Article citation information:
Laskowski P. Factors influencing axial
compressor stalls. Scientific Journal of
Silesian University of Technology. Series Transport. 2017, 95, 89-96. ISSN: 0209-3324.
DOI: https://doi.org/10.20858/sjsutst.2017.95.9.
Piotr LASKOWSKI[1]
FACTORS INFLUENCING AXIAL
COMPRESSOR STALLS
Summary. The
article describes the problem regarding stalls in axial compressors, which are
widely used in the engines of large and very large transportation aircrafts.
The stall phenomenon, unless cleared, poses a major danger to the integrity of
a compressor module, which in turn may result in uncontained engine failure and
loss of thrust, thereby seriously affecting flight safety. The costs of
maintenance to be performed may also be very high, due to irreversible
deterioration in some components. A practical assessment of the stall problem
and its countermeasures are discussed below.
Keywords: stall; axial compressor; performance.
1. Introduction
Modern transportation
aircraft are powered by dual- or triple-spool gas turbine engines. The purpose
of these engines is to deliver thrust or power to the propeller. As such,
turbine engines are divided into direct and indirect reaction engines. Direct reaction
fully ducted engines produce the thrust force by the significant acceleration
of stream flowing through them. To produce a mixture of fuel and air, air must
be first compressed. The pressure outlet to pressure inlet value is called the
compression ratio. To achieve high compression ratio values, which exceed 30 on
modern engines, multiple compressor stages are needed, given that the
single-stage axial compression ratio is 1.3...1.5. Radial compressors achieve
higher compression ratio values per stage and are more adaptive to harsh
operating conditions, which is an important feature when considering foreign
object damage resistance. Meanwhile, one disadvantage of this type of
compressor is that it is composed of no more than two stages, which restricts airflow.
Therefore, an axial design is used on fully ducted engines to enable high
compression ratio values. This, however, requires hundreds of kilograms per
second of the airflow, which in turn requires regulation to keep the airflow
stable. The more that air flows through the engine duct(s), the more that
regulating methods must be applied. The article below describes other factors
that influence the airflow and methods of flow regulation to avoid a stall in
the whole compressor working map.
A stall is an
unintentional and gas-dynamically unstable condition, caused by airflow
disturbance. This disturbance is not necessarily abrupt. A stall poses a danger
to safe engine operations, since it may cause engine damage, including
uncontained disintegration. Most common stall symptoms are sudden and wide
fluctuations in pressure, rotors’ rpm, exhaust gas temperature and airflow.
Fully ducted engines with a changeable exhaust nozzle throat experience
fluctuations in the convergent nozzle area as the first sign of a stall
occurrence.
This is caused by the
low-pressure compressor rotational speed and engine pressure ratio variation.
The convergent nozzle reacts directly to airflow changes, particularly to
bypass flow. Most of the thrust produced by a turbofan engine is delivered by
the bypass duct. Both high and low pressure rises in the compressors and
decreases in turbines, as a result of dependence on stream flow changes.
Therefore, a stall also means that these relations have been disturbed.
Fig. 1. Compressor map
2. STALL OCCURRENCE ORIGIN
In order to achieve a high
compression ratio, the axial compressor consists of several (up to 17) stages,
with the compression ratio at each stage being no more than 1.5. In modern
turbofan engines, the compression ratio of the engine reaches 52 (Trent 1000).
When producing such a pressure increase in a compressor, the main challenge is
to ensure its safe operation with an appropriate stall margin, alongside the
high compression ratio itself. Each stage of the axial compressor consists of
one rotor stage and one stator stage. Both the rotor and stator form a
divergent channel to enable a pressure rise; however, the compressor duct is
generally convergent in order to maintain the required axial velocity of the
airflow. The turbine delivers power to the rotor stages of the compressor,
which converts kinetic energy into potential energy, thereby decreasing volume
and axial velocity. In turn, a stall may occur due to a change in airflow
parameters, such as mass airflow, pressure, temperature or power delivered from
the turbine in the form of the rotational movement of the rotor.
With the fall in rpm, the first
compressor stages deliver the uncompressed stream to the latter stages. The air
volume and axial velocity exceed limits, which, alongside a decrease in the
cross section of the compressor duct, causes clogging of the latter stages. The
stream of air flows to the airfoils of blades and vanes at higher and higher
angles of incidence. When the critical value of the angle is exceeded, the
stream separates from the airfoil, which protrudes downstream, while air from
the latter stages moves back through those areas of lower pressure in a chaotic
way. When this phenomenon propagates through the adjoining stages, while
applied regulation methods fail to restore stabilized airflow, this situation
is referred to as stagnation.
The
velocity of the stream is tangential to the camber line of the airfoil under
optimum working conditions. Stalling conditions (i.e., when the airflow
separates from the airfoil) may occur both on its concave and its convex sides.
A stall on the convex side is more dangerous to the compressor and engine
operation, being more difficult to restore. In the latter stages of the compressor,
the stream may flow into airfoil at negative angles, which results in
separation on the concave side. However, this neither causes it to stall, nor
protrude through the compressor because the separation is neutralized by the
movement of the rotating airfoils.
Since the stall is caused by
inappropriate operations by the compressor, the likelihood of stall occurrence
increases with the compression ratio, in terms of its scope and rate of change.
The likelihood of stall is influenced by design parameters, the most
significant of which are as follows:
- Compressor’s sizes and
weight of its elements
- Angle of incidence, angle of
discharge of the blade, and their variation along the airfoil
- Axial and radial variation
in the parameters
- Minimum and maximum air
discharge of the stage
Rotor
speed, engine pressure ratio and mass airflow, as well as their mutual
relations, directly influence the stable compressor’s workings. The higher the
rotor speeds, the faster the axial velocity decreases, which practically means
that more compressor regulation methods should be applied, given the wide scope
of the rotor speed and engine pressure ratio. Furthermore, the applied methods
should be more precise.
Stall
occurrence is also influenced by the compressor’s aerodynamic characteristics,
as well as additionally inhibited by the engine inlet (with its equipment), the
volume of the combustion chamber and the engine exhaust nozzle surface.
3. STALL CAUSES
A stall is
initiated by one of the events specified below.
3.1. Uncontrolled
change in rotor speed
This phenomenon changes the angle of
incidence between the stream and the airfoil. The rpm fall causes the axial
velocity decrease, but in uncontrolled way since, in the first stages, it falls
more rapidly than in the latter stages. Therefore, the angle of incidence in
the first stages increases faster than in the latter stages in order to reach
critical value. As such, a stall takes place in the first stages during
deceleration and in the latter stage during acceleration in the compressor.
3.2. Uncontrolled
change in airflow
This may be the result of the
unintended opening or closing of the compressor bleed, particularly during its
latter stages. Air from these stages is used in systems where high pressure and
high temperature are needed. Such systems include the following: anti-ice,
environmental control or exhaust nozzle steering/control. The temperature of
the compressor outflow air reaches 500°C, while its pressure is a dozen times
higher than the ambient pressure. Such values impose stringent conditions on
compressor regulation. Under specific engine operating conditions, such as
engine start or take-off, bleed is restrained. However, bleed-off operations
enable maximum thrust and reduce the stall margin.
3.3. Temperature
and/or pressure change at the compressor inlet
A temperature rise results in an
axial velocity fall, causing a tendency towards increasing the angle of
incidence. This is true, when assuming a constant rotor speed; however, the
engine control system is designed to adjust the rotor operation to variable
ambient conditions. Rotor speed rises under higher temperatures, since more
power is required from the turbine to achieve the expected compression ratio
and adequate combustion. The rotor speed may vary up to 10% rpm in conditions
between -30 and +30°C at the same engine operating level. Due to variable and
various operation conditions, the engine control system uses rotor speed
signals corrected to the inlet temperature.
3.4. Fuel flow
scheduling and duct contamination
Contaminated compressor duct
elements have a minor influence on stall occurrence. But, as contamination has
more adverse effects on specific fuel consumption rises, periodical engine
cleaning is necessary to reduce the risk of stalls.
Stalls may be caused by an abrupt
change in fuel flow scheduling, particularly in the fuel governor, which
directly regulates the fuel pressure in fuel nozzles. Another important factor
is when an engine operates below or above the scheduled minimum or maximum
level. This may cause unpredicted effects, which exceed the compressor
operating diagram.
3.5. Influence of the
engine control system on safe compressor operations
This system controls not only fuel
scheduling, but also systems and components, such as variable guide vanes or
the exhaust nozzle. A stall may be caused by inappropriate parameter signals
(command or feedback) or false conditions while computing command signals. The
most important control signal with an influence on stall margin are rotor(s)
speed(s), compressor inlet pressure (station 2), exhaust nozzle inlet pressure
(station 6, on turbofan engines with a common exhaust nozzle), compressor inlet
temperature, low-pressure turbine inlet temperature and gas generator fuel
flow.
4. STALL SYMPTOMS
Stall symptoms may include one or
more of the phenomena listed below:
- Low- and/or high-pressure
rotor speed fluctuation - This is a reaction of the rotor to a chaotically
changing airflow.
- Low-pressure turbine inlet temperature
and/or exhaust gas temperature fluctuation with a strong tendency to increase
rapidly - A decrease in compressor outlet pressure will cause an increase in
turbine workload, caused by a rich fuel mixture, and therefore a rise in
temperature. This relation is primary. The turbine’s direct reaction will occur
sooner than the adjusted fuel flow scheduling by the fuel governor, which may
mean that it is not practically possible to respond to such abrupt variations
in stream flow. Furthermore, due to the compressor stalling, less air is
directed to the turbine(s) for cooling purposes because the compressor bleed is
most often the source of cooling air.
- Abrupt change in the exhaust
nozzle area - If the engine is equipped with an exhaust convergent nozzle with
a variable area, unstable area fluctuations will be the first observed symptom
of a stall by the engine operator, since this is a direct regulator of the
bypass airflow, which reacts immediately to low-pressure rotor speed changes
and fan air discharge.
- Rumbling
- Strong vibrations
- “Bang” – This is caused by
an intermittent air flow in the forward direction.
5. CONCLUSIONS
To ensure
the compressor’s safe operation with a sufficient stall margin, the following
systems should be used.
5.1. Variable stator
vanes
Variable stator vanes are positioned
upstream of the stage, where stalling is most possible due to the direct stream
at the optimum angle of incidence in relation to the following rotor stage at a
low rotor speed. During low rotor speed operations, the value of the angle is
no more than -30° for high-pressure compressor variable vanes, and
approximately -20° for low-pressure compressor variable vanes. This position is
called the cambered position. With rotor speed acceleration, slightly above idle
speed, the vanes’ axial position starts to change. Before maximum speed is
achieved, they reach the final “axial” position, which is in fact up to 50°
above the cambered position. Variable stator vanes are operated by a fuel
subsystem, but controlled by an engine control system. Typically, in a
low-pressure rotor, only inlet guide vanes are variable (actually, inlet guide
vanes are part of a fan module, but not the fan itself), while the amount of
regulated stages in a high-pressure compressor is at least two or three, or
even six or seven in older designs; this is also the case in which other
regulating systems are not applied or not enough. More stages are regulated in
high-pressure compressors because of their bigger influence on the stall
margin. Furthermore, variable stator vanes operate actively while accelerating,
decelerating and performing other specific flight phases to increase the stall
margin and enable smooth airflow adjustment, before the fuel flow schedule is
regulated. For example, when the throttle is rapidly retarded from the maximum
detent to the idle position, this feature will “camber” vanes to allow a
high-speed rotor to maintain high rpm, thereby avoiding abrupt changes in
airflow. This enables quick acceleration and deceleration. Only the first or
final stages of each compressor are regulated.
5.2. Variable inlet
guide vanes
Inlet guide vanes serve an identical
purpose to that described above. But, contrary to other variable vanes, they
may form a convergent duct. This will not cause a pressure increase, but will
allow an undisturbed stream to form, which is directed to the first-stage fan
rotor blades. Given the bigger values of stream velocities at the inlet, inlet
guide vanes may have variable arms attached. Like other compressor variables,
they are operated by a fuel-fed subsystem, which controls servomechanisms.
Servomechanisms receive a command signal from the engine control system,
sending out feedback on the current position. In modern designs, these signals
are electrical, and sent and received by a “full authority digital engine
control” system.
Contrary to high-pressure compressor
variable vanes, inlet variable vanes can only offer a dual position, which is
cambered from start-up, through idle, and then mild transition, starting from
above-idle speed.
The moment of position change
depends on the low-pressure compressor rotor speed, which is corrected with
regard to the compressor inlet temperature. Above the mid-range power setting,
when the position of the vanes is axially fixed, it is not adjusted until
spool-down time.
5.3. Exhaust nozzle
area control
It is necessary to control the area
of the exhaust nozzle if the engine is equipped with an afterburner. This
control only has an effect on engines with a chocked nozzle. The controlled
area is the end of the convergent section of the exhaust, while the divergent
section may remain as free-floating. Nozzle steering is not used on civilian
engines, since the velocity of the exhaust stream does not reach the sound
barrier level.
5.4. Compressor bleeds
Compressor bleeds, as stall
prevention systems, are used on various power settings, especially under
transient conditions. The air taken away from the compressor is regulated, depending
on the engine operating level, the flight envelope, the bleed interfaces
requests etc. The bleed is reduced while the engine is running at maximum rotor
speed. The bleed allows for the removal of “unwanted” (uncompressed) air in the
stages after the stall occurs, thereby re-establishing the pressure ratio
across the stage, as well as “requiring” more outflow from the preceding stage
and an increase in axial speed. During deceleration, for example, with a fall
in rotor speed, uncompressed air gathers at the latter stages of the
compressor, disturbing the airflow. The removal of air, in this case, allows
for the axial velocity to increase, which rectifies the stall phenomenon and
prevents stagnation. Bleeds from the compressors require more power delivery
from the turbine in order to realize the scheduled compression ratio, which
results in higher fuel consumption and higher turbine temperatures. Since
bleeds are considered as a loss in thermal cycles, efforts are needed to
replace them with another source of hot and high-pressure air. Compressor bleeds
are opened during the engine start to allow for smooth acceleration and ensure
a sufficient stall margin, as well as removing excess compressed air from
specific stages.
5.5. Multirotor
designs and active clearance control
Since the 1960s, dual-spool
constructions have superseded single-spool designs. The original reason for
this was to reduce specific fuel consumption, but it also resulted in increased
stall margins, as well as rotors spool with different speeds that were not
connected mechanically. This also led to higher compression ratios in modern
engines, and a reduction in the number of compressor stages and overall engine
length. To achieve high compression in single-spool aircraft, more and more
compressor stages were required, resulting in aerodynamic problems that
involved the mutual relation between axial and rotational velocities.
Indeed, the rotational speed of
low-pressure spools causes changes to a greater extent than high-pressure spool
changes during engine operations, which in turn facilitate quick and adequate
air delivery to the gas generator. For example, during deceleration, a
low-pressure compressor spools down more quickly than a high-pressure
compressor, which prevents air build-up during subsequent stages. The major
disadvantage of multi-spool designs is the requirement for more bearings, when
compared to single-spool designs. This means that the former designs are more
complex. This however has not stopped engine manufacturers from designing
triple-spool or free turbine constructions.
In terms of active clearance
control, radial clearance minimization not only increases compressor
efficiency, but also enables the adequate control of airflow throughout the
rotor stage.
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Received 13.02.2017; accepted in revised form 01.05.2017
Scientific Journal of Silesian
University of Technology. Series Transport is licensed under a Creative
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