Research
on the Rectangular
Lobed
Exhaust
Ejector /Mixer
By Hui HU,*2 Toshio KOBAYASHI,*2Tetsuo SAGA,*2 Nobuyuki
Huoxing LIU*3 and Shousheng WU*3
Key Words
Aero-Engine Exhaust System, Exhaust Ejector/Mixer
Jet Mixing Enhancement
Systems*
1
TANIGUCHI,*2
System, Rectangular Lobed Nozzle,
Abstract
Experimental research of seven rectangular exhaust ejector/mixer systems, which were the combinations of
four rectangular lobed nozzles with three rectangular mixing tubes, has been conducted to investigate the effect
of the geometry of rectangular lobed nozzles on the aerodynamic performances of exhaust ejector/mixer systems.
The experimental results showed that: rectangular aligned lobed nozzles have better pumping and mixing
enhancement abilities than rectangular staggered lobed nozzles, while they also cause bigger pressure losses.
Scalloping treatment on the lobe structure can improve the pumping ability and mixing enhancement performance
of a lobed nozzle, but it will also cause a big extra pressure loss. Among the seven tested exhaust ejector/mixer
systems, the exhaust ejector/mixer system which is a combination of the staggered lobed nozzle B and the
rectangular mixing tube III has the best aerodynamic performances.
Nomenclature
A
D
L
M
p
T
p
cross sectional area
diameter of the mixing tube
the required pressure recovery length along the
mixing tube
mass flow rate
pressure
temperature
velocity
density
cjPPT*
T*
cp
IF
parameter
(
Subscripts
a:
ambient
P:
primary
S:
secondary
flow
T:
total
flow
L1P:T
1 + rI>T*)(l + rI»
)
inlet of the mixing tube
exit of the mixing tube
Introduction
An exhaust ejector is a device, which converts a high
velocity fluid flow of given mass flow rate into a fluid
flow of lower velocity. This conversion is achieved by
the transfer of momentum and energy through viscous
interaction of the high velocity (primary) fluid flow with
a lower velocity (secondary) fluid flow within a mixing
tube (Fig. I ). During the past decades, this fluid dynamic
device has been further utilized to improve aircraft
performance in a variety of ways, including engine
component cooling, thrust augmentation, and exhaust
noise and infrared radiation reduction.
The conventional mixing of the primary and secondary
flow in a mixing tube occurs very slowly, which is
performed mainly by a small scale viscous mixing in a
shear layer. Thus, a conventional ejector requires a long
mixing tube to entrain the secondary flow, and a long
mixing tube results in large wall friction loss, extra weight
and higher cost. For this reason, a lobed exhaust
ejector/mixer system (Fig. 2), in which a lobed nozzle
was used as a primary nozzle, was proposed in the past
several years. It was found that a lobed nozzle can cause
large scale streamwise vortices to be shed at the trailing
edge of lobe structures, so the downstream of the flow
field is embedded with arrays of large scale streamwise
vortices, and a rapid exchange of momentum and energy
*1 Received May 12th, 1998.
*2 Institute of Industrial Science, The University of Tokyo, Tokyo, Japan.
*3 Jet Propulsion Laboratory, Department of Jet Propulsion, Beijing University of Aeronautics and Astronautics, Beijing, 100083, P. R. China
Trans. Japan Sac. Aera. Space Sci
188
pr;"",ry
nozzle
Vol. 41, No.
34
research, seven rectangular lobed ejector/mixer systems,
which were the combinations of four rectangular lobed
nozzles with three rectangular mixing tubes, were studied
experimentally to investigate the effect of the geometry
of rectangular lobed nozzles on the aerodynamic performances of rectangular lobed exhaust ejector/mixer
-
(PrT
systems.
\.TPT.
2.
Experimental Set-Up
Psr
Ts1
'ig.
:ig. 2.
The concept of a lobed exhaust ejector /mixer
system.
was achieved by means of intense mixing within a short
downstream with very little loss (Skebe et al.l).
On account of the above advantage, lobed exhaust
ejector/mixer systems have received great attention by
many researchers in the recent years (Refs. 1-10) and
they have been widely applied to the aero-engine area.
For example, on the commercial aero-engines, such as
JT8D-217C, CFM56-5C and RB211-524G/H, lobed exhaust ejector/mixer systems had been used to reduce
take-off jet noise and Specific Fuel Consumption (SFC)
(Presz6J). On the military aero-engines, such as military
helicopter "Tiger" developed by German-France, American RAH-66 "Comanche" and stealth fighter F-117
(WU11J), lobed exhaust ejector/mixer system had been
used to reduce the infrared radiation signals of the
exhaust system to improve the survivability of the
military aircraft in modem war .
It is well known that, a rectangular nozzle has better
mixing performance than a circular nozzle. A rectangular
lobed nozzle, which is formed by the combination of lobe
structures with a rectangular nozzle, seems to be much
more promising for mixing enhancement (Liu et al.12J).
Meanwhile, circular lobed exhaust ejector/mixer systems
have been widely studied in the past several years.
However, few researches were found on the rectangular
lobed ejector/mixer systems in which rectangular lobed
nozzles were used as primary nozzles. In the present
An experimental research was conducted on the low
speed exhaust ejector system test rig (corresponding to
the case of helicopter used aero-engine) in the Jet
Propulsion Laboratory ofBeijing University of Aeronautics and Astronautics (BUAA). The primary flow is
supplied by a compressor with a combustor, and its
flow rate and temperature can be adjusted. Figure 3 is
a schematic of the test section. The flowrate of the
primary flow is measured before the test section. The
total temperature and total pressure of the primary flow
are obtained by the thermocouples and total pressure
sensor located at the inlet of the tested exhaust ejector/
mixer system. The temperature and pressure fields at the
exit of the mixing tube are measured by a 12 point
temperature/pressure rake with a traverse mechanism.
The flowrate of the secondary flow can be obtained by
the flow nozzle installed at the top of the settling chamber
(Fig. 3) or/and by the above measured pressure and
temperature fields at the exit of the mixing tube. The
static pressure distributions along the mixing tube are
obtained by two rows of six static pressure tabs along
the mixing tube. One row is at the top of mixing tube
and the other row is at the side of mixing tube (see
Fig. 5). The signals of thermocouples and pressure
sensors are transferred to an IBM PC computer for data
acquirement and procession. During the test, the flowrate of the primary flow is about 1.Okg/s, and the exit
velocity of the primary flow is around 70 m/s. The detail
information of the experimental set-up can be obtained
from the Ref. 13.
It had been found that the geometrical parameters of
the lobed structures, such as the lobe height, the lobe
penetration angle, the form of the lobed structure, the
area ratio of the primary nozzle, mixing tube etc, can
affect the aerodynamic performance of a lobed ejector/
mixer system very much. The detail information can
be found in the papers of Skebe et al.,l) Presz et al.,3)
Eckerle et aC) and WU.11) Based on these previous
researches, all the rectangular lobed nozzles used in
present study are designed to have the same lobe
penetrate angle, lobe height and lobe form. Only the
effects of the lobed width, the way of lobe structure
arrangement and the scalloping treatment of the lobed
structures on the aerodynamic performance of a lobed
ejector /mixer system are discussed in the current paper .
The four rectangular lobed nozzles used in the pres-~
Feb. 1999
Ho Hu et alo : Rectangular Lobed Exhaust Ejector/Mixer
Systems
189
total
pressure/
temperature
rake
Fig.
~~r
l
Schematic of the test section.
3.
lobe width
!
-+--
outer lobe width
I
A. rectangular staggered lobed nozzle with bigger inner lobe width
e
B. rectangular staggered lobed nozzle with equal lobe width
--":I=.
~
c. rectangular aligned lobed nozzle with equal lobe width
Fig.
4.
D. rectangular aligned scalloped lobed nozzle with equal lobe width
Tested rectangular
ent research are shown in Fig. 4. Nozzle A and nozzle
B are the rectangular lobed nozzles with a staggered
arrangement of the two rows of the lobe structure. The
difference between the nozzle A and the nozzle B is that,
for the lobed nozzle A, the inner lobe width is two times
the outer lobe width, while the lobed nozzle B has the
equal value of inner lobe width and outer lobe width.
Nozzle C and nozzle D are rectangular lobed nozzles
with an aligned arrangement of the two rows of the
lobe structure. The rectangular lobed nozzle D is designed
the same as the rectangular lobed nozzle C but with a
scalloping treatment (Yu et al.14J) of the lobe structure.
All these four rectangular lobed nozzles have the same
exit area (6400mm2, equivalent diameter is about 90
mm). Rectangular lobed nozzle A has an aspect ratio
(AR) of 1.0, while the aspect ratios (AR) of lobed nozzle
B, lobed C and lobed D are same, which is 1.2.
The three mixing tubes used in the present study are
shown in Fig. 5. The mixing tube I and mixing tube II
lobed nozzles.
are simple rectangular tubes which have the same aspect
ratio (AR) as the rectangular lobed nozzle A (AR = 1.0)
and nozzle B (AR = 1.2). The rectangular mixing tube III
(AR = 1.2) is designed to have a multiple-stage cooling
structure and a 20 degree diffuser at the end of the tube.
The length of these three mixing tubes is the same, which
is 450 mm. All the mixing tubes have the same cross
section area (19,200mm2), therefore, the area ratios
between the rectangular lobed nozzles and mixing tubes
of the seven tested ejector/mixer systems are the same,
which is I: 3.
3.
Experimental Results and Discussions
During the experiment, cold tests (the temperature of
the primary flow is the same as the secondary flow, which
was 295 K) were conducted firstly to determine the
aerodynamic performances of seven rectangular lobed
exhaust ejector/mixer systems listed in Table I. Then,
Trans. Japan Sac. Aera. Space Sci.
190
Vo1. 41, No
34
I
rectangular simple mixing tube I
rectangular simple mixing tube
Fig.
5.
Cold
Combinations
A+I
A+II
B+I
4
B+II
B+111
6
'7
Note:
multiple-ring
Tested rectangular
mixing
cooling structure (AR=1.2)
tubes.
Aerodynamic performance of seven tested rectangular lobed exhaust ejector/mixer systems.
Table
ejector/mixer
system
rectangular mixing tube m with
(AR=1.2)
( AR=1.0)
Rectangular
exhaust
n
C+111
D+111
Pumping
coefficient
4i
0.87
0.84
0.87
0.87
0.98
1.02
1.34
Pressure loss
coefficient
JPPT*
1.251
1.231
1.245
1.226
1.245
1.342
2.298
Hot test Tp=573K
test
Combined
aerodynamic
parameter
IF
0.358
0.364
0.356
0.351
0.318
0.329
0.420
Static
Static
pressure
recovery
distance
pressure
recovery
distance
LID (at side)
LID (at top)
Average
Highest
temperature
temperature at
at the exit of the exit of the
the mixing
mixing tube (K )
tube (K)
.3
.8
.8
.8
.8
.8
.8
0.7
0.7
0.7
0,7
1.3
0.7
429.3
425.2
401.3
492.3
488.3
431.7
During the test, the used thermocouples have the range of300-1 073 K, accuracy for the full range is 0.1 %, and the used pressure transducers
have the range of 0-0.2 MPa, accuracy for the full range is 0.02%.
three representative rectangular lobed exhaust ejector
systems were selected from the seven for further experiment at hot test condition (the primary flow temperature at the inlet of the lobed nozzle was 573:!: 5 K,
while the temperature of the secondary flow was 295 K).
The studied aerodynamic performances of the exhaust
ejector/mixer systems include pumping coefficient,
pressure loss coefficient, combined aerodynamic parameter, static pressure recovery characteristics along the
mixing tube and the velocity and temperature distributions at the exit of the mixing tube.
3.1. Pumping coefficient (J)
The pumping coefficient
!1>was defined as the ratio of secondary flow rate to the
primary flow rate. The pumping coefficient of a conventional circular ejector with the same area ratio (I: 3)
of the primary nozzle and mixing tube is about 0.45
(Hu et al.13»). The pumping coefficient of a circular
lobed ejector/mixer system with the same area ratio
(1 : 3) and the same lobe configuration as used in current
study is about 0.81 (Hu et al.14»). However, from the
experimental results listed in Table 1, it can be seen that
the pumping coefficients of the seven tested rectangular
lobed ejector/mixer systems (0.84--1.34) are much higher
than that of a conventional circular ejector. That is, the
pumping abilities of the rectangular lobed ejector systems
are about 1 to 2 times that of the conventional ejector.
It can also be seen that a rectangular lobed ejector/mixer
system will have a much bigger pumping ability than a
circular lobed ejector/mixer system does for the same
lobe configuration. This may come from the difference
between the rectangular geometry and the circular
geometry of the primary nozzle.
Among the tested exhaust ejector/mixer systems, it
can also be seen, there is no apparent difference be-
Feb. 1999
H. Hu et a!
Rectangular Lobed Exhaust Ejector/Mixer
Systerr
100 u
n
{jf)j
staggered
Fig.
a)
arrangement
6.
aligned
The effect of the way of lobe structure
Conventionallobe
Fig. 7.
b)
arrangement
arrangement.
Scalloped lobe
Scalloping effects of lobe structure.
tween the ejector/mixer systems using nozzle A and
nozzle B as primary nozzles (system 1, system 2, system
3 and system 4) in terms of pumping coefficient. However ,
considering along with the pressure loss coefficient and
combined aerodynamic parameter (to be discussed later),
the ejector/mixer systems using nozzle B as primary
nozzle seems to be a bit better than the one using
nozzle A. Among the exhaust ejector/mixer systems with
nozzle B as primary nozzle (system 3, system 4 and system
5), the pumping coefficient of the exhaust ejector/mixer
system 5 (combination B + Ill) is the best. This can be
explained by presence of a diffuser at the end of the
rectangular mixing tube Ill, which can improve the
pumping ability of the ejector system (Skebe et al.1J).
Meanwhile, the mixing tube III also has a multiple-stage
cooling structure (Fig. 5), which makes the exhaust
ejector/mixer system 5 a multiple stage ejector system,
this also resulting in a bigger pumping coefficient.
From the comparison of the exhaust ejector/mixer
systems 5, 6, and 7 (combinations B+III,
C+III
and
D + Ill), it can be said that, the pumping abilities of
the aligned lobed nozzles (nozzle C and nozzle D) are
higher than that of the staggered lobed nozzle (lobed
nozzle B). The reason may be that the scale of the
streamwise vortices induced by aligned lobed nozzles will
be bigger than that of the staggered lobed nozzle B (Fig.
6) and larger streamwise vortices will give a higher
pumping ability (Presz et al.3J). From the comparison of
the exhaust ejector/mixer systems 6 and 7 (combinations of C + III and D + Ill), it can be suggested that
scalloping treatment of the lobe structure can improve
the pumping ability of the lobed nozzle greatly. The
reason may be that additional vortices can be generated
at the parallel sides of lobes by the scalloping treatment
of the lobe structure (Fig. 7 and Fig. 6 of the Ref. 15,
Yu et al.). This will result in a larger vortices roll-up and
enhances the "stir up effect" (Presz et al.3») of the
large-scale streamwise vortices, hence the lobed nozzle
D has the highest pumping ability.
3.2. Pressure loss coefficient JPPT*
The pressure
loss coefficient LlPPT*' which is defined as the ratio of the
pressure difference between the total pressure at the
inlet of the primary nozzle and the ambient pressure to the
dynamic pressure of primary flow, can indicate the power
loss of an aero-engine caused by the installation of an
ejector system. The bigger the pressure loss coefficient
is, the higher the engine power loss will be. From the
experimental results listed in Tablel, it can be seen
that; the exhaust ejector/mixer systems with rectangular
aligned lobed nozzles as primary nozzles (exhaust
ejector/mixer systems 6 and 1) have the bigger pressure
1oss coefficients than those with rectangular staggering
lobed nozzles as primary nozzles (exhaust ejector/mixer
system 1 to 5). The explanation of this is that the rectangular aligned lobed nozzles can induce larger scale
streamwise vortices as mentioned above (Fig. 6), which
will also cause bigger mixing loss. The exhaust ejector/
mixer system 7, which uses rectangular aligned scalloped lobed nozzle D as primary nozzle and can generate
additional vortices at the parallel sides of lobe structure and has the highest pumping coefficient among the
tested exhaust ejector/mixer systems, also has the biggest pressure loss coefficient.
3.3. Combined aerodynamic parameter III
An exhaust
Trans. Japan Soc. Aero. Space Sci
192
Vo1. 41, No.134
LID
ejector was always expected to have higher pumping
coefficient and smaller pressure loss coefficient. However, from the above experimental results, it can be
said that, the exhaust ejector/mixer system with higher
pumping coefficient always has bigger pressure loss. So,
a combined aerodynamic parameter 'P (Hu et al.16J),
which indicates the ratio of the momentum of exhausted
flow from an exhaust system with and without an exhaust
ejector/mixer system instillation, is introduced to evaluate the overall aerodynamic performance of an exhaust
ejector system.
JPPT*
+ <1>T*)(l
+ <1»
From the definition of the parameter IJI, it can be seen
that, a bigger pumping coefficient and a less pressure loss
coefficient will cause a smaller combined aerodynamic
parameter IJI. So, the smaller the parameter IJI is, the
better aerodynamic performance will be given by the
ejector system. From the combined aerodynamic parameter IJI values listed in Table 1, It can be seen that,
the exhaust ejector/mixer systems 6 and 7, which uses
rectangular aligned lobed nozzles (nozzle C and nozzle
D) as primary nozzles, have bigger combined aerodynamic parameter values than the one with rectangular
staggered lobed nozzles as primary nozzles (the exhaust
ejector/mixer systems I to 5).
Among the exhaust ejector/mixer systems with rectangular staggered lobed nozzles as primary nozzles
(the exhaust ejector/mixer systems I to 5), the one with
nozzle A (which has bigger inner lobe width) as primary
nozzle has a bigger combined aerodynamic parameter IJI
than the one with the rectangular lobed nozzle B as
primary nozzle. Meanwhile, the exhaust ejector systems
with the same aspect ratio (AR) of rectangular lobed
nozzle and mixing tube, i. e. system I (combination A+
I) and system 3 (combination B + II ), are better than the
hybrid systems (system 2 (combination A+II
) and
system 4 (combination B+I)). Among the seven tested
exhaust ejector/mixer systems, the system 5 (combination B + III), has the smallest combined aerodynamic parameter value, so it has the best combined aerodynamic
performance, while, the system 7 (combination D +III),
which has the highest pumping coefficient and the biggest
pressure loss coefficient, has the worst combined aerodynamic performance.
3.4. Characteristics of static pressure recovery performance
The static pressure recovery performance is
characterized by the static pressure distribution along
the mixing tube of an ejector system. It is well known
that, the faster the static pressure recovery is, the shorter
the mixing tube can be. This will be beneficial to the
size and weight of ejector systems. Two typical measured static pressure recovery characteristics are shown
in Fig. 7. Based on the figures, the required pressure
recovery distance (L/D) at which mixing flow reaches
near uniform condition for each ejector/mixer system
can be obtained, and the results are listed in Tablel.
From the data listed in Tablel, it can be seen that: the
required pressure recovery distance L/ D of the tested
combinations is about 0.7-1.8, which is much shorter
than that required by a conventional ejector (which is
about 4-6 (Presz et al.9». This means that, unlike the
conventional ejector, the required mixing tube length
can be reduced to 1/2 or 1/3 with a lobed nozzle as the
primary nozzle of the exhaust ejector /mixer system.
Waitz et al.17) also got a similar result.
3.5. Exit velocity and temperature distributions
The
velocity and temperature distributions at the exit of a
mixing tube can directly indicate the mixing efficiency of
the primary gas and pumped ambient air in the mixing
tube. Three typical measured velocity and temperature
distributions at the exit of the mixing tubes for the exhaust
ejector systems 5, 6 and 7 (combinations B + III, C + III
and D + III) are given in Figs. 9 and 10.
From the figures, it can be seen that the region of the
high speed and high temperature flow is bifurcated and
deviates from the center at the exit of mixing tube due
H. Hu et al. : Rectangular Lobed Exhaust Ejector/Mixer System
Feb. 1999
a. B+ ill combination
Fig. 9.
a. B+ m combination
Fig.
10.
b. C+ ill combination
Velocity distributions
Temperature
b. C+ m combination
distributions
c. D+ ill combination
(V2/Vp) at the exit of mixing tubes (cold test).
c. D+ m combination
(T 2T/T PT) at the exit of mixing
to the "stir up effect" of the streamwise vortices induced
by the lobed nozzles. Since the rectangular aligned lobed
nozzles (lobed nozzle C and lobed nozzle D) can generate
bigger scale streamwise vortices and has higher pumping
ability than the rectangular staggered lobed nozzles
(lobed nozzle A and lobed nozzle B), the size of the high
speed and high temperature at the exit of the mixing tube
of the exhaust ejector/mixer system 6 (combination
C+III, Fig. 9(b) and Fig. IO(b)) is less than that of the
exhaust ejector/mixer system 5 (combination B + III,
Fig. 9(a) and Fig. IO(a)). Furthermore, since the rectangular aligned scalloped lobed nozzle D can generate
additional vortices at the parallel sides of lobe structure
and enhance the roll-up and "stir up effect" of the
streamwise vortices, the bulk of low speed and cold flow
is engulfeds into the center of the mixing flow (Fig. 9(c)
and Fig. IO(c)).
From the comparison of the average and highest
temperature values at the exit of the mixing tube for the
exhaust ejector/mixer systems 5, 6 and 7 (combinations
B + III, C + III and D + III) listed in Table I, it also can
be seen that: the average and highest temperature values
of the systems with aligned lobed nozzles (combinations
C + III and D + III) as primary nozzle are smaller than
that of the combination B + III. This indicates that the
rectangular aligned lobed nozzle has one step higher
pumping ability and better mixing enhancement ability
than the rectangular staggered lobed nozzle. The rectangular aligned scalloped lobed nozzle D can generate
additional vortices and enhance the "stir up effect"
of the streamwise vortices. Thus, the exhaust ejector/
tubes (hot test).
mixer system (combination D + III) has the smallest
average and highest temperatures at the exit of the mixing tube.
4. Conclusion
From the above analysis and discussions, it can be
said that: compared with a conventional circular ejector ,
the exhaust ejector/mixer systems with rectangular lobed
nozzle as primary nozzle can improve pumping ability
200%-300%, and reduce the required mixing tube length
(L/D) to 1/2-1/3. The geometry of the lobed nozzles,
such as the lobe configuration and the way of lobe
arrangement can influence the aerodynamic performances of rectangular lobed exhaust ejector/mixer
systems.
Through the experimental research, the following
conclusions can be obtained:
(I) The way of the arrangement of the lobe structures
can affect the pumping ability and mixing enhancement
performance of the rectangular lobed nozzles very much.
Compared with the rectangular staggered lobed nozzles,
rectangular aligned lobed nozzles have higher pumping
coefficients and mixing enhancement abilities, but they
have bigger pressure losses. Thus, they are very fit for
the area where the higher pumping coefficient and mixing
enhancement are mainly required and the pressure loss
does not call for severe consideration.
(2) The scalloping treatment of the lobe structures
can improve the pumping ability greatly and mixing
enhancement of th.e lobed nozzle, but it will suffer a big
Trans. Japan Sac. Aera. Space Sci
194
extra pressure loss.
(3) Among the seven tested systems, the exhaust
ejector /mixer system 5 (combination B + III) has the
smallest combined aerodynamic parameter value, i.e. has
the best combined aerodynamic performance of the test~d
combinations. The exhaust ejector/mixer system 7
(combination D + 111), which has the highest pumping
coefficient and mixing enhancement ability, but suffers
the biggest pressure loss, has the worst combined
aerodynamic performance of the tested combinations.
Measurement
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Acknowledgements
The authors wish to thank Prof. Wei Fuqing and Ms. Li Li
of Beijing University of Aeronautics and Astronautics for help
in conducting the present study. The research fellowship
provided by Japan Society for Promotion of Science (JSPS) is
also gratefully acknowledged.
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