Method of casting to control the cooling air flow rate of the airfoil trailing edge

The flow rate of the cooling air at the trailing edge of the airfoil of a turbine blade for a gas turbine engine is established by casting the blade to include judiciously located radially spaced projections adjacent to or in the ribs in the trailing edge which serve to meter the flow in the channels formed between the ribs. The holes for forming the projections in the core that are used to form the internal cooling passages in the trailing edge are intentionally undersized. After the core is cleaned and flashed, the blade is cast in a mold and the ceramic core is leached out. The internal cooling passages in the trailing edge are flow tested to obtain the flow rate and pressure and this data is used to resize the projection. Resizing the projection is made by reforming the core with the same die, and with the information previously obtained in the flow test, the holes that form the projections are enlarged by drilling to obtain the desired flow rate. This process is repeated until the desired flow rate for each trailing edge channel is achieved. The die may be changed at this juncture to provide the ultimate dimensioned hole in the core to provide the necessary metering of the cooling air to attain the predetermined flow rate.

TECHNICAL FIELD 
This invention relates to cooled turbine blades for a gas turbine engine 
and particularly to means of tailoring the cooling air flow rate 
discharging through the trailing edge slots of the blade's airfoil and the 
method thereof. 
BACKGROUND ART 
As is well known, the core used to form the internal passages, ribs, and 
heat transfer enhancement means for investment cast air cooled turbine 
blades or stator vanes is fabricated from a ceramic material. This 
material inherently is brittle and tends to break if not designed and 
handled properly. Because of structural integrity considerations, the 
ribs, particularly at the trailing edge, contain certain constraints which 
are not necessarily advantages from a cooling aspect. Hence, for example, 
the flow cavities that serve to cool the trailing edge are typically an 
array of closely spaced flow channels defined by the ribs and serve to 
meter cooling flow to obtain optimized cooling effectiveness. Because of 
these constraints, these flow channels cannot be made sufficiently narrow 
and shallow to satisfy this requirement. The ceramic cores either become 
too fragile to handle or cannot survive the casting process. 
Notwithstanding the above deficiencies, it is also desirable to provide 
for the cooling effectiveness means by which the cooling flow rate can be 
adjusted. 
The problems attributed to in the above description are exemplified, for 
example, in U.S. Pat. Nos. 4,515,523 granted to W. E. North, et al on May 
7, 1985 and 4,526,512 granted to R. B. Hook on Jul. 2, 1985 which are also 
incorporated herein for reference. 
The U.S. Pat. No. 4,515,523 discloses the use of ribs for structural 
support of the trailing edge and pin fins and protuberances extending from 
the ribs for heat transfer enhancement. The U.S. Pat. No. 4,526,512 
discloses a spool for a flow control body disposed at the trailing edge of 
the airfoil of a stator vane for controlling the flow exiting the trailing 
edge flow channels. 
Obviously, the impediment to flow created by the heat transfer enhancing 
mechanism and the structural ribs to some extent control the flow through 
the trailing edge. Due to the nature of convective cooling, the slots and 
the opening between the pins, pedestals and the like are small and the 
tolerances occasioned in castings of this type produce large variations in 
these openings and hence the flow at different locations along the 
trailing edge vary considerably. This, of course, is a problem that needs 
to be corrected in order to attain maximum life out of the member being 
cooled as well as conserving cooling air, which impacts engine 
performance. Typically, in heretofore methods of fabricating the blade, 
the blade is flow tested in a well known manner, and pursuant to the 
results of the flow tests, the die for making the ceramic core is modified 
with the aim of modifying the core and correcting the flow or pressure 
deficiencies. This is an expensive and time-consuming process, and 
whenever time is allowed in a given development program, this procedure 
can be repeated, obviously compounding the expenses and time problems. 
We have found that we can obviate the problems outlined in the preceding 
paragraphs and provide a means and method for controlling the rate of flow 
of the cooling air in the trailing edge. To this end, we provide 
restrictions either in proximity to the structured ribs or in the ribs 
themselves that serve to meter the flow in the trailing edge slots or flow 
channels. The method of tuning in or tailoring the flow is by 
incorporating into the ceramic core the openings that will define these 
restrictions and undersizing these openings. After the blade is cast, the 
core is leached out. The blade is then flow tested to establish a datum 
from which the restrictor size required for proper flow and pressure is 
ascertained. Obtaining the restrictor size becomes very simple and routine 
merely by enlarging the openings in the core, which have been 
intentionally undersized, to the desired dimension. Hence, the core is 
modified to the new dimensions without having to adjust the core dies. The 
sizing can be done in the cleaning process of the core when the flashing 
is removed. 
SUMMARY OF INVENTION 
An object of this invention is to tune the cooling air flow rate of the 
trailing edge of the airfoil in a gas turbine engine by sizing the core 
used in fabricating the airfoil. 
A feature of this invention is to include restrictors in combination with 
the trailing edge ribs to meter the flow in the trailing edge channels. 
Another object of this invention is to include the method of tuning cooling 
air flow rate out of the trailing edge slots by making a ceramic core 
including undersized openings intended to be used as metering means, 
casting a blade with the core, leaching out the core, flow testing the 
blade to ascertain the flow and pressure through each of the trailing edge 
slots and enlarging the openings to provide metering means that attain the 
desired flow and pressure. 
The foregoing and other features and advantages of the present invention 
will become more apparent from the following description and accompanying 
drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Reference is now made to FIG. 1 which is a typical representation of an 
axial flow turbine blade generally illustrated by reference numeral 10 
comprising an airfoil section 12 and root section 14 separated by the 
platform 15. The blade is mounted in a disk which in turn is rotatably 
supported to the main engine shaft which is connected to the compressor of 
the gas turbine engine (not shown). A portion of the energy is extracted 
from the engine working fluid that impinges on the blade for powering the 
compressor and a portion is used in developing thrust for powering the 
aircraft. Obviously, since the turbine sees engine working fluid at or 
nearly at its highest temperature, the metal parts in this location of the 
engine require cooling by sophisticated cooling techniques. 
As viewed in FIGS. 1 and 2, the airfoil 12 comprises a tip 16, leading edge 
18, trailing edge 20, pressure surface 22, and suction surface 24. Cooling 
air typically bled from one of the several stages of the compressor (not 
shown) feeds two entry passages 26 and 28, which in turn feed cooling air 
to the leading edge portion 30, trailing edge portion 32, and the mid 
portion 41 which carries a series of serpentine passageways. What has been 
described is well known in the art, and since this invention pertains only 
to the trailing edge, for the sake of simplicity and convenience only the 
trailing edge portion will be described. 
As is apparent from FIGS. 2-4, cooling air from passageway 28 flows out of 
the trailing edge slots 34 by first flowing through apertures 37, cavity 
38, past flow restrictions 40, and channels 42 defined between the axially 
extending ribs 44 which connects to the walls 46 and 48 of the pressure 
side and suction side of the blade. According to this invention, flow 
restrictions 40 are located in proximity to the leading edge of the 
adjacent rib 44 and serve to meter cooling flow into the channels 42. 
Obviously, the space between restrictor 40 and rib 44 determine the flow 
area and by positioning the two relative to each other can be an effective 
way in which the rate of flow can be controlled. 
FIG. 6 is another embodiment in which the restrictions were made integral 
with the ribs and likewise can be used to meter the flow. As noted, the 
restrictor 48 projects from rib 44 into channel 42 and serves to restrict 
flow in the channel. (Like reference numerals refer to like elements in 
all the FIGS.) It is obvious that the diameter of restrictor 48, like the 
diameter of restrictor 40 (FIG. 4) will determine the flow area of the 
metering portion of channels 42. Restrictors 48 are located on alternate 
ribs since one restrictor controls the flow in two adjacent channels. Each 
restrictor may be located at various locations on each rib depending on 
the structural, flow and pressure characteristics of each application. 
The next portion of this description relates to the method of tuning or 
tailoring the flow rate in the trailing edge channel. As mentioned above, 
this is a simple procedure executed in the investment casting process. As 
the particular process is well known, for the sake of simplicity and 
convenience the details of the process are omitted from this disclosure. 
Suffice it to say that the internal details of the blade are cast by using 
a ceramic core generally illustrated by reference numeral 50 in FIGS. 5 
and 7 that is made by a die set. Obviously, when molten metal is poured in 
the mold, it will occupy the voids in the mold formed after the was is 
removed. 
It is apparent from viewing FIGS. 4-7 restrictors 40 and 48 in the blade 
appear as circular apertures or voids 40a and 48a in the core. 
The method of tuning in the flow rate contemplates undersizing the 
restrictors by dimensioning the diameters of the apertures 40a and 48a to 
a smaller size. After a blade is cast, the blade is then flow tested to 
determine the flow rates in each of the channels. Any deviation to flow 
rates and/or pressure can be corrected simply by increasing the diameter 
of each of the apertures 40a and 48a which in the next cast part will 
enlarge the restrictors 40 and 48. 
Obviously, since the flashing is cleaned after the ceramic core is formed 
with the die, it is a relatively simple task to drill or ream larger 
apertures in the core to correct the undersized dimension. Of course, in 
certain circumstances the die can be changed to reflect the new dimensions 
of the restrictors. 
Although this invention has been shown and described with respect to 
detailed embodiments thereof, it will be understood by those skilled in 
the art that various changes in form and detail thereof may be made 
without departing from the spirit and scope of the claimed invention.