US2258792A - Turbine blading - Google Patents

Description

0t.14,1941. w, R? 2,258,792 TURBINE BLADI Filed April 12,- 1941 T WI. H n n ATTORNEY F' IC1.4.

INVENTOR WINSTON R. Nsw. Y

Ill

Patented Oct. 14, 1941 Winston R. New, Springfield,

Pa., assignor to Westinghouse Electric a Manufacturing Company, East Pittsbur- Pennsylvania gh, la., a corporation of Application April 12, 1941, Serial No. seam My invention relates to reaction turbines, more particularly to those having elastic fluid supplied thereto at relatively high pressure and having a relatively small ratio of inlet pressure to back pressure, otherwise stated, a low pressure or expansion ratio; and it has for anobject to improve the eiliciency of such turbines by suitable choice of the blade gauging.

A more particular object of the invention is to improve the emciency of reaction turbines designed for nearly constant stage velocity ratio, and having a small expansion ratio, with steam supplied thereto at high absolute initial pressure. The principal effect of the invention is,

therefore, to improve the performance of highpressure units of multi-cylinder turbines and of topping turbines, where such machines are used for driving alternators, this use requiring constant speed to give a predetermined frequency and, largely on that account, permitting the designer to approach optimum stage velocity ratios independent of large changes in load. While a turbine driving an alternator may be regarded as a constant speed machine, the invention is not necessarily limited to one of this constant speed type for the reason that it may be applied to other high-pressure units, such as those of multi-cylinder marine turbines, where the steam is so admitted to the high-pressure unit that the ratio of blade speed to steam speed, that is, the velocity ratio, is maintained substantially constant. I

In many cases a reaction stage is the most eiilcient type and is to be preferred, where the volumetric'flows lead to favorable aspect ratios, h/l. The present invention is of particular interest in connection with turbines of large rating, designed for constant stage velocity ratio, relatively small pressure or expansion ratio, for example, high-pressure units of compound arrangements and topping turbines where emphasis is placed on the rated load performance, that is, is on base load performance.

To develop a desired power (law) a certain flow is needed, and, with preassigned fluidconditions at the inlet and the outlet, certain areas are required in the various stages. Passage area .is largely fixed by blade height, 'h, and gauging. o/s, where 0 is the opening between adjacent blades ands is the pitch. with turbines of the foregoing time. dense motive fluid is employed with the result that the loading for .each blade may be relatively high; and, on this account, the

blade section measured by'the chord dimension.

1, must be sufficiently large to provide adequate strength. For blade heights contemplated, the upper limit of aspect ratios, h/l, will be of the order of 3 or, perhaps, 4. The range of aspect ratios is relatively very much smaller for a turbine of the low expansion ratio type than for one of; the condensing type. Representative ranges of expansion ratios might be, for example, ten (10) in a turbine with an inlet pressure of 1000 lbs/sq. in. and aback pressure of,100 lbs/sq. in., and one thousand (1000) in a turbine with an inlet pressure of 500 lbs/sq. in. and a back pressure of 0.5 lb./sq. in. Heretofore, it has been common .practice to maintain a constant gauging, for example, 30%, and to calculate the blade heights or aspect ratios to give the needed flow area. I find that, for the required flow area ratios, h/lXo/s, (expressed dimensionlessly to conveniently restrict experiments and facilitate presentation of results) optimum stage efliciencies occur at various gaugings, and, if both the gauging and the blade height are varied, advantageous combinations may be chosen. Also, I find that, as the flow area ratio, h/lXo/s, diminishes, the rate of change of eiilciency with gauging, for favorable velocity ratios, increases.

Accordingly, a further object of the invention is to provide a reaction turbine of the favorable velocity ratio and low expansion ratio type, wherein thegauglng is smaller at the high-pressure end than at the low-pressure end thereof.

A'iurther object of the invention is to provide a reaction turbine 01 the type described, wherein, because the rate of change of efilciency with ga ng increases rapidly with decreasing flow area ratios, h/lXo/s, the gauging is selected to attain optimum stage emciency or nearly so. Conversely, as the necessary flow area ratio increases, the rate of change of efliciency with gauging decreases until values of the parameter h/lXo/s are reached where very little change in eiiiciency occurs with change in gauging so that, therebeyond to larger flow area ratios, the matter of a preferential gauging may be disregarded, and within wide limits, gauging may be arbitrarily selected so as to give the required flow area ratio-together with the aspect ratio.

These and other. objects are efiected by the invention 'as will be apparent from the following description and claims taken in connection with the accompanying drawing, forming a part of this application, in which:

Fig. l is a detail sectional view of alow expan-' sion ratio turbine employing the new arrangement of blading;

Fig. 2 is a view similar'to Fig. 1 but showing a conventional high expansion ratio type of turbine wherein employment of the invention is of less consequence;

Fig. 3 is a sectional view of a reaction turbine stage;

Fig. 4 is a detail side elevational view of the structure shown in Fig. 3; and

Fig. 5 is a diagrammatic view showing the discovered relation of flow area ratio to stage em- .ciency and gauging, all for favorable stage velocity ratios.

Referring now to the drawing, Figs. 3 and 4 will first be considered to make clear certain geometric quantitles of importance to the present invention. The following dimensions are indicated:

h=b1ade height l=chord o=minimum opening s=pitch As hereinbefore pointed out, the persent invention rests on the discovery that, at favorable velocity ratios, the rate of change of efficiency with gauging increases as the flow area ratio, h/lxo/s, diminishes. This means that the invention has greatest significance with constant velocity ratio turbines, such as high-pressure units of compound installations and topping turbines, where the aspect ratio is small, and, because of the low expansion or pressure ratio, it

does not change so greatly from the inlet to theexhaust. With large steam density and high power ratings, the blade loading is high and the blade section measured by the chord, I, must be large enough to give the required strength. Increasing l for this purpose diminishes the aspect ratio, h/l, and also the fiow area ratio. As shown in Fig. 5, I find that it becomes increasingly important, from the point of view of stage efficiency, to properly choose the gauging for the smaller flow area ratios. The blade height, h diminishes toward the high-pressure end, and, at the same time, for reasons of strength, as the high-pressure end is approached, the chord l of the blading must frequently be relatively wider than is the case with blading'of comparable heights in turbines of greater expansion ratio, such as shown in Fig. 2. Thus, the aspect ratio may be diminished toward the high-pressure end by both of the factors n and l; and the gauging must-be modified from the low-pressure end to the highpressure end, as hereinafter explained in detail, to obtain the best stage efliciency.

In Fig. 1, there is shown a turbine of the low expansion, or pressure, ratio type supplied with steam or elastic fluid at high initial absolute pressure, for example, at pressures of the order of 900 or 1000 lbs/sq. in. and above. The exhaust or back pressure is high enough to keep the expansion ratio low. If, for example, with an initial pressure of 1000 lbs/sq. in, the exhaust or back pressure is 100 lbs/sq. in.. then the expansion ratio is and, if the back pressure is 500 lbs/sq. in. for the same initial pressure, the expansion ratio is 2. Comparing the turbine of Fig. l with one of the high expansion ratio type, such as shown in Fig. 2, if, with the latter, the initial pressure is 500 lbs/sq. in. and the exhaust pressure V lb./sq. in. absolute, it will be seen that the pressure ratio is very much higher,

that is, 1000.. 7

Referring again to Fig. 1, the turbine includes rotor and cylinder elements ill and I I and a plurality of reaction stages 12 each including a stationary row of blades l3 followed by a row of moving blades H carried by the rotor.

With reaction turbines, it has been the practice to adhere to a fixed gauging, o/s, for example, throughout the turbine. I I find that this is detrimental to turbine eificiency in general, but particularlmto the efliciency of high-pressure units designed for favorable velocity ratio and low expansion ratio. With constant gauging heretofore commonly employed with reaction turbines, the necessary increase in flow area from the high-pressure to the low-pressure end of the machine is provided only by increase in blade efficiency, or nearly so.

height. I provide for variation in gauging from end-to-end of the turbine, the gauging being smaller at the high-pressure end than at the lowpressure end. As the fiow area of a blade row is fixed by the blade height and by the gauging, the decrease in gauging toward the high-pressure end gives somewhat greater blade heights than would otherwise be obtained. The important reason for varying the gauging is, for the required flow area ratio, to obtain the optimum As the flow area ratio, h/lXo/s, becomes smaller, the rate of change of efliciency with gauging becomes greater (Fig. 5).

From Fig. 5, it will be seen that, as the flow area ratio becomes greater, gauging variation has less and less effect on the efliciency until, at

large flow area ratios, the efliciency is not substantlally afi'ected by gauging variation within wide limits. The curve 0 representing a flow area ratio of 1/2, referred to stage efllciency and gauging coordinates, has a relatively much larger change in slope than the curves b and c, indicating flow area ratios of 2/2 and 3/2. That is, as the flow area ratio becomes smallen the rate of change of efllciency with gauging, for constant velocity ratios, increases. While the gauging'of the blading may vary from row torow, I prefer to arrange the blading in groups, I6, I1, I! and I! in Fig. 1, with the gauging of the blading of each held the same but decreasing group by group from the low-pressure to the high-pressure end.

While the invention has been shown in but one form, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof, and it is desired,

therefore, that only such limitations shall be placed thereupon as are specifically set forth in the appended claims.

What is claimed is:

i. In a full-peripheral admission reaction turbine designed for a relatively small expansion ratio and a narrow range of stage velocity ratios, a plurality of successive stages, each stage including a pair of rows of relatively movable blades; the blades having a height dimension 7:. and a chord dimension 1 sufficiently large in relation to height to give an aspect ratio, h/l, of the order of four as a maximum at the low-pressure end and decreasing therefrom toward the high-pressure end of the turbine because of decrease in o is the minimum opening between adiacent blades and s is the blade pitch; that, for the flow area ratio, h/lXo/s, necessary !or the development of the desired power, o'ptimum stage enlciency is attained by making the gauging smaller at the high-pressure end than at the low-pressure end of the turbine.

2. The combination as claimed in claim 31 gauging oi the group at the high-pressure end of the turbine is smaller than that at the group at plurality of groups and wherein thegauginz oi. the blading of each group is uniform and the the low-pressure end thereof.

4. The combination as claimed in claim 1 with the blading or turbine divided into a plurality ot'groups aflit wherein the gauging of the blading of each group is uniform and the wherein the gauging oi the blading increases 10 gauging oi the bladint. by groups. increases from from the high-pressure end to the low-pressure end of the turbine. A

3. The combination as claimed in claim 1 with the blading oi the turbine divided into a I the high-pressure and to the low-pressure end oi the turbine.

I WINSTON R. NEW.

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