Film cooling passages with step diffuser

A film cooling passage through the wall of a hollow airfoil for a gas turbine engine has a metering section communicating with the interior of the airfoil for directing a metered amount of coolant through the passage in a first direction, followed by a mixing section to create turbulence in the flow as it leaves the metering section, followed by a diffusing section leading to the passage outlet at the outer surface of the airfoil. The mixing section comprises a sudden jog or step in the flow path of the fluid to suddenly disrupt its forward momentum in the first direction and to create turbulence therein whereby the fluid is more readily able to spread out within the following diffusing section and thereby stay attached to more widely diverging diffusion section walls. Wider diffusion angles in the coolant passage permits the same amount of coolant to be spread out over a wider area of the surface of the airfoil.

DESCRIPTION 
1. Technical Field 
This invention relates to airfoils, and more particularly to film cooled 
airfoils. 
2. Background Art 
It is well known the external surface of airfoils may be cooled by 
conducting cooling air from an internal cavity to the external surface via 
a plurality of small passages. It is desired that the air exiting the 
passages remain entrained in the boundary layer on the surface of the 
airfoil for as long a distance as possible downstream of the passage to 
provide a protective film of cool air between the hot mainstream gas and 
the airfoil surface. The angle which the axis of the passage makes with 
the airfoil surface and its relation to the direction of hot gas flow over 
the airfoil surface at the passage breakout are important factors which 
influence film cooling effectiveness. Film cooling effectiveness is 
defined as the difference between the temperature of the main gas stream 
(Tg) and the temperature of the coolant film (Tf), divided by the 
temperature difference between the temperature of the main gas stream and 
the coolant temperature (Tc) at the passage exit (Tg-Tf)/(Tg-Tc). Film 
cooling effectiveness decreases rapidly with distance from the passage 
exit. Maintaining high film cooling effectiveness for as long a distance 
as possible over as large a surface area as possible is the main goal of 
airfoil film cooling. 
It is well known in the art, that the engine airfoils must be cooled using 
a minimum amount of cooling air, since the cooling air is working fluid 
which has been extracted from the compressor and its loss from the gas 
flow path rapidly reduces engine efficiency. Airfoil designers are faced 
with the problem of cooling all the engine airfoils using a specified, 
maximum cooling fluid flow rate. The amount of fluid which flows through 
each individual cooling passage from an internal cavity into the gas path 
is controlled by the minimum cross-sectional area (metering area) of the 
cooling passage. The metering area is typically located where the passage 
intersects the internal cavity. The total of the metering areas for all 
the cooling passages and orifices leading from the airfoil controls the 
total flow rate of coolant from the airfoil, assuming internal and 
external pressures are fixed or at least beyond the designer's control. 
The designer has the job of specifying the passage size and the spacing 
between passages, as well as the shape and orientation of the passages, 
such that all areas of the airfoil are maintained below critical design 
temperature limits determined by the airfoil material capability, maximum 
stress, and life requirement considerations. 
Ideally, it is desired to bathe 100% of the airfoil surface with a film of 
cooling air; however, the air leaving the passage exit generally forms a 
cooling film stripe no wider than or hardly wider than the dimension of 
the passage exit perpendicular to the gas flow. Limitations on the number, 
size, and spacing of cooling passages results in gaps in the protective 
film and/or areas of low film cooling effectiveness which may produce 
localized hot spots. Airfoil hot spots are one factor which limits the 
operating temperature of the engine. 
U.S. Pat. No. 3,527,543 to Howald uses divergently tapered passages of 
circular cross section to increase the entrainment of coolant in the 
boundary layer from a given passage. The passages are also preferably 
oriented in a plane extending in the longitudinal direction or partially 
toward the gas flow direction to spread the coolant longitudinally upon 
its exit from the passage as it moves downstream. Despite these features, 
it has been determined by smoke flow visualization tests and engine 
hardware inspection that the longitudinal width of the coolant film from 
an eliptical passage breakout (i.e. Howald) continues to expand 
longitudinally only about a maximum of one passage exit minor diameter 
after the coolant is ejected on the airfoil surface. This fact, coupled 
with typical longitudinal spacing of three to six diameters between 
passages, result in areas of airfoil surface between and downstream of 
longitudinally spaced passages which receive no cooling fluid from that 
row of passages. Conical, angled passages as described in Howald U.S. Pat. 
No. 3,527,543 provide at best probably no more than 70% coverage 
(percentage of the distance between the centers of adjacent hole breakouts 
which is covered by coolant). 
The velocity of the air leaving the cooling passage is dependent on the 
ratio of its pressure at the passage inlet to the pressure of the gas 
stream at the passage outlet. In general the higher the pressure ratio, 
the higher the exit velocity. Too high an exit velocity results in the 
cooling air penetrating into the gas stream and being carried away without 
providing effective film cooling. Too low a pressure ratio will result in 
gas stream ingestion into the cooling passage causing a complete loss of 
local airfoil cooling. Total loss of airfoil cooling usually has 
disastrous results, and because of this a margin of safety is usually 
maintained. This extra pressure for the safety margin drives the design 
toward the high pressure ratios. Tolerance of high pressure ratios is a 
desirable feature of film cooling designs. Diffusion of the cooling air 
flow by tapering the passage, as in the Howald patent discussed above is 
beneficial in providing this tolerance, but the narrow diffusion angles 
taught therein (12.degree. maximum included angle) require long passages 
and, therefore, thick airfoil walls to obtain the reductions in exit 
velocities often deemed most desirable to reduce the sensitivity of the 
film cooling design to pressure ratio. The same limitation exists with 
respect to the trapezoidally shaped diffusion passages described in 
Sidenstick, U.S. Pat. No. 4,197,443. The maximum included diffusion angles 
taught therein in two mutually perpendicular planes are 7.degree. and 
14.degree., respectively, in order to assure that separation of the 
cooling fluid from the tapered walls does not occur and the cooling fluid 
entirely fills the passage as it exits into the hot gas stream. With such 
limits on the diffusing angles, only thicker airfoil walls and angling of 
the passages in the airfoil spanwise direction can produce wider passage 
outlets and smaller gaps between passages in the longitudinal direction. 
Wide diffusion angles would be preferred instead, but cannot be achieved 
using prior art teachings. 
Japanese Patent No. 55-114806 shows, in its FIGS. 2 and 3 (reproduced 
herein as prior art FIGS. 18 and 19), a hollow airfoil having straight 
cylindrical passages disposed in a longitudinal row and emptying into a 
longitudinally extending slot formed in the external surface of the 
airfoil. While that patent appears to teach that the flow of cooling fluid 
from adjacent passages blends to form a film of cooling fluid of uniform 
thickness over the full length of the slot by the time the cooling fluid 
exits the slot and reaches the airfoil surface, our test experience 
indicates that the coolant fluid from the cylindrical passages moves 
downstream as a stripe of essentially constant width, which is 
substantially the diameter of the passage. Any diffusion which results in 
blending of adjacent stripes of coolant fluid occurs so far downstream 
that film cooling effectiveness at that point is well below what is 
required for most airfoil designs. 
U.S. Pat. No. 3,515,499 to Beer et al describes an airfoil made from a 
stack of etched wafers. The finished airfoil includes several areas having 
a plurality of longitudinally spaced apart passages leading from an 
internal cavity to a common, longitudinally extending slot from which the 
cooling air is said to issue to form a film of cooling air over the 
airfoil external surface. In FIG. 1 thereof each passage appears to 
converge from its inlet to a minimum cross-sectional area where it 
intersects the slot. In the alternate embodiment of FIG. 9, the passage 
appears to have a small, constant size which exits into a considerably 
wider slot. Both configurations are likely to have the same drawbacks as 
discussed with respect to the Japanese patent; that is, the cooling fluid 
will not uniformly fill the slot before it enters the main gas stream, and 
considerably less than 100% film coverage downstream of the slot is 
likely. 
Other publications relating to film cooling the external surface of an 
airfoil are: U.S. Pat. Nos. 2,149,510; 2,220,420; 2,489,683; and "Flight 
and Aircraft Engineer" No. 2460, Vol. 69, 3/16/56, pp. 292-295, all of 
which show the use of longitudinally extending slots for cooling either 
the leading edge or pressure and suction side airfoil surfaces. The slots 
shown therein extend completely through the airfoil wall to communicate 
directly with an internal cavity. Such slots are undesireable from a 
structural strength viewpoint; and they also require exceedingly large 
flow rates. 
U.S. Pat. No. 4,303,374 shows a configuration for cooling the exposed, 
cut-back surface of the trailing edge of an airfoil. The configuration 
includes a plurality of longitudinally spaced apart, diverging passages 
within the trailing edge. Adjacent passages meet at their outlet ends to 
form a continuous film of cooling air over the cut-back surface. 
A serial publication, "Advances in Heat Transfer" edited by T. F. Irvine, 
Jr. and J. P. Hartnett, Vol. 7, Academic Press (N.Y. 1971) includes a 
monograph titled Film Cooling, by Richard J. Goldstein, at pp. 321-379, 
which presents a survey of the art of film cooling. The survey shows 
elongated slots of different shapes extending entirely through the wall 
being cooled, and also passages of circular cross section extending 
through the wall. 
DISCLOSURE OF INVENTION 
One object of the present invention is an improved film cooling passage 
configuration for cooling a wall over which a hot gas stream is flowing. 
Yet another object of the present invention is an airfoil film cooling 
passage which, in a short diffusing distance, is able to spread a small 
amount of coolant as a film over a large area of the external surface of 
the airfoil. 
According to the present invention, a film cooling passage through a wall 
to be cooled has a metering section for directing a metered amount of 
coolant therethrough in a first direction, followed by a mixing section 
configured to create turbulence in the flow as it leaves the metering 
section and to at least partially disrupt its momentum in the first 
direction, followed by a diffusing section leading to the passage outlet 
at the outer surface of the wall over which a hot gas is to flow. 
As discussed in the Background of the Invention, it has been a goal of the 
prior art to take a small amount of coolant fluid from the cool side of a 
wall to be cooled and to spread it out as a thin film over as large an 
area of the hot surface of the wall as possible. To do this, it is 
desirable to make the cooling passage outlet as long as possible in a 
direction perpendicular to the flow of hot gases over the surface at the 
passage outlet; and then the coolant fluid must uniformly (in the ideal 
case) fill the entire passage at the outlet so as to create a film of 
coolant downstream of the outlet which film is as wide as the outlet is 
long. The prior art teaches that diffusing the coolant flow from a small 
area inlet or metering section to a large area outlet necessitates the use 
of relatively small diffusion angles (i.e., less than 14.degree.) to 
prevent separation from the diverging walls to assure that the coolant 
fluid fills the passage at the outlet. If one is limited to small 
diffusion angles, then long passage lengths are required to obtain large 
increases in passage outlet dimensions. When the wall to be cooled is 
thin, or has a concave curvature such as the wall of a hollow turbine 
airfoil, the length of the passage is severely limited. 
In accordance with the present invention, it has been found, surprisingly, 
that a mixing section interconnecting the outlet of the metering section 
to the entrance of the diffusing section of the cooling passage permits 
the use of much larger diffusion angles in the diffusing section than has 
been possible using prior art configurations. 
The mixing section may be of any configuration which creates turbulence in 
the coolant stream as it leaves the metering section, such as by rapidly 
expanding the flow to reduce and at least partially redirect the momentum 
of the stream such that the average velocity and cohesiveness of the 
coolant stream leaving the metering section outlet is diminished as it 
enters the diffusing section; and the mass flow of coolant no longer is 
uni-directional. This allows the coolant to more readily spread out within 
the diffusing section and to thereby stay attached to more widely 
diverging walls than was possible using prior art passage configurations. 
With the present invention a diffusing section having an included angle of 
60.degree. has been tested and becomes completely filled with coolant, 
which exits uniformly over the full extent of the outlet and forms a thin 
film of coolant on the hot surface. The use of angles of up to 80.degree. 
are believed possible. 
In a preferred embodiment of the present invention the mixing section is a 
step diffuser, wherein the coolant fluid exiting the metering section is 
suddenly expanded, such as by flowing over a step into a larger volume, as 
opposed to expanding by gradual diffusion. The sharp corner of the step is 
believed to create vortices which redirect part of the fluid momentum in a 
direction perpendicular to the original direction of flow from the 
metering section by the centrifugal forces created by the whirl of the 
vortex. 
The foregoing and other objects, features and advantages of the present 
invention will become more apparent in the light of the following detailed 
description of preferred embodiments thereof as illustrated in the 
accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
As an exemplary embodiment of the present invention consider the turbine 
blade of FIG. 1 generally represented by the reference numeral 10. With 
reference to FIGS. 1 and 2, the blade 10 comprises a hollow airfoil 12 
which extends in a spanwise or longitudinal direction from a root 14 which 
is integral therewith. A platform 16 is disposed at the base of the 
airfoil 12. The airfoil 12 comprises a wall 18 having an outer surface 20 
and an inner surface 22. The inner surface 22 defines a longitudinally 
extending internal cavity which is divided into a plurality of adjacent 
longitudinally extending compartments 24, 26, 28 by longitudinally 
extending ribs 30, 32. A passage 34 within the root 14 communicates with 
the compartment 24; and a passage 36 within the root 14 communicates with 
both compartments 26 and 28. When the blade 10 is operating in its 
intended environment, such as in the turbine section of a gas turbine 
engine, coolant from a suitable source, such as compressor bleed air, is 
fed into the passages 34, 36 and pressurizes the compartments 24, 26 and 
28. 
As shown in FIG. 1, the airfoil 12 includes a plurality of longitudinally 
extending rows of coolant passages, such as the passages 38, 40 on the 
suction side; the passages 42 in the vicinity of the leading edge of the 
airfoil; and the passages 44 on the pressure side. Each passage 38, 40, 
42, 44 extends from an inlet at the inner surface 22 of the airfoil to an 
outlet at the outer surface 20. These passages need not be identical 
within a row or from row to row. The airfoil shown in FIGS. 1 and 2 has 
been simplified for purposes of clarity, and the number of rows of 
passages, the number of passages in each row, and the spacing between 
rows, as shown in the drawing, is intended to be illustrative only and not 
limiting. 
The coolant passages may be formed by any suitable means. A preferred 
method is by the well known technique of electro-discharge machining (EDM) 
using an electrode having the shape of the passage to be formed. A 
plurality of passages may be simultaneously formed using a "comb" 
electrode such as shown in FIG. 9, which is simply an electrode comprised 
of a plurality of adjacent "teeth" 45, each having the shape of the 
passage to be formed, and joined together at a common base 47. The method 
for forming the passages is not considered to be an aspect of the present 
invention. 
Throughout the drawing the arrows 50 represent the direction of flow (i.e., 
steamlines) of hot gases over the surface of the airfoil. For purposes of 
the description of the present invention, the direction of flow of hot 
gases over either the pressure or suction side surfaces of the airfoil 
shall be considered the downstream direction. Thus, at any point on the 
suction or pressure side surface of the airfoil, the downstream direction 
is tangent to the surface of the airfoil and, except perhaps close to the 
airfoil tip or the airfoil base where atypical currents are generated, is 
substantially perpendicular to the spanwise direction of the airfoil. 
The improved coolant passages of the present invention are herein shown as 
the passages 38 disposed in a spanwise row through the suction side wall 
of the airfoil, and shown greatly enlarged in FIGS. 3 through 6. Although 
described as suction side coolant passages, their use is not limited to 
the suction side of the airfoil. It will be evident that the present 
invention is useful for cooling any relatively thin wall which has a 
pressurized compartment or chamber containing relatively cool fluid on one 
side of the wall and a hot fluid flowing over the surface of the other 
side of the wall which, in the area to be cooled, is at a lower pressure 
than the coolant fluid. 
Referring to FIGS. 3-6, the passage 38 includes, in series flow relation, a 
metering section 52, followed by a mixing section 54, followed by a 
diffusing section 56. In this embodiment, the metering section 52 is 
generally rectangular in cross-section; however, its specific 
cross-sectional shap is not critical to the present invention and may, for 
example, be circular or eliptical. By definition, the metering section 52 
is that portion of the coolant passage 38 having the smallest 
cross-sectional area perpendicular to the direction of flow through the 
passage 38, which is along the passage centerline 39 which passes through 
the geometric center of the cross-sectional area of the metering section 
52. The length B of the passage 38 is the length of the centerline 39 
between the points where it intersects the surfaces 20, 22. In this 
specification, flow along the centerline 39 is in the "axial direction". 
The metering section should have a constant cross-sectional area for no 
more than a distance equal to three times the effective diameter of the 
cross-sectional area of the metering section to reduce the cohesiveness of 
the mass of coolant exiting therefrom. The shorter the length the better, 
as long as the metering area is well defined. The inlet 58 to the metering 
section 52 communicates with the inlet 60 to the passage 38 at the inner 
surface 22 of the compartment 26 (FIG. 2) and receives a flow of coolant 
fluid therefrom. The outlet 62 of the metering section 52 is coincident 
with the inlet to the mixing section 54. In this exemplary embodiment, the 
mixing section comprises a step diffuser, as will be further described 
herein below. 
The outlet 64 of the mixing section is coincident with the inlet of the 
diffusing section 56. The diffusing section 56 includes a pair of spaced 
apart flat surfaces 66, 68. The surface 68 is parallel to the spanwise or 
longitudinal direction of the airfoil. The surface 68 is also parallel to 
the centerline 39. 
The surfaces 66, 68 intersect the outer surface 20 of the airfoil at angles 
.gamma..sub.1, .gamma..sub.2, respectively (FIG. 3). These angles are 
preferably shallow angles of no more than about 40.degree. (most 
preferably 30.degree. or less), in order to minimize penetration of the 
coolant stream into the hot gas stream in a direction perpendicular to the 
outer surface of the airfoil. Excessive penetration can result in the 
coolant being immediately swept away from the surface of the airfoil 
rather than forming a film of coolant entrained in the boundary layer of 
the airfoil surface downstream of the passage outlet. The intersection of 
the surfaces 66, 68 with the outer surface 20 define downstream and 
upstream edges 73, 75, respectively, of the passage outlet 71; and for 
this reason the surfaces 66, 68 are referred to as the downstream surface 
and upstream surface, respectively. Note that the downstream surface 66 
faces generally upstream, and upstream surface 68, faces generally 
downstream. It is preferred that the downstream surface 66 diverges from 
the centerline 39 toward the passage outlet 71. Preferably, the downstream 
surface diverges by an angle from the upstream surface 68 by an angle of 
between 5.degree. and 10.degree.. This reduces the angle .gamma..sub.1, 
which is desirable. Although not preferred, the surfaces 66, 68 may be 
parallel to each other; and such a configuration is intended to fall 
within the scope of the present invention. 
As best shown in FIGS. 4-6, the diffusing section 56 includes side surfaces 
70, 72 which face each other and extend between the surfaces 66, 68. Each 
side surface also extends from the mixing section outlet to the passage 
outlet along a straight path which diverges from the axial direction 39 by 
an angle .beta. (FIG. 6). Divergence angles .beta. of up to 30.degree. 
have been tested successfully, wherein a coolant film was produced having 
a width substantially equivalent to the full width of the passage outlet, 
meaning that the passage flowed "full". It is believed that with fine 
tuning divergence angles of up to 40.degree. may be used. 
As shown schematically in FIGS. 14 and 15, respectively, the side surfaces 
may also be convexly curved (70', 72') or be comprised of a plurality of 
straight sections (70" or 72"), each diverging from the axial direction by 
a greater angle than the preceding section. The effective angle of 
divergence in each case is .beta.' and .beta.", respectively. 
Referring to FIGS. 4a through 4c, each side surface 70, 72 blends (as at 
74, 76) along its length with the downstream surface 66 as a smooth curve, 
as opposed to a sharp corner or filet radius (see FIGS. 10-12 discussed 
below). Preferably the diameter of the curved corner at the outlet of the 
passage 38 (FIG. 4a) is on the order of the distance between the surfaces 
66, 68 at the outlet. The diameter is reduced gradually as the passage 
tapers down toward the diffusing section inlet. The corners preferably 
form segments of an oblique cone along the length of the diffusing 
section. The apex of the cones are preferably located at points C (FIGS. 3 
and 4(a)). The smoothly curved corners create counterrotating vortices 
which further aid in uniformly filling the diffusing section with coolant, 
enabling the use of larger divergence angles .beta. than would otherwise 
be possible with the normal filet radii or sharp corners used by the prior 
art. This aspect of the present invention is further described and claimed 
in commonly owned, co-pending application U.S. Ser. No. 812,104, now a 
U.S. Pat. No. 4,684,323, titled "Improved Film Cooling Passages with 
Curved Corners" by the same inventor as the present application and filed 
on even date herewith. Such application is incorporated herein by 
reference. 
The primary feature of the present invention is the mixing section 54 of 
the passage 38. The function of the mixing section 54 is to reduce and 
redirect the momentum of the coolant stream as it exits the metering 
section 52 before it reaches the diffusing section 56. In prior art 
passages such as shown in FIGS. 10-12 the coolant stream tends to remain a 
relatively cohesive, unidirectional mass which is difficult to redirect 
along the diverging surfaces 110, 112 of the diffusing section simply by 
viscous forces within the boundary layers. In accordance with the present 
invention, the redirection and reduction of the momentum of the coolant 
stream is brought about by increasing the cross-sectional area of the 
passage substantially step-wise, upstream of the diffusing section inlet, 
in a direction generally away from (i.e., opposite) the downstream 
direction 50 of the mainstream flow. In the embodiment of FIG. 3 this is 
toward and generally perpendicular to the plane of the upstream surface 
68. 
More specifically, in the embodiment of FIG. 3 the passage 38 has a step 90 
at the metering section outlet 62. The step is perpendicular to the 
surface 68. The step 90 has a sharp corner or edge 92 which is believed to 
generate vortices along its length which gives a portion of the coolant 
fluid a component of velocity in a direction parallel to the edge 92. 
Alternate configurations for the step 90 are shown in FIG. 16. In FIG. 
16(a) the step 90 is an undercut. In FIG. 16(b) the sharp inside corner or 
filet radius is replaced with a larger radius. In FIG. 16(c) the step 90 
slopes toward the passage outlet, but at a sufficiently small angle that 
the flow from the metering section does not remain attached to the surface 
of the step downstream of the edge 92. Vortices are thus generated along 
the edge as in the embodiments of FIGS. 16(a) and (b). The slope of the 
step 90 must be selected such that the change in cross-sectional area at 
the metering section exit is rapid enough to generate the turbulence 
necessary to produce the desired results in the diffusing section 56 of 
the passage. 
Preferably the passage cross-sectional area should increase suddenly at the 
metering section outlet to at least 1.5 times the metering section 
cross-sectional area. Also, the distance from the metering section outlet 
62 to the upstream edge 75 of the passage outlet is preferably no greater 
than four times the equivalent diameter of the metering section 
cross-sectional area because the diffusing effects created by the vortices 
in the mixing section 54 decay rapidly, and the coolant flow tends to 
return to a cohesive state if the distance between the mixing section 
outlet 64 and the airfoil surface 20 becomes too large. Furthermore, the 
distance between the step and the inlet to the diffusing section also 
should not be so long as to allow the flow to re-coalesce with its 
momentum once again directed substantially only axially. Long mixing 
sections may, therefore, require a second step 93 or sudden further 
increase in cross-sectional area, as shown in FIG. 17. 
Although the mixing section feature of the cooling passage of exemplary 
embodiment of FIGS. 3-6 is combined with the feature of a large curvature 
joining the side surfaces with the downstream surface in the diffusing 
section, these features provide benefits independent of each other; and 
the present invention is not limited to the combination of the two. Thus, 
the use of the mixing section with a diffusing section having standard 
filet radii (i.e., sharp corners), as shown in FIGS. 7 and 8, also 
provides significant improvements over the prior art. FIGS. 7 and 8 are 
views corresponding, respectively, to FIG. 4(a) and FIG. 6, which show an 
alternate configuration for the diffusing section 56 of FIG. 3 wherein the 
corners are all "sharp". The graph of FIG. 13 shows tests results for such 
alternate configuration as well as for a "baseline" configuration 
described below. 
In FIG. 13 the horizontal axis is a dimensionless parameter P whose value 
is the ratio of the distance .times. from the outlet of the cooling 
passage (in the direction of the mainstream gas flow over the outlet) to a 
number directly related to the mass flow rate of cooling air exiting the 
passage. The vertical axis is a measure of the film cooling effectiveness 
E (as hereinabove defined) measured at a distance .times. downstream of 
the passage outlet. The maximum possible cooling effectiveness is 1.0. 
Because P is directly related to distance from the passage outlet, and 
since the distance downstream of the outlet is the only variable in these 
tests, P may be considered as a measure of distance downstream of the 
passage outlet. 
The curve labeled A is for the baseline coolant passage as shown in FIGS. 
10-12. The baseline configuration is used for comparison purposes and is 
similar to the coolant passages described in Sidenstick, U.S. Pat. No. 
4,197,443 except the divergence angles are 10.degree.. The baseline test 
piece was a flat plate having a thickness of 1.4 inch. A baseline passage 
was machined into the plate. The flow of hot gas over the passage outlet 
was perpendicular to the downstream edge of the passage outlet. The 
constant cross-section metering section of the passage had a length L of 
0.475 inch; a width W of 0.450 inch; and a height H of 0.300 inch. The 
diffusing section 102 of each passage had an upstream surface 104 parallel 
to the axial direction 106 of the passage. The corners of the diffusing 
section 102 were all "sharp". The diffusing section 102 also had a 
downstream surface 108 which diverged at an angle of 10.degree. from the 
axial direction. The side surfaces 110, 112 each diverged from the axial 
direction at an angle of 10.degree. which we determined avoids separation 
(i.e., such that the passage flows "full" and produces a coolant film of 
substantially the same width as the passage outlet) despite the teaching 
of Sidenstick which suggests maximum divergence angles of 7.degree.. The 
angle between the axial direction and the outer surface 114 of the test 
piece was 35.degree.. 
The curve B represents data for a single coolant passage according to the 
present invention and shaped as shown in FIGS. 7 and 8, which passage, in 
cross-section taken along the line A--A, appears the same as in the view 
shown in FIG. 3. The thickness of the flat plate test piece was 0.9 inch. 
The angle .gamma. was 30.degree. and .theta. was 10.degree.. The angles 
.beta. were 30.degree.. The metering section had a length of 0.5 inch, a 
width W of 0.41 inch, and a height H of 0.28 inch. The mixing section had 
a length of 0.5 inch in the axial direction, a width of 0.41 inch and 
height of 0.42 inch. The mass flow rate of coolant through the passages 
represented by curves A and B was the same and constant during the test. 
The passages flowed "full" as evidenced by smoke flow visualization tests. 
The graph shows that at 20 units distance downstream of the passage outlet 
the film cooling effectiveness of the present invention is about 0.05 
greater than that of the baseline configuration; and at 40 units distance 
the difference is about 0.03. To put this in perspective, assuming a 
coolant temperature at the passage outlet of 1200.degree. F. and a 
mainstream gas temperature of 2600.degree. F., a 0.02 increase in cooling 
effectiveness translates into about a 28.degree. F. decrease in the 
temperature of the coolant film for the same mass flow rate of coolant. 
In addition to better cooling effectiveness directly downstream of each 
passage outlet, the wider divergence angles of the passages of the present 
invention result in spreading the same amount of coolant over a 
significantly greater area than the baseline configuration using the same 
metering section cross-sectional area and a passage length on the order of 
about half the length required by the baseline configuration. This permits 
the use of coolant passages with small ratios of length (B) to metering 
section effective diameter (D), which is particularly advantageous when 
the wall to be cooled is very thin. 
In small airfoils with thin walls (e.g., 0.03 inch thick), where the sum of 
the cross-sectional area of the metering sections of all the coolant 
passages is restricted, and the minimum size of each metering section is 
limited by practical considerations to about 0.015 inch diameter, the 
present invention permits the passage outlets of a spanwise row of 
passages to be more closely spaced from one another than if prior art 
passages were used. Thus, over the same spanwise length of the airfoil, 
even the best shaped passages of the prior art provide considerably less 
coverage than passages of the present invention, for the same total mass 
flow rate of coolant. 
In the embodiment hereinabove described, and as clearly shown in FIG. 1, 
each passage 38 of a spanwise row of passages 38 breaks out at the surface 
20 of the airfoil to form an outlet completely separate from each of the 
other outlets. The present invention contemplates that adjacent passages 
may be sufficiently close together and formed in such a manner that the 
passages (more specifically, the side walls) intersect each other below 
the surface 20, whereby a continuous outlet slot is formed at the surface 
20 which runs the length of the row of passages. During operation this 
slot becomes filled with coolant; and the film of coolant formed on the 
surface 20 downstream of the slot is continuous in the longitudinal 
direction over the length of the row, eliminating gaps in film coverage 
resulting from the gaps between passage outlets. This is more fully 
described and claimed in commonly owned, copending patent application Ser. 
No. 812,103, now a U.S. Pat. No. 4,664,597, titled "Improved Film Cooling 
Slots for Airfoils" by Thomas A. Auxier, Edward C. Hill, and Leon R. 
Anderson filed on even date herewith. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail of 
the invention may be made without departing from the spirit and scope 
thereof.