22 Time-Resolved Raman Spectroscopy in a Stratified-Charge Engine J. RAY SMITH Laser Probes for Combustion Chemistry Downloaded from pubs.acs.org by YORK UNIV on 12/03/18. For personal use only.
Sandia Laboratories, Livermore, CA 94550
The objectives of this research were to develop techniques to measure both the mean and fluctuating nitrogen density and temperature in a combusting stratified charge engine. Such data is necessary for analytical engine model verification. The method chosen to achieve these measurements was spontaneous vibrational Raman scattering by a pulsed frequency doubled YAG laser to get a time resolution of 10 nsec. The nitrogen density was determined from the Stokes signal and the temperature was determined from the ratio of the anti-Stokes to Stokes signal. Setchell demonstrated the feasibility of using time-averaged Raman scattering in a combusting homogeneous charge engine. A stratified charge engine with good optical access was recently developed, and its precombustion fuel-air distributions were determined by time-averaged Raman spectroscopy. The latter engine's precombustion velocity and turbulence fields were measured by laser Doppler velocimetry and its performance and emissions were quantified by conventional methods. The same engine design was used in the present study. 1
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The short duration of the l a s e r pulse at 5321 Angstroms precluded any movement of the spectrometer g r a t i n g t o allow s p e c t r a l d e t a i l s to be r e s o l v e d nor were there s u f f i c i e n t photons to use a multichannel d e t e c t o r . Therefore the spectrometer g r a t i n g was f i x e d and the e n t i r e nitrogen Stokes band was i n t e g r a t e d by a p h o t o m u l t i p l i e r tube (PMT). S i m i l a r l y , the anti-Stokes spectrum was i n t e g r a t e d by a second p h o t o m u l t i p l i e r . The major problem that must be solved i n making Stokes nitrogen d e n s i t y measurements i n a t u r b u l e n t flow was pointed out by S e t c h e l l i t o be the temperature dependence of the Raman s c a t t e r e d Stokes i n t e n s i t y . Because the t r a n s i t i o n p r o b a b i l i t y i s p r o p o r t i o n a l t o (v + 1) where ν i s the i n i t i a l v i b r a t i o n a l s t a t e , the i n t e g r a t e d Raman s c a t t e r e d i n t e n s i t y i s not a unique f u n c t i o n of number d e n s i t y . However, a t h e o r e t i c a l a n a l y s i s of the
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Stokes v i b r a t i o n a l Raman spectrum of n i t r o g e n has l e d t o a method o f making n i t r o g e n number d e n s i t y measurements t h a t are e s s e n t i a l l y independent of temperature. A v a r i e t y o f spectrometer s l i t c o n v o l u t i o n s and c e n t e r wavelength s e t t i n g s were s t u d i e d t o determine t h e i r i n f l u e n c e on the i n t e g r a t e d Stokes Raman i n t e n s i t y versus temperature. As p o i n t e d out by Leonardo the b a s i c approach i s t o s e l e c t spectrometer s e t t i n g s which balance the increased t r a n s i t i o n p r o b a b i l i t y o f higher v i b r a t i o n a l s t a t e s against the decrease i n p o p u l a t i o n o f the ground s t a t e as the temperature r i s e s . F i g u r e 1 i s an example o f the r e s u l t s f o r a t r a p e z o i d a l s l i t o f 10 by 50 Angstroms with the c e n t e r wavelength v a r i e d from 6070 t o 6073 Angstroms. By s e t t i n g the spectrometer at 6072 Angstroms, i t was p o s s i b l e t o have the Stokes i n t e n s i t y vary by only ±2% while the temperature v a r i e d from 300 t o 1970 K e l v i n s . The s i g n i f i c a n c e o f t h i s r e s u l t i s t h a t i t i s not necessary t o make simultaneous temperature measurements i n a t u r b u l e n t flow f i e l d i n order t o make d e n s i t y measurements. A s i m i l a r a n a l y s i s o f the a n t i - S t o k e s t o Stokes i n t e n s i t y r a t i o expected from n i t r o g e n as a f u n c t i o n o f bandwidth c e n t e r p o s i t i o n o f the a n t i - S t o k e s s p e c t r a i s shown i n F i g u r e 2. I t appears extremely d i f f i c u l t t o use spontaneous v i b r a t i o n a l Raman s c a t t e r i n g f o r determining temperatures o f l e s s than 1000 Κ due to the small a n t i - S t o k e s s i g n a l . Although temperatures i n an engine are well above t h i s l e v e l a f t e r the flame passes through the s c a t t e r i n g volume, the present d e t e c t i o n system does not have s u f f i c i e n t background r e j e c t i o n t o make temperature measurements with good s i g n a l t o noise r a t i o s unless the s t r a t i f i c a t i o n i s small and the equivalence r a t i o l e s s than 0.8. The engine i n t h i s work was designed t o s i m p l i f y the f l u i d mechanics f o r modeling purposes. The i n t a k e and exhaust v a l v e s , f u e l i n j e c t o r , spark plug and l a s e r input/output windows were l o c a t e d i n the c y l i n d e r s i d e w a l l s i n t h e c l e a r a n c e volume above the p i s t o n . The head c o n t a i n e d a 70 mm diameter, c l e a r aperture window. The l a s e r beam was passed through the small windows and s c a t t e r e d l i g h t was c o l l e c t e d a t r i g h t angles t o the beam through the l a r g e window i n the top o f the engine. The i n t a k e valve was shrouded which caused the a i r flow t o s w i r l . Propane at 3.35 MP a (485 p s i ) and 375 Κ was i n j e c t e d r a d i a l l y toward the c e n t e r o f the c y l i n d e r . The o v e r a l l equivalence r a t i o o f the data presented was 0.7 and the engine speed was 900 rpm. The experimental arrangement i s shown i n F i g u r e 3. A s h a f t encoder was used t o synchronize the l a s e r pulse t o the chosen crank angle. The Raman s c a t t e r e d l i g h t was imaged onto the s l i t of a 3/4 meter, s i n g l e spectrometer and detected by the cooled PMT's. Only the f i r s t s i x stages o f the PMT were used i n order to maintain l i n e a r i t y over a wide dynamic range. The PMT output charge was i n t e g r a t e d by a p r e a m p l i f i e r , pulse shaped by a spectroscopy a m p l i f i e r , d i g i t i z e d , and c o l l e c t e d by a m i n i computer. The Raman s i g n a l was normalized by the l a s e r pulse energy, and by accounting f o r the a m p l i f i e r g a i n , the number o f
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Relative Stokes vibrational Raman intensity for nitrogen for a trape zoidal slit function and various center positions
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Figure 2. Intensity ratio of anti-Stokes to Stokes vibrational Raman scattering for a trapezoidal slit function. Center position of Stokes bandpass at 6072 A.
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photoelectrons was recorded. One thousand measurements were taken during s u c c e s s i v e engine c y c l e s at each s e l e c t e d crank angle. Checks f o r p o s s i b l e s e n s i t i v i t y changes (due t o window t r a n s m i s s i o n ) were made d u r i n g the course of the data a c q u i s i t i o n . The maximum s e n s i t i v i t y v a r i a t i o n observed over a one hour p e r i o d of lean engine o p e r a t i o n was 2.4 percent change i n the mean value. Sub-microsecond time r e s o l u t i o n was achieved by using a Quanta-Ray DCR-1A l a s e r having a pulse width of 10 nsec. Although t h i s l a s e r was capable of producing i n excess of 250 m i l l i j o u l e s per p u l s e , only 50 m i l l i j o u l e s were used i n the experiment. Above t h i s energy l e v e l the chances of window damage are g r e a t l y i n c r e a s e d . A l l of the data presented were gathered with l e s s than 20 MW/cm^ power d e n s i t i e s on the input/output windows. Gas breakdown was avoided by t i l t i n g the f o c u s i n g lens r e l a t i v e t o the beam a x i s thus i n t r o d u c i n g a l a r g e degree of astigmatism. T h i s gave a s c a t t e r i n g volume of 0.5 mm diameter by 1.25 mm l e n g t h . The length was determined by the 4x m a g n i f i c a t i o n of the c o l l e c t i o n o p t i c s and the spectrometer entrance s l i t height of 5 mm. A check of the l i n e a r i t y of the Raman s i g n a l versus both n i t r o g e n d e n s i t y and l a s e r beam energy well beyond the ranges of the experiment was w i t h i n two percent. The Raman s c a t t e r i n g process i s very weak and the number of photoelectrons produced per pulse w i l l obey Poisson s t a t i s t i c s . I f more than 100 p h o t o e l e c t r o n s are produced i n each event, the u n c e r t a i n t y (one standard d e v i a t i o n ) i n the actual number of p h o t o e l e c t r o n s , N, may be approximated by /N and the f r a c t i o n a l u n c e r t a i n t y i s ση = /Ν. Since n i t r o g e n d e n s i t y f l u c t u a t i o n s , σ-f, are not r e l a t e d t o the p h o t o e l e c t r o n f l u c t u a t i o n s , σ ρ , they w i l l combine randomly t o give a s i g n a l f l u c t u a t i o n , 2 2 1/2 σ = (σ.ρ + Op) ' . T h e r e f o r e the f r a c t i o n a l f l u c t u a t i o n s may be assessed by t a k i n g a s u f f i c i e n t l y l a r g e sample of measurements t o compute o and u s i n g the mean Ν value of the s i g n a l t o compute σ ρ . T h i s technique works well provided the actual f l u c t u a t i o n s are at l e a s t h a l f the s i z e of the Poisson s t a t i s t i c a l f l u c t u a t i o n s . Below t h i s l e v e l the u n c e r t a i n t y i n σρ begins t o dominate the computed of value. In t h i s experiment 200 t o 800 photoelectrons were c o l l e c t e d from each l a s e r pulse which gave s t a t i s t i c a l u n c e r t a i n t i e s from 3.4 t o 7 percent. The mean values of the r e l a t i v e n i t r o g e n d e n s i t y versus crank angle at the c e n t e r of the combustion chamber are shown i n F i g u r e 4. A l s o shown (dashed l i n e ) f o r comparison i s the n i t r o g e n d e n s i t y expected from the p i s t o n motion. The d e n s i t y i s r e l a t i v e t o the n i t r o g e n d e n s i t y i n a i r at atmospheric pressure. I g n i t i o n occurred at 358 crank angle degrees and the flame a r r i v e d at the s c a t t e r i n g volume at about 382 degrees. One observes the compres s i o n of the unburned gases ahead o f the flame f r o n t a f t e r i g n i t i o n . The r e l a t i v e f l u c t u a t i o n s i n n i t r o g e n d e n s i t y versus crank angle are i n d i c a t e d by the s o l i d curve shown i n F i g u r e 5. P r i o r t o i g n i t i o n , the f l u c t u a t i o n l e v e l was q u i t e low. The f l u c t u a 5
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One standard deviation of fluctuations in nitrogen number density
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Histograms of nitrogen number density near time of flame arrival
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t i o n s reached a maximum near 382 degrees due t o a r r i v a l of the flame. The f l u c t u a t i o n s f e l l s l o w l y i n the post flame gases. The l a r g e peak value i s due t o the random v a r i a t i o n s i n the a r r i v a l time o f the flame f r o n t . Near 382 degrees some measure ments were made j u s t ahead o f the flame f r o n t where high d e n s i t i e s p r e v a i l and o t h e r measurements were made j u s t behind the flame f r o n t where low d e n s i t i e s p r e v a i l due t o high temperatures. C l e a r l y these l a r g e f l u c t u a t i o n s are due t o c y c l i c v a r i a t i o n s not t u r b u l e n t f l u c t u a t i o n s . The dashed curve i s an attempt t o remove t h i s c y c l i c v a r i a t i o n e f f e c t by using the most probable d e n s i t y value as the mean value o f a normal d i s t r i b u t i o n . The standard d e v i a t i o n of the d i s t r i b u t i o n i s determined from f i t t i n g the data to the s i d e o f the new mean t h a t has not been d i s t o r t e d by flame a r r i v a l . The r e d u c t i o n o f the apparent f l u c t u a t i o n s near the flame a r r i v a l crank angle i s dramatic. Both curves o f F i g u r e 5 have had the Poisson s t a t i s t i c a l f l u c t u a t i o n s s u b t r a c t e d . The histograms o f F i g u r e 6 represent the number o f measure ments versus the number o f photoelectrons at crank angles near flame a r r i v a l i n the s c a t t e r i n g volume. The b i n width i s ten photoelectrons and the t o t a l number o f events i s 1,000 f o r each histogram. The normal shape o f the histogram at 372 degrees i s t y p i c a l o f those from 300 t o 372 and 390 t o 420 degrees. At 380 degrees e a r l y flame a r r i v a l caused the long t a i l i n the d i s t r i b u t i o n below the most probable d e n s i t y . S i m i l a r l y , the d i s t o r t i o n of the d i s t r i b u t i o n t o h i g h e r d e n s i t y values at 384 degrees was due t o l a t e flame a r r i v a l s . T h i s e x p l a n a t i o n o f the e f f e c t s o f c y c l i c v a r i a t i o n on the d i s t r i b u t i o n j u s t i f i e s the attempt t o separate them from the real d e n s i t y f l u c t u a t i o n s . The Stokes s i g n a l - t o - n o i s e r a t i o s were o f the order o f t h i r t y t o one even when the flame was i n the s c a t t e r i n g volume. It i s l i k e l y t h a t the i n c r e a s e i n f l u c t u a t i o n s immediately behind the flame f r o n t was flame induced t u r b u l e n c e . Improved f l u c t u a t i o n measurements are expected by using the temperature d e r i v e d from the a n t i - S t o k e s channel f o r c o n d i t i o n a l sampling o f the d e n s i t y data. However, t h i s method cannot be used u n t i l the 5 microsecond i n t e g r a t i o n time o f present d e t e c t i o n e l e c t r o n i c s i s shortened t o reduce the background l u m i n o s i t y s i g n a l on the anti-Stokes channel. Acknowledgment T h i s w o r k s u p p o r t e d b y DOE a n d M o t o r V e h i c l e M a n u f a c t u r e r s Association.
References 1.
Setchell, R. Ε., 18th Annual Rocky Mountain Spectroscopy Conference, University of Denver, Aug. 2-3, 1976. 2. Johnston, S. C., SAE paper 790433, FEB. 1979.
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Johnston, S. C., Robinson, C. W., Rorke, W. S., Smith, J. R., and Witze, P. O., SAE paper 790092, Feb. 1979. 4. Setchell, R. E., 17th Aerospace Sciences Meeting, New Orleans, LA, Jan. 1979. 5. Leonard, D. Α., Project SQUID, Tech. Rep. AVCO-1-PU, 1972.
Received February 1, 1980.
