Gas turbine airfoil

Details Gas turbine airfoil Gas turbine airfoil

2001-06-11

2003-10-21

Look, Edward K. (Department: 3745)

Fluid reaction surfaces (i.e., impellers)

With heating, cooling or thermal insulation means

Changing state mass within or fluid flow through working...

C029S889721

Reexamination Certificate

active

06634858

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a gas turbine airfoil with internal serpentine passages for cooling purposes.
BACKGROUND OF THE INVENTION
Turbine airfoils are subjected to the very high temperatures of the hot gas driving the turbine. In order to prevent damage to the airfoils due to the high temperatures and assure a reasonable lifetime the airfoils are cooled externally and internally by a cooling medium, typically cooling air bled from the compressor of the gas turbine. Internal cooling of the airfoil is realized by several passages within the airfoil between the pressure sidewall and the suction sidewall of the airfoil. The passages typically extend spanwise from the root of the airfoil to its tip. Some of the passages consist of a single passage with an exit port near the tip of the airfoil and/or several film cooling holes on the edge or on the side wall of the airfoil. Other passages follow a serpentine path allowing the cooling air to flow for example from the root to the tip and around a 180° turn. From the tip it extends towards the root and around a further 180° turn that directs it again toward the tip where it finally exits through exit ports or film cooling holes. Serpentine cooling passages of this type are disclosed for example in U.S. Pat. No. 5,403,159. They allow for a high internal heat transfer with a minimum amount of cooling air.
FIG. 1
shows a radial cross-section of a typical airfoil
1
of the state of the art with several internal passages extending radially inward and outward between a root section
2
and a tip
3
. A first internal passage
4
extends from an entry opening
5
in the root section
2
radially outward to the tip
3
of the airfoil. Cooling air can flow from the root section
2
through the passage and exit via several cooling slots
6
along the trailing edge
7
as well as through a tip hole
8
. A second internal passage
10
extends from an entry opening
11
radially outward along the leading edge
12
of the airfoil. Cooling air flows through this passage
10
and exits via a tip hole
13
and through several rows of film cooling holes
31
drilled through the leading edge
12
of the airfoil. A serpentine passage comprises an entry opening
14
at the radially inner end of the root section, a first passage
15
a
extending radially outward with a tip hole
17
. At the tip a 180° turn
16
leads to a passage
15
b
extending radially inward. At the radially inner end of the passage a second 180° turn
18
leads to a third passage
15
c
extending radially outward to a tip hole
19
. Cooling air flowing through the straight and serpentine passages cool the airfoil from within by impingement cooling and exits through the film cooling holes on the edges of the airfoil
1
and/or through the tip holes. Other typical airfoils have several serpentine cooling passages or serpentine passages comprising five passages with four turns.
Airfoils with internal serpentine geometry for the cooling passages are typically manufactured by an investment casting process, which utilizes a ceramic core to define the individual internal passages. Following the casting the ceramic core is removed from the airfoil by a leaching process. The film cooling holes on the edges and sidewalls of the airfoil are then realized by a laser drilling process. This process involves, previous to the actual drilling, the insertion of a backing or blocking material which limits the laser radiation to the desired locations of the film cooling holes and prevents damage to the passage walls and other inner surfaces of the airfoil. Such a method is disclosed for example in U.S. Pat. No. 5,773,790. It uses a wax material as a blocking material.
During the process of casting the internal passages it is often difficult to maintain the separation of the passages in the cores due to thermal strains caused by differential heating and cooling rates of the core and surrounding metal.
A current practice to maintain the separation of the serpentine passages
15
a,b,c
and to support the core during the casting process utilizes conically shaped features in the core. These conical features are formed as part of the core and extend from the root section through an opening in the wall of the 180° turn
18
and into the passages
15
b
and
15
c
. After the part is cast and the core is leached out, the conical feature is closed off with a spherically shaped plug
30
that is brazed into place.
The conical feature maintains a near constant cross-sectional area and outer radius of the outer wall through the 180° turn in order to minimize pressure loss. Typical measured pressure losses through the turns are usually >1.5 times the dynamic pressure of the cooling air stream entering the turn.
However, the conical feature presents a weak spot in the core where it can break resulting in movement of the passages
15
b
and
15
c
, the turn
18
, and the root section of the passage
15
a.
Following the casting process and the leaching out of the core material, a backing material must be inserted into the cooling passages for the laser drilling of film cooling holes.
As the passages
15
b
and
15
c
following the 180° turns are not easily accessible from either end, it is difficult to fill these passages with backing material. In current practice this problem is circumvented with the use of a liquid wax, which is typically hot injected into the opening
14
until wax is seen exiting from the tip hole
19
. After the completion of the laser drilling, the waxen backing material is removed from the airfoil by heating the airfoil and burning the wax. This practice has shown however, that the use of wax as a backing material does not sufficiently absorb the laser energy, and therefore provides only limited protection from so-called back wall strike.
SUMMARY OF THE INVENTION
This invention provides an airfoil and a method of manufacturing the airfoil that comprises internal cooling fluid passages arranged in a serpentine path having one or more radially outward extending passages and one or more radially inward extending passages, which are connected by turns of approximately 180°. In particular, the invention provides an airfoil and a method of manufacturing such an airfoil that enables improved maintenance of the passage separation during casting and the use of a blocking material for a laser drilling process that provides greater shielding compared to the blocking material used in current manufacturing methods.
An airfoil comprises internal cooling air passages arranged in a serpentine manner having one or more radially outward extending passages and one or more radially inward extending passages, and turns of approximately 180° providing fluid connection between a radially inward and a radially outward extending passage. A radially inward extending passage in a serpentine passage is defined by the inner surfaces of the pressure and suction sidewalls of the airfoil. A first wall and a second wall separate the radially inward extending passage from neighboring passages. A radially outward extending passage in a serpentine passage following the radially inward extending passage in the fluid flow direction is defined by the inner surfaces of the pressure and suction sidewalls of the airfoil, the second wall, and a third wall. The third wall can be a separating wall to a further cooling passage or the leading or trailing edge wall of the airfoil.
According to the invention, each radially inward extending passage is in fluid connection with the next radially outward extending passage in the direction of the cooling fluid flow by means of a root turn. This root turn is defined by the first wall of the radially inward extending passage and the third wall of the radially outward extending passage, which both extend to the radially inner end of the root section of the airfoil. The root turn is further defined by a member that closes off the root turn at the radial inner end of the root section of the airfoil and is attached to the radially inner ends of the first and third walls of th

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