Circumferentially Grooved Shroud Liner

1. Field of the Invention

The present invention relates to turbines, and more particularly, to gas or steam turbines.

2. Description of the Prior Art

Rotor blades in a turbine, especially in smaller turbines, are normally unshrouded. The root of the blade is fixed to a hub, but the outer radial end or tip of the turbine rotor blade is free. A liner of cylindrical shape is normally provided in the stationary housing acting as a shroud for the blades. However, since the shroud liner is stationary, it is necessary to leave a gap or tolerance between the tip of the blade and the shroud liner so as to avoid rubbing. It is necessary to provide a sufficient gap between the tip and the shroud in order to provide for the differences in the expansion of the respective metal components. A minimum practical clearance has been found to be 1% of the blade height. However, when such gaps are provided, the gases on the high pressure side of the blade tend to leak over the tip of the blade at a relatively higher velocity than the rotating velocity of the blade to thereby interfere with the low pressure side of the blade and deteriorate the flow pattern of the gases on the low pressure side increasing, for instance, separation of the gas flow from the surface of the blade on the low pressure side thereof. Furthermore, the gap causes some of the gas to bypass the rotor, and thus not contribute to the work, since it is not turning relative to the rotor blades.

Various improvements and developments have been made to reduce the actual gap between the tip of the blade and the shroud liner as, for example, the ablative seal described in U.S. Pat. No. 3,836,156, issued Sept. 17, 1974, Hector B. Dunthorne, and assigned to the applicant. This helps to reduce the size of the gap, but does not eliminate the tip leakage. One common method for reducing the amount of tip leakage flow, as well as its disruptive influence on the flow on blade low pressure surface, is to equip the rotor tip with a shroud.

Each blade has at its tip a segment of a ring, such that when assembled in a rotor disc, these shroud segments form a continuous ring which prevents flow from within rotor blade passages from leaking around the blade tips. Due to the necessary tip clearance, some flow will still leak past the rotor blades, but at least it does not disrupt the mainstream flow on blade low pressure surfaces.

However, any such shroud causes a relatively large amount of metal to be added to rotor blade tips. This is most undesirable in the case of first stage turbine blades, because of the high gas temperature at the exit of the combustion chamber immediately preceding the turbine. The additional mass of the blade created at the tip causes the centrifugal stresses in the rotor blades to be substantially increased, with the result of a much reduced rotor blade life; while there are ways to alleviate this problem, such as a reduced gas temperature, or blade roots with very large metal areas, hence thick discs, considerations of overall engine efficiency and weight usually dictate the elimination of such shrouds on first stage blades.

It is an aim of the present invention to provide a mechanical means of aerodynamically controlling and reducing the leakage of the gases from the high pressure side of the blade to the low pressure side of the blade over the tip thereof without using shrouds at the blade tips.

A construction in accordance with the present invention includes a stationary shroud surrounding a rotor provided with a plurality of radially extending blades, the stationary shroud including a plurality of spaced-apart lands, the thickness of each land and the spacing therebetween and the depth of the openings so formed being selected so as to redirect the flow leakage.

Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration, a preferred embodiment thereof, and in which:

FIG. 1 is a fragmentary perspective view of a turbine wheel with a stationary shroud associated therewith;

FIG. 2 is a fragmentary radial cross-section showing the shroud and a tip of a typical blade;

FIG. 3 is a fragmentary cross-section taken along line 3--3 of FIG. 2;

FIG. 4 is a fragmentary radial cross-section similar to FIG. 2 but showing another embodiment of the shroud;

FIG. 5 is a radial fragmentary cross-section similar to FIG. 2 but showing yet another embodiment thereof;

FIG. 6 is a radial fragmentary cross-section taken along line 6--6 of FIG. 7, similar to FIG. 2, showing still yet another embodiment thereof;

FIG. 7 is a plan view looking outwardly radially at the shroud of FIG. 6;

FIG. 8 is a fragmentary radial cross-section similar to FIG. 2 but showing yet another embodiment; and

FIG. 9 is a schematic cross-section of a turbine blade of the present invention.

FIG. 10 is a graph of the turbine difference in efficiency 7 against t/d; and

FIG. 11 is a graph of the turbine difference in efficiency 7 against the inclination angle of the grooves.

Referring now to the drawings, there is shown a turbine wheel 10 with a plurality of blades 12 each having a high pressure or concave side 14 and a low pressure or convex side 16. Each of the blades is fixed to a hub 18. Each blade has a free end referred to as tip 20. A cylindrical housing surrounding the rotor includes a cylindrical stationary shroud 22. The shroud 22 is spaced from the tips 20 of the blades 16 by the gap "x". This distance "x" represents the necessary clearance to allow for differences in expansion of the respective metals.

The shroud, in the present embodiment, is provided with a plurality of parallel grooves 24 having a bight 28 and by thin lands 26. In a typical example of a turbine which was tested, the parameters were as follows:

The groove parameters would, as a function, be determined by the size and shape of the blades as well as the gap "x".

It has been found that, in addition to the advantages mentioned above, there is the added advantage that there is less rub area for the blade tips and the geometry of the shroud is better suited for blade containment in the case of accidental dislodgement of the blades.

In the embodiment shown in FIGS. 1, 2 and 3, the bight 28 of the groove 24 is semi-circular in cross-section, and the grooves 24 run circumferentially in planes at right angles to the axis of the turbine.

In the embodiment shown in FIG. 4, a fragmentary blade 112 having a tip 120, and fixed shroud 122 are illustrated the grooves 124 formed by lands 126 are shallower than those shown in FIGS. 1 and 2, and in this case, the depth of the groove was measured at between 0.100" and 0.050". The structure of the grooved shroud is the same as in the embodiment described with respect to FIG. 2.

It has further been discovered that the depth of the groove in the shroud should not be too deep as it will affect the efficiency of the engine in a detrimental manner. It has been discovered through tests that there appears to be a relationship between the thickness of the blade tip taken at t in FIG. 9 or the widest part of the blade at the tip thereof and the depth of the groove. It would appear that the efficiency of the engine will be increased if the ratio of the thickness of the blade to the depth of the groove in the shroud is greater than 1. In other words, t/d>1. Table A attached at the end of the specification shows graphically this phenomenon. It is conceivable that in the case of very thin blade tips, small caps or mini-shrouds could be provided at the tip thereof in order to effectively widen the tip of the blade. The following results were taken from actual tests, and the efficiency is measured against a turbine having a shroud without grooves but with the remaining characteristics the same.

t=thickness

w=width

d=depth

.eta.=efficiency

.DELTA..eta.=difference in efficiency

The tests shown above, it is noted, were taken with a groove width being constant, that is, at 0.050 inches in width.

In both embodiments, the axial extent which the grooves cover is equal to the blade chord length, that is, 0.540". FIG. 5 illustrates an embodiment where the shroud is referred to at 222 while the blade 212 includes a blade tip 220, with grooves 224 in the shroud 222. The grooves 224, however, extend over only a fraction of the axial chord length of the tip 220 in the shroud 222. The values with respect to the depth of the grooves, thickness of land, etc., are the same as with the embodiment shown in FIG. 2. Of course, the overall axial coverage of the grooves is now a fraction of the normal value with the other embodiments.

It has been found that with turbine rotor blades, separation of flow from the low pressure surface is generally present, near the tip of the blade. This separation is caused by the flow leakage over the blade tip, as discussed. Tip sections of different turbine blade design have different surface pressure distribution, and thus different positions of flow separation when tip leakage flow is present. Since the grooves redirect and retard the tip leakage flow, the optimum location (and direction, for that matter) of the grooves is likely to differ from one blade design to the next. For example, in the case of a blade tip having a large negative pressure gradient near its trailing edge, grooves are placed over the trailing edge portion of the blade, to delay separation, as shown in FIG. 5. In the case of a blade having a negative pressure gradient near its leading edge, the optimum placement of grooves would be over the forward portion of the blade.

Reference will now be made to FIGS. 6 and 7 showing the groove at angles to the radial plane taken through the turbine. Referring to FIGS. 6 and 7, there is shown a shroud 322 with grooves 324. A blade 312 with a tip 320 is shown. The values are similar to the embodiment shown in FIG. 2 with the exception that the lands are 0.030" in width while the groove widths are between 0.080" and 0.140". These dimensions are taken at right angles to the lands as opposed to the axial direction. The angle .theta., that is, the angle of the grooves, could be determined by computing: 90.degree. less the inlet flow angle. In other words, the setting of angle .theta. would be the complementary angle to the mean inlet flow angle, that is, the angle of inlet flow measured relative to the rotating blades.

In the embodiment shown in FIG. 8, there are grooves 524 at an angle to the radial plane, and this angle .phi. can be between 0.degree. to 40.degree. from the radial plane x'. The other dimensions can be similar to the other data with respect to the previously mentioned embodiments. The angle of inclined grooves 524 or lands 526 provides a further control of the flow in the grooves, that is, a changed flow co-efficient.

Table B which follows this specification shows the increase in efficiency of the turbine as the angle of inclination from the radial plane x' increases in a direction upstream of the flow. In specific tests, referring to FIG. 8 and Table B:

a=0.020"

b=0.050"

d=0.150"

.phi.=20.degree. to 30.degree.

As can be seen from Table B, the preferred angles of inclination are between 20.degree. and 30.degree. although improved results were obtained with angles of inclination from 10.degree. to 40.degree.. These comparisons are made with an ungrooved shroud.

By way of explaining the aerodynamics, reference will be made to FIGS. 1 and 3. Next to the stationary shroud 22, the primary stream of hot gas forms a boundary layer. The moving blades 12 continuously cut into this slower moving boundary layer, which has the effect of opposing the tip leakage flow, in effect forming a partial aerodynamic seal. This phenomenon occurs on all unshrouded blades.

The grooves formed by lands 26 have the effect of thickening this boundary layer, improving the effectiveness of this "seal" (by virtue of the larger surface area scrubbed by the gas in the tip region), and by directing this boundary layer more exactly in the direction of the prevailing tip leakage.

This direction, however, will not be the same for all possible turbine rotor blades, but will depend on the inlet and exit angles .beta. of the blade.

Many variations of the grooves can be obtained as can be seen from the few embodiments described above.