Air compressor

Abstract

This invention provides an air compressor, including a housing and a rotor assembly. The housing has a first end plate, and the rotor assembly includes a shaft. One end of the shaft is connected to a first impeller. The hub of the first impeller has a first sealing protrusion ring on the side facing the first end plate. The first end plate has a first sealing ring groove on the side facing the first impeller. The first sealing protrusion ring is located within the first sealing ring groove, and a labyrinth seal structure can be formed between the first sealing protrusion ring and the first sealing ring groove. According to this invention, a larger sealing length can be achieved without increasing the axial length of the shaft, thereby effectively reducing interstage gas leakage and improving the performance of the air compressor.

Description

Technical Field

[0001] This invention belongs to the field of compressor manufacturing technology, and specifically relates to an air compressor. Background Technology

[0002] With the increasing demand for energy conservation and environmental protection, and driven by national policies and the rapid development of hydrogen fuel cell technology, the research and development of automotive fuel cell systems has made significant progress. The air compressor, as the "heart" of the automotive fuel cell system, is also a current research hotspot. The direction and magnitude of the axial force in the air compressor are crucial to the design of the gas bearings. Excessive interstage leakage reduces compressor performance. For two-stage air compressors, improper rotor assembly design can result in excessive axial force with changing direction depending on operating conditions, reducing the safety and stability of the gas axial bearings. Due to the limitation of the main shaft length, the sealing length of the two-stage main shafts is insufficient, leading to significant interstage leakage and affecting compressor performance. Summary of the Invention

[0003] Therefore, the present invention provides an air compressor that can overcome the contradiction between the sealing performance and shaft length design of the air compressor spindle in related technologies, which leads to the air compressor needing a larger sealing length when it has higher performance, resulting in the spindle bearing being too long.

[0004] To address the aforementioned problems, the present invention provides an air compressor, comprising a housing and a rotor assembly. The housing has a first end plate, and the rotor assembly includes a rotating shaft. One end of the rotating shaft is connected to a first impeller. The hub of the first impeller is provided with a first sealing protrusion on the side facing the first end plate. The first end plate is provided with a first sealing ring groove on the side facing the first impeller. The first sealing protrusion is located within the first sealing ring groove, and a labyrinth seal structure can be formed between the first sealing protrusion and the first sealing ring groove.

[0005] In some embodiments, the first sealing ring groove has a first sealing comb structure on its groove wall.

[0006] In some embodiments, on the axial section of the first impeller, the first sealing ring is formed into a U-shape protruding towards the first sealing ring groove side, and on the axial section of the first end plate, the first sealing comb structure is formed into a U-shape protruding away from the first impeller side, and / or, the first sealing comb structure is composed of Na first sealing teeth spaced axially along the rotating shaft, where 8≤Na≤11.

[0007] In some embodiments, the housing further includes a second end plate disposed opposite to the first end plate, and the other end of the rotating shaft is connected to a second impeller. The hub of the second impeller is provided with a second sealing protrusion on the side facing the second end plate, and the second end plate is provided with a second sealing ring groove on the side facing the second impeller. The second sealing protrusion is located in the second sealing ring groove, and a labyrinth seal structure can be formed between the second sealing protrusion and the second sealing ring groove.

[0008] In some embodiments, the second sealing ring groove has a second sealing comb tooth structure on its groove wall; and / or, the first end plate includes a first outer ring plate and a first inner ring plate assembled together, the inner hole wall of the first outer ring plate has a first radially outer sealing tooth corresponding to the first sealing comb tooth structure, the outer circumferential wall of the first inner ring plate has a first radially inner sealing tooth corresponding to the first sealing comb tooth structure; and / or, the second end plate includes a second outer ring plate and a second inner ring plate assembled together, the inner hole wall of the second outer ring plate has a second radially outer sealing tooth corresponding to the second sealing comb tooth structure, and the outer circumferential wall of the second inner ring plate has a second radially inner sealing tooth corresponding to the second sealing comb tooth structure.

[0009] In some embodiments, on the axial section of the second impeller, the second sealing ring is shaped like a convex shape protruding towards the side of the second sealing ring groove; on the axial section of the second end plate, the second sealing comb structure is shaped like a convex shape protruding away from the side of the second impeller; and / or, the second sealing comb structure is composed of Nb second sealing teeth spaced axially along the rotating shaft, where 11≤Nb≤14.

[0010] In some embodiments, Na = 10; or Nb = 12; or the ratio of the height of the first sealing tooth or the pitch of the second sealing tooth is 1.3 to 1.5.

[0011] In some embodiments, the ratio of the height of the first sealing tooth or the pitch of the second sealing tooth is 1.44.

[0012] In some embodiments, the first impeller is a low-pressure stage impeller, the second impeller is a high-pressure stage impeller, and the dimensions of the first impeller and the second impeller, and / or the dimensions of the first sealing ring and the second sealing ring are configured such that when the first impeller and the second impeller are simultaneously in a compressed state, the axial force of the shaft is directed from the first impeller to the second impeller.

[0013] In some embodiments, the protrusion height a2 of the first sealing ring is 5mm to 7mm, and the protrusion height b2 of the second sealing ring is 8mm to 10mm.

[0014] In some embodiments, the protrusion height a2 of the first sealing ring is 6 mm, and the protrusion height b2 of the second sealing ring is 9 mm.

[0015] In some embodiments, when the axial section of the first sealing ring is U-shaped, the first sealing ring includes a first small-diameter ring segment near the first end plate and a first large-diameter ring segment sandwiched between the first small-diameter ring segment and the hub of the first impeller, the protrusion height of the first large-diameter ring segment is a1, and a1 / a2 is 0.58 to 0.62; or, when the axial section of the second sealing ring is U-shaped, the second sealing ring includes a second small-diameter ring segment near the second end plate and a second large-diameter ring segment sandwiched between the second small-diameter ring segment and the hub of the second impeller, the protrusion height of the second large-diameter ring segment is b1, and b1 / b2 is 0.58 to 0.62.

[0016] In some embodiments, a1 / a2 = b1 / b2 = 0.6; and / or, the diameter ratio d1 / d2 of the first sealing ring and the second sealing ring is 1.17 to 1.22.

[0017] In some embodiments, d1 / d2 = 1.18.

[0018] In some embodiments, the outlet diameter D1 of the first impeller is greater than the outlet diameter D2 of the second impeller.

[0019] In some embodiments, D1 / D2 is 1.08 to 1.13; and / or, the hub ratio l1 / D1 of the first impeller is 0.22 to 0.25, and the hub ratio l2 / D2 of the second impeller is 0.23 to 0.25; and / or, the wheel diameter ratio c1 / D1 of the first impeller is 0.58 to 0.62, and the wheel diameter ratio c2 / D2 of the second impeller is 0.55 to 0.59.

[0020] In some embodiments, D1 / D2 = 1.09; and / or, l1 / D1 = 0.23; and / or, l2 / D2 = 0.24; and / or, c1 / D1 = 0.61; and / or, c2 / D2 = 0.56.

[0021] The present invention provides an air compressor in which a first sealing ring groove constructed on the first end plate and a first sealing convex ring constructed on the first impeller form a labyrinth seal structure after being inserted and accommodated. This structure can have a larger sealing length without increasing the axial length of the rotating shaft, thereby effectively reducing interstage leakage of gas and improving the performance of the air compressor. Attached Figure Description

[0022] Figure 1This is a schematic diagram of the internal structure of an air compressor according to an embodiment of the present invention (the volute housing corresponding to the first impeller and the second impeller, the corresponding connecting pipes and other components are omitted);

[0023] Figure 2 for Figure 1 A schematic diagram of the labyrinth seal structure formed between the sealing convex ring and the sealing ring groove to attenuate the airflow.

[0024] Figure 3 for Figure 1 A schematic diagram of the shaft cross-section of the first impeller, with some structural dimensions marked in the diagram;

[0025] Figure 4 for Figure 1 A partial structural diagram of the first sealing ring, with some structural dimensions marked in the diagram;

[0026] Figure 5 for Figure 1 A schematic diagram of the shaft cross-section of the second impeller, with some structural dimensions marked in the diagram;

[0027] Figure 6 for Figure 1 The diagram shows a partial structural schematic of the second sealing ring, with some structural dimensions marked on it.

[0028] The reference numerals in the attached figures are as follows:

[0029] 1. Housing; 11. First end plate; 111. First sealing ring groove; 112. First sealing tooth; 114. First outer ring plate; 115. First inner ring plate; 12. Second end plate; 121. Second sealing ring groove; 122. Second sealing tooth; 124. Second outer ring plate; 125. Second inner ring plate; 21. Rotating shaft; 22. First impeller; 221. First sealing convex ring; 23. Second impeller; 231. Second sealing convex ring; 100. Motor stator; 101. Thrust plate; 102. Gas axial bearing; 103. Locking nut. Detailed Implementation

[0030] See also Figures 1 to 6As shown, the present invention provides an air compressor, particularly an ultra-high-speed oil-free two-stage air compressor for an air-suspension fuel cell engine, comprising a housing 1 and a rotor assembly. The housing 1 is equipped with a motor stator and the rotor assembly. One axial end of the housing 1 has a first end plate 11. The rotor assembly includes a rotating shaft 21, one end of which is connected to a first impeller 22 (specifically, a semi-open impeller). The hub of the first impeller 22 is provided with a first sealing protrusion 221 on the side facing the first end plate 11 (which can be understood as the back side of the hub). The first end plate 11 is provided with a first sealing ring groove 111 on the side facing the first impeller 22. The first sealing protrusion 221 is located in the first sealing ring groove 111, and a labyrinth seal structure can be formed between the first sealing protrusion 221 and the first sealing ring groove 111. In this technical solution, the first sealing ring groove 111 constructed on the first end plate 11 and the first sealing convex ring 221 constructed on the first impeller 22 form a labyrinth seal structure after being inserted and accommodated. This structure can have a larger sealing length without increasing the axial length of the rotating shaft 21, thereby effectively reducing interstage leakage of gas and improving the performance of the air compressor.

[0031] In some embodiments, the first sealing ring groove 111 has a first sealing comb structure 112 on its groove wall. That is, preferably, the first sealing comb structure 112 is constructed on the first end plate 11. In this way, the first sealing comb structure 112 can be replaced when it wears out, without having to replace the corresponding impeller, thus effectively reducing the maintenance cost of the air compressor.

[0032] In some embodiments, on the axial section of the first impeller 22 (i.e., the section passing through the central axis of the first impeller 22), the first sealing ring 221 is shaped like a convex U-shape protruding towards the first sealing ring groove 111. On the axial section of the first end plate 11, the first sealing comb structure 112 is shaped like a convex U-shape protruding away from the first impeller 22. Thus, the labyrinth seal structure forms a stepped structure along the axial direction of the rotating shaft 21, which can reduce the straight-through effect and air leakage loss. For the same sealing length, the sealing effect is better. Experimental verification shows that the interstage gas leakage of the labyrinth seal structure using this structure is reduced by approximately 25% compared to the smooth labyrinth seal structure, thereby further improving the performance of the air compressor. See also... Figure 2As shown, when gas flows through the aforementioned stepped labyrinth seal structure, the gas is throttled, the pressure and temperature of the gas flow decrease while the flow velocity increases. The volume of the gas in the cavity increases, the velocity decreases and a vortex is formed. The temperature of the gas in the cavity returns to its pre-throttling state, while the pressure does not recover. Each time the gas passes through a gap and cavity, the gas flow is throttled and expanded. As the number of gaps and cavities through which the gas flows increases, the gas velocity and pressure drop become greater and greater. When the pressure drops to atmospheric pressure, the gas no longer flows out, thus achieving gas sealing.

[0033] In some embodiments, the housing 1 also has a second end plate 12 disposed opposite to the first end plate 11, and the other end of the rotating shaft 21 is connected to a second impeller 23 (specifically, a semi-open impeller). The hub of the second impeller 23 is provided with a second sealing convex ring 231 on the side facing the second end plate 12, and the second end plate 12 is provided with a second sealing ring groove 121 on the side facing the second impeller 23. The second sealing convex ring 231 is located in the second sealing ring groove 121, and a labyrinth seal structure can be formed between the second sealing convex ring 231 and the second sealing ring groove 121. That is, the first impeller 22 and the second impeller 23 are located opposite each other at the two ends of the rotating shaft 21. Correspondingly, the first impeller 22 is the first-stage impeller, and the second impeller 23 is the second-stage impeller. This can significantly reduce the axial force of the two-stage air compressor, prevent the rotor assembly from moving back and forth axially, and also reduce the design difficulty of the gas axial bearing and improve the stability of the axial bearing. The second sealing ring groove 121 may also have a second sealing comb structure 122 on its groove wall to improve the gas sealing performance at the second impeller 23. Similar to the first sealing comb structure 112, on the axial section of the second impeller 23, the second sealing convex ring 231 is convex in shape facing the second sealing ring groove 121, and on the axial section of the second end plate 12, the second sealing comb structure 122 is convex in shape facing away from the second impeller 23.

[0034] In some embodiments, the first sealing comb structure 112 is composed of Na first sealing teeth 112 spaced axially along the rotating shaft 21, where 8 ≤ Na ≤ 11, preferably Na = 10. The second sealing comb structure 122 is composed of Nb second sealing teeth 122 spaced axially along the rotating shaft 21, where 11 ≤ Nb ≤ 14, preferably Nb = 12, which can effectively reduce air leakage. The shapes of the first sealing teeth 112 and the second sealing teeth 122 can be varied, such as any one of asymmetrical trapezoids, symmetrical trapezoids, rectangles, and triangles, with asymmetrical trapezoidal sealing teeth being preferred. The ratio of the height to the pitch of the first sealing tooth 112 or the second sealing tooth 122 is 1.3 to 1.5, preferably 1.44, which can further reduce air leakage.

[0035] In some embodiments, the first end plate 11 includes a first outer ring plate 114 and a first inner ring plate 115 assembled together. The inner wall of the first outer ring plate 114 has a first radially outer sealing tooth corresponding to the first sealing comb structure 112, and the outer circumferential wall of the first inner ring plate 115 has a first radially inner sealing tooth corresponding to the first sealing comb structure 112. Alternatively, the second end plate 12 includes a second outer ring plate 124 and a second inner ring plate 125 assembled together. The inner wall of the second outer ring plate 124 has a second radially outer sealing tooth corresponding to the second sealing comb structure 122, and the outer circumferential wall of the second inner ring plate 125 has a second radially inner sealing tooth corresponding to the second sealing comb structure 122. Assembling the corresponding outer ring plate and inner ring plate to form a corresponding sealing comb structure can reduce the difficulty of manufacturing the first sealing tooth 112 and the second sealing tooth 122. After manufacturing the corresponding sealing teeth, the corresponding outer ring plate and inner ring plate can be assembled, for example, by bolting. This is simple and convenient.

[0036] As mentioned above, the first impeller 22 is a low-pressure stage impeller, and the second impeller 23 is a high-pressure stage impeller. The dimensions of the first impeller 22 and the second impeller 23, and / or the dimensions of the first sealing ring 221 and the second sealing ring 231, are configured such that when the first impeller 22 and the second impeller 23 are simultaneously under compression, the axial force of the rotating shaft 21 is directed from the first impeller 22 to the second impeller 23. By designing the structural dimensions of the impellers and the corresponding sealing rings, the axial force borne by the rotating shaft 21 is reasonably oriented towards the second impeller 23, thereby effectively curbing the increasing trend of the gap between the second impeller 23 and the corresponding volute inner wall, thus ensuring that the leakage on the high-pressure stage side is at a low level and ensuring the performance of the air compressor. It is understood that the magnitude of the aforementioned axial force should be within the adjustable range of the gas axial bearing 102 on the rotating shaft 21, so that the gas axial bearing 102 can normally perform its axial displacement adjustment function.

[0037] Specifically, the protrusion height a2 of the first sealing ring 221 is 5mm to 7mm, preferably 6mm, and the protrusion height b2 of the second sealing ring 231 is 8mm to 10mm, preferably 9mm. This ensures that the corresponding labyrinth seal structure can adapt to the sealing requirements of low-pressure and high-pressure stages, thereby improving the sealing effect. When the axial section of the first sealing ring 221 is U-shaped, the first sealing ring 221 includes a first small-diameter ring segment near the first end plate 11 and a first large-diameter ring segment sandwiched between the first small-diameter ring segment and the hub of the first impeller 22. The protrusion height of the first large-diameter ring segment is a1, and a1 / a2 is 0.58 to 0.62 (preferably a1 / a2 = 0.6). Alternatively, when the axial section of the second sealing ring 231 is U-shaped, the second sealing ring 231 includes a second small-diameter ring segment near the second end plate 12 and a second large-diameter ring segment sandwiched between the second small-diameter ring segment and the hub of the second impeller 23. The protrusion height of the second large-diameter ring segment is b1, and b1 / b2 is 0.58 to 0.62 (preferably b1 / b2 = 0.6). This structural design ensures that the axial force of the rotating shaft 21 is directed towards the second impeller 23. Ideally, the diameter ratio d1 / d2 of the first sealing ring 221 and the second sealing ring 231 is 1.17 to 1.22 (preferably d1 / d2 = 1.18), which can ensure that the axial force of the rotating shaft 21 can be directed to the side of the second blade 23.

[0038] As mentioned earlier, the axial force is related to the impeller outlet diameter, impeller inlet diameter, hub diameter, and convex seal ring diameter. Therefore, these structural dimensions can all change the axial force. Specifically, the pressure difference ratio between the compression chamber corresponding to the first impeller 22 and the compression chamber corresponding to the second impeller 23 is 0.45:0.55. Following the method of matching a large impeller with a small pressure difference and a small impeller with a large pressure difference, the outlet diameter D1 of the first impeller 22 is larger than the outlet diameter D2 of the second impeller 23. In some embodiments, D1 / D2 is 1.08 to 1.13 (preferably). The first impeller 22 has a hub ratio l1 / D1 of 0.22 to 0.25 (preferably l1 / D1 = 0.23), and the second impeller 23 has a hub ratio l2 / D2 of 0.23 to 0.25 (preferably l2 / D2 = 0.24). The first impeller 22 has a diameter ratio c1 / D1 of 0.58 to 0.62 (preferably c1 / D1 = 0.61), and the second impeller 23 has a diameter ratio c2 / D2 of 0.55 to 0.59 (preferably c2 / D2 = 0.56).

[0039] It will be readily understood by those skilled in the art that the aforementioned advantageous methods can be freely combined and superimposed without conflict.

[0040] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention. The above are merely preferred embodiments of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the protection scope of the present invention.

Claims

1. An air compressor characterized by, The utility model provides a kind of centrifugal compressor, including casing (1), rotor assembly, the casing (1) with first end plate (11), the rotor assembly includes rotating shaft (21), one end of the rotating shaft (21) is connected with first impeller (22), the first impeller (22) hub is equipped with first sealing convex ring (221) towards the first end plate (11) side, the first end plate (11) has first sealing ring groove (111) towards the first impeller (22) side, the first sealing convex ring (221) is in the first sealing ring groove (111), and labyrinth seal structure can be formed between the first sealing convex ring (221) and the first sealing ring groove (111);The casing (1) also has second end plate (12) with the first end plate (11) opposite arrangement, the other end of the rotating shaft (21) is also connected with second impeller (23), the second impeller (23) hub is equipped with second sealing convex ring (231) towards the second end plate (12) side, the second end plate (12) has second sealing ring groove (121) towards the second impeller (23) side, the second sealing convex ring (231) is in the second sealing ring groove (121), and labyrinth seal structure can be formed between the second sealing convex ring (231) and the second sealing ring groove (121);The first impeller (22) is low pressure stage impeller, the second impeller (23) is high pressure stage impeller, the size of the first impeller (22) and the size of the second impeller (23) and / or the size of the first sealing convex ring (221), second sealing convex ring (231) is configured to make the axial force direction of the rotating shaft (21) be by the first impeller (22) to the second impeller (23) when the first impeller (22), second impeller (23) is in compression state simultaneously;Second sealing comb tooth structure has on the groove wall of the second sealing ring groove (121).

2. The air compressor of claim 1, wherein, First sealing comb tooth structure has on the groove wall of the first sealing ring groove (111).

3. The air compressor of claim 2, wherein, On the shaft section of the first impeller (22), the first sealing convex ring (221) is convex to the side of the first sealing ring groove (111) and is in the shape of a Chinese character, on the shaft section of the first end plate (11), the first sealing comb tooth structure is convex to the side away from the first impeller (22) and is in the shape of a Chinese character, and the first sealing comb tooth structure is composed of Na first sealing teeth (112) arranged along the axial direction of the rotating shaft (21), 8≤Na≤11.

4. The air compressor of claim 3, wherein, The first end plate (11) includes a first outer ring plate (114) and a first inner ring plate (115) assembled and connected, the inner hole wall of the first outer ring plate (114) has a first radially outer sealing tooth corresponding to the first sealing comb tooth structure, the outer circumferential wall of the first inner ring plate (115) has a first radially inner sealing tooth corresponding to the first sealing comb tooth structure, and the first radially outer sealing tooth and the first radially inner sealing tooth are both composed of a plurality of first sealing teeth (112).

5. The air compressor of claim 4, wherein, In the axial section of the second impeller (23), the second sealing convex ring (231) is convex in the shape of a Chinese character "D" towards the side of the second sealing ring groove (121); in the axial section of the second end plate (12), the second sealing comb structure is convex in the shape of a Chinese character "D" away from the side of the second impeller (23); and the second sealing comb structure is composed of Nb second sealing teeth (122) arranged along the axial direction of the rotating shaft (21) at intervals, 11≤Nb≤14.

6. The air compressor of claim 5, wherein, The second end plate (12) comprises a second outer ring plate (124) and a second inner ring plate (125) assembled together, the inner hole wall of the second outer ring plate (124) has second radially outer sealing teeth corresponding to the second sealing comb structure, and the outer circumferential wall of the second inner ring plate (125) has second radially inner sealing teeth corresponding to the second sealing comb structure, both the second radially outer sealing teeth and the second radially inner sealing teeth are composed of a plurality of the second sealing teeth (122).

7. The air compressor of claim 5, wherein, Na=10; or, Nb=12; or, the ratio of the tooth height to the tooth pitch of the first sealing teeth (112) or the second sealing teeth (122) is 1.3~1.

5.

8. The air compressor of claim 7, wherein, The ratio of the tooth height to the tooth pitch of the first sealing teeth (112) or the second sealing teeth (122) is 1.

44.

9. The air compressor of claim 1, wherein, The protruding height a2 of the first sealing convex ring (221) is 5mm~7mm, and the protruding height b2 of the second sealing convex ring (231) is 8mm~10mm.

10. The air compressor of claim 9, wherein, The protruding height a2 of the first sealing convex ring (221) is 6mm, and the protruding height b2 of the second sealing convex ring (231) is 9mm.

11. The air compressor of claim 9, wherein, When the axial section of the first sealing convex ring (221) is in the shape of a Chinese character "D", the first sealing convex ring (221) comprises a first small-diameter ring segment close to the side of the first end plate (11) and a first large-diameter ring segment sandwiched between the first small-diameter ring segment and the hub of the first impeller (22), the protruding height of the first large-diameter ring segment is a1, and a1 / a2 is 0.58~0.62; or, when the axial section of the second sealing convex ring (231) is in the shape of a Chinese character "D", the second sealing convex ring (231) comprises a second small-diameter ring segment close to the side of the second end plate (12) and a second large-diameter ring segment sandwiched between the second small-diameter ring segment and the hub of the second impeller (23), the protruding height of the second large-diameter ring segment is b1, and b1 / b2 is 0.58~0.

62.

12. The air compressor of claim 11, wherein, a1 / a2=b1 / b2=0.6; and / or, the diameter ratio d1 / d2 of the first sealing convex ring (221) to the second sealing convex ring (231) is 1.17~1.

22.

13. The air compressor of claim 12, wherein, d1 / d2=1.

18.

14. The air compressor of claim 1, wherein, The outlet diameter D1 of the first impeller (22) is greater than the outlet diameter D2 of the second impeller (23).

15. The air compressor of claim 14, wherein, D1 / D2 is 1.08 to 1.13; and / or, a hub ratio li / D1 of the first impeller (22) is 0.22 to 0.25, a hub ratio l2 / D2 of the second impeller (23) is 0.23 to 0.25; and / or, a wheel diameter ratio cl / D1 of the first impeller (22) is 0.58 to 0.62, a wheel diameter ratio c2 / D2 of the second impeller (23) is 0.55 to 0.

59.

16. The air compressor of claim 15, wherein, D1 / D2 = 1.09; and / or, li / D1 = 0.23; and / or, l2 / D2 = 0.24; and / or, cl / D1 = 0.61; and / or, c2 / D2 = 0.56.

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