Tunable excimer lasers are being used to produce species-, space-, and
time-resolved images of complex gaseous media. These media may be analyzed for
composition, density, temperature, or flow velocities. The techniques are, in
general, highly selective, sensitive, and nonintrusive and are being made
possible by recent technological developments in these UV lasers and in
intensified cameras, imaging spectrographs, and fast digital image processing.
We describe the needs for laser diagnostics in combustion, the physical
mechanisms, the relevant spectroscopy, typical experimental setups, and
equipment considerations. Precision and accuracy are discussed on the basis of
some simple, but realistic, calculations intended to guide the experimentalist
in design considerations and to reveal potential sources of errors in the
often difficult conversion of raw data to values for such quantitative
parameters as densities or temperatures. Finally we present an overview of
previous results, select some examples that show the power of tunable excimer
laser diagnostics in combustion, and present some suggestions for future
directions.
Peter Andresen, Gerard Meijer, Harald Schlüter, Heiner Voges, Andrea Koch, Werner Hentschel, Winfried Oppermann, and Erhard Rothe Appl. Opt. 29(16) 2392-2404 (1990)
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Number of PE’s,
Ze, Generated
under Different Focusing Conditions for RS, VRS, RRS, and Various LIF Processes
for Four Different Combustion Situationsa
Combustion Situation
Flame Cold
Flame Hot
IC Engine
Comments
Cold
Hot
T (K)
300
2100
600
2100
n (cm-3)
2.5 (19)
3.5 (18)
2.5 (20)
2.5 (20)
Zt (RS)
6.7 (14)
1 (14)
6.7 (15)
6.7 (15)
Zi (VRS)
5.2 (14)
7.1 (13)
5.2 (15)
5.2 (15)
Q (s-1)
1.6 (9)
5.9 (8)
2.2 (10)
4.2 (10)
RS
1.7 (5)
2.4 (4)
1.7 (6)
1.7 (6)
Laser sheet
RS
1.7 (7)
2.4 (6)
1.7 (8)
1.7 (8)
Laser line
N2, VRS
125
17
1.3 (3)
1.3 (3)
Laser sheet
N2, VRS
1.3 (4)
1.7 (3)
1.3 (5)
1.3 (5)
Laser line
O2 VRS
3.5 (3)
0
3.5 (4)
0
Laser line
HC, VRS
1.4 (4)
0
1.4 (5)
0
Laser line
H2O, VRS
800
770
9 (3)
5 (4)
Laser line
CO2, VRS
0
600
0
4.5 (4)
Laser line
N2, RRS
1.7 (3)
2.2 (2)
1.6 (4)
1.6 (4)
Laser sheet
N2, RRS
1.7 (3)
2.2 (4)
1.6 (6)
1.6 (6)
Laser line
O2, RRS
1.2 (5)
0
1.2 (6)
0
Laser line
CO2, RRS
0
3.4 (4)
0
3.8 (6)
Laser line
OH, LIF, v″ = 0, 308 nm
≈0
2.7 (4)
≈0
2.5 (4)
OH, LIPF, v″ = 0, 248 nm
0
1.5 (3)
0
2 (4)
NO, LIF, v″ = 1, 193 nm
0
5.2 (2)
0
5.2 (2)
Natural
NO, LIF, v″ = 0, 193 nm
0
23
0
23
Natural
NO, LIF, v″ = 0, 226 nm
0
3.3 (2)
0
3.2 (2)
Natural
NO, LIF, v″ = 0, 193 nm
1.2 (4)
2.3 (3)
8.7 (3)
2.3 (3)
Tracer
NO, LIF, v″ = 1, 193 nm
0
5.2 (4)
0
5.2 (4)
Tracer
NO, LIF, v″ = 0, 226 nm
1.7 (5)
3.3 (4)
1.2 (5)
3.3 (4)
Tracer
The mole fractions correspond roughly to
stoichiometric combustion of iso-octane with air (see
Table 2). The first five rows have
T, the total density n, the total
number of species Zt,
and the number of N2 molecules
Zi
in V (300 µm)3, and a
simplified (see text) quench rate Q for LIF. For RS,
VRS, and RRS, the laser is assumed to be at 248 nm. Laser sheet and laser line
refer to focusing cases (a) and (b), respectively (see text).
NO(n) and NO(t) refer to natural
composition or to a tracer. For VRS and RRS the numbers are for
zz detection. These should be multiplied by 7/4 if
unpolarized detection is used.
They refer to scattering from one
vibrational state, one rotational state, and all states, respectively, of the
given species. For VRS and RRS the numbers are for zz
detection (i.e., acquisition with E⊥)
and from J″ → J′.
These cross sections have all been extrapolated from those at 337.1 nm by
means of the expression (ν̅laser -
ΔνVRS)4 (see text).
In some cases this is not reliable (see text). The tabulated values should
be multiplied by 7/4 if unpolarized detection is used. Also tabulated
are the VRS wave-number shifts and the corresponding emission wavelengths.
The species with different ν are for different vibrational
modes.
Table 5
Absolute Temperature Errors ΔT (K) for Different Combustion Conditions, as Calculated
from Eqs. (41) and
(43)
NA means not applicable because the
molecule is not present.
Table 6
Species Excitation for LIF, LIPF, or Resonance-Enhanced Multiphoton Ionization (REMPI) with Tunable Excimer Light or with Stimulated Raman-Shifted Light in the First or the Second Stokes (S1 or S2) or the First Anti-Stokes (AS1) in H2 or D2a
For laser line focus in a hot flame
(conditions as in Table 3).
Conversion to temperature requires gas
composition; improvements by higher collection and detection efficiency are
possible.
Simultaneous measurement of
many majority species and temperature with spatial resolution along a line.
Only density in a single
quantum state measured.
Table 8
Excitation Parameters for LIF of OH and NO Excited by
Different Wavelengths
Transition
A (s-1)
P (s-1)
λlaser (nm)
ν̅e (cm-1)
τ (s)
q(v″, v′)
σint (cm2 cm-1)
γ(a) (s-1)
γ(b) (s-1)
OH(A–X) 0 ← 0
1 (6)
0
308
32500
1 (-6)
0.9056
4.4 (-16)
4.4 (10)
4.4 (12)
OH(A–X) 3 ← 0
1 (6)
1 (10)
248
40320
1 (-6)
0.0005
6.1 (-19)
6.1 (7)
6.1 (9)
NO(D–X) 0 ← 1
4 (7)
0
193
51800
2.5 (-8)
0.26
2.6 (-15)
2.6 (11)
2.6 (13)
NO(B–X) 7 ← 0
3.6 (5)
0
193
51800
2.8 (-6)
0.028
2.5 (-18)
2.5 (8)
2.5 (10)
NO(A–X) 0 ← 0
5 (6)
0
226
44250
2.0 (-7)
0.167
2.8 (-16)
2.8 (10)
2.8 (12)
Table 9
Parameters Used for the Calculation of the Number Ze of PE’s Given in
Table 3
Parameter
Flame Cold
Flame Hot
IC Engine
Cold
Hot
Detected
T, K
300
2100
600
2100
Zt
6.7 (14)
1 (14)
6.7 (15)
6.7 (15)
χm
0
1 (-2)
0
1 (-2)
OH
χm
1 (-4)
1 (-4)
NO(n)
χm
1 (-2)
1 (-2)
1 (-2)
1 (-2)
NO(t)
f(T)
2 (-2)
2 (-2)
OH, v″ = 0
f(T)
≈0
1 (-3)
≈0
1 (-3)
NO, v″ = 1
f(T)
1 (-2)
5 (-3)
1 (-2)
5 (-3)
NO, v″ = 0
Zi
0
2 (10)
0
1.3 (12)
OH, v″ = 0
Zi
0
1 (7)
0
6.7 (8)
NO(n), v″ = 1
Zi
0
1 (9)
0
6.7 (10)
NO(t), v″ = 1
Zi
0
5 (7)
0
3.4 (9)
NO(n), v″ = 0
Zi
6.7 (10)
5 (9)
6.7 (11)
3.4 (11)
NO(t), v″ = 0
θ
6.4 (-4)
1.7 (-3)
4.5 (-5)
2.4 (-5)
OH, v″ = 0, 308 nm
θ
8.6 (-5)
9.4 (-5)
3.1 (-5)
1.9 (-5)
OH, v″ = 0, 248 nm
θ
2.5 (-2)
6.3 (-2)
1.8 (-3)
9.6 (-4)
NO, v″ = 1, 193 nm
θ
2.3 (-4)
6.1 (-4)
1.6 (-5)
8.6 (-6)
NO, v″ = 0, 193 nm
θ
3.2 (-3)
8.4 (-3)
2.3 (-4)
1.2 (-4)
NO, v″ = 0, 226 nm
dw/dΩ
2.5 (-5)
6.7 (-5)
1.8 (-6)
9.6 (-7)
OH, v″ = 0, 308 nm
dw/dΩ
3.5 (-6)
3.8 (-6)
1.2 (-6)
7.8 (-7)
OH, v″ = 0, 248 nm
dw/dΩ
1 (-3)
2.5 (-3)
7 (-5)
3.8 (-5)
NO, v″ = 1, 193 nm
dw/dΩ
9 (-6)
2.4 (-5)
6 (-7)
3.5 (-7)
NO, v″ = 0, 193 nm
dw/dΩ
1.3 (-4)
3.3 (-4)
9 (-6)
4.8 (-6)
NO, v″ = 0, 226 nm
Tables (9)
Table 1
Manufacturer’s Specifications for the Largest
Lambda Physik Tunable Laser, the LPX 350, in its Standard Version
Number of PE’s,
Ze, Generated
under Different Focusing Conditions for RS, VRS, RRS, and Various LIF Processes
for Four Different Combustion Situationsa
Combustion Situation
Flame Cold
Flame Hot
IC Engine
Comments
Cold
Hot
T (K)
300
2100
600
2100
n (cm-3)
2.5 (19)
3.5 (18)
2.5 (20)
2.5 (20)
Zt (RS)
6.7 (14)
1 (14)
6.7 (15)
6.7 (15)
Zi (VRS)
5.2 (14)
7.1 (13)
5.2 (15)
5.2 (15)
Q (s-1)
1.6 (9)
5.9 (8)
2.2 (10)
4.2 (10)
RS
1.7 (5)
2.4 (4)
1.7 (6)
1.7 (6)
Laser sheet
RS
1.7 (7)
2.4 (6)
1.7 (8)
1.7 (8)
Laser line
N2, VRS
125
17
1.3 (3)
1.3 (3)
Laser sheet
N2, VRS
1.3 (4)
1.7 (3)
1.3 (5)
1.3 (5)
Laser line
O2 VRS
3.5 (3)
0
3.5 (4)
0
Laser line
HC, VRS
1.4 (4)
0
1.4 (5)
0
Laser line
H2O, VRS
800
770
9 (3)
5 (4)
Laser line
CO2, VRS
0
600
0
4.5 (4)
Laser line
N2, RRS
1.7 (3)
2.2 (2)
1.6 (4)
1.6 (4)
Laser sheet
N2, RRS
1.7 (3)
2.2 (4)
1.6 (6)
1.6 (6)
Laser line
O2, RRS
1.2 (5)
0
1.2 (6)
0
Laser line
CO2, RRS
0
3.4 (4)
0
3.8 (6)
Laser line
OH, LIF, v″ = 0, 308 nm
≈0
2.7 (4)
≈0
2.5 (4)
OH, LIPF, v″ = 0, 248 nm
0
1.5 (3)
0
2 (4)
NO, LIF, v″ = 1, 193 nm
0
5.2 (2)
0
5.2 (2)
Natural
NO, LIF, v″ = 0, 193 nm
0
23
0
23
Natural
NO, LIF, v″ = 0, 226 nm
0
3.3 (2)
0
3.2 (2)
Natural
NO, LIF, v″ = 0, 193 nm
1.2 (4)
2.3 (3)
8.7 (3)
2.3 (3)
Tracer
NO, LIF, v″ = 1, 193 nm
0
5.2 (4)
0
5.2 (4)
Tracer
NO, LIF, v″ = 0, 226 nm
1.7 (5)
3.3 (4)
1.2 (5)
3.3 (4)
Tracer
The mole fractions correspond roughly to
stoichiometric combustion of iso-octane with air (see
Table 2). The first five rows have
T, the total density n, the total
number of species Zt,
and the number of N2 molecules
Zi
in V (300 µm)3, and a
simplified (see text) quench rate Q for LIF. For RS,
VRS, and RRS, the laser is assumed to be at 248 nm. Laser sheet and laser line
refer to focusing cases (a) and (b), respectively (see text).
NO(n) and NO(t) refer to natural
composition or to a tracer. For VRS and RRS the numbers are for
zz detection. These should be multiplied by 7/4 if
unpolarized detection is used.
They refer to scattering from one
vibrational state, one rotational state, and all states, respectively, of the
given species. For VRS and RRS the numbers are for zz
detection (i.e., acquisition with E⊥)
and from J″ → J′.
These cross sections have all been extrapolated from those at 337.1 nm by
means of the expression (ν̅laser -
ΔνVRS)4 (see text).
In some cases this is not reliable (see text). The tabulated values should
be multiplied by 7/4 if unpolarized detection is used. Also tabulated
are the VRS wave-number shifts and the corresponding emission wavelengths.
The species with different ν are for different vibrational
modes.
Table 5
Absolute Temperature Errors ΔT (K) for Different Combustion Conditions, as Calculated
from Eqs. (41) and
(43)
NA means not applicable because the
molecule is not present.
Table 6
Species Excitation for LIF, LIPF, or Resonance-Enhanced Multiphoton Ionization (REMPI) with Tunable Excimer Light or with Stimulated Raman-Shifted Light in the First or the Second Stokes (S1 or S2) or the First Anti-Stokes (AS1) in H2 or D2a
For laser line focus in a hot flame
(conditions as in Table 3).
Conversion to temperature requires gas
composition; improvements by higher collection and detection efficiency are
possible.
Simultaneous measurement of
many majority species and temperature with spatial resolution along a line.
Only density in a single
quantum state measured.
Table 8
Excitation Parameters for LIF of OH and NO Excited by
Different Wavelengths
Transition
A (s-1)
P (s-1)
λlaser (nm)
ν̅e (cm-1)
τ (s)
q(v″, v′)
σint (cm2 cm-1)
γ(a) (s-1)
γ(b) (s-1)
OH(A–X) 0 ← 0
1 (6)
0
308
32500
1 (-6)
0.9056
4.4 (-16)
4.4 (10)
4.4 (12)
OH(A–X) 3 ← 0
1 (6)
1 (10)
248
40320
1 (-6)
0.0005
6.1 (-19)
6.1 (7)
6.1 (9)
NO(D–X) 0 ← 1
4 (7)
0
193
51800
2.5 (-8)
0.26
2.6 (-15)
2.6 (11)
2.6 (13)
NO(B–X) 7 ← 0
3.6 (5)
0
193
51800
2.8 (-6)
0.028
2.5 (-18)
2.5 (8)
2.5 (10)
NO(A–X) 0 ← 0
5 (6)
0
226
44250
2.0 (-7)
0.167
2.8 (-16)
2.8 (10)
2.8 (12)
Table 9
Parameters Used for the Calculation of the Number Ze of PE’s Given in
Table 3
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