Laser-induced fluorescence measurement of sodium in flames

COMBUSTION AND FLAME 33, 4 7 - 53 (1978)

47

Laser-Induced Fluorescence Measurement of Sodium in Flames JOHN W. DAILY and CALVIN CHAN

Department of MechanicalEngineering, Universityof California,Berkeley, California94 720

In the present work the use of Saturated Laser-Induced Fluorescence Spectroscopy to measure sodinmatom concentrations in flames is described. A fiashlamp pumped-dye laser is used to excite the sodium, and

the fluorescence signal is observed at 90° forming a focal volume about 27 t~m by 1 ram. A linear curve of growth is obtained in an atmospheric flame for concentration in the range 0.008-0.2 PPM.

I. INTRODUCTION The study of reacting gas flows has traditionally been limited by a lack of experimental data. The measurement of temperatures and species concentrations in complex flames with sufficient spatial and time resolution would be especially desirable. In this paper we discuss the application of LaserInduced Fluorescence Spectroscopy (LIFS) to measurement of species concentrations in flames, reporting on preliminary work done with sodium. The method consists of illuminating the gas with a pulsed laser tuned to an absorption line of the species of interest. The species is excited and fluoresces. The fluorescence is observed at 90 ° to the laser beam, the volume formed by the intersection of the laser beam and the collection optics defining the spatial resolution. If a relationship between the number density of the fluorescing states and the total number density can be found, then the fluorescence is a measure of the total number density. Furthermore, if a measurement can be made with a single pulse, then turbulence measurements are possible. LIFS has been discussed by many workers as a possible method for measuring species concentrations. The large signals obtainable with LIFS have been contrasted, however, with the difficulties posed by quenching. Quenching is the reduction in fluorescence signal caused by the fact that the collisional de-excitation rates for most species are Copyright ©1978 by The Combustion Institute Published by Elsevier North-Holland, Inc.

much greater than radiative decay rates. As a result, the relationships between various energy-level populations is complicated, and the quenching rates must be known. A solution to this dilemma proposed by several workers [1-4] for flames is to saturate the pumped transition, resulting in direct coupling of the two energy levels and thereby greatly simplifying population balances. That saturation occurs and can be used as the basis for a measurement has been verified by Rodrigo and Measures [5] observing potassium fluorescence in a plasma; Omenetto, et al. [3] while exciting indium, thallium, and strontium in flames; and Baronavski andMcDonald [6] while measuring C2 in an oxy-acetylene flame. We have chosen sodium to work with first because of its simple structure and large fluorescence signal. In ongoing work in our laboratory we are investigating the use of LIFS to measure radical species in turbulent flames. II. T H E O R Y Fluorescence Signal

To apply the technique we must develop the relationship between the species concentration and the fluorescence signal. If we ignore the effect of fluorescence trapping, the fluorescence signal for any one atomic transition becomes

IF

=

enhu ~ I2e V~Ari

(1)

O010-2180/78/0033-0047501.25

48

J. W. DAILY and C. CHAN

where h is Planck's constant, v the transition frequency, A the Einstein transition probability, f~e the collection optics solid angle, Ve the collection volume, and N l the excited state number density. e is the detector quantum efficiency and r/ the overall efficiency of the collection optics. The sodium atom may be treated as a three-level system. We excite to level 3. Under saturation conditions there is a fixed relationship between the third level and the first level so that (Daily [4] ):

N8 =g2N1

(2)

gl

The continuity relationship

~vl +~v2 +N3 =~%a

(3)

must also be satisfied. As a result, we have for the total sodium number density:

tions. By using only the leading edge and peak of the fluorescence pulse we find that chemical decay has no effect on the curve of growth.

Dynamic Range The dynamic range of the method will be determined by detectability limit considerations at the low number density end and radiative trapping at high number densities. As discussed by Daily [8], the detectability limit will be determined by either statistical effects or by interferences due to scattering from molecules and particles and incandescence of particles due to laser heating. In the present experiment, the detectability limit is determined by the noise characteristics Of our analog detection system. For linear analog systems the signal-to.noise ratio for a statistical photon source is [9] SNR =

/'31 NNa

=

(1 + g l /ga ) erlhv(A s l / 4 ~r)~2e Ve +

/21 enhv(A2 x/4~r)ac Vc

(4)

AS discussed by Daily [7], these relations do not give a complete picture of the fluorescence signal. Because of the intensity distribution across the beam, Equation (1) must be integrated across the focal volume. In the following, we assume that a significant fraction of the beam is saturated, and thus, the fluorescence signal is insensitive to laser intensity [7].

Chemical Effects The neglect of chemical reactions in the analysis of the fluorescence signal is only an approximation. In fact, chemical reactions are very important in flames and establish the population of any given species through a kinetic balance. Only if the laser pulse is very short compared to the characteristic chemical reaction time may chemistry by ignored. Chemical decay has the effect of chopping the tail end of the fluorescence pulse. In our experiments we have observed decay times ranging from one to several microseconds depending on flame condi-

~/2ar

where ~ is the quantum efficiency of the detector, lp the photon flux, and Af the bandpass. For SNR = 10 and A f = 2 MHz typical of our system, the detectability limit for the excited state becomes 0.002 PPM which is approximately that observed. Photon counting or gated integration over the pulse width would reduce this limit by about a factor of five. Trapping will modify the fluorescence spectrum according to I~F(L ) = e--r(v)Ive(O) where L is the distance from the focal volume to the edge of the flame and

r(v) --

fo L a~ clx -~ a~L

is the optical depth. For sodium in our flame, r(v) ~ 1 corresponds to a number density of about 1 PPM. As discussed below, we see trapping becoming important at N~r a = 0.2 PPM. Thus we observe a linear dynamic range of about two orders of magnitude.

LIFS MEASUREMENT OF Na IN FLAMES

49

LAM FA O OER IN MPE SR UA RBES M NP TTO

CMX-4 DYELASER

P,N HOLE

I

A

\I E "V?F

D,O0E

~ DUALBEAM{

MONOCHROMATOR

I + I o uo co l

Fig. 1 Experimental setup.

III. EXPERIMENTAL APPARATUS Optics The experimental apparatus is illustrated in Fig. 1. The laser, a Chromatix CMX-4 flashlamp pumpeddye laser, is focussed into the flame by a pianoconvex lens. At the focal volume, the beam has a diameter of about 0.4 mm leading to peak intensifies several orders of magnitude greater than that required for saturation. The light reflected off the lens is sent to a pin diode which monitors the beam intensity. The pulse width is about 1 /asec and each pulse has an average energy of about 3.0 mJ. The wavelength of the laser beam is monitored by reflecting the beam into a diffuser placed in front of the inlet slit of a monochromator. For fine tuning the laser is adjusted to maximize the fluorescence signal. The fluorescence signal is observed at 90 ° to the laser beam. F/3.5 optics focus the active volume onto the inlet slit of the monochromator which is used as a broad bandpass filter passing one of the two lines observed to the photomultiplier. The overall bandpass for the monochromator is about 3 A. The collected light is rotated 90 ° by two mirrors so that the image of the active volume lies parallel to the inlet slit. The slit has a height of 5 m m and a width of 135 #m which corresponds to an area of 1 mm by 27/am

in the active volume. (The exact shape and size of the focal volume are not ciritical so long as the apparatus is calibrated.) The signal which corresponds to the voltage across a 1 kl2 resistor placed across the output terminals of the photomultiplier is then displayed on an oscilloscope. The reported results are thus all based on the fluorescence signal from single laser pulses, averaged by eye over several pulses.

Flame Measurements are made in the equilibrium region of a small atmospheric pressure flat-flame burner. The schematic for the burner is shown in Fig. 2. The burner has a diameter of 1.25 cm. The combustion zone, the blue region, is stabilized on top of an array of stainless-steel capilary tubes having 0.147 cm OD and 0.023 cm wall thickness. No inert gas shielding has been used for the present experiment. The sodium is introduced by passing the air supply through a medical aspirator filled with water in which NaCI is dissolved. In order to calibrate the flame conditions, the sodium atom number density and the flame temperature are measured spectroscopically: the number density by absorption and emission, and the temperature by line reversal. The two measurements can effectively be made at the same time. The fuel is methane

J. W. DAILY and C. CHAN

50

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16

"e~

0

H O L E S

F U E L AIR

o

12

l I r

tO I-.. w Z a:

8

m ~E 0

o

6

/

INERT GAS

I

~d

u Z

• 1.27

\o\

14

CAPILLARY TUBES

RATIO

I

+

4

I'

r~

f

/

o

'

f

f o/

AIR

17

8= FUEL

I

I

IB

1.9

\\\\\

\"

Fig. 2 Experimental burner.

mixed with air at an equivalence ratio of 1.27 which provided the most stable flame.

W.

EXPERIMENTAL RESULTS

I

I

2.1

2.2

,

x 103='K

TEMPERATURE

""

.

I

2.0

Fig. 3 Burner temperature distribution.

up to total absorptances of about 0.08. That this departure from linearity is caused by trapping may be seen in Figure 5 which shows the fluorescence distribution across the burner for a variety of sodium number densities. Thus, a linear curve of growth can only be expected below a number

Calibration and Temperature Measurements

2.0 >

In order to calibrate the system, each fluorescence measurement was preceded by an absorption and omission measurement of sodium number density and by a line reversal temperature measurement. Figure 3 shows the reversal temperature as a function of distance above the flame for an equivalence ratio of 1.27. The measurements show the constant-temperature zone above the flame typical in burners of our type. All floureseence measurements were made in this constant temperature zone at a temperature of 2080°K.

_~ +

i.o

o

~,~ 0.5

o

;or °

0,2 0

Figure 4 shows the fluorescence signal as a function of total absorptance. As can be seen, the curve is linear, within experimental uncertainty,

/ /O

o/ @

/a

/ 0.05

/o

./ o

/

/

J

Fluorescence Measurements

o~ / ,(

./ 2

v ,~

o D o

0.02 i

I

I

I

I

I

I

0002

0.005

001

OD2

0.05

OI

0.2

TOTAL ABSORPTION FACTOR

Fig. 4 Total fluorescence versus total absorptance.

51

LIFS MEASUREMENT OF Na IN FLAMES

i

0.7

FLAME ZONE

0.6

J o~

0.5

/S

0 to

0.4

~ 7

03

~ z

0.2

k~ ~ 0

i;:o°,'i:

::::.o=

i

0.1

0.0 -6

×

I

I

I

I

I

I

I

I

-4

-2

0

2

4

6

8

I0

POSITION

OF

LASER

BEAM

WITHIN

FLAME ~ m.m.

Fig. 5 Effect of trapping on the fluorescence signal.

density of about 0.2 PPM which corresponds to a total absorptance below about 0.08. Figure 4 is not a true curve of growth because total absorptance is only linearly related to number density in the thin limit. The actual curve of growth can be expected to curve downward when trapping becomes important. The curve of growth is plotted in Figure 6. For analytic purposes, the calibration curve can be extended to much higher number densities. For combustion studies in which non-uniform or unsteady flames are being probed, however, such an extension is not possible. The error bars shown on Figs. 3 - 7 are for the single sample uncertainties calcualted for our conditions and represent the +95% confidence estimate for each variable not including the statistical noise discussed above. At high values of sodium number densities, uncertainty in flame conditions dominates the uncertainty in the fluorescence signal. This is due primarily to changes in the fuel-air ratio which fluctuates during the experiment due to aspirator instabilities, and to rotameter drift and flow turbulence. In an experiment in which the pulse intensity

was measured by photon counting or gated integration, the number density fluctuations that could be measured would be limited by statistical uncertainty. Except for the lowest number densities this would allow observations of fluctuations greater than less than 1%.

Excited State Energy Transfer One might expect that the aP1/2 and aPs/2 levels of sodium would remain in collisional equilibrium at approximately the translational temperature during pumping. At typical flame temperatures this would mean that the two levels are effectively equally populated to within the ratio of the degeneracies. That this is approximately so may be seen from Fig. 7 which shows the ratio of the fluorescence signals from each of the excited state. There is, however, a systematic difference depending on the state which is pumped. This has been explained by van Vitert, et al. [ 10], by accounting for the large value of the quenching rates for the excited states. Note that the scatter in the value of this ratio is much larger than that for the sum (1 + gx/ga)Ial + 121 which means that in flames for

J. W. DAILY and C. CHAN

52 0.5

,0,

=E

a. a.

000 0 0.2

>109 z

° 0 0

oo

o o

O.I

o

(r 0.05

o

o

°°

o

~E

o~ p o

z ~E o

0.02 o

o o

0 .OI

O3

D

0.005 &

I 0.05

0.02

TOTAL

I

I

I

I

l

O.I

0.2

0.5

1.0

ZO

FLUORESCENCE

SIGNAL

V

Fig. 6 Curve of growth. which conditions might vary both signals really do need to be measured if a linear curve of growth is to be obtained. V. SUMMARY A N D CONCLUSIONS The method of Saturated Laser-Induced Fluorescence Spectroscopy has been used to measure sodium atom concentrations in a flame. The curve of growth, calibrated by conventional absorption m

3.0

and emission spectroscopy, is shown to be linear until fluorescence trapping becomes important. Detectability is limited by statistical noise in the analog detection system. The linear region extends from .008 PPM to .2 PPM at the flame conditions in the experiment. Photon counting or gated integration should reduce the detectability limit by a factor of five. Since each laser pulse results in a satisfactory signal, turbulence quantities can be measured including all stationary statistics and

-

[] o

o

o W U

2.0

°o

o

o 0[3

o ~ i~

bJ

w ~" 0

~ o

o

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o

o o

~

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o []

o o

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o

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,~

10

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u --

EXCITED

AT 5 8 9 0 , ~

u. 0

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EXCITED

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o

0.o

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""

I

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I

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0.1~1

0.002

O005

O.OI

0.02

TOTAL Fig. 7 Ratio

ABSORPTION

of fluorescence

I 0.05

FACTOR from

3P3/2

to 3Pl/2.

1 O.I

I 0.2

~,

LIFS MEASUREMENT O F Na IN FLAMES frequency response up to the repetition rate o f the laser.

REFERENCES 1. Measures, R. M.,Z Appl. Phys. 39, 5232 (1968). 2. Piepmeir, E. H., Spectrochimica Acta 27B, 431 (1972)o 3. Omenetto, N., Hart, L. P., Benetti, P., and Winefordner, J. D., Spectrochimica Acta 28B, 301 (1973). 4. Daily, J. W.,Applied Optics, 16, 568 (1977).

53 5. Rodtigo, A. B. and Measures, R. M.,IEEEZ Quant. Elec. OE-9, 972 (1973). 6. Baronavski, A. P. and McDonald, J. R., £ Chem. Phys. 66, 3300 (1977). 7. Daily, J. W.,Applied Optics 17, 225 (1978). 8. Daily, J. W.,Applied Optics 17, 1610 (1978). 9. RCA Photomultiplier Manual, RCA Technical Series PT-61 (1970). 10. Van Vitert, B.,Van Calcar, IL A., Hollander, T. J., and Alkemade, C. Th. J., Utrecht, private communication (1977).

Received 30 July 1977; revised 23 January 1978