The present invention relates to an electrode for electrochemical machining interior surfaces of cooling passages for a turbine bucket, a method therefor and a turbine bucket having the machined interior surface.
Electrochemical machining known as shaped-tube electrochemical machining (STEM) is used for drilling small, deep holes in electrically conductive materials. STEM is a non-contact electrochemical drilling process for producing holes with high aspect ratios such as 300:1. The aspect ratio is the ratio of the length or depth of the hole to the largest lateral dimension, e.g., diameter, of the hole which, in certain specific applications, can be as small as a few millimeters. STEM is used, e.g., to form the deep holes used as cooling passages for the buckets in gas turbines.
Gas turbine efficiencies are directly proportional to the temperature of turbine gases flowing along the hot gas path and driving the turbine blades. Gas turbines typically have operating temperatures on the order of 2700xc2x0 F. To withstand these high temperatures, the buckets are manufactured from advanced materials and typically include smooth bore cooling passages for flowing a cooling medium for cooling the buckets. The cooling medium is typically compressor discharge air. The passages also conventionally extend from the bucket root to the bucket tip. While smooth-bore passages have been utilized, turbulence promoters, e.g., turbulators, are used in many gas turbine buckets to enhance the internal heat transfer coefficient. The heat transfer enhancement can be as high as 2.5 times as compared with smooth-bore passages for the same cooling flow rate. Turbulators conventionally comprise internal ridges or roughened surfaces along the interior surfaces of the cooling passages and are typically cast inside the cooling passages using ceramic cores. In many currently used turbines, however, many of the buckets have interior cooling passages with smooth interior wall surfaces formed by the casting process and therefore do not obtain the enhanced cooling effects otherwise available with turbulators.
The STEM technique identified above and described in the above-identified applications incorporated herein by reference employ an electrode having an insulating dielectric material or coating applied on the electrode surface in a pattern which, in conjunction with an electrolyte and the application of an electrical current between the electrode and the workpiece (bucket) displaces, i.e., dissolves, metal from the adjacent parts of the cooling passage wall to form projections and grooves along the interior surface. That is, the metallic portions of the interior surface of the cooling passage wall directly adjacent the insulated portions of the electrode when inserted into the preformed hole are not electrochemically removed, while the portions thereof directly adjacent the non-insulated portions of the electrode are electrochemically removed to form grooves in the interior wall portions of the cooling passage defining the projections therebetween. Certain of the foregoing applications are directed to the electrochemical machining process for forming projections and grooves along the interior wall surfaces.
In accordance with a preferred embodiment of the present invention, there is disclosed an electrochemical machining process for forming axially spaced rows of projections, i.e., turbulators, along the interior wall surfaces of the cooling passages with gaps in each row to form cooling passages in buckets having enhanced heat transfer characteristics, i.e., enhanced internal heat transfer coefficients. To accomplish the foregoing, the present invention provides an electrode having an insulating dielectric coating about its entire exterior surface. Portions of the coating on the electrode are removed to form a desired turbulator pattern. That is, retained insulated portions of the electrode will correspond to the locations of the projections to be formed along the interior wall surface of the cooling passages of the bucket. The non-insulated portions of the electrode correspond in location to the grooves and gaps to be formed along the interior surfaces of the cooling passages. For example, the insulated portions of the electrode are spaced 0.100 inches apart in an axial direction along the length of the electrode (corresponding to the radial direction of the bucket) and may have a width on the order of 0.010 inches. By inserting the patterned electrode into the preformed (predrilled or cast) cooling passage and circulating an electrolyte, the application of an electric current between the electrode and the workpiece (bucket) electrochemically removes material along the interior wall surface of the cooling passage adjacent non-insulated portions of the electrode to form the grooves or gaps therein. The material of the interior surface of the cooling passage adjacent the insulated portions of the electrode are not removed and therefore form the projections extending toward the axis of the hole.
A particular advantage of the turbulator pattern described above is that the electrode is relatively easy to fabricate. Moreover, the resulting pattern of turbulators along the interior surface of the cooling passages have greatly enhanced heat transfer characteristics. Particularly, the radial flow of cooling medium during turbine operation from the bucket root toward the bucket tip interacts with the bucket rotation and creates a coriolis effect to flow the cooling medium from the leading face (suction side) toward the trailing face (pressure side). The combination of this secondary flow with the radial flow forms a complicated flow field within the cooling passage having enhanced heat transfer characteristics. The radial flow sheds vortices from the turbulators and the interruptions, i.e., gaps, and the secondary flow circulates the vortices around the passage walls to enhance the surface heat transfer coefficient.
In a preferred embodiment according to the present invention, there is provided a process for forming interrupted raised projections and grooves therebetween perimetrically about an interior surface of a preformed elongated hole in a workpiece, comprising the steps of (a) locating within the hole an electrode having electrical insulating material arranged in axially spaced, perimetrically extending rows, interrupted by at least one gap, forming a pattern of insulated and non-insulated portions of the electrode about an outer surface of the electrode in general opposition to intended locations of the projections and grooves, respectively, about the interior surface of the hole, (b) flowing an electrolyte between the electrode and the interior surface of the hole of the workpiece and (c) passing an electric current between the electrode and the workpiece to form the perimetrically interrupted axially spaced rows of projections and grooves along the interior surface of the hole.
In a further preferred embodiment of the present invention, there is provided an electrode for forming interrupted raised projections and grooves therebetween perimetrically about an interior surface of an elongated hole in a workpiece, comprising an elongated electrode body having a long axis and coated with a dielectric material in a pattern having a plurality of axially spaced rows extending perimetrically about the body in planes generally normal to the axis, each row having at least one gap at a predetermined perimetric location along the row, surface area portions of the electrode body between said rows of dielectric material and in the gaps being exposed for electrical contact with an electrolyte upon insertion of the electrode body into the hole and application of an electrical current between the electrode body and the workpiece.
In a still further preferred embodiment according to the present invention, there is provided a bucket for a turbine having at least one generally elongated cooling passage extending along the bucket generally from the bucket root to the bucket tip and having an axis, the cooling passage having a perimetrical interior surface with rows of axially spaced raised projections extending generally perpendicular to and toward the axis, each projection being interrupted by a gap defining a recess extending generally in a direction away from the axis.