Absorption Spectroscopy of Combustion Gases Using a Tunable IR

35 Absorption Spectroscopy of Combustion Gases Using a Tunable IR Diode Laser

Downloaded by IMPERIAL COLLEGE LONDON on September 15, 2017 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch035

R. K. HANSON, P. L. VARGHESE, S. M. SCHOENUNG, and P. K. FALCONE High Temperature Gasdynamics Laboratory, Department of Mechanical Engineering, Stanford University, Stanford, CA 94305

Experimental studies of combustion chemistry require measurements of species concentrations, often under conditions where in situ spectroscopic techniques are desirable or necessary. Among other new methods, tunable laser absorption spectroscopy using infrared diode lasers offers prospects for improved accuracy and specificity in concentration measurements, when a line-of-sight technique is appropriate. The present paper discusses diode laser techniques as applied to a flat flame burner and to a room temperature absorption cell. The cell experiments are used to determine the absorption band strength which is needed to properly interpret high temperature experiments. Preliminary results are reported for CO concentration measurements in a flame, the fundamental band strength of CO at STP, collision halfwidths of CO under flame conditions, and the temperature dependence of CO and NO collision halfwidths in combustion gases. Experimental Arrangement Details of the experimental arrangement and procedures have been described in a series of previous papers dealing with flames ( 1 , 2 , 3 , 4 ) and shock tube flows (5,6). A s c h e m a t i c o f t h e s i n g l e beam o p t i c a l s y s t e m u s u a l l y employed i s shown i n F i g u r e 1. I n t h i s a r r a n g e m e n t , t h e l a s e r beam p a s s e s t h r o u g h t h e f l a m e , i n t o a monochromator f o r l a s e r mode s e l e c t i o n and w a v e l e n g t h i d e n t i f i c a t i o n , a n d i s t h e n s p l i t i n t o two beams: one p a s s i n g t h r o u g h a room t e m p e r a t u r e a b s o r p t i o n c e l l and t h e o t h e r t h r o u g h a F a b r y P e r o t étalon u s e d f o r m e a s u r i n g changes i n w a v e l e n g t h . T h i s conf i g u r a t i o n i s s u i t a b l e f o r e i t h e r f l a m e measurements o r a b s o r p t i o n c e l l e x p e r i m e n t s . The l a s e r i s r e p e t i t i v e l y m o d u l a t e d u s i n g a s a w t o o t h c u r r e n t waveform. I n t h e p r e s e n t e x p e r i m e n t s w i t h s t e a d y a b s o r p t i o n c o n d i t i o n s , t h e d e t e c t o r ( D i and D2) o u t p u t s i g n a l s a r e r e c o r d e d w i t h a s i g n a l a v e r a g e r (PAR 4 2 0 2 ) . The p r o b l e m o f l a s e r power v a r i a t i o n s w i t h w a v e l e n g t h i s overcome e i t h e r b y t a k i n g t h e d i f f e r e n c e b e t w e e n s e p a r a t e l y r e c o r d e d a b s o r b e d and n o n - a b s o r b e d t r a n s m i t t e d i n t e n s i t y r e c o r d s

0-8412-0570-l/80/47-134-413$05.00/0 © 1980 American Chemical Society Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


414

LASER PROBES FOR COMBUSTION

CHEMISTRY

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ω, cm"

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TIME I.A SYNCHRONOUS! TRIGGER

Figure 1.

CURRENT SUPPLY TIME

Optical arrangement for tunable diode laser absorption spectroscopy

Crosley; Laser Probes for Combustion Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


35.

HANSON E T A L .

Tunable

IR

Diode

415

Laser

o r b y e m p l o y i n g a d o u b l e - b e a m o p t i c a l s y s t e m (4) w h i c h g i v e s t h e r e a l - t i m e d i f f e r e n c e b e t w e e n t h e a b s o r b e d beam a n d a r e f e r e n c e ( n o n - a b s o r b e d ) beam. R e p e t i t i o n r a t e s f o r l a s e r m o d u l a t i o n c a n be v a r i e d t o o v e r 5 kHz b u t a r e t y p i c a l l y 200-500 Hz i n t h e c u r r e n t work. S i n c e t h e r e c o r d e d s i g n a l i s a c t u a l l y a m e a s u r e o f t h e absorbed i n t e n s i t y , ΔΙ = 1° - I where I and 1° a r e r e s p e c t i v e l y t h e t r a n s m i t t e d i n t e n s i t i e s w i t h and w i t h o u t a b s o r p t i o n , t h e q u a n t i t y 1° must a l s o be measured i n o r d e r t o o b t a i n t h e q u a n t i t y o f i n t e r e s t , t h e t r a n s m i s s i v i t y 1/1° across the f u l l y resolved absorption l i n e . 1° c a n be measured d i r e c t l y ( w i t h no a b s o r p t i o n p r e s e n t ) e i t h e r b y c h o p p i n g t h e l a s e r beam o r by b i a s i n g t h e d e t e c t o r o u t p u t ( w i t h t h e v o l t a g e s o u r c e E) t o g i v e z e r o s i g n a l w i t h t h e l a s e r beam b l o c k e d . Two f l a t f l a m e b u r n e r s h a v e b e e n e m p l o y e d , a 4 cm x 10 cm b u r n e r w i t h a c e r a m i c - l i n e d chimney f o r NO measurements (4) and a 2.6 cm x 8.6 cm o p e n - f a c e d b u r n e r w i t h a n i t r o g e n s h r o u d f l o w f o r CO measurements. B o t h b u r n e r s o p e r a t e a t a t m o s p h e r i c p r e s s u r e w i t h laminar, premixed methane-air mixtures. These b u r n e r s work s a t i s f a c t o r i l y over a broad range o f f u e l - a i r e q u i v a l e n c e r a t i o s , b u t b o t h have c o l d b o u n d a r y r e g i o n s w h i c h c a u s e n o n - u n i f o r m c o n d i t i o n s a l o n g t h e o p t i c a l a x i s t h a t c a n be i m p o r t a n t i n t h e d a t a a n a l y s i s (4). Absorption

Theory

The t h e o r y r e q u i r e d t o i n t e r p r e t t h e e x p e r i m e n t a l a b s o r p t i o n d a t a i s w e l l e s t a b l i s h e d . The g o v e r n i n g e q u a t i o n w h i c h l i n k s t h e measured t r a n s m i s s i v i t y , Tv, a t wavenumber V, t o t h e a b s o r b i n g s p e c i e s c o n c e n t r a t i o n and i t s a b s o r p t i o n l i n e p a r a m e t e r s i s t h e Bouguer-Lambert l a w o f a b s o r p t i o n T

=

v

(Ι/Ι·)

=

ν

exp [ - ^

β Ρ. d x ] ν

where Ρ 4 i s t h e p a r t i a l p r e s s u r e (atm) o f t h e a b s o r b i n g s p e c i e s and L i s t h e p a t h l e n g t h (cm) a c r o s s t h e f l o w . The a b s o r p t i o n coefficient i s t h e p r o d u c t o f t h e l i n e s t r e n g t h S (cm~2 atm"1) f o r t h e t r a n s i t i o n o f i n t e r e s t and t h e l i n e s h a p e f a c t o r g(v-v ) ( c m ) , w h i c h i s a f u n c t i o n o f t h e n o n - r e s o n a n c e , V-V ; Q

i.e.,

Q

3

V

= S g(V-V ), Q

where

f

v

v

g ( " o ) dV

Ξ

1

line

line Thus t h e l i n e s t r e n g t h , sometimes c a l l e d t h e l i n e i n t e n s i t y , i s s i m p l y t h e area under a curve o f t h e a b s o r p t i o n c o e f f i c i e n t . When


416

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u n i f o r m c o n d i t i o n s a l o n g t h e l i n e o f s i g h t c a n be assumed, t h e a b s o r p t i o n law reduces simply t o


T

v

=

exp(-S g(V-V ) Ρ

L) .

Q

When a b s o r p t i o n l i n e s o v e r l a p , t h e a b s o r p t i o n c o e f f i c i e n t i s a summation: 3v = Σ g^ . I t i s c l e a r from t h e above e q u a t i o n t h a t b o t h t h e l i n e s t r e n g t h and t h e l i n e s h a p e f a c t o r must be known t o c o n v e r t a mea surement o f t r a n s m i s s i v i t y t o a s p e c i e s p a r t i a l p r e s s u r e . I n o u r work, t h e l i n e s h a p e f a c t o r i s d e t e r m i n e d d i r e c t l y i n each e x p e r i ment by r e c o r d i n g t h e f u l l y r e s o l v e d a b s o r p t i o n p r o f i l e . The l i n e s t r e n g t h f o r a g i v e n v i b r a t i o n - r o t a t i o n t r a n s i t i o n i s a known f u n c t i o n (7) o f t h e f l a m e t e m p e r a t u r e and t h e band s t r e n g t h e v a l u a t e d a t a r e f e r e n c e t e m p e r a t u r e , t y p i c a l l y 273.2 K. Controlled laser a b s o r p t i o n e x p e r i m e n t s a t room t e m p e r a t u r e w i t h known l e v e l s o f CO a r e c o n d u c t e d t o d e t e r m i n e t h e band s t r e n g t h , w h i c h i s u s e d t o g e t h e r w i t h a measured t e m p e r a t u r e t o s p e c i f y t h e i n d i v i d u a l l i n e strengths at flame c o n d i t i o n s . I n t h e c a s e o f a t r a n s i t i o n ( v " + 1, J ± 1 «- v , J ) i n t h e f u n d a m e n t a l band (Δν = 1) o f CO, t h e r e l a t i o n b e t w e e n t h e l i n e strength, S, t h e band s t r e n g t h , S°, and t h e t e m p e r a t u r e , T, i s : M

S

vW'+l

=

s

[ °(

STp

2

n

n

fl

11

n

)( 73.2/T)](V /v)(v +l){exp(-T (v ,J )hc/kT]}. o

e

J

•[1 - exp(-hVc/kT)][S /Q(T)] where S S

J

= J ,

Ρ b r a n c h ( J " - 1 «- J " )

= J + 1 ,

R b r a n c h ( J + 1 +• J " )

M

and Q(T)

-

J ( 2 J + 1) e x p ( - T ( v , J ) h c / k T ) . v,j H e r e S°(STP) i s t h e f u n d a m e n t a l band s t r e n g t h a t 273.2 Κ and Q i s t h e p a r t i t i o n f u n c t i o n f o r v i b r a t i o n and r o t a t i o n . T (v,J) i s the energy of the (v,J) s t a t e , i n cm~l; V i s t h e wavenumber a t l i n e c e n t e r f o r t h e s p e c i f i c t r a n s i t i o n , and V i s an a v e r a g e wavenumber f o r t h e b a n d , u s u a l l y t a k e n as t h e b a n d - c e n t e r v a l u e a l t h o u g h s m a l l c o r r e c t i o n s c a n be c a l c u l a t e d ( 8 ) . The q u a n t i t i e s h, c and k a r e P l a n c k ' s c o n s t a n t , t h e speed o f l i g h t and Boltzmann's c o n s t a n t , r e s p e c t i v e l y . F o r most p u r p o s e s , i t i s s u f f i c i e n t l y a c c u r a t e (an e r r o r o f a few p e r c e n t o r l e s s ) t o u s e r i g i d - r o t o r , harmonic o s c i l l a t o r r e l a t i o n s f o r the p a r t i t i o n f u n c tion. R e c e n t d e t e r m i n a t i o n s o f t h e CO and band s t r e n g t h by o t h e r w o r k e r s have been i n t h e range S = 260-280 cm" - atm"~l a t 273.2 Κ ( 9 ) . The l i n e s h a p e f u n c t i o n g(V-V ) i s d e f i n e d assuming a V o i g t p r o f i l e (7) w h i c h a l l o w s f o r a c o m b i n a t i o n o f D o p p l e r and c o l l i s i o n e

e

Q

e

2

Q


35.

HANSON E T A L .

Tunable

IR

Diode

Laser

All

l i n e b r o a d e n i n g . The V o i g t f u n c t i o n i s computed (10) a s a f u n c t i o n of the parameter a, where a

-

The D o p p l e r - b r o a d e n e d


Δν

linewidth =

0

1

(An 2 )

7.16

/

2

Av /Av c

D

.

(FWHM) i s g i v e n by

x 10"

7

(T/M

c o

)

1 / 2

V

Q

where Τ i s t h e t e m p e r a t u r e (K) and Μςο i s the molecular weight of t h e a b s o r b i n g s p e c i e s (gm/mole). The c o l l i s i o n - b r o a d e n e d l i n e w i d t h i s e x p r e s s e d i n t e r m s o f t h e c o l l i s i o n h a l f w i d t h 2γ, i . e . , the c o l l i s i o n - b r o a d e n e d l i n e w i d t h (FWHM) p e r u n i t p r e s s u r e o f t h e b r o a d e n i n g s p e c i e s , and t h e p r e s s u r e P: Av

c

=

1

(2γ,

cm""

1

- a t m " ) ( P , atm)

.

The c o l l i s i o n h a l f w i d t h f o r a g i v e n t r a n s i t i o n i s a f u n c t i o n o f t e m p e r a t u r e and t h e b r o a d e n i n g s p e c i e s . In the present diode l a s e r e x p e r i m e n t s , t h e t e m p e r a t u r e (and h e n c e AVp) i s known so that i t i s s t r a i g h t f o r w a r d to i n f e r v a l u e s f o r the parameters a and A\>c and h e n c e 2γ, from the observed a b s o r p t i o n l i n e w i d t h s . The t e m p e r a t u r e dependence o f t h e c o l l i s i o n h a l f w i d t h , 2γ(Τ), i s o f f u n d a m e n t a l and p r a c t i c a l i n t e r e s t and h a s n o t p r e v i o u s l y b e e n i n v e s t i g a t e d a t e l e v a t e d t e m p e r a t u r e s . I n t h e p a s t , most d e t e r m i n a t i o n s o f c o l l i s i o n h a l f w i d t h h a v e b e e n made n e a r room t e m p e r a t u r e , and h i g h - t e m p e r a t u r e v a l u e s have b e e n o b t a i n e d by e x t r a p o l a t i o n , u s u a l l y a s s u m i n g a T-0.5 t e m p e r a t u r e dependence so t h a t 2γ

=

0

2γ°(300/Τ) ·

5

where 2γ° i s t h e c o l l i s i o n h a l f w i d t h a t 300 K. T h i s temperature dependence i s b a s e d on h a r d - s p h e r e c o l l i s i o n t h e o r y a r g u m e n t s and i s known t o be i n c o r r e c t . Tunable diode l a s e r s p e c t r o s c o p y a p p l i e d t o a v a r i e t y o f gas c o n d i t i o n s , i n c l u d i n g a room t e m p e r a t u r e s t a t i c c e l l , s h o c k t u b e f l o w s 05,6) and f l a m e s , t h e r e f o r e p r o v i d e s a u n i q u e o p p o r t u n i t y t o s t u d y the t e m p e r a t u r e dependence o f 2γ. The p r e s e n t p a p e r p r o v i d e s i n i t i a l r e s u l t s f o r s e l e c t e d CO and NO t r a n s i t i o n s i n c o m b u s t i o n gas m i x t u r e s . Room T e m p e r a t u r e C e l l

Experiments

An e x t e n s i v e s e r i e s o f room t e m p e r a t u r e e x p e r i m e n t s r e c e n t l y has b e e n c o m p l e t e d i n o u r l a b o r a t o r y t o d e t e r m i n e t h e f u n d a m e n t a l band s t r e n g t h s o f CO and NO and t o measure CO and NO c o l l i s i o n h a l f w i d t h s f o r N2> A r and c o m b u s t i o n gas b r o a d e n i n g as a f u n c t i o n of r o t a t i o n a l and v i b r a t i o n a l quantum number. Some p r e l i m i n a r y r e s u l t s f o r CO a r e r e p o r t e d h e r e . I n t h e s e e x p e r i m e n t s , a gas m i x t u r e o f known p r o p o r t i o n s was



418

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FOR

COMBUSTION

CHEMISTRY

i n t r o d u c e d i n t o t h e room t e m p e r a t u r e c e l l , and t h e t e m p e r a t u r e and p r e s s u r e were m e a s u r e d . The s i g n a l - a v e r a g e d a b s o r p t i o n and n o n a b s o r p t i o n r e c o r d s w e r e d i f f e r e n c e d (by t h e PAR 4202) and t h e o u t p u t was d i s p l a y e d on a c h a r t r e c o r d e r t o g e t h e r w i t h t h e o u t p u t o f t h e F a b r y - P e r o t étalon. The n o n - a b s o r b e d s i g n a l , 1°, was meas u r e d s e p a r a t e l y . A c a r e f u l c a l i b r a t i o n o f a l l s y s t e m components was p e r f o r m e d . D a t a w e r e r e d u c e d by o b t a i n i n g a c o m p u t e r generated best V o i g t f i t t o each a b s o r p t i o n l i n e p r o f i l e . A t y p i c a l r e s u l t f o r CO h i g h l y d i l u t e i n N2 i s shown i n F i g u r e 2. The c e l l c o n d i t i o n s w e r e : Τ = 294.2 Κ, Ρ = 51.5 t o r r , CO m o l e f r a c t i o n = 0.00350, c e l l l e n g t h = 1 5 . 0 cm. The q u a n t i t y p l o t t e d i s : S g PÇQ L v e r s u s t h e n o r m a l i z e d n o n - r e s o n a n c e , Δ = (V-V )/2(£n 2 ) / A v , f o r t h e ν = 1 + 0, R ( l ) t r a n s i t i o n a t 2150.86 cm" . The b e s t V o i g t f i t , shown as t h e s o l i d c u r v e , y i e l d s v a l u e s of S(294.2 Κ) = 4.86 c m ^ - a t m " , a = 1.73 and 2γ(294.2 K ) _ = 0.153 c m ^ - a t m " . D e t a i l s o f t h e e x p e r i m e n t a l p r o c e d u r e s and a d d i t i o n a l r e s u l t s w i l l be p u b l i s h e d s e p a r a t e l y (11). E a c h l i n e s t r e n g t h d e t e r m i n a t i o n was c o n v e r t e d t o a v a l u e f o r t h e band s t r e n g t h a t 273.2 K; p r e l i m i n a r y r e s u l t s f o r S°(STP) a s a f u n c t i o n o f t h e r o t a t i o n a l quantum number a r e shown i n F i g u r e 3. Each of the p o i n t s p l o t t e d a c t u a l l y r e p r e s e n t s the average of several determinations; experimental scatter i n S f o r a given l i n e was l e s s t h a n ± 3%. The a v e r a g e o f t h e d e t e r m i n a t i o n s a t 10 values of m i s S°(STP) = 279.4 cm" -atm"!, w h i c h i s i n good agreement w i t h r e c e n t w o r k b y V a r a n a s i and S a r a n g i (S°(STP) = 273 ± 10 and 277 ± 4 c m ~ - a t m ~ ) u s i n g a d i f f e r e n t e x p e r i m e n t a l t e c h n i q u e ( 9 ) . I t s h o u l d be n o t e d t h a t o u r d a t a a r e f o r s p e c i f i c a b s o r p t i o n l i n e s of CO - ^ 0 , and we h a v e n o t y e t a p p l i e d a c o r r e c t i o n t o account f o r the presence of other i s o t o p e s i n the CO s a m p l e . T h i s i s e q u i v a l e n t t o a s s u m i n g n e g l i g i b l e l e v e l s o f o t h e r CO i s o t o p e s , w h i c h i s i n e r r o r by a b o u t 1%. 1

2

0

D

1

1

1

C Q

N

e

2

2

1

1

F l a t Flame Burner

Experiments

E x p e r i m e n t s a r e c u r r e n t l y i n p r o g r e s s t o measure CO and NO c o n c e n t r a t i o n s i n a f l a t f l a m e b u r n e r by d i o d e l a s e r s p e c t r o s c o p y . C o m p a r a t i v e measurements a r e a l s o b e i n g made u s i n g m i c r o p r o b e s a m p l i n g w i t h s u b s e q u e n t a n a l y s i s by n o n - d i s p e r s i v e i n f r a r e d and chemiluminescent techniques. Some p r e l i m i n a r y l a s e r a b s o r p t i o n r e s u l t s f o r CO a r e r e p o r t e d h e r e ; i n i t i a l r e s u l t s f o r NO h a v e been p u b l i s h e d s e p a r a t e l y ( 4 ) . A l s o r e p o r t e d a r e i n i t i a l d a t a f o r c o l l i s i o n h a l f w i d t h s i n combustion gases. The e x p e r i m e n t a l and d a t a r e d u c t i o n p r o c e d u r e s a r e e s s e n t i a l l y t h e same as f o r t h e s t a t i c c e l l e x p e r i m e n t s . The gas t e m p e r a t u r e i s o b t a i n e d u s i n g a f i n e w i r e , r a d i a t i o n - c o r r e c t e d thermo couple. The c o l d m i x i n g l a y e r a t e a c h f l a m e b o u n d a r y i s a c c o u n t e d f o r by u s i n g an e f f e c t i v e p a t h l e n g t h (8.0 - 8.2 cm, d e p e n d i n g on the f u e l - a i r e q u i v a l e n c e r a t i o ) which d i f f e r s s l i g h t l y from the a c t u a l b u r n e r l e n g t h o f 8.6 cm. F u e l - a i r equivalence r a t i o s of


HANSON E T A L .

Tunable

IR Diode

Laser

419

m CO

Ο CM


in ο

27.0 2

(ν-ν )/(2(*η2)^ Δ ν ) 0

ρ

Figure 2. Voigt fit to absorption line profile for CO (v = 1

ο < GO \ ο

ο ο

Ο

ο

•

Ο

ο

CD *

ι— 15

10

τ 25

~ι— 20

Figure 3. CO fundamental band strength at 273.2 Κ determined in room-tempera ture cell experiments: (A), M = J: P-branch; (0),J + I: R-branch. S° = 279.4 cm atm ave

4



35.

HANSON E T A L .

Tunable IR Diode

421

Laser

φ = 0.8 - 1.4 a r e b e i n g i n v e s t i g a t e d . M e a s u r e m e n t s a r e made i n t h e p o s t f l a m e g a s e s 1 cm above t h e b u r n e r s u r f a c e . The r e c o r d e d s i g n a l s r e p r e s e n t t h e a v e r a g e o f 8-32 l a s e r m o d u l a t i o n c y c l e s . T y p i c a l e x p e r i m e n t a l r e s u l t s and t h e b e s t V o i g t f i t f o r a CO a b s o r p t i o n l i n e i n a f l a m e w i t h φ = 1.24, Τ = 1850 Κ a r e shown i n F i g u r e 4. The o b s e r v e d t r a n s i t i o n i s t h e ( v = l « - 0 , P ( 7 ) ) l i n e o f CO a t 2115.63 cm-1. The l i n e s t r e n g t h S i s c a l c u l a t e d from the p r e v i o u s l y d e t e r m i n e d band s t r e n g t h , and t h e p a t h l e n g t h L i s known, so t h e unknown p a r a m e t e r s a r e PÇQ a n d g w h i c h c a n b o t h be i n f e r r e d f r o m t h e b e s t V o i g t f i t a n a l y s i s . The r e s u l t s a r e P = 0.0421 atm, a = 2.66 and 2 Y ( 1 8 5 0 ) - o m b g a s = 0-0393 cm~l - a t m . A s w i t h t h e s t a t i c c e l l e x p e r i m e n t s , t h e V o i g t p r o f i l e i s s e e n t o p r o v i d e a good f i t t h r o u g h o u t t h e o b s e r v e d portion of the absorption l i n e . I n f u e l - r i c h f l a m e s , t h e CO s h o u l d be i n l o c a l c h e m i c a l e q u i l i b r i u m , and h e n c e t h e p a r t i a l p r e s s u r e o f CO c a n be c a l c u l a t e d f r o m t h e l o c a l t e m p e r a t u r e and t h e m e a s u r e d f u e l and a i r flowrates. T h u s , a c o m p a r i s o n b e t w e e n m e a s u r e d a n d c a l c u l a t e d CO l e v e l s can serve as a v a l i d a t i o n o f t h e diode l a s e r technique f o r f l a m e measurements. Such a c o m p a r i s o n i s shown i n F i g u r e 5 f o r e q u i v a l e n c e r a t i o s i n t h e r a n g e φ = 1.04 - 1.37. The d a t a p o i n t s shown r e p r e s e n t t h e a v e r a g e o f s e v e r a l o b s e r v a t i o n s on s e p a r a t e l i n e s i n c l u d i n g g r o u n d s t a t e ( v " = 0) and e x c i t e d s t a t e ( v " = 1) transitions. The agreement i s c o n s i s t e n t l y w i t h i n t h e e x p e r i m e n t a l u n c e r t a i n t y o f ± 5%. R e s u l t s s h o w i n g t h e dependence o f t h e CO c o l l i s i o n h a l f w i d t h i n c o m b u s t i o n g a s e s on t h e v i b r a t i o n a l and r o t a t i o n a l quantum numbers a r e shown i n F i g u r e 6. The d a t a were o b t a i n e d w i t h a f l a m e t e m p e r a t u r e o f 1875 Κ and e q u i v a l e n c e r a t i o s i n t h e r a n g e 1.2 - 1.4. A l t h o u g h t o o f e w d a t a p o i n t s a r e a v a i l a b l e f o r a detailed a n a l y s i s , i t i s c l e a r that 2γ d e c r e a s e s w i t h i n c r e a s i n g m and t h a t v a l u e s f o r 2γ a r e n e a r l y e q u a l ( w i t h i n 5%) f o r g r o u n d s t a t e and e x c i t e d s t a t e t r a n s i t i o n s . The t e m p e r a t u r e dependence o f t h e c o l l i s i o n h a l f w i d t h f o r combustion gas b r o a d e n i n g i s a l s o o f i n t e r e s t . Results f o r s p e c i f i c t r a n s i t i o n s i n CO a n d NO a r e g i v e n i n T a b l e I . I n t h e C 0

C O

C

1

TABLE I .

TEMPERATURE DEPENDENCE OF COLLISION HALFWIDTHS

A. CO [ν = 1+-0, P ( 7 ) ) ] 2γ (300 K )

_

C 0

2γ (1850 K )

C

1

= 0.131 cm" /atm

N 2

_

0

C

o

m

b

G

a

s

η

= 0.039 cnTVatm

( C H / a i r , φ = 1.24) 4

= 0.67

Β. NO [Ω = 3/2, ν = 1+Ό, R ( 1 3 / 2 ) ] 2γ (300 K )

N

0

_

C

0

_

o

m

b

G

a

s

G

a

= 0.137 cm

/atm CH /air, 4

2γ (1700 K ) η

N

C

o

m

b

s

φ = 0.65

= 0.043 = 0.67


422

LASER PROBES FOR COMBUSTION CHEMISTRY

IP


m

-24.0

-16.0

-8.0

0.0

8.0

16.0

24.0

( v - v ) / ( 2 U n 2 ) ^ Δν ) 0

0

Figure 4. Voigt fit to absorption line profile for CO (v = 1 *- 0, P(7) at 2115.63 cm' ) in CH -air fiat flame. Flame conditions are: Τ = 1850 K,V = 1 atm, and φ = 1.24. Inferred results are P o = 0.0421 atm, a = 2.66 and 2γ(1850) = 0.0393 cm atm . 1

h

C

1

1



35.

HANSON E T A L .

Figure 5.

Tunable

IR

Diode

423

Laser

CO partial pressure in CH^-air flat flame as a function of equivalence ratio: Τ = 1875 ± 25 K;? = 1 atm.


fuel-air

424


LASER PROBES FOR COMBUSTION CHEMISTRY

U4*

L

.uio

4

()| g^8L)X2



35.

HANSON E T AL.

Tunable

IR

Diode

Laser

425

c a s e o f t h e CO t r a n s i t i o n , t h e c o l l i s i o n h a l f w i d t h o b t a i n e d i n a f l a m e i s compared w i t h t h e room t e m p e r a t u r e v a l u e f o r b r o a d e n i n g by N2. T h i s procedure should not i n t r o d u c e a l a r g e e r r o r s i n c e the c o m b u s t i o n p r o d u c t s a r e p r i m a r i l y N£. I n t h e c a s e o f NO, t h e c o l l i s i o n h a l f w i d t h was a c t u a l l y m e a s u r e d i n t h e same c o m b u s t i o n gas s a m p l e , f i r s t i n t h e h i g h t e m p e r a t u r e g a s e s u s i n g t h e i n s i t u t e c h n i q u e , and s u b s e q u e n t l y i n l o w t e m p e r a t u r e g a s e s w h i c h w e r e e x t r a c t e d by a s a m p l i n g p r o b e and s e n t t o t h e a b s o r p t i o n c e l l . The t e m p e r a t u r e dependence c a n be e x p r e s s e d i n t e r m s o f an e x p o n e n t n, where 2γ(Τ)

=

2γ°(300/Τ)

η

w i t h t h e r e s u l t , f o r b o t h t h e CO and NO t r a n s i t i o n s s t u d i e d , η = 0.67. The e x a c t agreement f o r t h e s e CO and NO l i n e s must be c o n s i d e r e d f o r t u i t o u s , but the s i m i l a r i t y of the r e s u l t s w i t h the v a l u e f o u n d i n a p r e v i o u s s h o c k t u b e s t u d y o f CO b r o a d e n i n g by A r (6) s u g g e s t s t h a t η ~ 0.7 may p r o v i d e a r e a s o n a b l e e s t i m a t e f o r t e m p e r a t u r e d e p e n d e n c e . Work i s p r e s e n t l y underway t o d e t e r m i n e η f o r a l a r g e r number o f CO and NO t r a n s i t i o n s w i t h N2, A r and c o m b u s t i o n gas b r o a d e n i n g . Such d a t a s h o u l d be o f u s e t o t h e o r e t i c i a n s i n t e r e s t e d i n m o d e l l i n g c o l l i s i o n a l i n t e r a c t i o n s of molecules at elevated temperatures. C o n c l u d i n g Remarks These e x p e r i m e n t s demonstrate t h a t t u n a b l e d i o d e l a s e r a b s o r p t i o n s p e c t r o s c o p y i s w e l l s u i t e d f o r i n s i t u measurements o f s p e c i e s c o n c e n t r a t i o n s i n c o m b u s t i o n f l o w s , when a l i n e - o f - s i g h t t e c h n i q u e i s a p p r o p r i a t e , and f o r a c c u r a t e measurements o f s p e c t r o s c o p i c parameters needed t o c h a r a c t e r i z e h i g h - t e m p e r a t u r e absorption lines. The t e c h n i q u e i s s e n s i t i v e , s p e c i e s s p e c i f i c and a p p l i c a b l e t o a l a r g e number o f i m p o r t a n t c o m b u s t i o n s p e c i e s i n c l u d i n g r e a c t i v e i n t e r m e d i a t e s , and h e n c e i t s h o u l d p r o v e t o be a u s e f u l t o o l i n f u t u r e s t u d i e s of combustion chemistry. The p o t e n t i a l of tunable l a s e r absorption spectroscopy i n p a r t i c l e l a d e n f l o w s s h o u l d a l s o be n o t e d ( 1 2 ) , i n t h a t m o d u l a t i o n o f t h e l a s e r w a v e l e n g t h on and o f f an a b s o r p t i o n l i n e a l l o w s s i m p l e d i s c r i m i n a t i o n a g a i n s t c o n t i n u u m e x t i n c t i o n by p a r t i c l e s . Acknowledgements T h i s w o r k h a s been s u p p o r t e d by t h e D e p a r t m e n t o f E n e r g y u n d e r g r a n t EY-76-S-03-0328, PA58 and by t h e A i r F o r c e O f f i c e o f S c i e n t i f i c R e s e a r c h u n d e r C o n t r a c t F49620-78C-0026.


LASER PROBES FOR COMBUSTION

426

CHEMISTRY


Literature Cited 1.

Hanson, R. Κ., Kuntz, P. Α., and Kruger, C. Η., "HighResolution Spectroscopy of Combustion Gases Using a Tunable IR Diode Laser"; Applied Optics, 1977, 16, 2045.

2.

Hanson, R. K. and Kuntz-Falcone, P. Α., "Temperature Measure ment Technique for High-Temperature Gases Using a Tunable Diode Laser"; Applied Optics, 1978, 17, 2477.

3. Hanson, R. Κ., "Combustion Gas Measurements Using Tunable Laser Absorption Spectroscopy"; AIAA Preparint No. 79-0086, 17th Aerospace Sciences Meeting, New Orleans, Jan. 1979. 4.

Falcone, P. Κ., Hanson, R. Κ., and Kruger, C. Η., "Measurement of Nitric Oxide in Combustion Gases Using a Tunable Diode Laser"; Paper No. 79-53, Western States Section/Combustion Institute Meeting, Berkeley, Ca., Oct. 1979.

5.

Hanson, R. Κ., "Shock Tube Spectroscopy: Advanced Instrumen tation with a Tunable Diode Laser"; Applied Optics, 1977, 16, 1479.

6.

Hanson, R. Κ., "High-Resolution Spectroscopy of Shock-Heated Gases Using a Tunable Infrared Diode Laser"; in "Shock Tube and Wave Research"; University of Washington Press, 1978, p. 432.

7. Penner, S. S., "Quantitative Molecular Spectroscopy and Gas Emissivities"; Addison-Wesley: Reading, Mass., 1959. 8.

See Ref. 7, Chapter 7, Section 5, and the references listed there.

9. Varanasi, P. and Sarangi, S., "Measurements of Intensities and Nitrogen-Broadened Linewidths in the CO Fundamental at Low Temperatures"; Jour. of Quant. Spectrosc. Radiat. Transfer, 1975, 15, 473. 10. Drayson, S. R., "Rapid Computation of the Voigt Profile"; Jour. of Quant. Spectrosc. Radiat. Transfer, 1976, 16, 611. 11. Varghese, P. L. and Hanson, R. Κ., "Diode Laser Measurements of CO Collision Halfwidths and Fundamental Band Strength at Room Temperature"; to be published. 12. Hanson, R. Κ., "Absorption Spectroscopy in Sooting Flames Using a Tunable Diode Laser"; submitted to Applied Optics. R E C E I V E D February

1, 1 9 8 0 .


Absorption Spectroscopy of Combustion Gases Using a Tunable IR

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