阅读说明：本技术 制动鼓 (Brake drum ) 是由 博帕蒂·马尼 孙达尔·维格内什·阿拉瓦达 于 2020-05-19 设计创作，主要内容包括：一种用于车辆的鼓式制动器的制动鼓(24)。制动鼓(24)包括制动表面(26),其适于接收鼓式制动器(22)的至少一个制动蹄。制动表面(26)具有在周向方向(C)上的周向延伸部和在轴向方向(A)上的轴向延伸部。制动鼓(24)还包括至少部分地包围制动表面(26)的外表面(32)。制动鼓(24)还包括冷却装置(30),该冷却装置(30)包括位于制动表面(26)与外表面(32)之间并且至少部分地在轴向方向(A)上延伸的一组冷却管道(34,36,38)。制动鼓(24)包括位于所述制动表面(26)的轴向延伸部内的内轴向截面(I)和外轴向截面(II),其中,当制动鼓(24)安装到车辆(10)时,如从轴向方向(A)观察的,内轴向截面(I)定位成比外轴向截面(II)更靠近中心平面。冷却装置(30)还延伸穿过内轴向截面(I)和外轴向截面(II)中的每一个。在径向上位于该组冷却管道(34,36,38)与制动表面(26)之间的制动鼓(24)的材料在内轴向截面(I)处具有内轴向总热导率,并且在径向上位于该组冷却管道(34,36,38)与制动表面(26)之间的制动鼓(24)的材料在外轴向截面(II)处具有外轴向总热导率。内轴向总热导率不同于外轴向总热导率。(A brake drum (24) for a drum brake of a vehicle. The brake drum (24) includes a braking surface (26) adapted to receive at least one brake shoe of the drum brake (22). The braking surface (26) has a circumferential extension in the circumferential direction (C) and an axial extension in the axial direction (a). The brake drum (24) also includes an outer surface (32) at least partially surrounding the braking surface (26). The brake drum (24) further comprises a cooling device (30), the cooling device (30) comprising a set of cooling ducts (34, 36, 38) located between the braking surface (26) and the outer surface (32) and extending at least partially in the axial direction (a). The brake drum (24) comprises an inner axial section (I) and an outer axial section (II) within the axial extension of said braking surface (26), wherein the inner axial section (I) is located closer to the centre plane than the outer axial section (II) when the brake drum (24) is mounted to the vehicle (10) as seen in the axial direction (a). The cooling device (30) also extends through each of the inner axial section (I) and the outer axial section (II). The material of the brake drum (24) located radially between the set of cooling ducts (34, 36, 38) and the braking surface (26) has an inner axial total thermal conductivity at an inner axial cross-section (I), and the material of the brake drum (24) located radially between the set of cooling ducts (34, 36, 38) and the braking surface (26) has an outer axial total thermal conductivity at an outer axial cross-section (II). The inner axial total thermal conductivity is different from the outer axial total thermal conductivity.)

1. A brake drum (24) for a drum brake (22) of a vehicle (10), the vehicle (10) having a longitudinal centre plane (P) extending in a longitudinal direction (L) and a vertical direction (V) and dividing the vehicle (10) into a first longitudinal half and a second longitudinal half, the longitudinal direction (L) extending in a direction parallel to an intended direction of travel of the vehicle (10), the brake drum (24) comprising a brake surface (26), the brake surface (26) being adapted to receive at least one brake shoe of the drum brake (22), the brake surface (26) having a circumferential extension in the circumferential direction (C) and an axial extension in the axial direction (A), the brake drum (24) further comprising an outer surface (32) at least partially surrounding the brake surface (26), the brake drum (24) further comprising a cooling device (30), the cooling device (30) comprising a set of cooling ducts (34, 36, 38) located between the braking surface (26) and the outer surface (32) and extending at least partially in the axial direction (A), the brake drum (24) comprising an inner axial section (I) and an outer axial section (II) located within the axial extension of the braking surface (26), wherein the inner axial section (I) is located closer to the centre plane than the outer axial section (II) as seen from the axial direction (A) when the brake drum (24) is mounted to the vehicle (10), the cooling device (30) further extending through each of the inner axial section (I) and the outer axial section (II), radially located in the set of cooling ducts (34), 36, 38) and the braking surface (26) has an inner axial total thermal conductivity at the inner axial section (I), and the material of the brake drum (24) radially between the set of cooling ducts (34, 36, 38) and the braking surface (26) has an outer axial total thermal conductivity at the outer axial section (II), characterized in that the inner axial total thermal conductivity is different from the outer axial total thermal conductivity.

2. The brake drum (24) of claim 1, wherein the inner axial total thermal conductivity is less than the outer axial total thermal conductivity.

3. The brake drum (24) according to claim 1 or claim 2, wherein the set of cooling ducts (34, 36, 38) collectively have an inner axial cooling cross-sectional area (A) at the inner axial cross-section (I)_I) The set of cooling ducts (34, 36, 38) having in common an outer axial cooling cross-sectional area (A) at the outer axial cross-section (II)_II) Said inner axial cooling cross-sectional area (A)_I) Different from the external axial cooling cross-sectional area (A)_II)。

4. The brake drum (24) of claim 3, wherein the inner axial cooling cross-sectional area (A)_I) Is smaller than the external axial cooling cross-sectional area (A)_II)。

5. Brake drum (24) according to claim 3 or claim 4, wherein the inner axial cooling cross-sectional area (A)_I) And the outer axial cooling cross-sectional area (A)_II) Of the larger one is larger than the inner axial cooling cross-sectional area (A)_I) And the outer axial cooling cross-sectional area (A)_II) Is at least 30%, preferably at least 40% larger.

6. The brake drum (24) according to claim 4 or claim 5, wherein the two or more cooling duct portions (38', 38 ") of the set of cooling ducts (34, 36, 38) at the outer axial section (II) are connected to a common cooling duct portion (38"') of the set of cooling ducts (34, 36, 38) at the inner axial section (I).

7. Brake drum (24) according to any one of the preceding claims, wherein the set of cooling ducts (34, 36, 38) has an average radial distance (r) from the braking surface (26) at the inner axial section (I)_I) Said group of coolingThe ducts (34, 36, 38) having an average radial distance (r) from the braking surface (26) at the outer axial section (II)_II) The average radial distance (r) of the inner axial section (I)_I) The average radial distance (r) being different from the outer axial section (II)_II)。

8. Brake drum (24) according to claim 7, wherein the average radial distance (r) of the inner axial section (I)_I) Greater than the average radial distance (r) of the outer axial section (II)_II)。

9. Brake drum (24) according to claim 7 or claim 8, wherein the average radial distance (r) of the inner axial section (I)_I) And the average radial distance (r) of the outer axial section (II)_II) Is greater than the average radial distance (r) of the inner axial section (I)_I) And the average radial distance (r) of the outer axial section (II)_II) Is at least 30%, preferably at least 40% greater.

10. Brake drum (24) according to any one of the preceding claims, wherein at least one cooling duct of the set of cooling ducts (34, 36, 38) extends in axial direction over at least 90%, preferably over 100%, of the axial extension of the braking surface (26).

11. Brake drum (24) according to any one of the preceding claims, wherein the distance between the inner axial section (I) and the outer axial section (II) in the axial direction (A) is at least 10%, preferably at least 20%, of the axial extension of the braking surface (26).

12. Brake drum (24) according to any one of the preceding claims, wherein the greater one of the total axial thermal conductivity and the total axial thermal conductivity is at least 30%, preferably at least 40% greater than the other one of the total axial thermal conductivity and the total axial thermal conductivity.

13. A drum brake (22) for a vehicle (10), the drum brake (22) comprising a brake shoe (28) and a brake drum (24) according to any one of the preceding claims.

14. A vehicle (10) comprising a brake drum (24) according to any one of claims 1-12 and/or a drum brake (22) according to claim 13.

Technical Field

The present invention relates to a brake drum for a drum brake of a vehicle. Furthermore, the invention relates to a drum brake for a vehicle. The invention further relates to a vehicle.

The invention may be applied to heavy vehicles such as trucks, buses and construction equipment. Although the invention will be described in relation to a truck, the invention is not limited to this particular vehicle, but may also be used in other vehicles, such as buses and construction equipment.

Background

The vehicle may include a plurality of wheels. Furthermore, one or more wheels may be equipped with drum brakes. A drum brake comprises a brake drum and brake shoes adapted to abut a braking surface of the brake drum for braking a wheel.

To control the cooling of the brake drum, US 2003/0178270 a1 proposes to arrange ventilation openings in the brake drum. However, US 2003/0178270 a1 may result in an undesirably large amount of cooling of certain areas of the brake drum.

Disclosure of Invention

It is an object of the present invention to provide a brake drum whose cooling can be controlled in a suitable manner.

This object is achieved by a brake drum according to claim 1.

The invention thus relates to a brake drum for a drum brake of a vehicle. The vehicle has a longitudinal center plane extending in a longitudinal direction and a vertical direction and dividing the vehicle into a first longitudinal half and a second longitudinal half. The longitudinal direction extends in a direction parallel to the intended direction of travel of the vehicle. The brake drum includes a braking surface adapted to receive at least one brake shoe of the drum brake. The braking surface has a circumferential extension in the circumferential direction and an axial extension in the axial direction. The brake drum also includes an outer surface at least partially surrounding the braking surface.

The brake drum further comprises a cooling device comprising a set of cooling ducts located between the braking surface and the outer surface and extending at least partially in the axial direction. The brake drum includes an inner axial cross-section and an outer axial cross-section within the axial extension of the braking surface, wherein the inner axial cross-section is located closer to the center plane than the outer axial cross-section when the brake drum is mounted to the vehicle as viewed from the axial direction. The cooling means also extends through each of the inner and outer axial cross-sections.

The material of the brake drum, which is located radially between the set of cooling ducts and the braking surface, has an inner axial total thermal conductivity at the inner axial section. The material of the brake drum, which is located radially between the set of cooling ducts and the braking surface, has an outer axial total thermal conductivity at an outer axial cross-section.

According to the invention, the total thermal conductivity in the inner axial direction is different from the total thermal conductivity in the outer axial direction.

In general, the term "thermal conductivity" of a plate may be defined as the amount of heat that passes through a plate of a particular area and thickness per unit time when the temperatures of opposite sides of the plate differ by 1 kelvin. The thermal conductivity depends on the thermal conductivity of the material of the plate, the thickness of the plate and the area of the plate.

Thus, as used herein, the term "axial total thermal conductivity" refers to the amount of heat that passes through the material of the brake drum that is radially between the set of cooling conduits and the braking surface, in a unit of time, for a predetermined length unit of the brake drum cross-section in the axial direction, assuming the brake drum cross-section is constant, when the temperatures of the opposite faces of the brake drum (i.e., the braking surface and the surface of the cooling conduits) differ by 1 kelvin. The "axial total thermal conductivity" depends on the thermal conductivity of the material, the thickness of the material, and the area of the material.

Due to the fact that the total thermal conductivity in the inner axial direction differs from the total thermal conductivity in the outer axial direction, the cooling of the brake drum can be controlled in a suitable manner. For example, depending on the design of a drum brake having a brake drum and at least one brake shoe, it may be desirable for one part of the brake drum to cool more than another part thereof, and such controlled cooling may be achieved by the above-mentioned difference in total thermal conductivity.

Optionally, the inner axial total thermal conductivity is less than the outer axial total thermal conductivity.

Due to the fact that the total thermal conductivity in the inner axial direction is smaller than the total thermal conductivity in the outer axial direction, the cooling of the braking surface towards the outside (i.e. away from the above-mentioned centre plane) will be larger than the cooling towards the inside (i.e. towards the above-mentioned centre plane). This in turn means a suitable braking capability of a drum brake (in which the brake shoes are adapted to abut the inner side of the braking surface) since the braking effect obtained when the brake shoes abut the braking surface can benefit from the brake surface contacting the brake shoes not being excessively cooled. In other words, the braking effect of the drum brake may be appropriate if the temperature of the cooling surface adapted to receive the brake shoes is at or above a certain temperature.

Optionally, the set of cooling ducts collectively have an inner axial cooling cross-sectional area at the inner axial cross-section. The set of cooling ducts collectively have an outer axial cooling cross-sectional area at an outer axial cross-section. The inner axial cooling cross-sectional area is different from the outer axial cooling cross-sectional area.

Arranging the cooling ducts such that the inner axial cooling cross-sectional area is different from the outer axial cooling cross-sectional area means that the inner axial total thermal conductivity is different from the outer axial total thermal conductivity. Thus, by varying the total cross-sectional area between two axial sections of the brake drum means a preferred cooling of the brake drum.

Optionally, the inner axial cooling cross-sectional area is smaller than the outer axial cooling cross-sectional area. Arranging the cooling ducts such that the inner axial cooling cross-sectional area is smaller than the outer axial cooling cross-sectional area means that the inner axial total thermal conductivity is smaller than the outer axial total thermal conductivity.

Optionally, the larger of the inner and outer axial cooling cross-sectional areas is at least 30%, preferably at least 40% larger than the other of the inner and outer axial cooling cross-sectional areas.

Optionally, the two or more cooling duct portions of the set of cooling ducts at the outer axial cross-section are connected to a common cooling duct portion of the set of cooling ducts at the inner axial cross-section.

Optionally, the set of cooling ducts has an average radial distance from the braking surface at the inner axial cross-section and the set of cooling ducts has an average radial distance from the braking surface at the outer axial cross-section. The average radial distance of the inner axial cross-section is different from the average radial distance of the outer axial cross-section.

The thermal conductivity of the material between the cooling conduit and the braking surface depends on the thickness of the material. Thus, arranging the cooling ducts such that the average radial distance of the inner axial cross-section differs from the average radial distance of the outer axial cross-section means that the inner and outer cross-sections have different thermal conductivities.

Optionally, the average radial distance of the inner axial cross-section is greater than the average radial distance of the outer axial cross-section. Arranging the cooling ducts such that the average radial distance of the inner axial cross-section is larger than the average radial distance of the outer axial cross-section means that the inner axial total thermal conductivity is smaller than the outer axial total thermal conductivity.

Optionally, the larger of the average radial distance of the inner axial cross-section and the average radial distance of the outer axial cross-section is at least 30%, preferably at least 40% larger than the other of the average radial distance of the inner axial cross-section and the average radial distance of the outer axial cross-section.

Optionally, at least one cooling duct of the set of cooling ducts extends in the axial direction over at least 90%, preferably 100%, of the axial extension of the braking surface. This extension of the set of cooling ducts implies a proper cooling of the braking surface.

Optionally, the distance between the inner axial section and the outer axial section in the axial direction is at least 10%, preferably at least 20% of the axial extension of the braking surface.

Optionally, the greater of the inner and outer axial total thermal conductivities is at least 30%, preferably at least 40% greater than the other of the inner and outer axial total thermal conductivities.

A second aspect of the invention relates to a drum brake for a vehicle. A drum brake comprises a brake shoe and a brake drum according to the first aspect of the invention.

A third aspect of the invention relates to a vehicle comprising a brake drum according to the first aspect of the invention and/or a drum brake according to the second aspect of the invention.

Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.

Drawings

The following is a more detailed description of embodiments of the invention, reference being made to the accompanying drawings by way of example.

In the drawings:

figure 1 is a schematic side view of a vehicle,

figure 2 is a schematic partially sectioned perspective view of a wheel,

FIG. 3 is a schematic perspective partial cross-sectional view of an embodiment of a brake drum, an

FIG. 4 is a cross-sectional side view of another embodiment of a brake drum.

Detailed Description

The invention will be described below in relation to a vehicle in the form of a truck 10 such as that shown in figure 1. The truck 10 should be seen as an example of a vehicle which may comprise a control unit according to the invention, on which the method of the invention may be performed. However, the present invention may be implemented in a variety of different types of vehicles. Merely by way of example, the invention may be implemented in a truck, trailer, car, bus, work machine (such as a wheel loader), or any other type of construction equipment.

The vehicle 10 of fig. 1 includes a set of wheels 12, 14 adapted to be supported by a ground surface 16. While the embodiment of the vehicle 10 of FIG. 1 includes a pair of front wheels 12 and a pair of rear wheels 14, it is of course contemplated that other embodiments of the vehicle 10 may include fewer or more wheels.

Furthermore, fig. 1 shows that the vehicle 10 has a longitudinal center plane P which extends in the longitudinal direction L and the vertical direction V and divides the vehicle into a first longitudinal half and a second longitudinal half. Fig. 1 also shows that the vehicle 10 has an extension in a transverse direction T, which is perpendicular to each of the longitudinal direction L and the vertical direction V.

Fig. 2 shows a portion of the wheel 12. The wheel 12 of fig. 2 is illustrated as one of the front wheels in the set of wheels of fig. 1, but the wheel of fig. 2 may of course be used for other vehicles as well as other vehicle types.

The wheel 12 shown in fig. 2 includes a hub 18, the hub 18 being adapted to be connected to an axle (not shown) of a vehicle (not shown in fig. 2). The hub 18 is connected to a rim 20 adapted to receive a tire (not shown). Further, the wheel 12 of fig. 2 includes a drum brake 22, which drum brake 22 in turn includes a brake drum 24 having a braking surface 26. The drum brake 22 further comprises brake shoes 28, and the brake surface 26 is adapted to receive the brake shoes 28.

Further, the brake drum 24 includes a cooling device 30, which cooling device 30 includes a set of cooling conduits through which a cooling fluid (such as air) may flow for cooling the brake drum 24. Preferably, and as shown in fig. 2, at least one cooling duct of the set of cooling ducts extends in the axial direction over at least 90%, preferably over 100%, of the axial extension of the braking surface 26.

Fig. 3 shows an embodiment of a brake drum 24 for a drum brake 22. The brake drum 24 includes a braking surface 26, the braking surface 26 being adapted to receive at least one brake shoe of a drum brake (not shown in FIG. 3). The braking surface 26 has a circumferential extension in the circumferential direction C and an axial extension in the axial direction a. For example only, and as shown in fig. 3, the axial direction a may be parallel to the transverse direction T. The brake drum 24 also includes an outer surface 32 that at least partially surrounds the braking surface 26.

The brake drum 24 further includes a cooling device 30, the cooling device 30 including a set of cooling conduits 34, 36, 38 located between the braking surface 26 and the outer surface 32 and extending at least partially in the axial direction a. Thus, the cooling device 30 extends radially (i.e. in the radial direction R) outside the braking surface 26. Thus, the cooling device 30 does not include any openings in the braking surface 26.

The brake drum 24 includes an inner axial section I and an outer axial section II within the axial extension of the braking surface 26. When the brake drum 24 is mounted to the vehicle 10, the inner axial section I is located closer to the center plane P than the outer axial section II, as viewed from the axial direction a. The cooling device 30 extends through each of the inner axial section I and the outer axial section II.

For example only, the braking surface 26 may have a braking surface extension in the axial direction a, and the distance between the inner axial cross section I and the outer axial cross section II in the axial direction is at least 10%, preferably at least 20% of the braking surface extension.

Furthermore, the material of the brake drum 26 located radially between the set of cooling ducts 34, 36, 38 and the braking surface 26 has an inner axial total thermal conductivity at the inner axial section I. The material of the brake drum 26 located radially between the set of cooling ducts 34, 36, 38 and the braking surface 26 has an outer axial total thermal conductivity at the outer axial section II. The inner axial total thermal conductivity is different from the outer axial total thermal conductivity. For example only, the greater of the inner and outer axial total thermal conductivities may be at least 30%, preferably at least 40% greater than the other of the inner and outer axial total thermal conductivities.

As will be explained further below, in the embodiment shown in fig. 3, the inner axial total thermal conductivity is less than the outer axial total thermal conductivity. However, it is also contemplated that in other embodiments of the invention, the inner axial total thermal conductivity may be greater than the outer axial total thermal conductivity.

As indicated in the summary of the invention, the term "thermal conductivity" of a plate can be defined as the amount of heat that passes through a plate of a particular area and thickness per unit time when the temperatures of opposite sides of the plate differ by 1 kelvin. The thermal conductivity of the plate depends on the thermal conductivity of the material of the plate, the thickness of the plate and the area of the plate. In general, the term "thermal conductivity" of a plate may be defined in terms of kA/L, where:

k-the thermal conductivity of the material of the plate;

a is the area of the plate, and

l is the thickness of the plate.

Similarly, the term "axial total thermal conductivity" refers to the heat of the material passing through the brake drum radially between the set of cooling ducts of the cooling device and the braking surface, in a unit time-for a predetermined length unit of the section of the brake drum in the axial direction, assuming a constant section of the brake drum-when the temperatures of the opposite faces of the brake drum (i.e. the braking surface and the surface of the cooling ducts) differ by 1 kelvin.

The axial total thermal conductivity of each of the two sections can be determined in a number of ways. For example only, the axial total thermal conductivity may be determined by generating a computer model (such as a finite element model) of each of the two cross sections and applying a temperature difference of 1 kelvin from the braking surface to the surface of the cooling conduit in order to determine a value representing the axial total thermal conductivity.

Alternatively, a simplified model may be used to determine the axial total thermal conductivity, for example according to kwd/L_avWherein:

k — the thermal conductivity of the material of the brake drum 26;

w — the total width of the cooling duct measured along the circumference of the brake drum 26;

d is a predetermined length unit in the axial direction, and

L_avthe average thickness between the braking surface and the cooling duct.

It should be noted that the predetermined length unit d in the axial direction may be set to any positive value as long as the inner axial cross section I and the outer axial cross section II take the same value.

As can be appreciated from the above, the axial overall thermal conductivity increases with increasing overall width w of the cooling duct. In addition, the total thermal conductivity in the axial direction is dependent on the average thicknessL_avIs increased. Furthermore, by using different materials with different thermal conductivities for the two sections I, II, different values of the axial total thermal conductivity of the two sections I, II can be obtained. Of course, any combination of the above three options is contemplated.

The above-mentioned difference in the axial total thermal conductivity can be achieved in a number of different ways. In the embodiment shown in fig. 3, the difference in axial total thermal conductivity is obtained by different cross-sectional areas at the inner and outer axial sections, respectively. In other words, the difference in axial overall thermal conductivity may be obtained by different overall widths of the cooling ducts.

Thus, in the embodiment of FIG. 3, the set of cooling ducts collectively have an inner axial cooling cross-sectional area A at the inner axial cross-section I_I. Furthermore, the group of cooling ducts has an outer axial cooling cross-sectional area A in common at the outer axial cross-section II_II. Internal axial cooling cross-sectional area A_IDifferent from the external axial cooling cross-sectional area A_II. By way of non-limiting example, the inner axial cooling cross-sectional area A_IAnd the outer axial cooling cross-sectional area A_IIMay be larger than said inner axial cooling cross-sectional area a_IAnd the outer axial cooling cross-sectional area A_IIIs at least 30%, preferably at least 40% larger. In the embodiment of FIG. 3, the inner axial cooling cross-sectional area A_IIs smaller than the external axial cooling section area A_II。

Different axial cooling cross-sectional area A_I、A_IICan be obtained in a number of different ways. For example only, one or more of the cooling ducts 34, 36, 38 may have a different cross-sectional area along the axial direction a. By way of non-limiting example, one or more of the cooling ducts 34, 36, 38 may be funnel-shaped and thus wider at the outer axial section II than at the inner axial section I, or vice versa.

However, fig. 3 shows an embodiment of the brake drum 26 in which two or more cooling duct portions 38', 38 "of the set of cooling ducts at the outer axial section II are connected to a common cooling duct portion 38"' of the set of cooling ducts at the inner axial section I. Thus, the cooling duct 38 in the embodiment of fig. 3 is substantially Y-shaped.

Instead of or in addition to arranging cooling ducts 34, 36, 38 with different cross-sectional areas in the axial direction a, different axial total thermal conductivities may be obtained in other ways.

For example, as mentioned above, the total thermal conductivity in the axial direction is a function of the average thickness L_avIs increased. To this end, referring to FIG. 4, a cross-sectional view of another embodiment of the brake drum 26 in the V-T plane is shown. In the embodiment of fig. 4, the set of cooling ducts 34, 36, 38 has an average radial distance r from the braking surface 26 at the inner axial section I_I. The above average distance r_IOnly one cooling duct 34 is shown in fig. 4. Furthermore, the set of cooling ducts has an average radial distance r from the braking surface at the outer axial section II_II. Average radial distance r of inner axial section I_IMean radial distance r different from the outer axial section II_II。

By way of example only, the average radial distance r of the inner axial section I _IAverage radial distance r from outer axial section II_IILarger than the average radial distance r of the inner axial section I_IAverage radial distance r from outer axial section II_IIIs at least 30%, preferably at least 40% greater.

In the embodiment of fig. 4, the average radial distance r of the inner axial section I_IGreater than the mean radial distance r of the outer axial section II_II. However, it is also conceivable for the average radial distance r to be the inner axial section I_ILess than the mean radial distance r of the outer axial section II_IIExamples of (1).

It is to be understood that the invention is not limited to the embodiments described above and shown in the drawings; rather, the skilled person will recognise that many variations and modifications may be made within the scope of the appended claims. By way of example only, while FIG. 3 illustrates an embodiment having different cross-sectional areas at the inner and outer axial cross-sections I and II, FIG. 4 illustrates an embodiment having different average radial distances r_I、r_IIBut of course the present invention is envisagedEmbodiments of the invention may include a combination of different cross-sectional areas and different average radial distances between two cross-sections I, II.