Patent Application
20160169006
The present invention relates to the technology of gas turbines. It refers to a rotating blade for a gas turbine according to the preamble of claim 1.
Rotating gas turbine blades with a tip shroud (used primarily to reduce over-tip leakage flow) normally use one or more fins to improve gas sealing against the corresponding stator heat shield and often are hollow with two or more internal passages within the airfoil (e.g. for cooling and/or weight reduction purposes).
During a casting process (usually investment casting using a ceramic mould and a ceramic core) these passages are produced by a core, which requires holding in position by so-called core exits, which connect the core to the mould and leave openings in the blade after removal of the core (usually by leaching and/or an abrasive/erosive process). Such openings in a blade are normally at the blade's root end (where cooling air may enter the blade's internal passages) and at the tip end, i.e. through the tip shroud, where they may interfere with any fins of the shroud and thereby compromise a fin's sealing function and mechanical stability.
Additionally, the fins have the largest distance from the rotational axis and therefore exert in conjunction with the mass of the tip shroud itself a relatively high centrifugal stress onto the tip end of the airfoil with local peak stresses at the base of the fins, which limits the life time of the tip shroud and the fins.
Small core exits at the tip compromise mechanical core stability (potential scrap at casting, potential reduction in wall thickness control), may require a more complex cooling design and manufacture for an airfoil trailing edge (TE) and/or pressure side (PS) release of cooling medium, and may reduce life time caused by additional notches generated by the airfoil TE and/or PS release of cooling medium.
A potential countermeasure is to cool or additionally cool the tip shroud and fins to improve mechanical properties of the materials, but this consumes cooling air, which reduces turbine efficiency and power, and may not be readily possible due to other constraints (cooling air delivery to the required area, complexity, and cost).
An alternative potential countermeasure is to eliminate or significantly reduce the size of a blade's tip shroud. However, this will cause an over-tip leakage, which reduces turbine efficiency and power.
It is an object of the present invention to provide a rotating blade for a gas turbine, which avoids the drawbacks of known blades and has an improved stability and life time without sacrificing turbine efficiency.
This and other objects are obtained by a blade according to claim 1.
The rotating blade according to the invention comprises an airfoil extending in a longitudinal direction and having a leading edge and a trailing edge, whereby said airfoil is bordered at its outer end by a tip shroud, whereby said airfoil comprises two or more internal passages, which run in longitudinal direction and are separated by solid webs, and whereby a plurality of shroud fins is arranged on top of said tip shroud to improve gas sealing against a corresponding stator heat shield.
The blade is characterized in that the position of each of said shroud fins is selected to be exclusively above one of said webs and/or a leading edge wall.
According to an embodiment of the invention most of said shroud fins are straight, i.e. aligned with the longitudinal axis of said blade, in order to avoid a reduction of space for core exits provided in said tip shroud.
Specifically, a shroud fin provided at the leading edge of said blade has an inclination towards said leading edge in order to achieve good sealing against the corresponding stator heat shield.
According to another embodiment of the invention, on an upper surface of said tip shroud between said shroud fins one or more stiffener fins are provided to increase the stiffness of said tip shroud for reduction of mechanical stress and radial clearances.
Specifically, said airfoil has a camber line, and said stiffener fins are oriented perpendicular to said airfoil camber line.
Also, said stiffener fins may have a variable height to provide maximum stiffness with minimum weight to improve mechanical stability against tip shroud bending due to the centrifugal force.
According to a further embodiment of the invention, on an upper surface of said tip shroud and behind a shroud fin provided at the leading edge of said blade, one or more small fins are provided to increase the heat transfer to the colder surrounding medium for increased cooling of a floor of said tip shroud.
Specifically, said small fins are aligned with the rotating direction of the blade to minimise a breaking effect and improve the mechanical stability of tip shroud against bending upwards due to the centrifugal force.
The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.
At the outer end, the aerodynamical section of airfoil 11 is bordered by a tip shroud 14, which is shown in more detail in
Through the interior of airfoil 11 run in longitudinal direction two or more internal passages 15a, 15b and 15b, which are used to cool blade 10 by means of a cooling medium (e.g. cooling air). Heat transfer between the walls of airfoil 11 and the cooling medium is improved by providing ribs 16a, 16b and 16c on the walls of inner passages 15a, 15b and 15b. Inner passages 15a, 15b and 15b are separated by so-called solid webs 23 and 24.
Three shroud fins 18a, 18b and 18c are arranged on top of tip shroud 14. Shroud fins 18a, 18b and 18c are each part of a circumferential ring, which is composed of respective shroud fins of all blades of one turbine stage. These rings are used to improve gas sealing against the corresponding stator heat shield.
For tip shroud 14 of rotating gas turbine blade 10 with two or more internal passages 15a, 15b and 15c, which are separated by solid webs 23 and 24, the position and inclination of shroud fins 18a, 18b and 18c are selected to be above any webs 23, 24 or the leading edge wall (shroud fin 18c), but not above an internal passage 15a, 15b or 15c.
This selection provides increased space for core exits 17a, 17b and 17c (a core is used to produce the internal passages during a casting process and requires holding in position by so-called core exits, which connect the core to the mould) through the tip shroud 14 without interference with the shroud fins 18a, 18b and 18c, and improves life time of the shroud 14, as shroud fins 18a, 18b and 18c, which are primarily centrifugally loaded, are mechanically better supported by the solid webs 23, 24 or solid airfoil directly below and thereby in line with the centrifugal load due to the shroud fins.
Additionally, an inclination of shroud fin 18c towards the airfoil's leading edge (LE) 11a (see dashed line) achieves good sealing against the corresponding stator heat shield (as the differential in gas pressure across the LE fin 18c is larger than for any other subsequent fin), while other shroud fins 18b or 18a in the middle (fin 18b) or towards the trailing edge (TE) 11b (fin 18a) are straight (i.e. aligned with the blade's longitudinal axis; see dashed lines), thereby avoiding a reduction of space for core exits 17a, 17b and 17c.
Furthermore, rotating gas turbine blades 10 with a tip shroud 14 (used primarily to reduce over-tip leakage flow) often require increased fillets underneath of the shroud or increase of the shroud platform thickness to ensure the shroud stiffness and life time. However, increase of the fillet could lead to additional aerodynamic losses and the platform thickness increase leads to significant shroud weight increase and is not very efficient for stiffness improvement.
Thus, for a rotating gas turbine blade 10 with a tip shroud 14, on the upper surface of the shroud between the shroud fins 18a, 18b and 18c, one or more stiffener fins 19 and 20 are provided to increase the stiffness of the shroud for reduction of mechanical stress and radial clearances, which in turn extends the blade's life time and the turbine performance (see
Furthermore, rotating gas turbine blades 10 with a tip shroud 14 often require cooling of tip shroud 14 to ensure the life time. However, cooling in particular of the outer portions of a shroud towards (concave) pressure side (PS) or (convex) suction side (SS) is difficult, as potential design solutions are complex and expensive to manufacture, and/or cause additional notches which locally intensify stress and thereby limit life time.
Thus, for a rotating gas turbine blade 10 with a tip shroud 14, on the upper surface of the shroud and behind shroud fin 18c towards the blade's leading edge (LE) 11a one or more small fins 21, 22 are provided to increase the heat transfer to the colder surrounding medium (mixture of cooling medium and hot gas above tip shroud 14) for increased cooling of the tip shroud's floor, which in turn extends the blade's lifetime due to improved mechanical properties of the shroud material (see
Small fins 21, 22 are preferably aligned with the rotating direction of the blade to minimise a breaking effect, which might reduce the gas turbine's efficiency and power, and additionally to improve the mechanical stability of tip shroud 14 against bending upwards due to the centrifugal force. As the small fins 21, 22 are positive material on the upper surface of the shroud; they do not introduce any significant local notches.
| Number | Date | Country | Kind |
|---|---|---|---|
| 14198315.5 | Dec 2014 | EP | regional |
