US3627447A

US3627447A - Radial turbines

Info

Publication number: US3627447A
Application number: US807599A
Authority: US
Inventors: Ulo Okapuu
Original assignee: United Aircraft of Canada Ltd
Current assignee: Pratt and Whitney Canada Corp
Priority date: 1969-03-17
Filing date: 1969-03-17
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: GB1294593A; DE2014113A1

Abstract

A rotor for a centripetal turbine having a rotatable hub, star blades extending radially from the hub and exducer blades extending substantially radially from the hub but curved away from the direction of rotation of the hub. A gap or channel is provided between the trailing edge of each star blade and the leading edge of each corresponding exducer blade to direct a jet of fluid over the convex surface of the exducer blade to reenergize the boundary layer formed thereon during operation of the rotor.

Description

United States Patent [72] Inventor Ulo Okapuu St. Lambert, Quebec, Canada [21 Appl. No. 807,599 [22] Filed Mar. 17, 1969 [45] Patented Dec. 14, 1971 [73] Assignee United Aircraft of Canada Limited Longueuil, Quebec, Canada [54] RADIAL TURBINES 7 Claims, 7 Drawing Figs.

[52] U.S. Cl 416/227, 416/91,415/D1G.1 [51 int. Cl ..F04d 29/26, FOld H00 [50] Field 01 Search 230/134, 122 BL;416/l88, 182,183, 91, 227,185

[56] References Cited UNITED STATES PATENTS 1,622,930 3/1927 Von Karman et al......... 230/122 1,744,709 1/1930 Moody 230/122 2,576,700 11/1951 Schneider 230 /134 Von Der Nuell et al. Sheets Johns Meisser Whitaker 8/1965 Price l/l966 Cooper FOREIGN PATENTS 10/1922 Great Britain 4/1962 France Primary Examiner-Henry F. Raduazo AtI0rneyAlan Swabey ABSTRACT: A rotor for a centripetal turbine having a rotatable hub, star blades extending radially from the hub and exducer blades extending substantially radially from the hub but curved away from the direction 'of rotation of the hub. A gap or channel is provided between the trailing edge of each star blade and the leading edge of each corresponding exducer blade to direct a jet of fluid over the convex surface of the exducer blade to reenergize the boundary layer formed thereon during operation of the rotor.

mam) uricmem $621,441

INVENTOR Ulo OKAPUU A TTORNFY RADIAL TURBINES BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improvements in radial turbines and particularly in the rotor construction of such turbines.

The invention is particularly directed toward improving the aerodynamic efficiency of radial turbines.

2. Description of Prior Art The radial turbine that the invention particularly relates to, is of the centripetal type having a rotor comprising blades extending generally radially from a rotatable hub.

Hot gas or other elastic fluid flows radially inwardly at high velocity at one end of the rotor and is directed by the blades and hub outwardly from the rotor at its other end in a substan tially axial direction. At least the trailing portion of each blade of the rotor is curved. The hot gas, contacting the surfaces of the blades, rotates the rotor.

As the gas flows over the curved trailing portion of the convex side of each blade, a boundary layer is formed. This boundary layer reduces the aerodynamic efficiency of the rotor.

SUMMARY OF THE INVENTION It is a purpose of the present invention to provide a radial turbine rotor having improved aerodynamic efficiency which is constructed to reduce the drag effect created by the boundary layer fonned on the convex portion of the blades during operation of the rotor.

Means are provided in the blades of the rotor for directing a portion of the gas flowing between the blades from the high pressure or concave side of each blade onto the low-pressure or convex side of the blade to reduce the boundary layer formed and thus increase the efiiciency.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail having reference to the accompanying drawings, wherein:

FIG. 1, labeled as PRIOR ART," is a cross-sectional view of the blade construction presently used;

FIG. 2 is a cross-sectional view of a blade construction incorporating the invention in one embodiment;

FIG. 3 is a cross-sectional view of a blade construction incorporating a further embodiment of the invention;

FIG. 4 is a partial crosssectional view of a rotor having blades incorporating a preferred embodiment of the present invention;

FIG. 5 is a partial cross-sectional view of the rotor shown in FIG. 4 taken along line V-V.

FIG. 6 is a cross-sectional view of the preferred embodiment taken along line VI Vl in FIG. 4.

FIG. 7 is a further cross-sectional view taken along line VII-VII of FIG. 4.

DETAILED DESCRIPTION OF THE PRIOR ART Rotary turbines previously known in the art have a rotor with a plurality of equally spaced blades extending substantially radially from a hub. In centripetal turbines at least a trailing portion of each blade is curved away from the direction of rotation. The blades direct the gas flow, rotating the rotor, from a radially inward direction to a substantially axial outward direction. Because of the curvature of at least the trailing portion of the blade, away from the direction of rotation, it is general practice to build the centripetal turbine in two sections. This simplifies its construction. The first section comprises a plurality of equally spaced inlet or star blades extending radially from a first hub. The second section comprises a plurality of equally spaced, curved exducer blades extending from a second hub. The star blades are equal in number to the exducer blades. The exducer blades extend substantially radially from the hub but are curved away from the direction of rotation of the rotor to provide a concave surface and a convex surface. The two hubs are axially joined together to provide a unitary rotary structure with the trailing edge of each star blade aligned with the leading edge of each exducer blade.

During operation of the rotor, one surface of each aligned star and exducer blade is under high pressure and the other surface is under low pressure. The low-pressure surface during gas flow is on the convex portion of the exducer blade.

FIG. 1 shows the above-described blade construction with the trailing edge 1 of the star blade 3 aligned with the leading edge 5 of the curved exducer blade 7. During operation of the rotor the gas flows along the blades, as shown by arrows B, in a direction from the backface edge 13 of the star blade to the trailing edge 15 of the exducer blade. The blades rotate in the direction shown by arrow A. A high-pressure region is formed, during rotation, adjacent concave surface 9 of the exducer blade and adjacent surface 8 on the star blade and a low-pressure region is formed adjacent opposite surface 10 on the star blade and convex surface I 1 on the exducer blade.

During operation, a boundary layer 17, shown in dotted lines in FIG. 1, is formed along the low-pressure surface of the blades. The boundary layer thickens rapidly downstream of the beginning of curvature of the exducer blade 7 as shown. This boundary layer reduces the efficiency of the fluid flow through the blades and thus the efiiciency of the turbine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To reduce or eliminate the loss in efficiency due to the boundary layer formed, the present invention provides means to reduce or reenergize the boundary layer. These means can comprise providing a gap between the trailing edge of the star blade and the leading edge of the exducer blade and formed to direct a jet of high-pressure fluid over the Iowpressure surface on the exducer blade. The high-pressure jet blowing through the gap tends to follow the convex surface of the exducer blade and reenergizes the boundary layer thereon by accelerating the flow immediately adjacent the convex surface of the exducer blade to a velocity approximately equal to that of the flow outside the boundary layer.

FIG. 2 illustrates such a construction. The trailing edge 21 of the star blade 23 is spaced in the axial direction from the leading edge 25 of exducer blade 27 to form a gap or slot 29. Both the trailing edge 21 and leading edge 25 are angled away from the direction of gas flow on the high-pressure side to fonn an angled gap for directing a portion of the high-pressure gas flow as a jet over the low-pressure convex surface 31 of the exducer blade. A portion of this jet, shown in FIG. 2 by dotted lines, wipes away or reenergizes a substantial portion of the boundary layer formed on the convex surface 31 of the exducer blade 27 and thus increases the efficiency of the turbine. The slot 29 is of a width to provide sufficient jet flow to substantially reduce the boundary layer and extends at an angle to the axis of rotation of a hub carrying the blades. The embodiment shown in FIG. 2, while satisfactorily improving the efficiency of a turbine incorporating the slot construction, has the disadvantage that a portion of the jet directed through the slot 29 has a component extending in a direction away from the convex surface 31 of the exducer blade due to the an gling of the slot with respect to the convex surface.

FIG. 3 shows a further embodiment of the invention to provide more efficient removal of the boundary layer than the embodiment shown in FIG. 2. In this embodiment, the leading edge 41 of the exducer blade 43 is offset, preferably radially, with respect to the trailing edge 45 of the star blade 47. The exducer blade 43 and its leading edge 41 is offset radially away from the direction of rotation shown by arrow A to provide a gap or slot 49 between the two blades through which a jet of high-pressure fluid from the high-pressure side of the blades may flow onto and over the convex surface 51 of the exducer blade to remove or reenergize the boundary layer formed thereon. This offset construction is particularly easy to arrive at when the turbine rotor is formed in two sections. The exducer section is merely offset or rotated with respect to the star section prior to assembly to provide the gap between the trailing edge of each star blade and the leading edge of each exducer blade. This offset construction is more efficient than the gap construction shown in FIG. 2 since a major portion of the jet flowing through the slot 49 passes parallel to, and over, the convex surface 51 of the exducer blade from the highpressure surface 53 of the star blade 47.

A preferred embodiment of the invention is shown in FIGS. 4 to 7 inclusive. The exducer blades 61, carried by hub section 63, are offset, preferably radially, with respect to the star blades 65, carried by hub section 67, in a direction away from the direction of rotation shown by arrow A to provide a gap or slot therebetween. The hubs 63, 67, are joined in abutting axial relation by a bolt 64 and nut 66 connection passing through the axis of the hubs as is well known. The slot directs a jet of high-pressure fluid onto the low-pressure or convex side 71 of each exducer blade 61. However, in order to more smoothly direct the fluid jet onto the low-pressure surface 71, at least a portion of the slot is elongated to form a channel 69 which extends substantially in the same plane as the plane of at least the trailing portion of the star blade 65 and the leading portion of the exducer blade 61. The channel 69 is formed by extending the trailing edge 73 of the star blade 65 rearwardly so that a trailing portion of the star blade overlaps a leading portion of the exducer blade 61. The trailing portion of the star blade 65 extends rearwardly, in the direction of the gas flow, of the plane defining the abutting faces of the hub sections 63, 67.

The boundary layer, formed on the star and exducer blades, on the low-pressure side, particularly on the convex side of the exducer blade, is usually more pronounced at the shroud edge 75 of the blades than at their root edge 77 due to the fact that the shroud edge 75 of the exducer blade is curved more away from the direction of rotation than the root edge as is shown when comparing the cross-sectional views of FIGS. 6 and 7. The trailing edge 73 of the star blade can therefore be angled or cut back from the shroud edge 75 of the star blade if desired to provide a greater overlap at the shroud edge 75 of the blade than at the root edge 77 adjacent its hub 63. As shown in FIG. 4, this provides more efficient reenergization of the boundary layer at the shroud edge 75 of the blade, due to the channel 69 directing the jet parallel to the convex surface of the exducer blade, than at the root edge 77 where a short gap similar to that shown in FIG. 3 is formed.

Alternatively, of course, the leading edge of the exducer blade may be extended forwardly past the plane defining the joint between the star and exducer hubs to overlap the trailing portion of the star blade to provide the channel. However, it is preferred that the trailing edge of the star blades be extended rearwardly rather than extending the leading edge of the exducer blade forwardly. Extending the leading edge of the exducer blade forwardly would necessitate that the height of the leading edge 79 of the exducer blade 61 be increased to follow the contour of shroud edge 75 which could lead to vibration problems, particularly since the turbines to which the invention is particularly suited run at speeds of 40,000 r.p.m. and above.

As clearly seen in FIG. 6, the channel 69, formed between the blades, extends in a direction substantially parallel to the direction of flow and thus directs substantially all of the jet of high-pressure fluid, shown in dotted lines, along the low-pressure or convex side 71 of the exducer blade, as compared to the previous embodiments, where the jet emerged out the convex side at an angle to the surface.

The offsetting of the exducer blades with respect to the star blades to form a gap or channel is particularly adapted to the aerodynamic flow conditions in a centripetal rotor. Because the shroud edge 75 of the exducer blade 61 is curved more than the root edge 77 adjacent the hub 63, the boundary layer effect increases progressively as you move toward the shroud edge 75 of the blade. Thus, by merely radially offsetting the hub 63 containing the exducer blades with respect to the hub 65 containing the star blades about the hub axes, a wedgeshaped slot or channel 69 is formed as clearly shown in FIG. 6, which directs a greater proportion of flow of high-pressure fluid adjacent the shroud edge 75 of the blade than adjacent the root edge 77 which is needed to wipe off the boundary layer. More flow is required adjacent the upper edge because the boundary layer effect is more pronounced.

The slot or channel 69 must extend inwardly from the shroud edges 75 of the blades. It may extend down to the root edge 77 of the blades, as shown in FIG. 5, or only part way down from the shroud edge with the portion of the blades below the bottom of the slot joined, depending on whether the boundary layer is excessively thick only adjacent the shroud edges of the blades or over the entire low-pressure surface. If an excessively thick boundary layer in a particular turbine is fonned only at the shroud edge, the slot need only extend part way down from the shroud blade edge to provide a jet capable of reenergizing it. n

In a preferred embodiment, the width of the slot 49 or channel 69 formed adjacent the shroud edges of the blades should be between 5 and 35 percent, and preferably 20 percent of the pitch P, between the shroud edges 75 of adjacent star blades in the region of the slot or channel.

The invention has been particularly described with respect to a rotary turbine formed in two sections, the star and exducer sections, which are assembled together as shown in FIG. 4. However, the invention works equally well if the rotor is made in one piece. In the embodiment shown in FIG. 2 for example, the star 23 and exducer 27 blades could be formed as one integral blade having star and exducer portions. A saw cut can then be made in the blade at or adjacent the beginning of curvature of the exducer portion of the blade to form the slot 29. The cut would extend inwardly from the shroud edge of the blade radially toward or to the root edge of the blade.

While the use of the slot or channel in a turbine blade has been particularly described with respect to a centripetal-type gas turbine, the slot or channel could be used in the blades of any turbine to reduce the drag effect created when a boundary layer is fonned on the low-pressure side of a blade and thus improve the efficiency of the turbine.

I claim:

1. A centripetal-type turbine rotor havinga rotatable hub, a first set of equally spaced blades extending radially from the hub, and having free radially extending trailing edges, a second set of equally spaced blades extending substantially radially from the hub and located downstream from the first blades, and having free radially extending leading edges, the number of blades in the first set equal to the number of blades in the second set, the second set of blades curved away from the direction of rotation of the hub to provide a convex surface and a concave surface with the degree of curvature of the second set of blades being greater at the outer end of the blade than at the root end, the trailing edge of each first blade radially offset in the direction of rotation from the leading edge of each second blade a relatively small portion of the spacing between adjacent blades to provide a tapering gap between the blades for directing a jet of fluid over the convex surface of each second blade, the gap being widest at the outer end of the blades and narrowest at the root end of the blades, said gap constituting a means whereby a greater portion of fluid flows through the gap at the outer portion than at the inner portion to energize the boundary layer on the convex surfaces of the second set of blades.

2. A rotor as claimed in claim 1 wherein the trailing edge of each first blade extends rearwardly past the leading edge of the second blades to thereby lengthen the gap into a channel for directing a jet of fluid onto the convex side of the second blades.

3. A rotor as claimed in claim 2 wherein the trailing edge of each first blade is angled to provide a greater overlap of the second blade at the shroud edges of the blades than at their root edges.

4. A rotor as claimed in claim 2 wherein the leading edge of the second blade is extended forwardly of the plane defining which is between 5 and 35 percent of the pitch of the first set of blades.

7. A centripetal-type turbine rotor as in claim 1 in which the rotatable hub comprises two hub elements in axial, abutting relationship with the first set of blades on one hub element and the second set of blades on the other hub element.

Claims

1. A centripetal-type turbine rotor having a rotatable hub, a first set of equally spaced blades extending radially from the hub, and having free radially extending trailing edges, a second set of equally spaced blades extending substantially radially from the hub and located downstream from the first blades, and having free radially extending leading edges, the number of blades in the first set equal to the number of blades in the second set, the second set of blades curved away from the direction of rotation of the hub to provide a convex surface and a concave surface with the degree of curvature of the second set of blades being greater at the outer end of the blade than at the root end, the trailing edge of each first blade radially offset in the direction of rotation from the leading edge of each second blade a relatively small portion of the spacing between adjacent blades to provide a tapering gap between the blades for directing a jet of fluid over the convex surface of each second blade, the gap being widest at the outer end of the blades and narrowest at the root end of the blades, said gap constituting a means whereby a greater portion of fluid flows through the gap at the outer portion than at the inner portion to energize the boundary layer on the convex surfaces of the second set of blades.

4. A rotor as claimed in claim 2 wherein the leading edge of the second blade is extended forwardly of the plane defining the joint between the two hubs and the trailing edge of the first blades to thereby lengthen the gap into a channel for directing a jet of fluid onto the convex side of the second blades.

5. A rotor as claimed in claim 4 wherein the leading edge of each second blade is angled to provide a greater overlap of the first blade at the shroud edges of the blades than at their root edges.

6. A rotor as claimed in claim 1 wherein the gap has a width which is between 5 and 35 percent of the pitch of the first set of blades.