Patent Description:
During the operation of the electrical energy production plants, the blades of gas turbines are constantly exposed to a hot gas flow coming from the combustion chamber.

The temperature of the hot gas flowing in the gas turbine affects the performance of the plant. In particular, performances of the plant increase with an increasing temperature of the hot gas flowing inside the turbine.

However, the increase of the temperature of the hot gas flowing in the gas turbine is limited by the thermal resistance of the material constituting the blades.

To overcome this kind of limitation, in recent years, a cooling system for the blades has been adopted. Normally cooling air extracted from the compressor or coming from a dedicated cooling air source is driven through the blades. Examples of blades provided with a cooling system are disclosed in documents <CIT> or <CIT>.

However, introducing a large amount of cooling air into the blades of the gas turbine would lead to excessive thermodynamic losses. Another cooling system is disclosed in document <CIT>.

The object of the present invention is therefore to provide a blade having an optimized cooling system, capable of improving the thermal resistance of the blades, allowing a further increase of the temperature of the gases flowing in the gas turbine and reducing the thermodynamic losses thus consequently improving the plant performances.

According to the present invention, there is provided a blade for a gas turbine as claimed in claim <NUM>.

Thanks to the presence of a plurality of inlet holes having a defined passage area the flow rate of the cooling fluid exiting through the discharge arrangement is properly regulated. While thanks to the fact that the plurality of cooling fluid flows join at the outlet common slot the film cooling efficiency is improved. The external face of the outer wall, in fact, is lapped by a cooling flow which is wide and homogeneous.

As the efficiency of the cooling is increased, a lower cooling fluid flow rate can be drawn for cooling the blades. This lead to a significant increase in the efficiency of the plant as the cooling fluid is normally drawn from the compressor of the plant.

According to the present invention, the discharge arrangement comprises a plurality of connecting channels, each of which is configured to connect a respective hole with the outlet common slot.

According to a preferred embodiment of the present invention, the inlet holes are substantially identical to each other.

According to a preferred embodiment of the present invention, the connecting channels are substantially identical to each other.

According to the present invention, each connecting channel have an inlet section and an outlet section; the inlet section of each connecting channel being in contact with the respective inlet hole and the outlet section of each connecting channel being in contact with the outlet common slot.

According to the present invention, the passage area of each inlet hole is smaller than the passage area of the inlet section of the respective connecting channels.

According to a preferred embodiment of the present invention, the connecting channels are diverging toward the outlet common slot.

According to a preferred embodiment of the present invention, the passage area of the inlet holes is constant. According to a preferred embodiment of the present invention, the inlet holes are substantially aligned along a direction which is substantially a straight line extending from a base of the airfoil to a tip of the airfoil.

According to a preferred embodiment of the present invention, the inlet holes are arranged equally spaced from each other.

According to a preferred embodiment of the present invention, the discharge arrangement comprises at least two discharge groups; each discharge group comprises the plurality of inlet holes, the outlet common slot and the plurality of connecting channels.

According to a preferred embodiment of the present invention, the inlet holes of each discharge group are equally spaced one from the other.

According to a preferred embodiment of the present invention, the discharge groups are equally spaced one from the other.

It is furthermore another object of the present invention to provide a plant for electric power production having an improved power efficiency.

According to said object the present invention relates to a plant for electric power production comprising at least one gas turbine, which extends along a longitudinal axis and comprises at least one row of blades circumferentially spaced and extending radially outwardly from a respective supporting disc of the gas turbine; at least one of the blades of the row being of the type claimed in anyone of the claims <NUM>-<NUM>.

According to a preferred embodiment of the present invention, the plant comprises at least one compressor which is connected to the gas turbine by a suction line configured to draw cooling air from the compressor and supply it to the cooling arrangement of the at least one blade.

The present invention will now be described with reference to the accompanying drawings, which illustrate some non-limitative embodiment, in which:.

In <FIG>, reference numeral <NUM> indicates a gas turbine plant for electrical energy production comprising a compressor <NUM>, a combustor <NUM>, a gas turbine <NUM> and a generator <NUM>, which transforms the mechanical power supplied by turbine <NUM> into electrical power to be supplied to an electrical grid <NUM>, connected to the generator <NUM> via a switch <NUM>.

A variant not shown provides for plant <NUM> to be of the combined cycle type and including, in addition to the gas turbine <NUM> and generator <NUM>, also a steam turbine.

The gas turbine <NUM> extends along a longitudinal axis A and is provided with a shaft <NUM> (also extending along axis A) to which compressor <NUM> and generator <NUM> are also connected.

Gas turbine <NUM> comprises an expansion channel <NUM> wherein the hot gas working fluid coming from the combustor <NUM> flows in a direction D.

The expansion channel <NUM> has a section which radially increases along the axis A in the direction D.

In the expansion channel <NUM> a plurality of stages <NUM> spaced along the longitudinal axis A is arranged. Each stage <NUM> comprises a row of fixed blades and a row of rotating blades (not illustrated in <FIG>). Each row comprises circumferentially spaced blades extending radially outwardly from a respective supporting disc.

In <FIG> a blade <NUM> of a stage <NUM> of the gas turbine <NUM> is represented.

Preferably, blade <NUM> is a rotating blade, but it is clear that the present invention can also be applied to stator blades.

The blade <NUM> comprises a root <NUM>, an airfoil <NUM> and a platform <NUM>.

The root <NUM> is configured to be coupled to a supporting disc (not illustrated in the accompanying figures) of the gas turbine <NUM>. In particular, the disc has a plurality of axial seats, which are circumferentially spaced and engaged by respective roots <NUM> of the rotating blades <NUM>.

The airfoil <NUM> extends from the root <NUM> and is provided with base <NUM> coupled to the root <NUM> and a tip <NUM> which, in use, is radially opposite to the base <NUM>.

The airfoil <NUM> is completely housed in the expansion channel <NUM> and defines the aerodynamic profile of the rotating blade <NUM>.

The airfoil <NUM> has a concave pressure side <NUM> (better visible in <FIG> and <FIG>) and a convex suction side <NUM>, which, in use, extend axially between a leading edge <NUM> and a trailing edge <NUM> and radially between the base <NUM> and the tip <NUM>.

The leading edge <NUM> is arranged upstream of the trailing edge <NUM> along the direction D of the hot working fluid in the expansion channel <NUM>.

The platform <NUM> is arranged between the root <NUM> and the airfoil <NUM>.

Blade <NUM> is provided with a cooling arrangement <NUM>. The cooling arrangement <NUM> comprise a plurality of feeding channels <NUM> made in the root <NUM> and a plurality of cooling paths <NUM> (not illustrated in <FIG> and better visible in <FIG> and <FIG>) made in the airfoil <NUM>.

The feeding channels <NUM> are supplied with a cooling fluid coming from a cooling fluid source <NUM>.

Preferably, the cooling fluid source <NUM> is a portion of the compressor <NUM>. In <FIG> a suction line <NUM> dedicated to the suction of cooling air from the compressor <NUM> and connected to the gas turbine <NUM> is shown.

Preferably, each feeding channel <NUM> is coupled to a respective cooling path <NUM>. According to a variant not shown each feeding channels can be coupled to more than one cooling path.

In the non-limiting embodiment here disclosed and illustrated, the feeding channels <NUM> are four and the cooling paths <NUM> are four.

With reference to <FIG>, the cooling arrangement <NUM> comprises a suction side cooling path 31a mainly dedicated to the cooling of the suction side <NUM>, a pressure side cooling path 31b mainly dedicated to the cooling of the pressure side <NUM>, a leading edge cooling path 31c mainly dedicated to the cooling of the leading edge <NUM> and a trailing edge cooling path 31d mainly dedicated to the cooling of the trailing edge <NUM>.

In <FIG> a dashed line is used to schematically indicate the cooling path 31a, a dashed-dotted line is used to schematically indicate the pressure side cooling path 31b, a dotted line is used to schematically indicate the leading edge cooling path 31c, a solid line is used to schematically indicate the trailing edge cooling path 31d. The airfoil <NUM> comprises an outer wall <NUM> and an inner wall <NUM>.

The outer wall <NUM> defines at least in part the aerodynamic profile of the blade <NUM> and has an external face <NUM> which, in use, is arranged in contact with the hot gas working fluid flowing in the expansion channel <NUM>.

The inner wall <NUM> is enclosed by the outer wall <NUM> and may have cooling and structural functions.

In particular the inner wall <NUM> defines an inner central cooling chamber <NUM>, through which, in use, the cooling fluid coming from a respective feeding channel <NUM> flows as will be detailed in the following.

The suction side cooling path 31a is defined between the inner wall <NUM> and the outer wall <NUM> and extends at least partially along the suction side <NUM>.

The suction side cooling path 31a comprises at least one inlet <NUM> (better visible in <FIG>) and at least one discharge arrangement <NUM>.

The inlet <NUM> being arranged closer to the trailing edge <NUM> than the discharge arrangement <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the suction side cooling path 31a comprises one inlet <NUM>, which is defined by an aperture located at the base <NUM> of the airfoil <NUM> and in fluidic communication with the respective feeding channel <NUM> of the root <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the suction side cooling path 31a comprises one discharge arrangement <NUM> which will be described in detail later.

In more detail, the suction side cooling path 31a comprises a plurality of suction side cooling chambers <NUM> which are in fluidic communication and arranged side by side between the inner wall <NUM> and the outer wall <NUM> along the suction side <NUM>.

Each of the suction side cooling chambers <NUM> extends substantially along a direction going from the base <NUM> toward the tip <NUM>.

The plurality of suction side cooling chambers <NUM> comprises a suction side inlet chamber 42a, which is the suction side cooling chamber <NUM> closest to the trailing edge <NUM>, and a suction side discharge chamber 42b, which is the suction side cooling chamber <NUM> closest to the leading edge <NUM>.

The suction side inlet chamber 42a comprises the inlet <NUM> and the suction side discharge chamber 42b comprises the discharge arrangement <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the suction side cooling path 31a comprises three suction side cooling chambers <NUM>. In other words, between the suction side inlet chamber 42a and the suction side discharge chamber 42b only one suction side intermediate chamber 42c is arranged.

In use, the cooling fluid coming from the respective feeding channel <NUM> of the root <NUM> flows along the suction side inlet chamber 41a, along the suction side intermediate chamber 42c along the suction side discharge chamber 42b and exits through the discharge arrangement <NUM> of the suction side discharge chamber 42b.

In other words, the flow of the cooling fluid along the suction side cooling path 31a is a counter-current flow with respect to the flow of the hot gas working fluid in the expansion channel <NUM> having direction D.

The pressure side cooling path 31b is defined between the inner wall <NUM> and the outer wall <NUM> and extends at least partially along the pressure side <NUM>.

The pressure side cooling path 31b comprises at least one inlet <NUM> (better visible in <FIG>) and at least one discharge arrangement <NUM>.

The discharge arrangement <NUM> being arranged closer to the trailing edge <NUM> than the inlet <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the pressure side cooling path 31b comprises one inlet <NUM>, which is defined by an aperture located at the base <NUM> of the airfoil <NUM> and in fluidic communication with the respective feeding channel <NUM> of the root <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the pressure side cooling path 31b comprises two discharge arrangements <NUM>, which will be described in detail later.

In more detail, the pressure side cooling path 31b comprises a plurality of pressure side cooling chambers <NUM> which are in fluidic communication and arranged side by side between the inner wall <NUM> and the outer wall <NUM> along the pressure side <NUM>.

Each of the pressure side cooling chambers <NUM> extends substantially along a direction going from the base <NUM> toward the tip <NUM>.

The plurality of pressure side cooling chambers <NUM> comprises a pressure side inlet chamber 47a, which is the pressure side cooling chamber <NUM> closest to the leading edge <NUM>, and at least one pressure side discharge chamber 47b, which is the pressure side cooling chamber <NUM> closest to the trailing edge <NUM>.

The pressure side inlet chamber 47a comprises the inlet <NUM> and the pressure side discharge chamber 47b comprises at least one discharge arrangement <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the pressure side cooling path 31b comprises three pressure side cooling chambers <NUM>: one pressure side inlet chamber 47a and two subsequent discharge chambers 47b, each of which is provided with at least one discharge arrangement <NUM>.

In use, the cooling fluid coming from the respective feeding channel <NUM> of the root <NUM> flows along the pressure side inlet chamber 47a, along the pressure side discharge chamber 47b adjacent to the pressure side inlet chamber 47a and along the pressure side discharge chamber 47b closest to the trailing edge <NUM> and exits through the two discharge arrangements <NUM> of the pressure side discharge chambers 47b.

In other words, the flow of the cooling fluid along the pressure side cooling path 31b is a co-current flow with respect to the flow of the hot gas working fluid in the expansion channel <NUM> having direction D.

The leading edge cooling path 31c is defined by the inner central cooling chamber <NUM> and by a leading edge cooling chamber <NUM> arranged between the inner wall <NUM> and the outer wall <NUM> at the leading edge <NUM>. The inner central cooling chamber <NUM> being in fluidic communication with the leading edge cooling chamber <NUM> by at least one connecting aperture <NUM>.

The inner central cooling chamber <NUM> and the leading edge cooling chamber <NUM> extend substantially along a direction going from the base <NUM> toward the tip <NUM>.

The leading edge cooling path 31c comprising at least one inlet <NUM> (better visible in <FIG>) and at least one discharge arrangement <NUM>.

The discharge arrangement <NUM> being arranged closer to the leading edge <NUM> than the inlet <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the leading edge cooling path 31c comprises one inlet <NUM>, which is defined by an aperture located at the base <NUM> of the airfoil <NUM> and in fluidic communication with the respective feeding channel <NUM> of the root <NUM>.

In the non-limiting embodiment here disclosed and the leading edge cooling path 31c comprises a plurality of discharge arrangements <NUM>, which will be described in detail later. Preferably the discharge arrangements <NUM> are at least three: at least one discharge arrangement <NUM> directed toward the leading edge <NUM>, at least one discharge arrangement <NUM> directed toward the suction side <NUM> and at least one discharge arrangement <NUM> directed toward the pressure side <NUM>.

In more detail, the leading edge cooling chamber <NUM> comprises the discharge arrangements <NUM>, while the inner central cooling chamber <NUM> comprises the inlet <NUM>.

In use, the cooling fluid coming from the respective feeding channel <NUM> of the root <NUM> flows along the inner central cooling chamber <NUM>, through the connecting aperture <NUM>, along leading edge cooling chamber <NUM> and exits through the discharge arrangements <NUM> of the leading edge cooling chamber <NUM>.

In other words, the flow of the cooling fluid along the leading edge cooling path 31c is a co-current flow with respect to the flow of the hot gas working fluid in the expansion channel <NUM> having direction D.

The trailing edge cooling path 31d is defined by a trailing edge cooling chamber <NUM> arranged between the inlet <NUM> of suction side cooling path 31a and the trailing edge <NUM>.

The trailing edge cooling chamber <NUM> extends substantially along a direction going from the base <NUM> toward the tip <NUM>. The trailing edge cooling path 31d comprises at least one inlet <NUM> (better visible in <FIG>) and at least one discharge arrangement <NUM>.

The discharge arrangement <NUM> being arranged on the pressure side <NUM> and configured to direct the flow toward the trailing edge <NUM>.

In the non-limiting embodiment here disclosed and illustrated, the trailing edge cooling path 31d comprises one inlet <NUM>, which is defined by an aperture located at the base <NUM> of the airfoil <NUM> and in fluidic communication with the respective feeding channel <NUM> of the root <NUM>.

In the non-limiting embodiment here disclosed and illustrated the trailing edge cooling path 31d comprises one discharge arrangement <NUM>, which will be described in detail later.

In more detail, the trailing edge cooling chamber <NUM> comprises the discharge arrangements <NUM> and the inlet <NUM>. In use, the cooling fluid coming from the respective feeding channel <NUM> of the root <NUM> flows along the trailing edge cooling chamber <NUM> and exits through the discharge arrangement <NUM> toward the trailing edge <NUM>.

The suction side cooling chambers <NUM>, the pressure side cooling chambers <NUM>, the leading edge cooling chamber <NUM> and the trailing edge cooling chamber <NUM> can be optionally provided with at least one turbulator in order to improve the cooling effect.

In particular, the suction side cooling chambers <NUM>, the pressure side cooling chambers <NUM> and the trailing edge cooling chamber <NUM> may comprise turbulators defined by ribs which project from at least one internal face of the respective chamber and are angled with respect to the direction of the cooling fluid inside the chamber. Preferably said turbulators project from three adjacent internal faces of the respective chamber.

The leading edge cooling chamber <NUM> may comprise a plurality of turbulators defined by ribs projecting from at least one internal face of the leading edge cooling chamber <NUM>. Said ribs have a trapezoidal-shaped section. Preferably said turbulators are arranged staggered with respect to the inlet holes of the cooling arrangements <NUM> at least on the two internal faces of the leading edge cooling chamber <NUM> which are respectively closest to the pressure side <NUM> and to the suction side <NUM>.

In <FIG> and <FIG> the shape of a discharging arrangement is illustrated.

Preferably, the discharging arrangement <NUM> of the suction side cooling path 31a, the discharge arrangements <NUM> of the pressure side cooling path 31b, the discharge arrangements <NUM> of the leading edge cooling path 31c and the discharge arrangement <NUM> of the trailing edge cooling path 31d have all the structure illustrated in <FIG> and <FIG>.

According to a variant not illustrated, at least one of the discharge arrangement <NUM><NUM><NUM><NUM> have the structure illustrated in <FIG> and <FIG>.

In <FIG> and <FIG> only the discharge arrangement <NUM> is illustrated. However, as the structure of the remaining discharge arrangements <NUM><NUM><NUM> is substantially identical to the structure of the discharge arrangement <NUM>, the following considerations can be considered valid also for the discharge arrangements <NUM><NUM><NUM>.

Discharge arrangement <NUM> extends through the outer wall <NUM> from the respective internal face of the pressure side discharge cooling chamber 47b to the external face <NUM> of the outer wall <NUM>.

With reference to <FIG>, the discharge arrangement <NUM> comprises a plurality of inlet holes <NUM>, an outlet common slot <NUM> and a plurality of connecting channels <NUM>, each of which is configured to connect a respective hole <NUM> with the outlet common slot <NUM>.

Preferably the inlet holes <NUM> are identical to each other.

Preferably the connecting channels <NUM> are identical to each other.

The connecting channels <NUM> are diverging toward the outlet common slot <NUM>. In other words, the connecting channels <NUM> have a passage area which gradually increases toward the outlet common slot <NUM>.

The increase of the passage area starts from an inlet section <NUM> of the connecting channels <NUM> and ends at an outlet section <NUM> of the connecting channels <NUM>. The inlet section <NUM> of each connecting channel <NUM> being in contact with the respective inlet hole <NUM> and the outlet section <NUM> of each connecting channel <NUM> being in contact with the outlet common slot <NUM>.

The passage area of the inlet holes <NUM> is constant.

As better visible in <FIG>, the passage area of the inlet hole <NUM> is smaller than the passage area of the inlet section <NUM> of the respective connecting channels <NUM>.

In the non-limiting example here disclosed and illustrated the passage area of the inlet section <NUM> of the respective connecting channels <NUM> is <NUM>-<NUM>% greater than the passage area of the inlet hole <NUM>.

Moreover, in the non-limiting example here disclosed and illustrated, the area ratio between inlet section <NUM> and outlet section <NUM> of the connecting channels <NUM> is comprised between <NUM>,<NUM> to <NUM>.

Preferably, the inlet holes <NUM> are substantially aligned along a direction F on the respective internal face of the pressure side discharge cooling chamber 47b. Preferably, the inlet holes <NUM> are arranged equally spaced from each other.

Preferably, the outlet common slot <NUM> is substantially aligned along a direction parallel to direction F.

Direction F is substantially a straight line extending from the base <NUM> to tip <NUM> of the airfoil <NUM>.

With reference to <FIG>, the discharge arrangement <NUM> extends along a main axis G which is inclined with respect to the external face <NUM> of the outer wall with an angle α.

In other words, the inlet holes <NUM> and the connecting channels <NUM> and the outlet common slot <NUM> extends along said main axis G as shown in the cross section of <FIG>. The depth DH of the inlet holes <NUM> is <NUM>-<NUM>% of the total depth Dtot of the outer wall <NUM>; wherein both depth DH and depth Dtot are measured along the main axis G.

The depth DC of the connecting channels <NUM> is <NUM>%-<NUM>% of the total depth Dtot of the outer wall <NUM>; wherein both depth DC and depth Dtot are measured along the main axis G. Depth DS of the outlet common slot <NUM> is <NUM>-<NUM>% of the total depth Dtot of the outer wall <NUM>; wherein both depth DS and depth Dtot are measured along the main axis G.

Obviously the angle α of inclination and the total depth of the outer wall <NUM> measured along the main axis G can be different for each one of the discharge arrangements <NUM><NUM><NUM><NUM>.

In the non-limiting example here disclosed and illustrated, the angle α of discharge arrangement <NUM> is equal or greater than the angle α of discharge arrangement <NUM>.

In use, the cooling fluid coming from the respective pressure side discharge cooling chamber 47b is divided by the plurality of inlet holes <NUM>, flows into the respective connecting channels <NUM> and joins at the outlet common slot <NUM>. A single wide and homogenous flow of cooling fluid exits from the outlet common slot <NUM> as indicated also by the arrow in <FIG>.

The presence of a plurality of inlet holes <NUM> having a defined passage area regulates the flow rate of the cooling fluid exiting through the discharge arrangement <NUM>.

The presence of an outlet common slot <NUM> improves the film cooling efficiency as the external face <NUM> of the outer wall <NUM> is lapped by a cooling flow which is wide and homogeneous.

Due to the increased cooling efficiency lower amounts of cooling air is required for the blade. Due to this the overall efficiency of the gas turbine is increased.

In <FIG> a further embodiment of the discharge arrangement <NUM> is illustrated. The same reference numbers used for the cooling arrangement <NUM> of <FIG> and <FIG> are used also in <FIG> for indicating similar or identical parts.

According to said embodiment, the discharge arrangement <NUM> comprises at least two discharge groups <NUM>
In the non-limiting example here disclosed and illustrated, the discharge arrangement <NUM> comprises three discharge groups <NUM>.

Each discharge group <NUM> comprises a plurality of inlet holes <NUM>, an outlet common slot <NUM> and a plurality of connecting channels <NUM>, each of which is configured to connect a respective hole <NUM> with the outlet common slot <NUM>.

In particular, each connecting channel <NUM> have an inlet section <NUM> and an outlet section <NUM>; the inlet section <NUM> of each connecting channel <NUM> being in contact with the respective inlet hole <NUM> and the outlet section <NUM> of each connecting channel <NUM> being in contact with the outlet common slot <NUM>. The passage area of each inlet hole <NUM> is preferably smaller than the passage area of the inlet section <NUM> of the respective connecting channels <NUM> analogously to the embodiment illustrated in <FIG>.

In the non-limiting example here disclosed and illustrated, each discharge group <NUM> comprises a three inlet holes <NUM>, an outlet common slot <NUM> and three connecting channels <NUM>, each of which is configured to connect a respective hole <NUM> with the outlet common slot <NUM>.

The inlet holes <NUM> of each group <NUM> are equally spaced one from the other.

The discharge groups <NUM> are spaced one from the other. Preferably the discharge groups <NUM> are equally spaced one from the other.

In use the cooling fluid coming from the respective pressure side discharge cooling chamber 47b is divided by the plurality of inlet holes <NUM>, flows into the respective connecting channels <NUM> and joins at the outlet common slots <NUM>. In the non-limiting example here disclosed and illustrated, three homogenous flows of cooling fluid exits from the outlet common slots <NUM> as indicated also by the arrows in <FIG>.

Claim 1:
Blade for a gas turbine (<NUM>) comprising:
an airfoil (<NUM>) having a leading edge (<NUM>), a trailing edge (<NUM>), a pressure side (<NUM>) and a suction side (<NUM>); the airfoil (<NUM>) comprising an outer wall (<NUM>) and an inner wall (<NUM>) substantially enclosed by the outer wall (<NUM>); wherein the inner wall (<NUM>) surrounds an inner central cooling chamber (<NUM>), through which, in use, cooling fluid coming from a respectivefeeding channel (<NUM>) flows; and
a cooling arrangement (<NUM>) which comprises at least one cooling path (31a; 31b; 31c; 31d) between the outer wall (<NUM>) and the inner wall (<NUM>); the cooling path (31a; 31b; 31c; 31d) having at least one inlet (<NUM>; <NUM>; <NUM>; <NUM>) and at least one discharge arrangement (<NUM>; <NUM>; <NUM>; <NUM>); wherein the discharge arrangement (<NUM>; <NUM>; <NUM>; <NUM>) extends through the outer wall (<NUM>) and comprises a plurality of inlet holes (<NUM>; <NUM>)and at least one outlet common slot (<NUM>; <NUM>);wherein the discharge arrangement comprises a plurality of connecting channels (<NUM>; <NUM>), each of which is configured to connect a respective hole (<NUM>;<NUM>) with the outlet common slot (<NUM>; <NUM>); wherein each connecting channel (<NUM>; <NUM>) has an inlet section (<NUM>; <NUM>) and an outlet section (<NUM>; <NUM>); the inlet section (<NUM>; <NUM>) of each connecting channel (<NUM>; <NUM>) being in contact with the respective inlet hole (<NUM>; <NUM>) and the outlet section (<NUM>; <NUM>) of each connecting channel (<NUM>; <NUM>) being in contact with the outlet common slot (<NUM>; <NUM>); wherein the passage area of each inlet hole (<NUM>; <NUM>) is smaller than the passage area of the inlet section (<NUM>; <NUM>) of the respective connecting channels (<NUM>; <NUM>).