversing stage turbine blade 4 than the bend air outlet 7.

[0045] The specific design steps for each reverse stage turbine blade are as follows:

[0046] (1) According to the internal flow of the reverse stage turbine, the axial and radial positions of the start of airflow separation on the suction surface of the moving blade, etc., the axial position, circumferential position and number of the elbow air inlet hole 6 and the elbow air outlet hole 7 are selected. According to the thickness and chord length of the reverse stage turbine blade 4, the radius R1 of the elbow air inlet hole 6 and the radius R2 of the elbow air outlet hole 7, the diameter R3 of the elbow 5, the spacing b1 between two adjacent elbow air inlet holes 6, and the distance b2 between the first air inlet hole and the leading edge of the reverse stage turbine blade 4 are determined.

[0047] (2) According to the internal flow of the reverse-stage turbine 2 and the actual structure of the reverse-stage turbine blade 4, the sizes of the elbow air inlet 6, the elbow air outlet 7 and the elbow 5 are determined. The elbow air inlet 6 is close to the leading edge and closer to the high-pressure incoming gas to facilitate the inflow of high-pressure gas. The elbow air outlet 7 is as far as possible in the axial direction to prevent the high-pressure gas flowing out of the elbow 5 from tilting toward the end wall and the casing and affecting the mainstream flow. At the same time, the actual positions of the elbow air inlet 6 and the elbow air outlet 7 and the radius of the elbow 5 can be adjusted by means of experiments or numerical simulation methods to ensure that the addition of the elbow 5 can maximize the efficiency and power of the reverse turbine.

[0048] (3) The position and depth H of the groove 9 are selected according to the internal flow of the reverse-stage turbine 2, the position where the airflow separation on the suction surface of the reverse-stage turbine blade 4 begins, etc. The groove 9 can reduce the number of dimples 8 on the surface of the reverse-stage turbine blade 4, reduce the treatment of the surface of the reverse-stage turbine blade 4, and can also more effectively promote the transition of the boundary layer in the flow field.

[0049] (4) According to the flow conditions inside the reverse-stage turbine 2 after adding the groove 9, the span length D, spacing S, axial position, circumferential position and number of the dimples 8 are selected, and a certain number of dimples are added before and after the groove to more effectively control the separation flow in coordination with the control of the groove 9 and the bend 5 on the separation flow.

[0050] It is to be understood that the present invention is described by some embodiments, and it is known to those skilled in the art that various changes or equivalent substitutions may be made to these features and embodiments without departing from the spirit and scope of the present invention. In addition, under the teachings of the present invention, these features and embodiments may be modified to adapt to specific circumstances and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the scope of protection of the present invention.

Claims

1. A turbine capable of direct reversing with a double-layer blade adaptive separation flow control structure, comprising a forward-stage turbine (1) and a reverse-stage turbine (2), wherein the first-stage moving blades of the forward-stage turbine (1) and the reverse-stage turbine (2) are upper and lower double-layer moving blades, wherein the lower-layer moving blades are a plurality of forward-stage turbine blades (3), and the upper-layer moving blades are a plurality of reverse-stage turbine blades (4), characterized in that: Each of the reverse-stage turbine blades (4) is provided with a suppression structure, the suppression structure comprising: A plurality of curved pipes (5) are arranged inside the reverse-stage turbine blade (4), one end of the plurality of curved pipes (5) passes through the top end surface of the reverse-stage turbine blade (4) and communicates with the outside of the reverse-stage turbine blade (4), so that the top end surface of the reverse-stage turbine blade (4) forms a plurality of curved pipe air inlet holes (6) along the axial direction, and the other end of the plurality of curved pipes (5) passes through the suction surface of the reverse-stage turbine blade (4) and communicates with the outside of the reverse-stage turbine blade (4), so that the suction surface of the reverse-stage turbine blade (4) forms a plurality of curved pipe air outlet holes (7) along its radial direction; A groove (9) is radially arranged on the suction surface of the reverse stage turbine blade (4); A plurality of dimples (8) are radially arranged on the suction surface of the reverse stage turbine blade (4) and are located close to the groove (9); The high-pressure airflow on the top surface of the reverse-stage turbine blade (4) is guided by a plurality of curved pipes (5), so that the high-pressure airflow enters from each curved pipe air inlet (6) and flows out from the corresponding curved pipe air outlet (7), taking away part of the separated flow on the suction surface of the reverse-stage turbine blade (4), and cooperates with the grooves (9) and the plurality of dimples (8) to promote the process of breaking up and turning the vortex, thereby suppressing the separated flow on the suction surface of the reverse-stage turbine blade (4).

2. The direct reversing turbine with a double-blade adaptive separation flow control structure according to claim 1, characterized in that: The lengths of the plurality of curved pipes (5) corresponding to the plurality of curved pipe air outlet holes (7) increase sequentially along the direction from the top end surface to the bottom end surface of the reverse stage turbine blade (4).

3. The direct reversing turbine with a double-blade adaptive separation flow control structure according to claim 1, characterized in that: The positions of the plurality of curved pipe air inlet holes (6) are close to the pressure surface of the reverse stage turbine blades (4).

4. The direct reversing turbine with a double-blade adaptive separation flow control structure according to claim 1, characterized in that: The radius of each curved pipe air inlet hole (6) and the radius of each curved pipe air outlet hole (7) are both 1% to 3% of the axial chord length of the reverse-stage turbine blade (4).

5. The direct reversing turbine with a double-layer blade adaptive separation flow control structure according to claim 4, characterized in that: The distance between two adjacent curved pipe air inlet holes (6) is 1.0 to 5.0 times the diameter of the curved pipe air inlet hole (6).

6. The direct reversing turbine with a double-layer blade adaptive separation flow control structure according to claim 4, characterized in that: The axial spacing between two adjacent curved pipe air outlet holes (7) is 1.0 to 3.0 times the diameter of the curved pipe air outlet holes (7), and the radial spacing between two adjacent curved pipe air outlet holes (7) is 1.0 to 10.0 times the diameter of the curved pipe air outlet holes (7).

7. The direct reversing turbine with a double-blade adaptive separation flow control structure according to claim 1, characterized in that: The diameter of each of the curved pipes (5) gradually changes in a direction from the corresponding curved pipe air inlet hole (6) to the corresponding curved pipe air outlet hole (7).

8. The direct reversing turbine with a double-layer blade adaptive separation flow control structure according to claim 1, characterized in that: The distance between the curved pipe air inlet hole (6) near the leading edge of the reverse-stage turbine blade (4) and the leading edge of the reverse-stage turbine blade (4) is 5% to 80% of the axial chord length of the reverse-stage turbine blade (4), and the distance between the curved pipe air inlet hole (6) near the trailing edge of the reverse-stage turbine blade (4) and the trailing edge of the reverse-stage turbine blade (4) is 20% to 95% of the axial chord length of the reverse-stage turbine blade (4).

9. The direct reversing turbine with a double-layer blade adaptive separation flow control structure according to claim 1, characterized in that: The depth of the groove (9) is 1% to 5% of the thickness of the reverse-stage turbine blade (4), and the groove (9) is located at a position 20% to 60% of the axial direction of the reverse-stage turbine blade (4).

10. The direct reversing turbine with a double-layer blade adaptive separation flow control structure according to claim 1, characterized in that: The depth of each dimple (8) is 1% to 5% of the thickness of the reverse-stage turbine blade (4), the spanwise distance of each dimple (8) is 1.0 to 5.0 times the depth of the dimple (8), and the radial spacing between two adjacent dimples (8) is 1.0 to 3.0 times the spanwise distance of each dimple (8).

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