US1853137A - Draft tube - Google Patents

Description

April 12, 1932.

L. F. MOODY DRAFT TUBE .Driginal Filed May 8, 1925 2 Sheets-Sheet l 0200mm umn Fm Z tuod mas 3,137

L. F. MGODY April 12, 1932.

DRAFT TUBE Original Filed May 8, 1925 2 Sheets-Sheet 2 PatentedA'pr. 12, 1932' UNITED STATES:

ars-ur ()FFICE.

LEWIS FER-RY MOODY; OEFHILADELPHIA, PENNSYLVANIA.

DRAFT TUBE Original application filed May 8, 1925,Seria1-N0. 28,868. Divided and this application filed February 18,

1928. Serial This invention relates to draft tubes for hydraulic turbines and the like and particularly to draft tubes wherein the flow received in one direction is turned and discharged in a another direction at an angle to the first.

The object of the invention is to provide a draft tube which will smoothly and efficiently decelerate the flow within a relatively smallspace. The smoothness of the deceleration not only helps in increasing eflicl ency but also "avoids any tendency toward v1bration sometimes set up by a flow at high Velocity or under reduced pressure. The close spacing of a series of turbine units n a powerhouse also frequently limits the dimensions of the draft tube, particularly in transverse width, so that there is no room for extensive lateral spreading of the tube without interference with adjacent units or requiring a prohibitively wide spacing of the units.

I have found that by bending the discharge stream as a whole andat the same time changing its cross sectional form and properly controlling the rate of velocity I deceleration throughout the stream,;the regular and eiii ciently smooth turning of the entire flow may be attained within very narrow space restrictions,in a conduit of simple construction.

In the accompanying drawings illustrating the invention Fig. 1 is a sectional view through a draft tube illustrating the invention.

Fig. 2is a plan view of the same with successive sectional shapes indicated at corre sponding points of the tube, each cross section indicated at I, II, III, IV, V, VI, VII, VIII and IX of Fig. 1 being indicated in outline turned into the horizontal plane.-

W Fig. 3 is a typical graph showing the velocity of the flow at successive points in the tube.

Fig. 4 is a diagrammatic representation of a comparative straight conical tube having the same velocity values as those in the bent tube of Fig. 1, shown in half section.

Figs. 5 and 6 are views similar to Figs. 1 and 2 but illustrating another form of tube. Inthe embodiment of the invention shown in Figs. 1' and 2 the flow leaving the turbine runner enters the draft tube at 20, is turned in the bending portion 21 of the tube and passes out to tailwater through the discharge 1 portion 22.

From the entrance section, designated I at the center line, to the section II the tube is substantially straight and circular in section. Beyond section II the curvature definitely begins and continues with a center line designated ILIII, IV, V, VI and VII. Throughout the turn the sectional formprogressively changes, as shown in Fig. 2, from substantially circular at section II to a trans versely elongated section at VII. The side walls 23 and 24 spread or diverge (Fig. 2) while the inner surface 25 and outer surface tin 26 of the bend (Fig. 1) at first remain nearly smoothly bends and changes in shape in a natural manner and without tendency to set up. cross currents or eddies.

This change in direction and form of the stream is accompanied by a reduction in the velocity so as tosimultaneously convert velocity head into pressure head. At first this deceleration of the velocity may be relatively large and approach closely to the permissible efficient deceleration. in a straight draft tube.

As the tube curves however the rate of de, celeration is reduced throughout the major portion of the bend. As shown in the diagram Fig. 8, for instance, plotting velocityv against distance along the center line, the velocity is quite rapidly reduced until near section IIIthe graph begins to curve so as to,

become less and less steep, indicating a constantly' decreasing rate of deceleration. As

section VI is approachedthe graph flexes toward an increase in steepness indicating a slight increasein the rate of deceleration as the bendincreases in radius near its end. The

shape of the velocity graph of any given tube will depend upon the individual form and curvature of the tube. In general the lower the rate of deceleration, i. e., the flatter the curve, the less will be the likelihood of irregularities in the flow or any separation of the flow from the tube walls. The more rapid the curvature of the tube walls the less will be the tendency of the flow to follow them. Consequently the portions of a bend that are relatively abrupt and of small radius of curvature will have the least deceleration and the more rapidly the radius of curvature decreases the more rapidly will the rate of deceleration decrease.

In the process of decelerating the velocity of flow of water in a passage enlarging in the direction of flow, the angle of divergence of the side walls from the initial direction of flow is much more effective on the elements of the flowing stream which are adjacent to the side walls than it is upon the central portions of the flow which are more remote from the side walls and therefore do not feel the influence of their angle to the same extent. The result is that in a straight, enlarging conduit the central elements of the flow are decelerated much more slowly and less effectively than the elements in contact with the wall, and the result of the enlargement is therefore to set up an unequal distribution of velocity across the conduit, the velocity at the center remaining higher than at the side wall. If the angle of divergence of the side walls is too great, this unequal distribution of flow may be carried so far as to result in eddies or backward flow around the walls with a consequent loss of energy and inefficient deceleration.

In a conduit containing a bend or elbow, a similar effect is caused even when the area of the conduit remains constant in passing around the elbow. The effect of the bending of the stream is to set up centrifugal forces which build up an increase of pressure around the outside of the bend and cause a reduction of pressure at the inside of the bend with a consequent tendency toward an increase of velocity at the inside of the bend and a decrease at the outside, so that the effect of a bend in a channel is to cause a greater and greater inequality of velocity distribution as the flow passes around the bend. If the curvature of the conduit is too rapid, the flow will tend to leave the inner surface of the bend leaving eddies or backward flow in this region, or an effect similar to too rapid enlargement of a straight conduit. It is therefore seen that the effect of gradual enlargement in a conduit and the effect of curvature of a conduit without enlargement are similar in creating non-uniform distribution of velocity transverse to the flow.

Since the effects of enlargement and of curvature of the centerline of the conduit are thus in general similar in creating non-uniform distribution of velocity across a section, it follows that when we desire both to decelerate velocity and to turn it around a bend at the same time, it is necessary to limit the rate of enlargement in proper relation to the rate of curvature of the conduit so that the two effects when superimposed will not result in extreme variation of velocity across the conduit or the creation of backward flow or eddies.

As the flow progresses around a bend or elbow of the conduit, therefore, it is necessary progressively to decrease the rate of deceleration in passing from the entrance to the discharge section of the bend and at the point where the greatest distortion of fiow would be caused by the curvature of the conduit, for instance, adjacent the discharge end of the elbow, it is necessary to reduce the rate of deceleration to a very low value approaching a condition of constant velocity. After the flow has been deflected into a new direction. the rate of deceleration can then be gradually increased so that at points more distant from the elbow the rate of deceleration can approach more and more closely to that permissible in a straight conduit.

In Fig. 4: the successive areas of the tube of Fig. 1 are represented by the radii of equivalent circular sections laid off along a straight center line so as to show the form of the wall of an equivalent straight tube of circular cross section. This indicates a relatively large angle of flare a at entrance which angle begins to decrease at section III and continues this decrease to a point of flexure near section VI after which the angle begins a slight divergence to VII, after which it increases to a substantially constant value 01 for the straight outlet portion 22.

To convert velocity head into pressure head it is desirable to have as great a reduction of velocity as possible throughout the tube as a whole. To this end the straight or substantially straight portions of the tube have a relatively large angle of flarefor instance corresponding to 5 or even 7 in an equivalent straight tube such as indicated in Fig. 4. The bend of the tube has as much velocity reduction as can be effected during the process of deflecting the flow through an angle, consistently with the maintenance of smooth conditions of flow. I have found that while these conditions do not permit a high rate of deceleration at the bend the average per missible deceleration, and consequently the total deceleration, in the bend may be considerable and consistent with the mainten ance of highly smooth and efficient flow conditions. In Fig. 4, for instance, the flare of the straight tube walls corresponding to the bend of the tube of Fig. 1 decreases to between 1 and 1 and then slightly increases as the end of the elbow is approached and the curvature of the passages decreases. This flare around the curve gives an increase in area between the entrance section II and the Simultaneously with this increase inarea the cross sectional shape of the tube changes to widen out laterally and decrease the distance between the inner wall and outer wall so that the distance Z) between these walls at the discharge VII is less than the dimensiona of the substantially circular entrance section II'and in general this dimension 6 will be not over two-thirds of dimension at, nor less than one fourth of a.

The sh, peof the bend in plan view (Fig. 2) Willvary depending for instance on the available space for the widening of the bend at the turn. Where permissible this lateral spreading may be carried out to comparatively large widths but the conditions of installation will often make it advisable or necessary to retain the lateral width of the bend within dimensions permitting only a relatively gradual lateral spread. This will be the case when a portion of the draft tube structure is already in place in the plant or where it is desired to modify an existing draft tube so that its form and area transition may be made to conform to the principles here explained.

In the bend shown in Figs. 1 and 2 the tube expands laterally with a moderate increase in width from point to point indicated by the side surface contour lines 28 and this lateral expansion is such as to give these contour. lines 28 an outward convexity away from the axis at the entrance sections I,

II,v III followed by 'a flexure leading to a convexity towardthe axis for thesections IV, V, VI and this is succeeded by reverse flexure adjacent section VI to a slight convexity as the bend merges with the final outlet passage 22. The outlet passage 22, as shown in Fig. 2, expands laterally at a lowerrate than the lateral expansion of the bend, or the outlet passage may be constructed so as not to expand laterally at all.

In case it is permissible to widen the draft tube passage, the same successive area and velocity relations may be maintained around the bend by bringing the inner and outer Surfaces 25, 26 closer together. This wiil reduce the distance from the inner to the outer surface and may change the side contour lines to be generally convex away from the center.

The inner surface 25 of the bend is shown as are shaped in section' II and flat with transverse'straight line elements of increasing length in sections III to VII where it merges into the fiat roof 29 of the outlet passage 22.

Figs. 5 and 6 are views corresponding to Figs. 1 and 2 and illustrating a modified tube with decreased lateral spreading of the tube around the bend. In these Figs. 5 and 6 the, reference numerals of Figs. 1 and'2 indicate corresponding parts by corresponding numerals primed. The inner and outer walls 25, 26 of the bend of this modified tube do not approach each other as rapidlyas the walls 25, '26 of the draft tube of Fig. 1, and the sections III, IV, V, VI and VII of the modified tube are reduced in lateral Width, the successive areas and velocities being close ly analogous in the two tubes.

The draft tube of this invention is also pari ticularly advantageous in not carrying the excavation upstream any farther than necessary, so that the power house foundation 1S not unduly cut into. In many plants, and

particularly in the case of low or moderate head plants where the power house forms part of the dam, any excessive cutting away of the power house foundation is especially objectionable. It. is, therefore, often desirable from structural considerations to use a cbmparatively long radius of the outer surface 26 and 26 of the bend, joining the upstream wall of the vertical portion and the floor of the horizontal portion. It is still more important for hydraulic reasons not to have too short a radius of curvature for the inside surface 25 and 26' of the bend to give a gradual'turning of the stream Where thevelocity is high and the pressure low as is the case along this surface. .The radius of the outer surface of the bend must also, of course, not be so long as to impair the proper relative spacing between the inner and outer surfaces and the desired sequence of areas around the bend as pointed out above. The ratio of the radius of curvature r of the inner surface of the bend in the plane of the bend to the diameter a at the beginning of the bend, that is, V

will be approximately as follows:

In a tube having a wide, lateral extension,

will have a minimum valueof approximate: 1y .60, and this radius of curvature 7* should not be substantially less than .60 of the initial diameter in any of these tubes. In tubes hav may reach values as high as 1.5 eorresponding to a long and deep tube so that in most cases the radius 1* will not greatly exceed the diameter a.

While this invention has been illustrated and described in connection with a vertical shaft turbine and with the draft tube turning the flow toward the horizontal direction, it is obvious that the invention may be used in connection with turbines having inclined or horizontal shafts and is not confined to the specific embodiments shown and described.

In a horizontal shaft single or double runner turbine the entrance portion of the tube will in general be horizontal and the dis charge portion directed vertically downward. This form of tube is advantageous in its structural simplicity, minimizing the cost of form work when moulded in concrete, and the Weight and dimensions when built of metal.

This application is a division of my application, Serial No. 28,868, filed May 8, 1925, and is directed to the species shown in Figs. 1 to 6 of said parent application.

I claim:

1. A draft tube comprising a bent portion having inner and outer surfaces approaching each other in the direction of flow and both of said surfaces being substantially transversely flat for a portion of their length, and side surfaces spreading apart laterally so that the cross sectional shape of the flow is progressively lengthened in a lateral direction throughout substantially the whole of the bend, said tube at first increasing in crosssectional area at a decreasing rate in the direction of flow and then increasing in crosssectional area at an increasing rate.

2. An elbow draft tube comprising an entrance and a discharge connected by a gradually curving bend of relatively large radius, said bend commencing substantially near the beginning of said entrance and, in the direction of flow, increases in cross-sectional area at a decreasing rate.

'3. An elbow draft tube comprising entrance and discharge portions connected by a gradually curving bend of relatively large radius, said bend commencing substantially near the beginning of said entrance portion and at first increasing in cross-sectional area at a decreasing rate in the direction of flow and then increasing in cross-sectional area at an increasing rate before the end of the bend is reached.

4. An elbow draft tube as set forth in claim 2 further characterized by having the inner and outer surfaces of the bend spaced from each other by a decreasing distance.

5. An elbow draft tube as set forth in claim 2 further characterized by having the inner and outer surfaces of the bend spaced from each other by a constantly decreasing distance.

6. An elbow draft tube as set forth in claim 2 further characterized by having the side walls of the bend spread laterally.

7 An elbow draft tube comprising a long sweeping curved passage connecting into a discharge passage, the curvature of said curving passage starting substantially adjacent to the draft tube entrance, a portion of said curved passage increasing in cross-sectional area at a decreasingrate in the direct-ion of the flow.

8. An elbow draft tube having a bent portion proportioned so that the ratio of the radius of curvature of the inner surface of the bend in the plane of the bend to the diameter at the beginning of the bend is a value not less than substantially six-tenths.

9. An elbow draft tube having a bent portion proportioned so that the ratio of the radius of curvature of the inner surface of the bend in the plane of the bend to'the diameter at the beginning of the bend is a value not exceeding a substantially one and one-half.

LElVIS FERRY MOODY.

Images