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1
ISABE-2015-20020
Influence of Secondary Flow within Integrated Engine Inlets on the Performance and Stability of a Jet Engine
Rudolf P. M. Rademakers, Stefan Bindl, Reinhard Niehuis
University of the German Federal Armed Forces Munich, Germany
Institute of Jet Propulsion
Abstract
Engine inlet distortion can have a significant influence
on the performance, stability, and durability of a jet en-
gine. Total pressure distortion has been under investi-
gation for decades. The consideration of inlet swirl dis-
tortion is relatively rare within this field of research, but
especially the interactions between both pressure and
swirl distortion and subsequent influences on the pro-
pulsion system are not truly understood.
Direct-connect experiments were conducted at the en-
gine test bed of the Institute of Jet Propulsion. Distor-
tion screens and a delta-wing were integrated within the
engine inlet system to generate distortion patterns as
they typically occur within s-duct engine inlets. The in-
fluences of pressure, twin-swirl, and combined pres-
sure-swirl distortions on both the performance and the
stability of a jet engine were analyzed. In this paper,
performance and stability are mainly assessed by means
of specific fuel consumption and surge margin of the
low pressure compressor, respectively.
A clear linear relation between inlet distortion and both
the engineβs performance and stability is recognizable
if solely the pressure distortion is considered. Depend-
ing on the operating point a twin-swirl distortion can
have a positive or negative influence on the stability,
however, does not have a significant influence on the
performance of the propulsion system. Assessment of
combined pressure-swirl distortion is only possible to a
limited extent since dedicated distortion descriptors are
not available. An additional twin-swirl enhances the
stability margin by improving the total pressure distri-
bution within the engine intake plane. On the other
hand engine performance is not significantly influenced
by adding twin-swirl to an existing pressure distortion.
Nomenclature
Symbols
π΄πππ [β] effective cross-sectional intake area
π΄ππ΄ [Β°] angle of attack
π [π] diameter of the engineβs intake plane
οΏ½ΜοΏ½ [ / ππ
π ] corrected engine mass flow
ππΏπ [%] corrected relative spool speed (LPC)
pπ(1,21) [β] inlet-compressor pressure ratio
Ξpπ,π΄πΌπ [%] total pressure loss
ππΉπΆ [ / π
πππ ] specific fuel consumption
πππΏππΆ [%] surge margin of the LPC
Abbreviations
AIP Aerodynamic Interface Plane
AIR Aerospace Information Report
ARP Aerospace Recommended Practice
EOP Engine Operating Point
LPC Low Pressure Compressor
SAE Society of Automotive Engineers
1. Introduction
Pressure and swirl distortion typically occur within a
bent inlet of an integrated airframe-propulsion system.
Such distortions influence the performance, stability,
and durability of the propulsion system.
A lot of knowledge was gained during the last decades
with respect to inlet total pressure distortion. The SAE
Aerospace Information Report (AIR) 1419 (2013) is a
well-known document summarizing this knowledge,
which finally led to the Aerospace Recommended Prac-
tices (ARP) 1420 (2002) being universally accepted for
the assessment of inlet pressure distortion. The consid-
eration of swirl distortion is becoming more important
due to increasing demands on integrated airframe-pro-
pulsion systems. The SAE addressed this by the intro-
duction of the AIR 5686 (2010) document. It summa-
rizes the current knowledge of different kinds of swirl
distortion and their influence on the stability of the
downstream propulsion system. The interactions be-
tween both pressure and swirl distortion and the result-
ant influence on the propulsion system is not suffi-
ciently understood and accordingly indicated in the
SAE AIR 5686 (2010) as a major subject for future re-
search. Much work has yet to be conducted before an
ARP, based on the AIR 5686, can be issued.
2
Figure 1. Larzac 04 test vehicle
Engine tests with twin-swirl inlet distortion were pre-
sented by e.g. Pazur and Fottner (1991) and experi-
ments with a modified JT15D-1 engine and a distortion
screen by e.g. Lucas et al. (2014). Nevertheless, to the
authors knowledge this is the first publication present-
ing experimental investigations with a wide range of
pressure, twin-swirl, and combined pressure-swirl dis-
tortion patterns and an assessment of their influence on
both the performance and stability of a jet engine. The
results are meant to contribute to the establishment of
an ARP, which relates to the AIR5686.
2. Experimental set-up
2.1 Test vehicle
The Larzac 04 C5 jet engine (see Fig. 1) has an exten-
sive instrumentation in its two-stage low pressure com-
pressor (LPC) and is thus well-suited for investigations
regarding inlet flow distortions. The experimental set-
up is schematically displayed in Fig. 2 and described in
the following.
2.2 Airmeter
Probes to measure total (ππ1) and static pressure (π1) as
well as total temperature (ππ1) are installed in the air-
meter (see Fig. 2, no. 1) to determine the engine mass
flow. The probes are installed in a sufficient distance
upstream of the distortion generators such that up-
stream propagating flow phenomena cannot influence
the engine mass flow measurement.
2.3 Distortion generators
A housing for the installation of arbitrary distortion
screens (see Fig. 2, no. 2) is positioned about 3 β π up-
stream of the compressor system to generate a pressure
Figure 2. Experimental Test set-up
distortion. This position is adequate according to Bailey
and OβBrien (2013) since a damping or mixture of the
distortion is not expected to occur. Six different screen
configurations (see Fig. 4) were designed for the exper-
iments presented here. The screens were designed to
evoke different kinds of pressure distortions, which
typically occur in bent engine inlet configurations. Ta-
ble 1 gives more detailed information about the geom-
etry of the screens.
A delta-wing (see Fig. 2, no. 3) generates a counter ro-
tating twin-swirl distortion as it typically occurs in en-
gine inlet ducts with an s-bend in the vertical plane.
This device is installed about 2.5 β π upstream of the
compressor system. The wing has a leading edge sweep
of 60Β° and a wingspan of approximately 0.8 β π. The
angle of attack (π΄ππ΄) of the wing can be repositioned
to alter the intensity of the vortices.
Beale et al. (2002) described and recommended both
devices, which have been utilized for current experi-
mental investigations. Nevertheless, the assessment of
phenomena such as side-winds, which can turn a twin-
swirl within an s-duct into a bulk swirl, are not covered
with the described set-up. If swirl is mentioned in the
following it always concerns a twin-swirl as it is evoked
by the delta-wing.
2.4 Traversable measurement rake
A measurement rake (see Fig. 2 no. 4) is installed be-
tween the distortion generators and the compressor sys-
tem. The rake consists of eight five-hole probes in-
stalled in equal distance to each other along the rake
and moreover, the rake can be displaced in radial and
circumferential direction. Hence, flow data can be ob-
tained for a large distribution of measurement positions
by displacing the rake. This measurement plane is po-
sitioned about 1.0 β π upstream of the LPC and is called
Aerodynamic Interface Plane (AIP) in the following.
3
(1)
Figure 3. Positioning of the five-hole probes
The traversing procedure for an optimal pressure as
well as swirl distortion evaluation was defined using an
in-house tool. It was decided to measure 144 positions
within the AIP as it is schematically shown in Fig. 3.
According to Rademakers et al. (2014) this results in
acceptable errors for the evaluated distortion de-
scriptors while the engine operating time is limited to a
minimum.
2.5 Engine instrumentation
Extensive instrumentation is integrated in the Larzac 04
test vehicle. In the following it is solely the instrumen-
tation within the LPC (see Fig. 2 no. 5) outlined, which
was applied during current investigations.
Three vanes within the first stator stage and two vanes
within the first row of the second stator stage are
equipped with pitot probes. The probes are integrated
in the leading edge of the stator (seven probes at the
vanes of the first stator stage and five probes at the
vanes of the first row of the second stator) and tangen-
tially aligned with the chord of the statorβs profile at the
leading edge. The position of the probes in radial direc-
tion was set in such way that they cover circular rings
with equal areas. A rake with six pitot probes is in-
stalled behind the second row of the second stator stage
for the determination of the total pressure at the exit of
the LPC (ππ21). The latter can be resolved with a single
rake. This was assured after a comparison of pressure
measurements with the stator-instrumentation and the
ππ21-probe, which proved that any inlet distortion is
evenly spread over the cross-sectional area of the sec-
ond stator stage.
2.6 Bypass nozzle aperture
The unmixed nozzle configuration is an important fea-
ture of the Larzac 04 jet engine for the investigations
presented here. It enables an independent throttling of
both the core and the bypass flow. The bypass nozzle
aperture (see Fig. 2 no. 6) can reduce the cross-sectional
area of the nozzle till 10% of the design area to force
the LPC into stall. In case that surge occurs an emer-
gency shut-off can open the throttling device instantly.
Further details of the controlling and operating mode of
this bypass throttle are specified by HΓΆss et al. (1998).
3. Definitions
3.1 Distortion descriptors
The pressure loss coefficient
βππ,π΄πΌπ = (ππ1 β ππ,π΄πΌπΜ Μ Μ Μ Μ Μ Μ
ππ1
) β 100%
is used to illustrate the distortion patterns. It indicates
the percentage of surface averaged total pressure loss
within the AIP related to the total pressure being meas-
ured in the airmeter. This parameter is often used for a
preliminary characterization of an inlet flow distortion.
A broad spectrum of distortion descriptors can be found
in the open literature. The descriptors being described
in SAE ARP 1420 (2002) are well-established for the
evaluation of pressure distortion and their robustness
has been verified during numerous experimental inves-
tigations, which are summarized in SAE AIR 1419
(2013).
Bouldin and Sheoran (2002) proposed descriptors for
the evaluation of a pure swirl distortion. They use a
similar approach as it was used for the pressure distor-
tion descriptors in the SAE ARP1420. These swirl de-
scriptors are applicable to characterize a wide range of
swirl patterns.
Nevertheless, none of the previously mentioned para-
meters is truly dedicated to evaluate the flow within
highly bent engine inlet systems because complex in-
teractions between pressure and swirl distortion occur
in such ducts. It is not sufficient to analyze both pres-
sure and swirl distortion separately. In the AIR5686
document the SAE calls on the industry to deepen
knowledge for the establishment of an ARP with re-
spect to swirl and pressure-swirl distortion assessment.
4
(2)
(4)
(5)
(3)
Case Screen
size
Screen
blockage
π¨πππ
[-]
π¨πΆπ¨
[Β°]
Sym-
bol
0 n/a n/a 1 0Β°
0s1 n/a n/a 1 12Β°
0s2 n/a n/a 1 24Β°
1 120Β° (r) 42% 0.895 0Β°
2 80Β° (r) 63% 0.895 0Β°
3 135Β° 42% 0.843 0Β°
3s1 135Β° 42% 0.843 12Β°
4 120Β° (r) 63% 0.843 0Β°
4s1 120Β° (r) 63% 0.843 12Β°
5 90Β° 63% 0.843 0Β°
5s1 90Β° 63% 0.843 12Β°
5s1 90Β° 63% 0.843 24Β°
6 135Β° 63% 0.764 0Β°
6s1 135Β° 63% 0.764 12Β°
6s2 135Β° 63% 0.764 24Β° Table 1. Overview of all test cases
With the investigations presented in this paper it is
meant to contribute to the establishment of an ARP,
which is associated with to the AIR5686.
3.2 Engine parameters
Direct-connect experiments with a jet engine enable an
assessment of the engineβs behaviour without putting
the focus on solely the distortion pattern. Several pa-
rameters are introduced to evaluate both stability and
performance of the test vehicle in the following.
The determined engine mass flow is corrected with the
inlet conditions in the airmeter and furthermore the
pressure and temperature at International Standard At-
mosphere conditions. In the following this corrected
engine mass flow
οΏ½ΜοΏ½πππππππ‘ππ = οΏ½ΜοΏ½ππππ π’πππ β βππ1
ππ1
βππΌππ΄
βππΌππ΄
is denoted as οΏ½ΜοΏ½ in the following.
The inlet-compressor pressure ratio
ππ(1,21) = ππ21
ππ1
puts the pressure downstream of the LPC (ππ,21) in re-
lation to ππ1 and thus considers the inlet and the LPC
Figure 4. Distortion screens
as a single system to enable a comparison of all test
cases in a single compressor performance map (see e.g.
Fig. 6 or 7). The respective inlet-compressor system is
indicated with a black dashed box in Fig. 2.
The surge margin
πππΏππΆ =
β[(ππ(1,21))ππ β (ππ(1,21))ππΏ]2
+ [οΏ½ΜοΏ½ππΏ β οΏ½ΜοΏ½ππ]2
describes the stability of the LPC as function of both
ππ(1,21) and οΏ½ΜοΏ½ at both the operating line (OL) and the
surge margin (SM). This is best practice since the in-
vestigated compressor maps encompass relatively flat
speed lines (see e.g. Fig. 6 or 7) and secondly, differing
gradients of the speed lines are included in the perfor-
mance assessment. The surge margin of the reference
case (Case 0) is set to πππΏππΆ = 100% and all other test
cases were set in relation to this reference case.
The specific fuel consumption
ππΉπΆ = οΏ½ΜοΏ½ππ’ππ
πΉπβππ’π π‘
is applied to evaluate the efficiency of the entire pro-
pulsion system.
4. Test cases
In Tab. 1 all 15 test cases under consideration here are
summarized. For the reference case (Case 0) a distor-
tion screen was not installed and the delta-wing was set
to π΄ππ΄ = 0Β°. Six different screens were designed to
5
Figure 5. Pressure distortion patterns within the
AIP for all test cases at NLc = 90%
simulate distortion patterns as they are likely to occur
in highly bent engine inlet ducts. The screens are sche-
matically displayed in Fig. 4 and geometrical properties
are summarized in Tab. 1. The screens in Cases 1, 2,
Figure 6. Pressure distortion configurations within
the LPC performance map at π΅π³π = ππ%
Figure 7. Pressure distortion configurations within
the LPC performance map at π΅π³π = ππ%
and 4 cover a reduced area in radial direction, which is
denoted with (r) in Tab. 1. The delta-wing was utilized
to add a twin-swirl to the inlet flow. According to
Genssler et al. (1987) this delta-wing at π΄ππ΄ = 12Β°
evokes a moderate twin-swirl representing the swirl
within an s-duct with modest bends. A strong swirl rep-
resenting the upper maximum of swirl distortion, which
could occur in a highly bent s-duct is simulated by the
delta-wing at π΄ππ΄ = 24Β°.
Experiments were conducted at the spool speed settings
ππΏπ = 76% and 90%. ππΏπ = 90% is the design oper-
ating point of the LPC as it was depicted by StΓΆΓel et
al. (2013) and thus this engine operating point (EOP)
was preferred for current investigations instead of the
engineβs full thrust setting.
6
Figure 8. Compressor stability with pressure
distortion at π΅π³π = ππ%
Figure 9. Compressor stability with pressure
distortion at π΅π³π = ππ%
5. Results
5.1 Visualization of the test cases
The distortion pattern of each test case at ππΏπ = 90%
is displayed in Fig. 5 by means of the pressure loss co-
efficient. The distortion patterns do not change qualita-
tively over the operating range of the test vehicle as it
was reported by Rademakers et al. (2015).
Seven plots on the left hand side display all cases with
the delta-wing set to π΄ππ΄ = 0Β°. A slight pressure dis-
tortion due to the installation of the delta-wing is still
visible in the center of the AIP. For five cases (shown
in middle of Fig. 5) the delta-wing at π΄ππ΄ = 12Β° gen-
erates a moderate twin-swirl distortion, which super-
poses the pressure distortion. These cases are indicated
with the designation βs1β. For three test cases shown on
the right hand side the delta-wing was set to π΄ππ΄ =
24Β°. Latter cases are labelled with βs2β.
5.2 Influence of pressure distortion on the stability
and the performance of a jet engine
This subsection only evaluates the influence of pressure
distortion and thus all test cases with the designation
βs1β and βs2β are not considered. A first visualization
of the influences of all pressure distortion patterns on
the stability and performance of the LPC at ππΏπ =
76% and 90% is shown in the performance maps in
Fig. 6 and 7, respectively.
The colored speed lines depict the time-variant unfil-
tered data during throttling of the bypass nozzle while
the spool speed of the LPC was kept constant. The sym-
bols indicate the new surge line for the respective case.
The influence of pressure distortion on both the pres-
sure ratio and the surge margin in all six cases is clearly
visible for both EOPs. The operating point ππΏπ = 90%
is the design point of the compressor system and for that
reason more sensitive to inlet distortion. Hence, the
speed lines are spread over a larger area within the map.
Stability and performance are evaluated individually in
the following by means of πππΏππΆ and ππΉπΆ, respec-
tively, for a proper assessment of both characteristics.
5.2.1 Engine stability assessment
Figure 8 and 9 show the πππΏππΆ as function of the mean
pressure distortion within the AIP for seven test cases
at ππΏπ = 76% and 90%, respectively.
There is a linear relation recognizable between both pa-
rameters. The same relation is distinguishable if other
pressure distortion descriptors (e.g. the descriptors
from the SAE ARP 1420) are applied for assessment,
see Rademakers et al. (2015). This states the robustness
of those distortion descriptors and enables the para-
metrization of pressure distortion patterns occurring
within s-shaped engine inlets.
While assessing both Fig. 8 and 9 it can be noticed that
the Cases 1, 2, and 4 gain in surge margin relatively to
the other cases. These test cases cover distortion pat-
terns within the tip area. At its design point the blading
near the hub of the LPC seems to be most sensitive to
inlet pressure distortion.
SM
LP
C [%
] S
ML
PC
[%
]
ΞpT,AIP,mean [%]
ΞpT,AIP,mean [%]
7
Figure 10. Engine performance with pressure
distortion at NLc = 76%
Figure 11. Engine performance with pressure
distortion at NLc = 90%
5.2.2 Engine performance assessment
Figure 10 and 11 show the ππΉπΆ as function of the mean
pressure distortion within the AIP for seven test set-ups
at ππΏπ = 76% and 90%, respectively. The linear rela-
tion between the jet engine performance and the distor-
tion enables a general parametrization of pressure dis-
tortion, especially because the linear relation remains
unchanged if other distortion descriptors are used for
assessment. The latter is not shown in separate graphs
for the sake of clarity. Instead it is referred to Rademak-
ers et al. (2015) for the interested reader.
As a conclusion it can be stated that the stability and
performance of a jet engine, which is exposed solely to
an inlet pressure distortion, can be predicted well as
long as the upstream total pressure distortion pattern is
known in advance.
Figure 12. Combined pressure-swirl distortion
configurations within the LPC performance map at
π΅π³π = ππ%
Figure 13. Combined pressure-swirl distortion
configurations within the LPC performance map at
π΅π³π = ππ%
5.3 Influence of swirl and combined pressure-swirl
distortion on the stability and the performance of a
jet engine
A dedicated coefficient to assess the results regarding
combined pressure-swirl distortion is not available.
Bouldin and Sheoran (2002) proposed descriptors for
the evaluation of swirl distortion and the SAE AIR
5686 document describes a procedure to combine sev-
eral pressure and swirl descriptors. However, every sin-
gle coefficient within this combined pressure-swirl dis-
tortion descriptor has to be weighted with an additional
sensitivity coefficient. These sensitivity coefficients are
not yet generally defined.
For a first visualization of the test cases with combined
pressure-swirl distortion the speed lines of five cases
ΞpT,AIP,mean [%]
ΞpT,AIP,mean [%]
8
(6)
are shown in Fig. 12 and 13 at ππΏπ = 76% and 90%,
respectively, in the compressor performance map.
Both Cases 3 and 6 (moderate and severe pressure dis-
tortion, respectively) were already shown in Fig. 6 and
7. An additional moderate swirl distortion (Case 3-s1)
does not displace the speed line of Case 3 into a lower
region of the compressor performance map, however,
enhances the surge margin for both investigated spool
speed settings. In the case of a severe pressure distor-
tion (Case 6) the speed line shifts into a lower region of
the LPC map with an additional swirl distortion. The
surge margin is on the other hand significantly ex-
tended, especially at ππΏπ = 90%. It seems that an ad-
ditional swirl distortion mainly influences the stability
of the compressor system in a positive way for both
cases with moderate and severe swirl.
The enhancement of the surge margin is mainly ex-
plained by the position of the distortion generators
within the engine inlet of the test set-up. The delta-wing
is positioned downstream of the distortion screen. The
vortices induced by the delta-wing spread the pressure
distortion over the AIP as it is apparent in the plots of
Fig. 5. A spreading of the pressure distortion pattern has
a greater impact than the negative effect of an addi-
tional swirl distortion by means of reducing the surge
margin. The other test cases with combined pressure-
swirl distortion are not shown for the sake of clarity.
Cases 4 and 4-s1 show the same characteristics as
Cases 3 and 3-s1. Cases 5, 5-s1, and 5-s2 are further-
more comparable to Cases 6, 6-s1, and 6-s, respectively.
For a proper performance and stability assessment both
characteristics are evaluated individually in the follow-
ing subsections.
5.3.1 Engine stability assessment
In the case of a combined pressure-swirl distortion it is
not adequate to assess the influence of both phenomena
separately. The interactions between both flow distor-
tions have to be considered as it is described in the SAE
AIR 5686 documents (see Eq. 6) meaning that the surge
margin is influenced by the pressure distortion (βπππ),
by the swirl distortion (βπππ), and the interactions be-
tween both pressure and swirl distortion (βπππ+π).
βππ = βπππ + βπππ + βπππ+π
Figure 14. Stability of the compressor system with
pressure, swirl, and combined pressure-swirl
distortion at π΅π³π = ππ%
Figure 15. Stability of the compressor system with
pressure, swirl, and combined pressure-swirl
distortion at π΅π³π = ππ%
The LPC surge margin for all test cases is depicted in
Fig. 14 and 15 for both investigated spool speed set-
tings. There is a strong relation between an increasing
pressure distortion and a degrading stability margin as
already shown in Fig. 8 and 9. The coefficient βπππ
within Eq. 6 is thus always negative and relatively easy
to determine.
According to the numerical investigations by Davis et
al. (2008) and Sheoran et al. (2012) the surge margin is
also reduced if it is solely the twin-swirl being consid-
ered. This suggests that βπππ in Eq. 6 is negative as
well. This agrees with the test results for ππΏπ = 90%
(Fig. 15), however, for the reduced spool speed setting
in Fig. 14 the surge margin is barely decreased by a
moderate twin-swirl distortion (Case 0-s1) and even en-
SM
LP
C [%
] S
ML
PC
[%
]
9
(7)
Figure 16. Performance of the engine with a
combined pressure-swirl distortion at π΅π³π = ππ%
Figure 17. Performance of the engine with a
combined pressure-swirl distortion at π΅π³π = ππ%
hanced in the case of a severe twin-swirl distortion
(Case 0-s2).
The surge margin increases in all test cases where an
initial pressure distortion is superposed with a swirl dis-
tortion, which yields a positive βπππ+π coefficient. In
some of the cases with a reduced spool speed the
βπππ+π even repeals the negative influence of both
βπππ and βπππ (see Case 3-s1, Case 5-s2, and Case
6-s2).
Analyzing the influence of swirl distortion and espe-
cially the interactions between pressure and swirl dis-
tortion within an engine inlet on degrading stability
margin is extremely complicated. Once again it is noted
that during the current work only twin-swirl has been
considered. Other types of swirl should be considered
for a completion of the SAE AIR 5686 and a future es-
tablishment of a corresponding ARP.
Current investigations also show potential advantages
of passive flow control in s-duct inlets. A negative ef-
fect of an additional swirl distortion is more than com-
pensated by an improved distribution of pressure losses
within the AIP for the cases investigated here. The ad-
vantages of passive flow control devices were shown
by e.g. Owens et al. (2006) and Tournier et al. (2006)
during wind-tunnels tests. The real potential of passive
flow control in a modern engine inlet can only be deter-
mined by experimental investigation with a jet engine.
5.3.2 Engine performance assessment
The influence of swirl as well as combined pressure-
swirl distortion on the performance of the propulsion
system is not particularly considered in the SAE AIR
5686 document. For an optimized inlet-propulsion sys-
tem, however, the influence on the performance has to
be considered. In this paper the performance of the en-
tire propulsion system is assessed by means of the ππΉπΆ.
At first it is distinguished between the influence of pres-
sure, swirl, and the interaction between both types of
distortion as shown in Eq. 7 similar to the approach in
the preceding subsection.
βππΉπΆ = βππΉπΆπ + βππΉπΆπ + βππΉπΆπ+π
The ππΉπΆ as function of the mean pressure loss in the
AIP for all test cases is depicted in Fig. 16 and 17 for
ππΏπ = 76% and 90%, respectively. There is a strong
relationship between an increasing pressure distortion
and a degrading stability margin as it was already
shown in Fig. 10 and 11. The coefficient βππΉπΆπ within
Eq. 7 is hence negative and relatively straightforward
to identify. A strong swirl distortion (Case 0-s2) results
in an increased ππΉπΆ. It is not reasonable to consider this
case in both Fig. 16 and 17, since there was no signifi-
cant pressure distortion and thus βππ,π΄πΌπ,ππππ cannot be
used for an assessment here.
The same linear relationship between the ππΉπΆ and the
distortion exists if all test cases βs1β and βs2β are in-
cluded in the same graph. It seems that the additional
swirl distortion itself does not have any influence on the
performance of the propulsion system. In fact, the ad-
ditional swirl distortion changes the pressure distortion
ΞpT,AIP,mean [%]
ΞpT,AIP,mean [%]
10
pattern, which again has an influence on the perfor-
mance. This perceptions yields that that βππΉπΆ β
βππΉπΆπ and thus |βππΉπΆπ| + |βππΉπΆπ+π| β 0.
6. Conclusions
Great efforts were made in the last decades to assess the
influences of pressure distortion within engine-inlet
systems. However, the effects of twin-swirl distortion
on a propulsion system are only roughly known and not
generally parametrized. Especially the understanding
of interactions between pressure and swirl distortion
and the subsequent influence on the stability and per-
formance of a propulsion system is extremely limited.
In this paper pressure, twin-swirl, and combined pres-
sure-swirl distortions have been investigated experi-
mentally with the Larzac 04 jet engine. The distortion
patterns were chosen to represent flow distortions as
they typically occur within s-duct engine inlet systems.
The following conclusions were drawn from the exper-
imental results:
1) There is a clear linear relationship between s-duct
type pressure distortion and both the stability (πππΏππΆ)
and performance (ππΉπΆ) of a jet engine.
2) If it is solely twin-swirl being considered, the inlet
distortion can have a negative as well as a positive in-
fluence on the surge margin of the compressor system.
At the design point of the compressor system a twin-
swirl degrades the surge margin.
3) Is a pressure distortion superposed with a twin-swirl
distortion the surge margin is enhanced due to an redis-
tribution of the total pressure distribution in the AIP.
This indicates the major influence of pressure distortion
compared to twin-swirl by means of surge margin re-
duction and furthermore, that a separate assessment of
both phenomena is not sufficient in case of a combined
pressure-swirl distortion.
4) A twin-swirl distortion decreases the efficiency of
the compressor system and subsequently decreases the
ππΉπΆ of the propulsion system. The performance is not
significantly influenced if a pressure distortion is super-
posed with a moderate twin-swirl distortion. An addi-
tional severe twin-swirl distortion does have an influ-
ence, but only because it changes the total pressure dis-
tortion pattern.
5) Pressure distortion is the dominating factor by means
of both engine stability and performance as long as the
swirl angles are moderate. From this it can be con-
cluded that the design focus of passive flow control de-
vices should be a reduction of critical pressure distor-
tion within the AIP and not the reduction of secondary
flow phenomena. Additional twin-swirl can be ac-
cepted when the alteration of the pressure distortion
pattern is positive. The real potential of passive flow
control in a modern engine inlet, however, can only be
determined within a dedicated testing set-up.
7. Acknowledgement
The authors would like to express their appreciation to
the members of the instituteβs technical staff Heinz
Hampel, Georg KΓΆttner, and Waldi Weigel. Their ef-
forts for the preparation of the experimental test set-up
and conducting of engine tests at the engine test facility
are acknowledged.
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