Gas turbine engine

A gas turbine engine in which a centrifugal diffusor is located downstream of a centrifugal compressor. The compressor air flow from the centrifugal diffusor is deflected into an axial direction by a substantially 90 degree elbow. The air flow is further decelerated in an axial-flow stator cascade upstream of the combustion chamber. A main bearing of the gas generator is arranged immediately downstream of the centrifugal compressor. This bearing is supplied from the outside through vanes of the axial-flow stator cascade. The axial-flow stator cascade is divided into groups of vanes, so that each group had a number of relatively small guide vanes and one relatively large guide vane. The small guide vanes are designed from the view point of fluid mechanics consideration, while the larger guide vane of a group is hollow for supplying the bearing. The large guide vane has a substantially longer and a substantially thicker vane profile than the small vanes. Slow ducts are provided between the small vanes on the one hand, and between each small guide vane and a large guide vane on the other hand. The flow ducts are substantially of identical geometric dimensions. The large vane and small guide vanes within the cascade assembly may be arranged so that half the relative length of profile of all vanes extend in substantially one plane.

BACKGROUND OF THE INVENTION 
This invention relates to a gas turbine engine having a centrifugal 
compressor and immediately downstream of it a centrifugal diffusor from 
which the compressor air flow is deflected into an axial direction via a 
substantially 90 degree elbow and is further decelerated in an axial-flow 
stator cascade arranged upstream of the combustion chamber. Arranged 
immediately downstream of the centrifugal compressor, is a main bearing of 
the gas generator. This main bearing is supplied from the outside through 
the vanes of the axial-flow stator cascade. 
Highly-stressed centrifugal compressors, especially centrifugal compressors 
of gas turbine engines, are normally fitted with two stators for maximum 
conversion of the dynamic pressure downstream of the impeller into static 
pressure by deceleration of the flow. Following a first centrifugal stator 
cascade often fitted with wedge-shaped vanes, the flow is deflected, 
especially with gas turbine engines, through 90.degree. and is then 
decelerated in a second axial-flow cascade. The airfoil sections used in 
this second cascade roughly corresponds to those of an outlet stator 
cascade with highly-stressed axial-flow compressors. 
If the general design of the engine calls for a main bearing immediately 
downstream of the centrifugal compressor--which is often recommended for 
reasons of efficiency and performance--all bearing supply lines (fresh 
oil, return oil, sealing air and possibly bearing chamber venting) must 
necessarily be routed through the flow duct. When this is the case it is 
generally impossible to route these supply lines through the radially 
wetted portion of the stator with its thin, wedge-shaped vanes. When the 
bearing is supplied through freely exposed lines running through the flow 
duct downstream of the axial-flow stator cascade, these will cause 
irregular flow, normally to the great detriment of component assemblies 
downstream of the compressor, as perhaps the combustion chamber of a gas 
turbine engine. When the bearing is supplied through the vanes of the 
axial-flow portion of the stator, the form of the airfoil sections of the 
cascade is generally less than ideal. This is aggravated by the fact that 
it invariably takes a group of blades to serve any one function, as e.g. 
for draining the oil, because each cascade section has only little free 
cross-sectional area available. At the same time there is an unfavorable 
ratio of circumference to cross-sectional area of the partial ducts, which 
is a considerable disadvantage especially for the oil ducts (high heat 
transfer, oil heating). This compels considerable complexity of design 
when splitting the various streams into a number of partial streams. This 
effort is duplicated when the various partial streams are subsequently 
gathered into the respective main stream. 
A broad object of the present invention is to improve conventional gas 
turbine engines of this generic category such that the main bearing 
downstream of the centrifugal compressor is optimally supplied while 
ensuring proper aerodynamic conditions for the axia-flow stator cascade. 
Another object of the present invention is to provide an improved gas 
turbine engine of the foregoing character, which is substantially simple 
in construction and may be economically fabricated. 
A further object of the present invention is to provided an arrangement, as 
described, which has a substantially long operating life. 
SUMMARY OF THE INVENTION 
The objects of the present invention are achieved by providing an 
arrangement where the axial-flow stator cascade is split into groups of 
vanes each consisting of a number of relatively small guide vanes and one 
relatively large guide vane. The small vanes are designed strictly from 
the mechanics of fluids aspect and the large vane of the group are hollow 
to supply the bearing and have a much longer profile as well as a 
substantially greater absolute thickness of profile. The flow ducts 
between the small vanes, on the one hand, and between each small vane and 
a large vane, on the other, exhibit essentially identical geometric 
dimensions. 
In a further embodiment of the present invention the large and the small 
guide vanes within the stator cascade are arranged such that half of the 
relative length of profile of all vanes extends approximately in one 
plane. 
In a still further advantageous embodiment of the present invention the 
guide vanes of the axial-flow stator cascade are welded or brazed to a 
double-walled, preferably cast shell serving as a bearing support of the 
main bearing such that the large guide vane of each group of vanes is 
fitted exactly above a section of the bearing shell that is formed as a 
supply duct. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
With reference now to FIG. 1 the gas generator of the gas turbine engine 
comprises a centrifugal compressor 1 and downstream of it a centrifugal 
diffusor 2. The compressor air flow from diffusor 2 is deflected into an 
axial direction by means of a 90 degree elbow 3 and ducted to an 
axial-flow stator cascade 4 downstream of the elbow 3. 
The axial-flow stator cascade 4 issues into an annular duct 10 arranged 
between outer casing components 5, 6 and 7 and the flame tube 8 of a 
reverse-flow combustion chamber 9, with the annular duct supplying the 
combustion chamber with combustion, mixing and cooling air. 
The guide vane and rotor blade of a drive turbine 11 of the centrifugal 
compressor 1 are indicated by the numerals 12, 13 and 14, 15, 
respectively. 
The centrifugal compressor 1 and the compressor drive turbine 11 are 
arranged on a common gas generator shaft 16. The main bearing of the gas 
generator shaft 16 at the compressor end is indicated by the numeral 17. 
As it will further become apparent from FIG. 1, the outer and inner bearing 
chamber 18, 19 of the main bearing 17, inclusive of the associated seal 
carriers 20, 21 opposite the gas generator shaft 16 are formed by a 
double-walled bearing shell 22, 22' which thus serves as a bearing support 
of main bearing 17. This bearing shell 22, 22'--cf. FIG. 2--is arranged 
coaxially with the longitudinal centerline 23 of the engine, designed as a 
rigid box construction to resist endwise forces, and provided with supply 
ducts 24 (FIG. 2) at the centrifugal compressor end for the supply of the 
bearing. The supply ducts 24--originating at the longitudinal 
centerline--may be directed outwardly in stellate or radial arrangement 
and spaced equally to serve the following exemplary functions: bearing 
chamber venting, fresh oil supply, return oil discharge. 
The supply ducts 24 may be formed by ribs 25 (FIG. 1) associated with one 
or the other of the two shell members 22 or 22' and simultaneously 
providing a spacing axially between the shell members 22, 22'. 
Considering the above special construction of the gas turbine engine the 
supply of the main bearing 17 from the outside is effected through the 
vanes of the axial-flow cascade 4. For this purpose the axial-flow stator 
cascade 4 is split into groups of vanes each consisting of a number of 
relatively small guide vanes 26 and one relatively large guide vane 27. 
The small vanes 26 are designed strictly from the mechanics of fluids 
aspect while the large vane 27 is made hollow and, compared with the small 
vanes 26, has a clearly longer vane profile and an essentially greater 
absolute thickness of profile. In this arrangement the flow ducts between 
the small vanes 26, on the one hand, and between one each small vane and a 
large vane 27, on the other, exhibit essentially identical geometric 
dimensions. 
In the interest of aerodynamically favorable conditions, the large vane 27 
and the small vanes 26 of this group of vanes are aranged within the 
stator cascade such that half the relative length of profile of all vanes 
26, 27 extends in approximately one plane. 
As it will further become apparent from FIG. 3, the small guide vanes 26 
and the large guide vane 27 exhibit a common radius of curvature on the 
pressure side on the one hand and on the suctions side on the other, which 
applies to all axial planes concerned. 
In a further advantageous aspect of the present invention the large and 
small guide vanes are precision castings. 
In a further aspect of the present invention, the small guide vanes are DCA 
or NACA sections of small thickness-chord ratios, the DCA section being a 
double circular arc section for subsonic or transonic flows, and the NACA 
section being section series developed by NACA for mostly subsonic flows. 
The guide vanes 26, 27 forming part of the respective groups of vanes of 
the axial-flow stator cascade 4 are optionally brazed or welded to the 
members 22, 22' of the bearing shell, thus inseparably joining the two 
members 22, 22' of the bearing shell. 
In the absence of special considerations, such as extremely lightweight 
construction, the two members 22, 22' can be made as castings with the 
supply ducts 24 integrated into the castings in the form of, perhaps, 
cored passages. 
Although not shown on the drawings, the centrifugal compressor of FIGS. 1 
and 2 may be preceded by a multiple-stage axial-flow compressor driven by 
a mechanically independent turbine downstream of the compressor drive 
turbine 11, where the shaft of the second turbine is carried through the 
interior of the tubular gas generator shaft. 
Without further analysis, the foreging will so fully reveal the gist of the 
present invention that others can, by applying current knowledge, readily 
adapted for various applications without omitting features that, from the 
standpoint of prior art, fairly constitute essential characteristics of 
the generic or specific aspects of this invention, and therefore, such 
adaptations should and are intended to be comprehended within the meaning 
and range of equivalence of the following claims.