CA2913026A1

CA2913026A1 - Compressor impellers

Info

Publication number: CA2913026A1
Application number: CA2913026A
Authority: CA
Inventors: Alberto Scotti Del Greco; Libero Tapinassi
Original assignee: Nuovo Pignone SRL
Current assignee: Nuovo Pignone SRL
Priority date: 2013-06-13
Filing date: 2014-06-11
Publication date: 2014-12-18
Also published as: CN105556129A; JP2016521821A; ITCO20130024A1; BR112015029639A2; MX2015016450A; AU2014280238A1; EP3008345B1; EP3008345A1; RU2015150305A; WO2014198790A1; US20160123345A1; KR20160019418A; RU2667855C2

Abstract

An impeller (46) includes a hub (62) having a direction of rotation, a plurality of impeller blades 60 extending from the hub (62), each blade having a downstream end (721), an upstream end (68), a leading surface (74) facing the direction of rotation of the hub, and a trailing surface (76) facing opposite to the direction of rotation of the hub. The impeller (46) further includes a secondary flow reducer (80) extending towards the downstream end (72) and the upstream end (68) of the at least one of the plurality of impeller blades (60), the secondary flow reducer (80) defining first and second surfaces intersecting one of the leading surface and the trailing surface of the at least one of the plurality of impeller blades as well as a third surface between the first and second surfaces.

Description

COMPRESSOR IMPELLERS
BACKGROUND
TECHNICAL FIELD
The present invention relates generally to compressors and, more specifically, to secondary flow of process fluid proximate to compressor impeller blades.
DISCUSSION OF THE BACKGROUND
A compressor is a machine which increases the pressure of a process fluid, e.g., a gas, through the use of mechanical energy. Compressors are used in a number of different applications, including operating as an initial stage of a gas turbine engine.
Among the various types of compressors are the so-called centrifugal compressors, in which mechanical energy operates on process fluid input to the compressor by way of centrifugal acceleration, e.g., by rotating a centrifugal impeller (sometimes also called a "rotor") by which the process fluid is passing. More generally, centrifugal compressors can be said to be part of a class of machinery known as "turbo machines"
or "turbo rotating machines".
Centrifugal compressors can be fitted with a single impeller, i.e., a single stage configuration, or with a plurality of impellers in series, in which case they are frequently referred to as multistage compressors. Each of the stages of a centrifugal compressor typically includes an inlet conduit for the flow of process fluid to be compressed, an impeller including blades which are capable of imparting kinetic energy to the input process fluid and a diffuser which converts the kinetic energy of the process fluid flowing away from the rotor into pressure energy.
The flow of the process fluid from the inlet to the diffuser may be categorized as primary or secondary. Primary flow is desirable and may be considered to be efficient progression of the process fluid through the compressor. Conversely, secondary flows are undesirable and may require the compressor to perform additional work to achieve the demanded pressure rise in the process fluid. Secondary flows are potentially troublesome not only during a compression process, stage or stages, but also, thereafter, when downstream components of the compressor are exposed and potentially compromised or otherwise prevented from performing optimally by such flows.
While a large percentage of the process fluid may move by way of a primary flow through the compressor, at least some portion of the process fluid may move by way of a secondary flow, particularly process fluid in close proximity to the impeller blades. For example, some portion of the process fluid flow may form a boundary layer near the face of an impeller blade and slow down relative to other portions of the process fluid being compressed. As another example, some portions of the flow may migrate transversely to a desired flow across the impeller blades. These portions may cause or be part of a secondary flow.
To address the problem of secondary flow, there has been a focus on the design of impeller blade shapes. As a result, blade shapes have evolved to the point where proposed changes oftentimes result in only incremental gains in compressor efficiency and/or performance. Moreover, these changes are oftentimes difficult and expensive to implement particularly where the design of other compressor components must be changed to accommodate the proposed changes to the shape of the impeller blades.
Consequently, there may be a resistance to proposed change in compressor design, particularly in the design of impeller blade shape. Therefore, what is needed is a solution to the problem of secondary flows which is more readily accepted for integration to both new and existing impeller blade designs and further which may preserve the overall shape of a given impeller blade design.
SUMMARY
According to an exemplary embodiment, an impeller includes a hub having a direction of rotation, a plurality of impeller blades extending from the hub, each blade having a downstream end, an upstream end, a leading surface facing the direction of rotation of the hub, and a trailing surface facing opposite to the direction of rotation of the hub. The impeller further includes a secondary flow reducer extending towards the downstream end and the upstream end of the at least one of the plurality of impeller blades, the secondary flow reducer defining first and second surfaces intersecting one

2 of the leading surface and the trailing surface of the at least one of the plurality of impeller blades. The secondary flow reducer further defines a third surface between the first and second surfaces.
According to another exemplary embodiment, a turbo machine includes a rotor assembly including at least one impeller, a bearing connected to, and for rotatably supporting, the rotor assembly, and a stator. The at least one impeller includes a hub having a plurality of impeller blades. At least one of the impeller blades includes a plurality of ribs for reducing a secondary flow proximate to the at least one impeller blade.
According to another exemplary embodiment, a method of configuring an impeller blade surface to provide reduced secondary flow can include the steps of identifying an ideal streamline of the impeller blade surface and adding a rib to the blade surface coincident with the streamline, the rib defining first and second surfaces intersecting the surface and defining a third surface between the first and second surfaces.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
Figure 1 depicts an exemplary centrifugal compressor.
Figure 2 depicts an exemplary impeller of the centrifugal compressor of Figure 1.
Figure 3 shows a cross-sectional view of the impeller shown in Fig. 2.
Figure 4 shows an impeller blade of the impeller shown in Figs. 2 and 3 including secondary flow reducers according to an exemplary embodiment.
Figure 5 shows a loss coefficient diagram for an impeller including secondary flow reducers according to the exemplary embodiment of Figure 4, as compared to a conventional impeller.

3 Figure 6 shows a work coefficient and flow coefficient diagram for the impeller of Figure 4.
Figure 7 shows a flow angle versus blade span diagram for the impeller of Figure 4, as compared to a conventional impeller.
Figure 8 shows process fluid flow voracity at an exit of an impeller without secondary flow reducers.
Figure 9 shows process fluid flow vorticity at an exit of the impeller with secondary flow reducers.
Figure 10 shows streamlines on an impeller blade without secondary flow reducers.
Figure 11 shows streamlines on the impeller blade with secondary flow reducers.
Figure 12 shows a partial cross sectional view of the secondary flow reducer shown in Fig. 4.
Figure 13 shows a partial cross sectional view of another exemplary embodiment.
Figure 14 shows a partial cross sectional view of another exemplary embodiment.
Figure 15 shows a partial cross sectional view of another exemplary embodiment.
Figure 16 shows a partial cross sectional view of another exemplary embodiment.
Figure 17 shows a partial cross sectional view of another exemplary embodiment.
Figure 18 shows a partial cross sectional view of another exemplary embodiment.
Figure 19 is a flowchart illustrating a method of configuring an impeller blade surface to resist secondary flow according to an exemplary embodiment.
DETAILED DESCRIPTION
The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or

4 similar elements. The following detailed description does not limit the invention.
Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a turbo machine that has a stator and a rotor. However, the embodiments to be discussed next are not limited to these exemplary systems, but may be applied to other systems.
Reference throughout the specification to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
To provide some context for the subsequent discussion relating to the reduction of secondary flows within a compressor according to these exemplary embodiments, Figure 1 schematically illustrates a multistage, centrifugal compressor 40 in which impellers 46 are employed in the compression process. Therein, the compressor includes a box or housing (stator) 42 within which is mounted a rotating compressor shaft 44 that is provided with a plurality of centrifugal rotors or impellers 46. The rotor assembly 48 includes the shaft 44 and rotors 46 and is supported radially and axially through bearings 50 which are disposed on either side of the rotor assembly 48.
The multistage centrifugal compressor operates to take an input process gas from duct inlet 52, to accelerate the process gas particles through operation of the rotor assembly 48, and to subsequently deliver the process gas through various interstage ducts 54 at an output pressure which is higher than its input pressure. The process gas may, for example, be any one of carbon dioxide, hydrogen sulfide, butane, methane, ethane, propane, liquefied natural gas, or a combination thereof. Between the impellers 46 and the bearings 50, sealing systems (not shown) are provided to prevent the process gas from flowing to the bearings 50. The housing 42 is configured so as to cover both the

5 bearings 50 and the sealing systems, so as to prevent the escape of gas from the centrifugal compressor 40.
A more detailed, but purely exemplary, illustration of an impeller 46 is provided in Figs. 2 and 3. Therein it can be seen that the impeller 46 has a plurality of impeller blades 60 extending from hub 62 to a shroud 64. Each impeller blade 60 includes an upstream end 68, a downstream end 72, a leading surface 74 (Fig. 3), and a trailing surface 76 (Fig. 3).
A detailed view of the trailing surface 76 of a pair of impeller blades 60 is shown in the exemplary embodiment of Fig. 4. Each trailing surface 76 includes a plurality of secondary flow reducers 80 extending towards downstream end 72 (Fig. 2) and upstream end 68 (Fig. 2) of each impeller blade 60.
As shown in Fig. 4, the secondary flow reducers 80 may be equally spaced from each other as well as from the hub 60 and shroud 64. Moreover, each secondary flow reducer 80 may terminate at a location on the trailing surface 76 which is spaced from the upstream end 68 of blade 60. As further shown in Fig. 4, each secondary flow reducer 80 may also include a tapering portion 96 which tapers to the location of termination on the trailing surface 76.
As may be further appreciated from Fig. 4, each secondary flow reducer 80 may be coincident with, or follow, an ideal streamline of process fluid progressing across surface 76. An ideal streamline may be established in theory, through experimental observation, or other criteria. For example, in existing impeller design applications, an ideal streamline may be established by creating a line on the blade surface 76 which is congruent to the intersection of the blade 60 with the shroud 64. An ideal streamline may also be established during the design process using flow equations as, for example, disclosed in U.S. Patent No. 6,654,710, the disclosure of which is incorporated herein by reference. Additionally, an ideal streamline, as that term is used herein, can refer to a streamline which is substantially parallel to lines associated with the endwalls of the impeller blade.

6 A streamline may be unique with respect to other streamlines on the same surface, for example, a streamline proximate to hub 62 may be different from a streamline proximate to shroud 64. As another example, a single secondary flow reducer 80 may define more than a single streamline, for example, a secondary flow reducer 80 may branch into two interconnected streamlines.
The secondary flow reducers 80 in Fig. 4 are configured to inhibit the migration of process fluid, which may be part of a secondary flow boundary layer on blade 60, between hub 62 and shroud 64. Each secondary flow reducer 80 is also configured to funnel and/or concentrate the flow of process fluid between secondary flow reducers 80. The resulting enhanced flow may be characterized not only by reduced secondary flow but also by greater flow uniformity at the downstream end 72 of impeller blade 60.
In a test involving the simulated operation of impeller blade 60 including the exemplary secondary flow reducers shown in Fig. 4, an improvement in compressor performance was realized. Fig. 5 shows a line plot 84 of the loss coefficient achieved by blade 60 including secondary flow reducers 80 as well as a line plot 82 of the loss coefficient of a blade 60 without secondary flow reducers 80. Specifically, the abscissa shows loss coefficient and the ordinate shows fractional distance from the hub to the downstream end 72 of blade 60. The area between the line plots 82 and 84 shows a significant reduction in loss coefficient of an impeller 60 with secondary flow reducers 80 according to the exemplary embodiment of Fig. 4.
Fig. 6 shows further results of the calculations. Specifically, flow coefficient which is defined as the volume flow of the impeller with respect to a standard volume flow is shown on the abscissa and work coefficient which is defined as the power input to the compressor with respect to a standard power input is shown on the ordinate.
The plot line 86 shows calculation results of impeller 60 without secondary flow reducers 80 and the plot line 88 shows the calculation results of impeller 60 with secondary flow reducers 80. Here again, the data shows an improvement in compressor performance associated with the inclusion of secondary flow reducers 80 on the blades of impeller 60. Specifically, the impeller 60 with secondary flow reducers 80 provides both an

7 improved work coefficient and an improved flow coefficient versus an impeller without flow reducers 80.
In the graph of Fig. 7, the flow angle of the process fluid leaving impeller 46 is shown on the abscissa and the distance across the span from the hub 62 to the shroud 64 is shown on the ordinate. The plot line 118 shows the calculation results for an impeller 60 without secondary flow reducers 80 and the plot line 122 shows the calculation results of impeller 60 with secondary flow reducers 80. As may be appreciated from a comparison between plot line 118 and plot line 122, the inclusion of secondary flow reducers 80 decreases the angle at which process fluid leaves the blade particularly at points on the impeller closer to the shroud 64 thereby reducing secondary flow and generating greater flow uniformity at the blade exit 72.
Figs. 8 and 9 show the magnitude of fluid vorticity proximate to the shroud 64. For the impeller blades 60 without secondary flow reducers shown in Fig. 8, the magnitude of the flow vorticity is shown by the shaded regions 124. In Fig. 9, the magnitude of the flow vorticity for impeller blades including secondary flow reducers is shown in the regions 126. Regions 126 are smaller than regions 124 thereby indicating the improved performance of an impeller 60 which includes secondary flow reducers 80 according to the present invention.
Fig. 10 shows simulated streamlines 128 of process fluid across a blade 60 without secondary flow reducers 80 and Fig. 10 shows simulated streamlines 132 of process fluid across blade 60 with secondary flow reducers 80. As may be appreciated from Figs. 10 and 11, a higher percentage of streamlines 132 extend to the downstream end 72 of the impeller 60 including secondary flow reducers 80 than the streamlines 128 of the impeller without secondary flow reducers 80. Thus, Figs. 10 and 11 further show that secondary flow reducers 80 have a channeling or "combing" effect on the process fluid flow which causes a higher percentage of the process fluid to flow along the ideal streamlines 132.
A more detailed view of a secondary flow reducer 80 is shown in Fig. 12.
Therein, secondary flow reducer 80 includes a first surface 102 intersecting the trailing surface 76 of impeller blade 60, and a second surface 104 spaced apart from first surface 102

8 and also intersecting the trailing surface 76 of impeller blade 60. A third surface 106 of secondary flow reducer 80 extends from the first surface 102 to the second surface 104.
In the exemplary embodiment of Fig. 12, the intersection 103 of the first surface 102 with blade surface 76 defines a first curvilinear line and the intersection 105 of the second surface 104 with blade surface 76 defines a second curvilinear line. In the exemplary embodiment of Fig. 12, the curvilinear lines defined by intersection and intersection 105 are parallel, however, in other exemplary embodiments the intersection 103 of first surface 102 with blade surface 76 and the intersection 105 of second surface 104 with blade surface 76 may define non-parallel lines, for example, each intersection 103 and 105 may define a line having a wave pattern.
As also shown in Fig. 12, surface 102 and surface 104 extend away from the direction of rotation 108 of impeller 46 to third surface 106 thereby forming a rib 78.
In alternative exemplary embodiments, such as that shown in Fig. 13, surface 102 and surface 104 may extend toward the direction of rotation 108 and thus, the third surface 106 may form the floor of a channel 98 in the trailing surface 76 of impeller blade 60.
Moreover, first surface 102 and/or second surface 104 may be planar or non-planar, for example, concave, convex, or other surface. Further, in the alternative exemplary embodiment shown in Fig. 14, a secondary flow reducer 80 in the form of a rib 78 is included on the leading surface 74 of impeller 80. Similarly in Fig. 15, a secondary flow reducer 80 in the form of a channel 98 is included on the leading surface 74 of impeller 80.
In the exemplary embodiment of Fig. 12, surface 102 and surface 104 converge towards surface 106 and thus define a tapering rib 78. Alternatively, surfaces 102 and 104 may be parallel to each other or diverge towards surface 106, i.e.
surfaces 102 and 104 may be closer to each other at the intersection 103 or 105 with blade surface 76 than at the transition to third surface 106.
As further shown in Figs. 4 and 12, third surface 106 defines a rounded profile 110. In alternative exemplary embodiments, third surface 106 may define other profiles. For

9 example, and as shown in Fig. 16, third surface 106 may be defined by the extension and intersection of surfaces 102 and 104 to a pointed profile 112. In Figs. 17 and 18, third surface 106 is shown having a polygonal profile 114 and a concave profile 116, respectively. The profile of third surface 106 may be configured to further enhance the capacity of the secondary flow reducer 80 to inhibit and/or reduce a secondary flow, to enhance the uniformity of process fluid flow, and/or to otherwise improve the efficiency and performance of compressor 40.
In addition to improving the flow of process fluid, another benefit associated with secondary flow reducers according to exemplary embodiments is the ease of integration with various impeller blade designs. Specifically, and as may be appreciated from Fig. 4, the third surface 106 of the secondary flow reducers 80 are congruent to the surface 76 of blade 60. Thus, secondary flow reducers 80 may be implemented without introducing a change to the overall shape or design of the impeller blade 60. In addition to preserving the overall shape of the impeller blade, secondary flow reducers 80 may also provide a lower cost solution for reducing secondary flow, and otherwise improve the efficiency and performance of a compressor.
Thus, according to an exemplary embodiment shown in Fig. 19, a method (1000) of configuring an impeller blade surface to provide reduced secondary flow can include the steps of identifying (1002) an ideal streamline of the impeller blade surface and adding (1004) a rib to the blade surface coincident with the streamline, the rib defining first and second surfaces intersecting the blade surface, the rib further defining a third surface between the first and second surfaces.
The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article "a" is intended to include one or more items.

Claims

1. An impeller comprising:
a hub having a direction of rotation;
a plurality of impeller blades extending from said hub, each said blade having a downstream end, an upstream end, a leading surface facing said direction of rotation of said hub, and a trailing surface facing opposite to said direction of rotation of said hub; and a secondary flow reducer extending towards said downstream end and said upstream end of said at least one of said plurality of impeller blades, said secondary flow reducer defining first and second surfaces intersecting one of said leading surface and said trailing surface of said at least one of said plurality of impeller blades and a third surface between said first and second surfaces.

2. The impeller of claim 1, further comprising at least one of the plurality of impeller blades with at least one secondary flow reducer on said leading surface and said trailing surface, the flow reducer being designed following an ideal streamline.

3. The impeller of claim 2, wherein said ideal streamline is substantially parallel to lines associated with the endwalls of the at least one of the impeller blades.

4. The impeller of any preceding claim, wherein said first surface and said second surface intersect said trailing surface of said at least one impeller blade and extend opposite to said direction of rotation of said hub to said third surface thereby forming a rib on said trailing surface.

5. The impeller of any preceding claim, wherein said at least one secondary flow reducer comprises a plurality of secondary flow reducers on said trailing surface.

6. The impeller of claim 5, wherein said third surface of each said plurality of secondary flow reducers is congruent to said trailing surface thereby preserving an overall shape of said at least one of said plurality of impeller blades.

7. The impeller of claim 5 or claim 6, wherein said plurality of secondary flow reducers is evenly distributed across said trailing surface.

8. The impeller of claim 6, wherein, for each of said plurality of secondary flow reducers, a curvilinear line defined by said intersection of said first surface with said trailing surface is congruent to a curvilinear line defined by said intersection of said second surface with said trailing surface.

9. A turbo machine comprising:
a rotor assembly including at least one impeller of any one of the preceding claims;
a bearing connected to, and for supporting the rotor assembly; and a stator.

10. A turbo machine comprising:
a rotor assembly including at least one impeller;
a bearing connected to, and for rotatably supporting, the rotor assembly; and a stator, wherein said at least one impeller includes:
a hub having a plurality of impeller blades; and a plurality of ribs on at least one of said plurality of impeller blades for reducing a secondary flow proximate to said at least one impeller blade.

11. A method of configuring an impeller blade surface to provide reduced secondary flow, said method comprising:

identifying an ideal streamline of said impeller blade surface; and, adding a rib to said blade surface coincident with said streamline, said rib defining first and second surfaces intersecting said blade surface and a third surface between said first and second surfaces.