Apparatus For Protection Of Rotor Vanes Against Surface Erosion Caused By Impingement Of High Speed Liquid Drops

Abstract

The surface of a structural part such as for example the leading edge portion of a turbine vane or airplane wing is protected against erosion which would otherwise be caused by impingement of high speed liquid drops thereon by positioning an array of flexible baffles in front of the surface. These baffles which are slightly spaced from the surface of the vane or wing serve to intercept the liquid drops resulting in a division thereof into smaller drops which are simultaneously decelerated as a result of flexation of the baffles.

Description

This invention relates to an improved device for the protection of the vanes of a rotor against erosion which are imperiled by liquid drops impinging thereon at high speed.

When liquid drops impinge on metals or other materials at very high speeds, they cause erosion, that is, small particles of material break off after a certain time. The erosion effect depends mainly on the speed of impingement of the drops, their diameter, the angle of incidence on the surface of the structural part, and the specific properties of the eroding material. Due to the removal, the surface first becomes rough, then peaks opposed to the impinging drops form, which are now removed more slowly but continue to be removed. The friction losses of a flow along such eroded surfaces are correspondingly great.

Low pressure vanes on steam turbines, running temporarily or permanently in the saturated steam zone, are very susceptible to erosion. Especially the radially external portion of the entrance edge may be eroded intensively, and this leads to a loss of efficiency. It should be taken into consideration also that eroded vanes can hardly be repaired.

For protection against erosion, therefore, steam turbine vanes are produced either of an erosionproof material, e.g. hardened 12 percent chromium steel, or they are provided at the entrance edge with a soldered on hard metal plate (stellite). When using titanium or plastic vanes, hardening is not possible. The application of stellite shields involves difficulties in the case of titanium vanes because of the temperature influences during brazing, and in the case of plastic vanes because of the weight. Also, in the case of stellites, the properties of a material determinant for the erosion resistance, namely strength combined with toughness, can hardly be increased any further, so that at increased circumferential speeds greater erosions are to be expected even with stellites.

When airplanes fly through rain clouds at supersonic speed, the wing noses are so intensively eroded by the rain drops that already after several times the planking consisting of a light metal alloy must be exchanged. The only remedy is seen in increasing the erosion resistance of the metals used.

The principal object underlying the present invention is to protect against the erosion of structural parts endangered by drop impact without having to subject the material to a special treatment and without having to replace it in the specially affected areas by a more erosion-resistant material.

This problem is solved according to the invention in that the drops are divided and, at the same time, decelerated before they impinge on the structural parts.

A device for carrying out this method is characterized by baffle bodies of small mass which are arranged, viewed in the direction of incidence of the drops, ahead of the endangered structural parts.

Instead of a fixed, stiff, solid erosion shield, there are used, according to the invention, baffle bodies which, due to their small mass, and because they do not rest on anything except for their attachment to the structural part to be protected, are flexible and hence can be elastically deflected in the direction of incidence or laterally thereto when a drop impinges. This yielding of the baffle bodies upon impingement of a drop gives the same effect as a lower impingement speed. Due to the narrow and usually also curved impingement surface offered to the drops, the angle of incidence is more favorable than in the case of impingement normal to the impingement surface. The drops are divided and so slowed in their velocity that the erosion effect on the imperiled structural parts behind the baffle bodies is greatly reduced.

If these structural parts are, for example, the impellers of a low-pressure turbine, running in the saturated steam zone, they can be protected by preceding fixed baffle bodies installed in the flow path, e.g. a wire mesh. However, the effect is greater if the baffle bodies co-rotate in the same direction of rotation as the vanes, preferably at the same speed. They may be fastened either to the rotor or to the vanes themselves. As the vanes are endangered by drop impact mainly toward the tip, it may sometimes be sufficient to protect only their radially external portion. The stress of the baffle bodies caused by the centrifugal force is thereby greatly reduced; yet it is advantageous to taper them conically in radial direction outwardly.

The drops impinge close to the stagnation or ram point of the profile, and therefore, if the baffle bodies are fastened on the rotor or on the vanes, they are provided only in this region. Also it is not necessary to protect each vane separately, as five or six vanes are covered together and are sufficiently protected by baffle bodies mounted in front.

The baffle bodies may be made, for example, of cold-drawn stainless steel. Experiments have shown that the erosion effect greatly decreases with decreasing wire diameter. A thickness of the baffle bodies of 1 mm is to be regarded practically as the upper limit. A few millimeters distance of the baffle bodies from the imperiled structural parts are sufficient for adequate protection.

Two examples of construction of the invention are illustrated in simplified form in the accompanying drawings, wherein:

FIG. 1 shows a steam turbine vane in front view;

FIG. 2, a single baffle body on a larger scale; and

FIG. 3, an airplane wing in partial view.

According to FIG. 1, the entrance edge of the turbine vane 1 is protected at its radially external portion by wires 2, which are secured to the ribs 3,4. They extend at a small distance from the vane approximately parallel with one another. Because of the high stress exerted by the centrifugal forces, a careful attachment to the ribs 3 is necessary.

An example of this is illustrated in FIG. 2. It shows a single wire 2 greatly enlarged, especially the thickness being much exaggerated. The outwardly tapering wire is welded into the rib 3, while at rib 4 it is fastened only by spot welding, because there only relatively small forces must be absorbed. According to the occurring stress, the wire may alternatively be jointed to the ribs by brazing or otherwise.

Instead of providing the rib 3, the profile may be recessed at this point, the wires extending in the original profile line, whereby the necessary spacing and the desired flexibility are established. Also, the wires may extend over the full length of the vane blade and may be held, for example, in a bottom plate, which is fastened to the root of the vane. Another possibility, already mentioned, consists in fastening the wires directly to the rotor, that is, in front of the vane root.

FIG. 3 shows another possible use of the baffle bodies. Before the nose of an airplane wing 5 a wire grid 6 is applied, which protects the planking against impinging rain drops.

Other uses of the baffle bodies are conceivable wherever high speeds occur, especially greater than Mach 1, and liquid drops are contained in the gaseous medium. Exchangeability of the baffle bodies is of advantage in most cases.

Claims

We claim:

1. A device for protecting the vanes of a rotor against surface erosion caused by impingement of liquid drops thereon at high speed which comprises an array of thin flexible baffles positioned in front of and spaced from the surface of each vane, said baffles serving to intercept and decelerate the droplets prior to striking the vane surface as a result of flexation of said baffles, and said baffles also serving to divide the droplets into smaller masses, thereby to reduce their impact force upon the vane surface.

2. A device as defined in claim 1 for protecting the vanes of a rotor against surface erosion wherein said baffles are constituted by an array of parallel spaced thin flexible wires enveloping the leading edge portion of the vane and extending in a radially outward direction, said wires being spaced from the vane surface and being secured thereto only at their ends.

3. A device as defined in claim 2 for protecting the vanes of a rotor against surface erosion wherein said wires are tapered in the radially outward direction.

4. A device as defined in claim 1 for protecting the vanes of a rotor against surface erosion wherein said baffles are constituted by the wires of a thin flexible wire grid, said grid enveloping the leading edge portion of the vane and being secured thereto in spaced relation.