Electric heating – Metal heating – By arc
Reexamination Certificate
1999-03-22
2001-10-23
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C219S121720, C219S121800
Reexamination Certificate
active
06307175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a noncircular cooling bore, in particular for the film cooling of a wall in a hot-gas environment. The invention also relates to a method of producing a noncircular cooling bore.
2. Discussion of Background
To increase the output and the efficiency, increasingly higher turbine inlet temperatures are being used in modern gas-turbine plants. In order to protect the turbine blades from the increased hot-gas temperatures, they must be intensively cooled. At correspondingly high inlet temperatures, purely convective cooling is no longer sufficient. The film-cooling method is therefore often used. In this case, the turbine blades are protected from the hot gas by a cooling film. To this end, openings, for example bores, through which the cooling air is blown out, are made in the blades.
In order to achieve as high a cooling effect as possible, the cooling air which is blown out must be deflected as rapidly as possible and flow in a protective manner along the profile surface. In order to also protect the zones lying between the bores, rapid lateral spreading of the cooling air is also necessary. This may be achieved by the cooling-air bores having a diffuser, which on account of the lateral widening permits a wider area of the surface to be covered. To further improve the mixing behavior, geometrical diffuser forms in which the bore is widened not only laterally but also on the downstream side of the bore are used.
For example, publication EP-B-228 338 describes a cooled wall having a cooling-medium passage, the diffuser section of which widens laterally toward the cooling-medium outlet and the downstream flat surface of which diverges away from the axis.
The blow-out rates in the case of these geometrical diffuser forms are small, so that there is little risk of the cooling air passing through the flow boundary layer. The cooling efficiency can therefore be increased considerably compared with a cylindrical bore.
The accuracy with which the workpieces to be provided with cooling holes must be produced represents a significant cost factor. Large wall tolerances of up to 10% or even up to 20% permit the components to be produced cost-effectively. On the other hand, the fluctuations in the wall thickness lead to variations in the opening ratio of the cooling bores as a function of the wall thickness. The accompanying non-uniform film-cooling effectiveness leads either to the costly redesign of the weakest points or to the occurrence of overheated spots on the wall surface, a factor which drastically reduces the service life of the component.
The production of such holes by a spark-machining machining method, as described, for instance, in publication U.S. Pat. No. 4,197,443, has, in addition to the high production costs, the disadvantage that the use of a spark-machining grid, even in the case of small surface tolerances, leads to greatly varying opening ratios of the individual cooling holes. In addition, the spark-machining method cannot be used in the case of ceramically coated surfaces, since the latter are electrically insulating. In this case, the cooling holes must be produced before the coating. The subsequent coating generally covers part of the diffuser opening, as a result of which the cooling properties of the holes are affected. It then becomes necessary to remove the obstructing material in a further step of the method. For example, publication U.S. Pat. No. 5,216,808 describes a method of producing or repairing a gas-turbine component. In this case, after a protective coating has been applied to the component, a UV laser beam is directed toward the position of a film-cooling hole in order to remove obstructing coating material athermally.
In the laser drilling of turbine blades, two drilling methods are mainly used. In percussion drilling, a hole is bored to the nominal diameter by a number of laser pulses with a beam axis fixed relative to the workpiece. With this method, however, only cylindrical holes are easy to produce. In the trepanning drilling method, a finely focused laser beam is moved relative to the workpiece and the hole is thus cut out. In the production of cooling holes having a diffuser by a laser-drilling method, the problem occurs that the length of the cylindrical air-inlet passage also increases as the wall thickness increases. This inlet passage is damaged by the laser beam during the cutting-out of the widening diffuser. The sharp-edged damage which occurs constitutes a serious strength problem. In addition, the inlet opening and thus the flow through the cooling bore change. For this reason, the trepanning method for cooling holes having a diffuser can only be used in the case of small wall thicknesses.
Publication U.S. Pat. No. 5,609,779 discloses a method of forming an opening in a metallic component wall, the opening having a widening diffuser. The noncircular diffuser is produced by an Nd:YAG laser beam being directed within a few laser pulses in an accelerated manner from the center line of the opening to the edge of the diffuser. The pulse rate and the power of the laser are selected in such a way that the metal is vaporized by the laser beam. A disadvantage is that the diffusers which are produced turn out to be very variable with such a method. However, uniform effectiveness of the cooling openings is imperative in modern gas turbines on account of the close dimensioning of the components.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide a novel method with which a cooling bore can be formed in a wall in a cost-effective, accurate and highly flexible manner. In particular, the method is to permit the cooling bore to be formed irrespective of the production tolerances of the wall thickness and is to be suitable for all wall thicknesses. Furthermore, a cooling bore which can be produced in a cost-effective and flexible manner is to be provided. This object is achieved by the method of forming a cooling bore as claimed in claim
1
and the noncircular cooling bore as claimed in claim
13
.
The method according to the invention for forming a cooling bore in a wall of a workpiece, the cooling bore, in the flow sequence, having a feed section of constant cross-sectional area and a diffuser section widening toward an outlet at an outer surface of the wall, comprises the following steps:
A) selecting the shape and size of the cross-sectional area and an axis of the feed section; B) selecting the depth of the diffuser section and the shape and size of its discharge area at the outlet; C) producing a throughbore having a cross-sectional area which lies within the cross-sectional area, selected in step A, of the feed section; and D) cutting out the diffuser section with a beam- or jet-drilling method, the drilling beam or jet being directed in such a way that, in the region of the feed section, it remains essentially within the cross-sectional area selected in step A.
The invention is accordingly based on the idea of cutting out the diffuser section of a cooling bore with a drilling beam or jet in such a way that the feed section is not damaged or is only slightly damaged, as a result of which the cooling bore obtains high strength.
The flow of the cooling medium through the cooling bore during operation establishes a direction of flow in the cooling bore. The shape and size of the cross-sectional area of the feed section determine the quantity of cooling medium flowing through. The method according to the invention offers the advantage that the cooling bore can be cut in an accurate and flexible manner by the use of a drilling-beam or drilling-jet method, in particular a laser-drilling method. The method is suitable for uncoated components as well as for metallically or ceramically coated components. In the latter case, the cooling bores can be produced after the coating in a single operation. It is not necessary to drill the holes before the coating and to expose the obstructed openings again after the coating. Damage to the feed section i
Blöchlinger Michael
Luketic Ivan
Semmler Klaus
Wilfert Gunter
ABB Research Ltd.
Burns Doane Swecker & Mathis L.L.P.
Cooke Colleen
Dunn Tom
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