GAS TURBINE BURNER

Abstract

A gas turbine burner comprising: a combustion tube (11) forming a combustion chamber (9) on an inner surface; and a fuel injection device (13) provided on an upper section of the combustion tube (11) and configured to inject fuel into the combustion chamber (9), the fuel injection device (13) comprising a fuel injection section (23) with a plurality of fuel injection holes, each configured to inject fuel in a direction containing a component perpendicular to an axial direction of the combustion chamber (9), and with a common fuel supply chamber configured to supply fuel to the plurality of fuel injection holes, and an air guide section (25) with an air guide groove (33) configured to direct air for combustion to the fuel injected from each fuel injection hole (29).wherein the fuel injection section (23) has an air guide surface (35) designed to direct the air for combustion and which is located forward in the axial direction of the combustion chamber (9) relative to the fuel injection hole (29), a fuel injection opening of the fuel injection hole (29) is provided on a lower wall surface of a stepped recess which is recessed in a step shape from the air guide surface (35), and the stepped recess (37) extends to the rear end of the fuel injection section (23).

Description

CROSS-REFERENCE TO RELATED REGISTRATION

[0001] The present application is based on and claims priority in accordance with the Paris Convention of Japanese patent application No. 2021-090376, filed on May 28, 2021, the entire disclosure of which is incorporated herein by reference as part of this application. BACKGROUND OF THE INVENTION (Field of Invention)

[0002] The present invention relates to a burner used in a gas turbine engine. (Description of the state of the art)

[0003] In recent years, a gas turbine engine using hydrogen as fuel has been proposed as a means of achieving a so-called low-carbon society. It goes without saying that fuels with a high combustion rate, such as hydrogen-containing fuels, reach high combustion temperatures and therefore readily produce NOx. Furthermore, the combustion of fuels with high combustion rates can lead to a backfire phenomenon, in which a flame generated in the combustion chamber travels back towards the burner.

[0004] Accordingly, a burner is proposed for achieving low-NOx combustion and preventing backfire when using highly reactive gases such as hydrogen as fuel. This burner employs a fuel injection device configured such that a plurality of annular fuel injection elements are arranged concentrically. The fuel is injected radially from multiple fuel injection holes provided in each annular element, and air flows towards the combustion chamber side in a direction essentially perpendicular to the fuel injected from each fuel injection hole (see, for example, patent document 1). With this burner structure, the occurrence of local high-temperature combustion is suppressed by the dispersed multi-point injection of the fuel, thereby achieving low-NOx combustion.In addition, air is supplied to the injected fuel towards the combustion chamber side, thereby suppressing the occurrence of a backfire phenomenon. [State of the art document][Patent document]

[0005] [Patent Document 1] WO2015 / 182154 OVERVIEW OF THE INVENTION

[0006] However, in the burner disclosed in patent document 1, the diameter of each fuel injection hole is small, and therefore, when using a fuel (e.g., a mixed fuel of hydrogen gas and natural gas) whose reactivity is not as high as that of pure hydrogen gas, it is difficult to maintain a stable flame.

[0007] Accordingly, to solve the above problem, it is an object of the present invention to enable the stable maintenance of a flame even when using fuel with comparatively low reactivity in a burner with multi-point injection, which can achieve low NOx combustion and prevent backfire.

[0008] To solve the aforementioned problem, a gas turbine burner according to the present invention comprises: a combustion tube forming a combustion chamber on its inner surface; and a fuel injection device provided on an upper section of the combustion tube and configured to inject fuel into the combustion chamber. The fuel injection device has a fuel injection section with a plurality of fuel injection holes, each configured to inject fuel in a direction containing a component perpendicular to an axial direction of the combustion chamber, and a common fuel supply chamber configured to supply fuel to the plurality of fuel injection holes, and an air guide section with an air guide groove configured to direct air for combustion to the fuel injected from each fuel injection hole.The fuel injection section has an air guide surface designed to direct the air for combustion, and which is oriented axially forward of the combustion chamber relative to the fuel injection hole. A fuel injection port of the fuel injection hole is provided on a lower wall surface of a stepped recess, which is recessed from the air guide surface in a stepped shape.

[0009] In this configuration, the stepped recess is provided on the fuel injection section and the fuel injection opening of the fuel injection hole is formed on the lower wall surface of the stepped recess, making it possible to maintain a stable flame even when using fuel with low reactivity, as will be described in detail later.

[0010] All combinations of at least two designs disclosed in the appended claims and / or the description and / or the appended drawings should also be considered to fall within the scope of the present invention. In particular, any combination of two or more of the appended claims should likewise be considered to fall within the scope of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention will be better understood by the following description of preferred embodiments in conjunction with the accompanying drawings. However, the embodiments and the drawings serve only illustrative and explanatory purposes and are not to be understood as limiting the scope of the present invention in any way as defined in the accompanying claims. In the accompanying drawings, the same reference numerals are used to designate identical parts in the multiple views, and: Fig. Figure 1 shows a block diagram illustrating a schematic configuration of a gas turbine engine in which a burner according to an embodiment of the present invention is used; Fig. Figure 2 shows a partially cutaway perspective view showing the burner according to the embodiment of the present invention; Fig.Figure 3 shows a front view illustrating an example of a burner in the Fig. 2 fuel injection device used; Fig. Figure 4 shows a partially enlarged front view depicting the fuel injection device in Fig. 3; Fig. Figure 5 shows a partially enlarged vertical sectional view depicting the fuel injection device in Fig. 3; Fig. Figure 6 shows a front view illustrating another example of a burner in the Fig. 2 fuel injection device used; Fig. Figure 7 shows a partially enlarged perspective view depicting the fuel injection device in Fig. 3; Fig. Figure 8 shows a partially enlarged top view showing the fuel injection device in Fig. 3; and Fig.Figure 9 shows a result of a CFD combustion analysis of the combustion process in the burner in Fig. 2. Fuel injection device used. DESCRIPTION OF EXECUTION FORMS

[0012] In the following, embodiments of the present invention are described with reference to the drawings, although the present invention is not limited to these embodiments.

[0013] Fig.Figure 1 shows a schematic embodiment of a gas turbine engine GT (hereinafter referred to as a gas turbine for simplicity), in which a burner according to an embodiment of the present invention is used. The gas turbine GT is configured such that supplied air is compressed by a compressor 1 and directed to a burner 3, fuel is injected into the burner 3 and combusted, and a turbine 5 is driven by the resulting high-temperature and high-pressure combustion gas G. The turbine 5 is connected to the compressor 1 via a rotating shaft 7, and the compressor 1 is driven by the turbine 5. The power of the gas turbine GT drives a load L, e.g., a rotor of an aircraft or an electric generator. The burner 3 is, for example, a tube-type burner in which a plurality of burner tubes are arranged annularly around the axis of the gas turbine GT.The type of burner 3 is not limited to the tube type and can, for example, be a ring type.

[0014] As in Fig. As shown in Figure 2, the burner 3 has a combustion tube 11, which forms a combustion chamber 9 on the inside, and a fuel injection device 13, which is attached to an upper section (most upstream section) 13a of the combustion tube 11 and injects fuel and air into the combustion chamber 9. The fuel injected from the fuel injection device 13 is ignited by means of an igniter P provided on the combustion tube 11, thereby forming a flame in the combustion chamber 9. The combustion tube 11 and the fuel injection device 13 are housed concentrically in a substantially cylindrical casing H, which acts as an outer tube of the burner 3. Here, as in Fig.As shown in Figure 2, in the burner 3, the side of the combustion chamber 9 along an axial direction C of the combustion chamber 9 is referred to as the "back" and the opposite side as the "front". In the following description, the axial direction C of the combustion chamber 9 will be referred to as the "axial direction C" for the sake of simplicity.

[0015] In the present embodiment, the burner 3 is designed as a counterflow type, in which the flow directions of air A and of the fuel gas G are opposite to each other. This means that the burner 3 has an air inlet passage 17, which is formed between the housing H and the combustion tube 11, and a support tube 15, which extends forward in a tubular shape from the combustion tube 11. The air supplied by the compressor 1 ( Fig.1) Compressed air A is directed through the air inlet passage 17 into the combustion chamber 9 in the opposite direction to the flow direction of the fuel gas G. The burner 3 can be of an axial flow type, in which the flow directions of the air A and the fuel gas G are the same. A plurality of air inlet holes 19 are provided at a front end section of a circumferential wall of the support tube 15, arranged circumferentially. The air A sent through the air inlet passage 17 passes through the air inlet holes 19 to be introduced into an air supply passage 21 formed in the support tube 15, and the air A is then directed towards the fuel injection device 13 at the rear.

[0016] As in Fig.As shown in Figure 3, the fuel injection device 13 has a fuel injection section 23 and an air guide section 25. In the present embodiment, the fuel injection section 23 and the air guide section 25 are each annular in shape. In this example, a plurality of (four in the example shown) fuel injection sections 23 with different diameters are arranged concentrically to each other and concentrically to the burner 3. Fig. 2), and annular air guide sections 25, which correspond to the shapes of the respective fuel injectors, are provided on the inner circumferential side and the outer circumferential side of each fuel injection section 23. The fuel is supplied to each fuel injection section 23 of the fuel injection device 13 via a fuel supply passage 27 ( Fig. 2).

[0017] As in Fig.As shown in Figure 4, the fuel injection section 23 has a plurality of fuel injection holes 29 for injecting the fuel and a common fuel supply chamber 31 for supplying the fuel F to the plurality of fuel injection holes 29.

[0018] In the present embodiment, a large number of fuel injection holes 29 are arranged at equal intervals in the circumferential direction in each fuel injection section 23. Each fuel injection hole 29 is configured to inject fuel in a direction containing a component perpendicular to the axial direction C. In particular, in this example, as shown in Fig.As shown in Figure 5, each fuel injection hole 29 is configured to inject the fuel F in a direction (in this example, a radial direction R of the annular fuel injection section 23) perpendicular to the axial direction C. More precisely, a vertical cross-section of the fuel injection section 23 in this example has a rectangular shape overall, and the fuel injection holes 29 are configured as holes opening on a wall surface facing outwards in the radial direction R of the fuel injection section 23 and / or on a wall surface facing inwards in the radial direction R.

[0019] As in Fig. As shown in Figure 4, the air guide section 25 has air guide grooves 33 that direct the air A to the respective fuel injection holes 29 for combustion. In particular, in this example, as shown in Fig.As shown in Figure 5, the air guide section 25 is formed as a plate-shaped element in a direction parallel to a plane perpendicular to the axial direction C and is located on the front side relative to the fuel injection hole 29 of the fuel injection section 23 (i.e., on the upstream side in the direction of air flow A). The air guide section 25, provided as described above, is partially cut out in a region on the side of the fuel injection section 23, as shown in Figure 5. Fig. Figure 4 shows how the air guide grooves 33 are formed. In the present embodiment, one air guide groove 33 is provided for each fuel injection hole 29.

[0020] In the fuel injection device 13, configured as described above, the fuel F injected from each fuel injection hole 29 of the fuel injection section 23 is premixed with the air A guided through the air guide groove 33 of the air guide section 25 and then injected as a premixed gas into the combustion chamber 9. In this way, the air A is guided by the air guide section 25 from the upstream side in axial direction C to the fuel F injected from each fuel injection hole 29, causing the fuel F and the air A to cross paths in directions substantially perpendicular to each other. This allows the fuel F and the air A to be mixed uniformly outside the fuel injection device 13.

[0021] The injection direction of the fuel F from the fuel injection hole 29 of the fuel injection section 23 can be any direction that contains a component perpendicular to the axial direction C, and can, for example, be inclined in axial direction C in a range of ±10° relative to the direction perpendicular to the axial direction C.

[0022] The overall design of the fuel injection device 13 is not limited to the preceding example. For example, the shape of the fuel injection section 23 is not limited to that shown in Fig. The ring-shaped form shown in 3 is limited and can be a rectangular shape in a front view (viewed in the axial direction C of the combustion chamber 9), as in a Fig.6. Modification shown. It is not necessary to provide a plurality of fuel injection sections 23 and a plurality of corresponding air guide sections 25, and one fuel injection section 23 and one air guide section 25 may be provided.

[0023] Next, a structure around the fuel injection hole 29 of the fuel injection section 23 will be described in detail.

[0024] As in Fig.As shown in Figure 5, the fuel injection section 23 in the present embodiment has an air guide surface 35 that directs air A on the front side relative to the fuel injection hole 29. The air guide surface 35 is designed as a flat surface extending parallel to the axial direction C. On the rear side of the air guide surface 35, a stepped recess 37 is formed, which is recessed from the air guide surface 35 in a stepped shape. In this example, the stepped recess 37 extends to the rear end of the fuel injection section 23. A fuel injection opening 29a of the fuel injection hole 29 is formed on a lower wall surface 39 of the stepped recess 37. For the sake of simplicity, in the following description, a direction towards the lower wall side of the stepped recess 37 can be referred to as the "downward direction" and a direction towards the side of the air guide surface 35 as the "upward direction".

[0025] It has been confirmed that the formation of the fuel injection opening 29a of the fuel injection hole 29 on the lower wall surface 39 of the stepped recess 37 enables the stable maintenance of a flame even when using fuel F with low reactivity, as described later. This is assumed to be due to the following effects. (1) The side walls of the stepped recess 37 safely guide the fuel F injected from the fuel injection opening 29 towards the upward and rearward side on which the air A flows. (2) The flow of air A, which has flowed along the smooth air guide surface 35, is disturbed by the stepped recess 37, so that a vortex is formed, which promotes the mixing with the fuel F. (3) The presence of the stepped recess 37 protects the fuel F injected from the fuel injection port 29 from a strong flow of air A.

[0026] In this example, as in Fig.As shown in Figure 7, the stepped recess 37 is formed in a shape with a plurality of steps (two steps in this example) and has a lower wall surface 39 at the lowest step and a lower wall surface 40 at the first step. In the present embodiment, the fuel injection opening 29a of the fuel injection hole 29 is formed on the lower wall surface 39 in the lowest step. In a case where the stepped recess 37 has a single-step structure, the fuel injection opening 29a of the fuel injection hole 29 is formed on the lower wall surface 40 in the first step. For the sake of simplicity, in the following description, the lowest step portion of the stepped recess 37 can be referred to as the first step section 37a, and a second step portion can be referred to as a second step section 37b.

[0027] In the present embodiment, both side walls 41 of the stepped recess 37 are formed in such a way that the distance between the two side walls 41 gradually increases from the front end of the stepped recess 37 to its rear end (i.e., to the side of the combustion chamber 9). More precisely, as in Fig. As shown in Figure 8, the two side walls 41 of the stepped recess 37 extend such that they form parts of two equal sides of a virtual isosceles triangle, the underside of which is the rear end of the fuel injection section in a top view. In the example shown, a front end section 43 of the stepped recess 37, corresponding to the vertex of the virtual isosceles triangle, is formed in a curved, forward-cut shape.

[0028] By forming both side walls 41 of the stepped recess 37 in such a way that the distance between the two side walls 41 gradually increases from the front end to the rear end, as described above, the flow velocity of the mixed gas stream of air A and fuel F injected from the fuel injection port 29 gradually decreases due to the expansion of the flow path as the mixed gas moves towards the side of the combustion chamber 9. Similarly, by forming the stepped recess 37 in a shape with a plurality of steps, the flow velocity of the fuel F injected from the fuel injection port 29 gradually decreases due to the expansion of the flow path as the fuel F moves upwards. This further promotes the mixing of fuel F and air A.

[0029] In a top view, the range of an angle α formed by either side wall 41 is preferably 0° < α ≤ 80°, particularly preferably 20° ≤ α ≤ 60°, and even more preferably 25° ≤ α ≤ 40°. In the example shown, the angles α1, α2 between the side walls 41 at the respective steps in the multi-stepped recess 37 are the same α, but if the stepped recess 37 has a plurality of steps, the angles between the side walls 41 at the respective steps can be different.

[0030] It is not necessary to form both side walls 41 of the stepped recess 37 in such a way that the distance between the two side walls 41 gradually increases from the front end to the rear end as described above, and α can be 0° or can be a negative angle (an angle such that the distance between the two side walls 41 gradually narrows from the front end to the rear end). In a top view, the shapes of the two side walls 41 need not be straight, as shown in the drawings, but can, for example, be curved.

[0031] It is not necessary for the stepped recess 37 to have a plurality of steps, and the stepped recess 37 may have only one step. In a case where the stepped recess 37 has a plurality of steps, the number of steps is not limited to two, as shown in the drawings, but may also be three or more.

[0032] The position of the fuel injection port 29a of the fuel injection hole 29 in the lower wall surface 39 at the lowest step of the stepped recess 37 is not particularly restricted. However, as described later, it has been confirmed that a flame can be maintained more stably if the distance D between the center point O of the fuel injection port 29a of the fuel injection hole 29 and the front endpoint at the lowest step of the stepped recess 37 is shorter. This is because the effects (1) to (3) described above are more pronounced by the stepped recess 37 when the fuel injection port 29 is located near the rear end wall of the stepped recess 37. Therefore, if the hole diameter of the fuel injection port 29 is denoted by d, the range of the distance D is preferably D ≤ 2d and can be D ≤ d or D = d / 2 (i.e.,, the front end of the fuel injection opening 29 coincides with the front endpoint at the lowest stage of the stepped recess 37).

[0033] The position of the stepped recess 37 relative to the air guide section 25 is not particularly restricted. However, the flow velocity of the combustion air A is greatest near the air guide groove 33 of the air guide section 25, which is why the fuel F is preferably injected near the air guide groove 33. Furthermore, from the perspective of backfire prevention, the entry of the fuel F into the air A preferably occurs on the downstream side of the air guide section 25. From this perspective, the front end of the stepped recess 37 is preferably directed rearward with respect to the front end of the air guide section 25.Furthermore, the front end of the stepped recess 37 is preferably located in the region of the thickness (the dimension in the axial direction C) of the air guide section 25, and the front end of the stepped recess 37 preferably coincides with the middle position of the thickness of the air guide section 25. In addition, the entire stepped recess 37 is preferably enclosed in the width-direction region of the air guide groove 33.

[0034] Specific dimensions of each part of the stepped recess 37 are selected according to the specifications required for the burner 3, such as power, size, and fuel F to be used. For example, the hole diameter d of the fuel injection port 29, in the case of the fuel injection port 29 described above for multi-point injection, can be approximately 0.5 mm to 1.0 mm. In this case, the axial dimension C and the lateral dimension of the stepped recess 37 can be several millimeters.

[0035] Various dimensions of the fuel injection device, differing from those described above, according to the [reference to be added]. Fig. The dimensions shown in the embodiment 7 can be specified as required and can, for example, be set within the following range. However, these dimensions are not limited to the following ranges.

[0036] A distance c from the center point O of the combustion injection opening 29a of the fuel injection hole 29 to the rear end of the lower wall surface 39 of the first stage section 37a must be not less than 1.5 mm and not more than 4.0 mm. A height h1 of the first stage section 37a must be not less than 0.1 mm and not more than 1.5 mm, and a height h2 of the second stage section 37a must be not less than 0.2 mm and not more than 3.0 mm. A distance D1 from the center point O of the combustion injection opening 29a of the fuel injection hole 29 to the front endpoint of the first stage section 37a must be not less than 0.4 mm and not more than 2.6 mm, and a similar distance D2 for the second stage section 37b must be not less than 0.2 mm and not more than 1.9 mm.A radius of curvature r1 of a curved part at the front end of the first step section 37a must be not less than 0.6 mm and not more than 2.0 mm, and a similar radius of curvature r2 for the second step section 37b must be not less than 0.2 mm and not more than 1.5 mm.

[0037] The effects of the burner 3 according to the present embodiment, which is designed as described above, are described with reference to a result of a CFB combustion analysis.

[0038] In the CFD combustion analysis, a conventional fuel injection device without the stepped recess 37 was used as a comparison example, and the in Fig. The fuel injection device 13 shown in Figure 7 was used as an example. The side walls 41 of the stepped recess 37 in the example were formed in a shape (α = 30°) that widens straight towards the rear, as shown in Figure 7. Fig. 8 shown. [Table 1] Specifications Example comparative example Number of steps in the stepped recess 2 no stepped recess Distance between the center of the fuel injection hole and the front end of the stepped recess (d: hole diameter) d / 2

[0039] The fuels used were 100% hydrogen gas (hereinafter referred to simply as "hydrogen fuel") and a mixed fuel of hydrogen gas and natural gas (volume ratio of hydrogen gas and natural gas = 60:40; hereinafter referred to simply as "mixed fuel"), and a comparison of temperatures at rated load, part load, and no load was performed for each fuel. Conditions under which these fuels were burned in the aforementioned devices were simulated, and temperature distributions were compared, the results of which were presented in Fig. 9 is shown.

[0040] As in Fig.As shown in Figure 9, a flame could be maintained in the fuel injection device according to the comparative example in the case of hydrogen fuel, but in the case of mixed fuel, a flame could not be maintained at any of the temperatures. On the other hand, a flame could be maintained in the fuel injection device in the example for all temperatures, including the temperature at no load. Thus, it was confirmed that by providing the stepped recess in the fuel injection section, the flame retention performance for low-reactivity fuel is significantly improved, and in particular, by providing the fuel injection hole at the front end of the lower wall surface of the stepped recess, a significantly favorable flame retention performance is demonstrated.

[0041] The embodiment in which the distance D between the center point O of the fuel injection opening 29a of the fuel injection hole 29 and the front endpoint at the lowest step of the stepped recess 37 is defined as D = d / 2 (i.e. the front end of the fuel injection hole 29 coincides with the front endpoint at the lowest step of the stepped recess 37) can be combined with an embodiment in which the stepped recess 37 has one step.

[0042] The type of fuel F used in the burner 3 according to the present embodiment is not particularly limited. However, as described above, the provision of the stepped recess 37 in the fuel injection section 23 significantly improves the flame retention performance for fuel F with lower reactivity than hydrogen gas. Therefore, for example, it is possible to ensure stable operation and simultaneously reduce fuel costs by using the hydrogen-natural gas mixture used in the CFD combustion analysis mentioned above.

[0043] As described above, with the gas turbine burner 3 according to the present embodiment it is possible to maintain a stable flame even when fuel F with comparatively low reactivity is used in the burner 3 with multi-point injection, thereby achieving low NOx combustion and preventing backfire.

[0044] Although the present invention has been described above in connection with preferred embodiments with reference to the accompanying drawings, numerous additions, modifications, or omissions can be made without departing from the essential nature of the present invention. Accordingly, such additions, modifications, or omissions are to be interpreted as falling within the scope of the present invention. [Reference sign] 3 burners 9 Combustion chamber 11 Burner tube 13 Fuel injection device 23 Fuel injection section 25 Air duct section 29 Fuel injection hole 33 Air guide groove 35 air distribution area 37-step recess 39 lower wall surface of the stepped recess 41 Side wall of the stepped recess G gas turbine engine P igniter QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] JP 2021090376

[0001] WO 2015182154

[0005]

Claims

[1] Gas turbine burner having: a combustion tube forming a combustion chamber on an inner side; and a fuel injection device provided at an upper portion of the combustion tube and configured to inject fuel into the combustion chamber, wherein the fuel injection device comprises a fuel injection portion having a plurality of fuel injection holes each configured to inject the fuel in a direction including a component perpendicular to an axial direction of the combustion chamber, and a common fuel supply chamber configured to supply the fuel into the plurality of fuel injection holes, and an air guide portion having an air guide groove configured to guide air for combustion to the fuel injected from each fuel injection hole, wherein the fuel injection portion has an air guide surface configured to guide the air for combustion and located forward in the axial direction of the combustion chamber relative to the fuel injection hole, and a fuel injection opening of the fuel injection hole is provided on a lower wall surface of a stepped recess recessed in a step shape from the air guide surface. [2] A gas turbine combustor according to claim 1, wherein both side walls of the stepped recess are formed in such a shape that a distance between both side walls increases from a front end of the stepped recess to a rear end thereof. [3] A gas turbine combustor according to claim 1 or 2, wherein the stepped recess is formed in a shape recessed with a plurality of steps. [4] A gas turbine combustor according to any one of claims 1 to 3, wherein a relationship between a hole diameter d of the fuel injection hole and a distance D between a center of the fuel injection port and a front end of a bottom wall of the stepped recess is d / 2 ≤ D ≤ 2d. [5] A gas turbine combustor according to claim 4, wherein the fuel injection hole is formed at such a position that a front end of the fuel injection port coincides with the front end of the bottom wall of the stepped recess. [6] Gas turbine burner according to one of claims 2 and 3 to 5, which relate to claim 2, wherein an angle α formed by both side walls is in a range of 0° < α ≤ 80°.