Conventional turbine blades comprises at their airfoil tips when cantilevered often fin shaped elements which are also known as squealer tips. The fins are usually rather extensions of the suction side and pressure side walls of the airfoil in radial direction and border a space with low depth there between. The bordered space is also known as squealer tip pocket. Further, the known turbine blades are hollow for establishing internal cooling of the airfoil for achieving an appropriate lifetime and are usually manufactured by investment casting using casting cores within a casting fixture. The casting cores represent the cooling cavities and leave them behind in the casted blade . To locate the casting core reliable within the casting fixture it is known to use additionally core ties which are just pins attached on the one hand to the casting fixture and on the other hand at the radial outer end of the cores. However, these core ties leave in the casted turbine blade openings connecting the internal cavities with the squealer pocket. These core tie holes in the airfoil tip act often simultaneously as dust holes, especially when the internal cooling structure of the turbine blade comprises turnaround sections beneath the airfoil tip. In detail the dust holes are located such, that the particles carried with the cooling air could leave the turbine blade due to their inertia and without changing their flow direction while the remaining cooling air turned by the turnaround section into opposite flow direction. Conventionally core tie holes are cylindrical shaped so that the cooling air discharges straight out of the holes into the above mentioned squealer tip pocket. Up to now the discharged air does not contribute to the tip cooling very well. Therefore, the aim of the invention is the provision of a turbine blade with increased squealer tip pocket cooling.

The object of the invention is achieved by the independent claims. The dependant claims described advantages, developments and modification of the invention. Their features could be combined arbitrarily.

In accordance with the invention there is provided a turbine blade for a rotor of a gas turbine, comprising successively along a radial direction of said gas turbine, a blade root for attaching the turbine blade to a turbine rotor, a plat- form, an aerodynamically shaped hollow airfoil comprising a suction side wall and a pressure side wall extending with respect to the direction of a hot gas from a leading edge to trailing edge flow and extending transversely thereof from said platform to a wall of an airfoil tip and at least one squealer tip, preferred two squealer tips extending from said airfoil tip wall in radially outward direction, wherein the airfoil comprises at least one cooling cavity extending in accordance to a cooling fluid flow direction from said platform to said airfoil tip wall, that at least one cooling cav- ity being in fluid connection by a dust hole or core tie hole with a space bordered by the outwardly directed surface of said airfoil tip wall, wherein respectively the dust hole or the core tie hole is at least partly inclined with respect to a radial direction of said gas turbine and/or comprises a shaped outlet area.

The invention is based on the knowledge, that the air discharging through the dust hole or through the core tie hole could also be intensively used as cooling air for cooling the tip wall . Hence the idea is to manipulate the direction of the discharged cooling flow by said holes by either inclining the main hole axis of said hole, a certain portion (e.g. at the hole outlet) thereof or by applying a shaped outlet area. An outlet area is shaped, if the outlet area of the respective hole, which is arranged in the plane of the surface of the tip wall, is not circular. In a preferred embodiment by shaping the hole as a cone and further preferred leaned into a direction to the trailing edge . The angle a of inclination with regard to the radial direction should be in a range from 10° to 75°, most preferably between 20° and 30°. Even if it is preferred that the in- clination is directed towards the trailing edge for providing a most effective outlet cooling flow, but the inclination could also be directed towards the leading edge for disturbing and reducing secondary flows of hot gas streaming as tip leakages between airfoil tip and casing.

In a further preferred embodiment alternatively or in combination with the before mentioned features an inclination of the respective hole can be directed either towards the pressure side or towards the suction side of the airfoil for re- ducing the turbine tip leakage flow. This feature allows locally increased fin cooling. Leaning toward the suction side improves castabilities while leaning towards the pressure side supports cooling and tip leakage issues. The new geometries of the respective holes can be manufactured during casting of the turbine blade or turbine vane or during subsequent machining.

For all embodiments in common the main benefit of the pro- posed features are that the cooling air exiting the dust hole or the core tie hole can also be used for a dedicated cooling of the squealer tip and/or tip wall instead of only releasing it. This could increase the lifetime of the turbine blade in the tip area.

Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which : Figure 1 shows a top view onto the airfoil tip of a turbine blade according to an exemplary embodiment and Figure 2 shows in a longitudinal cross section the turbine blade tip of figure 1.

The illustration in the drawings is only in schematic form. Is it noted that in different figures similar or identical elements may be provided with the same reference signs.

Figure 1 shows in a perspective view a turbine blade 10 in an exemplary embodiment onto its tip 28. Along a radial direction 11 (Fig. 2) of a gas turbine the turbine blade 10 com- prises a blade root (not shown) , a platform 12 and an aerody- namically shaped airfoil 14 comprising a suction side wall 16 and a pressure side wall 18 extending with respect to a direction of a hot gas from a leading edge 20 to a trailing edge 22. Both the suction side wall 16 and the pressure side wall 18 are attached to the platform 12 via a rounded transition region 24 which is also known as fillet. The airfoil 14 extends transversely to the direction of said hot gas from said platform 12 to a wall 26 of the airfoil tip 28. From said tip wall 26 two fins 30 extending in radially outward direction as an extension of said suction side wall 16 and pressure side wall 18 as squealer tip. The two fins 30 surrounds a space radially outwardly located of the wall 26 with relatively low depth which is also known as squealer tip pocked 35. The outer surfaces of the fins 30 and the pressure side and suction side walls 16, 18 are aligned and do not include any step or edge. In the displayed exemplary embodiment the inner side surface of fin 30 located on the suction side wall 16 and facing the squealers tip pocket 35 extends stepwise in radial direction for providing a lower step 32 in which a row of cooling holes 34 discharges. In the displayed exemplary embodiment three core tie holes 36 discharges in the outer surface of wall 26. The size of their minimal cross section is about at least three times larger than the size of the cross section of the cooling holes 34. Further the core tie holes 36 comprises shaped outlet areas 38 which are not circular but profiled rather elongated with an upstream end and a downstream in view of the direction of the cooling air flow leaving the core tie hole 36. It is preferred that both ends locally are shaped identical .

Figure 2 shows a longitudinal cross section through the turbine blade according to figure 1.

The airfoil 14 of the turbine blade 10 is hollow and compris- es in the displayed exemplary embodiment three cooling cavities 44 which can be individual identified in the drawings and the subsequent description by suffixes a, b and c. The same applies for the core tie holes 36, which are located at the radial outer end of the cooling channels 44 within the airfoil tip wall 26, and their main axis 46.

During manufacturing of the casted turbine blade 10 the corresponding casting fixture carries multiple casting cores which represents later the cooling channels 44 in the finally manufactured turbine blade 10. To keep said casting cores reliable in a proper position the invention proposes to use shaped core ties leading to core tie holes 36 having a shaped profile as displayed in figure 2. The shaped profile is in an first example an asymmetric cone, which main axis 46a is leaned about an angle a of 25° into a direction towards the trailing edge 22, leading to the core tie hole 36a. A second exemplary embodiment of an inventive core tie hole is referenced by sign 36b which, comprises at the radial inner end a cylindrical section followed by an inclined section. The an- gle a shows the inclination of the modified core tie hole with regard to the radial axis 11. The inclination seems similar to film cooling holes but with - as seen in top view ac- cording to figure 1 - the core tie holes 36 merges via a radius 33 into the outer surface of wall 26.

Each of the displayed core tie hole 36 and especially the core tie hole 36c due to its location relatively to the channel 44c acts simultaneously as a dust hole since they enables particles carried with the cooling air to be discharged from the turbine blade through their inertia.

Although the present invention has been described in detail with reference to the preferred embodiment, it is to be understood that the present invention is not limited by the disclosed examples and that numerous additional modifications and variations could be made there by a person skilled in the art without departing from the scope of the invention.

It should be noted that the use of "a" or "an" throughout this application does not exclude a plurality and "comprising" does not exclude other steps or elements. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting in the scope of the claims .