7

NASA Technical Memorandum 100498

.

A S I M P L I F I E D CARS MEASUREMENT SYSTEM FOR RAPID DETERMINATION

OF TEMPERATURE AND OXYGEN CONCENTRATION

(NASA-TH-loa498) A S IHPLXFXED CARS N88- 12353 HEASURERENT SYSTEM FOR RAPID DETERRIWA'IXON OF TEMPERATURE A N D O X Y G E N CONCENTRATIOH {NASA) 32 p A v a i l : NTIS NC A03/19F A01 Unclas

CSCL 20R G3/72 Ct106516

SHOICHI FUJI!

AUGUST 1987

National Aeronautics and Space Administration

Langley Research Center Hampton, Virginia 23665

https://ntrs.nasa.gov/search.jsp?R=19880002971 2020-03-20T08:39:16+00:00Zbrought to you by COREView metadata, citation and similar papers at core.ac.uk

provided by NASA Technical Reports Server

SUMMARY

.

A new spectroscopic concept fo r t h e r a p i d determinat ion of temperature and oxygen concent r a t i o n by CARS (Coherent Ant i -Stokes Raman Spectroscopy ) was described. The r a t i o of two spect ra l regions i n t h e broadband Q-branch spectrum was detected by p h o t o m u l t i p l i e r s i n a monochromator, which r a t i o depends on tem- perature and species concentrat ion. theory was made us ing a f l a t flame burner and an e l e c t r i c furnace, w i t h reason- ab le resu l ts . Various o p t i c a l techniques for alignment were introduced i n c l u d i n g a h i g h l y e f f i c i e n t , s tab le dye o s c i l l a t o r . The combination o f t h e spectroscopic concept and t h e o p t i c a l techniques w i l l make t h e CARS measurement system r a p i d i n data processing and simple i n o p t i c a l parts.

The comparison of t h e measured data w i t h

i

NOMENCLATURE

T

X ( 3 )

X'

X"

X~~

02

Aw

W 1

02

W 3

gas temperature, i n Kelvin

third-order s u s c e p t i b i l i t y

unsymmet r i ca 1 term

symmetrical term

nonresonant term

angle between t h e pump and Stokes beam

separation d i stance between two hemici r c l e beams

pump frequency

Stokes frequency

CARS frequency

.

ii

I NTRODUCT I ON

CARS (Coherent Ant i -Stokes Raman Spectroscopy ) techniques have been widely app l ied i n combustion diagnost ics. o f t e n chosen because o f t h e i r l a r g e concentrat ions a v a i l a b l e as an i n e r t gas. I n combustion gases, t h e concentrat ion o f d iatomic molecules such as 02 and CO may be e a s i l y determined by CARS because of t h e i r simple t h e o r e t i c a l spectrum.

For CARS thermometry, n i t rogen molecules are

Most measurements have been c a r r i e d out by m u l t i p l e x CARS where t h e broad- band o f ~2 i s employed. CARS has many advantages i n i t s appl icat ion. It has exce l len t d i s c r i m i n a t i o n against luminous or f luorescence backgrounds. I n contrast , the present author has found a few drawbacks dur ing a ser ies o f prac- t i c a l appl icat ions. By use o f one o p t i c a l mult ichannel analyzer, i t takes appreciable t ime t o record t h e i n t e n s i t y i n mult ichannels and then compare t h e measured data w i t h theory. As t h e r e p e t i t i o n r a t e o f t h e l a s e r increases, i t w i l l be unable t o f u r n i s h t h e instantaneous t ime-vary ing temperature and con- c e n t r a t i o n on a rea l - t ime basis. spec t ra l p r o f i l e s may need a photodetector and analyzer w i t h many channels and a monochromator w i t h s i g n i f i c a n t l y h igh resolut ion. Such devices are q u i t e expen- sive. dures, which r e s u l t s i n rea l - t ime determinations of temperature and concentrat ion. Various o p t i c a l techniques are a l s o introduced. The com- b ina t ions o f t h e new spectroscopic concept and these new o p t i c a l techniques w i l l make t h e CARS system f a s t i n data processing and simple i n op t ics , w i t h a remarkable decrease i n o p t i c a l and e l e c t r o n i c instruments costs.

The a c q u i s i t i o n and storage of broadband

This paper proposes a new method t o s i m p l i f y t h e spectroscopic proce-

R A P I D TEMPERATURE DETERMINATION BY NITROGEN CARS

The CARS i n t e n s i t y depends on t h e squared t h i r d - o r d e r s u s c e p t i b i l i t y o f molecules i n t h e gas, which can be w r i t t e n as

Ni t rogen molecules are o f t e n chosen f o r CARS temperature measurements because of t h e i r l a r g e concentrat ions i n hot and chemical r e a c t i n g flows. For such l a r g e

t h e resonant symmetric term I xll Y i s dominant and other terms and 4 XI XNR are n e g l i g i b l e . The spect ra l p r o f i l e s o f

depends only on temperature through t h e Maxwell-Boltzman e q u i l i b r i u m d i s t r i b u t i o n f o r t h e Q-branch spectrum. The comparison o f experimental p r o f i l e s w i t h theory gives in fo rmat ion on temperature. sweep t h e vol tages o f i n t e n s i t y i n t h e mult ichannels and then compare t h e measured data w i t h theory i n the whole Q-branch range. Because o f t h e amount o f t ime necessary t o do t h i s comparison, rea l - t ime measurements are not possible.

However, i t takes some t ime t o

Instead o f us ing t h e whole Q-branch spectrum, t h e r a t i o o f p a r t i a l i n t e g r a l s suc s hot /co ld bands, warm/cold bands and t h e i r combinations have been repor ted '1 17 . However, t h e i n t e g r a t i o n s were performed numer ica l ly on t h e

measured data a f t e r t he o p t i c a l mult ichannel processes, which s t i l l d i d not g ive t h e rea l - t ime data reduct ion. one p o s s i b i l i t y f o r speeding up t h e process but w i t h ra the r l a rge errors?2{. As another a l t e rna t i ve , a new spectroscopic concept i s proposed herein.

The peak r a t i o of ho t /co ld bands might p r v de

The basic idea s t a r t s by s p l i t t i n g the CARS s igna l us ing spectrometer e x i t s l i t s #1 and #2, as shown i n f i g u r e 1. i n t e n s i t y r a t i o s o f s l i t #1 t o #2 could be o p t i c a l l y obtained.

By changing the two s l i t widths, var ious

A propane-air mixed f l a t flame burner was employed i n the experiment. use o f the burner, a n i t rogen spectrum a t h igh temperature was obtained by scanning t h e monochromator. average o f over 100 pulses. value o f t he s l i t f unc t i on i n t h e computer code, as i nd i ca ted i n f i g u r e 2. The value obtained from t h e best f it was 5.5 cm-l. peratures w i t h t h i s s l i t f unc t i on were convoluted i n f i g u r e 3, where the widths o f s l i t s #1 and #2 are a l so i l l u s t r a t e d along the ho r i zon ta l axis. The in ten - s i t y r a t i o f o r s l i t #2 t o s l i t #1 was ca l cu la ted i n the temperature range from 300K t o 2500K and p l o t t e d i n f i g u r e 4 and i s denoted by CASE(1). The width o f s l i t #1 was se lected t o g ive t h e curve s e n s i t i v i t y t o t h e temperature change. To see the e f f e c t o f s l i t width, the w id th o f s l i t #1 was s l i g h t l y widened. r e s u l t i s drawn i n f i g u r e 4 as CASE(2).

By

The r e s u l t s are shown i n f i g u r e 2. The best f i t curve was obtained by ad jus t i ng the

We made an

The spectra a t three tem-

The

To compare the present method w i t h the convent ional whole-spectrum method,

The 700mJ/pulse Nd:YAG l a s e r w i t h 20Hz r e p e t i t i o n simultaneous measurements were performed. s i s t e d o f two o p t i c a l paths. ra te , homemade dye o s c i l l a t o r , f l a t flame burner and computer were common t o both paths. mi r ro r . One o f the CARS s igna ls was passed through the monochromator w i t h a h a l f m i r r o r inser ted , detected by a p h o t o m u l t i p l i e r fo l lowed by a sample-hold c i r c u i t and then fed t o the computer. Th is i s t he path f o r the new concept. The other s ignal was processed by t h e monochromator, s i l i c o n - i n t e n s i f i e d ar ray de tec tor and o p t i c a l mult ichannel analyzer. a standard CARS measurement. It i s f u r t h e r noted t h a t t h e monochromator asso- c i a t e d w i t h the two pho tomu l t i p l i e rs had much less r e s o l u t i o n (0.03nm, FWHM), compared t o t h a t (0.007nm) o f t he monochromator used i n the convent ional arrangement.

The t e s t setup shown i n f i g u r e 5 con-

The CARS s ignal was generated w i t h i n t h e burner and s p l i t by a h a l f

The l a t t e r descr ibes t h e l i neup f o r

Extensive and r igorous comparisons were made fo r t he measured temperatures by the convent ional CARS, and by t h e two-photomul t ip l ie r method. The r e s u l t s a re summarized i n f i g u r e 6. The s t r a i g h t l i n e r e l a t i o n s h i p va l i da tes t h e new method. of CASE(1) and CASE(2). Note t h a t a l l data were obtained from t h e average over 100 pulses.

It was not s e n s i t i v e t o the change o f s l i t widths w i t h i n t h e data range

R A P I D CONCENTRATION DETERMINATION R Y OXYGEN CARS

I n a combustion gas, oxygen i s sometimes detected i n low concen r a t i o n s because i t i s react ive. are neglected and the s u s c e p t i b i l i t y can be w r i t t e n as,

I n such a s i t u a t i o n , t h e two terms o f I XI and( x'l 2

I 2

Equation (2 ) i n d i c a t m a s i g n i f i c a n t domination o f the nonresonant term which imp l i es a d i f f i c u l t y i n de tec t ing low concentrat ions. coming t h i s d i f f i c u l t y i s t o use a p o l a r i z a t i o n technique f o r r e j e c t i n g XNR. However, t he disadvantages i n us ing po la r i zed CARS are twofo ld . One i s associated w i t h the considerable a t tenuat ion o f s igna ls under the cond i t ions o f extremely low s igna l - to -no ise ra t i o . i s f u r t h e r complicated by the add i t i on o f expensive p o l a r i z a t i o n devices. Instead o f r e j e c t i n g the nonresonant term, the i n t e n s i t y r a t i o method was again empl oyed.

XNR, One method o f over-

The other i s t ha t t he measurement system

F igure 7 shows a t y p i c a l computed 02 spectrum of the Q-branch a t a con- cen t ra t i on o f 3% i n volume f o r a temperature of 1500K. of 5.5cm-1 as t h a t obtained i n the t e m p e r l y r e experiment gas used w i t h a nonresonant s u s c e p t i b i l i t y o f 10.97 X 10’ X (273.15/T)Cm /erg. The suscep- t i b i l i t y g iven by equation (2) depends on both temperature and concentrat ion. We f i r s t ca l cu la ted the r a t i o o f numerical i n t e g r a l s #2/#1 and next those #3/#1 a t var ious temperatures and concentrat ions. f i g u r e s 8 and 9. Therefore, t h e i n t e g r a l r a t i o shown i n f i g u r e 8 can be considered the hot/nonresonant r a t i o , which i s ra the r i n s e n s i t i v e t o the change of temperature beyond 1200K but s t i l l s e n s i t i v e t o the concentrat ion va r ia t i on . hand, the r a t i o o f #3 t o #1 i n f i g u r e 9 i s the warm/resonant r a t i o and i s use fu l f o r concent r a t i on determi na t i ons bel ow 1200K.

The same s l i t f unc t i on

The r e s u l t s a re p l o t t e d i n The i n t e g r a l over #1 represents t h e magnitude o f WR.

On t h e o ther

A small e l e c t r i c furnace was used t o compare t h e measured and ca l cu la ted r a t i o s and i s i l l u s t r a t e d i n f i g u r e 10. t h e CARS temperature measurement w i t h i n the furnace and determined t h e r e l a - t i o n s h i p between the gas temperature and the w a l l thermocouple p r i o r t o a l l t es ts . The u n i f o r m i t y o f gas temperature w i t h i n the furnace space was a l so ascertained. Ne, was in t roduced i n t o the furnace and heated up t o the predetermined temperature. A t an 02 concentrat ion o f 8%, the data were obtained i n t h e temperature range o f 300K up t o 900K a t lOOK steps. A t 4%, th ree data p o i n t s a t 300K, 600K and 900K were co l lected. Data o f 1% concentrat ion were obtained only near room tem- pera ture (300K) and the maximum temperature o f 900K. The measured data are p l o t t e d i n f i g u r e 9. average. The agreement w i t h theory was reasonable. v ide measured data beyond 900K due t o the furnace c a p a b i l i t y .

Using pure n i t rogen gas, we f i r s t made

I n t h e oxygen de tec t ion tes ts , t he 02 gas, d i l u t e d by

A l l data were obtained from t h e 100 pulse ensemble It was not poss ib le t o pro-

It i s fu r the r noted t h a t t h e present method o f determining temperature and concentrat ion assumes an e q u i l i b r i u m s t a t e between the r o t a t i o n a l and v ib ra - t i o n a l degrees o f freedom i n the gas t o be measured. bu len t flames would be the next step i nc lud ing a r igorous eva lua t ion o f measurement uncer ta in ty .

The a p p l i c a t i o n t o t u r -

3

OPTICAL ALIGNMENT TECHNIQUES

MAC A1 i gnment

c o l l i n e a r method i s popular and easy t o i m

takes some t ime t o burner. USED CARS([! may be a usefu l method f o r ob ta in ing h igh reso lu t i on but not i n general use because i t requ i res t h e unstable resonant c a v i t y a t t h e l a s e r source t o produce a doughnut-shaped beam.

Various methods have been proposed t o accomplish CARS phase matching. The ment but not adequate fo r ob ta in ing

h igh s p a t i a l reso lu t ion . Although BOXCARS M provides f o r h igh reso lu t ion , i t a l i z e t h e prec ise alignment i n a p r a c t i c a l large-scale

As another method, MAC (Minimum Angle Crossed-beam) CARS i s proposed i n f i g u r e 11. The hemic i rc les of 01 and y , which were made by k n i f e edge inse r t i on , were focused and overlapped only i n a small area. present method would be a k i n d o f misalignment. mari ly made i n the h igh i n t e n s i t y beam center t o avo id the rap id decrease of s ignal l e v e l s due t o mismatch.

I n p r i n c i p l e , t h e However, the over lap was p r i -

A small Bunsen burner was used t o determine t h e r e s o l u t i o n c h a r a c t e r i s t i c s o f the MAC method. was t o make t h e beams tangent t o the burner per imeter and the o ther t o do i t normally. The measured temperatures were compared i n f i g u r e 12 for the d i f - f e ren t beam separat ions i n both tangen t ia l and normal modes. t i o n o f 0.2mm i n the tangen t ia l mode and D.6mm i n the normal mode gave almost t h e same temperature p r o f i l e s along the burner center- to-outside. The small separat ion o f 0.3mm revealed lower temperatures i n the h igh gradient region, which r e f l e c t e d t h e poor s p a t i a l r e s o l u t i o n and t h a t l a r g e po r t i ons o f t h e s igna l might be from the co lder spots. r e s u l t much in f luenced by the co lde r gas.

There were two ways t o determine the temperature f i e l d . One

The beam separa-

Zero beam separat ion gave an inaccurate

New Beam S p l i t t i n g Method

I n t h e CARS measurement, a s p l i t o f the q beam i s sometimes required. It i s proposed t h a t a pr ism be used t o separate the beam i n t o two hemic i rc les, as i l l u s t r a t e d i n f i g u r e 13. appeared as a hemic i rc le as i t was generated i n the dye c e l l w i t h t h e hemic i r c le pump beam. c i r c l e s o f o and ~ 2 . One more fea ture i s t h a t t he t ransverse movement o f t he pr ism w i l l change the beam s p l i t r a t i o . I n o ther words, t he i n t e n s i t y r a t i o o f 02 t o WT can be var ied a r b i t r a r i l y t o ob ta in the optimum s ignal , which would be p a r t i c u l a r l y advantageous under adverse environments.

It i s o f i n t e r e s t t o note t h a t t he l a s e r beam ~2

This f a c t i s advantageous f o r the MAC alignment us ing t h e hemi-

- High E f f i c i e n c y Dye O s c i l l a t o r

It i s important t o have a dye o s c i l l a t o r w i t h a h igh conversion e f f i c i e n c y i n t h e s i m p l i f i e d CARS measurement system, otherwise excess l a s e r power as we l l as dye a m p l i f i e r would be required. wavelength i s another important fac to r , i n broadband operat ion, t o avoid a reference c e l l f o r i n s t a b i l i t y compensation.

The s t a b i l i t y of the l a s e r i n t e n s i t y vs.

4

Figure 14 i l l u s t r a t e s the geometrical re la t i onsh ips fo r t he dye lase r emission. Our experience has shown t h a t t he angle 02 between t h e pump and Stokes beams has an e f f e c t on the i n t e n s i t y and center-frequency s h i f t f o r Rhodamine 610, shown i n f i gu re 15. a maximum of 45%. The resonant distance L2 a l so was an e f f e c t i v e parameter, as shown i n f i g u r e 16. Resides the geometrical parameters, t he concentrat ions of dye s o l u t i o n and of N40H, an a d d i t i v e t o the so lu t ion , have some e f fec ts on the dye l a s i n g c h a r a c t e r i s t i c s ( f i gu res 17 and 18). i n f i g u r e 19 fo r Rhodamine 640. poss ib le t o ob ta in data i n more than lnm range a t one time. A m u l t i p l e x CARS genera l ly uses l ess than 2nm o f the Stokes-beam range. o s c i l l a t o r was stable, f l a t , and h igh l y e f f i c i e n t over t h i s range.

Note t h a t the conversion e f f i c i ency reached

9 The output p r o f i l e i s shown

Due t o the g r a t i n g d ispers ion, i t was not

Our homemade dye

SIMPLIFIED CARS MEASUREMENT SYSTEM

I n f i g u r e 20, a s i m p l i f i e d N CARS temperature measurement system i s i l l u s t r a t e d . output a t p o s i t i o n B was almost a hemic i rc le beam whose upper p a r t was cu t o f f by a k n i f e edge i n f r o n t of p o s i t i o n C.

l a s e r beams a t p o s i t i o n F were m u l t i r e f l e c t e d on the two opposi te s ides o f t he d i c h r o i c m i r ro r , which had 80% transmission a t 532nm and 97% r e f l e c t i o n a t 473nrn. The i n t e r e s t i n g feature of the present MAC va r ian t i s t h a t t h e s p a t i a l r e s o l u t i o n may be adjusted and changed by s l i d i n g t h e k n i f e edge p o s i t i o n i n f r o n t o f p o s i t i o n F. I f the lower p a r t i s more cu t o f f , the r e s o l u t i o n becomes f i n e bu t w i t h a lower s ignal - to-noise r a t i o .

The l a s e r beam a t 5 5 2nm was s p l i t by the prism.

M 3 C method and the s ignal was generated i n the lower p a r t o f p o s i t i o n E. The

The dye l a s e r

A t p o s i t i o n D, t he over lap o f w 1 and was done i n the lower p a r t o f the hemic i rc le . This i s a v a r i a t i o n o f the

I n t h e case of de tec t ing oxygen, we need another set o f o p t i c a l p a r t s t o ob ta in temperature and concentrat ion a t t he same time, and t h e focal and c o l l e c - t i v e lens can be i n common w i t h the temperature measurement system.

CONCLUDING REMARKS

A spectroscopic concept f o r the r a p i d determinat ion of temperature and oxy- gen concentrat ion has been discussed. t h e Q-branch spectrum was employed and the ca lcu la ted r e s u l t s were compared w i t h t h e measured data from a f l a t flame burner and an e l e c t r i c furnace.

The r a t i o of two i n t e n s i t y i n t e g r a l s i n

A new geometrical alignment, dubbed MAC CARS, was in t roduced w i t h exper i - mental data from a Bunsen burner. The two hemic i rc les of w 1 and w2 were overlapped only i n the small por t ion. s p a t i a l resolut ion. With a small beam separation, t h e l a r g e r con t r i bu t i ons t o t h e s igna l were from the co lder spots due t o l a r g e r a x i a l ex ten ts o f t he sampling volume i n the h igh temperature gradient.

Large beam separat ion gives r i s e t o f i ne

The use o f a pr ism i n separat ing the pump beam might decrease t h e number o f lenses and m i r r o r s necessary f o r the CARS o p t i c a l alignment and prov ide t h e ade- quate character i n the beam shape f o r t he MAC method.

5

An example has been designed and proposed f o r the o p t i c a l alignment p a r t o f t he s i m p l i f i e d CARS measurement system, where t h e MAC va r ian t was used. se ts o f such a system w i l l be requi red t o make t h e simultaneous measurement o f temperature and oxygen concentrat ion a t t he same p o i n t i n space, w i t h the common foca l and c o l l e c t i v e lens.

Two

It i s f u r t h e r necessary t h a t when such systems are appl ied t o t u r b u l e n t flames, the measurement e r ro rs w i l l have t o be examined ca re fu l l y .

6

ACKNOWLEOGEMENTS

A p a r t o f t,,,: work was w n e wh i le the author he ld a Nat ional Research Council -NASA Langley Research Associateship. Dr. Reginald 3. Exton was the research advisor. l lsefu l d iscussions and encouragements by Dr. Richard R. A n t c l i f f and Mr . O l i n J a r r e t t and other i n d i v i d u a l s a t t he Langley Research Center are acknowledged. The assistance o f Or. Burton Northam i n he lp ing t o arrange t h e NRC res ident associateship i s acknowledged.

Most o f the experiments c i t e d were performed by Mitsuo Gomi a t t h e Nat ional Aerospace Laboratory, Tokyo.

7

I REFERENCES

1. Eckbreth, A. C.; Dobhs, G. M.; and Shufflebeam, 3. H.: "CARS Temperature and Species Measurements i n Augmented Engine Exhausts," AIAA/ASME/SAE 19th J o i n t Propuls ion Conf., Paper 83-1294, June 1983.

2. F u j i i , S.; Gomi, M.; and Jin, Y.: "Instantaneous CARS Thermometry i n Turbulent Flames," Comb. Flame, 48, 1983, pp. 232-240.

3. Eckbreth, A. C.: "BOXCARS: Crossed-Ream Phase-Matched CARS Generation i n Gases," Appl. Phys. Lett. , 32-7, A p r i l 1978, pp. 421-423.

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Fig . 12 Measured temperatures i n Bunsen burner

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Fig. 20 Optical conf igurat ion f o r proposed s i m p l i f i e d CARS measurement system - -

I 28

Standard Bibliographic Page

I . Report No.

4. Title and Subtitle

2. Government Accession No. NASA TM- 100498

A SIMPLIFIED CARS MEASUREMENT SYSTEM FOR RAPID DETERMINATION OF TEMPERATURE AND OXYGEN CONCENTRATION

3. Recipient’r Catalog No.

5. Report Date August 1987

6. Performing Organization Code

16. Abstract A new spec t roscopic concept f o r t h e rap id de te rmina t ion of temperature and

The r a t i o of two s p e c t r a l reg ions i n t h e broadband Q-branch oxygen concent ra t ion by CARS (Coherent Anti-Stokes Raman Spectroscopy) w a s descr ibed . spectrum w a s de t ec t ed by pho tomul t ip l i e r s i n a monochromator, which r a t i o depends on temperature and s p e c i e s concent ra t ion . measured d a t a wi th theory was made us ing a f l a t f lame burner and an e lec t r ic furnace , wi th reasonable r e s u l t s . Various o p t i c a l techniques f o r alignment w e r e introduced inc luding a h igh ly e f f i c i e n t , s t a b l e dye o s c i l l a t o r . The combination of t h e spec t roscopic concept and t h e o p t i c a l t echniques w i l l make t h e CARS measurement system rap id i n da t a processing and s imple i n o p t i c a l p a r t s .

The comparison of t h e

7. Author(s)

Shoichi Fuj ii -

3. Performing Organization Name and Address

National Aeronaut ics and Space Adminis t ra t ion Langley Research Center Hampton, VA 23665-5225

12. Sponsoring Agency Name and Address

Nat iona l Aeronautics and Space Adminis t ra t ion Washington, DC 20546-0001

’

-- I

7. Key Words (Suggested by Authors(8))

Laser d i a g n o s t i c s CARS

8. Performing Organization Report No.

10. Work Unit No. 505-62-81-03

11. Contract or Grant No.

13. Type of €&port and Period Covered Technical Memorandum

14. Sponsoring Agency Code

18. Distribution Statement

Unclas s i f i ed - Unlimited

19. Security Classif.(of this report) 20. Security Classif.(of this page) 21. No. of Pages 31 Unc las s i f i ed Unclass i f ied

Subjec t Category 72

22. Price A03