FLAME TRAP FILTER

Abstract

A flame trap filter including a grid structure, wherein the grid structure determines grid openings which are bordered by intersecting strip sections, and/or wherein the grid structure is formed by a laid scrim. A method for producing a grid structure of a flame trap filter includes the step of allowing a substance to solidify on a substrate, in order to form at least one strip section of the grid structure. Alternatively or in addition, the method includes the step of compressing a material for the flame trap filter or a semi-finished product of the flame trap filter at points, e.g. by mechanically deforming the material or the semi-finished product, in order to form at least one strip section of the grid structure.

Claims

1. A flame trap filter having at least one grid structure, wherein the at least one grid structure defines grid openings that are limited by web sections intersecting one another and/or wherein the at least one grid structure is formed by a laid scrim.

2. The flame trap filter according to claim 1, wherein the flame trap filter forms with further elements, a pressure relief body for a protected housing for an electrical operating means.

3. The flame trap filter according to claim 1, wherein the at least one grid structure is planar and/or wherein the web sections are located in one plane.

4. The flame trap filter according to claim 1, wherein the flame trap filter comprises a number of multiple layers, wherein the at least one grid structure forms an additional layer between two layers of the number of multiple layers or wherein the at least one grid structure forms a terminal layer on a side of the flame trap filter facing opposite a flow direction or a terminal layer on a side of an arrangement of the number of multiple layers facing in a flow direction.

5. The flame trap filter according to claim 4, wherein the number of multiple layers are woven layers or wherein the number of multiple layers are grid structures respectively that are limited by the web sections intersecting one another and/or wherein the at least one grid structure is formed by one laid scrim respectively.

6. The flame trap filter according to claim 4, wherein the at least one grid structure is connected with the number of multiple layers, completely two-dimensionally or in sections by means of sintering, gluing and/or in a form-fit manner.

7. The flame trap filter according to claim 1, wherein the at least one grid structure is manufactured by laser cutting or stamping from a foil or a sheet material.

8. The flame trap filter according to claim 4, wherein the number of multiple layers comprise openings, the opening area of which is defined in an opening area range, wherein the opening area of each of the grid openings of the at least one grid structure is multiple times larger than the largest opening area within the opening area range.

9. The flame trap filter according to claim 8, wherein the opening area of each of the grid openings is about at least five times or at least ten times larger than a largest opening area of the opening area range.

10. The flame trap filter according to claim 1, wherein the at least one grid structure is formed by compressing a material for the flame trap filter or a semi-finished product of the flame trap filter, wherein the locations at which the material or the semi-finished product is compressed, form the web sections of the at least one grid structure.

11. The flame trap filter according to claim 10, wherein the compressing has been carried out on one side facing a flow or on one side facing in a flow direction or on both sides.

12. The flame trap filter according to claim 1, wherein a thickness of the flame trap filter measured at a web section of the web sections is less than a thickness measured at a grid opening of the grid openings of the at least one grid structure.

13. The flame trap filter according to claim 1, wherein the web sections are manufactured by solidification of a substance on a substrate.

14. The flame trap filter according to claim 12, wherein the web sections are created by an additive manufacturing process.

15. The flame trap filter according to claim 13, wherein the substance closes openings of a layer of the flame trap filter.

16. The flame trap filter according to claim 1, wherein the flame trap filter comprises multiple layers that are free from connection locations within a projection of the grid openings of the at least one grid structure in or against a flooding direction.

17. A pressure relief device having a flame trap filter according to claim 1.

18. A method for manufacturing a grid structure of a flame trap filter according to claim 1, wherein a substance is solidified on a substrate, and/or wherein a material for the flame trap filter or a semi-finished product of the flame trap filter is compressed at locations.

19. The flame trap filter according to claim 2, wherein the at least one grid structure is planar and/or wherein the web sections are located in one plane.

20. The flame trap filter according to claim 19, wherein the flame trap filter comprises a number of multiple layers, wherein the at least one grid structure forms an additional layer between two layers of the number of multiple layers or wherein the at least one grid structure forms a terminal layer on a side of the flame trap filter facing opposite a flow direction or a terminal layer on a side of an arrangement of the number of multiple layers facing in a flow direction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Additional exemplary features and embodiments of the inventive flame trap filter, the pressure relief body as well as the method are derived from the dependent claims, the subsequent description as well as the figures. The drawings show:

[0038] FIG. 1—a perspective view of an embodiment of an opened and apart therefrom, explosion-protected housing having two pressure relief devices with one inventive flame trap filter in each case,

[0039] FIG. 2a—a perspective view of an embodiment of the inventive flame trap filter,

[0040] FIG. 2b—a top view of a part of the flame trap filter according to FIG. 2a,

[0041] FIG. 2c—a top view of a part of the flame trap filter according to a further embodiment,

[0042] FIG. 2d—a top view of a part of a flame trap filter according to yet another embodiment,

[0043] FIG. 3—a perspective view of another embodiment of an inventive flame trap filter,

[0044] FIG. 4a—a sectional view through a yet additional embodiment of an inventive flame trap filter, wherein the flooding direction is within the sectional plane,

[0045] FIG. 4b—a top view of a part of the flame trap filter according to FIG. 4a,

[0046] FIG. 5—a perspective view of a semi-finished product of an example of a fourth embodiment of the inventive flame trap filter.

DETAILED DESCRIPTION

[0047] Embodiments of explosion-protected housings are explosion protected according to the explosion protection category “flameproof enclosure”. Inventive housings can comprise one or more pressure relief devices having at least one inventive flame trap filter in each case.

[0048] FIG. 1 shows an embodiment of such an explosion-protected housing 10 with removed cover. The housing 10 defines an interior 11. The housing 10 comprises two pressure relief openings 12a, 12b. These are closed in a flameproof manner by pressure relief bodies 13a, 13b having at least one inventive flame trap filter 14a, 14b in each case. A gas exchange through the flame trap filter 14a, 14b is in principle possible, e.g. in order to achieve pressure compensation between the interior 11 of housing 10 and its environment. In case of an explosion in interior 11 of housing 10, however, gases or particles can escape from housing 10 through flame trap filters 14a or 14b only cooled down, such that a gas or particles cannot ignite the atmosphere outside housing 10. In the illustrated embodiment the flame trap filter 14a is arranged in a first pressure relief body 13a such that the inventive grid structure 15a for dividing the pressure relief opening 12a in multiple partial openings 16 is facing the interior 11 of the explosion-protected housing 10. In the case of the first pressure relief body 13a the grid structure 15a is accordingly passed by a flow prior to at least one additional layer of the flame trap filter 14a. The same also applies to the flame trap filter 14b of the second pressure relief body 13b, the grid structure of which is covered by at least one additional layer of the flame trap filter 14b in the view shown in FIG. 1. Alternatively, the at least one flame trap filter 14a, 14b can be orientated such that the inventive grid structure faces outwardly, such that the grid structure is passed by flow after gas has flown through the at least one additional layer of the flame trap filter of the second pressure relief device. In yet further embodiments the grid structure can form an intermediate layer between at least two additional layers of the flame protection filter (not illustrated).

[0049] In each case each pressure relief body 13a, 13b divides the pressure relief opening 12a, 12b in the housing 10 into partial openings 16.

[0050] The sub-division of the pressure relief opening 12a, 12b in partial openings 16 is carried out by the web sections 17 (compare e.g. FIG. 2a) of grid structure 15a. Because the web sections are not monolithic parts of wall 18 of housing 10 that limits the pressure relief opening 12a or 12b, a cumbersome processing of the housing wall 18 for manufacturing of grid structure 15a for spreading the thermal input and/or the gas flow is not required. It is rather sufficient to form a pressure relief opening 12a, 12b with continuous opening surface in the wall 18 of housing 10 that is only subdivided in partial openings 16 by a flameproof closing of the pressure relief opening 12a or 12b by means of the pressure relief bodies 13a, 13b.

[0051] With the grid structure 15a created by web sections 17 a pressure relief flow outwardly out of interior 11 of housing 10 is subdivided in partial flows, whereby the thermal input in the pressure relief body 13a (the same applies for the pressure relief body 13b) is distributed over a larger area and thus the effectiveness of the pressure relief body 13a for pressure relief is increased.

[0052] In an embodiment the grid openings of the grid structure 15 can be limited by intersecting web sections 17i, 17ii. Web section 17i, 17ii are arranged transverse, preferably orthogonal to a flooding direction D of the pressure relief body 13a, 13b. Web sections 17i, 17ii can be straight or curved. Web sections 17i, 17ii can be rod-shaped. Web sections 17i, 17ii can form a perforated grid. Preferably web sections 17i, 17ii extend in a common plane. The grid structure 15a is preferably planar and/or the intersecting web sections 17i, 17ii are arranged in one plane.

[0053] A regular grid structure 15a that is formed by intersecting web sections 17i, 17ii is exemplarily shown in a perspective view in FIG. 2. Index a or b is omitted in the following, because the first flame trap filter 14a and/or the second flame trap filter can be carried out according to one of the subsequently described embodiments, for example. The grid structure 15a shown in FIG. 2 comprises rectangular, here square, grid openings 19 that form the partial openings 16 of the pressure relief opening 12. The grid openings 19, apart from the grid openings at the edge of the grid structure 15, comprise preferably the same area. The grid structure 15 is illustrated in FIG. 2 as a terminal layer that either faces a gas flow direction D from the interior 11 of housing 10 or faces in gas flow direction D out of the interior 11 outwardly. It is, however, alternatively or in addition also possible that a grid structure 15 having grid openings 19 that are formed by web sections 17i, 17ii intersecting one another, forms an intermediate layer.

[0054] In the illustrated embodiment the outer contour of the grid structure 15 is round. The grid structure 15 can, however, also be polygonal or angular, e.g. rectangular or square. Preferably the shape of the outer contour is geometrically similar to the shape of the outer contour of the pressure relief body 13 and/or the pressure relief opening 12.

[0055] For example, the grid structure 15 can be manufactured from a sheet metal or foil. Intersecting web sections 17i, 17ii can be formed, for example, in that grid openings 19 are cut off the foil or sheet metal, e.g. by laser cutting and/or stamping. The web sections 17i, 17ii can be made of metal or plastic. The grid structure 15 according to FIG. 2 can be cut or stamped out of a sheet metal part or a metal foil, for example. In doing so, the grid opening areas 19 are manufactured under material losses.

[0056] In a modification the grid structure 15 can be formed by an expanded grid, particularly expanded metal (not shown). For manufacturing of an expanded grid, meshes are created by offset cuts without material loss under concurrent expanding deformation. These meshes are at least limited at two sides by web sections intersecting one another. Without additional measures, however, pairs of web sections intersecting one another do not extend parallel to a common plane, but even if the grid structure as a whole extends in a grid plane, the pairs of intersecting web sections are extending obliquely to this grid plane. By mechanical deformation transverse to the grid plane, all pairs can be deformed such that they extend parallel to the grid plane.

[0057] Due to intersecting web sections 17i, 17ii, heat introduced at one point in the grid structure 15 cannot only be conducted substantially by the web in which the heat is introduced, as in case of a woven fabric, but the heat conduction can be partly taken by the intersecting web section at the intersections of two web sections 17i, 17ii. If the webs or web sections 17i, 17ii are straight, short webs are obtained and as a result a remarkably good heat conductivity can be obtained. Contrary thereto, a web has to have a wavy shape in a woven fabric in order to alternatingly extend above or below of crossing rods. This elongates the web compared with a straight web in case of equal area of the flame trap filter.

[0058] Due to the intersecting web sections 17i, 17ii, the grid opening limited by the intersecting web sections 17i, 17ii is delimited from an adjacent grid opening. A cross flow from one grid opening in the plane of the grid structure is thus impeded or blocked. The intersecting web sections 17i, 17ii thus form flooding screen openings or channels for a separation of the gas flow in partial flows at least within the plane of the grid structure 15. This is contrary to a woven layer in which the webs cross, but not intersect such that less flow resistance encounters a cross flow in the woven layer.

[0059] The flame trap filter 14 comprises additional layers or sub-layers 20.2 to 20.8 that can form grids respectively. These additional layers 20.2 to 20.8 can be woven layers. Alternatively, one or more additional layers 20.2 to 20.8 can be formed from a laid scrim and/or intersecting web sections. It is alternatively or additionally also possible that at least one or all additional layers 20.2 to 20.8 are manufactured from entangled fiber material or felt material. The flame trap filter 14 can be formed by an arrangement of planes 20.1 to 20.8 that are not connected with one another. Preferably the grid structure 15 (layer 20.1) and the additional layers 20.2 to 20.8 are, however, connected with one another. For example, the layers 20.1 to 20.8 can be glued with one another, can be connected with one another by sintering, can be connected with one another by mechanical deformation, by screwing or the like. In other embodiments the grid structure 15 is not connected with the finer grids of layers 20.2 to 20.8, wherein the finer grids of layers 20.2 to 20.8 are connected with one another or not connected with one another. Embodiments are possible in which more or less than eight layers 20.1 to 20.8 are arranged adjacent or on top of each other and namely in contact with one another or not in contact with one another.

[0060] Grid layers 20.2 to 20.8 present in addition to the grid structure 15 comprise a plurality of openings 21, the opening area of which is defined within an opening area range. The opening area of the grid openings 19 of the grid structure 15 is preferably multiple times larger, preferably at least five times or even at least ten times or even at least thirty times or even at least one hundred times larger than the largest opening area within the opening area range. An example for this is shown in FIG. 2b. It illustrates a top view of a part of the inventive flame trap filter 14 according to FIG. 2a. The section A shown in FIG. 2b is illustrated in dashed lines in FIG. 2a. As illustrated, layer 20.2 arranged below the grid structure 15 forms openings 21 having an opening area that is multiple times smaller, in the present embodiment far more than ten times smaller than the opening area of a grid opening of the grid structure 15. In doing so, a coarse division of the pressure relief flow in partial flows is achieved by grid structure 15. The flame trap filter 14 can be configured such that the grid structure 15 furthermore does not contribute to the flameproof condition. Rather remaining layers 20.2 to 20.8 can together already provide flameproof condition. The grid structure 15 serves only for distribution of the gas flow over the entire area of the flame trap filter 14.

[0061] While seven layers 20.2 to 20.8 are illustrated in the illustrated embodiment that together guarantee the flameproof condition, it could also be more or less layers (e.g. only one).

[0062] Alternatively or additionally and different to the illustrated embodiment, more than only one layer 20.1 can form the inventive grid structure 15. For example, a flame trap filter 14 can have at least two layers (not illustrated) that respectively form inventive grid structures 15. The multiple, at least two grid structures provided exemplarily subdivide the flow through the flame trap filter subsequently in coarse partial flows. Between at least two grid structures one or more finer grids can be arranged. With the at least two grid structures the gas flow can be controlled or guided.

[0063] The opening width (e.g. mesh width) of the multiplicity of openings 21 provided in the additional layers 20.2 to 20.8 are preferably defined within an opening width range. Preferably the width of the web sections 17i, 17ii of grid structure 15, as apparent from FIG. 2b, is larger than the largest opening width within the opening width range. Preferably the width of each web section 17i, 17ii covers at least one or at least two openings 21 of layer 20.2 arranged below the web sections 17i, 17ii. “Below” refers to a respective—potentially virtual—orientation of flame trap filter 14.

[0064] FIG. 2c shows a section A of another embodiment of the inventive flame trap filter 14. FIG. 2c illustrates a top view of the embodiment in part. Different to the embodiment illustrated in FIG. 2a in which a perforated grid with square openings is shown, the grid structure of the embodiment illustrated in FIG. 2c is a perforated grid, e.g. perforated sheet metal or perforated foil, having circular grid openings 19. Apart therefrom the description referring to the embodiment according to FIGS. 2a and 2b can be used for the explanation of the embodiment according to FIG. 2c.

[0065] FIG. 2d shows a section A of a further embodiment of an inventive flame trap filter 14. FIG. 2c illustrates a top view of the embodiment in part. The grid structure 15 can comprise grid openings 19 arranged in regular (as e.g. illustrated in FIG. 2a) or irregular distances that are formed in the embodiment according to FIG. 2d by meshes 19 between two longitudinal warp elements 29 and two longitudinal weft elements 30, wherein the meshes 19 have a mesh width in a second mesh width range. Between the meshes 19 webs 17i, 17ii extend that are formed of an arrangement of multiple longitudinal warp elements extending adjacent to one another or an arrangement of multiple longitudinal weft elements 30 extending adjacent to one another. The warp elements 29 and/or weft elements 30 can be particularly wire-, thread- or strip-shaped. Within the arrangement meshes 31 are formed between adjacent weft elements 30 and warp elements 29 having a mesh width in a first mesh width range. The mesh width in the second mesh width range is larger than each mesh width in the first mesh width range. The mesh widths in the second mesh width range are preferably at least five times larger or even at least ten times larger than the larges mesh width in the first mesh width range. The first mesh width range can particularly comprise the mesh width zero. Accordingly, zero meshes can form meshes 31 in the arrangements.

[0066] The grid structure according to FIG. 2d comprises intersecting web sections 17i, 17ii that limit a grid opening 19. The webs 17i are formed of an arrangement of multiple longitudinal warp elements 29 in each case, wherein distant thereto multiple transverse, preferably orthogonal, extending longitudinal weft elements 30 are woven—preferably in plain weave—forming a further web 17ii, wherein the web 17i, 17ii limit a mesh 19 (grid opening) having a mesh width in the second mesh width range and the weft and warp elements limit meshes in the intersection ranges of the arrangement have a mesh width less than the mesh width of mesh 19, for example mesh width zero, between intersecting arrangements. As illustrated, longitudinal weft elements 30 or longitudinal warp elements 29 within the arrangement are alternatingly arranged above or below singular warp elements 29 or weft elements 30 extending transverse thereto between two grid openings 19.

[0067] The mesh width of the grid opening 19 is preferably and as apparent from FIG. 2d larger than the width b of each web section 17i, 17ii that limit the grid opening 19, e.g. at least five times larger or at least ten times larger. This applies accordingly for other inventive embodiments, e.g. for the embodiments that are illustrated in the further figures.

[0068] In the view according to FIG. 2d, a layer 20.2 having openings 21 that are smaller than the grid opening 19 is schematically illustrated below the grid structure 15. For this and for additional optional features reference can be made to the description referring to the further embodiments.

[0069] FIG. 3 shows an example of another embodiment of an inventive pressure relief body 13. The pressure relief body can have the inventive flame trap filter 14 or can consist thereof. For the flame trap filter 14 reference can be made to the above description as long as it is not described otherwise in the following. The grid structure 15 is formed of a laid scrim in the embodiment illustrated in FIG. 3. The laid scrim comprises a layer of webs 17i orientated in a first direction R1 and a second layer of webs 17ii orientated in a second direction R2 transverse to the first direction R1, e.g. orthogonal. In doing so, rectangular, particularly square, or other polygonal grid openings 19 can be created. For example, the laid scrim can be formed by arranging of sheet strips, e.g. sheet metal strips, in the first direction R1 and the arrangement of additional sheet strips in the second direction R2. As illustrated, preferably no web 17i, 17ii changes the layer, but all webs 17i or 17ii are arranged on one side over the entire length of the arrangement of webs 17ii or 17i orientated transverse thereto. The sheet strips of different orientation are preferably connected with one another at the crossing locations, e.g. glued, sintered, welded or the like. The webs 17i and/or 17ii of one or both layers are preferably straight in order to provide a connection to the edge of the flame trap filter 14 that is as short as possible in order to distribute heat in the pressure relief body 13 and/or discharge heat in the housing wall 18.

[0070] Another possibility for configuration of the inventive grid structure is to compress a material for the flame trap filter 14 or a semi-finished product of the flame trap filter 14 by mechanical deformation at locations 22 that together form a grid structure 15. The locations 22 at which the material or semi-finished product is compressed by mechanical deformation form the web sections 17i, 17ii of the grid structure 15. The web sections 17i, 17ii thus intersect. Preferably the web sections 17i, 17ii are arranged in one plane. A wave structure is obtained having a period that corresponds to the opening width of the grid structure 15. A respectively manufactured embodiment is shown in part cross-sectional illustration in FIG. 4a. The flow or flooding direction D is within the cross-section plane. FIG. 4b shows a section of a top view on the side of the flame trap filter 14 structured by compression.

[0071] Between the webs 17i, 17ii less compressed cushion-shaped areas 23 are formed such that as a whole a quilt structure results. These areas 23 form the grid openings 19, because the permeability of the flame trap filter 14 through the webs 17i, 17ii is compared with the permeability through the areas 23 at least remarkably reduced. The locations 22 can be compressed so intensively that the webs 17i, 17ii are gas impermeable. As illustrated in the embodiment according to FIG. 4, side 24 of flame trap filter 14 having the quilt structure is opposed to flow direction D. The opposite side 25 is flat. In alternative embodiments the quilt-like three-dimensionally structured side 24 is orientated in flooding direction D. In again other embodiments the material for the flame trap filter 14 and/or the semi-finished product is compressed from both sides 24, 25 such that two opposite orientated sides 24, 25 result that can be three-dimensionally structured, e.g. quilt-like.

[0072] The material can comprise one or multiple layers, particularly woven layers. The material can comprise exclusively woven layers, for example. By mechanical deformation, however, a flame trap grid is created in total having a grid structure of web sections 17i, 17ii intersecting one another.

[0073] In embodiments as shown in FIG. 4, the thickness dl of flame trap filter 14 measured at web sections 17i, 17ii is less than thickness d2 measured in a grid opening 19 of grid structure 15. Measuring at a web section 17i, 17ii means that the web section 17i, 17ii is measured in straight line (parallel to flooding direction) between the two reference points, the distance of which is determined. The same applies for the grid opening 19.

[0074] FIG. 5 shows an example of a still further embodiment. For explanation the description above can be applied as long as it is not otherwise indicated in the following description. In the further embodiment the grid structure 15 is at least partly manufactured by solidifying a substance on a substrate 26. The substrate 26 can be an aid that has to be removed again for providing the flame trap filter 14. As an alternative—and as illustrated—a substrate 26 can form at least one layer of the flame trap filter 14.

[0075] In the embodiment illustrated in FIG. 5 webs 17ii of one layer of one direction R1 are already manufactured. A web 17ii extending in a direction orientated transverse thereto, preferably orthogonal is manufactured by means of an instrument I in that material 27 is applied on the flame trap filter semi-finished product 28 and is solidified there to form a web 17i. The material 27 can be, e.g. silicone or adhesive. It is also possible to solidify at least partly liquid or soft material on the substrate 26. For example, the web sections 17i, 17ii can be formed by weld beads. A possibility is to apply metal powder on substrate 26 and to at least partly melt them, e.g. by means of a laser, such that the metal particles connect during solidification. The web sections 17i, 17ii can be particularly manufactured by laser deposit welding or by another manufacturing method suitable for additive manufacturing of webs. Thereby, besides metal, also plastic or materials based on natural substances can be introduced or applied. If webs 17i, 17ii are created, a laid scrim of webs 17i, 17ii is obtained.

[0076] Independent from the configuration in particularly preferred embodiments, some layers or all layers 20.2 to 20.8 that can be present in addition to the grid structure 15 in the flame trap filter 14 are free from connection locations within the projection of each area of grid openings 19 of grid structure 15 in flooding direction D. In other words, the space that is passed by the virtual shift of an area of grid opening 19 in flooding direction D is preferably free of connection locations. From this a highly increased flow resistance results at the web sections 17i, 17ii, however, a remarkably lesser flow resistance between web sections 17i, 17ii.

[0077] According to the invention, a flame trap filter 14, 14a, 14b having a grid structure 15, 15a is provided, wherein the grid structure 15, 15a defines grid openings 19 that are limited by web sections 17i, 17ii intersecting one another and/or wherein the grid structure 15, 15a is formed by a laid scrim. An inventive method for manufacturing a grid structure 15, 15a of a flame trap filter 14, 14a, 14b comprises the step of solidifying a substance 27 on a substrate 26 in order to form at least one web section 17i, 17ii of grid structure 15, 15a. As an alternative or in addition, the method can, for example, comprise the step of compressing a material for the flame trap filter 14, 14a, 14b or a semi-finished product 28 of flame trap filter 14, 14a, 14b at locations 23—for example by mechanical deformation of the material or the semi-finished product 28, in order to form at least one web section 17i, 17ii of grid structure 15, 15a.

TABLE-US-00001 List of Reference Signs: 10 housing 11 interior 12, 12a pressure relief opening 12b pressure relief opening 13, 13a first pressure relief body 13b second pressure relief body 14, 14a (first) flame trap filter 14b second flame trap filter 15, 15a grid structure 16 partial opening 17, 17i, 17ii web section 18 wall 19 grid opening 20.1-20.8 layers 21 opening 22 locations 23 area 24 side 25 side 26 substrate 27 material 28 semi-finished product 29 warp element 30 weft element 31 mesh D flooding direction R1 first direction R2 second direction d1 thickness d2 thickness b width I instrument A section