Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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25188-31
AXIAL FLOW COMPRRSSOR S~RGE MARGIN IMPROVEMENT
This invention relates to gas turbine engines and more
particularly to axial flow compressors for such engines.
An axial flow compresso~ generally comprises one or more
rotor assemblies that carry blades of aerofoil sec-tion, the rotor
assemblies are carried within a casing within which are located
stator blades~ The compressor is a multi-stage unit as the amount
of work done (pressure increase) by each stage is small, a stage
consists of a row of rotating blades followed by a row of stator
blades. The reason for the small pressure increase across each
stage is that the rate of diffusion and the deflection angle of
the blades must be limited if losses due to air breakaway of the
blades and subsequent blade stall are to be avoided.
The condition known as stall, or surge, occurs when the
smooth flow oE air through the compressor is disturbed. Although
the two terms "stall" and "surge" are often used synonymo-usly
there is a difEerence which is mainly a matter of degree. A stall
may affect only one stage or even group oE stages but a compressor
surge generally refers to a complete flow breakdown through the
compressor.
The value of airflow and pressure ratio at which a surge
occurs is termed the "surge point". This point is a
characteristic of each compressor speed, and a line which joins
all the surge points, called the surge line, defines the maximum
stable airflow which can be obtained at any ro-tational speed. A
compressor is designed to have a good safety margin (Region A)
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25188-31
between the air flow and the pressure ratlo at which it will
normally be operated (the working llne), and the airflow and
pressure ratio at which a sure will occur.
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131~
For satiqfactory operation of a compressor stage,
it is well known that it, and also the adjacent stages
of the blades, must be carefully matched as each stage
possesses its own individual airflow characteristics.
Thus it is extr~mely difficult to design a compressor to
operate satisfactorily over a wide range of operating
conditions such as an aircraft engine encounters.
Outside the design conditions, the gas flow around
the blade tends to degenerate into a violent turbulence
and the smooth pattern of flow through the stage or
stages is destroyedO The gas flow through the compres-
sor usually deteriorates and becomes a rapidly rotating
annulus of pressurised gas about th~ tips of ~one com-
pressor blade ~tage or gxoup of stagesO If a ~omplete
breakdown of flow occurs through all the stages of the
"- compressor such that all the stages of blades becomes
~stalledn the compressor will "surgen.
. ~ ,
The transition from stall to surge can be so rapid
as to be unnoticed, or on the other hand, a stall may be
so weak as to produce only slight vibration or poor
acceleration or deceleration characteristics. A more
severe compressor stall is indicated by a rise in
turbine gas temperature, and vibration or "coughing" of
the compressor. A surge is evident by a bang of varying
severity from the engine compressor and a rise in
turbine gas temperature.
3 It is necessary to use a system of airflow control
to ensure the efficient operation of an engine over a
wide speed range and to maintain the safety margin
referred to above. A well known method of control is
described in British Patent l,518,293 and consists of
providing the compressor casing of such an engine with a
circumferential row of slots inclined to the axis of
rotation of the rotor blade row and disposed within its
internal cylindrical surface adjacent to at least one
blade row. The sl~s have an axial length substantially
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greater than that of the blade row, and terminate
downstream of the blade row.
An object of the present invention is to provide a
form of compres~or casing treatment which optimises both
the geometry and position of the slot re`lative to the
blade, in order to obtain a stall marg~n-improvement
without exces~ive loss of compressor efficiency.
10Accordingly the present invention provide~ an axial
flow compressor, comprising a casing having an internal
cylindrical surface, in which is mounted a rotor
! carrying at least one row of generally radially
extending blades~ each of said blades having a leading
edge which describes an arc upon rotation of said rotor
and a trailing edge which describes an arc upon rotation
of said rotor, one or more slots disposed within the
internal cylindrical surface of the casing adjacent the
~ of at least one of said blade rows, each of -~aid
slo~s having a leading end and a trailing end,
c~aracterised in that the leading ends of the slots
extend axially upstream of the arc described by the
leading edges of the blades and the trailing ends of the
slots lie in the same plane as, or axially upstream of,
~he arc describecl by the trailing edges of the blades.
) Preferably the base surface of each inclined slot is
shaped to allow a smooth exit of high pressure fluid
from the slot.
30Additivnally each slot is disposed such that its
sidewalls are arranged at an angle to a radial line
through the centre of the casing and so extend non-
radially into the internal cylindrical surface of the
casing with respect to the rotor axis, and the angle of
inclination of the slot may be substantially equal to
the exit angle of the fluid leaving the blades.
Tests have shown an improvement in surge margin can
be obtained by altering the ratio ~f the distance
1 3 ~ 8 6
between the slots to the slot width, measured circumfer-
entially around the compressor casing. This ratio is
known as the closed to open ratio or (m/M) as shown in
Figure 4. Improvements may also be made in the surge
5 margin by alte~ing the axial position of the slot such
- that the leading edge of the slot leads the leading edge of the blade by an amount termed the overhang.
It was expected that the best overall improvement
10 in the compressor characteri~tics would be obtained by
combining the m/M ratio with the overhang which indi-
vidually provided the best surge margin improvement.
? Further test~ ~howed, however~ that this was not the
case and that in fact the best overall impro~ement in
15 the compressor characteristics was obtained by combining
the previous best known overhang with an (m~M) ratio
somewhat higher.
~ ~. The present invention will now be more particularly
20 described, by way of example only, and with reference to
t~he àccompanying drawings, in which:
Figure l shows a pictorial side elevation of a gas
turbine engine having a broken away compressor casing
25 portion di~closing a diagrammatic embodiment of the
present invention.
Figure 2 illustrates in more detail the casing
treatment shown in the broken away portion of Figure l.
3o
Figure 3 shows a view in the direction of arrows
D-D at Figure 2.
Figure 4 is a cross-sectional view of the slots in
35 the direction of arrows K-X in Figure 3.
Figure 5 is a graph of surge margin improvement
(line W) and efficiency deficit (line X) plotted against
the clo~ed to open ratio (m/M~ for a zero overhang
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casing treatment.
Figure 6 is a graph of surge margin improvement
(line Y) and efficiency deficit (line Z) plotted against
5overhang for a~.slotted casing treatment having a closed
to open ratio of 0.58.
Figure 7 is a graph of pres6ure ratio against mass
flow for a typical compxe~sor, clearly illu~trating the
10surge line, the working ].ine and the safety margin
between the two (region A). - -
, . . .
?Figures 8, 9 and 10 illustrate three alternative
shapes of slot.
Referring to Figure 1 of ~he drawings, a gas
turbine en~ine shown generally at 10 comprises in flow
serie~ a low pressure compressor 12, a high pressure
co~pressor 1~, combustion equipment 16, a high pressure
Z0 tur~ine 18, a low pressure turbine 20 and.exhaust nozzle
~~. The low pressure compressor 12 and low pressure
turbine 20, and the high pressure compressor 14 and high
pressure turbine 18 are each rotatably mounted upon a
co-axially arranged shaft assembly not shown in the
25 drawing~. A diagrammatic view of an embodiment of the
present invention i8 shown within the broken portion of
the low pressure compressor casing 24.
Figure 2 of the drawings shows a cross-sectional
30 view in greater detail of that shown diagrammatically at
Figure 1 and comprises a portion of low pressure com
pressor blade 26 having a leading edge 26(a) and a
trailing edge 26(b) on one sta~e of the low pressure
compressor 12. A compressor casing 24 is arranged
35 radially outwardly of the low pressure compressor 12, a
portion of which is shown at 28. A circumférentially
extending array of inclined slot-~, one of which is shown
at 30, are provided within the internal cylindrical
surface 32 of the compressor casing portion 28. Bach
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slot 30 has a depth B and an axial length C and is
shaped and positioned such that the leading end 30(a) of
the slot 30 extends axially upstream of the arc de~
scribed by the blade leading end 26(b).
Referring to Figure 3, the skew angle~ ~ of the
inclined slot is arranged to be substantially the same
as the gas outlet angle of the compressor blade 26. The
gas outlet angle being that angle at which the compres-
sor gas leaves the row of compressor blades, and isusually substantially 35~ Thi~ angle is obviously also
the same angle as that of the gas inlet angle of the
adjacent down~tream stator blade row Inot shown).
Dimension H defines the axlal length of the ~blade 26
measured between its leading edge 26(a3 and its trailing
edge 26(b) along an axis parallel to the centreline of
the compressor I-I.
~ As will,be seen from Figure~2 of the drawings the
bas`~ 34 of each slot 30 is substantially flat except for
t~e tralfi~g end 30(b) which is tapered at an angle
arranged to be approximately 45 to the compressor
longitudinal axis. It will be appreciated however that
alternative surfacas may be incorporated, for example,
the slots 30 may be formed with a concaved bottom
) surface or with a taper at both ends in order to effect
a smoother passage of air through the slots 3n. The
longitudinal sidewalls 36 of each slot 30 are inclined
to the radial plane as shown in Figure 4.
3o
Figure 4 of the drawings shows a cross-sectional
view taken on line KK of Figure 3. The slots 30 extend
non radially into the compressor casing 28 at an angle 0
relative to a radial axis R of the compressor 12. This
angle 0 being so arranged that the slots 30 collect
pressurised gas from the compressor blade 26. The
direction of travel of the compressor blade 26 being
indicated by arrow S. The slot closea to open ratio is
illustrated by dimensions m and M respectively.
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It has been found that the slots 30 provided within
the low pressure casing 28 can provide a degree of
control or in fact eliminate a "stall" and thus substan-
tially reduce the likelihood of "surge" occurring.
The following results are given as examples of the ''
benefit3 obtain2ble Eor a set of blades as tested.
The sl~t axial length C wa~ arranged to be equal to
the axial length ~ of the blade 26 measured at its
radially outermost portion approximately 12mm (0.47
inches). The optimum overhang A of the slot 30 was i , "
found to be equal to approximately 23% of the bladeR 26
axial length H measured at its radially outermost
portion. It is reasonable to expect similar ~enefits
will be achieved on blade~ of other dimensions 7 n which
the overhang A of the slot 2~ is similarly arranged to
be approximately equal to 23% of the blades axial
-le~th.
~,.
~ n '`à ~first,test, with a casing treatment having
zero overhang, there was found to be a definite advan-
tage in improved surge margin by reducing the closed to
open ratio (m/M) to as low a value as 0.42. This is
clearly illustrated in Figure 5 (line W). However, as
) indisated in this Figure (line X) the efficiency deficit
increases with reducing closed to open ratio. At the
best recorded closed to open ratio of 0042, giving the
maximum surge margin improvement of 63~, the deficits in
flow (not shown) and efficiency were in the region of
1.1% and 1.4~ respectively.
A second test showed that for a casing treatment
having a given m/M ratio a further benefit in surge
margin improvement was obtainable by'' altering the slot
overhang such that the leading edge of the slot leads
the leading edge of the blade. The greatest benefit was
obtained with an overhang of between 2.54mm and 4.6mm
(0.1" and 0.18n) and have a surge maxgin improvement of
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64%.
It was reasonable expected that the best overall
improvement in surge margin would be achieved by com-
bining the previously best overhang from Test 2 with thebest open to closed ratio frorn Test 1.
A third test, however, showed that this was not the
case and that the same maximum improvement in surge
margin was obtained by combining the previou~ best
overhang with a closed to open ratio ~omewhat higher
than Test 1 and that this combination gave a r~duced -i
deficit in flow and eficiency.
The advantage of the overhang is that it gives the
same (i.e. maximum) surge margin improvement at higher
- m/M value with a corresponding reduction of the flow and
efficiency deficits.
,~ .
~ The optimum combination was found to be one having
~n--m7M ra~io of 0.58 and an overhang of approximately
2.8mm (0.11 inches). Figure 6 is a graph of stall
margin improvement (line Y) and efficiency deficit (line
Z) plotted against overhang for a slotted casing treat-
ment having a closed to open ratio of 0.58. The rise in
) stall margin improvement is clearly illustrated by line
Y; there being a rapid rise in improvement between zero
and 2.5mm (0.10 inches) overhang whilst the maximum
improvement is achieved between 2.8mm (0.11 inches) and
3 4.6mm ~0.18 inches) overhang. The corresponding reduc-
tion in efficiency deficit is clearly illustrated by
line Z which has a rapid reduction in deficit between
zero overhang and 2.5mm (0.1 inches) overhang, the
minimum value being reached with an overhang between
2.54mm (0.1 inches~ and 4.6mm (0.18 inches). Region C
marked on the graph indicates the optimism performance
conditions. That is to say for a slot having a closed
to open ratio of 0.58 and an overhang of approximately
2.8mm (0.11 inches~ a stall margin improvement of 64~
. . .
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can be obtained with an efficiency deficit of just 0.3%
and a rPduction in flow (not shown) of just 1%.
Whilst there i8 no actual increase in ~tall margin
improvement be~ween the ~econd and third teqts (both
64%) the thir~ te~t has the advantage of sub~tantial
reductions in both the eficiency deficit and flow
defect over the ~econd.
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