A mounting structure for a combustion chamber dynamic pressure sensor

Abstract

The present invention discloses a mounting structure for a combustion chamber dynamic pressure sensor, wherein the combustion chamber wall of the combustion chamber includes a first wall layer 1 and a second wall layer 2 arranged outside the first wall layer 1, and is characterized in that the mounting structure includes: a first mounting hole, the first mounting hole is arranged on the first wall layer 1 and penetrates the first wall layer 1; a second mounting hole, the second mounting hole is arranged on the second wall layer 2 and penetrates the second wall layer 2; a mounting assembly, the mounting assembly includes a mounting seat 3, the mounting seat 3 is used to carry a dynamic pressure sensor 12, the mounting seat 3 is installed in the first mounting hole, the second mounting hole allows the dynamic pressure sensor 12 to pass through the second wall layer 2 to allow the dynamic pressure sensor 12 to directly measure the pulsating pressure inside the combustion chamber, and a gap is formed between the second mounting hole and the dynamic pressure sensor 12; a gap compensation device, the gap compensation device is installed on the outside of the second wall layer 2, and the gap compensation device at least partially blocks the gap outside the gap to compensate for the intake amount.

Description

Technical Field

[0001] The present invention relates to a combustion chamber technology of a gas turbine, and in particular to a combustion chamber dynamic pressure sensor mounting structure. Background Art

[0002] During the operation of heavy-duty gas turbines, the heat release pulsation of the flame can easily couple with the inherent acoustic pulsation of the combustion chamber to form acoustic pressure resonance, causing combustion thermoacoustic instability, affecting emissions and combustion stability, and even causing damage to the gas turbine in severe cases. Therefore, combustion pulsation pressure is very important for combustion state assessment, abnormality analysis and combustion system improvement, and is an important parameter for gas turbine operation monitoring.

[0003] Combustion chamber dynamic pressure can be measured directly or indirectly. Patent CN213019742U discloses an indirect measurement method, in which a pulsating pressure sensor is mounted on the combustion chamber housing via a designed mounting bracket. The bracket consists of two hollow cylindrical bodies joined at their ends. The upper opening of the bracket forms a cooling air inlet, providing direct access to the cooling gas, while an upper side opening provides access for the sensor cable. The lower opening of the bracket forms a connection for the pressure supply line, into which the pulsating pressure sensor is inserted, with the measuring end sealingly fitting the pressure supply line.

[0004] Indirect measurement compromises accuracy and is unsuitable for the high-precision requirements of combustion monitoring. Direct measurement offers high precision and requires placing sensors directly within the combustion chamber structure. However, due to the complex design of the combustion chamber, particularly at the transition section and guide bushing, where measurement points have double-walled surfaces with cooling holes, the additional mounting structure must be minimally disruptive to the combustion chamber structure, making design difficult.

[0005] In view of the above technical problems, the present invention is specially introduced. Summary of the Invention

[0006] The main purpose of the present invention is to provide a mounting structure that can directly measure the pulsating pressure of a combustion chamber without destroying the structure of the combustion chamber itself.

[0007] To achieve the above-mentioned objectives, the present invention provides a mounting structure for a combustion chamber dynamic pressure sensor, wherein the combustion chamber wall of the combustion chamber includes a first wall layer and a second wall layer arranged outside the first wall layer, and the mounting structure includes: a first mounting hole, the first mounting hole being arranged on the first wall layer and penetrating the first wall layer; a second mounting hole, the second mounting hole being arranged on the second wall layer and penetrating the second wall layer; a mounting assembly, the mounting assembly including: a mounting seat, the mounting seat being used to support the dynamic pressure sensor, the mounting seat being installed in the first mounting hole, the second mounting hole allowing the dynamic pressure sensor to pass through the second wall layer to allow the dynamic pressure sensor to directly measure the pulsating pressure inside the combustion chamber, and a gap being formed between the second mounting hole and the dynamic pressure sensor; and a gap compensation device, the gap compensation device being installed outside the second wall layer, the gap compensation device at least partially blocking the gap outside the gap to compensate for the intake air volume. The mounting structure of the present invention takes into account the double-layer structure of the combustion chamber and the possible expansion that may result, and provides a basis for the dynamic pressure sensor to be able to directly measure without affecting the combustion chamber structure itself.

[0008] The following is a further optimization of the above scheme by the present invention:

[0009] Preferably, the gap compensation device is a floating portion, and the floating portion can float along the axial direction and the radial direction of the dynamic pressure sensor due to thermal expansion in the combustion chamber.

[0010] Preferably, the mounting base includes an upper end surface and a lower end surface, wherein the lower end surface passes through the first wall layer and is flush with the inner surface of the first wall layer; the upper end surface is open toward the second wall layer and toward the outside of the combustion chamber. The lower end surface is flush with the first wall layer to avoid forming a cavity with the first wall layer, thereby ensuring more accurate measurement.

[0011] Preferably, the mounting seat is an annular structure, including a first inner annular surface and a first outer annular surface, the first inner annular surface is mounted on the dynamic pressure sensor, and the first outer annular surface cooperates with the first mounting hole.

[0012] Preferably, the first outer annular surface is connected to the first mounting hole by welding.

[0013] Preferably, the dynamic pressure sensor is fixed in the mounting seat.

[0014] Preferably, the first inner annular surface is connected to the dynamic pressure sensor via a mounting thread.

[0015] Preferably, the mounting seat further includes a cooling hole, and the cooling hole is used to cool the dynamic pressure sensor.

[0016] Preferably, the number of cooling holes is 3 to 5, and the cooling holes are evenly distributed along the first outer annular surface.

[0017] Preferably, the cooling hole diameter is 2 to 5 mm.

[0018] Preferably, the floating portion includes a floating ring and a first limiting seat and a second limiting seat for limiting the displacement of the floating ring, and the first limiting seat and the second limiting seat are installed on the second wall layer.

[0019] Preferably, the first limiting seat and the second limiting seat are mounted on the second wall layer by welding.

[0020] Preferably, the floating ring is a sheet structure with a thickness of t and a diameter of d, and includes a second inner ring surface and a second outer ring surface. The second inner ring surface cooperates with the dynamic pressure sensor, and the second outer ring surface cooperates with the first limit seat and the second limit seat.

[0021] Preferably, the first limiting seat and / or the second limiting seat is an L-shaped or Z-shaped structure.

[0022] Preferably, the first limiting seat includes a first vertical accommodating edge and a first horizontal accommodating edge; and the second limiting seat includes a second vertical accommodating edge and a second horizontal accommodating edge.

[0023] Preferably, the first horizontal accommodating edge and the second horizontal accommodating edge are in the same plane, and the distance from the second wall layer is (t+1) mm to (t+2) mm.

[0024] Preferably, the distance between the first horizontal accommodating edge and the second horizontal accommodating edge is (d+10) mm to (d+16) mm.

[0025] Preferably, the first limiting seat and / or the second limiting seat comprises a cable fixing portion to fix the cable of the dynamic pressure sensor, so as to prevent the cable from being cut due to large swings during the operation of the gas turbine.

[0026] Preferably, the cable fixing portion extends outward from the first vertical accommodating edge and / or the second vertical accommodating edge to form a U-shaped structure.

[0027] Based on the above technical solution, the dynamic pressure sensor installation structure of the present invention has at least the following beneficial effects:

[0028] 1. The installation components can ensure that the gas turbine is always safely and reliably fixed during operation without affecting the structural design of the combustion chamber itself;

[0029] 2. The floating ring 5 of the installation assembly does not affect the overall gas distribution;

[0030] 3. The size of the cooling hole 11 on the mounting base 3 has been analyzed by aerodynamics, and the cooling air volume generated will not affect the overall air volume distribution;

[0031] 4. The bottom surface of the dynamic pressure sensor 12 mounting base 3 is flush with the inner wall of the combustion chamber, without forming a cavity, meeting the requirements of pulsating pressure measurement;

[0032] 5. The anti-loosening holes 7 on the first limiting seat 4 and the second limiting seat 6 ensure that the dynamic pressure sensor 12 is always securely fixed;

[0033] 6. The cable of the dynamic pressure sensor 12 is fixed on the fixing seat to prevent the cable from being cut due to large swing during the operation of the gas turbine. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The accompanying drawings, which constitute part of the present invention, are intended to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are intended to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0035] Figure 1 This is a schematic diagram of the dynamic pressure sensor installation structure

[0036] Figure 2 It is a structural diagram of the dynamic pressure sensor mounting base and the first layer wall;

[0037] Figure 3 is a three-dimensional schematic diagram of the mounting base;

[0038] Figure 4 This is a schematic diagram of the cooperation between the dynamic pressure sensor and the mounting base;

[0039] Figure 5 It is a cross-sectional view of the floating portion;

[0040] Figure 6 It is a three-dimensional schematic diagram of the floating part.

[0041] The above drawings include the following reference numerals:

[0042] 1. First wall layer; 2. Second wall layer; 3. Mounting seat; 4. First limit seat; 5. Floating ring; 6. Second limit seat; 7. Anti-loosening hole; 8. Cable fixing hole; 9. Floating gap; 10. Mounting thread; 11. Cooling hole; 12. Dynamic pressure sensor;

[0043] a. First vertical accommodation edge; b. First horizontal accommodation edge; c. Second horizontal accommodation edge; d. Second vertical accommodation edge. DETAILED DESCRIPTION

[0044] It should be noted that, unless there is a conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other. The present invention will be described in detail below with reference to the accompanying drawings and in conjunction with the embodiments. The present invention will be further described in detail below with reference to specific embodiments, which are not to be construed as limiting the scope of protection claimed in the present invention.

[0045] In this description, it should be noted that, unless otherwise specified or limited, the terms "installed," "connected," and "connected" should be understood broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to direct connections, indirect connections through an intermediate medium, or internal connections between two components. Those skilled in the art will understand the specific meanings of these terms in the present invention. Furthermore, in the description of the present invention, unless otherwise specified, "plurality" means two or more.

[0046] The unique environment of a heavy-duty gas turbine's combustion chamber necessitates direct measurement of its fluctuating pressure. Direct measurement utilizes a dynamic pressure sensor 12, which directly measures the fluctuating pressure of the combustion flame within the combustion chamber, avoiding the inaccuracies associated with indirect measurement using a pressure lead pipe. However, the combustion chamber's structural design is complex and places corresponding demands on overall gas volume distribution. Therefore, the design of the dynamic pressure sensor 12 mounting assembly must ensure that the sensor 12 remains securely and reliably fixed during gas turbine operation while also ensuring that it does not affect the combustion chamber structure itself.

[0047] Based on the use environment of real-time monitoring of pulsating pressure during the operation of the combustion chamber of a heavy-duty gas turbine, the present invention proposes a dynamic pressure sensor 12 installation structure that can directly measure the dynamic pressure of the combustion chamber and meet the high-precision requirements for pulsating pressure monitoring. Figures 1-6 As shown, the combustion chamber wall of the combustion chamber includes a first wall layer 1 and a second wall layer 2 disposed outside the first wall layer 1. The mounting structure includes a first mounting hole, a second mounting hole, and a mounting assembly. The first mounting hole is disposed in the first wall layer 1 and penetrates the first wall layer 1; the second mounting hole is disposed in the second wall layer 2 and penetrates the second wall layer 2. The mounting assembly includes a mounting seat 3 and a gap compensation device. The mounting seat 3 is used to support the dynamic pressure sensor 12 and is installed in the first mounting hole. The second mounting hole allows the dynamic pressure sensor 12 to pass through the second wall layer 2, allowing the dynamic pressure sensor 12 to directly measure the pulsating pressure within the combustion chamber. A gap is formed between the second mounting hole and the dynamic pressure sensor 12. The gap compensation device is installed outside the second wall layer 2, at least partially blocking the gap outside the gap to compensate for the intake air volume. First wall layer 1, second wall layer 2: First wall layer 1 encloses second wall layer 2, second wall layer 2. The first wall layer 1 refers to the flame tube wall, transition section wall, etc., and the second wall layer 2 refers to the outermost structure of the combustion chamber, such as the guide bushing.

[0048] The mounting assembly of the present invention utilizes the combustion chamber's first wall layer 1, such as the flame tube or transition section, as the mounting surface for the dynamic pressure sensor 12. It utilizes the combustion chamber's second wall layer 2, i.e., the outermost structure of the combustion chamber, such as the flow guide bushing, as the mounting surface for the floating portion. This provides a foundation for the dynamic pressure sensor 12 to directly measure pressure without affecting the combustion chamber structure, taking into account the combustion chamber's dual-layer structure and the resulting expansion.

[0049] The mounting assembly for the combustion chamber dynamic pressure sensor 12 enables it to directly measure the pulsating pressure inside the first wall layer 1. A gap compensation device covers the gap to meet the combustion chamber's intake air volume requirements. The gap compensation device is a floating portion that floats along the axial and radial directions of the dynamic pressure sensor 12 in response to thermal expansion within the combustion chamber.

[0050] like Figure 1-Figure 3 As shown, the mounting base 3 of the mounting assembly includes an upper end face and a lower end face, wherein the upper end face is the end face away from the combustion chamber, and the lower end face is the end face close to the combustion chamber. The lower end face is flush with the first wall layer 1 to avoid forming a cavity with the first wall layer 1, thereby making the measurement more accurate. The upper end face of the mounting base 3 passes through the second wall layer 2 and opens to the outside of the combustion chamber to install the dynamic pressure sensor 12. Figure 3 As shown, during the installation of the dynamic pressure sensor 12 , the dynamic pressure sensor 12 also needs to be flush with the first wall layer 1 .

[0051] like Figure 2 As shown, the mounting base 3 is welded to the measurement position of the combustion chamber's preset dynamic pressure sensor 12. A hole is opened at the corresponding position in the combustion chamber, and the lower surface of the mounting base 3 is flush with the inner wall surface of the combustion chamber. The mounting base 3 is an annular structure, including a first inner annular surface and a first outer annular surface. The first inner annular surface is mounted on the dynamic pressure sensor 12, while the first outer annular surface of the mounting base 3 mates with the first mounting hole of the first wall layer 1. Furthermore, the first outer annular surface is connected to the first mounting hole of the first wall layer 1 by welding.

[0052] like Figure 4 As shown, the dynamic pressure sensor 12 is fixed in the mounting base 3, and the first inner annular surface is connected to the dynamic pressure sensor 12 via the mounting thread 10. The lower end surface of the dynamic pressure sensor 12 component is flush with the bottom surface of the mounting base 3 structure, forming a cavity with the wall surface, thereby meeting the requirements of pulsating pressure measurement.

[0053] Mounting base 3 also includes cooling holes 11, which are used to cool dynamic pressure sensor 12. Because the pressure outside the combustion chamber is higher than the pressure inside the combustion chamber, a flow is generated to cool dynamic pressure sensor 12. The size of cooling holes 11 has been determined through aerodynamic analysis, and the cooling air volume generated will not affect the overall air volume distribution. Cooling holes 11 are evenly distributed along the first outer annular surface, with a diameter of 2 to 5 mm and a number of 3 to 5. Preferably, the number of cooling holes 11 can be designed to be 4, with a diameter of 3 mm.

[0054] For the floating part, if Figure 6 As shown, it includes a floating ring 5 and a first limit seat 4 and a second limit seat 6 for limiting the displacement of the floating ring 5. The first limit seat 4 and the second limit seat 6 are installed on the second wall layer 2. Preferably, the first limit seat 4 and the second limit seat 6 are welded to the second wall layer 2. For the second wall layer 2, in order to install the dynamic pressure sensor 12, a larger hole needs to be opened; at the same time, due to the displacement caused by thermal expansion of the second wall layer 2 itself, a certain amount of displacement needs to be reserved to prevent the sensor from being cut off during the operation of the gas turbine. In this way, a large hole for installing the dynamic pressure sensor 12 is formed on the second wall layer 2, and this large hole will affect the overall gas distribution. Therefore, it is necessary to design a floating ring 5 and place it on the outer surface of the second wall layer 2 to seal the annular gap formed by the first outer ring surface and the second wall layer 2.

[0055] The present invention uses a floating ring 5 as a floating device. The floating ring 5 is a sheet-like structure that can float on the outer surface of the second wall layer 2 and float freely with thermal expansion. Figure 6 As can be seen, the floating ring 5 is located outside the outermost structure of the combustion chamber, in a relatively low ambient temperature. This makes it less susceptible to damage and deformation, providing a more effective seal for the annular aperture. Furthermore, as a floating device, the floating ring expands with the hot gases in the combustion chamber, preventing structural damage caused by thermal stress. The floating ring 5 has a thickness of t and a diameter of d. It includes a second inner annular surface and a second outer annular surface. The second inner annular surface mates with the dynamic pressure sensor 12, while the second outer annular surface engages with the first and second stopper seats 4 and 6.

[0056] The first limit seat 4 and / or the second limit seat 6 have an L-shaped or Z-shaped structure. The first limit seat 4 includes a first vertical receiving edge a and a first horizontal receiving edge b; the second limit seat 6 includes a second vertical receiving edge d and a second horizontal receiving edge c. The first horizontal receiving edge b and the second horizontal receiving edge c are coplanar. The floating ring 5 forms a floating gap 9 with the first and second limit seats 4 and 6.

[0057] After simulations were performed to calculate the maximum thermal expansion of the floating ring 5, the distances between the first and second horizontal receiving edges b and c and the second wall layer 2 were designed to be (t+1) mm to (t+2) mm. The distance between the first and second horizontal receiving edges b and c was designed to be (d+10) mm to (d+16) mm. During floating, the floating plate will not exceed the fixed range of the first and second stopper seats 4 and 6.

[0058] Because the combustion chamber of a gas turbine generates significant vibration during operation, anti-loosening holes 7 are designed on the first and second stopper seats 4 and 6. During installation, after the dynamic pressure sensor 12 is mounted on the mounting seat 3, the sensor and anti-loosening holes 7 are fixed with wire to limit displacement in the direction perpendicular to the surface, ensuring that the sensor is always securely fixed.

[0059] Due to the high air velocity outside the combustion chamber, direct measurement requires the cable of the dynamic pressure sensor 12 to be routed through a lead hole to the outside of the gas turbine. The first retaining seat 4 and / or the second retaining seat 6 include a cable securing portion to secure the cable of the dynamic pressure sensor 12. The cable securing portion extends outward from the first vertical receiving side a and / or the second vertical receiving side d to form a U-shaped structure. Figure 5 The second limit seat 6 includes a cable fixing portion, which includes a cable fixing hole 8. The cable of the dynamic pressure sensor 12 is fixed to the fixing seat through the cable fixing hole 8 to prevent the cable from being cut due to large swings during operation of the gas turbine.

[0060] The mounting assembly of the present invention can ensure that the cooling and gas distribution of the combustion chamber are less affected, and also take into account the characteristics of heavy-duty combustion engine combustion chamber with large vibration, ensuring that the dynamic pressure sensor 12 is always installed and reliable during use. At the same time, the cable is fixed to prevent it from being cut by airflow during use.

[0061] In summary, from the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0062] 1. The installation components can ensure that the gas turbine is always safely and reliably fixed during operation without affecting the structural design of the combustion chamber itself;

[0063] 2. The floating ring 5 of the installation assembly does not affect the overall gas distribution;

[0064] 3. The size of the cooling hole 11 on the mounting base 3 has been analyzed by aerodynamics, and the cooling air volume generated will not affect the overall air volume distribution;

[0065] 4. The bottom surface of the dynamic pressure sensor 12 mounting base 3 is flush with the inner wall of the combustion chamber, without forming a cavity, meeting the requirements of pulsating pressure measurement;

[0066] 5. The anti-loosening holes 7 on the first limiting seat 4 and the second limiting seat 6 ensure that the dynamic pressure sensor 12 is always securely fixed;

[0067] 6. The cable of the dynamic pressure sensor 12 is fixed on the fixing seat to prevent the cable from being cut due to large swing during the operation of the gas turbine.

[0068] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A mounting structure for a combustion chamber dynamic pressure sensor, wherein the combustion chamber wall of the combustion chamber comprises a first wall layer (1) and a second wall layer (2) arranged outside the first wall layer (1), characterized in that: The mounting structure comprises: a first mounting hole, the first mounting hole being arranged on the first wall layer (1) and penetrating the first wall layer (1); a second mounting hole, the second mounting hole being arranged on the second wall layer (2) and penetrating the second wall layer (2); a mounting assembly, the mounting assembly comprising a mounting seat (3), the mounting seat (3) being used to carry the dynamic pressure sensor (12), the mounting seat (3) being mounted in the first mounting hole, the second mounting hole allowing the dynamic pressure sensor (12) to pass through the second wall layer (2) to allow the dynamic pressure sensor (12) to directly measure the pulsating pressure inside the combustion chamber, and a gap being formed between the second mounting hole and the dynamic pressure sensor (12); a gap compensation device, the gap compensation device being mounted on the outside of the second wall layer (2), the gap compensation device at least partially shielding the gap outside the gap to compensate for the intake air amount; The gap compensation device is a floating part, and the floating part can float along the axial direction and radial direction of the dynamic pressure sensor according to the thermal expansion in the combustion chamber; the floating part includes a floating ring (5) and a first limit seat (4) and a second limit seat (6) for limiting the displacement of the floating ring (5), and the first limit seat (4) and the second limit seat (6) are installed on the second wall layer (2).

2. The mounting structure according to claim 1, wherein: The mounting seat (3) comprises an upper end surface and a lower end surface, the lower end surface passes through the first wall layer (1) and is flush with the inner surface of the first wall layer (1); the upper end surface is open toward the second wall layer (2) and toward the outside of the combustion chamber.

3. The mounting structure according to claim 2, wherein: The mounting seat (3) is an annular structure, comprising a first inner annular surface and a first outer annular surface, the first inner annular surface being mounted with the dynamic pressure sensor (12), and the first outer annular surface being matched with the first mounting hole.

4. The mounting structure according to claim 3, wherein: The first outer annular surface is connected to the first mounting hole by welding.

5. The mounting structure according to claim 4, wherein: The dynamic pressure sensor (12) is fixed in the mounting seat (3).

6. The mounting structure according to claim 5, characterized in that: The first inner annular surface is connected to the dynamic pressure sensor (12) via a mounting thread (10).

7. The mounting structure according to claim 6, wherein: The mounting seat (3) further comprises a cooling hole (11), wherein the cooling hole (11) is used to cool the dynamic pressure sensor (12).

8. The mounting structure according to claim 7, characterized in that: The number of the cooling holes (11) is 3 to 5, and the cooling holes (11) are evenly distributed along the first outer ring surface.

9. The mounting structure according to claim 8, characterized in that: The cooling hole (11) has a diameter of 2 to 5 mm.

10. The mounting structure according to claim 9, characterized in that: The first limiting seat (4) and the second limiting seat (6) are mounted on the second wall layer (2) by welding.

11. The mounting structure according to claim 10, wherein: The floating ring (5) is a sheet-like structure, has a thickness of t and a diameter of d, and comprises a second inner ring surface and a second outer ring surface, wherein the second inner ring surface cooperates with the dynamic pressure sensor (12), and the second outer ring surface cooperates with the first limit seat (4) and the second limit seat (6).

12. The mounting structure according to claim 11, wherein: The first limiting seat (4) and / or the second limiting seat (6) are of L-shaped or Z-shaped structure.

13. The mounting structure according to claim 12, wherein: The first limiting seat (4) includes a first vertical accommodating edge and a first horizontal accommodating edge; and the second limiting seat (6) includes a second vertical accommodating edge and a second horizontal accommodating edge.

14. The mounting structure according to claim 13, wherein: The first horizontal accommodating edge and the second horizontal accommodating edge are in the same plane, and the distance from the second wall layer (2) is (t+1) mm to (t+2) mm.

15. The mounting structure according to claim 14, wherein: The distance between the first horizontal accommodating side and the second horizontal accommodating side is (d+10) mm to (d+16) mm.

16. The mounting structure according to claim 15, characterized in that: The first limiting seat (4) and / or the second limiting seat (6) comprises a cable fixing portion for fixing the cable of the dynamic pressure sensor (12).

17. The mounting structure according to claim 16, wherein: The cable fixing portion extends outward from the first vertical accommodating edge and / or the second vertical accommodating edge to form a U-shaped structure.

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