FIELD AND BACKGROUND OF THE INVENTION
[0001] The present invention relates to turbines and, more particularly, to a double rimmed
centrifugal-centripetal turbine, suitable for applications, such as hand-held pneumatic
tools, in which space is at a premium, and which can be fabricated by casting.
[0002] In devices with small turbo drives, such as hand-held pneumatic tools, it often is
necessary to minimize the maximum rotational (idling) speed. One conventional method
of reducing a turbine's idling speed, increasing its diameter, is obviously not applicable
to small turbo drives. In addition, small turbines have unique features relative to
large turbines. Some of these features, such as large gaps between the inlet nozzle
and the turbine wheel and above the turbine blades, relative to the overall dimensions
of the turbine, and relatively large tangential length of the turbine blades, lead
to a loss of efficiency because of leakage of working fluid. These features also make
the use of jet stages and the use of multi-rimmed active velocity stages with more
than three blade rims extremely complicated. Considering these limitations, the most
efficient designs of small turbines are the radial-axial design, with centripetal
entrance of working fluid and axial exit of working fluid; and the exclusively radial
design, either centripetal or centrifugal, with energy transferred from the working
fluid as the working fluid moves in a single plane.
[0003] Another significant difference between large turbines and small turbines is related
to their manufacture. The rotors of small turbines preferably are manufactured in
one piece. Most of the known manufacturing processes, such as electroerosion, or milling
on a numerically controlled milling machine, are relatively expensive. It is with
regard to manufacture that radial turbines have an advantage over axial and radial-axial
turbines. The components of axial and radial-axial turbines cannot be manufactured
by the relatively simple and inexpensive process of knock-down transfer casting without
radical design changes that reduce the efficiency of these turbines. By contrast,
radial turbine designs are known, for example, the single-rimmed design taught by
Kirby in European Patent Application 0 353 856 A1, whose components can be produced
by pressure casting in a knock-down transfer mold. In Kirby's design, the flow of
the working fluid is either exclusively centrifugal or exclusively centripetal.
[0004] The most efficient method for reducing the idling speed of a turbine is by using
double rimmed stages. This method can lower the idling speed of an axial turbine by
as much as 40% to 50%. Double rimmed turbine velocity stages also are widely used
in radial turbines. The double rimmed design has two disadvantages for radial turbines,
as compared to axial turbines. In a centripetal radial design, the inlet nozzle must
be situated radially beyond the rotor, increasing the radial size of the design. In
a centrifugal radial design, the main transfer of kinetic energy from the working
fluid to the turbine rotor occurs at a smaller radius than in an axial turbine of
similar outer dimensions, decreasing the torque available from this design. Thus,
although ease of manufacture favors the radial design for small turbines, the incorporation
of a double rimmed stage in such a turbine to reduce its idling speed is expected
to carry a penalty in reduced efficiency.
[0005] Kotlyar et al., in Russian patent 2,008,435, teach a compact double-rimmed centrifugal-centripetal
radial turbine in which the rotor blades are near the periphery of the rotor, optimizing
the energy transfer in the centrifugal stage, and the flow of the working fluid is
transformed from centrifugal to centripetal by a circumferential shelf that is incorporated
in the rotor. The geometry of this shelf, however, makes it impossible to fabricate
the rotor by knock-down transfer molding.
[0006] To sum up, it has not been possible heretofore to combine efficiency, small size,
ease of manufacture, and low idle speed in the same turbine.
[0007] There is thus a widely recognized need for, and it would be highly advantageous to
have, a compact efficient turbine with a relatively low idle speed that is easy to
manufacture.
SUMMARY OF THE INVENTION
[0008] According to the present invention there is provided a turbine for exchanging kinetic
energy with a working fluid flowing therethrough with a velocity having an azimuthal
component including: (a) a turbine wheel having a proximal side, a distal side and
an outer edge, the proximal side having a first plurality of blades thereon; (b) a
housing, the turbine wheel being rotatably mounted within the housing, the housing
including a chamber extending at least partially circumferentially around the outer
edge of the turbine wheel; and (c) a mechanism for directing the working fluid axially
from the proximal side of the turbine wheel past the outer edge of the turbine wheel
to the distal side of the turbine wheel via substantially all of the circumferential
extent of the chamber while reversing the azimuthal component of the velocity of the
working fluid.
[0009] The turbine of the present invention achieves high power by using a turbine wheel
having blades on both its proximal side and its distal side and by directing the working
fluid centrifugally on the proximal side of the turbine wheel and centripetally on
the distal side of the turbine wheel. Both the radial component and the azimuthal
component of the velocity of the working fluid are reversed at the outer edge of the
turbine wheel. This is and of itself is not new, having been taught, for example,
in UK Patent No. 252,706. Unlike the turbine of UK 252,706, however, which uses cored
U-passages in the turbine housing adjacent to the outer edge of the turbine wheel
to redirect the flow of the working fluid, the present invention allows the working
fluid to flow around essentially the entire extent of the outer edge of the turbine
wheel, and uses guide vanes in a chamber circumferential to the turbine wheel to reverse
the radial and azimuthal components of the velocity of the working fluid. Preferably,
to minimize frictional losses as the moving working fluid and the moving turbine wheel
pass each other in different directions, a barrier, attached to the guide vanes, is
provided to keep the working fluid from contacting the outer edge of the turbine wheel.
Preferably, the guide vanes are spaced and shaped, as described below, to allow all
parts of the turbine of the present invention to be fabricated by casting.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The invention is herein described, by way of example only, with reference to the
accompanying drawings, wherein:
FIG. 1 is a central axial section through a turbine according to the present invention;
FIG. 2 is a proximal transverse section through the turbine of FIG. 1;
FIG. 3 is a peripheral axial section through the turbine of FIG. 1;
FIG. 4 is a distal transverse section through the turbine of FIG. 1;
FIG. 5A illustrates a portion of a guide vane assembly that cannot be fabricated by
casting;
FIG. 5B illustrates a portion of another guide vane assembly that cannot be fabricated
by casting.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] The present invention is of a turbine, of high power and low idling speed, which
can be used in applications such as the powering of hand-held pneumatic tools and
whose parts can be fabricated by casting.
[0012] The principles and operation of a turbine according to the present invention may
be better understood with reference to the drawings and the accompanying description.
[0013] Referring now to the drawings, Figure 1 is an axial cross section of a turbine
10 according to the present invention. Figures 2 and 4 are transverse cross sections
of turbine
10 along cuts A-A and C-C respectively. Figure 3 is an axial cross section of turbine
10 along cut B-B.
[0014] The main parts of turbine
10 are a housing
12, a turbine wheel
20, a nozzle assembly
40 and a stationary axial guide vane assembly
50. Housing
12 includes a proximal portion
14 and a distal portion
16. Turbine wheel
20 is rotatably mounted within distal portion
16 of housing
12 on bearings
30, and is rigidly attached to a shaft
28. Turbine wheel
20 and shaft
28 together constitute the rotor of turbine
10. Nozzle assembly
40 and guide vane assembly
50 are described in more detail below.
[0015] Turbine wheel
20 includes an outer cylindrical portion
25, the proximal surface of which is a first rim
23, and the distal surface of which is a second rim
27. On first rim
23 are mounted a first set of blades
32. Similarly, on second rim
27 are mounted a second set of blades
36. The radially outermost portion of housing
12 defines a chamber
18 whose peripheral circumference is defined by an inner circumferential surface
19 of housing
12. The inner surface of chamber
18 also includes, in addition to circumferential surface
19, a proximal surface
19' and a distal surface
19''. Adjacent to inner circumferential surface
19 and opposite outer edge
24 of turbine wheel
20 is guide vane assembly
50, which includes a set of guide vanes
34 projecting radially inward from surface
19 and a circumferential barrier
52, radially inward from guide vanes
34, to which guide vanes
34 are rigidly attached and from which guide vanes
34 project radially outward. Note that barrier
52 surrounds, but does not contact, outer edge
24 of turbine wheel
20.
[0016] Proximally to turbine wheel
20 and mounted on proximal portion
14 of housing
12 is nozzle assembly
40. As best seen in Figure 2, nozzle assembly
40 includes three azimuthally pointing nozzles.
[0017] In operation, a working fluid such as air is introduced under pressure into proximal
portion
14 of housing
12. The working fluid flows through nozzles
42, which, along with surface
19', direct the working fluid azimuthally and centrifugally at first blades
32. In passing first blades
32, the working fluid transfers some of its kinetic energy to turbine wheel
20, causing turbine wheel
20 and shaft
28 to rotate. The working fluid proceeds into chamber
18, where surface
19 and guide vanes
34 redirect the flow of the working fluid: surface
19 reverses the radial component of the velocity of the working fluid, and guide vanes
34 reverse the azimuthal component of the velocity of the working fluid. Barrier
52 intervenes between the working fluid in chamber
18 and outer edge
24 of turbine wheel
20, inhibiting the loss of energy due to friction that would result if the working fluid
were to flow directly past outer edge
24 of turbine wheel
20. The working fluid now is directed by surface
19'' to flow centripetally past second blades
36, thereby transferring almost all of the remaining kinetic energy of the working fluid
to turbine wheel
20. Finally, the working fluid leaves turbine
10 via exit ports
44 in distal portion
16 of housing
12.
[0018] One important feature of turbine
10 is that both first blades
32 and second blades
36 are mounted on rims
23 and
27, respectively, of outer cylindrical portion
25 of turbine wheel
20, adjacent to outer edge
24 of turbine wheel
20. Mounting blades
32 and
36 at the outermost possible radial positions on turbine wheel
20 minimizes the idling speed of turbine
10.
[0019] It will be appreciated that proximal portion
14 of housing
12, distal portion
16 of housing
12, turbine wheel
20 and nozzle assembly
40 all can be fabricated by knock-down transfer casting. Guide vane assembly
50 also can be fabricated by knock-down transfer casting, provided that any line, such
as line
60 in Figure 3, that is drawn in the axial direction through guide vane assembly
50, intersects at most one guide vane
34, and intersects that guide vane
34 only once. This can be better understood with reference to the counterexamples in
figures 5A and 5B, of guide vane assemblies that cannot be fabricated by casting.
Figure 5A shows a portion of a guide vane assembly
50' whose guide vanes
34' overlap azimuthally, so that some axial lines
62 intersect two guide vanes
34'. Figure 5B shows a portion of a guide vane assembly
50'' whose guide vanes
34'' are sufficiently curved to be re-entrant in the axial direction, so that same axial
lines
64 intersect the same guide vane
34'' twice. This ease of fabrication of guide vane assembly
50 is purchased at the expense of a certain loss of efficiency in the redirection of
the working fluid in chamber
18; but because the purpose of guide vane assembly
50 is to redirect the flow of the working fluid, not to extract energy from the working
fluid, this loss of efficiency is minimal compared to the efficiency that is lost
in adapting the rotors of axial and radial-axial designs to manufacture by knock-down
transfer casting.
[0020] While the invention has been described with respect to a limited number of embodiments,
it will be appreciated that many variations, modifications and other applications
of the invention may be made.
1. A turbine for exchanging kinetic energy with a working fluid flowing therethrough
with a velocity having an azimuthal component, comprising:
(a) a turbine wheel having a proximal side, a distal side and an outer edge, said
proximal side having a first plurality of blades thereon;
(b) a housing, said turbine wheel being rotatably mounted within said housing, said
housing including a chamber extending at least partially circumferentially around
said outer edge of said turbine wheel; and
(c) a mechanism for directing the working fluid axially from said proximal side of
said turbine wheel past said outer edge of said turbine wheel to said distal side
of said turbine wheel via substantially all of said circumferential extent of said
chamber while reversing the azimuthal component of the velocity of the working fluid.
2. The turbine of claim 1, wherein said chamber includes a circumferential surface, and
wherein said mechanism includes a plurality of guide vanes extending radially inward
from said circumferential surface.
3. The turbine of claim 2, wherein said mechanism for directing the working fluid axially
includes a mechanism for isolating said outer edge of said turbine wheel from said
working fluid.
4. The turbine of claim 3, wherein said mechanism for isolating said outer edge of said
turbine wheel from said working fluid includes a circumferential barrier rigidly attached
to said guide vanes between said guide vanes and said turbine wheel.
5. The turbine of claim 2, wherein any axial line through said chamber intersects at
most one of said guide vanes.
6. The turbine of claim 5, wherein said axial line that intersects one of said guide
vanes intersects said guide vane only once.
7. The turbine of claim 1, further comprising:
(d) a mechanism for directing the working fluid centrifugally at said first plurality
of blades.
8. The turbine of claim 7, wherein said mechanism for directing the working fluid centrifugally
at said first plurality of blades includes a plurality of nozzles, said first plurality
of blades being positioned circumferentially around said nozzles.
9. The turbine of claim 7, wherein said mechanism for directing the working fluid centrifugally
at said first plurality of blades includes a proximal surface, in said chamber, opposite
said first plurality of blades.
10. The turbine of claim 1,
wherein said distal side has a second plurality of blades thereon, the turbine further comprising:
(d) a mechanism for directing the working fluid centripetally at said second plurality
of blades.