Impingement cooling components, turbine blades and gas turbines

Abstract

The present invention discloses an impingement cooling assembly and a turbine blade using the impingement cooling assembly, and a gas turbine using the impingement cooling assembly, wherein the impingement cooling assembly comprises a first plate and a second plate, wherein the first plate is provided with a plurality of impingement holes, wherein the impingement holes are suitable for allowing cooling airflow to pass through the first plate and form an impingement jet; the second plate is spaced apart from the first plate, and an airflow channel is formed between the second plate and the first plate, wherein the airflow channel is suitable for allowing cross flow to flow; the second plate is provided with a plurality of spoilers on one side facing the first plate, wherein the plurality of spoilers are arranged one by one at the upstream of the plurality of impingement holes, and the spoilers are suitable for disturbing the cross flow upstream of each impingement hole to reduce the disturbance of the cross flow to the impingement jet. The cooling impingement assembly of the embodiment of the present invention has the advantage of a better cooling effect.

Description

Technical Field

[0001] The present invention relates to the technical field of blade cooling, and in particular to an impingement cooling assembly, a turbine blade using the impingement cooling assembly, and a gas turbine using the impingement cooling assembly. Background Art

[0002] Gas turbine blades work in harsh environments such as high temperature and high speed. The blades need to be cooled during operation to ensure stable operation of the gas turbine. Cooling impulse technology is a commonly used internal cooling technology for turbine blades and is widely used in turbine vane cooling. The cooling effect of the cooling impulse assembly in the related art is poor. Summary of the invention

[0003] The present invention is based on the inventor's discovery and understanding of the following facts and problems:

[0004] In the cooling impingement technology in the related art, when the cooling airflow exchanges heat with the impingement target surface, the boundary layer flow generated by the upstream impingement jet affects the downstream impingement jet, thereby reducing the cooling effect of the cooling impingement component.

[0005] The present invention aims to solve one of the technical problems in the related art at least to a certain extent. To this end, an embodiment of the present invention provides an impingement cooling assembly, which has the advantage of a good cooling effect.

[0006] An embodiment of the present invention further provides a turbine blade.

[0007] An embodiment of the present invention further provides a gas turbine.

[0008] The impingement cooling assembly of an embodiment of the present invention includes a first plate, the first plate is provided with a plurality of impingement holes, the impingement holes are suitable for allowing cooling airflow to pass through the first plate and form an impingement jet; a second plate, the second plate is arranged at an interval with the first plate, and an airflow channel is formed between the second plate and the first plate, the airflow channel is suitable for cross-flow circulation, and the second plate is provided with a plurality of spoilers on a side facing the first plate, the plurality of spoilers are arranged one by one upstream of the plurality of impingement holes, and the spoilers are suitable for disturbing the cross flow upstream of each impingement hole to weaken the disturbance of the cross flow to the impingement jet.

[0009] The cooling impact assembly of the embodiment of the present invention has the advantage of better cooling effect.

[0010] In some embodiments, a plurality of recesses are provided on a side of the second plate facing the first plate, and the recesses correspond to the impact holes.

[0011] In some embodiments, the recessed portion is a groove, the groove extends along the flow direction of the cross flow, and a plurality of the grooves are arranged at intervals along a direction orthogonal to the flow direction of the cross flow, and the spoiler is disposed in the groove.

[0012] In some embodiments, a width dimension of the groove is greater than a width dimension of the spoiler.

[0013] In some embodiments, a plurality of protrusions are provided on a side of the second plate facing the first plate, the protrusions extend along the flow direction of the cross flow, the plurality of protrusions are arranged at intervals along a direction orthogonal to the flow direction of the cross flow, and the plurality of protrusions are arranged at intervals with the plurality of grooves.

[0014] In some embodiments, the spoiler includes a windward section and a leeward section, the leeward section is arranged downstream of the windward section along the cross-flow direction, the height dimension of the windward section gradually increases along the cross-flow direction, and the height dimension of the leeward section gradually decreases along the cross-flow direction.

[0015] In some embodiments, the width dimension of the windward segment gradually increases along the cross flow direction, and the width dimension of the leeward segment gradually decreases along the cross flow direction.

[0016] In some embodiments, a recess is provided downstream of the leeward section along the cross flow direction, and the recess is suitable for forming a vortex downstream of the leeward section.

[0017] In some embodiments, the impact holes are arranged at intervals along the width direction of the airflow channel to form a plurality of hole rows, the plurality of hole rows comprising a first row and a second row, the second row being located downstream of the airflow of the first row in the airflow channel, the plurality of impact holes in the second row being staggered with the plurality of impact holes in the first row in the width direction of the airflow channel.

[0018] The turbine blade of the embodiment of the present invention comprises an impingement cooling assembly, and the impingement cooling assembly may be the impingement cooling assembly according to any one of the above embodiments.

[0019] In some embodiments, the turbine blade includes an impingement plate and a heat exchange plate, at least a portion of the impingement plate forms the first plate, and at least a portion of the heat exchange plate forms the second plate.

[0020] The gas turbine of the embodiment of the present invention may include the cooling impingement assembly described in any of the above embodiments, or may include the turbine blades described in any of the above embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a schematic structural diagram of the first plate of the impingement cooling assembly according to an embodiment of the present invention.

[0022] Figure 2 yes Figure 1 Schematic cross-sectional view at AA in the middle.

[0023] Figure 3 Schematic diagram of a spoiler of an impingement cooling assembly according to an embodiment of the present invention.

[0024] Figure 4 It is a schematic diagram of a spoiler of an impingement cooling assembly according to another embodiment of the present invention.

[0025] Reference numerals:

[0026] First plate 1; impact hole 11; first row 01; second row 02;

[0027] The second plate 2 comprises a concave portion 21 , a groove 211 , a spoiler 22 , a windward section 221 , a leeward section 222 , a cavity 223 , and a convex portion 23 . DETAILED DESCRIPTION

[0028] Embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0029] Combine the following Figures 1 to 4 An impingement cooling assembly according to an embodiment of the present invention is described.

[0030] The impingement cooling assembly according to the embodiment of the present invention comprises a first plate 1 and a second plate 2 .

[0031] The first plate 1 is provided with a plurality of impact holes 11, and the impact holes 11 are suitable for allowing the cooling airflow to pass through the first plate 1 and form an impact jet. Specifically, the cooling airflow flows from the side of the first plate 1 away from the second plate 2 through the impact holes 11 to the side of the first plate 1 facing the second plate 2. When the cooling airflow passes through the impact holes 11, the cross-flow area becomes smaller and the flow velocity increases, thereby forming an impact jet perpendicular to the first plate 1.

[0032] The second plate 2 is arranged at an interval with the first plate 1, and an air flow channel is formed between the second plate 2 and the first plate 1, and the air flow channel is suitable for cross flow circulation. A plurality of spoilers 22 are provided on the side of the second plate 2 facing the first plate 1, and the plurality of spoilers 22 are arranged one by one upstream of the plurality of impact holes 11, and the spoilers 22 are suitable for disturbing the cross flow upstream of each impact hole 11 to weaken the disturbance of the cross flow to the impact jet.

[0033] Specifically, the second plate 2 is arranged in parallel with the first plate 1, and an air flow channel is formed between the second plate 2 and the first plate 1. The impinging jet flows along the thickness direction of the air flow channel, penetrates the air flow channel and impinges on the second plate 2. After the impinging jet contacts the second plate 2, a cross flow is formed. Figure 1 Flow in direction b.

[0034] The flow direction of the cross flow is perpendicular to the flow direction of the impact jet. The spoiler 22 is provided on the second plate 2. The spoiler 22 protrudes from the second plate 2 into the air flow channel. When the cross flow flows through the surface of the spoiler 22, the cross flow is disturbed by the spoiler 22, thereby destroying the laminar structure of the cross flow.

[0035] Therefore, the spoiler 22 can disturb the cross flow upstream of the impinging jet to prevent the cross flow from affecting the air flow velocity at the contact point between the impinging jet and the second plate 2, thereby improving the heat exchange effect between the impinging jet and the second plate 2.

[0036] The cooling impingement assembly of the embodiment of the present invention provides a spoiler 22 upstream of the impingement jet to disturb the cross flow upstream of the impingement jet, thereby reducing the effect of the cross flow on the impingement jet, so that the impingement jet can completely impinge on the second plate 2. Therefore, the impingement cooling assembly of the embodiment of the present invention increases the air flow velocity at the contact point between the impingement jet and the second plate 2, increases the heat exchange efficiency between the impingement jet and the second plate 2, and thus has the advantage of better cooling effect.

[0037] In some embodiments, a plurality of recesses 21 are provided on a side of the second plate 2 facing the first plate 1 , and the recesses 21 correspond to the impact holes 11 .

[0038] Specifically, one side of the second plate 2 facing the first plate 1 is an impact target surface, the impact jet flows from the impact hole 11 to the impact target surface and exchanges heat on the impact target surface, the recess 21 is recessed from the air flow channel toward the second plate 2, and the recess 21 is formed on the side of the second plate 2 facing the first plate 1, and the projection of the impact hole 11 on the second plate 2 falls in the recess 21, so that the impact jet can directly flow into the cavity 223 formed by the recess 21, and the impact jet flows along the recess 21.

[0039] As a result, the flow direction of the impinging jet after being disturbed by the concave portion 21 intersects with the flow direction of the cross flow, and the tail end of the impinging jet is along Figure 2 The middle c direction extends into the cross flow, thereby destroying the laminar structure of the cross flow, increasing the turbulence of the cross flow, reducing the influence of the cross flow on the impinging jet, thereby increasing the flow velocity of the impinging jet flowing to the second plate 2, and improving the heat exchange effect between the impinging jet and the second plate 2.

[0040] In some embodiments, the recess 21 is a groove 211 , the groove 211 extends along the flow direction of the cross flow, and a plurality of grooves 211 are arranged at intervals along a direction orthogonal to the flow direction of the cross flow, and the spoiler 22 is disposed in the groove 211 .

[0041] Specifically, the recess 21 extends along the flow direction of the cross flow and forms a groove 211. There are multiple grooves 211, and the multiple grooves 211 are arranged in parallel and spaced apart along the width direction of the cross flow. The boundary layer airflow between the cross flow and the second plate 2 flows in the groove 211 along the extension direction of the groove 211 under the guidance of the multiple grooves 211.

[0042] The spoiler 22 is disposed in the groove 211 , and the spoiler 22 protrudes from the wall of the groove 211 into the groove 211 . When the cross-flow boundary layer airflow in the groove 211 flows to the spoiler 22 , the cross-flow boundary layer airflow is disturbed by the spoiler 22 , forming turbulence in the groove 211 .

[0043] As a result, the airflow in the groove 211 is disturbed by the spoiler 22 to form turbulence, thereby reducing the interference of the airflow in the groove 211 on the impingement jet, increasing the flow velocity of the impingement jet close to the second plate 2, and improving the heat exchange effect between the impingement jet and the second plate 2.

[0044] In some embodiments, the width of the groove 211 is greater than the width of the spoiler 22. Figures 1 to 4 As shown, direction a is perpendicular to the cross flow direction and parallel to the second plate 2. Direction a is the width direction of the groove 211 and the width direction of the spoiler 22. The width dimension of the groove 211 is greater than the width dimension of the spoiler 22, so that the bottom end of the spoiler 22 is completely located in the groove 211.

[0045] Therefore, when the spoiler 22 disturbs the boundary layer airflow in the groove 211, the groove 211 is prevented from guiding the boundary layer airflow to the outside of the groove 211, so that the boundary layer airflow in the groove 211 is completely disturbed by the spoiler 22, further improving the turbulence of the airflow in the groove 211 when it flows to the impact jet.

[0046] In some embodiments, a plurality of protrusions 23 are provided on the side of the second plate 2 facing the first plate 1 , the protrusions 23 extend along the flow direction of the cross flow, the plurality of protrusions 23 are arranged at intervals along a direction orthogonal to the flow direction of the cross flow, and the plurality of protrusions 23 are arranged at intervals with the plurality of grooves 211 .

[0047] Specifically, Figure 3 and Figure 4 As shown, the grooves 211 are arranged in parallel and spaced apart, and a convex portion 23 is provided between two adjacent grooves 211 . The extension direction of the convex portion 23 is the same as that of the grooves 211 . The convex portion 23 protrudes from the impact target surface of the second plate 2 toward the airflow channel.

[0048] Therefore, when the airflow in the groove 211 is disturbed by the spoiler 22, the airflow in the groove 211 can overflow to the outside of the groove 211, thereby affecting the disturbance effect of the airflow in the adjacent groove 211. By setting a plurality of protrusions 23, the airflow in two adjacent grooves 211 can be isolated, thereby preventing the airflow in a certain groove 211 from affecting the disturbance effect of the airflow in the adjacent groove 211.

[0049] In some embodiments, the spoiler 22 includes a windward section 221 and a leeward section 222. The leeward section 222 is arranged downstream of the windward section 221 along the cross-flow direction. The height dimension of the windward section 221 gradually increases along the cross-flow direction, and the height dimension of the leeward section 222 gradually decreases along the cross-flow direction.

[0050] Specifically, the windward section 221 is arranged on the upstream side of the spoiler 22 along the cross flow direction, and the leeward section 222 is arranged on the downstream side of the spoiler 22 along the cross flow direction. When the cross flow passes through the spoiler 22, it first contacts the windward section 221 and then contacts the leeward section 222. The height of the windward section 221 gradually increases along the cross flow direction.

[0051] Therefore, the windward section 221 has a slope to adapt to the flow direction of the airflow, thereby reducing the resistance of the spoiler 22 to the airflow, facilitating increasing the overall flow velocity of the cross flow, thereby improving the heat exchange effect between the cross flow and the second plate 2.

[0052] In some embodiments, the width dimension of the windward segment 221 gradually increases along the cross-flow direction, and the width dimension of the leeward segment 222 gradually decreases along the cross-flow direction.

[0053] Specifically, the dimension of the windward section 221 perpendicular to the cross flow direction gradually increases along the cross flow direction, the dimension of the leeward section 222 perpendicular to the cross flow direction gradually decreases along the cross flow direction, and the width dimension of the spoiler 22 first increases and then decreases in the direction along the cross flow direction, forming a water drop shape.

[0054] Therefore, the droplet-shaped spoiler 22 can reduce the resistance of the spoiler 22 to the cooling airflow, reduce the resistance of the spoiler 22 to the cross flow, thereby increasing the flow rate of the cooling airflow to increase the heat exchange efficiency between the cooling airflow and the first plate 1 and the second plate 2.

[0055] In some embodiments, a cavity 223 is provided downstream of the leeward section 222 along the cross-flow direction, and the cavity 223 is suitable for forming a vortex downstream of the leeward section 222 .

[0056] Specifically, the recess 223 is located on the downstream side of the spoiler 22 close to the cross flow, and the recess 223 is concave toward the spoiler 22, so that part of the air flowing through the surface of the spoiler 22 forms a vortex in the recess 223. Thus, the recess 223 can increase the turbulence of the cooling airflow, thereby improving the heat exchange efficiency between the boundary layer airflow and the second plate 2.

[0057] In some embodiments, the impact holes 11 are arranged at intervals along the width direction of the airflow channel to form a plurality of hole rows, the plurality of hole rows including a first row 01 and a second row 02, the second row 02 is located downstream of the first row 01 in the airflow channel, and the plurality of impact holes 11 in the second row 02 are staggered with the plurality of impact holes 11 in the first row 01 in the width direction of the airflow channel.

[0058] Specifically, Figure 1 As shown, there are multiple first rows 01 and multiple second rows 02, and the multiple first rows 01 and the multiple second rows 02 are alternately arranged along the flow direction of the cross flow, the cooling airflow flows through the impact holes 11 and forms impact jets, so that the impact jets also have multiple rows, and the multiple rows of impact jets impact from the first plate 1 to the second plate 2 to cool the second plate 2.

[0059] The plurality of impact holes 11 in the second row 02 are arranged alternately with the plurality of impact holes 11 in the first row 01 , that is, each impact hole 11 in the second row 02 is located downstream of the midpoint of the geometric center connecting line of two adjacent impact holes 11 in the first row 01 .

[0060] Thus, the impact holes 11 of the first row 01 are arranged alternately with the impact holes 11 of the second row 02 , so as to prevent the impact jets generated by the impact holes 11 of the first row 01 from affecting the impact jets generated by the impact holes 11 of the second row 02 .

[0061] It can be understood that, since the spoilers 22 are located downstream of the impact holes 11 and the spoilers 22 correspond to the impact holes 11 one by one, the spoilers 22 also have multiple rows, and the multiple rows of spoilers 22 are staggered along the cross-flow direction.

[0062] Next, a turbine blade according to an embodiment of the present invention will be described.

[0063] The turbine blade of the embodiment of the present invention comprises an impingement cooling assembly, and the impingement cooling assembly may be an impingement cooling assembly according to any one of the above embodiments.

[0064] Specifically, a first plate 1 and a second plate 2 are provided in the turbine blade, the first plate 1 and the second plate 2 are arranged in parallel and spaced apart, and an air flow channel is provided between the first plate 1 and the second plate 2, so that the impingement cooling assembly can use the impingement jet to cool the second plate 2 of the turbine blade.

[0065] Therefore, since the impingement cooling assembly of the embodiment of the invention has a higher cooling efficiency, the turbine blades of the embodiment of the invention have a higher cooling efficiency, thereby reducing the temperature of the turbine blades of the embodiment of the invention during operation, so that the turbine blades of the embodiment of the invention have higher working stability.

[0066] In some embodiments, the turbine blade includes an impingement plate and a heat exchange plate, at least a portion of the impingement plate forms a first plate 1 , and at least a portion of the heat exchange plate forms a second plate 2 .

[0067] Specifically, the turbine blade includes an impact plate and a heat exchange plate. The heat exchange plate is located outside the turbine blade and forms a part of the turbine blade shell. When the turbine blade is working, one side of the heat exchange plate is high-temperature combustion gas, and the impact plate is located on the other side of the heat exchange plate.

[0068] The first plate 1 is a part of the impact plate, and the second plate 2 is a part of the heat exchange plate, so that the cooling airflow forms an impact jet when passing through the impact plate and flows to the second plate 2. The air flow rate at the contact point between the impact jet and the second plate 2 is relatively high, thereby effectively reducing the temperature of the second plate 2, thereby reducing the overall temperature of the turbine blades, and enabling the turbine blades to work stably at high temperatures.

[0069] Next, a gas turbine according to an embodiment of the present invention will be described.

[0070] The gas turbine of the embodiment of the present invention comprises the turbine blades of the above embodiment. Specifically, the turbine blades are arranged at intervals along the circumferential direction on the outer peripheral side of the turbine of the gas turbine. When the gas turbine is working, the gas turbine introduces cooling airflow to the side of the impact plate of the turbine blade away from the heat exchange plate. When the cooling airflow flows through the impact plate, an impact jet is generated and contacts the heat exchange plate. The impact jet exchanges heat with the heat exchange plate to reduce the temperature of the blade, thereby increasing the life of the turbine blade, so that the gas turbine of the embodiment of the present invention can continue to work safely and stably.

[0071] In the description of the present invention, it is to be understood that the terms “center”, “longitudinal”, “lateral”, “length”, “width”, “thickness”, “up”, “down”, “front”, “back”, “left”, “right”, “vertical”, “horizontal”, “top”, “bottom”, “inside”, “outside”, “clockwise”, “counterclockwise”, “axial”, “radial”, “circumferential”, etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the present invention.

[0072] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0073] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral one; it can be a mechanical connection, an electrical connection, or communication with each other; it can be a direct connection, or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements, unless otherwise clearly defined. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0074] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may mean that the first and second features are in direct contact, or the first and second features are in indirect contact through an intermediate medium. Moreover, a first feature being "above", "above" or "above" a second feature may mean that the first feature is directly above or obliquely above the second feature, or simply means that the first feature is higher in level than the second feature. A first feature being "below", "below" or "below" a second feature may mean that the first feature is directly below or obliquely below the second feature, or simply means that the first feature is lower in level than the second feature.

[0075] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the described specific features, structures, materials or characteristics may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

[0076] Although the above embodiments have been shown and described, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those of ordinary skill in the art are all within the scope of protection of the present invention.

Claims

1. An impingement cooling assembly, characterized in that: include: a first plate, wherein the first plate is provided with a plurality of impingement holes, wherein the impingement holes are suitable for allowing a cooling airflow to pass through the first plate and form an impingement jet; A second plate, the second plate is arranged at an interval with the first plate, and an air flow channel is formed between the second plate and the first plate, the air flow channel is suitable for cross flow circulation, a plurality of spoilers are provided on the side of the second plate facing the first plate, the plurality of spoilers are arranged one by one at the upstream of the plurality of impact holes, the spoilers are suitable for disturbing the cross flow upstream of each impact hole to weaken the disturbance of the cross flow to the impact jet, a plurality of recesses are provided on the side of the second plate facing the first plate, the recesses correspond to the impact holes, the recesses are grooves, and the grooves are arranged along the The invention relates to a cross-flow device, wherein the plurality of grooves are arranged at intervals along a direction orthogonal to the direction of the cross-flow, the spoiler is arranged in the groove, the width of the groove is greater than the width of the spoiler, the spoiler comprises a windward section and a leeward section, the leeward section is arranged downstream of the windward section along the cross-flow direction, the height of the windward section gradually increases along the cross-flow direction, the height of the leeward section gradually decreases along the cross-flow direction, the width of the windward section gradually increases along the cross-flow direction, and the width of the leeward section gradually decreases along the cross-flow direction.

2. The impingement cooling assembly according to claim 1, characterized in that A plurality of protrusions are provided on one side of the second plate facing the first plate. The protrusions extend along the flow direction of the cross flow. The plurality of protrusions are arranged at intervals along a direction orthogonal to the flow direction of the cross flow, and the plurality of protrusions are arranged at intervals from the plurality of grooves.

3. The impingement cooling assembly according to claim 1, characterized in that A recess is provided downstream of the leeward section along the cross flow direction, and the recess is suitable for forming a vortex downstream of the leeward section.

4. The impingement cooling assembly according to claim 1, characterized in that The impact holes are arranged at intervals along the width direction of the airflow channel to form a plurality of hole rows, the plurality of hole rows comprising a first row and a second row, the second row being located downstream of the first row in the airflow of the airflow channel, the plurality of impact holes in the second row being staggered with the plurality of impact holes in the first row in the width direction of the airflow channel.

5. A turbine blade, characterized in that: Comprising the impingement cooling assembly according to any one of claims 1-4.

6. The turbine blade according to claim 5, characterized in that It comprises an impact plate and an impact target surface, wherein at least a part of the impact plate forms the first plate, and at least a part of the heat exchange plate forms the second plate.

7. A gas turbine, characterized in that: The invention comprises an impingement cooling assembly as claimed in any one of claims 1 to 4, or comprises a turbine blade as claimed in any one of claims 5 to 6.

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