Turbine blade

technical field

0001The disclosure relates to a turbine blade for a fluid rotary machine with a blade which is delimited by a concave pressure side wall and a convex suction side wall which enclose a cavity which is delimited by the pressure and suction side walls and by an intermediate wall extending in the longitudinal direction and connecting the suction and pressure side walls internally.

State of the art

0002Turbine blades of the above-mentioned type are heat-resistant components which are used in particular within turbine stages of gas turbine arrangements and, in the form of guide or rotor blades, are exposed to the hot gases escaping directly from the combustion chamber.

0003The heat resistance of such turbine blades is due, on the one hand, to the use of heat-resistant materials and, on the other hand, to a highly efficient cooling of the turbine blades directly exposed to the hot gases, which have corresponding cavities for the purpose of continuous flow and exposure to a coolant, preferably cooling air, which are connected to a coolant feed system of the gas turbine arrangement. which provides cooling air for cooling all heat-exposed gas turbine components during gas turbine operation, especially the turbine blades.

0004Conventional turbine blades have a blade root, to which the blade is directly or indirectly connected radially. The blade has a concave pressure side wall and a convex suction side wall, which are integrally connected in the area of ​​the blade leading edge and between which a gap is defined, which is supplied with cooling air from the blade root side for cooling purposes. The term "radially" refers to the turbine blade extension in the assembled state within the gas turbine arrangement, which is oriented radially to the axis of rotation of the rotor unit. In order to ensure the supply and distribution of cooling air within the space enclosed between the suction side wall and the pressure side wall for optimized cooling of the turbine blade, the space is provided with radially extending partition walls, each of which separates cavities oriented radially within the blade, some of which have fluidic connections. At suitable locations along the cavities, passage openings are provided in the suction or pressure side wall, in the area of ​​the turbine blade leading and/or trailing edge or at the turbine blade tip so that the cooling air can escape to the outside into the hot gas channel of the turbine stage.

0005A gas turbine blade optimized for cooling purposes is the<patcit id="pcit0001" dnum="EP1319803A2"><text>EP 1 319 803 A2</text></patcit> which provides a large number of radially oriented cooling channel cavities within the turbine blade, each of which is fluidically connected in a meandering manner and through which more or less cooling air flows depending on the blade areas subject to different levels of heat. In particular, the area of ​​the blade leading edge, which experiences the greatest flow and heat exposure of the hot gases, must be cooled in a particularly efficient manner. For this purpose, a cavity extends along the inner wall of the blade leading edge, which is delimited by the suction and pressure side walls, which come together at the blade leading edge, and by an intermediate wall, which connects the suction and pressure sides with each other on the inner wall, and which is fed with cooling air from the blade root side. The cooling air flowing through the cavity usually reaches the outside in the area of ​​the blade tip. In order to improve the heat transfer between the blade wall and the cooling air flowing through the cavity, structures that swirl the cooling air flow are also provided along the wall areas enclosing the cavity.

0006Another preferred cooling of the blade leading edge area of ​​a turbine blade is in the<patcit id="pcit0002" dnum="US5688104A"><text>US 5,688,104</text></patcit> described. A cavity runs along the leading edge of the blade, which is delimited by the suction and pressure side walls, which come together at the leading edge of the blade, and by an intermediate wall, which rigidly connects the suction and pressure side walls within the blade. The cavity running along the leading edge of the blade is fed with cooling air, which enters the cavity exclusively through cooling channel openings provided within the intermediate wall. The rectilinear intermediate wall is provided with a plurality of individual through-channels in the radial longitudinal extension, through which cooling air from an adjacent radially extending cooling channel along the blade enters in the form of impingement cooling in the direction of the blade leading edge within the cavity described above. In order to discharge the cooling air introduced into the cavity, film cooling openings are provided along the leading edge of the blade, directed towards the suction and pressure side outer walls, through which the cooling air introduced within the cavity is discharged to the pressure and suction side outer walls, forming a film cooling.

0007In order to improve the cooling effect, particularly of the leading edge of a turbine blade, it is possible to use known cooling techniques to increase the cooling air supply on the one hand and to optimize the cooling mechanisms of the impingement cooling on the other.

0008Turbine blades which have the cooling measures described above for the purpose of optimised heat resistance, particularly in the area of ​​the blade leading edge, often show fatigue phenomena in the blade leading edge area along the pressure and suction side walls, which manifest themselves in the final stage as crack formation. The reason for such crack formation lies in the occurrence of thermo-mechanical stresses within the suction and pressure side walls in the blade leading edge area, which are caused by high temperature differences between the hot gas-exposed blade leading edge and the cooling air-exposed inner wall areas of the blade. In particular, in the case of transient operating conditions of the gas turbine arrangement, such as those that occur during start-up or during load changes in the turbine stage, temperature differences of approximately 1000°C can occur between the blade leading edge exposed to hot gas and the intermediate and inner wall sections exposed to cooling air. It is obvious that with such large temperature differences, considerable thermo-mechanical stresses occur within the suction side and pressure side walls along the blade leading edge, which lead to considerable material stresses, as mentioned above.

0009From the<patcit id="pcit0003" dnum="EP0447320A1"><text>EP 0 447 320 A1</text></patcit> A blade with suction and pressure side walls is known, into which an intermediate wall is inserted which connects the suction and pressure side walls and has holes for impact cooling of the side wall.

0010From the<patcit id="pcit0004" dnum="EP2107215A1"><text>EP 2107215 A1</text></patcit>, <patcit id="pcit0005" dnum="US3191908A"><text>US 3191908</text></patcit> and the<patcit id="pcit0006" dnum="JP2002242607A"><text>JP 2002 242607 A</text></patcit> Blades are known with an intermediate wall connecting the suction and pressure side walls and having holes for impact cooling of the side wall.

0011The<patcit id="pcit0007" dnum="EP0806546A1"><text>EP 0 806 546 A1</text></patcit> discloses a turbine blade with a ceramic leading edge insert, which is placed on the leading edge of the blade profile and is impingement air cooled.

0012From the<patcit id="pcit0008" dnum="EP2258925A2"><text>EP 2258 925 A2</text></patcit> and the<patcit id="pcit0009" dnum="US5246340A"><text>US 5246340</text></patcit> Blades are known with an intermediate wall connecting the suction and pressure side walls, which has holes for introducing a flow of a cooling medium into the front cooling channel.

presentation of Revelation

0013The disclosure is based on the object of providing a turbine blade according to claim 1 for a rotary flow machine with a blade which is delimited by a concave pressure side wall and a convex suction side wall, which are connected in the region of a blade leading edge which can be assigned to the blade and enclose a cavity extending along the longitudinal extent of the blade leading edge, which is delimited internally by the pressure and suction side walls in the area of ​​the blade leading edge and by an intermediate wall extending longitudinally to the blade leading edge and connecting the suction and pressure side walls internally, in such a way that the fatigue phenomena caused by temperature differences in the area of ​​the blade leading edge are reduced or even completely avoided, in order to improve the service life of turbine blades that are exposed to high levels of heat. The measures required for this should not impair the cooling measures that are already known, but rather improve and support them. The measures required for this should also not require any costly or complex manufacturing expenditure.

0014A turbine blade for a turbo-rotation machine according to the solution has a blade which is bounded by a concave pressure side wall and a convex suction side wall. These walls are connected in the region of a blade leading edge that can be assigned to the blade and enclose a cavity that extends in the longitudinal extension of the blade leading edge, which is delimited internally by the pressure and suction side walls in the region of the blade leading edge and by an intermediate wall that extends longitudinally to the blade leading edge and connects the suction and pressure side walls internally. This intermediate wall and the suction and/or pressure side wall are a continuous part. This is typically manufactured as a cast part. The disclosed turbine blade is characterized in that the intermediate wall in the connection area to the suction and/or pressure side wall has a perforation at least in sections in order to increase the elasticity. Perforation is understood to mean a large number of holes. These are typically arranged along a line. Typically, this line is straight, at least in sections. For example, three or more holes can be arranged along a straight line. In particular, this increases the elasticity of the partition wall. Due to the elastic connection area, the partition wall has a less stiffening effect on the entire blade, so that the tension between the pressure and suction side walls is also reduced. The connection area of ​​the partition wall to the suction and/or pressure side wall is the area of ​​the partition wall adjacent to the suction and/or pressure side wall. The connection area can extend up to a quarter of the distance between the suction and pressure side walls. Typically, the connection area extends to a distance that is smaller than the thickness of the partition wall or smaller than one to two times the thickness of the partition wall. According to one design, the connection area is limited to a curve or groove in the transition from the partition wall to the suction and/or pressure side wall. According to another design, the connection area is limited to an area from the side wall that corresponds to twice the radius of the curve or groove in the transition from the partition wall to the suction and/or pressure side wall.

0015The disclosure is based on the finding that the fatigue crack formation in the blade leading edge area of ​​turbine blades exposed to hot gases is primarily due to the fact that the thermally induced expansion and contraction tendency of the pressure and suction side wall areas in the blade leading edge area is counteracted by the rigidity of the partition wall, which is constantly flowing with cooling air, which is arranged downstream of the blade leading edge immediately within the blade and firmly connects the suction side wall and pressure side wall, mechanically counteracts this, whereby the strongly heated, heat-exposed suction and pressure side wall areas experience increased internal mechanical stress, which in turn results in high material stress, which ultimately leads to fatigue phenomena that reduce the service life. In order to counteract the mechanical stress which causes the fatigue phenomena and which acts on the pressure and suction side wall areas along the blade leading edge, the intermediate wall which is arranged immediately downstream of the blade leading edge and which, together with the inner walls of the pressure and suction side walls, delimits the cavity running along the blade leading edge, is modified according to the solution in such a way that the intermediate wall or the connection area of ​​the intermediate wall experiences elasticity, whereby the thermally induced expansion and contraction tendencies of the suction side and pressure side wall areas along the blade leading edge can be at least partially accommodated. For this purpose, in contrast to the conventional rigid wall connection between the partition wall and the suction and pressure side walls, the partition wall has a perforation at least in a connection area to the side wall, through which the elasticity described above can be realized. According to one embodiment, the perforation comprises a series of cylindrical holes. According to a further embodiment, the perforation comprises a series of elongated holes or slots, the longer side of which extends parallel to the adjacent suction or pressure side wall.

0016The connection between the intermediate wall and the side wall results in relatively thick accumulations of material whose surface-to-volume ratio is much smaller than in a free wall section. On the inside, the connection also impedes the flow of the walls, so that the temperature of the blade material in the connection area changes more slowly in the event of transient changes in the hot gas or cooling air temperatures than the material temperatures in a free wall section. This leads to additional thermal stresses, which are reduced by the perforation.

0017Typically, the connection area between the partition wall and the suction and/or pressure side wall is even designed with a curve or groove. This groove is a manufacturing feature of cast blades. On the one hand, it reduces the concentration of stress at the wall connection, but on the other hand, the groove increases the accumulation of material in the connection area between the partition wall and the suction and/or pressure side wall. The perforation in the connection area improves the heat transfer on the inside of the walls so that transient temperature changes can be better followed. In order to further counteract the effect of material accumulation and to improve the heat transfer in the connection area, according to one embodiment the perforation runs at least partially through the cove.

0018The partition wall has at least one curved wall section extending from the suction side wall to the pressure side wall or vice versa, deviating from a straight wall course. This curvature increases the elasticity, so that a flexible partition wall is produced, particularly in combination with the perforated connection area of ​​the partition wall.

0019In a preferred embodiment, the intermediate wall which faces directly towards the blade leading edge and which connects the suction and pressure side inner walls to one another has a "V" or "U"-shaped wall cross-section which preferably extends over the entire radial length of the intermediate wall. A curvature of the intermediate wall which is designed in accordance with the solution and which runs from the suction to the pressure side wall or extends in reverse and enables a curvature-related wall elasticity in precisely this spatial direction, allows the tendency of the suction and pressure side walls to space themselves apart relative to one another to yield in the event of a thermally induced expansion of the suction and pressure side walls in the blade leading edge area by elastic stretching of the curved intermediate wall. In the opposite case of thermally induced material shrinkage, which leads to a reduction in the mutual distance between the pressure and suction side walls in the blade leading edge area, the curved intermediate wall is able to follow the decreasing wall distance by increasing the wall curvature.

0020According to a further embodiment, the turbine blade has at least in sections a perforation at the base of the "V-" or "U-shaped" cross-section of the intermediate wall, which runs parallel to the perforation of the connection region in order to increase the elasticity. Overall, this results in a hinge-like structure for the partition wall between the two legs of the "V" or "U"-shaped cross-section, which allows the legs to rotate around the perforations and thus compensates for changes in the mutual distance between the pressure and suction side walls.

0021Due to the flexibility of the partition wall explained above, the mutual distance between the pressure and suction side walls in the blade leading edge area can be adjusted depending on the temperature level without harmful mechanical stresses occurring within the pressure and suction side walls, particularly in the connection area to the inner partition wall.

0022It is of course conceivable to design the partition wall in question with curved wall contours that deviate from the "V" or "U" wall cross-section shape. However, all such wall sections designed according to the solution have in common that they have a curvature-related wall elasticity and are flexibly connected to the outer walls through the perforation.

0023To further improve the wall elasticity, a preferred embodiment provides for the partition wall to be designed at least in some areas with an equal or preferably smaller partition wall thickness compared to the wall thicknesses of the suction and pressure side walls in the blade leading edge area. It is not necessarily necessary for the partition wall to have a constant wall thickness along its entire wall cross section. In this way, the thickness of the partition wall, the elasticity of the perforated connection area and the curvature behavior of the partition wall can be optimally coordinated with one another in such a way that a particularly suitable wall elasticity can be achieved. If particularly high wall elasticities are to be achieved, particularly strongly curved and/or suitably thin wall sections along the partition wall are suitable.

0024Furthermore, the proposed solution of an intermediate wall with a perforated connection area is not necessarily limited to the intermediate wall directly facing the blade leading edge. It is of course also possible to design additional intermediate walls within the blade profile with perforations or perforations and curved in accordance with the solution in order to be able to give way to thermally induced shrinkage or expansion effects relating to the pressure and suction side walls without stress. It has proven to be particularly advantageous that the "V" or "V-shaped" walls are perforated. "U"-shaped wall curvature of the intermediate wall directly facing the blade leading edge is designed and arranged such that the convex wall side of the "V" or "U"-shaped wall section faces the area of ​​the blade leading edge.

0025Furthermore, it is advantageous to form the curvature contour of the intermediate wall extending from the suction to the pressure side wall or in the opposite direction in such a way that the convex wall side of the intermediate wall facing the blade leading edge is formed and arranged largely parallel to the suction and pressure side wall delimiting the cavity and connected to the blade leading edge. Such a design is particularly advantageous when implementing so-called impingement cooling, as will be shown in the further explanations with reference to a relevant embodiment. In this case, it is possible to direct impingement cooling air flows through passage channels introduced within the partition wall to specific inner wall areas in the blade leading edge area. In this way, temperature-related material stresses can be effectively counteracted by optimized cooling of the blade leading edge area.

0026In order to achieve sufficient flexibility, according to one embodiment, a row of holes is considered to be a perforation in which the proportion of hole lengths in the perforation direction is at least 30% of the total length of the perforated area. For high flexibility, according to another embodiment, the proportion of hole lengths is at least 50% of the total length of the perforated area. This is e.g. realized by a series of cylindrical holes, each spaced twice the diameter. Particularly in designs with elongated holes or slots, a proportion of the hole lengths can exceed 70% of the total length of the perforated area.

0027The connection area of ​​the partition wall to the pressure or suction side wall comprises, for example, up to 20% of the wall distance between the two side walls. Typically, the connection area extends one or two wall thicknesses of the partition wall in the connection direction of the partition wall.

Short description of the characters

0028Preferred embodiments of the disclosure are described below with reference to the drawings, which are for illustrative purposes only and are not to be interpreted as limiting. In the drawings:<dl id="dl0001"><dt>Fig. 1</dt><dd>Illustration of the schematic arrangement of turbine guide vanes and turbine rotor blades within a turbine stage,</dd><dt>Fig. 2</dt><dd>representative profile through a turbine blade and</dd><dt>Fig. 3a, b, c</dt><dd>alternative variants for the formation of a perforation in an intermediate wall in the area of ​​the blade leading edge,</dd><dt>Fig. 4a - d</dt><dd>alternative variants for the formation of an intermediate wall in the area of ​​the blade leading edge.</dd></dl>

Detailed description

0029In<figref idref="f0001">Fig. 1</figref> A guide vane 2 and a rotor blade 3 are shown in a schematic representation, as they are arranged along a row of guide vanes and rotor blades in a turbine stage 1 (not further illustrated). It is assumed that the guide vane 2 and the rotor blade 3 come into contact with a hot gas flow H, which flows over the respective blades 4 of the guide vane 2 and the rotor blade 3 from left to right in the illustration. The blades 4 of the guide vanes and rotor blades 2, 3 protrude into the hot gas channel of the turbine stage 1 of a gas turbine arrangement, which is delimited by radially inner shrouds 2i, 3i and by the radially outer shrouds 2a of the guide vanes 2 and by radially outer heat accumulation segments 3a. The rotor blade 3 is mounted on a rotor unit R (not shown in detail) which is mounted so as to be rotatable about a rotation axis A.

0030In<figref idref="f0002">Fig. 2</figref> is a cross-sectional view of a guide or rotor blade, which extends along a<figref idref="f0001">Fig. 1</figref> The typical blade profile of a turbine guide vane or turbine rotor blade is characterized by an aerodynamically profiled blade 4, which is bounded on both sides by a convex suction side wall 7 and a concave pressure side wall 6. The convex suction side wall 7 and the concave pressure side wall 6 are joined together in one piece in the area of ​​the blade leading edge 5, which, as already explained at the beginning, is directly exposed to the hot gas flow passing through the turbine stage of a gas turbine arrangement. It is obvious that the turbine blade area along the blade leading edge 5 is subjected to particularly high thermal stress.

0031To cool the turbine blade exposed to the hot gases, radially oriented cavities 9, 10, 11 etc. are provided within the blade 4, which are flushed with cooling air. The individual cavities 9, 10, 11 etc. are separated from one another by partition walls 8, 12, 13 etc. Depending on the design and shape of the turbine blade, the individual cooling channels 9, 10, 11 etc. communicate with one another.

0032In order to solve the problem described at the beginning of fatigue-related crack formation in the suction and pressure side walls 6, 7 near the blade leading edge 5, the foremost intermediate wall 8 in the connection area to the suction 7 and/or pressure side walls 6 is provided with a perforation 16 at least in sections. Embodiments of perforations 16 are shown in the<figref idref="f0002">Fig. 3a, b and c</figref> shown.

0033A first embodiment is shown in the<figref idref="f0002">Fig. 3a</figref> shown. One perforation 16 is provided in the connection area of ​​the intermediate wall 8 to the suction and pressure side walls 6, 7. The perforations in the example shown are a series of cylindrical holes 18 that are arranged parallel to the suction and pressure side walls 6, 7. The perforation 16 on the pressure side wall 6 runs in the example only over a section of the intermediate wall 8.

0034A second embodiment is shown in the<figref idref="f0002">Fig. 3b</figref> shown. One perforation 16 is provided in the connection area of ​​the intermediate wall 8 to the suction and pressure side walls 6, 7. The perforations in this example are a series of elongated holes 19 which are arranged parallel to the suction and pressure side walls 6, 7 and whose longer side extends parallel to the adjacent suction 7 or pressure side wall 6.

0035In the third embodiment of the<figref idref="f0002">Fig. 3c</figref> is in addition to the perforations 16 of the<figref idref="f0002">Fig. 3b</figref> In the example shown, a central perforation 20 is also provided, which runs parallel to the suction and pressure side walls 6, 7 in the middle of the intermediate wall 8. Together with the perforations 16 in the connection area to the suction and pressure side walls 6, 7, a two-part intermediate wall 8 is formed, which can be flexibly folded together.

0036For a better illustration of the partition wall design, please refer to<figref idref="f0003">Fig. 4a</figref> illustrated detailed embodiment, which shows the blade profile in the blade leading edge area. The<figref idref="f0003">Fig. 4a</figref> shows a perforation 16 in the connection area of ​​the suction side wall 7 and in the connection area of ​​the pressure side wall 6. The main direction of the material expansion or shrinkage tendency 21 of the side walls 6, 7 runs in the example essentially parallel to the extension of the intermediate wall 8.

0037In contrast to a linear education, as is the case in<figref idref="f0001">Fig. 1</figref>, <figref idref="f0002">2</figref>, <figref idref="f0002">3</figref> and<figref idref="f0003">4a</figref> As is the case with partition walls 8, 12, 13,<figref idref="f0003">Fig. 4b</figref> an embodiment with a curved intermediate wall 8 is shown. The intermediate wall 8 has a U-shaped wall cross-section, which is integrally connected on both sides to the suction side wall 6 and the pressure side wall 7. The U-shaped wall design of the intermediate wall 8 gives the blade profile area additional elastic deformability in such a way that the thermally induced material expansion or contraction is compensated for. The shrinkage tendency of the suction and pressure side walls can be accommodated by the wall distance w not being fixed, as before, but being variable within certain limits which are determined by the shape and curvature elasticity of the intermediate wall 8 and the elasticity of the perforation 16.

0038In<figref idref="f0003">Fig. 4c</figref> an embodiment with an additional central perforation 20 is shown in detail. This divides the intermediate wall 8 into two legs, which run towards each other at an angle starting from the connection area to the side walls 6, 7, whereby the angle can be flexibly changed by the central perforation 20 and thus expansion-related changes in the distance between the pressure and suction side walls can be easily compensated.

0039Next is in<figref idref="f0003">Fig. 4c</figref> an example of a possible film cooling arrangement is shown. Cooling air passes out of the cavity 9 through the film cooling holes 14 and forms a cooling air film that rests on the surface of the suction side 6 and pressure side outer wall 7. The U-shaped intermediate wall 8, which is integrally connected on both sides to the inner wall of the suction side wall 7 and the pressure side wall 6, preferably has a convex-sided wall profile which faces the blade leading edge 5 and is formed largely parallel to the suction side wall 7 and the pressure side wall 6 which delimit the cavity 9 and are integrally connected to the blade leading edge 5. In this example, the cooling air enters the front cavity 9 at least partially through the perforations 16 and central perforation 20.

0040Another embodiment with details on cooling is shown in<figref idref="f0003">Fig. 4d</figref> shown. Here, the intermediate wall has perforations 16 in the connection areas to the suction 7 and pressure side wall 6. In addition to the perforations, it also has cooling air passage channels 15a, b, c, which serve for impingement air cooling of the inner wall side of the blade wall leading edge. In a particularly advantageous manner, the passage channels 15a, b, c can be divided into at least three groups with regard to their passage channel longitudinal extent and the flow direction predetermined thereby. A first group of passage channels 15a is characterized by a flow direction directed towards the suction side wall 7, a second group of passage channels 15b is characterized by a flow direction directed towards the blade leading edge and a third group of passage channels 15c is characterized by a flow direction directed towards the pressure side wall 6. The passage channels 15a, 15b and 15c are distributed along the entire radial extent in the intermediate wall 8 and thus ensure effective and individual cooling of the leading edge area of ​​the turbine blade. Of course, further passage channels can be attached to the intermediate wall 8 for the purpose of optimized impact cooling.

0041Furthermore, impingement air cooling can be combined with central perforation. Typically, impingement air cooling holes have a larger diameter, e.g. twice as large as perforation holes.

list of reference symbols

0042<dl id="dl0002" compact="compact"><dt>1</dt><dd>turbine stage</dd><dt>2</dt><dd>vane</dd><dt>2i</dt><dd>Inner shroud of the guide vane</dd><dt>2a</dt><dd>Outer shroud of the guide vane</dd><dt>3</dt><dd>blade</dd><dt>3i</dt><dd>Inner cover sheet of the rotor blade</dd><dt>3a</dt><dd>heat accumulation segment</dd><dt>4</dt><dd>blade</dd><dt>5</dt><dd>blade leading edge</dd><dt>6</dt><dd>Concave pressure sidewall</dd><dt>7</dt><dd>Convex suction side wall</dd><dt>8</dt><dd>partition wall</dd><dt>9</dt><dd>cavity</dd><dt>10,11</dt><dd>cavities</dd><dt>12,13</dt><dd>partition walls</dd><dt>14</dt><dd>film cooling holes</dd><dt>15</dt><dd>passage channels</dd><dt>16</dt><dd>perforation</dd><dt>17</dt><dd>cove</dd><dt>18</dt><dd>Hole</dd><dt>19</dt><dd>slot</dd><dt>20</dt><dd>center perforation</dd><dt>21</dt><dd>Main direction of material expansion or contraction</dd><dt>R</dt><dd>rotor unit</dd><dt>A</dt><dd>axis of rotation</dd><dt>E</dt><dd>elastic degree of freedom</dd><dt>W</dt><dd>wall distance</dd></dl>