NO-flow tagging by photodissociation of NO2. A new approach for measuring small-scale flow structures

Article abstract of DOI:10.1016/S0009-2614(99)00512-6

A new flow tagging technique was developed which visualizes small-scale flow structures by writing a spatial line of NO into an air flow homogeneously seeded with NO₂. The NO-line was generated by photodissociation of NO₂ at 308 nm using a XeCl excimer laser and was imaged by planar laser-induced fluorescence at various delays after its formation. With seeding levels of 600 ppm, signal-to-noise levels are shown to be sufficient for detecting the shifted structure at delays of up to 20 ms. The technique was used to resolve small scale turbulent flow structures within a quartz cell.

Full text of DOI:10.1016/S0009-2614(99)00512-6

2
5 June 1999
Chemical Physics Letters 307 Ž1999. 15–20
NO-flow tagging by photodissociation of NO . A new approach
2
for measuring small-scale flow structures
)
Claus Orlemann, Christof Schulz , J u¨ rgen Wolfrum
Physikalisch-Chemisches Institut, UniÕersit a¨ t Heidelberg, INF 253, 69120 Heidelberg, Germany
Received 12 April 1999
Abstract
A new flow tagging technique was developed which visualizes small-scale flow structures by writing a spatial line of NO
into an air flow homogeneously seeded with NO . The NO-line was generated by photodissociation of NO at 308 nm using
2
2
a XeCl excimer laser and was imaged by planar laser-induced fluorescence at various delays after its formation. With
seeding levels of 600 ppm, signal-to-noise levels are shown to be sufficient for detecting the shifted structure at delays of up
to 20 ms. The technique was used to resolve small scale turbulent flow structures within a quartz cell. q 1999 Elsevier
Science B.V. All rights reserved.
1
. Introduction
The contribution of the molecular tracer to the
total mass flow is generally negligible, avoiding a
significant alteration of the flow. On the other hand,
diffusion plays an important role and causes dilution
of tracers within the tagged volume element. There-
fore molecular tracers are restricted to investigations
at high flow velocities or small spatial scales but
they are the best choice for applications which aim
to study interactions between molecular and turbu-
lent mixing.
Three different applications of molecular tracers
are common: first, the Doppler shifts of LIF- or
Raman-signals can be used to evaluate velocities.
This has been shown in both seeded flows Že.g. by
LIF of an NO seeded supersonic flow w7x, LIF of
iodine w3x, LIF of sodium vapor w8x. and by Rayleigh
w5x or Raman w2x scattering in unseeded flows. In
A number of different approaches for measuring
flow patterns by molecular tracers have been pub-
lished w1–10x. In contrast to particle or droplet seed-
ing, molecular tracers show a better ability to follow
flows with high velocity gradients. This circumvents
typical problems with particle seeding where at high
speeds and high turbulence tracking of the gas flow
by particles may be poor. Homogeneously seeded
flows allow investigation close to walls, where elec-
trostatic forces may cause problems with window
fouling, and in the center of turbulence elements,
where it is difficult to inject particles due to centrifu-
gal forces. Furthermore, molecular tracers allow uni-
form seeding even in large-scale facilities.
inhomogeneously seeded flows, velocities or mixing
are visualized by the detection of concentration gra-
dients which are induced by partial or pulsed seeding
of the gas flow, e.g. using acetone w9x or NO w10x. In
)
Corresponding author. Fax: q49 6221 544255; e-mail:
christof.schulz@urz.uni-heidelberg.de
0
009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0009-2614Ž99.00512-6

1
6
C. Orlemann et al.rChemical Physics Letters 307 (1999) 15–20
y19
pulsed configurations, however, altering the flow by
the injection has to be carefully avoided. Due to the
statistic nature of the structure of the tracer clouds,
these techniques complicate measurements when
well-defined starting conditions are required. In con-
trast, flow tagging techniques are based on the move-
ment of structures which are generated within the
flow by photo-induced processes. For unseeded
flows, Miles et al. proposed RELIEF ŽRaman-excita-
tion and laser-induced electronic fluorescence. of O2
tion cross-section of NO2 Žs s 1.7 = 10
2
cm rmol w15x. is about a factor of 2.5 below its
maximum around 400 nm, which is compensated for
by the high power output of a XeCl excimer laser. At
y19
226 nm, the absorption cross-section is 3.9=10
2
cm rmol w15x, leading to the production of addi-
tional NO by the detection laser beam. This contribu-
tion, however, is negligible since the pulse energy
density of the detection beam is about three orders of
magnitude weaker than that of the photodissociation
beam.
w4x. Boedeker measured velocities by H O photolysis
2
and subsequent LIF detection of OH w1x, and Ribarov
et al. w6x showed an ozone tagging technique by
In the photodissociation process, NO is either
produced initially by
imaging an ArF-Laser written line of O3.
2
2
3
NO2
Ž
X A
.
qhn™NO
Ž
X P
.
qO
Ž
P
.
,
1
Ž .
1
In the study presented here, air flows were homo-
geneously seeded with NO at low concentrations Ža
or at high NO concentrations by the subsequent
2
2
few hundred ppm.. By photodissociation of NO2 ,
reaction of oxygen atoms
NO was formed, which can be imaged by planar LIF
at various delays after its generation. Whereas sev-
eral other tracers disappear rather quickly due to
energy transfer processes Žfrom vibrationally excited
molecules w4x. or high reactivity Žin the case of OH
w1x., due to small reaction cross-sections, NO is
stable on the timescale of interest. Therefore, using
NO, the displacements of volume elements can be
imaged at longer times and larger distances com-
pared to other techniques. Furthermore, with the
molecular weight of NO being between that of N2
3
2
2
3
O
Ž
P
.
qNO2
Ž
X A
.
™NO
Ž
X P
.
qO2
Ž
X S
.
.
1
Ž .
2
At atmospheric pressure and room temperature,
the non-thermal vibrational populations after pho-
todissociation w16–18x relax to the vibrational ground
state within few nanoseconds. Therefore, in non-re-
active flows, NO detection via LIF imaging requires
excitation from the vibrational ground state.
The shape of the spatial line of NO is determined
by the dissociation laser profile, but the NO-produc-
tion channel Ž2. may cause a broadening of the line
because of the diffusion of free oxygen atoms before
attacking other NO -molecules. However, due to the
and O and diffusion coefficients close to nitrogen,
2
NO turns out to be a good choice for characterizing
the small-scale behavior of turbulent air flows.
2
low abundance of NO2 in our experiments Ža few
hundred ppm. the contribution of channel Ž2. is
expected to be negligible and the overall dissociation
2
. Photophysics of NO2
A well-defined spatial line of NO can easily be
produced by photodissociation of NO . Jones and
2
Bayes w11x state from measurements at 300 K and
NO -pressures between 0.5 and 4 Torr that the over-
2
all dissociation quantum yield is constant and close
to 2. An increase in either the N or O mol fraction
2
2
was found to reduce the quantum yield due to altered
kinetics w12,13x. Photodissociation occurs at wave-
lengths shorter than 398 nm, which corresponds to
the NO dissociation limit at 3.115 eV w14x. Never-
2
Fig. 1. Layout of the quartz cell in which both static and flow
experiments were carried out. Main flow direction is from right to
left, dissociation and detection laser were coupled in from the left
side.
theless, absorption cross-sections of NO vary over
2
more than one order of magnitude for commercially
available UV laser sources. At 308 nm, the absorp-

C. Orlemann et al.rChemical Physics Letters 307 (1999) 15–20
17
quantum yield is expected to be close to unity with
NO formation occurring predominantly by reaction
Ž1.. With increasing pressure, in addition to mecha-
nisms Ž1. and Ž2. the formation of NO3 w12x via
three-body reactions has to be considered. Then, the
overall quantum yield of NO formation is further
reduced w19,20x by
bution of NO was detected with a gated Ž100 ns.
image-intensified slow scan CCD-camera ŽLaVision,
Flame Star III. equipped with an achromatic lens
ŽHalle, fs100 mm, f s2.. The arrangement of the
a
optical system is shown in Fig. 2.
4
. Generation of NO2
NO qNO™NO qNO .
3
Ž .
3
2
2
Handling and controlling constant flows of NO2
under atmospheric or higher pressure conditions is
3
. Experimental
difficult due to condensation of NO and formation
2
of N O . Therefore, to provide constant and repro-
Measurements were carried out in NO -seeded
2
x
2
flows passing through a T-shaped quartz cell ŽFig.
ducible seeding, NO was produced from the reac-
2
tion of NOqO by mixing NO ŽMesser Griesheim.
1
.. For photodissociation of NO , the beam of a
2
2
XeCl excimer laser Ž308 nm, Lambda Physik EMG
with air in a 5 l steel chamber. The individual flows
were controlled by mass flow controllers ŽTylan FC
2
62. with a maximum flow of 100 slm and 66 sccm
for air and NO, respectively. At standard conditions,
1
50 TMC. was focused by a combination of two
cylindrical lenses Ž fs2000 mm and 500 mm, re-
spectively.. The resulting photodissociation beam had
an average height of 0.8 mm and a width of 1 mm,
the ratio of equilibrium concentrations of NO and
2
2
NO lies far on the side of NO . To give the system
resulting in energy densities of up to 225 MWrcm ,
2
enough time to reach equilibrium, a second steel
chamber of 50 l was used to increase residence
times. The influence of the initial NO concentration
and residence time on the signal-to-background ratio
upon NO2 photodissociation and NO-LIF detection
was investigated.
which were varied with a dielectric attenuator. Imag-
ing of room temperature NO usually was carried out
with excitation of the A–XŽ0,0. transition at 226 nm
w21x. The beam Ž1.3 mJrpulse. of a XeF excimer-
pumped ŽLambda Physik LPX 200. frequency-dou-
bled dye-laser ŽLambda Physik, Scanmate IIe. was
formed to a light sheet of 0.1 mm=2.7 mm. LIF-
signals of the NO A–XŽ0,1., Ž0,2. and Ž0,3. transi-
tions Žat 235–258 nm. were separated from scattered
5
. Results and discussion
light by two dielectric mirrors acting as bandpass
filters ŽLaseroptik, HR 248 nm at 458, FWHM: 20
nm.. Delays between the formation and detection of
When detecting the NO generated by photodisso-
ciation of NO , the signal-to-background ratios are
2
influenced in several ways. Therefore, the flow rates
of NO and air had to be optimized regarding the
points:
the NO-line were controlled by a pulserdelay gener-
ator ŽStanford Research DG535.. The spatial distri-
Ø The flow of the initial NOrair mixture should be
slow enough to reach equilibrium inside the mix-
ing chamber.
Ø The total number density of NO and NO should
2
be as low as possible to reduce fluorescence
quenching and self absorption.
Ø Photodissociation of NO should be very effec-
2
tive Žclose to saturation. in order to give high NO
concentrations for increasing the signal-to-noise
ratio of the LIF-detection.
In the gas mixing system, for practical reasons,
the residence time of the mixture was limited to
Fig. 2. Layout of the optical arrangement.

1
8
C. Orlemann et al.rChemical Physics Letters 307 (1999) 15–20
some minutes. The ratio of NO-LIF signals with and
without photodissociation Žat 308 nm. indicates the
efficiency of the photodissociation process and deter-
mines the contrast between the dissociation induced
NO-line and the detection laser sheet induced back-
ground. This was examined at different residence
times of the seeding gases ŽNOrair mixture. inside
the reaction vessel. At various flow rates which
translate to average residence times from 1 up to 12
min, no significant increase in signal-to-background
ratio was observed. This indicates that under the
conditions used the NOrNO equilibrium was
2
Fig. 4. Temporal evolution of the spatial width ŽFWHM. of the
NO line.
reached within the residence times of the gas mixture
in the reaction vessels.
NO fluorescence is efficiently quenched by NO2.
Doping the flow with high concentrations of NO2
therefore does not necessarily increase signal intensi-
the photodissociation beam Ž308 nm, XeCl excimer
laser.. As can be seen, the NO-signal rises linearly
ties. Furthermore, increasing NO and NO concen-
with energy densities up to approximately 100
2
2
trations increases the attenuation of the laser beams
and signal light. For our system, we found an initial
flow of 27 slm air and 17 sccm NO gave optimal
MWrcm . Full saturation of the NO photodissocia-
2
2
tion was observed above 180 MWrcm . For the
experiment, a narrowband excimer laser was used.
Due to its high beam quality, this laser allowed the
beam to be focused tightly in order to cause pho-
todissociation only within a small, well defined vol-
ume.
performance. Thus, the NO concentration within the
2
flow system under investigation could be as low as
6
00 ppm, which is also important with respect to the
toxic and corrosive characteristics of NO . The en-
x
ergy density of the photodissociation laser beam
should be high enough to dissociate every NO2-
molecule within the beam without inducing optical
break-down and without damaging the windows. Fig.
An important issue concerning the application
range of a flow tagging technique is how far the
effects of diffusion, which broaden the written line,
are important. To test this, a NOrNO rair mixture
2
3
shows the dependence of the LIF intensity from
was filled into the quartz cell, allowing time to reach
chemical equilibrium. Within the cell, the variation
of the spatial width of the NO line was measured at
different delays after photodissociation ŽFig. 4.. Af-
the generated NO-line versus the energy density of
ter 20 ms, the NO-line is broadened to 2 mm, but it
still can clearly be discriminated against the back-
ground.
The new flow tagging technique was applied to
the investigation of a turbulent flow inside a quartz
cell. The narrow flow inlet tube Ždiameter: 4.5 mm.
caused a fairly high Reynolds number of 8500 at
flow rates of 27 slm. Vortex-street-like flow struc-
tures therefore allowed the testing of the abilities of
the new flow tagging technique. Fig. 5 shows
single-shot measurements at various temporal delays
after writing a NO-line by photodissociation. The
horizontal field of view is approximately 15 mm. In
data evaluation, the image of the shifted line is
divided by a reference image which shows energy
Fig. 3. Dependence of the NO-LIF intensity on the energy density
of the dissociation laser beam. The intensities represent average
values from a rectangular subregion around the center of the
generated NO line. Error bars indicate the standard deviation of
pixel intensities within this area.

C. Orlemann et al.rChemical Physics Letters 307 (1999) 15–20
19
line was segmented from the corrected image using a
thresholding and erosion algorithm.
The resulting images show the statistical behavior
of the highly turbulent flow. At a delay of 300 ms
between dissociation and detection, the first oscilla-
tions of the line can be seen. These oscillations
increase at longer delays, and pocket formation starts.
The three-dimensional nature of the flow leads to
interruption of the line when volume elements are
moving out of the plane of detection.
6
. Conclusions
The new NOrNO flow tagging technique has
2
been shown to be a promising tool for measuring
small-scale flow patterns. NO concentrations were
2
optimized in order to maximize the signal-to-back-
ground ratios. Initial NO concentrations of 600 ppm
2
were found to give the best results. Constant NO2
seeding was shown to be feasible, with NO being
2
formed within a 5 l mixing vessel and a 50 l delay
vessel from the reaction of NO with air. Due to the
long lifetime of the photochemically produced NO,
marked volume elements can be traced at delay
times as long as 20 ms. Dissociation laser energy
2
densities of 180 MWrcm per pulse generated with
a XeCl excimer laser were shown to lead to optimal
line-to-background contrast. A first application, mon-
itoring a turbulent flow inside an odd geometry
quartz cell shows highly resolved flow structures in
the sub-millimeter-range. Further improvements in
image processing routines Žcontour averaging. will
allow the evaluation of low intensity structures. This
will further extend the range of temporal delays
accessible by the technique. In combination with
highly focused dissociation laser beams, the spatial
resolution can be extended down to some tens of
microns. Highly resolved studies in turbulent flows
close to walls are under way.
Acknowledgements
Fig. 5. Variation of the structure of the NO-line generated by
photodissociation at various temporal delays.
This work has been funded by the Deutsche
Forschungsgemeinschaft within the Sonder-
forschungsbereich SFB 359 ‘‘Reaktive Str o¨ mungen,
Diffusion und Transport’’.
inhomogeneities within the lightsheet and the contri-
bution of background luminosity. The center of the

2
0
C. Orlemann et al.rChemical Physics Letters 307 (1999) 15–20
w12x H.W. Ford, N. Endow, J. Chem. Phys. 27 Ž1957. 1156.
References
w13x H.W. Ford, S. Jaffe, J. Chem. Phys. 38 Ž1963. 2935.
w1x L.R. Boedeker, Opt. Lett. 14 Ž1989. 473.
w14x G. Herzberg, Molecular Spectra and Molecular Structure, III.
Polyatomic Molecules, Krieger Publishing, Malabar, 1988.
w15x W.B. DeMore, S.P. Sander, C.J. Howard, A.R. Ravis-
w2x R.J. Exton, M.E. Hillard, Appl. Opt. 25 Ž1986. 14.
w3x B. Hiller, R.K. Hanson, Appl. Opt. 27 Ž1988. 33.
w4x R.B. Miles, C. Cohen, J. Connors, P. Howard, S. Huang, E.
Markovitz, G. Russell, Opt. Lett. 12 Ž1987. 861.
w5x R.B. Miles, W.R. Lempert, Ann. Rev. Fluid. Mech. 29
Ž1997. 285.
hankara, D.M. Golden, C.E. Kolb, R.F. Hampson, M.J.
Kurylo, M.J. Molina, NASA JPL Publication 97-4, 1997.
w16x H. Zacharias, K. Meier, K.H. Welge, in: J. Hinze ŽEd..,
Energy Storage and Redistribution in Molecules, Plenum,
w6x L.A. Ribarov, J.A. Wehrmeyer, F. Batliwala, R.W. Pitz, P.A.
New York, 1983, p. 107.
w17x A. Doughty, G. Hancock, E.L. Moore, Chem. Phys. Lett. 274
Ž1997. 58.
DeBarber, AIAA-Paper 98-0513, 1998.
w7x P.H. Paul, M.P. Lee, R.K. Hanson, Opt. Lett. 14 Ž1989. 417.
w8x R.B. Miles, E. Udd, M. Zimmermann, Appl. Phys. Lett. 32
Ž1978. 317.
w18x I.W.M. Smith, R.P. Tucket, C.J. Whitham, Chem. Phys. Lett.
200 Ž1992. 615.
w9x A. Lozano, S.H. Smith, M.G. Mungal, R.K. Hanson, AIAA
w19x H. Gaedtke, J. Troe, Ber. Bunsenges. Phys. Chem. 79 Ž1974.
J. 32 Ž1993. 217.
184.
w10x G. Gr u¨ nefeld, A. Gr a¨ ber, A. Diekmann, S. Kr u¨ ger, P. An-
dresen, Combust. Sci. and Tech. 135 Ž1998. 135.
w11x I.T.N. Jones, K.D. Bayes, J. Chem. Phys. 59 Ž1973. 4836.
w20x J. Troe, Ber. Bunsenges. Phys. Chem. 73 Ž1967. 906.
w21x J.M. Seitzman, G. Kychakoff, R.K. Hanson, Opt. Lett. 10
Ž1985. 439.