Download Simultaneous measurements of OH, mixture fraction and velocity

15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
Simultaneous measurements of OH, mixture fraction and velocity fields
to investigate flame stabilization enhancement by electric field
Emilien Varea1, Karine Criner2, Gilles Godard3, Pierre Vervisch4,
Armelle Cessou5
1: CORIA, UMR 6614 CNRS, Université et INSA de Rouen, St Etienne du Rouvray, France, [email protected]
2: CORIA, UMR 6614 CNRS, Université et INSA de Rouen, St Etienne du Rouvray, France, [email protected]
3: CORIA, UMR 6614 CNRS, Université et INSA de Rouen, St Etienne du Rouvray, France, [email protected]
4: CORIA, UMR 6614 CNRS, Université et INSA de Rouen, St Etienne du Rouvray, France, [email protected]
5: CORIA, UMR 6614 CNRS, Université et INSA de Rouen, St Etienne du Rouvray, France, [email protected]
Abstract: Simultaneous stereoscopic PIV, OH and acetone PLIF was performed to analyze the processes
involved in the enhancement of flame stabilization by electric fields. The new processes envisaged to
reduced pollutant emission and fuel consumption can lead to the reduction of flame stability. Electric field
can enhance the flame stabilization with flexibility, but the physical processes involved are not well
understood. Our purpose is thus to investigate the flow properties in front of the flame base of a lifted flame
when a continuous electric field is applied by the simultaneous use of three laser diagnostics. A statistical
sampling is acquired (1100 acquisitions) and automatic post-processings are developed to provide
conditional analysis of the velocity and mixture fraction fields in analysis windows attached to the
instantaneous flame bases. The instantaneous flame base is identified from OH LIF. Velocity field is
calculated in an analysis window placed at a fixed distance from the flame base height. The mixture fraction
is determined from acetone LIF in a window attached to the instantaneous flame base. The conditional
measurements when electric field is applied are compared with conditional measurements without electric
field. These associated measurements show that the velocity decreases with electric field. To analyze the
mixture in front of the flame, a joined analysis of velocity and mixture fraction is required to conclude that
two situations are encountered: the more likely case is stoichiometric mixture, the second less probable is a
richer mixture (increase of 40%) associated to faster flow.
1. Introduction
Improvement of combustion processes are required to reduce pollutant emission and fuel
consumption. The new processes can lead to the reduction of the flame stability, especially when
the global fuel-air ratio is reduced. Then, it is particularly interesting to find new systems providing
more flexibility in assisting flame stabilization than the usual processes (bluff-body, stabilizer,
quarl, swirl, …). Electric fields are known to enhance stabilization of turbulent flame (Lee et al.
2005;Sakhrieh et al. 2005;Won et al. 2007;Won et al. 2008). The physical processes involved are
not well identified yet, and several interpretations of the phenomenon are proposed: change in flame
chemistry (Jaggers and von Engel 1971;Marcum and Ganguly 2005), ionic wind (Belhi et al.
2009;Hu et al. 2000). Then further investigations are required. When high voltage is applied by
locating an electrode through the flame, the flame polarizes and acts as an electrode then if ionic
wind occurs, it can modify the velocity in front of the flame. Thus the investigation of the flow
properties in front of the flame can provide useful information to check the assumption of ionic
wind and better understand the lift-off process when the flame is submitted to an electric field.
Our purpose is to measure the properties of the turbulent flows at the flame base when the electric
field is applied, and to compare it to the flow properties without electric field. We present the
simultaneous use of three laser diagnostics and the post-processing to obtain the velocity and the
mixture fraction fields conditioned on the flame base. Measurements of flame location, mixture
fraction and velocity fields are performed by simultaneous OH-acetone PLIF and PIV. For flames
without electric field, simultaneous laser diagnostics, particularly PIV and PLIF, have already
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
shown their interest for investigating the flow properties conditioned on the flame base height (Han
and Mungal 2000;Maurey et al. 2000;Maurey et al. 1998;Muñiz and Mungal 1997;Schefer and
Goix 1998;Su et al. 2000). In our study, the configuration investigated consists in a lifted turbulent
jet flame. The flame is polarized by an electrode placed within the flame far downstream (~80D),
and at which a positive DC high voltage is applied. When the high voltage is applied, lift-off height
of the flame is reduced of 72%. To characterize the flow properties in front of the flame base,
simultaneous and instantaneous measurements of OH, mixture fraction and velocity fields are
performed by simultaneous PIV, OH and acetone PLIF. A high sampling of 1100 instantaneous
images is acquired to allow the statistical analysis of the quantities measured. Specific automatic
post-processings have been developed to determine velocity and mixture fraction conditioned on
the instantaneous flame base location.
The pdfs of lift-off height are measured with and without electric field. The flame height decreases
strongly when the electric field is applied. Without electric field our measurements of axial velocity
conditioned on the flame base are in agreement with the literature (Maurey, Cessou, Lecordier and
Stepowski 2000;Muñiz and Mungal 1997). When the electric field is applied, the axial velocity
conditioned on the flame base is reduced of 57%. To analyze the mixture in front of the flame, a
joined analysis of velocity and mixture fraction is required to conclude that two situations are
encountered: the more likely case is stoichiometric mixture, the second, less probable, is a richer
mixture (increase of 40%) associated to faster flow.
2. Experimental set-up
The facility consists in a central tube (ceramic, 5 mm inner diameter) fed with propane at 8.8 m/s,
surrounded by air coflow (50 mm inner diameter) at 0.5 m/s. The air flow can be fed with different
tangential components, providing different swirl numbers. In the present study the air flow is
without swirl, leading to a flame stabilized in a coflow. The electric field is applied to the flame by
inserting an electrode in the trailing diffusion flame (Figure 1), ~80D downstream. A high voltage
of 6 kV is applied to the electrode, then the overall flame is polarized at a voltage around 3 kV.
Figure 1. Application of electric field by an electrode inserts within the flame 80 D downstream of the injector
The jet and the air coflow are seeded with micron DEHS droplets for PIV measurements.
Stereoscopic PIV is applied to measure the three components of velocity, especially in the purpose
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
of investigate swirling flow. In the present paper, only results for flames without swirling coflow
are presented. The PIV apparatus consists of a two-pulse frequency-doubled Nd:YAG (Quantel,
Twins Ultra 120 mJ, CFR200PIV). The scattered light from particles is collected onto two interline
CCD cameras (LaVision, Flowmaster3, 12 bits, 1280×1024) disposed in a forward-backward
scattering configuration and equipped with Scheimpflug adapters. A field of view of
64.8×81.1 mm2 with a magnification of 63.3 µm/pixel.
Two LIF methods are used to visualize the flame and the mixture fraction field. The reaction zone is
visualized by OH LIF. OH LIF signal is a good marker of the marker of the interface between fresh
and burned gases in premixed flames and is a rather a marker of the lean side of a diffusion flame
(Watson et al. 1999). Due to its long life time OH LIF signal persists in the burned gases, but in the
first part of lifted jet flames, numerous studies have shown the suitability of this marker (Maurey,
Cessou, Lecordier and Stepowski 2000;Schefer et al. 1990;Seitzman et al. 1990), which provides
high level of signal and a mean to identify the leading-edge of the flame (Maurey, Cessou,
Lecordier and Stepowski 2000;Maurey, Cessou and Stepowski 1998). To measure the fuel
concentration, in the cold flow (i.e. upstream the flame), propane is seeded with acetone. PLIF of
acetone provides propane concentration field, and then mixture fraction field in the cold gas. Vapor
of acetone is added in propane by a CEM-liquid delivery system (Bronkhorst). The volumetric
percentage of acetone is 11%. This quite high value is imposed by the need of a large signal
dynamic to measure simultaneously in the same image high value of fuel mole fraction (χ=1) and
very small value (χ~0.04) close to the stoichiometry. The measurement of that low signal level with
correct signal-to-noise ratio imposed high signal level in the pure jet. If this percentage of acetone
can modified the chemical flame properties compared to a pure propane flame, and it is more
correct to consider that we burnt a propane-acetone mixture. The stoichiometric value of mixture
fraction is however not very modified by this addition of acetone, and then the radial location of the
flame in the mixing layer is not changed. OH radicals in flame and acetone in fresh gases are
excited through a laser sheet at 282.927 nm. Thus OH ground state is probed pumping the Q1(6)
line of (0,1) vibrational band of the (X2Π-A2Σ) electronic transition. The laser radiation (dt= 10 ns,
E~12 mJ) is provided by the second harmonic of a dye laser (TDL50, Quantel) pumped by the
second harmonic of a Nd:YAG (YG180, Quantel). The OH fluorescence is detected via the (1,1)
and (0,0) radiative relaxation bands. The strong elastic scattering from PIV seeding was suppressed
by a hig-pass filter (SCHOTT WG305, 3mm), the possible PAH LIF signal was reduced by a lowpass filter SWP606 (Melles Griot). The fluorescence from the laser sheet (60 mm high x 400 µm
thick) is imaged at 90° onto a gated intensified CCD camera (PiMax, Princeton Instrument, Gen II,
5 MHz, 512x512). The field of view is 74.8×74.8 mm2 with a pixel resolution of 6.84 pixel/mm.
SPIV and LIF set-up are associated as presented on the Figure 2. The PIV and LIF lasers are
combined by a dichroic mirror reflecting the UV laser beam and transmitting the green laser beam.
The two apparatus are synchronized to place the LIF laser pulse between the two PIV pulses. The
LIF laser is running at 10 Hz and provides a trigger signal to put in phase the 30 Hz PIV laser by
the Davis software. The LIF and PIV acquisitions are authorized when all the cameras are ready all
together.
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
Figure 2. Experimental set-up for simultaneous measurements of SPIV, OH and acetone LIF
3. Post-processing for conditional measurements
Our objective is to measure simultaneously the velocity and mixture fraction fields conditioned on
the location of the flame base (Figure 3), defined as the most upstream point of the OH LIF zones
(Maurey, Cessou, Lecordier and Stepowski 2000). The post-processing must begin by the
measurement of the coordinates of MB(hB,rB) on each LIF image. The LIF images contain both OH
and acetone LIF signals. Thus the discrimination of the two fluorescence signals is required. LIF
images are post-processed in three stages: the correction of the spatial inhomogenity of the laser
sheet, the discrimination of the OH and acetone LIF signals, the analysis of the signals to determine
MB from OH LIF and to measure Z from acetone LIF.
Figure 3. Window analysis anchored to the flame base defined from OH LIF
Before post-processing and analyzing the LIF images, the shot-to-shot laser energy fluctuations and
the relative spatial non-homogeneities of the laser sheet have to be corrected. The shot-to-shot
energy fluctuations are corrected by averaging the acetone LIF signal of a small region of interest
placed in the jet potential core region at the exit of the injector. From one shot to another the
changes in shape of the laser sheet profile are moderate. Then the laser sheet inhomogeneities are
corrected from an average normalized profile. The energy laser sheet profile is deduced from the
profile of spontaneous Raman scattering signal of liquid ethanol. The Raman signal is acquired just
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
after each series of acquisitions by placing a parallelipedic cell of quartz (10x10x100 mm) near the
injector. The instantaneous vertical profiles are extracted by averaging 8 columns of the cell image.
The shot-to-shot stability of the laser sheet profile is checked from these instantaneous profiles (5%
rms). The average normalized profiles calculated from these instantaneous profiles are not collected
simultaneously of the acquisition of the LIF images. Then it was also checked that the average laser
sheet profile is representative of the laser profile during the LIF acquisition by comparing the
Raman profiles to the acetone LIF profile in the potential core of the jet below the flame (h/D<5).
The LIF images are then divided line by line by the normalized Raman profile values. A median
filter of (3×3) is applied to the images.
OH and acetone fluorescence signals are collected on the same image. The OH fluorescence regions
characterized the reactive zones from each side of the jet centerline, and the acetone fluorescence
provides the mixture fraction field upstream the flame. At their boundary, the two fluorescence
regions are separated by a thin layer where the two signals decrease (Figure 4): for OH due to the
decrease of concentration at the flame boundary, for acetone due to pyrolysis (Clemens and Paul
1995). The LIF image is binarized from different threshold levels. The threshold level is decreased
up to three independent zones and their outlines are identified: two for the OH zones on each side of
the jet centerline and one for the acetone zone (Figure 5). From the both reaction zones identified on
the LIF images, two flame bases can be defined since the two measurements are independent
(Cessou and Stepowski 1996;Seitzman 1994).
Figure 4: Example of OH and acetone LIF image
Figure 5 : Example of outlines defined to determine
the instantaneous flame base on the left and on the
right of the jet centerline
From the coordinates of the flame base determined from OH LIF, velocity and mixture fraction
fields are determined within windows anchored at the flame base. The locations of the analysis
windows for velocity and mixture fraction change at each instantaneous acquisition. Figure 6 et
Figure 7 show examples of instantaneous velocity and mixture fraction fields anchored to the flame
base .
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
Figure 7: Example of instantaneous mixture
fraction field conditioned on the flame base
Figure 6 : Example of instantaneous velocity field
conditioned on the flame base
The PIV processing is performed with the LaVision software (DaVis 7.2), by standard FFTcorrelation with a multi-pass scheme consisting of two passes with interrogation window size of
32×32 followed by two passes with 16×16 and overlaps of 87%. The vector fields are validated
from median filter, peak ratio measurements and elimination of isolated vectors. Specific processing
is performed to measure the velocity field conditioned on flame base location, MB, defined from
OH LIF. The principle is to anchor an analysis window at MB. In the DaVis software this
conditional measurement is performed by defining a mask on each instantaneous acquisition,
corresponding to the analysis window. Stereoscopic viewing leads to perspective deformation of the
images of particles. In the PIV software used the correction of this deformation is performed once
the velocity field determined. The PIV software computes first the 2D-2C vector field for each
camera before computing the 2D3C-vector fields by stereoscopic reconstruction. The location of
MB is known on the corrected images, but not within the raw deformed images of particles. We
have placed the analysis window without taking account of the deformation and we have
interpolated the final result to obtain the velocity in front of MB. First a dynamic rectangular mask
of 125 × 50 pixels (7.8 mm×3 mm), is anchored 700 µm below the point MB and centered on rB.
This distance larger than the half-width of the PIV correlation window of the first pass, ensures the
measurement is always performed in a region of homogeneous seeding since the shift between the
base of the OH LIF region and that of the region with no Mie scattering signal is ±200 µm. Within
these small images the PIV processing is performed for the two stereoscopic pairs of particle
images. The deformed velocity field is reconstructed by Soloff calibration method. On the velocity
fields corrected from the deformation, we note that the maximum error we make on the vertical
position is 5 pixels (300 µm). On the horizontal location, the deformation can lead to a maximal
shift of 30 pixels (2 mm). Despite this shift the point MB is always included in the radial range
explored within the analysis window. Then the velocity field is interpolated to obtain data centered
on rB.
The mixture fraction fields are measured from the acetone LIF signal (Figure 5), corrected from the
laser shot-to-shot fluctuation and the spatial non-homogeneity of the laser sheet. The signal is
normalized by the signal value in the potential core to provide the fuel mole fraction, and the
mixture fraction is calculated. For each instantaneous LIF image, an analysis window of 50×25
pixels (7 mm×3.5 mm) is placed at 1 mm from the flame base. Thus the mixture fraction is
determined in the isothermal region upstream the flame where no change in fluorescence efficiency
of acetone due to temperature occurs. The conditional measurements of Z are performed only for
height smaller or equal to 55 mm. This reduction of sampling is due to the necessity of performing
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
the measurement in a part of the laser sheet where the signal level is high enough to obtain an
accurate value of Z. With DC, the flame is closer to the injector and it stays easily in an intense part
of the laser sheet, leading to weaker rejection.
Without DC, the pdf of the properties measured are identical on the left and right sides of the jet, all
the measurements were grouped together to double the number of samples.
4. Results
The instantaneous height and radius of the flame base are measured from OH LIF, with and without
electric field. The Figure 8 shows the pdfs of the flame height without and with electric field
respectively. These measurements are performed from OH LIF images obtained without acetone
LIF signal. Without electric field the flame height ranges between 30 mm and 70 mm, with a modal
value around 55 mm. With electric field the flame stabilizes distinctly more upstream, and the liftoff height ranges between 7 mm and 35 mm. In this case, the left and right measurements are shown,
since the flame is not symmetrical with electric field. The flame stabilizes at lower height on the left
side. This behavior comes from an non-homogeneity of the electric field due to an nonhomogeneity of the lip of the ceramic injector. For the lowest case the pdf presents a dissymmetry,
which is not a priori linked to an electric field effect but to the fact that the flame frequently
stabilizes at heights lower than the jet potential core, since the symmetry properties of the pdf of
lift-off height are linked to the region of the turbulent jet and to the mixing layer properties where
the flame stabilizes (Cessou et al. 2004). In the following only the measurement on the right are
presented. With electric field, the average height is clearly reduced from 55 mm to 20 mm.
Figure 8. decrease in lift-off height when DC electric field is applied. With DC, the measurements on the left and
the right of the jet centerline show the dissymmetry of the flame base.
The axial velocity conditioned on MB decreases when the electric field is applied (Figure 9).
Without electric field the velocities range between -0.2 and 4 m/s, with a modal value of 1.3 m/s
and a mean value of Ū=1.4 m/s With electric field, the velocities in front of MB range between 0.3 m/s and 2.5 m/s, with a modal and a mean value of 0.6 m/s.
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
Figure 9 : Decrease in the axial velocity conditioned on the flame base when DC electric field is applied
The mixture fraction conditioned on MB increases when the electric field is applied. Figure 10
shows the pdf of mixture fraction measured in front of the flame base. Without DC, Z ranges
between 0.03 and 0.2 with a modal value, Z=0.055, close to the stoichiometric value Zs=0.06. The
pdf of Z is non symmetrical and presents a tail to the larger values of Z. This dissymmetry leads to a
mean value of 0.08, larger than the modal the modal value. Then in the following we will use the
modal value for the analysis, considering its statistical representativity. With DC the pdf shifts
slightly to the large values of Z, and presents a modal value of Z = 0.07.
Figure 10 : shift of the mixture fraction in front of the flame base when DC electric field is applied
Since we measure simultaneously the axial velocity and the mixture fraction in front of the flame
base, the joined analysis of these quantities are performed. The joined histogram of the axial
velocity and mixture fraction conditioned on MB shows an intense peak at U = 0.65 m/s and Z =
0.055, and a less intense secondary peak at U = 0.95 and Z = 0.077 (Figure 11).
Figure 11 : Joined histogram of axial velocity and mixture fraction in front of the flame base
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
The comparison of the average field of Z conditioned on MB without and with DC fields shows that
the flame stabilizes in a thinner mixing layer when the electric field is applied (Figure 12). With DC
field the flame stabilizes at more upstream position, within a thin mixing layer where the higher
value of Z gradient than without DC can be expected. This result shows clearly that with the electric
the flame can persist within regions ordinarily very unfavorable for combustion, due to rapid
dissipation of the heat released by the flame.
Figure 12 : Average mixture fraction field within the analysis window anchored at the flame base without (left)
and with (right) electric field. When the electric field is applied, the flame stabilizes in a region where the mixing
layer is thinner and where higher Z gradients can be expected.
5. Discussion
For few years the enhancement of flame stabilization by electric field has known a renewed interest.
Our study shows how three simultaneous laser diagnostics provide very interesting results to
investigate the enhancement of flame stabilization by electric field. Very few studies using laser
diagnostics are dedicated to this flame behavior. We applied simultaneously PIV, acetone LIF and
OH LIF to measure the velocity, mixture fraction, to localize the reaction zone and provide
conditional measurements. 1100 instantaneous acquisitions of PIV and LIF are analyzed for flames
with and without electric field. The pdfs of lift-off height are measured with and without electric
field. The flame height decreases strongly when the electric field is applied. We conditioned the
measurement of velocity and mixture fraction on the flame base location (MB), by attaching an
analysis window to each instantaneous left and right flame bases identified from OH LIF images.
This conditional measurement provides information on the flow properties in the direct
neighborhood of the flame base. Other works have proposed to investigate the properties in front of
the leading-edge (Domingo 2002;Maurey, Cessou, Lecordier and Stepowski 2000). This type of
analysis requires oriented PIV calculation, which we do not have for SPIV measurements. However
previous works (Muñiz and Mungal 1997;Schefer and Goix 1998;Su, Han and Mungal 2000;Su et
al. 2006) have shown that measurements in the direct neighborhood of the flame base provide very
useful information for investigation of flame stabilization processes.
The conditional measurement of velocity shows that the mean axial velocity in front of the flame
base decreases when the electric field is applied from 1.4 to 0.6 m/s. The axial velocity in front of
the flame is closed to the “stoichiometric velocity” (Han and Mungal 2000), defined as
Us=ZsU0+(1-Zs)UCF, considering that the flame stabilizes along the stoichiometric surface within
the mixing layer. Without electric field our results are in agreement with the other similar works
with axial velocities between -0.2 and 4 m/s and a mean value of Ū=Us=1.4 m/s. When electric field
is applied the mean axial velocity is clearly smaller than Us, since Ū=0.6 m/s. This decrease can
support the assumption of ionic wind induced by the acceleration the charged species of the flames.
From the amplitude of the velocity decrease the force induced by the electric field could be
evaluated, knowing the density of charged species within the flame (Belhi, Domingo and Vervisch
2009) and also their production rate (Criner 2008). For this evaluation it is also needed to know if
the flame is still stabilized in a stoichiometric region when the electric field is applied, what it is
usually assumed.
The first analysis of the pdf of Z conditioned on MB leads to an inappropriate conclusion in showing
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
the mixture fraction increases when the electric field is applied. When the electric field is applied
the modal value shifts from 0.055 without DC field to 0.07 with DC field. However the joined
histogram of U and Z conditioned on MB shows two peaks, one at U= 0.6 m/s and Z= 0.055 and a
second less intense at U= 0.95 m/s and Z= 0.077. This result shows that the mixture fraction
conditioned on MB stays equal to the modal value measured without DC field on Figure 11. The
shift observed in Figure 10 comes from the projection of secondary peak onto the Z axis. The
presence of the secondary peak requires further investigations. It can come from the difficulty
encountered by the flame to stabilize so upstream, where the mixing layer is very thin (Figure 12).
A further analysis of our results will also show if this secondary peak corresponds to particular
locations of the flame base, since the values of hB and rB are measured simultaneously.
Concerning the laser diagnostic methods, the simultaneous use of three laser diagnostics provides
original results to better understand the physical processes involved in stabilization enhancement of
flame by electriec field. We have chosen to use only one camera to collect simultaneously the LIF
signals from OH and acetone. This configuration constitutes a compromise between the cost of the
set-up and the simplicity of the post-processing. An optical separation of the two LIF signals by
dichroic sheet and two cameras would simplify the post-processing to determine the flame base
location and then increase the number of data validated. Our configuration reduces the cost of the
set-up, but it imposes the discrimination of the OH and acetone LIF signals by post-processing. In
the present study, this discrimination is successful with a percentage of measurement validated
keeping high (80%).
Our uncertainty in Z measurement is between 5 and 10 % depending of the signal level. This level
of uncertainty is suitable for the statistical analysis performed in the present study. One part of this
uncertainty comes from the laser profile shot-to-shot fluctuations, while only the average laser sheet
profile is corrected. Our set-up presents however the interest of evaluating the laser sheet profile insitu, but it requires the precaution to verify the shot-to-shot stability of the laser profile shape to
obtain an average profile representative of the instantaneous profiles. If this stability were not
obtained a simultaneous laser-sheet profile estimation would be required.
The knowledge of dissipation rate (2D∇Z.∇Z) conditioned on stoichiometry or flame base is
required to better understand the process of flame stabilization. Therefore is data is also essential to
describe the stabilization process when electric field is applied. Since we measure 2D mixture
fraction field, we could complete our data of the estimation of the scalar dissipation. A priori our
set-up makes this quantity difficult to obtain, especially near stoichiometry where the signal is very
low. This low level of signal is linked to the low mole fraction of fuel, and thus fuel tracer, at the
stoichiometry compared to the level at Z=1 at the exit of the injector. The acquisition of the signal
in the potential core of the jet is needed to estimate the mole fraction, and then Z. Seeing that we
record on the same image the high level of signal at the exit of the injector and the low level at the
flame base far downstream, the level near the flame base is very low (~4% of the value at the
injector exit) and the spatial resolution is poor due to the wide field of view needed. Therefore an
accurate measurement of the Z gradient conditioned on the stoichiometry or the flame base, will be
very difficult to obtain.
6. Conclusion
Our measurements show that the velocity in front of the flame is reduced when the electric field is
applied, and that the mixture fraction keeps close to the stoichiometry.To obtain these results we
performed simultaneous measurements of PIV, OH and acetone LIF to investigate the enhancement
of stabilization of lifted jet flame by DC electric field. Since the studies of turbulent flames required
high statistical sampling, we acquired 1100 images for each operating condition and we developed
specific automatic post-processing to perform conditional measurement of the aerodynamic and
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15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
mixture properties of the flow. An analysis window is attached to each instantaneous flame base
identified from OH LIF. The conditional measurements of flame where electric field is applied are
compared with conditional measurements without electric field. Our measurements show that the
velocity in front of the flame base decreases when the electric field is applied. The joined histogram
of Z and U conditioned on the flame base show that the mixture fraction remains close to
stoichiometry but some larger values associated with larger value of U are also observed.
In the present paper, we present measurement conditioned on the flame base, but the measurements
of velocity and mixture fraction field between the injector and the flame base are also available.
Another interesting prospect will be to access to the velocity in front of the flame leading-edge as
performed in a previous work with 2 components PIV (Maurey, Cessou, Lecordier and Stepowski
2000). Such a measurement requires oriented PIV calculation which needs to be developed for
SPIV measurement.
Our results show that the simultaneous use of three laser diagnostics (SPIV, OH and acetone LIF)
provides a useful data base to better understand the process of stabilization enhancement by electric
field, to propose physical model, and to provide validation tool for numerical modeling.
7. Acknowledgment
This research project was supported by a contract with SNECMA Moteurs, and the CNRS.
8. References
Belhi M, Domingo P, Vervisch P (2009) Effect of Electric Field on Flame Stability Fourth
European combustion Meeting, ECM2007. Vienna, Austria,
Cessou A, Maurey C, Stepowski D (2004) Parametric and statistical investigation of the behavior of
a lifted flame over a turbulent free-jet structure. Combust Flame 137:458-477
Cessou A, Stepowski D (1996) Planar Laser Induced Fluorescence Measurement of OH in the
Stabilization Stage of a Spray Jet Flame. Combust Sci Technol 118:361-381
Clemens NT, Paul PH (1995) Effects of Heat Release on the Near Field Flow Structure of
Hydrogen Jet Diffusion Flames. Combust Flame 102:271-284
Criner K (2008) Stabilisation de flammes de diffusion turbulentes assistée par plasma hors-équilibre
et par champ électrique. University de Rouen,
Domingo P, Vervisch, L., Bray, K., (2002) Partially premixed flamelets in LES of nonpremixed
turbulent combustion. Combust Theory Modelling 6:529-551
Han D, Mungal MG (2000) Observations on the Transition From Flame Liftoff to Flame Blowout.
Proc Combust Inst: 28:537-543
Hu J, Rivin B, Sher E (2000) The effect of an electric field on the shape of co-flowing and candletype methane-air flames. Experimental Thermal and Fluid Science 21:124-133
Jaggers HC, von Engel A (1971) The effect of electric fields on the burning velocity of various
flames. Combust Flame 16:275-285
Lee SM, Park CS, Cha MS, Chung SH (2005) Effect of Electric Fields on the Lioff of Nonpremixed
Turbulent Jet Flames. IEEE Transactions on Plasma Science 33:1703-1709
Marcum SD, Ganguly BN (2005) Electric-field-induced flame speed modification. Combust Flame
143:27-36
Maurey C, Cessou A, Lecordier B, Stepowski D (2000) Statistical flow dynamic properties
conditioned on the oscillating stabilization location of turbulent lifted flame. Proc Comb Inst
28:545-551
Maurey C, Cessou A, Stepowski D (1998) Simultaneous P.I.V. and OH Planar L.I.F. in the
Stabilization Region of a Lifted-Jet Diffusion Flame. Ninth International Symposium on the
- 11 -
15th Int Symp on Applications of Laser Techniques to Fluid Mechanics
Lisbon, Portugal, 05-08 July, 2010
Applications of Laser techniques to Fluid Mechanics, Lisbon, Portugal, July 0:Muñiz L, Mungal MG (1997) Instantaneous Flame-Stabilization Velocities in Lifted-Jet Diffusion
Flames. Combust Flame 111:16-31
Sakhrieh A, Lins G, Dinkelacker F, Hammer T, Leipertz A, Branston DW (2005) The influence of
pressure on the control of premixed turbulent flames using an electric field. Combust Flame
143:313-322
Schefer RW, Goix PJ (1998) Mechanism of Flame Stabilization in Turbulent, Lifted-Jet Flames.
Combust Flame 112:559-574
Schefer RW, Namazian M, Kelly J (1990) CH, OH and CH4 Concentration Measurements in a
Lifted Turbulent-Jet Flame. Proc Combust Inst 23:669Seitzman JM, Ungüt A, Paul PH, Hanson RK (1990) Imaging and Characterization of OH
Structures in a Turbulent Nonpremixed Flame. Proc Combust Inst 23:1361Seitzman JM, Ungut, A., Paul, P.H., Hanson, R.K., (1994) Structural Characterization of a
Turbulent Nonpremixed Hydrogen-Air Flame with PLIF of OH. Western States Section,
The Combustion Institute, Fall Meeting, October 0:Su LK, Han D, Mungal MG (2000) Measurements of Velocity and Fuel Concentration in the
Stabilization Region of Lifted Jet Diffusion Flames. Proc Combust Inst 28:327-334
Su LK, Sun OS, Mungal MG (2006) Experimental investigation of stabilization mechanisms in
turbulent, lifted jet diffusion flames. Combust Flame 144:494-512
Watson KA, Lyons KM, Donbar JM, Carter CD (1999) Scalar and Velocity Field Measurements in
a Lifted CH4-Air Diffusion Flame. Combust Flame 117:257-271
Won SH, Cha MS, Park CS, Chung SH (2007) Effect of electric fields on reattachment and
propagation speed of tribrachial flames in laminar coflow jets. Proc Comb Inst 31:963-970
Won SH, Ryu SK, Kim MK, Cha MS, Chung SH (2008) Effect of electric fields on the propagation
speed of tribrachial flames in coflow jets. Combust Flame 152:496-506
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