The performance of a gas turbine engine is sometimes limited by the maximum temperature to which certain components can be exposed. These components, such as turbine blades and vanes, various seals and spacers, combustors, and other components, are sometimes actively cooled with air from the engine's compressor, so that the component metal temperatures can be maintained at acceptable levels while being exposed to gas temperatures that are in excess of permissible metal temperatures.
This active cooling comes with an inherent trade off. Although higher gas temperatures tend to improve the performance of the engine, the use of air from the compressor for active cooling tends to lower the performance of the engine. It is imperative that the scheme for active cooling use as little air as possible, so that performance gains are maximized.
In order to cool that various components efficiently, numerous component internal cooling flowpath designs have been used to optimally transfer heat from the component into the cooling air. Examples of such cooling flowpath designs can be found in U.S. Pat. Nos. 4,930,980 issued to North et al.; 4,236,870 issued to Hucul, Jr. et al.; 4,946,346 issued to Ito; 5,062,768 issued to Marriage; and British Patent No. 2,246,174A issued to Lings et al.
These flowpath designs provide cooling air to various regions of the component, and often use small features to enhance heat transfer. Examples of cooling flowpath designs with small features include U.S. Pat. Nos. 4,770,608 issued to Anderson et al.; 4,962,640 issued to Tobery; 5,281,084 issued to Noe et al.; and 5,193,975 issued to Bird et al. Small sized features such as pins, pedestals, walls, and passageways permit an increased number of such features to be included in the cooling flowpath. A large number of cooling pedestals, for example, increases the surface area of the component exposed to the cooling air. A small size for cooling passageways permits not only a larger number of passageways, but may also increase the convective heat transfer coefficient of the passageway.
However, use of small features such as pins, pedestals, walls, and passageways poses a problem during manufacturing. These features are often cast or etched onto one workpiece which must then be joined to another workpiece. Joining methods often include welding, brazing, or diffusion bonding. Examples of cooling flowpath designs in which portions of the workpieces are brazed include U.S. Pat. Nos. 5,263,820 issued to Tubbs; 4,786,234 issued to Readnour; 4,505,639 issued to Groess et al.; 4,482,295 issued to North et al.; 5,419,039, 5,392,515, and 5,405,242 issued to Auxier et al.; 5,383,766 issued to Przirembel et al.; 5,193,980 issued to Kaincz et al.; and British Patent Specifications No. 1,285,369 issued to Steel et al. and 1,299,904 to Curtiss Wright Corporation.
Brazing of flowpaths with small geometric features presents the problem of excess brazing material interfering with the intended use of the feature. For example, excessive brazing material within a cooling air passageway can reduce or completely eliminate airflow through the passageway, with subsequent local or general overheating of the gas turbine component. It is important that excessive brazing material be kept from areas adjacent to various cooling flowpath features. U.S. Pat. No. 4,507,051 issued to Lesgourgues et al. discusses brazing of actively cooled gas turbine components.
One manner of depositing brazing material is by electrophoresis. U.S. Pat. Nos. 5,336,382 issued to Bodin, and 5,411,582 issued to Bodine, describe use of electrophoresis with a mucilage remarkable for its high viscosity, the muciliage containing brazing powder with grains as large as 53 microns. Outer areas in which brazing material is not desired are masked by a suitable adhesive masking tape, and inner cavities are masked with silicone plugs.
What is needed is a method of brazing components in which brazing material can be accurately deposited near small features. The present invention provides this method in a novel and unobvious way.