The characteristics of a Trapped-Vortex (TV) combustor are presented. A vortex is trapped in the cavity established between two disks mounted in tandem. Fuel and air are injected directly into the cavity in such a way as to increase the vortex strength. Some air from the annular flow is also entrained into the recirculation zone of the vortex. Lean blow-out limits of the combustor are determined for a wide range of annular air flow rates. These data indicate that the lean blow-out limits are considerably lower for the TV combustor than for flames stabilized using swirl or bluff-bodies. The pressure loss through the annular duct is also low, being less than 2% for the flow conditions in this study. The instantaneous shape of the recirculation zone of the trapped vortex is measured using a two-color PIV technique. Temperature profiles obtained with CARS indicate a well mixed recirculation zone and demonstrate the impact of primary air injection on the local equivalence ratio

Gross, L. P.

Hsu, K.-Y.

Roquemore, W. M.

Trump, D. D.

English

NASA Technical Reports Server

N95- 70866 -
Characteristics of a Trapped-Vortex (TV) Combustor
K.-Y. Hsu, L. P. Goss and D. D. Trump
Systems Research Laboratories, Inc.
Dayton OH 45440-3696
W. M. Roquemore
Wright Laboratory
Wright-Patterson Air Force Base OH 45433-7103
ABSTRACT
The characteristics of a Trapped-Vortex (TV) combustor are presented. A vortex is trapped
in the cavity established between two disks mounted in tandem. Fuel and air are injected directly
into the cavity in such a way as to increase the vortex strength. Some air from the annular flow is
also entrained into the recirculation zone of the vortex. Lean blow-out limits of the combustor are
determined for a wide range of annular air flow rates. These data indicate that the lean blow-out
limits are considerably lower for the "IV combustor than for flames stabilized using swift or bluff-
bodies. The pressure loss through the annular duct is also low, being less than 2% for the flow
conditions in this study. The instantaneous shape of the recirculation zone of the trapped vortex is
measured using a two-color PIV technique. Temperature profiles obtained with CARS indicate a
well mixed recirculation zone and demonstrate the impact of primary air injection on the local
equivalence ratio.
INTRODUCTION
All practical combustors and commercial burners use recirculating flows to stabilize the
flame. Re,circulation zones provide regions of low velocity fluid where burning can take place.
The recirculation zones also provide mixing of fuel, air, and hot products and a continuous ignition
source that sustains the flame by transporting the hot mixture back toward the incoming reactants.
Swirl, bluff-bodies, and rearward facing steps or a combination of these are commonly used to
create reeirculation zones. Swirl is used in most practical combustors and burners; however, the
operating range of swirling flames is often not so large as desired. This is due to aerodynamic
instabilities of the recirculation zone. Bluff-bodies and rearward facing steps are also used to
create re.circulation zones that promote flame holding in much the same way as swirl-stabilized
flames. However, recirculation zones behind bluff-bodies and rearward facing steps are even less
stable and have longer flame lengths than those established using swirl. Also, recirculation zones
established by bluff-bodies or rearward facing steps do not entrain a large quantity of free stream
fluid. This limits their application to high speed, premixed combustion systems such as those
encountered in ramjets and gas turbine engine afterburners.
One technique for creating a stable recalculation zone is to trap a vortex between two disks
mounted in tandem. This technique has been investigated as a means of reducing drag of bluff-
bodies; however, it has never been considered as a means of stabilizing a flamel,2, 3. The
objective of this paper is to evaluate the TV concept for flame stabilization. A vortex is trapped in a
cavity and fuel and air are injected directly into the cavity in such a way that the vortex strength is
increased. This provides a stable environment that can sustain the flame over a wide range of
operating conditions.
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EXPERIMENTAL
The combustor used in this study consists of a forebody and an afterbody disk as shown in
Fig. 1 (a). The forebody, with diameter (df) of 70 ram, is located at the exit plane of the annular
air duct which has a diameter of 80 mm. The blockage ratio def'med as the area ratio between the
forebody and the annular air duct is 76%. An annular air velocity of 50 m/s can be achieved in the
facility. The annular and primary air flows originate from two air supplies and are controlled and
metered separately. Fuel and primary air are delivered through concentric tubes into separate
chambers in the afterbody which is 50.8 mm in diameter.
A schematic of the primary fuel and air injection plane is shown in Fig. 1 (b). Gaseous
propane fuel is injected through 8 orifices (1.75 mm in diameter) and the primary air is supplied
through 24 orifices (2.29 mm diameter) surrounding the fuel jets. The fuel and primary air jets are
located so as to reinforce the vortex created in the cavity. Direct injection of primary air serves the
following purposes : (1) provides direct control of local equivalence ratio inside the recirculation
zone; (2) enhances mixing by increasing the mixing region through distributed fuel and air jets; and
(3) provides cooling for the injector assembly. The separation between the forebody and the face
of injector plane, H, can also be varied to optimize the condition for trapping a vortex and
providing stable combustion. The pressure drop due to the annular air flow is measured by using
a water manometer. The lean-blow-out limits of the combustor are established for a range of flow
conditions. The effects of separation between forebody and afterbody and primary air on the
combustor performance are also examined.
The velocity fields are obtained for both cold and reacting flow conditions. The
instantaneous velocity field is obtained by employing the two-color Particle-Imaging Velocimetry
(PIV) 4,5 technique. A pulsed Nd:YAG laser and a YAG-pumped dye laser are used to generate
two different wavelengths to eliminate the ambiguity in flow direction. Cylindrical lenses are used
to form a laser sheet vertically across the centedine of the burner. The spherical particles (5 lain in
diameter) are seeded through primary air jets. A 35 mm camera mounted at a right angle to the
laser sheet is used to take images of particles using laser illumination. A synchronization circuit is
used to control the time separation between the two laser pulses. The typical time separation
between two lasers is about 30 Its for the flow conditions studied. A color scanner is used to
digitize the images which are stored in the computer for further analysis. The theory and
experimental setup of the PIV technique are presented in Ref. 4 and 5.
The radial temperature profiles at locations inside the recirculation zone are measured by
using Coherent Anti-Stokes Raman Spectroscopy (CARS). The CARS system used in this study
consists of a Nd:YAG-pumped broad-band dye laser for N2 excitation. A CCD camera interfaced
with the computer is used for single-shot broad-band detection. At each location, 1000 samples
were collected at a rate of 10 Hz, and the spectra are stored for data analysis. Computer software
was developed to obtain temperature information by curve-fitting the band-shape of the measured
N2 spectrum.
RESULTS AND DISCUSSION
An image of a flame stabilized in the TV combustor is shown in Fig. 2 (a). The flame is
stabilized within the cavity between the afterbody and the forebody. Its bright blue color indicates
low soot production. The overall equivalence ratio is about 0.06 which is considerably below the
lean blow-out limit of typical swirl stabilized flames. The annular air velocity is 33 ntis. The
primary equivalence ratio calculated using the fuel and primary air is about 1.2. The flame is
attached in the shear layer near the forebody. This is typical for all 'IV stabilized flames when a
small amount of primary air (5% of annular air) is injected into the recircul_ition zone. The
_ 19
instantaneous velocity field is shown in Fig. 2 (b) for the flow domain drawn in Fig. 2 (a). The
center of the recirculation zone is evident in Fig. 2 (b) and corresponds to the dark region in the
flame image. The plane of the velocity field also includes part of a fuel and primary air jet as is
evident from the large velocities near the afterbody face. In general, increasing the fuel flow rate
with the air flow rates fixed results in longer flame with a yellow sooting region near the vortex
center. Increasing the primary air flow reduces the flame length and eliminates the soot formation.
However, the general structure of the flame inside the cavity is essentially the same as that shown
in Figs. 2 (a) and 2 (b) for a wide range of conditions.
The pressure drop across the TV combustor is measured for both cold and reacting flows. A
150 mm long 80 mm diameter Pyrex tube was used to confine the flow. The measured pressure
drop does not include that across the fuel and primary air jets. The percentage of pressure drop
(dp/P) as a function of I-I/df (ratio of separation H and the diameter of the forebody df) for cold
flow conditions is shown in Fig. 3. The differential pressure between annular air and ambient is
denoted by dp, and P is the ambient pressure. The pressure drop (dp/P) across the TV combustor
increases as the velocity of the annular air is increased. A minimum pressure drop occurs at
H/df = 0.6 for three different annular air flow rates. This is in good agreement with the forebody
and afterbody separation found for maximum drag reduction using the TV concept 1,2,3.
The criteria for trapping a vortex are given in Ref. 1 where the afterbody diameter was found
to be 0.75df and the separation of the forebody and afterbody was 0.55df. Combusting flow
experiments were conducted to determine the optimum diameter of the afterbody and the
separation. It was found that the condition for trapping a vortex in cold flows is near optimum for
providing a visibly stable flame. This is not evident in Fig. 4 where the pressure drop as a
function of H/df, for different conditions of primary air, in both cold and reacting eases is shown.
The pressure drop is higher in eombusting flows than that in the cold experiments due to heat
release. Also, the pressure drop increases with the primary air flow rate for both cold and reacting
flows although it is more notable for the combusting cases. The heat release appears to modify the
flow pattern in such a way as to increase the effective blockage ratio and this resulted in a higher
pressure drop. This is evident in the combusting case since the flame increases in diameter as the
primary air flow is increased.
The effect of primary equivalence ratio on pressure drop for different H/df is shown in
Fig. 5. The primary air flow rate determines the local equivalence ratio and thus the heat release
rate in the re.circulation zone. The pressure drop is expected to be a maximum at the condition of
maximum heat release rate. This occurs for a primary equivalence ratio of unity for various H/df
and is consistent with the data in Fig. 5. The local equivalence ratio in the re.circulation zone may
be slightly leaner than the primary equivalence ratio due to entrained air from the annular flow.
The entrainment rate may also vary with flow conditions. However, the data in Fig. 5 suggest
that very little air is entrained into the recirculation zone when there is primary air flow.
The lean blow-out limits for the "IN combustor are illustrated in Figs 6 and 7. With zero
primary air flow rate, the overall equivalence ratio at lean blow-out increases with the annular air
velocity and with increasing H/df, as shown in Fig. 6. The slope of the lean blow-out versus H/df
curve changes at H/df around 0.65. This could indicate that air entrainment might increase with
large separation distance (H/df). The overall equivalence ratio at lean blow-out is less than 0.035
which illustrates the excellent stability of this combustor. The effect of primary air equivalence
ratio on the overall lean blow-out limit is shown in Fig. 7 for an H/df = 0.55. These data suggest
that for a fixed annular air flow rate there is a near linear relationship between the overall and
primary equivalence ratio. Furthermore, for a f'med primary equivalence ratio the overall
equivalence ratio at blow-out will be lower for higher annular air flow rates. It is also noted that
the slopes of the curves for high and low annular air flow rates are nearly the same. It is evident
u
7
7
20 _..
wthat by controlling the primary equivalence ratio, one can optimize conditions for minimum lean
blow-out for different annular air flow rates.
Direct-injection of primary air and fuel into the recireulation zone of the trapped vortex has a
major impact on flame stability and local equivalence ratio. This is illustrated by the temperature
prof'tles in Figs. 8, 9, and 10 for H/df = 0.59 at three different axial locations (z = 5, 25 and 39
mm) as measured from the forebody face. The overall equivalence ratio is about 0.2 with an
annular air velocity of 50 m/s and 25 slpm of fuel. Data are presented for primary air flow
conditions of 56 and 280 slpm, which correspond to primary equivalence ratios of 11.1 and 2.2,
respectively. In the figures, the mean temperature profiles are marked with open symbols. Closed
symbols denote the fluctuations in temperature. Near the forebody as shown in Fig. 8, the
temperature is relatively flat across the forebody with temperature fluctuations of about 200 K.
The radial temperature profiles at an axial location near the center of vortex are relatively flat in the
range ofr > 20 mm, as shown in Fig. 9. The jet-dominant region can be clearly seen at r = 18 mm
where fluctuation in temperature is higher than that at other locations. It is also noted that the flame
increases in diameter as the primary air is increased. The primary air has a significant impact on
the temperature profiles near the afterbody plane as noted in Fig. 10. The temperature in the fiat
region (r > 20 mm) increases by about 700 K as the primary air is increased. The maximum
temperature of 2200 K equivalent to stoichiometric flame temperature has been measured for high
primary flow conditions, however, the fluctuation in temperature is only about 200 K. The
fluctuations in temperature are lower under the leaner condition. (_. prim = 2..2) than under, the
rich condition (Oprimary = 11.1). In general, the temperature ts hlglier _sar_e pnmary eqtuvalence
ratio approaches-the stoichiometric condition, however;, lower temperature are observed in the
range of r < 20 mm where the primary air jets were dominant. The temperature is higher in the
downstream location near the injector plane, where additional entrainment of air is expected. The
flat temperature profiles indicate that the recirculation zone of the trapped vortex behaves like a well
stirred reactor for most conditions. However, the temperature fluctuations are relatively large
especially for the low primary air flow rate (high primary equivalence ratio). This suggest that the
mixing may not be complete in the recirculation zone.
SUMMARY
The TV combustor demonstrates extremely low lean blowout and pressure drop for a wide
range of flow conditions. The primary air has significant impact on the combustion characteristics,
namely, stability, local stoichiometry and lean blow-out. The direct control of the primary air can
maintain stable combustion over a very wide range of operating conditions that has not been
achieved by other combustors. Additional data on combustion efficiency and emissions will be
presented at the meeting.
REFERENCES
1. Mair, W. A., "The Effect of a Rear-Mounted Disc on the Drag of a Blunt-Based Body of
Revolution", The Aeronautical Quarterly, pp. 350-360, Nov 1965.
2. Little, Jr., B. H., Whipkey, R. R., "Locked Vortex Afterbodies, J. of Aircraft, Vol, 16,
No. 5, pp. 296-302, May 1979.
3. Gharib, M. and Roshko, A., "The Effect of Flow Oscillations on Cavity Drag", J. Fluid
Mech., Vol. 177, pp. 501-530, 1987.
4. Goss, L. P., Post M. E., Trump, D. D. and Sarka, B., "Two Color Particle Imaging
Velocimetry", J. Laser Appl., p.p. 36-42, 1991.
5. Post, M. E. and Goss, L. P., "Two-Color Particle-Imaging Velocimetry in Vortex
Structures", 31st Aerospace Sciences Meeting & Exhibit, AJAA-93-0412, 1993.
21
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Air
Air Jets_
(d=l'8mm) _o O°O 0 /
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Figure I. Schematic Diagrams of (a) Trapped-Vortex Combustor and (b) Injection Plane of Aftcrbody
lllii!iiiii iii i!  lllli
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30 m/s
(a) (b)
Figure 2. (a) Flanm Image and (b) Instantaneous Velocity Field
0
0
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0 0
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o o o U_ r
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O O
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XXX X
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0.5 1 1.5
It/dr
Figure 3. Pressure Drop versus H/df
(Cold Flow without Primary Air)
0.8 ' I ' I ' I ' I ' I ' I '
+ @
+ +
o , 8,,a $
0.7 o o o o o o
o o o
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, I , I , I , I ui I t--0.4
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
It/d t
Figure 4. Comparison of Pressure Drop between Cold
and Reacting Flows (_Overall - 0.12) at Umr = 33 m/s
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22
0.85
0.8
0.75
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0.65
0.6
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0.64
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I , I , I , I , I , I
I 2 3 4 5 6
Primary Equivalence Ratio
Figure 5. Pressure Drop versus Primary
Equivalence Ratio at Uair = 33 m/s
0.05
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,_ 0.03
0.02
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-
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0.2 0.4 0.6 0.8
H/d r
Figure 6. Effect of H/df on Lean Blow-out
under Various Annular Air Flow Conditions
w
L_
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0.14
O
m 0.12
@
o 0.10
a_
8 0.08
_ 0.06
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,, 50 m/s
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[] A
0
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[]
I , I , I I I , I , I , I
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Primary Equivalence Ratio @ LBO
J
0.8
Figure 7. Relationship between Overall and
Primary Equivalence Ratios at LBO (H]df = 0.55)
2400
2000
"-" 1600
t_ 1200
800
''''t''''1''''1 .... I''''1''''1'''
o I1.1
2.2
.oooOX_*6g_°*°o o
A &
A
&
&&&&&&&&&&&&&&
_Oe gIOQ e eO eo aOo g &
400 .... I .... I .... I,,,,I .... I .... lit _ 0
5 10 15 _ 25 30 35 40
RadiNIx_tion(mm)
, 1000
800
v
400 _"
2OO
Figure 8. TemperatureProfiles at z = 5 mm
(H/df = 0.59)
2400
2000
"_ 1600
1200
.'' '_I' '''I''''I ''''I''''I''''I'''
Primary
- o 11.1 tx A t_ _ _ tx A
tx 2.2
0_000000
OO o
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ee80_
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5 10 15 20 25 30 35 40
RadialLocation(nun)
, I000
a 800
°
iX"
6oog
A- 400 _
• 200
24_. .... I''''1 .... I''''1 .... I .... I''''
_o 11.1 _ _ _ n _ _ _
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. 00000 &"
_O0•O•_ • •
0
• 060
O
O •
_ ,,,,I .... I .... I .... I .... I,,,,l_xa.._ 0
5 10 35 4015 20 25 30
RadiaL o cation (nun)
Figure 9. Temperature Profiles at z = 25 mm
(H/df = 0.59)
Figure 10. Temperature Profiles at z = 39 mm
(H/df= 0.59)
1000
8OO
200
w
23


Characteristics of a trapped-vortex (TV) combustor

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