Electronic expansion valve

Abstract

It includes a valve seat portion (10), a gasket (20), and a valve head (30). The valve seat portion (10) has a valve chamber (11) and a first flow path (12). The gasket (20) is located within the valve seat portion (10) and is in sealing engagement with the valve seat portion (10). The gasket (20) is provided surrounding the first flow path (12). The gasket (20) has a second flow path (21). The first flow path (12) and the second flow path (21) are in communication. The valve head (30) is movably provided within the valve chamber (11) so as to open and close the second flow path (21), and is in communication with the valve chamber (11) when the second flow path (21) is opened. Here, the first flow path (12) and the second flow path (21) constitute a variable flow path. The flow areas at both ends of the variable flow path are S1 and S2 respectively, and the minimum flow area at the middle portion of the variable flow path is S3, where S3 < S1 and S3 < S2. An electronic expansion valve is disclosed. This structure enables the flow velocity to increase when the fluid flows through the variable flow path, effectively improving the flow capacity of the electronic expansion valve, increasing the Cv value, adopting the engagement between the gasket and the valve head to realize the opening and closing of the flow path, reducing the internal leakage of the electronic expansion valve, and ensuring the reliable conduction and cut-off functions of the electronic expansion valve.

Description

Technical Field

[0001] This application claims the priority of a patent application filed with the China National Intellectual Property Administration on September 20, 2022, with the application number 202211145110.8 and the invention title "Electronic Expansion Valve".

[0002] This application relates to the technical field of electronic expansion valves, and specifically to electronic expansion valves.

Background Art

[0003] Most electronic expansion valves may have internal leakage after being closed. For electronic expansion valves with a large diameter and low internal leakage, it is required that the internal leakage is very small when closed, that it has the functions of conduction and cut-off, and that the Cv value also needs to reach a certain requirement, and it is required that the Cv value is relatively large. The Cv value represents the flow capacity of the element for the liquid, that is, the flow coefficient, and is also called the Kv value.

[0004] In conventional electronic expansion valves, the functions of conduction and cut-off of the electronic expansion valve are realized by opening and closing the flow path by the movement of the valve head. The flow path engaged with the valve head is usually a straight flow path with a constant flow area. When the valve head opens the flow path, the fluid in the flow path is likely to generate a vortex at one end close to the valve head, and the vortex affects the flow capacity of the flow path, resulting in a decrease in the Cv value. Therefore, the flow path structure in the conventional electronic expansion valve has the problem of poor flow capacity. Summary of the Application

[0005] This application provides an electronic expansion valve to solve the problem of poor flow capacity of the electronic expansion valve in the prior art.

[0006] To solve the above problems, the present application provides an electronic expansion valve including: a valve seat portion having a valve chamber and a first flow path; a gasket located within the valve seat portion and engaged with the valve seat portion, provided so as to surround the first flow path, having a second flow path, and the first flow path and the second flow path being in communication with each other; and a valve head movably provided within the valve chamber so as to open and close the second flow path, the valve head being in communication with the valve chamber when the second flow path is opened, and the hardness of both the valve head and the valve seat portion being higher than the hardness of the gasket. Here, the first flow path and the second flow path constitute a variable flow path, the flow areas at both ends of the variable flow path are S1 and S2 respectively, and the minimum flow area at the middle portion of the variable flow path is S3, where S3 < S1 and S3 < S2.

[0007] Furthermore, the minimum flow area at the middle portion of the variable flow path is located in the first flow path and / or the second flow path.

[0008] Furthermore, the inner surface of the variable flow path is an arc surface, or the inner surface of the variable flow path includes tapered surfaces of a plurality of segments.

[0009] Furthermore, in the direction of the valve chamber of the first flow path, the inner surface of the second flow path includes a first annular surface, a second annular surface, and a third annular surface connected in sequence. The first annular surface is a cylindrical surface or a tapered surface, the second annular surface is a tapered surface, and the third annular surface is a cylindrical surface or a tapered surface. Here, the end with the larger opening of the tapered surface in the second flow path faces the valve chamber.

[0010] Furthermore, in a cross-section passing through the axis of the first flow path, the angle between the first annular surface and the axis of the first flow path is A1, the angle between the second annular surface and the axis of the first flow path is A2, and the angle between the third annular surface and the axis of the first flow path is A3, where A1 < A2 < A3.

[0011] Furthermore, 0 ≦ A1 ≦ 10°, 6° ≦ A2 ≦ 26°, and 40° ≦ A3 ≦ 60°.

[0012] ]>Furthermore, in the direction of the second channel toward the first channel, the inner surface of the first channel includes a fourth and fifth annular surface connected in order, the fourth annular surface being a cylindrical or tapered surface, and the fifth annular surface being a tapered surface, where one end of the tapered surface in the first channel with the larger opening is separated from the valve chamber.

[0013] Furthermore, in the cross-section passing through the axis of the second channel, the angle between the fourth annular surface and the axis of the second channel is B1, and the angle between the fifth annular surface and the axis of the second channel is B2, and B1 <B2である。

[0014] Furthermore, 0 ≤ B1 ≤ 10° and 2° ≤ B2 ≤ 25°.

[0015] Furthermore, the gasket has an annular sealing surface on one side facing the valve chamber, and this annular sealing surface surrounds the second flow path. The outer diameter of the valve head is larger than the inner diameter of the second flow path, and when the valve head and the annular sealing surface come into contact, the valve head closes the second flow path.

[0016] Furthermore, the valve seat portion includes a main valve seat and an auxiliary valve seat connected to each other, the main valve seat having a valve chamber, the auxiliary valve seat having a first flow path, the auxiliary valve seat further having an annular groove, and the gasket is fitted within the annular groove.

[0017] Furthermore, the sub-valve seat includes a body and an annular cylinder provided on the body, the body having a first flow path, the annular cylinder having an annular groove, the body being fixedly connected to the main valve seat, and one end of the annular cylinder away from the body being crimped to a gasket; the electronic expansion valve further includes a first connecting pipe and a second connecting pipe, the first connecting pipe being connected to the main valve seat and communicating with the valve chamber, the second connecting pipe being connected to the body and communicating with the first flow path, and the fluid area of ​​both the first connecting pipe and the second connecting pipe being greater than S3.

[0018] Furthermore, one side of the gasket facing the valve chamber has a first annular chamfer, the first annular chamfer is provided to surround the outside of the annular sealing surface, one side of the gasket facing the sub-valve seat has a second annular chamfer, the second annular chamfer is provided to surround the second flow path, and one side of the sub-valve seat facing the gasket has a third annular chamfer, the third annular chamfer is provided to surround the first flow path.

[0019] When the technical solution of the present application is applied, it includes a valve seat portion, a gasket and a valve head. The valve seat portion has a valve chamber and a first flow path. The gasket is located within the valve seat portion and engaged with the valve seat portion. The gasket is provided to surround the first flow path. The gasket has a second flow path. The first flow path and the second flow path are communicated. The valve head is movably provided within the valve chamber so as to open and close the second flow path, and is communicated with the valve chamber when the second flow path is opened. The hardness of both the valve head and the valve seat portion is higher than that of the gasket. Here, the first flow path and the second flow path constitute a variable flow path. The flow areas at both ends of the variable flow path are S1 and S2 respectively, and the minimum flow area in the middle of the variable flow path is S3, where S3 < S1 and S3 < S2. An electronic expansion valve is provided. In this aspect, the flow paths in the gasket and the valve seat portion constitute a variable flow path, and the minimum flow area in the middle of the variable flow path is smaller than the flow areas at both ends, that is, the variable flow path has a structure that is narrow in the middle and thick at both ends. When such a structure is adopted, compared with a straight flow path, when the fluid flows through the variable flow path, the flow velocity becomes faster. When the flow velocity becomes faster, the formation of vortices near the valve head can be avoided or reduced, so as to avoid or reduce the influence of vortices on the flow capacity. Therefore, this aspect effectively improves the flow capacity of the electronic expansion valve and increases the Cv value. In addition, this aspect adopts the engagement between the gasket and the valve head to realize the opening and closing of the flow path. Compared with the case of adopting a rigid structure, the sealing effect is good, the internal leakage of the electronic expansion valve is reduced, and the reliable conduction and cut-off functions of the electronic expansion valve are ensured.

Brief Description of the Drawings

[0020] The drawings in the specification, which constitute part of this application, are provided for further understanding of this application, and the schematic embodiments and descriptions thereof are for interpretation purposes only and do not improperly limit this application.

[0021] [Figure 1] A schematic diagram of the structure of the electronic expansion valve provided by the embodiment of this application is shown. [Figure 2] Figure 1 shows a schematic diagram of the assembly of the auxiliary valve seat and gasket. [Figure 3] A magnified view of a portion of Figure 2 is shown.

[0022] The above drawing includes the following reference numerals. 10 Valve seat, 11 Valve chamber, 12 First flow path, 121 Fourth annular surface, 122 Fifth annular surface, 13 Main valve seat, 14 Sub-valve seat, 141 Annular groove, 142 Body, 143 Annular cylinder, 20 Gasket, 21 Second flow path, 211 First annular surface, 212 Second annular surface, 213 Third annular surface, 22 Annular sealing surface, 30 Valve head, 41 First connecting pipe, 42 Second connecting pipe. [Modes for carrying out the invention]

[0023] The following describes the technical aspects of the embodiments of this application clearly and completely with reference to the drawings of the embodiments of this application, although it is clear that the embodiments described are only a selection of the embodiments of this application and not all embodiments. The following description of at least one exemplary embodiment is, in practice, merely descriptive and does not imply any limitation on this application or its application or use. All other embodiments that a person skilled in the art could obtain without creative effort based on the embodiments of this application are all within the scope of protection of this application.

[0024] As shown in FIGS. 1 to 3, an embodiment of the present application includes a valve seat portion 10 having a valve chamber 11 and a first flow path 12, and a gasket 20 located within the valve seat portion 10 and engaged with the valve seat portion 10. The gasket 20 is provided surrounding the first flow path 12, the gasket 20 has a second flow path 21, and the first flow path 12 and the second flow path 21 are in communication with each other. A valve head 30 is movably provided within the valve chamber 11 to open and close the second flow path 21, and when the second flow path 21 is opened, it is in communication with the valve chamber 11. The hardness of both the valve head 30 and the valve seat portion 10 is higher than the hardness of the gasket 20. Here, the first flow path 12 and the second flow path 21 constitute a variable flow path, the flow areas at both ends of the variable flow path are S1 and S2 respectively, and the minimum flow area at the middle portion of the variable flow path is S3, where S3 < S1 and S3 < S2. An electronic expansion valve is provided. Here, the gasket 20 may be made of a soft material such as rubber, and in this way, the sealing effect is good.

[0025] In this aspect, the second flow path 21 and the first flow path 12 in the gasket 20 and the valve seat portion 10 constitute a variable flow path. The minimum flow area S3 at the middle portion of the variable flow path is smaller than the flow areas at both ends, that is, the variable flow path has a structure that is narrow in the middle and wide at both ends. When such a structure is adopted, compared with a straight flow path, when the fluid flows through the minimum flow area of the variable flow path, the flow velocity becomes faster. When the flow velocity becomes faster, it is possible to avoid or reduce the formation of vortices in the vicinity of the valve head 30, so as to avoid or reduce the influence of vortices on the flow capacity. Therefore, this aspect effectively improves the flow capacity of the electronic expansion valve and increases the Cv value. Also, this aspect adopts the engagement between the gasket 20 and the valve head 30 to achieve the opening and closing of the flow path. Compared with the case of adopting a rigid structure, the sealing effect is good, reducing the internal leakage of the electronic expansion valve and ensuring the reliable conduction and cutoff functions of the electronic expansion valve. The variable flow path in this aspect utilizes the principle of a Laval nozzle.

[0026] Here, the position of the minimum flow area in the middle part of the variable flow path is located at a position close to the second flow path 21 of the first flow path 12, or at a position close to the first flow path 12 of the second flow path 21, or is simultaneously located at a position where the first flow path 12 and the second flow path 21 are close to each other.

[0027] In this embodiment, the inner surface of the variable diameter flow path may be an arcuate surface. In this way, the resistance of the fluid is small, the flow is relatively smooth. For example, the inner surface of the variable diameter flow path is an elliptical surface. Alternatively, the inner surface of the variable flow path includes a tapered surface of a plurality of segments. Adopting a tapered surface of a plurality of segments with a changing diameter can also achieve the effect of changing the diameter and increasing the flow velocity, and such a method can reduce the difficulty of processing.

[0028] As shown in FIGS. 2 and 3, in the direction toward the valve chamber 11 of the first flow path 12, the inner surface of the second flow path 21 includes a first annular surface 211, a second annular surface 212, and a third annular surface 213 that are sequentially connected. The first annular surface 211 is a cylindrical surface or a tapered surface, the second annular surface 212 is a tapered surface, and the third annular surface 213 is a cylindrical surface or a tapered surface. Here, the end with the larger opening of the tapered surface in the second flow path 21 faces the valve chamber 11. In this way, by providing the first annular surface 211, the second annular surface 212, and the third annular surface 213, it is realized that the flow area changes from small to large, and the processing becomes easy.

[0029] In a specific embodiment, in the cross-section passing through the axis of the first flow path 12, the included angle between the first annular surface 211 and the axis of the first flow path 12 is A1, the included angle between the second annular surface 212 and the axis of the first flow path 12 is A2, and the included angle between the third annular surface 213 and the axis of the first flow path 12 is A3. Here, A1 < A2 < A3. Here, the minimum flow area of the variable flow path is located in front of the first annular surface 211 or at the first annular surface 211. By providing as described above, the flow areas of the three annular surfaces are made from small to large in sequence, and a Laval nozzle is formed.

[0030] Specifically, 0≦A1≦10°, 6°≦A2≦26°, and 40°≦A3≦60°. When the included angles between the first annular surface 211, the second annular surface 212, the third annular surface 213 and the axis of the first flow path 12 are within the above ranges, the fluid passing through this structure can have a relatively high flow rate, avoiding or reducing the formation of eddy currents in the vicinity of the valve head 30, and increasing the Cv value of the electronic expansion valve.

[0031] As shown in FIGS. 2 and 3, in the direction of the second flow path 21 toward the first flow path 12, the inner surface of the first flow path 12 includes a fourth annular surface 121 and a fifth annular surface 122 connected in sequence. The fourth annular surface 121 is a cylindrical surface or a tapered surface, and the fifth annular surface 122 is a tapered surface. Here, one end with the larger opening of the tapered surface in the first flow path 12 is away from the valve chamber 11. The fluid enters the valve chamber from the first flow path 12. By providing it as described above, in the process of the fluid flowing in the first flow path 12, there is a process in which the flow area changes from large to small. In this way, when the flow rate of the fluid increases, it can avoid or reduce the formation of eddy currents when the fluid flows near the valve head 30, and increase the Cv value of the electronic expansion valve. Here, the fifth annular surface 122 may be a tapered surface composed of a plurality of segments connected in sequence.

[0032] Specifically, in the cross-section passing through the axis of the second flow path 21, the included angle between the fourth annular surface 121 and the axis of the second flow path 21 is B1, and the included angle between the fifth annular surface 122 and the axis of the second flow path 21 is B2, and B1 < B2. In this way, it is advantageous for realizing the change of the flow area and is also easy to process.

[0033] Here, 0≦B1≦10° and 2°≦B2≦25°. When the included angles between the fourth annular surface 121 and the fifth annular surface 122 and the axis of the second flow path 21 are within the above ranges, the fluid passing through this structure can have a relatively high flow rate, avoiding or reducing the formation of eddy currents in the vicinity of the valve head 30, and increasing the Cv value of the electronic expansion valve. In this embodiment, the second flow path 21 and the first flow path 12 are provided coaxially.

[0034] As shown in Figures 1 and 2, the gasket 20 has an annular sealing surface 22 on one side facing the valve chamber 11, and the annular sealing surface 22 is provided surrounding the second flow path 21. The outer diameter of the valve head 30 is larger than the inner diameter of the second flow path 21. When the valve head 30 and the annular sealing surface 22 come into contact, the valve head 30 closes the second flow path 21. In this way, the contact between the valve head 30 and the annular sealing surface 22 causes compression on the gasket 20, thereby increasing the contact area and resulting in a good sealing effect, which leads to low or no internal leakage after the valve is closed. Compared to the prior art, in this embodiment, the engagement structure with the valve head 30 is a soft seal structure, and the sealing surface is located on the end face of the gasket 20 rather than on the inner wall of the flow path. In this way, the inner wall of the flow path is not worn or damaged, the contact area is increased, and sealing is reliable.

[0035] In this embodiment, the valve seat portion 10 may be an integral structure, which results in relatively high structural strength, or the valve seat portion 10 may be a separate structure, which makes processing easier.

[0036] In one specific embodiment, the valve seat portion 10 includes a main valve seat 13 and an auxiliary valve seat 14 connected to each other. The main valve seat 13 has a valve chamber 11, the auxiliary valve seat 14 has a first flow path 12, and the auxiliary valve seat 14 further has an annular groove 141. The gasket 20 is installed in the annular groove 141. By making the valve seat portion 10 two separate parts, they can be machined individually, and the main valve seat 13 and the auxiliary valve seat 14 can be connected after the gasket 20 is installed in the annular groove 141, thus simplifying assembly.

[0037] Specifically, the sub-valve seat 14 includes a body 142 and an annular cylinder 143 provided on the body 142, the body 142 having a first flow path 12, the annular cylinder 143 having an annular groove 141, the body 142 being fixedly connected to the main valve seat 13, and one end of the annular cylinder 143 away from the body 142 being crimped to a gasket 20. The electronic expansion valve further includes a first connecting pipe 41 and a second connecting pipe 42, the first connecting pipe 41 being connected to the main valve seat 13 and communicating with the valve chamber 11, and the second connecting pipe 42 being connected to the body 142 and communicating with the first flow path 12, and the fluid area of ​​both the first connecting pipe 41 and the second connecting pipe 42 being greater than S3. The connection is secure when the gasket 20 is fixed using a crimping method. Here, a portion of the main body 142 is inserted into the main valve seat 13, and the two are restricted in the axial direction by a stepped structure. The main body 142 and the main valve seat 13 can be connected by methods such as interference fit or welding.

[0038] Furthermore, the gasket 20 has a first annular chamfer on one side facing the valve chamber 11, which surrounds the outside of the annular sealing surface 22; the gasket 20 has a second annular chamfer on one side facing the sub-valve seat 14, which surrounds the second flow path 21; and the sub-valve seat 14 has a third annular chamfer on one side facing the gasket 20, which surrounds the first flow path 12. By providing annular chamfers, the generation of burrs during processing can be avoided, and the accuracy of the electronic expansion valve can be ensured.

[0039] Selectively, in one specific embodiment, the valve head 30 is provided with a sealing surface and a flow guide surface. The sealing surface surrounds the flow guide surface, i.e., the sealing surface is annular, and the sealing surface is sealed and engaged with the annular sealing surface 22. The flow guide surface is a tapered surface. By providing a tapered flow guide surface, the resistance when fluid passes through can be reduced, and the fluid flow capacity can be improved. Furthermore, a avoidance groove is drilled in the valve head 30, and the avoidance groove is located between the sealing surface and the flow guide surface. When the valve head 30 is manufactured, it is unavoidable that burrs or local flanges will be generated. However, by providing the avoidance groove, the burrs or flanges generated during manufacturing will be located within the avoidance groove. In this way, it is possible to avoid the burrs or local flanges affecting the sealing engagement, and the sealing effect can be ensured.

[0040] The foregoing describes preferred embodiments of this application and is not intended to limit it. Those skilled in the art will know that this application is subject to various modifications and changes. Any modifications, equivalent substitutions, improvements, etc., made within the scope of the intent and principles of this application should be included within the scope of protection of this application.

Claims

1. A valve seat portion (10) having a valve chamber (11) and a first flow path (12), A gasket (20) located within the valve seat portion (10) and engaged with the valve seat portion (10), wherein the gasket (20) is provided surrounding the first passage (12), and the gasket (20) has a second passage (21), and the first passage (12) and the second passage (21) are in communication with the gasket (20), A valve head (30) is provided movably within the valve chamber (11) to open and close the second passage (21), and is in communication with the valve chamber (11) when the second passage (21) is opened, wherein the hardness of both the valve head (30) and the valve seat portion (10) is higher than the hardness of the gasket (20), Here, the first flow path (12) and the second flow path (21) constitute a variable diameter flow path, the flow areas at both ends of the variable diameter flow path are S1 and S2, respectively, the minimum flow area in the middle of the variable diameter flow path is S3, and S3 < S1 and S3 < S2. In the direction of the second flow path (21) toward the first flow path (12), the inner surface of the first flow path (12) includes a fourth annular surface (121) and a fifth annular surface (122) connected in order, the fourth annular surface (121) being a cylindrical or tapered surface, and the fifth annular surface (122) being a tapered surface, wherein one end of the tapered surface in the first flow path (12) with the larger opening is separated from the valve chamber (11), an electronic expansion valve.

2. The electronic expansion valve according to claim 1, wherein the minimum flow area in the intermediate portion of the variable diameter flow path is located in the first flow path (12) and / or the second flow path (21).

3. The electronic expansion valve according to claim 1, wherein the inner surface of the variable diameter flow path is an arc-shaped surface, or the inner surface of the variable diameter flow path includes tapered surfaces of multiple segments.

4. In the direction of the first flow path (12) toward the valve chamber (11), the inner surface of the second flow path (21) includes a first annular surface (211), a second annular surface (212), and a third annular surface (213) connected in order, wherein the first annular surface (211) is a cylindrical or tapered surface, the second annular surface (212) is a tapered surface, and the third annular surface (213) is a cylindrical or tapered surface, and where one end of the tapered surface in the second flow path (21) with the larger opening faces the valve chamber (11), as described in claim 1.

5. In a cross-section passing through the axis of the first channel (12), the angle between the first annular surface (211) and the axis of the first channel (12) is A1, the angle between the second annular surface (212) and the axis of the first channel (12) is A2, and the angle between the third annular surface (213) and the axis of the first channel (12) is A3, where, The electronic expansion valve according to claim 4, wherein A1 < A2 < A3.

6. The electronic expansion valve according to claim 5, wherein 0 ≤ A1 ≤ 10°, 6° ≤ A2 ≤ 26°, and 40° ≤ A3 ≤ 60°.

7. The electronic expansion valve according to claim 1, wherein in a cross-section passing through the axis of the second flow path (21), the angle between the fourth annular surface (121) and the axis of the second flow path (21) is B1, the angle between the fifth annular surface (122) and the axis of the second flow path (21) is B2, and B1 < B2.

8. The electronic expansion valve according to claim 7, wherein 0 ≤ B1 ≤ 10° and 2° ≤ B2 ≤ 25°.

9. The electronic expansion valve according to claim 1, wherein the gasket (20) has an annular sealing surface (22) on one side facing the valve chamber (11), the annular sealing surface (22) is provided surrounding the second flow path (21), the outer diameter of the valve head (30) is larger than the inner diameter of the second flow path (21), and when the valve head (30) and the annular sealing surface (22) come into contact, the valve head (30) closes the second flow path (21).

10. The electronic expansion valve according to claim 9, wherein the valve seat portion (10) includes a main valve seat (13) and a sub-valve seat (14) connected to each other, the main valve seat (13) having the valve chamber (11), the sub-valve seat (14) having the first flow path (12), the sub-valve seat (14) further having an annular groove (141), and the gasket (20) is installed in the annular groove (141).

11. The auxiliary valve seat (14) includes a body (142) and an annular cylinder (143) provided on the body (142), the body (142) having the first flow path (12), the annular cylinder (143) having the annular groove (141), the body (142) being fixedly connected to the main valve seat (13), and one end of the annular cylinder (143) away from the body (142) being crimped to the gasket (20), and the electronic expansion valve is the first The electronic expansion valve according to claim 10, further comprising a connecting pipe (41) and a second connecting pipe (42), wherein the first connecting pipe (41) is connected to the main valve seat (13) and communicates with the valve chamber (11), and the second connecting pipe (42) is connected to the body (142) and communicates with the first flow path (12), and the fluid area of ​​both the first connecting pipe (41) and the second connecting pipe (42) is greater than S3.

12. The electronic expansion valve according to claim 10, wherein the gasket (20) has a first annular chamfer on one side facing the valve chamber (11), the first annular chamfer is provided surrounding the outside of the annular sealing surface (22), the gasket (20) has a second annular chamfer on one side facing the sub-valve seat (14), the second annular chamfer is provided surrounding the second flow path (21), and the sub-valve seat (14) has a third annular chamfer on one side facing the gasket (20), the third annular chamfer is provided surrounding the first flow path (12).

13. The electronic expansion valve according to claim 1, wherein the valve head (30) has a sealing surface and a flow guide surface, the sealing surface is annular and surrounds the flow guide surface, the sealing surface is sealed and engaged with the end face of the gasket (20), and the flow guide surface is tapered so as to reduce fluid resistance.

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