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Fiberoptic Absorption/Fluorescence CombustionDiagnosticsMARK A. KIMBALL-LINNE a , GEORGE KYCHAKOFF a & RONALD K. HANSON aa Department of Mechanical Engineering, Stanford University , Stanford, California , 94305Published online: 27 Apr 2007.

To cite this article: MARK A. KIMBALL-LINNE , GEORGE KYCHAKOFF & RONALD K. HANSON (1986) FiberopticAbsorption/Fluorescence Combustion Diagnostics, Combustion Science and Technology, 50:4-6, 307-322, DOI:10.1080/00102208608923939

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Fiberoptic Absorption/Fluorescence Combustion Diagnostics

MARK A. KIMBALL-LINNE, GEORGE KYCHAKOFF and RONALD K. HANSON Department of Mechanical Engineering, Stanford University, Stanford, California 94305

( R~rewed December 17, 1985; in flnulform Murch 17, 1986)

Abstract-A new application of fiberoptics to spectroscopic measurement of minor species in combustion products is discussed. Fiberoptics could be applied to other optical combustion diagnostics as well. They provide optical access to enclosed combustion systems and allow sensitive equipment to be located remotely. Optical fibers and their use are dicussed in detail, as is probe design. The two most promising spectroscopic techniques for use with fiberoptics are absorption and fluorescence. Examples of both techniques are discussed in detail. We were able to detect 30 ppm of OH (at 310 nm) in a flow reactor using our fluorescence probe. Suggestions for improvement of the technique are given.

1 INTRODUCTION

Fiberoptic sensors capable of monitoring a wide variety of physical quantities have been developed (Giallorenzi et al., 1982). In this paper, we describe the use of fiberoptic techniques for measuring concentrations of minor species in combustion flows. The techniques described could be applied to fundamental laboratory studies, pollution monitoring, control of industrial processes, or the characterization of full scale devices such as gas turbine combustors or furnaces.

Optical waveguides served as input and output conduits for measurements as early as 1963, when Lee and Brodkey measured dye concentrations in water with an absorption probe. Proposals for absorption measurements using fiberoptic probes and incoherent sources were made as early as 1973 (Bowman, et al.). Fibers have been used for spectroscopic measurements by Eckbreth (1979) (collection of CARS data) and by Hirschfield et al. (1983) (laser induced fluorescence for species measurements in liquids). Fiberoptic tunable laser absorption and fluorescence probes have been developed by Kychakoff and Hanson (1981), Kimball-Linne et al. (1982) and Kychakoff et al. (1983). F~beroptics have also been used to make tunable laser absorption measurements in a shock-tube (Hanson et al., 1983, and Louge er ni., 1984).

Although this paper is concerned with the application of fiberoptics to species measurements, fiberoptics can be applied to other optical combustion diagnostics. For example, they have been used to make spectroradiometric temperature measurements in a particle laden combustion flow (Paul, 1984), and a temperature probe based on black body emission of platinum coated on a fiber end has been developed (Dils and Tichenor, 1984).

Laser spectroscopic techniques can be sensitive, species specific and capable of high spatial and temporal resolution. In their common laboratory configurations they are nonintrusive, thereby avoiding hydrodynamic or thermal disturbances. A large number of spectroscopic techniques have been developed for probing flames (see, for example, Bechtel and Chraplyvy, 1982; Eckbreth et al., 1979; and Crosley, 1980) but many of them are not applicable to minor species measurements, generally because the cross-section of the process used is too low. Two processes with the required sensitivity are absorption and fluorescence.

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308 M. A. KIMBALL-LINNE, G . KYCHAKOFF A N D R. K. HANSON

These methods have been demonstrated on clean, open laboratory burners with good optical access. Most combustion systems of practical interest, however, have poor optical access. Addition of windows can significantly disturb flow and temperature profiles, and hence perturb the phenomena under study. Moreover, hostile or dirty environments are not amenable to investigation with sensitive optics, detectors and electronics. Fiberoptics provide optical access without large openings and allow sensitive equipment to be located remotely.

In fluorescence/absorption probes, radiation from one of several possible sources, resonant with a transition of the atom or molecule under investigation, is transported into flame gases via an input fiber. This radiation is transmitted across the gas and is partially absorbed. The excited absorber can then emit radiation as fluorescence. The transmitted or fluorescent light, which contains species I concentration information, is collected and transported to a detection system via an output fiber. The detector signal can then be related to number density.

Fiberoptic probes can be inserted beyond boundary layers to minimize data interpretation problems in non-uniform regions. Beam steering by large refractive index gradients is also reduced. Two optical fibrers facing each other across an open space can be used as path limiting devices for absorption measurements, thus improving spatial resolution. Although water cooled fiber guides are intrusive, the optics collection volume is located some distance from the probe body, so the effect of flow disturbance can be minimized. Multiplexing renders simultaneous measurements at several locations possible. Fiberoptics are not electrical conductors, which may provide an additional benefit in some applications.

2 EXPERIMENTAL CONSIDERATIONS

This section discusses the practical aspects of the development and use of fiberoptic combustion diagnostics: optical fibers and probe design. We emphasize the use of fiberoptics in combustion probes as other topics ( e g , absorption measurement techniques) are discussed in detail elsewhere (see for example Bechtel and Chraplyvy, 1982; Eckbreth er (11.. 1979; and Crosley, 1980).

2.1 Opficol Fibers

Optical fibers are drawn or extruded from various glasses or crystals (depending upon the transmission wavelength of interest) to yield core diameters from 2 to 1000 pm. They guide light because the index of refraction decreases with radius, either suddenly (step index) or gradually (graded index). The change in refractive index is chosen so that a meridional ray entering the fiber end (see Figure 1) at an angle less than about 10" will, after refraction, be within the angle of total internal reflection when i t reaches the edge of the fiber core. That ray will be reflected internally across the core to the opposite wall where it is internally reflected again, and so on; the ray is trapped and guided down the fiber.

Transmission of radiation in waveguides occurs in radial and azimuthal modes (see Snyder and Love, 1983) which decay radially within the cladding. The number of propagating modes in a step index fiber can be estimated by (Suematsu and lga, 1982):

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FIBEROmIC COMBUSTION DIAGNOSTICS

where: M =number of modes k, = propagation constant = 2x/1 1 =wavelength in vacuum a =fiber core radius n, =index of refraction of core n,, = index of refraction of cladding.

Equation (1 ) indicates that larger fibers should sustain more modes.

RRY I S REFLECTED

FIGURE I Coupling ray\ into a fiher

Graded index fibers were developed for the communications industry because they minimize modal dispersion at high signal bandwidths. For the purposes of combustion diagnostics, step index fibers are less expensive and simpler to use, while graded index fibers offer no special advantages. Step index fibers have a uniform refractive index core surrounded by a cladding material with refractive index 1 to 5 percent less than that of the core, covered by a final outer protective jacket, typically of nylon or teflon. Large fibers will sustain many propagating modes, but small core fibers (2 to 10 p n ) , which propagate only one mode, are also available. These single-mode fibers are not subject to modal noise (discussed below) and can be used when coherent or diffraction limited beams are required at the exit. By varying the azimuthal dependence of step index cladding, a single- mode fiber can also be made to propagate only one polarization.

Light transmission down a fiber can be reduced by reflections at the entrance and exit interfaces, by scattering at the core cladding interface, by absorption in the cladding, and by attenuation (scattering plus absorption) in the core (Kapany, 1967). Losses at the faces are due to Fresnel reflection and are relatively small. Cladding losses are a function of the number of reflections experienced by a ray, and are therefore a function of the angle of incident light. Losses in the core are a function of wavelength, as they are primarily due to scattering and molecular absorption.

The principal limit imposed by fiberoptic materials is attenuation in the core, expressed in dB/km { [ - 10 log( P input/P output)lL], P= power, L= fiber length)

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310 M. A . KIMRALL-LINNE, G . KYCHAKOFF A N D R. K. HANSON

for low loss fibers, or percent transmission over a specified length for high loss fibers. Fiberoptic combustion diagnostics require fiber lengths only on the order of meters, so that relatively low transmission fibers can be used. Commercially available fiberoptics generally transmit well in the visible and near IR, with losses under 10 dB/km from 600 nm to 1.2 pm. These fibers are usually made of silica glasses. Attentuation in the IR is due to absorption in molecular vibrational bands (principally O H bands of impurities in the fiber core) or electronic absorption by transition metal or rare earth impurities. Strong OH band heads exist at about 0.95, 1.25 and 1.35 pm, with regions of high transmittance in between. Materials for longer wavelengths are in the research and development stage; fibers suitable for combustion diagnostics in the mid IR are becoming available.

Some effort has been made to produce good UV fibers, but losses are high compared with the 1R fibers of interest for communications. Attenuation in the UV is controlled by Rayleigh scattering, proportional to \/,I4. High purity fused silica is the material of choice; yielding 40 percent transmission at 350 nm (for a 1.5 m fiber) dropping to 20 percent at 290 nm and cutting off at about 200 nm.

Meridional ray analysis for a large step index fiber yields an expression for the numerical aperture (NA) (Kapany, 1967):

NA= n sin o,.= (n,,,' - n,.,2 )I", (2)

where: I I =index of refraction of air O,.=entrance angle, with respect to fiber axis

Rays exit a straight fiber at the same NA. A more accurate skew ray analysis shows that actual NA's will be slightly greater. Rays at entrance angles greater than 13,. will suffer losses at each reflection and hence will not be sustained, although for shorter fibers many of these rays will be transmitted.

Multi-mode fibers have NA's around 0.35 to 0.22 (I3=20" to 13"), corresponding to relatively large light gathering power (monochromators have NA=O.l). Theoretical coupling efficiency is limited by reflection losses which should be around 4 percent per surface. Coupling efficiency for a collimated beam (laser) and for a partially-diffuse source can be improved with focusing, with a lens of focal length (Snyder and Love, 1983):

where: f =focal length r , = lens radius.

Coupling efficiency for a diffuse (Lambertian) source cannot be improved with focusing. When using lasers, irradiances can be high enough to induce damage so that coupling efficiency may be limited by the damage threshold (around 3 GW/ cm' in fused silica fibers). Nonlinear processes such as inverse Raman may be induced as well. These problems are especially evident with pulsed lasers.

Single-mode fibers are used with narrow linewidth sources. Coupling presents greater difficulty, although coupling efficiencies of 85 percent are readily achieved. Single-mode fibers have NAs between 0.2 and 0.1 (12" to 6"). Care must be taken to successfully launch a beam into the fundamental mode (see for example Mostafavi er ul., 1975). Coupling into the fiber at the beam waist, so that the wave

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FIBEROPTIC COMBUSTION DIAGNOSTICS 311

fronts are planar, is most effective. This allows the waist to be slightly off center and/or off axis without sharply decreasing coupling efficiency. The beam waist should be the same size or slightly larger than the fiber core. A smaller waist will allow excitation of higher order modes which are then damped out, delivering less energy to the transmitted mode.

When a fiber is bent, the angle between the core-cladding interface and a propagating ray changes. As a result, previously bound rays become "leaky". They are either refracted at the core-cladding interface or they "tunnel"; tunneling rays are lost by frustrated total internal reflection, resulting from the azimuthal curvature of the fiber. The total power lost to leaky rays is small, even for small bend radii (Was 100, R = bend radius), but the output angle changes. Geometric analysis yields the following expression for this "focal ratio degradation" (Angel el al., 1977):

where: A 0 = change in output angle 8 = input angle R, =final bend radius R, =initial bend radius.

Focal ratio degradation can induce variation of the ouJput signal with fiber movement if fibers are not coupled by appropriate collection and focusing optics. To minimize this effect the full NA of the fiber (the upper limit of 8) should be filled. It may be possible to reduce focal ratio degradation by looping a fiber several times before it enters a probe. Small changes in the bend radius with probe movement may then be negligible.

Slight vibration of a multimode fiber will cause bound radiation to oscillate between spatial modes within the fiber (Wood, 1984; Hjelme and Mickelson, 1983). If a fiberoptic system contains an element that filters spatially, modal noise can introduce signal-to-noise problems. Focusing a fiber into a monochromator is a good example. The difference between fiber and monochromator numerical apertures introduces noise. As a second example, butting a fiber against a detector with responsivity that varies spatially will also introduce modal noise. A mode overlap length can be defined for a step index fiber as (Mickelson and Eriksrud, 1982):

If the source linewidth is much smaller than 61 (i.e., when using a laser), the fiber output is in the speckle regime, and modal noise can be a problem. When the source linewidth exceeds 61, the fiber output is in a continuum regime and modal noise is unimportant.

A large body of work is devoted to modal noise in the speckle regime. An important result is that modal noise is proportional to the inverse of the square root of the total number of modes. For example, Goodman and Rawson (1981) give:

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M. A. KIMBALL-LINNE, G. KYCHAKOFF AND R. K. HANSON

where: S/N = signal-to-noise ratio p =(A,/A,)ln

A, =detector area A, =fiber area.

For the purposes of combustion diagnostics, spatial filtering of radiation fields subject to modal noise should be avoided by appropriate focusing and collection optics. For example, the light exiting a collection fiber may require filtering to reject background interferences. Monochromators can be used for this purpose, but they can exacerbate modal noise. Fabry-Perot etalons and narrow band interference filters have much higher numerical appertures, and therefore reduce modal noise.

In order to couple light into a fiber efficiently, the fiber ends must be optically smooth. Optical surfaces can be achieved by one of two methods: by the score-and- pull technique (for 0 < a 6 200 pm) or by polishing (for 200 < a 9 200 pm). The score-and-pull technique consists of placing the fiber under tension, usually over a curved surface, and lightly scoring it with a commercially available tool (typically of saphire or diamond). A brittle fracture results, with the possibility of three surface quality regimes at the fiber end: a mirror (originating at the score), mist (at the mirror edge), and hackle zones. A successful fracture results if the mist and hackle zones are avoided. An empirical relation for the boundary between the mirror and mist zones is given by Bendow and Mitra (1979):

where: Z =local stress r = mirror/mist boundary k =empirical constant, 6.1 kg/mm3/? for fused silica.

For a good fracture, ZJr< k across the fiber. When the fiber is bent over a curved surface, Z is given by

where: x =distance from score T =average tension in fiber E =Young's modulus R = radius of bend.

Fiber cleavers are commercially available, hut we achieved better results by designing a cleaver based upon these equations, for the fiber diameters we used.

Large fibers are polished on optical polishing wheels. The cladding must first be removed several millimeters from the end. The outer jacket can be removed with wire strippers, and the cladding can be stripped with an appropriate solvent. We used surfonic si-sulfisol (from Surfonic Engineers, Layton, Utah) to strip silica

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FIBEROPTIC COMBUSTION DIAGNOSTICS 313

rubber cladding from fused silica fibers. Commercially available paint stripper is also effective on some fibers. Alternatively, the cladding can be carefully removed by hand and the remnants dissolved with an acid solution. The fiber is then held in a jig for polishing. Stainless steel or fused silica jigs are suggested, as softer materials can become imbedded in the fiber during polishing. Care must be taken to avoid stress concentrations on large fibers during preparation and in use. After polishing, we fixed our fiber ends in brass collets with epoxy.

Several methods for producing a lens on the fiber tip have been developed. In view of the discussion on focal ratio degradation and nodal noise, these techniques could prove useful. One approach is to melt the core with an arc or flame (Benson et al., 1975). A second technique uses UV sensitive photoresist (Bear, 1980). The photoresist is applied to the tip, baked, exposed to W radiation traveling down the fiber, developed, rinsed and baked. The fiber tip could also be drawn into a cone and then melted (Wenke and Zhu, 1983).

Laser radiation can be focused to the diffraction limit, and this can result in damage to a fiber input face, or more likely to the cladding. This problem is especially evident with pulsed lasers. One should perform initial alignment of the fiber under low power and ensure that maximum transmission has been reached before increasing the input power. With untried systems one can calculate the beam size at the fiber face and estimate what average power will damage it. Glass damage thresholds are discussed in the Handbook of Laser Science and Technology (CRC Press) and in the proceedings of the Boulder Damage Symposium (Laser Damage in Optical Materials, NBS special publications, Department of Commerce). Alternatively, one can insert a glass slide (or similarly inexpensive piece) into the beam and increase the power until damage occurs. That gives an upper limit on input power.

2.2 Probe Design

Optical fiber materials will not survive flame temperatures and therefore require cooling if inserted into combustion flows. We have found it useful to house the fiber within a metal tube, with a close fit for good thermal contact. This fiber-guide is oven-brazed onto the inside or outside wall of a tube containing circulating coolant. Hot water from building supplies is usually above the dew point of water in combustion products, which avoids condensation. The resulting fiberoptic probe is not unlike conventional water-cooled gas extraction probes in construction.

Fiber suppliers quote minimum bend radii of 3 mm for 200 ,urn core diameter fibers, increasing to 25 mm for 1000 pm core diameter fibers. We have found that small core fibers (50-100 p n ) can maintain their minimum bend radii for extended periods of time but that large core fibers (400-1000 pm) will fracture if held at quoted radii. We found that 1000 pm fibers will survive indefinitely at bend radii greater than 6 cm.

The fiber should be recessed within the fiberguide. This keeps the tip cleaner and gives some spatial filtering of background radiation. Co-axial gas purging has been used success full^ to keep fiber tips clean in a particle-laden MHD flow (Simons, 1982).

Boundary layers will develop on cooled probe bodies and may affect chemistry in the observation volume, biasing the measurement. In addition, temperature differences can interfere with number density determinations. Care should be taken to design a probe geometry that places the observation volume outside the area affected by flow disturbance (see the examples given below).

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314 M. A. KIMBALL-LINNE, C . KYCHAKOFF A N D R. K. HANSON

3 INTERPRETATION O F RESULTS

Absorption spectroscopy has been used to make Line-of-sight measurements in both major and minor species in flames. Wang (1976) reviewed applications of this technique. Visible and UV laser absorption measurements of OH, for example, have been made by Luck and Thielen (1978), Luck and Tsatsaronis (1979), Teets and Bechtel (1981), Revet et al. (1978), Catollica (1979), Kychakoff and Hanson (1981), Kychakoff et al. (1983), and Rea etal. (1984).

When narrow linewidth ( - 20 MHz) laser radiation passing through a gas is scanned in frequency across an absorption line, the transmitted signal yields an absorption profile. The shape of such a spectral profile is the result of both collisional and Doppler broadening. Analysis of an experimental absorption profile using Beer's law yields an absolute number density measurement (see for example, Wang, 1976).

Piepmier (1972) suggested the use of fluorescence for species measurements in flames. Daily (1977, 1978a, 1978b) examined the use of saturated fluorescence, demonstrated the validity of the rate equation approach and discussed problems such as the nonuniform intensity profile of the saturating beam. Chan and Daily (1980) have carried out OH measurements, for example, using saturated fluorescence. Lucht and Laurendau (1979) have made a careful study of the use of saturated fluorescence and introduced the semi-empirical balanced cross-rate model to ease the difficulty of interpreting the results. Lucht, Sweeney and Laurendau (1980) have also made saturated fluorescence measurements in flames. Cattolica and Schefer (1983) have made fluorescence measurements of O H concentrations in combustion boundary layers.

An atomic two-level model of fluorescence has been developed (Piepmier, 1972) from which the following expression for observed fluorescence power (valid for weak excitation, a spectrally broad laser and broadband detection) can be derived:

where: SF =observed fluorescence power 'I =optics collection efficiency 8 =,solid angle of collection optics y. =volume of region observed hv =photon energy A214A21 + C)) =the Stern-Vollmer ratio @ = collisional quenching rate A Z I =spontaneous emission rate

=stimulated absorption rate coefficient =fraction of molecules in the absorbing level = N,,,/N,, Eq. (71

N,. = iota1 number density

In the pulsed fluorescence experiment described in Section 4.2, a high resolution tunable laser absorption calibration measurement was used to'fold the unknown factors (7, v, 8, Q), the known parameters ( A , I,, A , , , B,,, v ) and the constants in Eq. (9) into a single calibration constant, the aim being to arrive at a simple linear

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relationship between SF and N,. This calibration is a weak function of temperature if one pumps the correct line. The effects of both temperature and concentration changes are discussed by Kimball-Linne (1984).

4 EXAMPLES OF FIBEROPTIC PROBE EXPERIMENTS

4.1 C W Absorption Probe Measurements of OH

The arrangement for an absorption experiment is shown in Figure 2. A commercially available ring dye laser (using Rhodamine 6G dye) pumped by an Ar ion laser (5 W at 514.5 nm) was followed by a laser noise reduction unit. The output was then frequency doubled using an external, temperature-tuned, 90" phase matched ADA crystal. This arrangement produced about 3-25 ,uW of cw W radiation which was repetitively scanned (at 4 Hz) across the R2(3) line in the (0,O) band of the A 2 Z + - X 2 1 1 transition of OH (at 307.7 nm). The laser bandwidth was less than 20 MHz.

The probe used in these experiments consisted of two identical sections (Figure 3a). Each fiber was enclosed in a water-cooled stainless steel jacket terminating in a conduction-cooled copper tip. This structure ended in an aluminum block which

OUTPUT FIBER

I FIBER PROBE

FLAT FLRME NOISE RDR ERTER CRYSTRL

SPECTRUM DETECTOR

RNRLYZER INPUT FIBER

SIGNRL OSCILLOSCOPE RVERRGER

FIGURE 2 Ahsorption fiberoptic prohc experimental arrangement.

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316 M. A. KIMBALL-LINNE. G. KYCHAKOFF AND R. K. HANSON

was supported by an optical mount. The 3-m long input fiber faced the 3-m output fiber across the absorption path length, which could be varied by moving either piece. Combustion products were supplied by a conventional premixed methane/ air, laminar flat-flame burner (FFB).

The effect of the probes in Figure 3a on the flow field was investigated in early experiments using sodium (Kychakoff and Hanson, 1981). Results indicated that flow disturbances were negligible for probe spacings greater than 0.3 cm. The concentration of OH at one centimeter above the burner surface was measured with an absorption length of 4.5 cm, and with variable stoichiometry. Single frequency scans yielded fully resolved absorption lines. The results shown in Figure 4 exhibit the same trends observed by other investigators (Cattolica, 1979; Liick and Tsatsaronia, 1979).

WATER IN- WATER

COOLING COPPER JACKET

T IP $in

jJ WATER l U OUT

OPTICAL F I B E R

ABSORPTION PATH

I+ --. PROBE T I P 5

FIGURE 3a Absorption probe (Kychakoff el a/., 1983)

OPTICAL WATER COOLED - ACCEPTANCE FIBERS BODY CONE FOR

FLUORESCENCE FLUORESCENCE C I C L I A I A8 I T -

IIRROR

FIGURE 3h Fluorrscencc prohe (Kychakoff rr 01.. 1984).

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. : CW ABSORPTION RESULTS \

.:PULSED FLUORESCENCE ' RESULTS

0 . 8 1.0

EQUIVALENCE RATIO $

FIGURE 4 Calibrated fluorescence and high resolution tunable cw laser absorption measurements, as a function of equivalence ratio (Kychakoff er a/., 1983).

4.2 Pulsed Fluorescence Probe

A pulsed fluorescence probe experiment is shown schematically in Figure 5. The commercially available laser system consisted of a pulsed dye laser pumped by an Nd: YAG laser and frequency doubled into the UV by a KDP crystal. The laser was tuned to the P,(8) line of the (0,O) band of the A 2 Z + - X 2 n system of OH (310 nm). The output of the laser (2 mJ/5 ns pulse) was diffusely focused so that roughly 60 percent entered a 3 m long, 1 mm 0.d. input fiber. Higher irradiances damaged the polished fiber end. Approximately 10 percent of the total laser output was transmitted out of the probe tip. Fluorescence induced in the gas adjacent to the tip was collected by a 3 m long, 1 mm 0.d. output fiber of the same type. The signal transported by the output fiber was focused into a monochromator which was tuned to reject laser scattering from surfaces (encountered in the flow reactor described below). The PMT signal was averaged on a boxcar averager; the boxcar aperture was centered on the fluorescence pulse peak with a width of 10 ns, averaging over 100 pulses at 10 Hz.

The fluorescence probe body consisted of a 35 cm long, 0.63 cm (1/4 in.) 0.d. stainless steel water-cooled tube. Two fiber guides (0.32 cm (1/8 in.) thick-walled stainless steel tubing) were brazed to opposite sides and ran the length of the probe, providing a spacing between fiber centers of about 1 cm. A schematic of the probe tip is shown in Figure 3b. The fluorescence induced in the gas adjacent to the tip was collected by a small platinum mirror mounted at 45" to the probe axis. The mirror reflected the collected light into the output fiber. The spatial resolution of this geometry, i.e., the maximum extent of the observation volume, was roughly 4 mm.

In order to account for the effect of long term drift in laser power, the average power was recorded and subsequently divided out of the experimental values for SF.

A general expression for the measured OH concentration is given by:

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where: S,(x, t ) = fluorescence signal as a function of position and time P, = laser power C = calibration factor

1 mrn INPUT FIBER / 1 3

lmrn OUTPUT FIBER U

FLUORESCENCE

MONO- CHROMATOR

:+: P M T n J

DIGITAL

- & I

DELAY LINE

FIGURE 5 Pulsed fluorescence fiheroptic prohe experimental arrangement

The fluorescence probe was tested on a premixed methane/air, laminar flat flame burner (FFB). OH fluorescence was induced by pumping the P,(8) line and a broadband signal was detected. The fluorescence signal was recorded as a function of equivalence ratio at a position 1 cm above the burner surface. These results were then compared with high resolution tunable cw laser absorption results (Section 4.1). The purpose was to investigate the correspondence of fluorescence and absorption measurements over a wide range of composition and temperature, and to establish the range of validity of fluorescence calibration by absorption. Both absorption and fluorescence data are presented in Figure 5. The fluorescence signal was calibrated at @ = 1, and was not corrected for changes in temperature or composition. The LIF results differ from the absorption results by less than + 20 percent. A calculation was performed to estimate the agreement one would expect using the collisional quenching rate data of Bechtel and Teets (1979). Fairchild el ol. (1982), and Morley (1982), together with measured temperatures. The high temperature data of Fairchild et al. (1982) are consistent with these results.

The pulsed fluorescence probe was used in a combustion-driven flow reactor (CDFR) to acquire OH information. This was part of a chemical kinetics investigation of Exxon's Thermal DeNO, process. discussed elsewhere (Kimball-Linne and Hanson, 1985).

Significant interference from laster backscatter within the enclosed reactor was encountered. A spectral scan of the rotational lines of the (0, 0 ) band showed that. fluorescence occured from the entire excited state rotational manifold. This

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FIBEROPTIC COMBUSTION DIAGNOSTICS 319

afforded the opportunity to detect at wavelengths different from that of the pump beam. Backscatter was filtered with a monochromator detuning scheme (Kimball- - Lime, 1984).

Two other approaches to the problem of laser backscatter can be suggested. First, one could detect broadband from a different vibrational transition. The detected band could be far enough from the laser wavelength to obviate the problem of interference. In the current example, this method would have unacceptably reduced the signal-to-noise ratio. Alternatively, one could narrow the laser linewidth and scan across the absorption line. Fully resolved profiles allow discrimination against background interferknce. Our lase; system did not have the capability to scan smoothly in wavelength at the time of these experiments, hence our decision to use monochromator scanning.

A representative curve of O H concentration versus vertical distance within the CDFR is given in Figure 6. Both a calibrated fluorescence measurement and an independent absorption measurement were performed at this flame condition. This affords a comparison of the O H absorption measurement (84 ppm) with a calibrated fluorescence measurement (97 ppm) taken at the same vertical position. The discrepancy between the two is well within the experimental uncertainty (due to backscatter) shown by error bars.

5 10 VERTICAL DlSTANCE (cml

FIGURE 6 Measured OH and temperature profiles in the combustion-driven flow reactor. Error hars are due to hackscatter in CDFR.

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4.3 Detection Limits

The greatest source of uncertainty in the fluorescence experiments discussed above was laser backscatter from the flow reactor walls. That uncertainty imposed a 30 ppm limit of OH detection. The backscatter problem can be reduced by using a larger combustor or one of the methods suggested in the previous section. It is useful, therefore, to consider other potential sources of uncertainty. Here we discuss two other major contributors to noise (modal noise and laser fluctuations) in both the pulsed fluorescence probe and the calibration absorption probe experiments described in Section 4.2. Focal ratio degradation was not a problem in these experiments as we did not move the fibers significantly.

Modal noise was probably a significant source of signal fluctuation in both the fluorescence and the absorption probes. Our 1000 pm, fused silica fibers sustain roughly M = 300 modes [see Eq. (I)] . A signal-to-modal-noise ratio can be estimated using Eq. (6), together with the above value for M. We used geometric optics (with a 1 mm fiber exit angle of 12") to estimate optical volumes. Geometric optics are more appropriate here than the Gaussian beam formulae since radiation occupies numerous spatial modes in the fiber. In the fluorescence probe system we had p= 0.7 (this includes the spatial filtering effect of monochromator slits). This corresponds to SIN (modal)= 17. By averaging over 100 pulses, the signal would improve by a factor of (100)"?, to give SIN (modal)= 170. For the absorption experimental arrangement we estimated p using the fiber numerical apperture to get SIN (modal) = I (for 9 per cent absorption of I,, at line center). Averaging over 50 scans would give SIN (modal)= 7. These values would impose detection limits on the order of 1 ppm for fluorescence and 20 ppm for absorption. We assumed calibration of the fluorescence signal at high concentrations, for best absorption signal quality.

A second major source of noise is fluctuation of the laser output. In the fluorescence experiment, SIN (laser)= 70 and in the absorption experiment, SIN (laser)= 1 (this was uncharacteristically low, the noise perfcraance of our laser has been improved since the experiments described here). In an absorption experiment, one measures a small dip in a large signal, while fluorescence signals are dc levels. These laser noise values combine with modal noise to give SIN (fluorescence) = 17 and SIN (absorption)= 1. After averaging (100 times for fluorescence, 50 times for absorption), SIN (f luorescence)~I70 and SIN (absorption)-7. These values compare fairly well with observed fluctuations: SIN (fluorescence)~80 and SIN (absorption)=K Here, SIN (fluorescence) would be dominated by modal noise but SIN (absorption) would be limited by both laser amplitude noise and modal noise.

5 CONCLUSION

Fiberoptic absorption/fluorescence combustion diagnostics have been demonstrated. These techniques are sensitive and species specific. Optical fibers give access to enclosed combustion systems and allow sensitive equipment to be located remotely. They minimize problems with nonuniform regions in combustion flows and can be used to improve spatial resolution of absorption measurements. We have emphasized species measurements in this paper, but new spectroscopic techniques which lend themselves to fiberoptics are emerging. Examples are the use of absorption and fluorescence to monitor temperature (see, for example, Seitzman el ( I / . , 1985; Hiller and Hanson, 1985). Single mode fibers offer the

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potential for LDV experiments in an enclosed flow passage (e.g., IC engine intake and exhaust) and the temperature measurement schemes mentioned in Section 1 are already proven.

These techniques lend themselves to research reactors burning gaseous, liquid or solid fuels. They could be applied to IC engines to characterize intake and exhaust conditions. The combustion chamber itself presents pressure and sooting problems which must be overcome before it can be probed with fiberoptics. Commercial combustors and furnaces lend themselves immediately to investigation or control with fiberoptics. Fairly simple and reliable systems can be designed using lamps. The combination of fiberoptics and semiconductor lasers may eventually offer the capability to monitor several species concentrations, pressure and temperature.

ACKNOWLEDGEMENT

This work was supported hy the National Science Foundation and the A i r Force Off iceof Scientific Research. The authors would like to thank Richard Booman for technical assistance and Philip Paul for many useful discussions. The present address o f M.A. K - L is Spectra-Physics. 1250 W. Middlefield Road, Mountain View. C A 94039-7013.

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