CARS Measurements in Simulated Practical Combustion

24 C A R S Measurements in Simulated Practical Combustion Environments GARY L. SWITZER—Systems Research Laboratories, Inc., Dayton, OH 45440

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0134.ch024

WILLIAM M. ROQUEMORE, ROYCE P. BRADLEY, and PAUL W. SCHREIBER—Air Force Aero Propulsion Laboratory, Wright-Patterson Air Force Base, OH 45433 WON B. ROH—Air Force Institute of Technology, Department of Physics, Wright-Patterson Air Force Base, OH 45433

The measurement of temporally and spatially resolved temperature and species concentrations in the combustion zone of reacting gases presents a formidable problem. The conventional approach to solving this problem has been the use of mechanical probing techniques to obtain time-averaged data. Unfortunately, the perturbation of the reactive media due to the presence of probes remains an unknown factor. For this and other reasons, optical techniques that have the potential for real-time non-intrusive measurements are very desirable. By using the physical processes of radiation scattering or fluorescence, data can be collected from a temporally and spatially resolved point. However, either the data analysis required or the relatively low signal intensities involved may present serious problems. In overcoming these limitations, coherent anti-Stokes Raman spectroscopy (CARS) has been considered as a promising new method for combustion diagnostics. The signal levels generated by CARS may be orders of magnitude greater than those of spontaneous Raman scattering, and the data analysis does not significantly depend upon collisional quenching rates as it does with most fluorescence techniques. Although CARS has been used to study laboratory flames (1,2), it has only recently been applied to large-scale combustion environments (3,4). This paper reports the results of CARS measurements performed in a highly turbulent sooting flame produced in a large-scale practical combustion-type environment. CARS measurements w e r e made i n a b l u f f - b o d y s t a b i l i z e d flame w i t h t u r b u l e n t and r e c i r c u l a t i n g f l o w c h a r a c t e r i s t i c s s i m i l a r t o t h o s e f o u n d i n many p r a c t i c a l c o m b u s t o r s . The comb u s t o r was o p e r a t e d 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 i n l e t a i r t e m p e r a t u r e s b e t w e e n 280 a n d 300K, a n a i r f l o w r a t e o f 0.5 k g / s e c , a n d a n u p s t r e a m R e y n o l d s number 1.5 * 1 0 . Gaseous p r o p a n e was i n j e c t e d f r o m a h o l l o w - c o n e n o z z l e l o c a t e d a t t h e c e n t e r o f t h e b l u f f - b o d y c o m b u s t o r a t a f l o w r a t e o f 7.06 k g / h r . The f l a m e c o n s i s t e d o f a b l u e cone o r i g i n a t i n g a t t h e n o z z l e f o l l o w e d b y a yellow-luminous t a i l . 5

0-8412-0570-1/80/47-134-303$05.00/0 ©

1980 A m e r i c a n Chemical Society

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


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The CARS s y s t e m u s e d t o m e a s u r e t e m p e r a t u r e and s p e c i e s c o n c e n t r a t i o n s i n t h e c o m b u s t o r zone i s composed o f a s i n g l e - m o d e r u b y - l a s e r o s c i l l a t o r - a m p l i f i e r w i t h a r e p e t i t i o n r a t e o f 1 Hz and a ruby-pumped, n e a r - i n f r a r e d b r o a d - b a n d dye l a s e r . The two l a s e r beams a r e combined c o l l i n e a r l y and f o c u s e d f i r s t i n t o a c e l l c o n t a i n i n g a n o n r e s o n a n t r e f e r e n c e gas and t h e n i n t o t h e s a m p l e v o l u m e ( a p p r o x i m a t e l y 30-μ diam. χ 2 cm) i n t h e c o m b u s t i o n region. The a n t i - S t o k e s beams p r o d u c e d i n t h e s a m p l e and r e f e r e n c e v o l u m e s a r e d i r e c t e d t o s p a t i a l l y s e p a r a t e d f o c i on t h e e n t r a n c e s l i t o f a s p e c t r o m e t e r and d e t e c t e d by s e p a r a t e p h o t o m u l t i p l i e r t u b e s . An o p t i o n a l means o f d e t e c t i o n i s p r o v i d e d f o r t h e s a m p l e s i g n a l i n t h e f o r m o f an o p t i c a l m u l t i c h a n n e l a n a l y z e r (OMA), w h i c h makes i t p o s s i b l e t o o b t a i n s i n g l e - p u l s e CARS spectra. Q-branch s p e c t r a o f N2 and O2 w e r e o b t a i n e d i n t h e r e a c t i o n zone o f t h e c o m b u s t o r d u r i n g a s i n g l e 15-ns p u l s e t h r o u g h t h e u s e o f t h e OMA and b r o a d - b a n d S t o k e s beam. An e x a m p l e o f t h e s e s i n g l e - s h o t s p e c t r a i s g i v e n f o r N2 i n F i g . 1. T h e s e s p e c t r a suggest that i t i s f e a s i b l e to o b t a i n simultaneous s i n g l e - p u l s e measurements o f t e m p e r a t u r e and s p e c i e s c o n c e n t r a t i o n i n t h i s type of combustion environment. Although not presented here, s i n g l e - s h o t t e m p e r a t u r e d e t e r m i n a t i o n s h a v e i n d e e d b e e n made d u r i n g r e c e n t measurements. S p a t i a l l y r e s o l v e d average temperatures were a r r i v e d a t t h r o u g h t h e use o f t i m e - a v e r a g e d N2 CARS s p e c t r a o b t a i n e d by s t e p p i n g t h e s p e c t r o m e t e r t h r o u g h t h e Q-branch s p e c t r a g e n e r a t e d i n the combustion r e g i o n . T e m p e r a t u r e was d e t e r m i n e d by compar i n g t h e m e a s u r e d , n o r m a l i z e d Q-branch s p e c t r u m , i n d i c a t e d by "+" i n F i g . 2, and t h e c a l c u l a t e d s p e c t r a g e n e r a t e d b y a d j u s t i n g t h e t e m p e r a t u r e u n t i l t h e b e s t f i t was o b t a i n e d . The m e a s u r e d tem p e r a t u r e p r o f i l e s a r e shown f o r t h e a x i a l and t h e Y and X r a d i a l d i m e n s i o n s i n F i g s . 3, 4, and 5, r e s p e c t i v e l y . I t s h o u l d be p o i n t e d out t h a t the apparent d i s c r e p a n c y i n the c e n t e r - l i n e t e m p e r a t u r e s o f F i g s . 3 and 5 i s t h e r e s u l t o f a s l i g h t m o d i f i c a t i o n i n the p o s i t i o n i n g of the f u e l n o z z l e r e l a t i v e to the face o f t h e b l u f f - b o d y c o m b u s t o r w h i c h was made b e t w e e n t h e m e a s u r e ment p e r i o d s . S i n g l e - s h o t i n t e g r a t e d Q-branch i n t e n s i t y measurements w e r e p e r f o r m e d t o o b t a i n t h e N and 0 m o l e c u l a r number d e n s i t i e s from the r e a c t i o n p r o d u c t s . These d a t a were reduced w i t h the a v e r a g e d CARS t e m p e r a t u r e s t o o b t a i n t h e s p e c i e s - c o n c e n t r a t i o n p r o f i l e s a l s o shown i n F i g s . 3-5. To o b t a i n c o m p a r a t i v e t e m p e r a t u r e d a t a , a P t 1 3 % - R h P t t h e r m o c o u p l e p r o v i d e d by NASA L e w i s R e s e a r c h C e n t e r was u s e d t o p r o f i l e t h e p r o p a n e f l a m e a t t h e Ζ = 50-cm d o w n s t r e a m p o s i tion. C o m p a r i s o n o f t h e CARS and r a d i a t i o n - c o r r e c t e d t h e r m o c o u p l e - d e r i v e d t e m p e r a t u r e p r o f i l e s i s shown i n F i g . 6. The agreement b e t w e e n CARS ( s o l i d c u r v e ) and t h e r m o c o u p l e ( d a s h e d l i n e ) t e m p e r a t u r e s a p p e a r s t o be q u i t e r e a s o n a b l e f o r l o c a t i o n s w i t h i n 3 cm o f t h e c o m b u s t o r c e n t e r l i n e . However, t h e CARS d a t a 2

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FREQUENCY (CM"') Applied Optics

Figure 2.

Overlay of computer-generated spectrum of N measured spectra (-\-) (3)

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Figure 3.

A xial profiles (3)


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Y radial profiles atX = 0 and Ζ = 7 cm ( 3)


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Comparison of CARS temperature ( ) and average thermocouple temperature ( ) at Υ = 0 and Ζ = 50 cm


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i n d i c a t e t h a t f o r d i s t a n c e s o f more t h a n 3 cm f r o m t h e c e n t e r l i n e , some a s y e t unknown e f f e c t s a r e c o n t r i b u t i n g t o t h e CARS i n f o r m a t i o n and r e s u l t , f o r e x a m p l e , i n t h e k n e e s shown i n t h e CARS t e m p e r a t u r e p r o f i l e . I n v e s t i g a t i o n s t o determine whether these d i s t u r b a n c e s i n t h e d a t a a r e a consequence o f o p t i c a l misalignment o r improper a v e r a g i n g caused by flame t u r b u l e n c e o r w h e t h e r t h e y a r e o f a more f u n d a m e n t a l n a t u r e a r e p r e s e n t l y underway. In c o n c l u s i o n , t h e data accumulated d u r i n g these experiments d e m o n s t r a t e t h a t t h r o u g h t h e u s e o f t h e CARS t e c h n i q u e , combust i o n d i a g n o s t i c s c a n b e p e r f o r m e d i n a l a r g e - s c a l e p r a c t i c a l comb u s t o r e n v i r o n m e n t . R e s u l t s a l s o i n d i c a t e t h e p o t e n t i a l o f CARS f o r t h e d e t e r m i n a t i o n o f t e m p o r a l l y r e s o l v e d temperature and s p e c i e s c o n c e n t r a t i o n . With t h e a i d o f such d a t a , p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n s can be o b t a i n e d from which t r u e t i m e a v e r a g e d q u a n t i t i e s may b e d e t e r m i n e d .

Literature Cited 1. Moya, F.; Druet, S. A. J.; and Taran, J. P. E. Opt. Commun., 1975, 13, 169. 2. Eckbreth, A. C.; Hall, R. J.; and Shirley, J. A. "Investigations of Coherent Anti-Stokes Raman Spectroscopy (CARS) for Combustion Diagnostics," Paper 79-0083, Presented at the 17th AIAA Conference on Aerospace Sciences, New Orleans, LA, June 15-17, 1979. 3. Switzer, G. L.; Roquemore, W. M.; Bradley, R. P.; Schreiber, P. W.; and Roh, W. B. "CARS Measurements in a Bluff-Body Stabilized Diffusion Flame," Appl. Opt., 1979, 18, 2343. 4. Eckbreth, A. C. "Spatially Precise Laser Diagnostics for Practical Combustor Probing," Presented at the 178th ACS National Meeting, Washington, D.C., September 10-14, 1979. RECEIVED

February 1, 1980.


CARS Measurements in Simulated Practical Combustion

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