Experimental Study of a Lifted LPP Flame.
LDV, CH Chemiluminescence, Acetone PLIF Measurements.
by
Guillaume Martins, Gilles Cabot*, Benoit Taupin, David Vauchelles, Abdelkrim Boukhalfa
gilles.cabot@coria.fr
CORIA UMR CNRS 6614
avenue de l’Université BP8
76801 Saint Etienne du Rouvray (France)
ABSTRACT
The purpose of this experimental work is to study lean premixed fuel/air swirl flames in order to identify the
factors governing flame stabilization. The experimental burner is composed of a cylindrical quartz combustion
chamber (200×80mm) placed in a pressurized and cooled casing with three large optical accesses. Downstream,
a converging nozzle with variable section allows to rise the pressure inside the combustion chamber up to 8 Bar.
The combustion air can be preheated up to 500°C. The fuel/air injector is a radial swirler injector. The premix
tube is a 70mm length and 10 mm diameter cylinder. The experimental device can operate either with natural gas
or with liquid n -heptane, since liquid fuel prevaporization is to be studied in a second step. Results of the present
study only concern natural gas combustion.
Velocity field is measured by LDV technique, reacting zone is visualized by CH chemiluminescence imaging,
mixing in the non reacting zone is qualified by acetone-PLIF, and dynamic pressure sensors are used.
For the two tested swirl numbers and for a fixed inlet velocity, a Corner Recirculation Zone (CRZ) can be
observed in LDV results for the non reacting flow. However the Inner Recirculation Zone (IRZ) appears only for
high swirl numbers.
For atmospheric pressure and Tinlet air = 20°C, a lifted flame with an unstable position is observed. The IRZ
strongly decreases lift height and extends burner lean operating limit. When equivalence ratio is decreased,
instability intensity increases and a frequency appears at 240Hz in pressure spectrum. A cyclic evolution along
pressure cycle is observed for the structure and the reaction intensity of flame foot.
INTRODUCTION
A basic problem in combustion is to achieve low pollutant emission with high combustion efficiency. Lean
Premix Prevaporized combustion for liquids fuels (LPP) is being considered as a promising solution for the
reduction of nitrogen oxides and soot particles. Soot particle formation occurring during droplet combustion
[Ohkubo et al. 1997] is decreased by fuel prevaporizing. Furthermore, the homogeneity of fuel-air mixture at the
exit of the premix tube leads to a decrease of local rich zones with high flame temperature, where NOx
production is governed by thermal NO mechanism [Mongia et al. 1996]. Unfortunately, operating at lean
equivalence ratios induces flame instabilities [Samaniengo et al. 1993], flash back [Plee et al. 1978 ] and CO
emissions. This kind of instabilities results fro m a coupling between pressure fluctuations and heat release. To
increase stability and to increase flame holding, high swirl injectors are often used [Anacleto et al. 2002].
Our experimental device is designed to operate with pressure inside the combustion chamber ranging up to 8 bar,
and premix temperature ranging up to 500°C. Pressure and temperature influences were studied previously for a
gaseous fuel injection device, including a axial swirler and a bluff-body for flame stabilization [Taupin 2003].
The purpose is now to operate in LPP conditions, without bluff-body and with a radial swirler. The device works
with liquid fuel, however, natural gas is used for simplification in the present study, since the purpose is to
analyze flame structure and swirling flow in the restricted combustion chamber as function of injection swirl
number.
EXPRIMENTAL SET-UP
Injector and Burner
The experimental configuration uses a radial swirl injector in a sudden-expansion dump combustor (80mm
diameter) which is made of a quartz tube for an optical access in the first part (200mm) and which is made of
steel in the last part (170mm). Air flows inside the swirler through 18 rectangular holes inclined at an angle of
45°. Fuel is injected radially 20 mm downstream and mixes with air in the 70 mm length and 10 mm diameter
premixing chamber. Swirler geometry can be modified by obturating some radial air inlets. For a constant mass
flow rate, reducing number of inlets increases tangential velocity and then increases swirl number. An estimation
of swirl number was performed with Fluent [2001] simulations, and was found to be S=0.46 for the basic 18
holes configuration and S=0.56 for the 6 holes configuration. Results will be presented in the following for these
two configurations, and will be referred as 10-18h and 10-6h. Reference mean axial velocity at injector exit will
be 70m/s.
Air
Inlet
102/3
8 mm 25m
α = 45°
Gas
Fuel
Inlet
Combustion air
Cooling air
Premixing Zone 10 mm
70 mm
Natural Gas
Radial injector
Figure 1 : Details of radial swirl injector
Chocked section
Flame tube
Figure 2 : Details of experimental set-up
A conical converging nozzle is placed at the co mbustion chamber exit to allow pressure modification up to 8 bar.
Dilution air flow is injected at 230 mm downstream air/fuel injection, in order to cool the quartz tube and to
dilute the hot combustion product. To limit the strain of pressure and temperature on the quartz tube, it was
placed in a pressurized square box equipped with three wide windows for optical access. Three mass flow
controllers are used to set the cooling dilution air mass flow, the combustion air and the natural gas flow rates,
full scale are respectively: 85g/s for air and 6g/s for gas. Combustion air can be preheated up to 500°C. This
experimental set-up allows a parametrical investigation of equivalence ratio (φ), pressure chamber (P) and air
inlet temperature (T) influences. Results presented in the present study were obtained for atmospheric pressure
and Tinlet =20°C.
It can be noted that several trials were performed for injector and burner design. During these trials, it was
observed that flash-back combustion regime could occur in the restricted exit chamber, for injection diameters
higher than 14mm.
Pressure measurements
Pressure measurements are obtained by a dynamic pressure sensor with a 45 kHz response (Kistler: 7061B),
which was calibrated between 0 and 5 bars. This sensor is coupled to a charge amplifier with a 70 kHz cut
frequency. The sensor is fixed at the end of the combustion chamber.
LDV measurements
The LDV measurement device is a two color, dual beam TSI system. Axial and radial velocity components are
measured using respectively the green and the blue lines from a 6 W Argon Ion laser. The LDV signal is
processed by an IFA 750 Digital Burst Correlator. The swirling incoming air jet was seeded with zirconium
oxide (mean diameter ∼2 µm) introduced in the flow by a rotative brush seeder.
CH Chemiluminescence
CH chemiluminescence images were obtained with an Intensified CCD camera (LaVision: Flamstar2, 14 bytes,
384×286 pixels 2 ). CH* emission was filtered with two colored filters, the first one is 180 nm band pass centered
on 410 nm and the other one is high pass with a cut frequency at 390 nm. CH formation occurs during fuel
decomposition and then CH emission is a marker for reaction zone [Donbar et al. 2000] [Higgins et al. 2001].
For collecting images, two different fields of view were used, a 142 x 80 mm field and a 38 x 51mm field.
Acetone P-LIF
Gaseous acetone seeded into the studied gas flow can be excited by a high energy pulse of UV laser in a thin
sheet. The resulting vis ible fluorescence of acetone provides an instantaneous image of the flow collected by an
intensified CCD camera. Acetone is seeded into gaseous fuel flow by bubbling into a isothermal acetone bath.
Below the decomposition temperature which is about 1000K [Miller 94], acetone is ideal for instantaneously
capturing two dimensional flow structure. In isobar, isothermal and non reactive flow, acetone PLIF allows
quantitative measurement of the mixing [Lozano et al. 1992]. In our case, signal vanishes in reacting zones, and
flow structure can be qualitatively examined in regions where mixture is cold and has not yet reacted. Laser
excitation at 248nm was provided by Lambda Physik LPX100 Excimer with a Kr-F mixture. The fluorescence
signal is captured using ICCD camera (LaVision: Flamstar2) and a 180 nm band pass filter centered at 410 nm.
The field of view was 38 x 51 mm.
For CH emission and P-LIF images and for each tested condition (Swirl number, Equivalence Ratio), 300
instantaneous images were taken, from wh ich a mean image and a r.m.s. image were calculated. During these
acquisitions, images obtained with the smaller field of view, close to the nozzle exit, were also triggered with the
pressure signal of the combustion chamber.
RESULTS AND DISCUSSIONS
Flame structure as function of swirl number
The studied burner produces a relatively stable and lifted flame. LDV velocity measurements and spontaneous
CH emission measurements with a CCD camera inform on internal flame structure.
•
LDV measurements
Velocity measurements have been performed in the non reacting flow but were not possible to achieve in the
reacting flow. In figure 3 are reported the obtained velocity fields respectively for injector 10-18h and for
injector 10-6h. Air flow rate is 6.65 g/s corresponding to a mean exit velocity equal to 70m/s.
In the case of injector 10-18h, a jet flow can be observed in the velocity cartography, and is confirmed by
centerline axial velocity decay presented in figure 4. A corner recirculation zone (CRZ) is also observed in the
dump region, due to sudden expansion geometry with a confinement ratio higher than 4 [Gupta 1984].
In the case of injector 10-6h, swirl is higher, and the flow no more behaves as a simple jet. An internal
recirculation zone (IRZ) is observed, making the annular jet expanding radially. Jet opening angle is about 30°
with the axis. This IRZ is generated by the swirl induced central depression and is known to appear for a swirl
number higher than a critical value [Lilley 1977]. The axis profile in figure 4 shows that IRZ closes up on the
axis at about 120mm from injection. A CRZ is also observed at burner base, with a lower size than for 10-18h
case (40mm versus 80 mm) but higher velocities. Size decrease and intensity increase certainly result from the
interaction of the annular conical jet with combustion chamber walls.
X (mm)
120
130
120
110
110
100
100
90
90
80
80
70
70
60
60
30
50
50
20
40
40
30
30
20
20
10
10
Uaxis (m/s)
X (mm)
130
Injector 10-1 8 h
Injector 10-6 h
50
40
10
0
0
20
40
R (mm)
-10
0
20
40
R (mm)
Figure 3: Velocity fields for 10-18h and 10-6h
injectors, isothermal flow
•
-20
0
20
40
60
80
100
120
X (mm)
Figure 4 : Axial velocity on chamber centerline for
both injectors, isothermal flow
Acetone P-LIF – mixing characterization
Fluorescence signal measurement of a fuel/air mixture seeded with acetone allows to qualify the region where
mixture is cold and has not yet reacted. Mean images obtained for the 2 injectors are presented in figure 5.
Measured field corresponds to a restricted 38 x 51mm area located close to nozzle exit.
It can be observed that for injector 10-18h, mixed air/fuel jet issues vertically, without radially dispersing, up to
about 3 cm. For injector 10-6h, signal strongly decreases after 1 cm, and radially disperses with a 30° cone
shape. Structures previously observed in the non reacting flows seem to be preserved, as simple jet structure and
supercritical swirl structure including a IRZ are compatible with the present images. It should be mentioned that
this result is not necessary obvious since thermal expansion due to combustion reaction decreases effective swirl
number in the case of reacting flow [Hillemanns et al. 1986][Weber et al. 1992].
Figure 5 : Mean acetone PLIF images for both injectors
Figure 6 : Mean CH emission images for both
injectors, white dash lines represent P-LIF fields
•
CH Chemiluminescence Images
Measurement of CH chemiluminescence allows to localize combustion reaction zone. Measurements have been
performed for both injectors, at 0.7 equivalence ratio and 70 m/s nozzle exit mean velocity.
Inje ctor 1 0-18 h
Inje ctor 1 0-9h
Inje ctor 1 0-6h
CH* signal (a.u.)
5500
5000
4500
4000
3500
Height of lift (mm)
It can be observed in figure 6 that flame is lifted for all swirl numbers. A significant modification of flame shape
can be observed between low swirl and higher swirl due to the modification of flow structure. For 10-18h
injector (low swirl), the bowl shape flame does not come up to chamber walls. On the contrary, for 10-6h
injector (higher swirl), a more compact and intense flame and a strong contact with walls are observed. When
mean CH chemiluminescence signal on chamber axis is examined, in figure 7, it can be observed that distance
between injector and maximum position varies between the studied configurations, from x=100mm for 10-18h
injector to x=50mm for 10-6h injector. Lift and reaction zone extent can be determined by stating a reference
threshold of 20% of maximum intensity. Lift and reaction zone extent both decrease when swirl is increased (fig.
7), from 50mm lift and 70mm extent for 10-18h injector to 25mm lift and 45mm extent for 10-6h injector.
Furthermore, the extra test of 10-9h injector shows a very similar behavior with the 10-6h injector, in terms of
lift, position and intensity of the flame (fig. 7). It seems that transition between the two lift positions occurs
abruptly (fig. 8) when critical swirl number is reached.
50
45
40
Critical Swirl Injector
3000
35
2500
2000
30
1500
25
1000
20
40
60
80
100
120 140
X (mm)
Figure 7: Axial profile of mean CH emission for
10-18h, 10-9h and 10-6h injectors
0.4410-18h
0.46 0.48 0.5 10-9h
0.52 0.5410-6h
0.56 0.58
Swirl number
increasing swirl number
Figure 8: Height of flame lift evolution versus
swirler geometry
Burner operating range
Burner operating range was studied as function of fuel flow rate and equivalence ratio, for both injectors. In all
studied conditions, flame is lifted. When equivalence ratio is decreased, it is generally observed that flame turns
from a relatively stable regime (referred as rich stable) to an unstable regime (lean unstable) before extinction.
Transition equivalence ratio between stable and unstable regimes will be referred as φunstable and lean extinction
limit will be referred as φlean-blow-off. Unstable regime is observed visually, and is quantified by measuring
pressure fluctuations inside combustion chamber. A temporal pressure evolution and corresponding frequency
spectrum obtained for an unstable case are reported in figures 9 and 10, where a 240Hz pressure oscillation can
be observed. Burner operating limits are reported in figure 11 for 10-6h injector. For low fuel flow rates, the
unstable regime is not observed before lean extinction. Extinction limit equivalence ratio (φlean-low-off ) decreases
when fuel flow rate is increased, from φblow -off = 0.67 for 0.1g/s to φblow -off = 0.63 for 0.4g/s. A decrease of
transition equivalence ratio φunstable is also observed when flow rate is increased, from φunstable=0.70 for 0.2 g/s to
φunstable=0.68 for 0.4 g/s.
Amplitude
Pressure Fluctuations (a.u.)
1.5
1.0
0.6
0.5
0.5
0.4
0.0
0.3
-0.5
0.2
-1.0
0.1
-1.5
0.00
0.01
0.02
0.03
0
0.04
0.05
Time (s)
Figure 10: Spectrum of pressure signal for 10-6h
injector (unstable case)
Equivalence Ratio
Figure 9: Temporal evolution of pressure inside
combustion chamber for 10-6h injector (unstable
case)
50 150 250 350 450 550 650 750 850 950
Frequency (Hz)
0.75
0.7
φblow-off
0.65
Injector 0-3
φblow- off
Injector 2-3
0.6
0.55
Figure 11: Burner operating range of 10-6h flame
as function of fuel flow rate
0.2
0.3
0.4
Fuel mass flow-rate (g/s)
Figure 12: Lean Blow Off limit of 10-18h and 106h flames evolution as function of fuel mass flow
rate
In the present case, the increase of fuel flow rate leads to an increase of burner operating range. In a similar
configuration but including a bluff-body, Taupin [2003] observed that operating range did not depend on fuel
flow rate since flame was anchored on bluff-body.
In order to study the effect of swirl number on burner operating domain, extinction limit φblow-off have been
reported in figure 12 for both injectors. As for 10-6h injector (higher swirl), extinction equivalence ratio φblow-off
decreases when fuel flow rate is increased for 10-18h injector (lower swirl), but higher values are observed for
φblow-off. This confirms the interest of IRZ in flame stabilization. IRZ allows hot and reacting gases to be
reinjected in the center of reaction zone, leading to flame stabilization for lower values of equivalence ratio.
CH emission
Mean
Instantaneous
Acetone P-LIF
Mean
Instantaneous
ϕ = 0°
ϕ = 45°
ϕ = 90°
ϕ = 135°
ϕ = 180°
ϕ = 225°
ϕ = 270°
ϕ = 315°
Figure 13: Mean and instantaneous phase locked images of CH emission and Acetone P-LIF of the flame foot
Phase locked measurements of acetone P-LIF and CH emission
The injector with the higher swirl (10-6h) is retained for this study. In unstable regime, acquisition are triggered
with the 240Hz oscillating pressure signal. Experimental conditions are φ=0.64, nozzle exit mean velocity at
70m/s, corresponding to fuel and air mass flow rates equal to 0.65g/s and 6.69g/s.
Instantaneous and mean images of CH chemiluminescence and acetone P-LIF are presented as function of
pressure phase. Both techniques are obtained for small field close to nozzle exit. The main part of the flame
develops downstream the measured region, since measurements are focused on the near nozzle region.
Mean evolution of mixing (P-LIF) and reaction (CH*) along the cycle are reported in figure 13, for 8 different
phases ϕ of the pressure cycle. ϕ = 0 corresponds to the minimum of pressure inside combustion chamber.
Concerning CH chemiluminescence images, for ϕ = 0, the main flame comes down inside the measurement zone
and has the closer value for distance to injector. When ϕ increases, flame moves downstream and the maximum
value for distance to injector is obtained for ϕ =180°. From ϕ =180°, a reaction zone with a light intensity comes
upstream, along a cone shape, up to injector. Intensity of this flame foot increases for ϕ =180° to ϕ =270°. For ϕ
= 315°, the flame foot disappears but main flame comes again in the measurement field. The behaviour of main
flame and of flame foot can be confirmed by the analysis of instantaneous images.
Concerning P-LIF images, a cyclic evolution is also observed with a minimum jet penetration distance at ϕ =90°
and maximum at ϕ =270°. A variation of intensity at the nozzle exit is also observed, with a maximum at ϕ =
180°. This variation can be interpreted as a fluctuation of injection equivalence ratio. Furthermore, two lobes are
observed in mean images as well as in instantaneous images, resulting from the interaction of the issuing jet with
the IRZ. It can be noticed that IRZ is physically at very short distance from nozzle exit, since the maximum for
this distance is about 1cm for ϕ =270°. For ϕ = 0, 45 and 90°, it seems that IRZ comes inside the injector.
A scenario is now proposed for the cyclic behaviour : when IRZ comes inside injector, the restriction of nozzle
exit area induces an increase of velocity. Resulting high strain rate, combined with lean equivalence ratio,
induces local extinction in the near nozzle region and a shift of main flame downstream. When IRZ moves out
the injector, nozzle exit velocity decreases and reaction can occur again in the near nozzle region, making main
flame shift upstream, dragging IRZ again towards injector.
Additional phenomenon might also be involved in flame oscillation. P-LIF signal intensity variation which is
observed at the injector exit might be attributed to equivalence ratio variations, which have an influence on flame
stability. This phenomenon was previously mentioned by Lieuwin an Zinn. [1998]. Furthermore, the coherence
observed in flame curvature at the jet base (instantaneous P-LIF images) might be due to a vortex shedding
phenomenon [Venkatraman 1999].
CONCLUSION
Flame structure and swirling flow in the restricted combustion chamber has been analyzed for two different
injection swirl number.
In the non reacting flow, a Corner Recirculation Zone is observed for both injectors. A structure transition occurs
when swirl is increased and an Inner Recirculation Zone is observed for the higher swirl injector.
The observed flow structures are maintained in the reacting flows, despite the decrease of effective swirl number
due to thermal expansion. Flame is always lifted, but is scattered and lifted far from injector for the lower swirl
while it is compact and relatively close by injector for the higher swirl. IRZ participates in flame holding and
allows operating at lower values of equivalence ratio.
However, this lean regime induces the apparition of a 240 Hz combustion instability. In CH emission and
acetone P-LIF phase locked images, a cyclic behaviour is observed at the flame base. The cyclic position of IRZ
relative to nozzle exit seems to be a key in flame stability, through local stretching and extinction.
REFERENCES
Anacleto P., Fernandes E., Heitor M. and Shtork S., "Characterization of a strong swirling flow with precessing
vortex core based on measurements of velocity and local pressure fluctuations" 11th International Symposium
on Application of Laser Techniques to Fluid Mechanics, (2002)
Donbar J., Driscoll J., Carter C., "Reaction zone structure in turbulent nonpremixed jet flames from CH-OH
PLIF Images" Combustion and Flame, vol.122, pp.1-19 (2000)
FLUENT, Version 6.0.20, Copyright 2001, Fluent Inc.
Gupta, L., Ed. Syred, Swirl flows energy and engineering science series (1984)
Higgins, B., McQuay, M., Lacas, F., Candel, S., "An experimental study of the effect of pressure and strain rate
on CH chemiluminescence of premixed fuel-lean methane/air flames" Fuel, vol.80, pp.1583-1591 (2001)
Hillemanns R., Lenze B., Leuckel W., "Flame stabilization and turbulent exchange in strongly swirling natural
gas flames", 21st Sy mposium International on Combustion, pp. 1445-1453, (1986)
Lieuwen,T., Zinn B. "The role of equivalence ratio oscillations in driving combustion instabilities in low NOx
gas turbines" 27th Symposium International on Combustion, pp. 1809-1816 (1998)
Lilley D., "Swirl flows in combustion : a review", AIAA Journal, vol.15, n°8, (1977)
Lozano A., Yip B. and Hanson R., "Acetone: a tracer for concentration measurements in gaseous flows by
planar-induced fluorescence", Exp. Fluids, vol.13, pp.369-376 (1992)
Miller M., "An experimental investigation of the effect compressibility on a turbulent reacting mixing layer"
Ph.D. Thesis, Stanford University, (1994)
Mongia R., Tomita E., Hsu F., Talbot L. and Dibble R., "Use of an optical probe for time-resolved in situ
measurement of local air-to-fuel ratio and extent of fuel mixing with applications to low NOx emissions in
premixed gas turbines", 26th Symposium International on Combustion, pp. 2749-2755 (1996)
Ohkubo Y.,Idota Y., Nomura Y., "Evaporating characteristics of fuel spray and low emission in a lean premixedprevaporization combustor for a 100 kW automotive ceramic gas turbine", Energy Conversion & Management,
vol.38 pp. 1297-1309 (1997)
Plee S., Mellor A., "Review of flashback reported in prevaporizing/premixing combustors", Combustion and
Flame, vol.32, pp.193-203 (1978)
Samaniengo J., Yip B., Poinsot T., Candel S., "Low-frequency combustion instability mechanisms in a siddump combustor" Combustion & Flame, vol.94, pp.363-380 (1993)
Taupin B., "Etude de la combustion turbulente à faible richesse, haute température et haute pression", Ph.D.
Thesis, Rouen University (2003)
Venkatarman K., Preston L., Simons D., Lee B., Lee G., Santavicca D., Journal of Propulsion and Power, vol. 15
n°6 (1999)
Weber R., Dugué J., "Combustion accelerated swirling flows in high confinements", Progress in Energy and
Combustion Science, vol.18, pp. 349-367 (1992)