Two-Dimensional Gas-Phase Temperature Measurements Using Fluorescence Lifetime Imaging

TU QIANG NI and LYNN A. MELTON* Department of Chemistry, University of Texa~ at Dallas, Richardson, Texas 75083

Two-dimensional (2D) temperature images of turbulent hot nitro- gen flows have been obtained by use of a novel technique involving fluorescence lifetime imaging. Nitrogen flows were doped with naph- thalene vapor. The fluorescence lifetime of naphthalene decreases from 180 ns at 25 °C to 35 ns at 450 °C, and this temperature dependence can be used to calculate the temperature from the flu- orescence lifetime in oxygen-free environments. With the current apparatus and with naphthalene as the fluorescent dopant, the error in the absolute temperature was estimated to be • 15 °C in the range of 25 to 450 °C. This application demonstrates that fluorescence lifetime imaging, which does not depend on the absolute concentra- tion on the fluorescent dopant, may be useful in a variety of tur- bulence, temperature, and other combustion-related studies.

Index Headings: Fluorescence; Lifetimes; Temperature; Imaging; Naphthalene.

I N T R O D U C T I O N

Temperature is one of the most important parameters in combust ion flows. Several laser diagnostic techniques have been developed for instantaneous and noninstrusive temperature measurements. These techniques include co- herent anti-Stokes Raman spectroscopy (CARS), laser- induced fluorescence (LIF) and Rayleigh scattering? -5 Because of its high sensitivity, LIF is particular attractive for temperature measurements. Indeed, two-dimensional (2D) temperature images have been obtained with a sin- gle laser shot. 6 In general, two excitation sources with different wavelengths are required to measure the relative population in two vibrational or rotational states in order to compute temperature. In this paper, we report another approach, based on fluorescence lifetime information, for determining temperature. This technique requires only one laser excitation frequency, and it can be applied to gas-phase and liquid systems.

The basis for the technique is the temperature depen- dence of fluorescence lifetimes. For an excited state, the deactivation process may involve both radiative and non- radiative pathways. The fluorescence lifetime, T, is deter- mined by the sum of all deactivation rates:

,1.-1 = kr -I- kor (1)

where kr and k,r are the radiative decay rate and nonra- diative decay rate, respectively. The nonradiative path- ways include predissociation, internal conversion, and in- tersystem crossing, and the rate constants for these pro- cesses are, in general, 7-~2 temperature-dependent.

Lifet ime measurements have been used for imaging purposes and for temperature measurements, but, to our knowledge, this work is the first to combine the two ob-

Received 22 September 1994; accepted 16 April 1996. * Author to whom correspondence should be sent.

jectives. Fluorescence lifetime imaging, using phase- modulated instrumentation, has been the subject of a re- cent review. 13 The temperature dependence of the fluo- rescence lifetime of a probe has been used to measure the temperature of living cells 14 and of liquid crystals? 5 Optical fiber temperature sensors have been devel- oped, 16-~8 and lifetime measurements have been used to determine the temperature of a piece of solid phosphor. 19

We have developed 1D and 2D fluorescence lifetime imaging systems and have used them to obtain oxygen diffusion patterns in liquids 2° and fuel/oxygen equiva- lence ratio images for turbulence methane jets. 21 I f the fluorescence lifetime imaging technique is applied to an oxygen-free system, in which temperature is the only fac- tor affecting fluorescence lifetime of a fluorescent dopant, noninstrusive temperature images can be captured in times which are short in comparison to flow times. In this paper, we describe the first use of fluorescence lifetime imaging to obtain 2D temperature images of gas-phase flows. In these experiments, nitrogen flows were seeded with naphthalene vapor as a fluorescent dopant, and tem- peratures ranging f rom 25 to 450 °C were determined.

EXPERIMENTAL

Basic Technique. The fluorescence lifetime imaging technique is a modification of Ashworth 's rapid lifetime determination (RLD) method? 2,23 I f the fluorescence is known to have a single exponential decay, the lifetime can be determined f rom two integrated intensity mea- surements (Fig. 1). With this scheme, it is possible to conduct fluorescence lifetime imaging by using dual 2D gatable detectors. To accomplish this, one makes sure that the first detector is gated on immediately after excitation for a period of ~t to accumulate fluorescence intensity Dl, while the second detector is delayed by At to accu- mulate fluorescence intensity D 2. The fluorescence life- time, thus, can be computed on a pixel-by-pixel basis f rom a simple equation:

At r - . (2)

ln(Di/D2)

Numerical analysis of Ashworth 's methods has been performed by Ballew and Demas, 23 and their work indi- cates that the rapid lifetime determination method is nu- merically stable for the range of 0.2 < ~r/At < 5. Although the analysis was performed for the case of 8t = At, in which the signal is maximal ly utilized, good precision in lifetime determination for systems where 8t is signifi- cantly shorter than At can still be obtained as long as the signal intensity is strong enough for accurate measure- ment of D 1 and D 2. Naphthalene, which serves as fluo- rescence dopant in this work, fluoresces strongly; the gas-

1112 Volume 50, Number 9, 1996 0003-7028/96/5009-111252.00/0 APPLIED SPECTROSCOPY © 1996 Society for Applied Spectroscopy

I(t)

Time FIG. 1. Ashworth's rapid lifetime determination method.

phase fluorescence quantum yield is estimated to be 0.38. 24 Indeed, a short detector integration time, gt, can help to avoid saturation of the image intensifiers.

I m a g i n g Sys tem. The cylindrical nitrogen/nitrogen co- flows under study were confined in a quartz tube (i.d. = 1.34 in.) to prevent interference by oxygen quenching (Fig. 2). The coaxial central flow (i.d. = 0.31 in.) was preheated with a firerod. The temperature at the exit of the central flow tube was monitored with a thermocouple (Omega, Model 199, type J). Both the central flow and the outer flow were seeded with naphthalene vapor by passing room-temperature nitrogen through drying tubes which contained naphthalene crystals. The estimated naphthalene partial pressure was about 0.2 TorrY The flow rates of the central and outer flows were monitored by flow meters (Matheson, Model 603 or 605).

The 2D fluorescence lifetime imaging system (Fig. 3) is similar to that used previously. 2j The fourth harmonic of a Q-switched Nd: YAG laser [Quantel 671C, 266 nm, 8-ns full width at ha l f -maximum (FWHM)] was used for the excitation. A small portion of the laser beam was directed to a fast photodiode (Motorola, MD510) to gen- erate trigger pulses for synchronization. The main laser beam (4 mJ/pulse) was expanded into a planar sheet ( -200-1xm thick × 25-cm tall) and passed through the nitrogen/nitrogen coflow transverse to the flow axis. At right angles and in opposite directions, the fluorescence f rom the naphthalene seeded flow was imaged onto two separate image intensifiers (Kentech Instruments Ltd., GOI /NS/UV) by quartz lenses (focal length = 50 mm) and a luminum front-surface mirrors. A 305-nm cutoff fil- ter (ESCO, WG305) was placed in front of each inten- sifier to prevent scattered excitation light f rom reaching the detectors. The output f rom the intensifiers was lens- coupled to a cooled charge-coupled device (CCD) camera (Photometrics, Star I) by a 35-mm camera lens (Nikon, f/1.4). Each of the two fluorescence images occupies a half-frame of the CCD image. The CCD readout was dig- itized and transferred to a lab computer for storage and later analysis.

The gate width for both intensifiers was set at 8 ns. The output f rom the photodiode was split to trigger the two pulsers. One of the intensifiers was gated on imme- diately after the excitation pulse; the other was delayed by 113 ns with respect to the first gate by use of a delay

- - Imaging Area

N2

Firerod -~_~. Sleel Wool

N2

Naphthalene Crystal FIG. 2. Schematic diagram of apparatus for vapor-phase temperature imaging of naphthalene-seeded nitrogen coflow.

cable between the photodiode and the second gate pulse generator.

C a l i b r a t i o n P r o c e d u r e . The fluorescence lifetime of naphthalene as a function of temperature was measured with the use of a flow fluorescence cell to minimize the influence of pyrolysis of naphthalene on the fluorescence intensity and lifetime. The fluorescence cell is made of brass. Four slots have been machined to provide for op- tical access. These slots are covered with quartz plates. A nitrogen flow (about l0 cc/s) passes through a drying tube, which contains naphthalene crystals, before enter- ing the fluorescence cell. The partial vapor pressure of naphthalene in the flow cell is estimated to be 0.2 Torr. 25 The gas mixture in the cell can be heated up to 500 °C. The gas temperature at fluorescence probe volume is monitored with a type-J thermocouple. In this experi-

F ow ~ Plane of Incident ~ / _"~" laser sheet

M / ...................... E J ~ " ................................ ~ M

/ ? 7 ..... - ....... ......... / \ / \ i l ~ \ i

Lens ~ ~ Lens it / ~ Prism / \

M i ", / i M " % - - 1 _ _ 1 ...... ....... L L I . - - - - / M

Lens

I °°° I FIG. 3. Block diagram of dual gated imaging system. M, front surface mirror; GI, gated image intensifier.

APPLIED SPECTROSCOPY 1113

C

E m

FIG. 4.

"4, 0.6-

0.4-

260°C

0

-0.2 0 5'0 160 150 2d0 250 300 350 400 450

Time (ns)

Naphthalene fluorescence decay curves at various temperatures.

ment, after being heated to the desired temperature, the naphthalene vapor was excited by a 266-nm laser pulse. The fluorescence was collected at 90 ° , and was focused onto the entrance slit of a monochromator (DIGIKROM 240). A 305-nm cutoff filter was placed in the front of the monochromator to prevent the scattered excitation light from reaching the detector. The fluorescence decay at 320 nm was monitored by a PMT (Hamamatsu R928) wired for fast response, and captured by a digital storage oscilloscope (Tektronics DSA 602).

Naphthalene (scintillation grade, 9 9 + % purity) was used as received. Nitrogen was purchased from Big Three Industries Gas (zero grade, 99.99% pure).

R E S U L T S A N D D I S C U S S I O N

F l u o r e s c e n c e Li fe t ime o f N a p h t h a l e n e as a F u n c t i o n o f T e m p e r a t u r e . The collisionless fluorescence lifetime and the fluorescence lifetimes in the presence of argon have been measured for naphthalene vapor by Beddard et al. 26 Their results suggest that the collisionless fluores- cence lifetime decreases with increasing excitation ener- gy. If the system is buffered with argon gas for vibra- tional deactivation, the fluorescence lifetimes with in- creasing argon pressure, for excitation wavelength rang- ing from 268 to 308 nm, approach 182 ns. In this work, nitrogen gas served as a vibrational deactivator. If the quenching of vibrational energy is diffusion-controlled, the relaxation of vibrational energy should be complete in less than 1 ns under 1 atm nitrogen.

Figure 4 shows that the fluorescence decays of naph- thalene vapor in the presence of 1 atm of nitrogen (266- nm excitation) are single exponential for temperatures ranging from room temperature to 400 °C. Thus the Ash- worth rapid lifetime determination method can be used.

The fluorescence lifetimes of naphthalene vapor, in the presence of 1 atm nitrogen, at various temperatures, are summarized in Table I. The lifetime of naphthalene at room temperature is slightly shorter than the literature value (182 ns) measured in argon gas. Since nitrogen should not quench the fluorescence, the difference may result from trace oxygen trapped in naphthalene crystals. The fluorescence lifetime of naphthalene decreases rap- idly with increasing temperature. This significant change of lifetime in the range from room temperature to 450 °C

T A B L E I. Fluorescence lifetimes of naphthalene vapor in the pres- ence of 1 atm nitrogen.

Exp. no. Temperature (°C) Lifet ime (ns)

1 26.5 162.0 2 37.4 157.8 3 48.3 147.9 4 55.8 141.6 5 77.6 127.3 6 93.9 117.6 7 110.5 113.7 8 138.8 99.6 9 159.9 91.0

10 181.6 84.5 11 204.1 77.9 12 232.0 68.9 13 260.2 62.8 14 288.4 55.9 15 315.6 53.2 16 342.2 46.2 17 370.1 42.6 18 398.6 39.5 19 426.2 34.7

makes it possible to image the gas-phase temperature by using naphthalene vapor as fluorescent dopant.

T e m p e r a t u r e I m a g e s o f N i t rogen Flows. The inten- sifiers used in this work were designed for fast gating. However, their dynamic range is limited. They begin to saturate at intensities corresponding to approximately 700 counts/pixel. We grouped four pixels (2 pixel X 2 pixel) in a "4-pixel" so that a linear dynamic range of 1-2500 was obtained. Because the gain of the intensifiers as well as the collection efficiencies of the lenses and interme- diate optics is unknown, the number of counts cannot be interpreted as the number of signal photons ,emitted from the sample. With this extended dynamic range, the un- certainty in the lifetimes is within 5%, estimated by using the following equation: 23

At f 1 o-~ - (In y)2 f f ~ + D--~ (3)

where Y = D2D1; ~r~ is the standard deviation of lifetime. Each CCD image contains a pair of intensified fluo-

rescence images (a delayed image and an undelayed im- age). To generate one temperature image, we took three sequential CCD images, i.e., (1) an image of combined background and dark current (B1 and B2), (2) a pair of fluorescence images from naphthalene-doped nitrogen flow at room temperature (D] and DD, and (3) a pair of fluorescence images from hot nitrogen flow doped with naphthalene vapor (D~ h and D~). The combined back- ground and dark current image was recorded with the laser beam blocked, and this image was subsequently subtracted from the fluorescence images. The image pairs from room-temperature naphthalene vapor were used to calculate the correction factor, f for each "4-pixel" to correct the combined effects of nonuniform intensifier gain and nonuniform CCD sensitivity. At room temper- ature, Eq. 2 for our system becomes

At ro = (4)

l n ( f ) - At ln(D~ - B~2 ) ~-o \D~ (5)

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300

250

200

150

100

50

0

I I I ! I I Temperature a b o (°c)

FtG. 5. Temperature images of naphthalene-seeded nitrogen coflows. The mean velocity in the outer flow is 7.3 cm/s; the mean velocity of the central flow: (a) 80 cm/s; (b) 170 cm/s; (c) 390 cm/s.

300

250

200

150

100

50

0

I I I l I I Temperature

a b c (°c) FIG. 6. Temperature images of naphthalene-seeded nitrogen coflows. The mean velocity of the central flow is 19 cm/s. The mean velocity of the outer flow: (a) 13 cm/s; (b) 26 cm/s; (e) 42 cm/s.

where % is the fluorescence lifetime of naphthalene vapor in nitrogen at room temperature. The fluorescence life- t ime image of naphthalene in heated nitrogen flow on a "4 -p ixe l " by "4-p ixe l" basis is calculated by the follow- ing equation:

At z = . (6)

- Bi) ln( f ) + ln(D~=~ ~ ~

By comparing the lifetimes with the calibration data shown in Table I, one can obtain a temperature image. Figures 5 and 6 show the temperature images of nitro- gen/nitrogen coflows. Each image consists of 132 "4- pixel" × 36 "4-p ixe l" . The scale factor is 28 "4-pix- el"/in. , which results in a viewing region of 4.7 X 1.3 in. The temperature at the exit of the inner tube, moni- tored with a thermocouple, was maintained at 270 °C. Figure 5 presents temperature images for three different inner flow rates with the mean velocity of outer flow fixed at 7.3 cm/s. At low inner flow rate (Fig. 5a), the temperature image shows a laminar flow structure. The temperature distributions at high inner flow rates show the turbulent mixing of hot and cold flows. Temperature images in Fig. 6 were obtained with a constant inner flow velocity and varying outer flow rates. Note that before inner/outer flow mixing, the temperature of the outer flows, especially near the center region, is higher than room temperature, because the outer flow was heated by the hot inner quartz tube.

The accuracy for temperature imaging depends on the accuracy of fluorescence lifetime data and the tempera- ture sensitivity of fluorescence dopant used. The standard deviation of temperature is related to the temperature sen- sitivity of fluorescence dopant by

~r T = (r~ (7)

where (r, is the standard deviation of fluorescence life- time, and T is the temperature of nitrogen flow. I f the fluorescence lifetime of a dopant decreases rapidly with increasing temperature, the temperature in that range can be imaged with high accuracy. As shown in Table I, the fluorescence lifetime of naphthalene vapor decreases

more rapidly with increasing temperature in the lower temperature region. At 25 °C, IdT/d'rl = 1.3, and at 400 °C, IdT/d'r I = 6.4. Thus, if (r~ is constant over the entire temperature range, then the temperature measurement is less accurate at high temperature. Lifetime imaging is accurate only in a limited lifetime range; 23 however, with the adjustment of the delay time (At) of the gate pulse of the second intensifier with respect to the first intensifier, short lifetimes, which correspond to higher temperature, can be measured more accurately. (In general, shorter de- lay time allows shorter lifetimes to be measured with bet- ter accuracy, with the penalty of increased error in longer lifetimes.) In this work, At was set at 113 ns, which gives the estimated standard deviations of + 1.l ns, and + 10 ns for the measurement of lifetimes of 35 ns and 180 ns, respectively. With the use of Eqs. 3 and 7, we estimated that the standard deviation for temperature imaging is less than + 15 °C in the range f rom 25 to 450 °C.

C O N C L U S I O N

The fluorescence lifetime imaging technique has been adapted to obtain single-shot 2D temperature images for naphthalene-doped nitrogen flows in which the tempera- ture varies f rom 25 to 270 °C. This method is sensitive, nonintrusive, and instantaneous, and it requires only one excitation wavelength. Since fluorescence lifetimes in the liquid phase and solid phases also depend on temperature, this thermometric method can probably be used to image temperature fields in liquids and solids. This technique may have other applications, such as for studying mixing and heat transfer in turbulent flows and for obtaining evaluations of the performance of thermal devices such as heat exchangers.

ACKNOWLEDGMENTS

Support through the Army Research Office (Grant DAAL03-91-G- 0148) and the Texas Higher Education Coordinating Board-Energy Re- search Application Program (Contract 27) is gratefully acknowledged.

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APPLIED SPECTROSCOPY 1115

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