The present invention relates to the general field of sets of blades for a turbine engine turbine, and more particularly to turbine blades having cooling circuits incorporated therein and made by the lost-wax casting technique.
In known manner, a turbine engine includes a combustion chamber in which air and fuel are mixed together prior to being burnt therein. The gas resulting from such combustion flows downstream from the combustion chamber and then feeds a high-pressure turbine and a low-pressure turbine. Each turbine comprises one or more stationary vane rows (known as nozzles) alternating with one or more moving blade rows (referred to as rotor wheels), in which rows the blades or vanes are spaced apart circumferentially all around the rotor of the turbine. Such turbine blades or vanes are subjected to the very high temperatures of the combustion gas, which temperatures reach values that are well above those that can be withstood without damage by the blades or vanes that are in direct contact with the gas, thereby having the consequence of limiting their lifetimes.
In order to solve this problem, it is known to provide such blades and vanes with internal cooling circuits presenting high levels of thermal effectiveness and seeking to reduce their temperatures by creating an organized flow of the air inside each blade or vane (e.g. simple direct feed cavities U-shaped or “trombone” cavities) together with perforations in the wall of the blade or vane for generating a protective film around it.
Nevertheless, that technology presents several drawbacks. Firstly, although circuits with trombone cavities present the advantage of maximizing the work done by the air passing through the circuit, that leads to considerable heating of the air, which results in a reduction in the thermal effectiveness of the holes situated at the end of the trombone cavity. In the same manner, configurations having leading edge cavities and trailing edge cavities with direct feed do not make it possible to provide an effective response at the high temperature levels usually observed at the tip of a blade. Finally, the various cavities are separated from the gas flow passage only by a wall of thickness that varies as a function of different zones of the airfoil. Given the constraints on the flow rate that can be devoted to cooling sets of blades or vanes, and given the current trend towards increasing temperatures in the gas passage, it is not possible to cool a blade or a vane effectively with a circuit of that type without significantly increasing the flow rate of the air, and thus penalizing the performance of the engine.
The shape and the number of cavities, and also the positions of the external holes 34, 36 and the shapes of the trailing edge slots 38 are shown by way of illustration, given that all of these elements are generally optimized so as to maximize thermal efficiency in the zones that are the most sensitive to the heat from the combustion gas in which the blades are immersed. The internal cavities are also often provided with turbulators (not shown) in order to increase heat exchange.
As described in application FR 2 961 552 in the name of the Applicant, high-pressure turbine blades and vanes are conventionally made by lost-wax casting, with the shapes of the circuits being made therein by positioning one or more ceramic cores (depending on complexity) in the mold and presenting outside surfaces that form the inside surfaces of the finished blade or vane.
In particular, the cooling circuits have a plurality of cavities, like those in
The present invention thus seeks to mitigate the drawbacks associated with manually assembling a plurality of separate cores by proposing a cooling circuit for a turbine blade that can be made using a single core so as to eliminate those assembly operations and bathtub finishing operations required by prior art circuits, while also guaranteeing an intercavity distance, corresponding to the thickness of the metal partition after casting the molten metal, in a manner that is more reliable than with present manual assemblies.
To this end, there is provided a ceramic core used for fabricating a hollow turbine blade for a turbine engine by using the lost-wax casting technique, the blade including at least one central cavity, a first lateral cavity arranged between said at least one central cavity and a suction side wall of the blade, and a second lateral cavity arranged between said at least one central cavity and a pressure side wall of the blade. The core is shaped to constitute said cavities as a single element and, in order to feed the insides of said cavities jointly with cooling air, it includes core portions that are to form said first and second lateral cavities and that are connected to a core portion that is to form said at least one central cavity, firstly in the core root via at least two ceramic junctions, and secondly at various heights up said core via a plurality of other ceramic junctions of position that defines the thickness of the internal partitions of the blade, while also ensuring additional cooling air for predetermined critical zones of said first and second lateral cavities.
In addition, a core portion for forming a bathtub and connected to said core portion that is to form at least one central cavity via ceramic junctions of positioning that defines the thickness of said bathtub, while ensuring that cooling air is discharged at the blade tip.
By means of these junctions via the body of the blade, the need for assembly contrivances at the blade tip is eliminated, thereby making it possible to obtain a cast bathtub having the same mechanical properties as the body of the blade. In addition, the main feed of the lateral cavities via their roots gives better control over the air stream and over the overall cooling of the outer walls of the finished airfoil, and in the core, the feeds to the various cavities can be joined as from injection, thereby further improving the mechanical strength of the cores.
In the intended embodiment, said predetermined critical zones are selected from the zones of said first and second lateral cavities that are subjected to the greatest thermomechanical stresses, and said ceramic junctions are of section determined to ensure the mechanical strength of said internal partitions while casting the molten metal.
The invention also provides both the method of fabricating a hollow turbine blade for a turbine engine using the lost-wax casting technique with a single-element core as explained above, and also any turbine engine turbine including a plurality of cooled blades fabricated using such a method.
Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings, which show an implementation having no limiting character, and in which:
The sizes of the various bridges are determined so as to avoid them breaking while the core 40 is being handled, which could make it unusable. In the example under consideration, the bridges are distributed by being spaced apart substantially regularly up the height of the core 40, and in particular in the first column 42 of the core.
In accordance with the invention, the core 40 also has sixth and seventh columns 60 and 62 arranged laterally and both spaced apart from the second and third columns 44 and 48 by determined spacing so as to leave room for creating a solid inter-cavity wall when casting molten metal. For purposes of holding these columns and imparting rigidity to the core assembly, the bottom end of the sixth column 60 is connected to the first column root 46, and the bottom end of the seventh column 62 is connected to the second column root 54, and multiple ceramic junctions of small section (see for example references 64, 66, 68 in
The presence of two column root connections (even through only the ceramic junction 70 at the root of the seventh column 62 is shown) has the consequence, after casting, that the lateral cavities 24, 26 are connected directly to the cooling air feed channel of the central cavities 20 and 22, thereby further improving the mechanical strength of the core and, in the finished airfoil, improving the feed via the root of the core so as to obtain better control over the internal stream of cooling air and over the overall cooling of the outer walls.
In
Once the single-element core has been made, the lost-wax method of fabricating the blade is conventional and consists initially in forming an injection mold in which the core is placed prior to injecting wax. The wax model as created in that way is then dipped in slurries constituted by ceramic suspensions in order to make a casting mold (also known as a shell mold). Finally, the wax is eliminated, and the shell mold is baked so that molten metal can then be cast into it.
Because of the ceramic junctions interconnecting the central columns and the lateral columns of the core, their relative spacing is controlled over the entire height of the blade. These junctions are also positioned in such a manner as to give rise, in the finished blade, to an additional supply of cool air from the central cavities towards the zones of the lateral cavities that are subjected to the greatest thermomechanical stresses, thereby also improving local thermal efficiency and the lifetime of the blade. In particular, these junctions are dimensioned and arranged in such a manner as to ensure:
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1552383 | Mar 2015 | FR | national |
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PCT/FR2016/050628 | 3/22/2016 | WO | 00 |
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WO2016/151234 | 9/29/2016 | WO | A |
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