16 Determination of Flame and Plasma Temperatures and Density Profiles by Means of Laser-Excited
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Fluorescence J. BRADSHAW, S. NIKDEL, R. REEVES, J. BOWER, Ν. OMENETTO, and J. D. WINEFORDNER Department of Chemistry, University of Florida, Gainesville, FL 32611
The fluorescence technique, like other methods based on scatter (elastic or inelastic), has been shown by us and others to be a reliable unperturbing method of measuring spatial/ temporal flame temperatures and species concentrations. To avoid the dependency of the fluorescence signal on the environment of the emitting species, it has been shown by several workers that optical saturation of the fluorescence process (i.e., the condi tion occurring when the photoinduced rates of absorption and emission dominate over the spontaneous emission and collisional quenching rates) is necessary. Pulsed dye lasers have sufficient spectral irradiances to saturate many transitions. Our work has so far been concerned with atomic transitions of probes (such as In, Pb, or Tl) aspirated into combustion flames and plasmas. 1-3
Concepts and Methods The temperature of a flame, plasma, or hot gas can be esti mated by using the steady state fluorescence expressions derived by Boutilier, et al for spectral continuum excitation. Several unique methods which can be used to measure soatial temoeratures (volumes < 10 mm2) have been developed by us and will be reported in detail in a oaper to be submitted for oublication.4 The methods are generally based uoon the introduction of inorganic 3-level orobes (Tl or Pb) into a flame and measuring the ratio of fluorescence signals resulting between levels 3 and 2 and 3 and 1 following excitation of level 3 via levels 1 or 2. Because of the restrictions regardina overall length of this report and be cause of the future availabilitv of the oubli shed paper! concern ing these new methods, we will here only give the aooroaches and several flame temperatures measured by the described methods. 1
Method 1. Linear 2-Line MethodS>& In this method, the ratio of fluorescence signals, B and B (the upper sub3+2 3+l 1+3 2+3 scripts represents the measured fluorescence transition and the lower subscripts reoresents the excitation transitions), is c
h
c
h
0-8412-0570-l/80/47-134-199$05.00/0 © 1980 American Chemical Society Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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LASER PROBES FOR COMBUSTION CHEMISTRY
measured. By c a l i b r a t i o n o f the s p e c t r o m e t r i c system and measur ing the r a t i o o f f l u o r e s c e n c e and e x c i t a t i o n i n t e n s i t y r a t i o s , the flame temperature can be determined from a simple e x p r e s s i o n . T h i s method r e q u i r e s c a l i b r a t i o n , l i n e a r behavior o f the f l u o r e s cence i n t e n s i t y with e x c i t a t i o n i n t e n s i t y and e x c i t a t i o n beam matching. In a d d i t i o n , e f f i c i e n t quenching s p e c i e s i n some flames (hydrocarbon f u e l s ) and o r e - and p o s t - f i l t e r e f f e c t s lead t o d e t e r i o r a t i o n o f the s i g n a l - t o - n o i s e r a t i o s . Laser e x c i t a t i o n i s advantageous f o r s p a t i a l measurements and improved s i g n a l - t o noise r a t i o s . Method 2. S a t u r a t i o n Method f o r Sequential Pumping. In t h i s method, atomic f l u o r e s c e n c e o f the i n o r g a n i c probe i s oroduced at 3+1 and a t 3+2 a f t e r e x c i t a t i o n a t 1+3 and/or 2+3 r e s p e c t i v e l y . However, i n t h i s case, i t i s necessary t o " s a t u r a t e " the e x c i t e d l e v e l , 3, i n order t o use the m e t h o d i c In a d d i t i o n , i n order for the flame temnerature to be evaluated i t i s necessary f o r the mixing f i r s t order r a t e constant, 1 20X) than the sum o f the t o t a l d e a c t i v a t i o n r a t e constants between l e v e l s 3 and 1 and a l s o between 3 and 2. T h i s method a l s o r e q u i r e s c a l i b r a t i o n o f the s p e c t r o m e t r i c measurement system, s a t u r a t i o n o f l e v e l 3, c o r r e c t i o n s o r m i n i m i z a t i o n o f s c a t t e r and post f i l t e r e f f e c t s , and beam matching o f 2 dye l a s e r beams are needed f o r the e x c i t a t i o n orocess. Method 3. S a t u r a t i o n Method With Peak D e t e c t i o n . In t h i s method, develoDed by Omenetto and Winefordnerî^, i t i s necessary to e x c i t e f l u o r e s c e n c e 3+1 with 1+3 and a s h o r t time l a t e r (< 1 ys) e x c i t e 3+1 with 2+3. In t h i s case the atomic system e f f e c t i v e l y acts on a 2 - l e v e l atom s i n c e e x c i t a t i o n and measure ment o f f l u o r e s c e n c e i s done a t the peak o f the e x c i t a t i o n pro f i l p r i o r to r e l a x a t i o n o f the system to a 3 - l e v e l steady s t a t e p r o c e s s ^ The temperature here i s r e l a t e d simply t o the r a t i o Br / B r and s t a t i s t i c a l weights o f the l e v e l s and i s independ3 + l ""3+1 1+3 2+3 ent o f n o n - r a d i a t i o n a l r a t e constants as i n the preceeding case and o f c a l i b r a t i o n as i n the two preceeding cases. On the other hand, t h i s method r e q u i r e s the use o f a f a s t r i s i n g l a s e r pulse to oerturb the i n o r g a n i c orobe to reach a 2 - l e v e l steady s t a t e and the use o f f a s t e l e c t r o n i c s to measure the f l u o r e s c e n c e p r i o r to r e l a x a t i o n o f the system t o a steady s t a t e i n v o l v i n g a l l three l e v e l s (1,2,3). T h i s method a l s o r e q u i r e s 2 s o a t i a l l y and geo m e t r i c a l l y matched dye l a s e r beams which w i l l cause the Drobe t o be r a p i d l y s a t u r a t e d . Methods 4 and 5. Two other novel methods f o r flame temperature measurement w i l l be reported uoon i n the f u l l paper t o be published,4 but no r e s u l t s w i l l be given here. One o f these methods (Method 4) i n v o l v e s s a t u r a t i o n o f l e v e l 3 v i a simultaneous r
pumping o f b o t h l-*3 a n d 2+3 a n d t a k i n g t h e r a t i o o f t h e
resulting
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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16.
Laser-Excited
BRADSHAW E T AL.
201
Fluorescence
3+1 f l u o r e s c e n c e and the 3+1 f l u o r e s c e n c e r e s u l t i n g when e x c i t i n g 2+3. T h i s method has most of the same p o t e n t i a l d i f f i c u l t i e s of Method 2. Method 5 i n v o l v e s the use of one l a s e r beam and l i n e a r or s a t u r a t i o n behavior: i n t h i s case, the r a t i o of the probe f l u o r e s c e n c e r e s u l t i n g at 3+1 with 2+3 e x c i t a t i o n and the l a s e r induced emission r e s u l t i n g at i+1 (i>3) with 2+3 e x c i t a t i o n . T h i s method has a number of advantages: ( i ) s a t u r a t i o n i s not necessarv; ( i i ) only one l a s e r wavelength i s needed; ( i i i ) no need to s n a t i a l l y match l a s e r beams: ( i v ) c a l i b r a t i o n of the f l u o r e s c e n c e spectrometer i s s t i l l needed but there i s no need to c a l i b r a t e the e x c i t a t i o n i n t e n s i t y : post f i l t e r and s c a t t e r e f f e c t s are minimal, and (v) temporal ( s i n g l e pulse) measurements of temoeratures are f e a s i b l e . In Method 1-4, by measurina the f l u o r e s c e n c e s i g n a l s c l o s e together (sav 1 y s ) , then températures corresponding to n e a r l y " f r o z e n " flame c o n d i t i o n s are o b t a i n a b l e . We are c u r r e n t l y in the process o f making such temporal temperature measurements; these r e s u l t s w i l l be published at a l a t e r date. By beam expan s i o n of the l a s e r beam(s) and i s o l a t i o n of the c e n t r a l homogeneous s e c t i o n , i t i s a l s o p o s s i b l e to r e s o l v e s p a t i a l l y small flame volumes, e.g., depending upon the spectrometer entrance s l i t or s l i t aperature, < 10 mnr. S o a t i a l d e n s i t y p r o f i l e s of atomic (and molecular) soecies can a l s o be made v i a s a t u r a t i o n f l u o r e s c e n c e approaches. For a " 2 - l e v e l " atom, l i k e Sr, a o l o t of 1/B vs 1/Ε (Bp i s the f l u o r e s c e n c e r a d i a n c e , i n J s~lm-2sn-l, and Ε i s the e x c i t a t i o n s p e c t r a l i r r a d i a n c e , i n J s lm"^nm-l) allows e s t i m a t i o n of the quantum e f f i c i e n c y , Y of the f l u o r e s c e n c e process (and thsu e s t i m a t i o n o f " r a d i a t i o n l e s s " r a t e constants) and the t o t a l num ber d e n s i t y n j , of the species of i n t e r e s t by means of F
λ
λ
_
where: ι i s the f l u o r e s c e n c e path l e n g t h , hv i s the f l u o r e s c e n c e (or e x c i t a t i o n photon energy) i n J , A i s the emission p r o b a b i l i t y , in s-1, the g's are the s t a t i s t i c a l weights of the 2 l e v e l s , h i s Planck's constant, and c i s the soeed of l i g h t . The o l o t of 1/Bp vs 1/Ε has a slooe which i n c l u d e s ( n y Y ) - l and an i n t e r c e p t which i n c l u d e s n y l . I f both Bp and Ε are measured i n absolute u n i t s , then ny and Y can be obtained. Even i f Bp i s measured i n r e l a t i v e u n i t s Y can be determined by m u l t i p l y i n g through by C and then c a l i b r a t i n g o r d i n a t e i n u n i t s of (g^ + 9^)/92^ ' atom i s a 3 (or m u l t i ) l e v e l system, then the r a d i a t i o n l e s s r a t e constants must be known and i n c l u d e d i n the expression f o r 1/Bp as a f u n c t i o n of 1/Ε unless the f o r t u n a t e circumstances a r i s e s where two of the 3 l e v e l s e s s e n t i a l l y coalesce i n t o a s i n g l e l e v e l and once again we have e s s e n t i a l l y a 2 - l e v e l atom. In t h i s case absolute measurement of both Bp and Ε i s necessary. λ
λ
t
λ
λ
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
i e
202
LASER PROBES FOR COMBUSTION CHEMISTRY
Table I. Measured Flame Temperatures a
b
H /0 /Ar » » Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 15, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch016
2
L i n e a r 2 l i n e Method (Source SDeçtral Radiance was 5 t o 1(P !»!/cnr nm) R a t i o taken was B 3+2/1+3
H / 0 > N ,b,c a
c
2
2
2
2
2200 + 30 K
d
1980 + 30 K
d
2120 + 30 K
3
1990 + 30 K
d
F r
r
3 1/2 3
S a t u r a t i o n Method-Sequential Pumoinq ( X f | = same i n both cases) (Source S p e c t r a l Radiance was 1 χ 107 W/cm nm) R a t i o taken was B 3+1/1+3 2
F h
r
a.
3+1/2-3
L i j n s e and Elsenaar (P.L. L i j n s e and R.J. E l s e n a a r , J_. Quant. Spectrosc. Radiât. T r a n s f e r , 12 (1972) 1115.) obtained temperatures o f 2136 Κ f o r H 7U?/Ar, 2/1/4 and 1970 Κ f o r H / 0 / N 1.9/0.95/4. 2
2
b.
2
2
Hoomayers (H.P. Hoomayers, Ph.D. T h e s i s , U n i v e r s i t y o f U t r e c h t , 1966.) obtained temneratures o f 2350 Κ f o r H / 0 / A r , 1.72/0.85/3.45 and 2160 Κ f o r H /0 /N , 1.72/0.85/3.45. The source o f the systematic e r r o r s i n the values measured by L i i n s e and E l s e n a a r , by Hoomayers, and by us i s not known. 2
2
2
2
2
c.
Line r e v e r s a l temperature measurements by us f o r the same flames were 50-100 Κ higher than our f l u o r e s c e n c e values.
d.
The random e r r o r o f + 30 Κ was due to shot noise on the s i g n a l .
Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
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16.
BRADSHAW ET AL.
Laser-Excited Fluorescence
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EXPERIMENTAL SYSTEM Out experimental system c o n s i s t e d o f a ^-pumped dual dye l a s e r (Molectron UV-14 with Molectron DL-400 and Lambda-Physik FL-2000), operated a t 20Hz, s o a t i a l f i l t e r s t o i s o l a t e the cen t r a l n o r t i o n o f the dye l a s e r beams, H2-02-Ar and H2-O2-N2 flames supported on a Meker type flame s h i e l d e d flame with Ar o r N2 outer sheaths, and a f l u o r e s c e n c e d e t e c t i o n system c o n s i s t i n g o f a 0.1 m g r a t i n g monochromator, a Hamamatsu R928 p h o t o m u l t i p l i e r tube, and a T e k t r o n i x 151 sampling o s c i l l o s c o p e with 0.5 s averaging time constant. A l l measurements were taken 1.5 cm above the burner too (previous s t u d i e s i n d i c a t e d the flame to be n e a r l y constant i n temperature with h e i g h t , 1-3 cm, and with width. The flames s t u d i e d i n t h i s r e p o r t i n c l u d e d : H2/02/Ar, 2/1/4 and H2/O2/N2, 2/1/4: the flows are r e l a t i v e volume r a t i o s at standard temperature and pressure f o r the unburnt gases. RESULTS AND DISCUSSION Flame temperatures were determined by both the l i n e a r 2 - l i n e method and by the s a t u r a t i o n method with s e q u e n t i a l pumping f o r both flames. The measured values are qiven i n Table I . Assuming the sum o f the r a d i a t i o n l e s s and r a d i a t i o n a l r a t e constants between l e v e l s 3 and 2 f o r i n o r g a n i c probe, l i k e T l , are much l e s s than the n o n - r a d i a t i o n a l mixing constant k 2 i , then the 2 - l e v e l e x p r e s s i o n r e l a t i n g 1/Bp t o 1/Ε a p p l i e s . Using t l r r e l a t i o n s h i p and the f o l l o w i n n measured parameters: s l i t a r e a , 0.5 χ 1.5 mm : s o l i d anale, 0.26 s r : f l u o r e s c e n c e deoth, l = 0.4 cm; 100 pom T l a s p i r a t e d ; (H2/O2/N2) = 1.1 x λ
2
3
1
2
10-1 2 d B f ^ H2/02/Ar) = 1.8 χ 10" W/cm s r , an< using A 3 1 = 0.41 χ 108 s * (taken from Wade, e t a l ) , then n j = 1.0 χ 10* cm" f o r the H /02/Ar flame. These values compare f a v o r a b l y with p o p u l a t i o n d e n s i t i e s measured f o r s i m i l a r flames and s i m i l a r a s p i r a t i o n c o n d i t i o n s . REFERENCES 1. G.D. Boutilier, M.B. Blackburn, J.M. Mermet, S.J. Weeks, H. Haraquchi, J.D. Winefordner, and N. Omenetto, Appl. Optics, 17 (1978) 2291. 2. NT Omenetto and J.D. Winefordner, Prog. Anal. Atomic. Spectrosc., 2 (1979)1. 3. N. Omenetto and J.D. Winefordner, Chapter 4 in Analytical Laser Spectroscopy, N. Omenetto, ed., Wiley, New York, 1979. 4. J.D. Bradshae, N. Omenetto, J.N. Bower, and J.D. Winefordner, AppI. Optics, (to be submitted). 5. N. Omenetto, R.F. Browner, J.D. Winefordner, G. Rossi, and P. Benetti, Anal. Chem., 44 (1972) 1683. 6. H. Haraquchi, B. Smith, S. Weeks, D.H. Johnson, and J.D. Winefordner, Appl. Spectrosc., 31. (1977) 156. 7. J.N. Bower, J.D. Bradshaw, N. Omenetto, and J.D. Winefordner, Appl. Optics, (to be submitted). 8. M.K. Wade, M. Czajkowski, and L. Krause, Canad. J. Phys., 56, (1978) 891. w / c m
s r a n
1
1
8
3
2
Received February 11, 1980. Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.
