US20150345301A1

US20150345301A1 - Rotor blade cooling flow

Info

Publication number: US20150345301A1
Application number: US14/289,908
Authority: US
Inventors: Xiuzhang James Zhang; James William Vehr; Haiping Wang
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 2014-05-29
Filing date: 2014-05-29
Publication date: 2015-12-03
Also published as: DE102015107844A1; JP2015224634A; CH709650A2; CN105317468A

Abstract

A rotor blade includes and airfoil. The airfoil includes pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and between a leading edge and a trialing edge. The tip includes a tip floor, a plurality of coolant outlets and a tip rail having a pressure side and suction side portions that extend radially outwardly from the tip floor. Cooling passages are circumscribed within the airfoil and are in fluid communication with one or more of the coolant outlets. A baffle extends radially outwardly from and transversely across the tip floor from the pressure side portion to the suction side portion to define first and second tip pockets. A slot is disposed along the suction side portion of the tip rail to provide for fluid communication out of one of the first or second tip pockets, thereby reducing pressure within the corresponding tip pocket.

Description

FIELD OF THE INVENTION

The present invention generally relates to a rotor blade for a turbine. More particularly, this invention involves a rotor blade having a tip configured for promoting coolant flow through the rotor blade.

BACKGROUND OF THE INVENTION

In an air-ingesting turbo machine (e.g., a gas turbine), air is pressurized by a compressor and then mixed with fuel and ignited within an annular array of combustors to generate hot gases of combustion. The hot gases flow from each combustor through a transition piece for flow along an annular hot gas path. Turbine stages are typically disposed along the hot gas path such that the hot gases flow across first-stage nozzles and rotor blades and across the nozzles and rotor blades of follow-on turbine stages. The rotor blades may be secured to a plurality of rotor disks which are coupled to a turbine rotor shaft, with each rotor disk being mounted to the rotor shaft.
A rotor blade generally includes an airfoil that extends radially outwardly from a substantially planar platform and a mounting portion that extends radially inwardly from the platform for securing the rotor blade to one of the rotor disks. A tip of the airfoil is typically spaced radially inwardly from a stationary shroud or seal of the turbine such that a small clearance gap is defined between the tip and the shroud. A plurality of cooling passages is defined within the airfoil for routing a coolant such as compressed air through the airfoil. In particular configurations, a plurality of coolant outlets are defined along the tip for routing the coolant out of the cooling passages at the tip.
The flow of the coolant through the cooling passages is primarily driven by a pressure difference defined between a supply pressure of the coolant and a static pressure which is typically defined at the tip of the airfoil at or just downstream from the coolant outlets. If the supply pressure is too low, for example, due to aerodynamic loading optimization, a decrease in operating speed and/or a change in turbine load requirements, a lower or reduced static pressure is required in order to meet cooling flow needs. Therefore, an improved rotor blade tip design which provides a lower or reduced static pressure at the tip to increase or enhance coolant flow through the airfoil would be useful.

BRIEF DESCRIPTION OF THE INVENTION

Aspects and advantages of the invention are set forth below in the following description, or may be obvious from the description, or may be learned through practice of the invention.
One embodiment of the present invention is a rotor blade having an airfoil. The airfoil includes pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and in chord between a leading edge and a trialing edge. The tip includes a tip floor and a plurality of coolant outlets disposed along the tip floor. The tip further includes a tip rail that extends radially outwardly from the tip floor. The tip rail has a pressure side portion and a suction side portion which are joined at the leading and trailing edges. A plurality of cooling passages is circumscribed within the airfoil for routing a coolant therethrough. Each or at least some of the cooling passages is/are in fluid communication with one or more of the coolant outlets. A baffle extends radially outwardly from and transversely across the tip floor from the pressure side portion to the suction side portion so as to define a first tip pocket and a second tip pocket. A slot is disposed along the suction side portion of the tip rail and provides for fluid communication out of one of the first or second tip pockets.
Another embodiment of the present invention is a system for promoting coolant flow through a rotor blade. The system includes a coolant source for supplying a pressurized coolant to a cooling passage inlet formed along the rotor blade. The rotor blade comprises a mounting portion which includes a mounting body. The mounting body is interconnectable with a rotor shaft. At least one of the cooling passage inlets is formed by the mounting body. An airfoil extends radially outwardly from the mounting portion and includes pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and in chord between a leading edge and a trialing edge. The tip includes a tip floor and a plurality of coolant outlets disposed along the tip floor. The tip further includes a tip rail that extends radially outwardly from the tip floor. The tip rail includes a pressure side portion and a suction side portion which are joined at the leading and trailing edges. A plurality of cooling passages is circumscribed within the airfoil for routing a coolant therethrough. Each cooling passage is in fluid communication with one or more of the coolant inlets. A baffle extends radially outwardly from and transversely across the tip floor from the pressure side portion to the suction side portion to define a first tip pocket and a second tip pocket. A slot is disposed along the suction side portion of the tip rail and provides for fluid communication out of one of the first or second tip pockets.
Another embodiment of the present invention is a gas turbine. The gas turbine includes a compressor, a combustor disposed downstream from the compressor and a turbine disposed downstream from the combustor. The turbine includes a rotor shaft that extends axially through the turbine. An outer casing circumferentially surrounds the rotor shaft to define a hot gas path therebetween. A plurality of rotor blades is interconnected to the rotor shaft, which together, define a stage of rotor blades. Each rotor blade comprises a mounting portion which includes a mounting body. The mounting body is interconnectable with a rotor shaft and at least one of the cooling passage inlets is formed in the mounting body. The rotor blade further includes an airfoil that is coupled to the mounting portion and that includes pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and in chord between a leading edge and a trialing edge. The tip includes a tip floor and a plurality of coolant outlets disposed along the tip floor. The tip further includes a tip rail that extends radially outwardly from the tip floor. The tip rail includes a pressure side portion and a suction side portion which are joined at the leading and trailing edges. A plurality of cooling passages is circumscribed within the airfoil for routing a coolant through the airfoil. Each cooling passage is in fluid communication with one or more of the coolant inlets. A baffle extends radially outwardly from and transversely across the tip floor from the pressure side portion to the suction side portion to define a first tip pocket and a second tip pocket. At least one coolant outlet is disposed along the tip floor within the first tip pocket and at least one coolant outlet is disposed along the tip floor within the second tip pocket. A slot is disposed along the suction side portion of the tip rail and provides for fluid communication out of one of the first or second tip pockets.
Those of ordinary skill in the art will better appreciate the features and aspects of such embodiments, and others, upon review of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof to one skilled in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures, in which:

FIG. 1 illustrates a functional diagram of an exemplary gas turbine as may incorporate at least one embodiment of the present invention;

FIG. 2 is a perspective view of an exemplary rotor blade as may incorporate various embodiments of the present disclosure;

FIG. 3 is an enlarged perspective view of a tip of an exemplary rotor blade, according to at least one embodiment of the invention;

FIG. 4 is an enlarged top view of the exemplary rotor blade tip as shown in FIG. 3;

FIG. 5 is an enlarged perspective view of an exemplary rotor blade, according to at least one embodiment of the invention;

FIG. 6 is an enlarged perspective view of an exemplary rotor blade, according to at least one embodiment of the invention;

FIG. 7 is an enlarged perspective view of an exemplary rotor blade, according to at least one embodiment of the invention; and

FIG. 8 is an enlarged perspective view of an exemplary rotor blade, according to at least one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to present embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. The detailed description uses numerical and letter designations to refer to features in the drawings. Like or similar designations in the drawings and description have been used to refer to like or similar parts of the invention. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows. The term “radially” refers to the relative direction that is substantially perpendicular to an axial centerline of a particular component, and the term “axially” refers to the relative direction that is substantially parallel and/or coaxially aligned to an axial centerline of a particular component.
Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. Although an industrial or land based gas turbine is shown and described herein, the present invention as shown and described herein is not limited to a land based and/or industrial gas turbine unless otherwise specified in the claims. For example, the invention as described herein may be used in any type of turbine including but not limited to a steam turbine, an aircraft gas turbine or marine gas turbine.
Referring now to the drawings, FIG. 1 illustrates a schematic diagram of one embodiment of a gas turbine 10. The gas turbine 10 generally includes an inlet section 12, a compressor section 14 disposed downstream of the inlet section 12, a plurality of combustors (not shown) within a combustor section 16 disposed downstream of the compressor section 14, a turbine section 18 disposed downstream of the combustor section 16 and an exhaust section 20 disposed downstream of the turbine section 18. Additionally, the gas turbine 10 may include one or more shafts 22 coupled between the compressor section 14 and the turbine section 18.
The turbine section 18 may generally include a rotor shaft 24 having a plurality of rotor disks 26 (one of which is shown) and a plurality of rotor blades 28 extending radially outwardly from and being interconnected to each rotor disk 26. Each rotor disk 26 may, in turn, be coupled to a portion of the rotor shaft 24 that extends through the turbine section 18. The turbine section 18 further includes an outer casing 30 that circumferentially surrounds the rotor shaft 24 and the rotor blades 28, thereby at least partially defining a hot gas path 32 through the turbine section 18.
During operation, a working fluid such as air flows through the inlet section 12 and into the compressor section 14 where the air is progressively compressed, thus providing pressurized air to the combustors of the combustion section 16. The pressurized air is mixed with fuel and burned within each combustor to produce hot gases of combustion 34. The hot gases of combustion 34 flow through the hot gas path 32 from the combustor section 16 to the turbine section 18, wherein energy (kinetic and/or thermal) is transferred from the hot gases 34 to the rotor blades 28, thus causing the rotor shaft 24 to rotate. The mechanical rotational energy may then be used to power the compressor section 14 and generate electricity. The hot gases of combustion 34 exiting the turbine section 18 may then be exhausted from the gas turbine 10 via the exhaust section 20.
FIG. 2 is a perspective view of an exemplary rotor blade 28 as may incorporate one or more embodiments of the present invention. As shown in FIG. 2, the rotor blade 28 generally includes a mounting or shank portion 36 having a mounting body 38 and an airfoil 40 that extends substantially radially outwardly from a substantially planar platform 42. The platform 42 generally serves as the radially inward boundary for the hot gases of combustion 34 flowing through the hot gas path 32 of the turbine section 18 (FIG. 1). As shown in FIG. 2, the mounting body 38 of the mounting or shank portion 36 may extend radially inwardly from the platform 42 and may include a root structure, such as a dovetail, configured to interconnect or secure the rotor blade 28 to the rotor disk 26 (FIG. 1).
The airfoil 40 includes a pressure side wall 44 and an opposing suction side wall 46. The pressure side wall 44 and the suction side wall 46 extend substantially radially outwardly from the platform 42 in span from a root 48 of the airfoil 40 which may be defined at an intersection between the airfoil 40 and the platform 42, and a tip 50 of the airfoil 40. The pressure side wall 44 and suction side wall 46 extend in chord between a leading edge 52 and a trialing edge 54 of the airfoil 40. The pressure side wall 44 generally comprises an aerodynamic, concave outer surface of the airfoil 40. Similarly, the suction side wall 46 may generally define an aerodynamic, convex outer surface of the airfoil 40. The tip 50 is disposed radially opposite the root. As such, the tip 50 may generally define the radially outermost portion of the rotor blade 28 and thus, may be configured to be positioned adjacent to a stationary shroud or seal (not shown) of the gas turbine 10.
As shown in FIG. 2, a plurality of cooling passages 56 (shown in dashed lines in FIG. 2) is circumscribed within the airfoil 40 for routing a coolant 58 through the airfoil 40 between the pressure side wall 44 and the suction side wall 46, thus providing convective cooling thereto. The coolant 58 may include a portion of the compressed air from the compressor section 14 (FIG. 1) and/or steam or any other suitable fluid or gas for cooling the airfoil 40. One or more cooling passage inlets 60 are disposed along the rotor blade 28. In one embodiment, one or more cooling passage inlets 60 are formed within, along or by the mounting body 38. The cooling passage inlets 60 are in fluid communication with at least one corresponding cooling passage 56.
FIG. 3 is an enlarged perspective view of the tip 50 of the airfoil 40 as shown in FIG. 2, according to one embodiment of the present invention. FIG. 4 is an enlarged top view of the tip 50 of the airfoil 40 as shown in FIGS. 2 and 3. As shown in FIGS. 3 and 4, the tip 50 includes a tip floor 62. The tip floor 62 generally extends between the pressure and suction side walls 44, 46 and the leading and trailing edges 52, 54 of the airfoil 40. A plurality of coolant outlets 64 is disposed along the tip floor 62. Each cooling passage 56 (FIG. 2) is in fluid communication with at least one of the coolant outlets 64.
As shown in FIG. 3, a tip rail 66 extends radially outwardly from the tip floor 64. The tip rail 66 comprises a pressure side portion 68 and a suction side portion 70. The pressure side portion 68 extends along a perimeter of the tip floor 62 and generally conforms in profile to the pressure side wall 44. The suction side portion 70 extends along the perimeter of the tip floor 62 and generally conforms in profile to the suction side wall 46. The pressure side portion 68 and the suction side portion 70 are joined and/or intersect at the leading edge 52 and at and/or proximate to the trailing edge 54.
In one embodiment, as shown in FIGS. 3 and 4, a baffle 72 extends radially outwardly from the tip floor 62. The baffle 72 extends across the tip floor 62 from the pressure side to the suction side portions 68, 70 of the tip rail 66. In one embodiment, the baffle 72, the tip rail 66 and the tip floor 62 define a first tip pocket 74 and a second tip pocket 76 along the tip 50 of the airfoil 40. The first tip pocket 74 is generally defined adjacent and/or proximate to the leading edge 52 of the airfoil 40. The second tip pocket 76 generally extends from the baffle 72 towards the trailing edge 54 of the airfoil 40. In various embodiments, at least a portion of the coolant outlets 64 are formed along the tip floor 62 within the first tip pocket 74 and at least a portion of the coolant outlets 64 are formed along the tip floor 62 within the second tip pocket 76.
In operation, the hot gases 34 are directed onto the pressure side wall 44 of the airfoil 40, thus creating a high pressure region 78 along the pressure side wall 44 of each rotor blade 28. As the rotor blades 28 rotate and/or as a portion of the hot gases 34 leaks over the tip 50, a reduced or low pressure (with respect to the high pressure region) region 80 develops along the suction side wall 46. Typically, the coolant 58 is supplied from a coolant source, such as the compressor section 14 (FIG. 1) to the cooling passages 56 through the coolant passage inlets 60 at various supply pressures which generally relate to the various operating modes of the gas turbine 10. The flow of the coolant 58 through the cooling passages 56 and out of the coolant outlets 64 at the tip 50 is primarily driven by a pressure difference defined between the supply pressure at the coolant passage inlets 60 and a static pressure which is typically defined at the tip 50 of the airfoil 40, particularly within the tip pockets 74, 76. If the supply pressure is too low, for example, due to aerodynamic loading optimization or a decrease in operating speed or change in turbine load requirements, a lower static pressure is required in order to meet cooling flow needs.
In various embodiments, as shown in FIGS. 3 and 4, a slot or opening 82 is formed along the suction side portion 70 of the tip rail 66. In one embodiment, the slot 82 is formed along the suction side portion 70 of the tip rail 66 to define a coolant flow path 84 that provides for fluid communication of the coolant 58 from the first tip pocket 74 into the low or reduced pressure region 80, thus reducing the static pressure within the first tip pocket 74, thereby promoting or enhancing coolant flow through the airfoil 40, particularly proximate to the leading edge 52.
FIGS. 5, 6, 7 and 8 are enlarged perspective views of the tip 50 of the airfoil 40, according to various embodiments of the present invention. In one embodiment, as shown in FIG. 5, a slot or opening 86 may be defined along the suction side portion 70 of the tip rail 66 so as to define a coolant flow path 88 that provides for fluid communication of the coolant 58 from the second tip pocket 76 into the low or reduced pressure region 80, thus reducing the static pressure with the second tip pocket 76, thereby promoting or enhancing coolant flow through the airfoil 40, particularly proximate to a mid-portion and/or the trialing edge 54 of the airfoil 40.
In one embodiment, as shown in FIG. 6, the tip 50 may include slot 82 which defines a first slot and slot 86 which defines a second slot 88 where both slots 82, 86 are defined along the suction side portion 70 of the tip rail 66 so as to define coolant flow paths 84, 88 that provide for fluid communication of the coolant 58 from the first and second tip pockets 74, 76 respectively into the low or reduced pressure region 80, thus reducing the static pressure within both the first and second tip pockets 74, 76, thereby promoting or enhancing coolant flow through the airfoil 40, proximate to both the leading edge 52, a mid-portion and/or the trialing edge 54 of the airfoil 40.
In one embodiment, as shown in FIG. 7, the tip 50 may include a secondary baffle 90 which extends radially outwardly from the tip floor 62 and from the pressure side to the suction side portions 68, 70 of the tip rail 66, thus defining a third tip pocket 92. A slot 94 may be defined along the suction side portion 70 of the tip rail 66 so as to define a coolant flow path 96 that provides for fluid communication of the coolant 58 from the third tip pocket 92 into the low or reduced pressure region 80, thus reducing the static pressure within the third tip pocket 92, thereby promoting or enhancing coolant flow through the airfoil 40, proximate to the trialing edge 54 of the airfoil 40.
In one embodiment, as shown in FIG. 8, the baffle 72 is configured to allow at least a portion of the coolant 58 to flow between the first and second tip pockets 74, 76. For example, a slot or opening 96 may be formed along the baffle 72, thus defining a flow path 98 therebetween. In addition or in the alternative, the baffle 72 may be sized from the tip floor 62 such that at least a portion of the coolant 58 that flows into the first tip pocket 74 is allowed to flow over a top portion 98 of the baffle 72, thus reducing the static pressure in the first tip pocket 74, thereby promoting or enhancing coolant flow through the airfoil 40, proximate to the leading edge 52 of the airfoil 40.
As described and illustrated herein, the present invention provides various technical benefits over existing rotor blade tip technologies. For example, the present invention provides lower static pressures for various cooling flows, especially for flows along the leading edge of the rotor blade airfoil. The lower static pressures are achieved by segregating the tip into separate tip pockets or regions, and connecting the different tip pockets with different pressure zones. The reduced static pressure at the tip may reduce the required coolant supply pressure at the cooling passage inlets, thus resulting in improved overall turbine performance.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other and examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.

Claims

What is claimed is:

1. A rotor blade, comprising:

an airfoil including pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and in chord between a leading edge and a trialing edge, the tip including a tip floor and a plurality of coolant outlets disposed along the tip floor, the tip further including a tip rail that extends radially outwardly from the tip floor, the tip rail having a pressure side portion and a suction side portion which are joined at the leading and trailing edges;

a plurality of cooling passages circumscribed within the airfoil for routing a coolant therethrough, each of the cooling passages being in fluid communication with one or more of the coolant outlets;

a baffle extending radially outwardly from and across the tip floor from the pressure side portion to the suction side portion to define a first tip pocket and a second tip pocket; and

a slot disposed along the suction side portion of the tip rail, wherein the slot provides for fluid communication out of one of the first or second tip pockets.

2. The rotor blade as in claim 1, wherein the slot extends radially outwardly from the tip floor.

3. The rotor blade as in claim 1, wherein at least one of the coolant outlets is disposed along the tip floor within the first tip pocket.

4. The rotor blade as in claim 1, wherein at least one of the coolant outlets is disposed along the tip floor within the second tip pocket.

5. The rotor blade as in claim 1, further comprising an opening formed in the baffle, wherein the opening provides for fluid communication between the first tip pocket and the second tip pocket.

6. The rotor blade as in claim 1, wherein the baffle is configured to allow a portion of the coolant to flow over a top portion of the baffle into an adjacent tip pocket.

7. The rotor blade as in claim 1, further comprising a secondary baffle extending radially outwardly from and across the tip floor from the pressure side portion to the suction side portion to define a third tip pocket.

8. The rotor blade as in claim 7, further comprising a slot disposed along the suction side portion of the tip rail, wherein the slot provides for fluid communication out of the third tip pocket.

9. A system for promoting coolant flow through a rotor blade, comprising:

a coolant source for supplying a pressurized coolant to a cooling passage inlet formed along the rotor blade; and

wherein the rotor blade comprises:

a mounting portion including a mounting body, the mounting body being interconnectable with a rotor shaft, at least one of the cooling passage inlets being formed by the mounting body;

an airfoil coupled to the mounting portion and including pressure and suction side walls which extend radially outwardly from a platform in span from a root to a tip and in chord between a leading edge and a trialing edge, the tip including a tip floor and a plurality of coolant outlets disposed along the tip floor, the tip further including a tip rail that extends radially outwardly from the tip floor, the tip rail having a pressure side portion and a suction side portion which are joined at the leading and trailing edges;

a plurality of cooling passages circumscribed within the airfoil for routing a coolant through the airfoil, each of the cooling passages being in fluid communication with one or more of the coolant inlets;

10. The system as in claim 9, wherein the slot extends radially outwardly from the tip floor.

11. The system as in claim 9, wherein at least one coolant outlet is disposed along the tip floor within the first tip pocket and at least one coolant outlet is disposed along the tip floor within the second tip pocket.

12. The system as in claim 9, further comprising a coolant opening formed in the baffle, wherein the coolant opening provides for fluid communication between the first tip pocket and the second tip pocket.

13. The system as in claim 9, wherein the baffle is configured to allow a portion of the coolant to flow over a top portion of the baffle into an adjacent tip pocket.

14. The system as in claim 9, further comprising a secondary baffle extending radially outwardly from and across the tip floor from the pressure side portion to the suction side portion to define a third tip pocket.

15. The system as in claim 14, further comprising a secondary slot disposed along the suction side portion of the tip rail, wherein the secondary slot provides for fluid communication out of the third tip pocket.

16. A gas turbine, comprising;

a compressor;

a combustor disposed downstream from the compressor;

a turbine disposed downstream from the combustor, the turbine including a rotor shaft extending axially through the turbine, an outer casing circumferentially surrounding the rotor shaft to define a hot gas path therebetween and a plurality of rotor blades interconnected to the rotor shaft and defining a stage of rotor blades, wherein each rotor blade comprises;

a baffle extending radially outwardly from and transversely across the tip floor from the pressure side portion to the suction side portion to define a first tip pocket and a second tip pocket, wherein at least one coolant outlet is disposed along the tip floor within the first tip pocket and at least one coolant outlet is disposed along the tip floor within the second tip pocket; and

17. The gas turbine as in claim 16, wherein the slot extends radially outwardly from the tip floor.

18. The gas turbine as in claim 16, further comprising a slot formed in the baffle, wherein the slot provides for fluid communication between the first tip pocket and the second tip pocket.

19. The gas turbine as in claim 16, wherein the baffle is sized to allow a portion of the coolant to flow over a top portion of the baffle into an adjacent tip pocket.

20. The gas turbine as in claim 16, further comprising a secondary baffle extending radially outwardly from and across the tip floor from the pressure side portion to the suction side portion to define a third tip pocket, wherein a secondary slot is disposed along the suction side portion of the tip rail, wherein the secondary slot provides for fluid communication out of the third tip pocket.