A Novel Technique for Quantitative Temperature Mapping in Liquid by Measuring the Lifetime of Laser Induced Phosphorescence

Transcription

1 The 0th International Symosium on Flow Visualization August 26-29, 2002, Kyoto, Jaan F02 A Novel Technique for Quantitative Temerature Maing in Liquid by Measuring the Lifetime of Laser Induced Phoshorescence Hui HU * and Manoochehr M. KOOCHESFAHANI * * Turbulent Mixing and Unsteady Aerodynamics Laboratory, Deartment of Mechanical Engineering, A22, Research Comlex Engineering, Michigan State University, East Lansing, 48824, Michigan. U.S.A., Fax: , huhui@egr.msu.edu Abstract: A unique method is described in the resent aer for quantitative maing of temerature field in a liquid flow by taking advantage of the hoshorescence lifetime deendence on temerature. This methodology reresents an extension of Molecular Tagging Velocimetry (MTV) technique. As in MTV measurements, the hoshorescence signal is imaged at two successive times within the lifetime of the tracer. The hoshorescence lifetime, estimated from the intensity ratio of the two images at each oint, is calibrated against the temerature. This rocedure eliminates the effects of the variation of the incident illumination. The unique method described in the resent aer also allows the simultaneous qualification of velocity and temerature fields in a fluid flow by using the same hoshorescent dye (-BrN Gβ-CD ROH) and same otical and equiment setu. While the dislacements of the tagged regions between two interrogations rovide the estimate of fluid velocity vectors, the intensity ratio of the two images reresents the fluid temerature distribution. The imlementation and alication of the new technique are demonstrated by conducting simultaneous temerature and velocity measurements in a ulsed cold jet discharging into hot ambient fluid. Keywords: temerature measurement, Molecular Tagging Velocimetry (MTV), simultaneous velocity and temerature measurement, hoshorescence lifetime measurement.. Introduction It is well known that both fluorescence and hoshorescence are molecular hotoluminescence henomena. When a substance is exosed to incident hotons, a fraction of the molecules in the aroriate lower energy level will absorb the incident light and are excited to a higher energy state. Some of the excited molecules may be dissociated. The rest return to the equilibrium ground state by emitting hotons or by transferring the excess of energy through non-radiative decay rocess (collisional quenching, intersystem crossing or internal conversion). The hotons can be sontaneously emitted on short time scales (fluorescence), or on much longer time scales when metastable electronic states are involved (hoshorescence). Comared with fluorescence, which tyically has a lifetime of order nanoseconds, hoshorescence has much longer lifetime and can last as long as microseconds to minutes. Since the emission intensity is a function of the temerature of the molecule for some substances, both fluorescence and hoshorescence of tracer molecules in a fluid flow may be used for temerature measurements by qualification of

2 hotoluminescence intensity. Laser Induced Fluorescence (LIF) technique has been widely used for both qualitative flow visualization and quantitative scalar measurements (temerature or scalar concentration) of fluid flows in recent years (Koochesfahani and Dimotakis, 985; Sakakibara et al. 997). For dilute solutions and low excitation energies, the LIF intensity is linearly roortional to the exciting light intensity, the concentration of the fluorescent dye and the quantum efficiency of the fluorescent molecule. For some dyes, such as Rhodamine B, the quantum efficiency is a function of temerature. Therefore, the fluorescence intensity may be considered, in rincile, to deend only on temerature if the excitation laser intensity is uniform and the fluorescent dye concentration remains constant in the measurement region. However, in ractice it is very difficult, if not imossible, to ensure a non-varying incident illumination light intensity distribution, esecially in flows with index of refraction variations. This may cause significant error in the measurement results. In order to overcome this roblem, intensity ratio based LIF techniques have been roosed recently, which include the two-dye LIF technique (Coeta and Roger, 998 and Sakakibara and Adrian, 999) and the two-emission-band LIF technique (Lavieille et al., 200). In the two-dye LIF technique, two fluorescent dyes, one temerature-sensitive and the other temerature-insensitive, are used. The fluorescence signal from the temeratureinsensitive dye rovides the intensity distribution of the incident illumination light in the measurement region, while the fluorescence signal from the temerature-sensitive dye contains the information of the temerature distribution. If the fluorescence signals from the two fluorescent dyes are detected simultaneously, the intensity ratio of the two fluorescence signals will be only the function of temerature and indeendent of incident illumination. Instead of using two fluorescent dyes, Lavieille et al. (200) demostrated a two-emissionband LIF technique by taking advantage of the temerature sensitivity of some fluorescent dyes (such as Rhodamine B) being deendent considerably on the the wavelegth. The LIF signal of the same fluorescent dye (Rhodamine B) at two different emission bands is recorded searately and simultaneously. LIF signal from one band is very temeraturesensitive, while the other is temerature-insensitive (or less temerature-sensitive). The intensity ratio of the corresonding fluorescence signals will be only a function of temerature and indeendent of the incident illumination. For these intensity-ratio LIF techniques at least two cameras with various otical filters are required for the image recording. Therefore, a very careful image registration or coordinate maing rocedure has to be conducetd during the exeriment in order to get the quantitative satial relation between the two images. In addition, other comlications need to be carefully considered such as the sectral conflicts, hotobleaching behavior of the two dyes, and the weak temerature deendence of the nominally temeratureinsensitive dye for the two-dye LIF technique. Since the LIF signal in a narrow sectrum band is always very weak, issues like signal to noise ratio level and the choices of the image recording system and otical filters need to be carefully considered for the two-emissionband LIF technique. Since long-lived hoshorescence is often quenched raidly by O 2, Laser Induced Phoshorescence has not been used as commonly as LIF for flow diagnostics in liquids until recently. New findings by Nocera and his grou (Ponce et al. 993, Mortellaro and Nocera 996 and Hartman et al. 996) show that suramolecules may be designed to exhibit longlived hoshorescence which is not quenched. A successful design by Ponce et al. (993) indicates that the quenching of a lumohore (-BrN) can be revented by mixing certain alcohols (indicated collectively by ROH ) with an aqueous solution of glucosyl-β-cyclodextrin (Gβ-CD) that contains the lumohore. The resulting long-lived green hoshorescence has a tyical lifetime u to several milliseconds. The availability of long-lived luminescent tracers has lead to the technique of Molecular Tagging Velocimetry (MTV) (Gendrich et al. 997). As the molecular counterart of Particle Image Velocimetry (PIV) technique, MTV utilizes molecules that can be turned into long-lived traces when excited by hotons of aroriate wavelength. Tyically a ulsed laser is used to tag the regions of interest, and those regions are interrogated at two successive times within the lifetime of the tracer. The measured dislacements of the tagged regions rovide the estimates of fluid velocity vectors. By using the -BrN Gβ-CD ROH comlex as the

3 long-lived tracers for MTV measurements in liquids, extensive studies have been conducted to investigate fundamental flow henomena and alied fluid engineering roblems in the ast several years (Koochesfahani, 999, Koochesfahani et al. 2000, Bohl et al. 200 and Lum et al 200). Thomson and Maynes (200) recently demonstrated the ossibility of temerature measurement in a liquid by using Laser Induced Phoshorescence of the -BrN Gβ- CD ROH comlex. The method described by Thomson and Maynes (200), however, is actually the hoshorescence version of the conventional single-dye LIF technique for temerature measurement, in that the emission intensity is used directly to infer the temerature. The satial and temoral variations of the incident light intensity would have to be searately corrected for. These are the same issues that motivated the develoment of intensity ratio based LIF techniques. In the resent aer, a unique method will be described for quantitative maing of temerature field in a liquid by taking advantage of the hoshorescence lifetime deendence on temerature. This methodology reresents an extension of the Molecular Tagging Velocimetry (MTV) technique. As in MTV measurements, the hoshorescence signal is imaged at two successive times within the lifetime of the tracer. The hoshorescence lifetime, estimated from the intensity ratio of the two images at each oint, is calibrated against the temerature. This rocedure eliminates the effects of the variation of the incident illumination. This methodology also allows the simultaneous qualification of velocity and temerature fields in a fluid flow by using the same hoshorescent dye (- BrN Gβ-CD ROH) and the same otical and equiment setu. While the dislacements of the tagged regions between two interrogations rovide the estimate of fluid velocity vectors, the intensity ratio of the two images reresents the fluid temerature distribution. In the sections that follow, the details of the new technique are described along with a demonstration of its alication to conduct simultaneous temetaure and velocity measurement in a ulsed cold jet discharging into hot ambient fluid. 2. Descrition of the exerimental technique 2. Technique basis According to quantum theory, the decrease of emission intensity follows an exonential law (Pringsheim, 949) and, therefore, the intensity decay of a hotoluminescence rocess (either flourescence or hoshorescence) can be exressed in the form I em t /τ = I e, () o where the lifetime τ refers to the time when the intensity dros to 37% (i.e. /e) of the initial intensity I o. For an excited state, the deactivation rocess may involve both radiative and nonradiactive athway. The lifetime of the hotoluminescence rocess, τ, is determined by the sum of all the deactivation rates: τ = k +, (2) r k nr where kr and knr are the radiative and non-radiative rate constants, resectively. According to the theory of the kinetics of hotoluminescence, these rate constants are, in general, temerature-deendant (Ferreudi, 987). The temerature deendence of the hoshorescence lifetime is the technique basis of the resent technique. The temerature deendance of the fluorescence lifetime of a robe has been used to measure the temerature of living cells (Chaman et al., 995) and of liquid crystals (Huang et al. 995). Since the fluorescence lifetime is always very short, which is in the order of nano seconds, the short fluorescence lifetime were usually measured by using oscilloscoebased instrumetations. Although Ni and Melton (996) has demonstrated the gase-hase

4 temerature measurement by using fluorescence lifetime imaging technique, the exerimental setu was relatively involved in order to image the short fluorescence lifetime. Brewster et al. (200) has reorted the ossiblity of the temerature measurement in liquid by measuring the hoshorescence lifetime of a water-soluble hoshorscent dye, which is similar to the comound used here. The work of Brewster et al. (200) is a singleoint feasibility study using oscilloscoe-based instrumetation. Futhermore, the comound utilized has a relatively short lifetime of 00 µs (at room temerature), nearly 40 times smaller than that we reort here, which make it suitable for velocimetry only for very highseed water flows. The work described in the resent aer reresents, to our knowledge, the first to conduct two-dimensional temerature field measurements in a liquid flow based on the direct imaging of hoshorescence lifetime with a conventional image detecting CCD camera Phoshorescence lifetime imaging The total hoshorescence intensity at a given oint I is given by I = I C ε Φ, (3) i where Ii is the local incident laser intensity, C the concentration of hoshorescence dye, ε the absortion coefficient, and Φ the hoshorescence quantum efficiency. The total hoshorescence intensity can also be searately determined from the integration of Equation () over all time, resulting in I = τ. (4) I o Now consider caturing the hoshorescence emission by a gated intensified CCD detector where the integration starts at a delay time to after the laser excitation ulse with a gate eriod δ t. The hoshorescence signal D generated by the detector is then given by to + δ t t /τ D = Io e dt. (5) to Using the equations given above, it can be shown that D δ t / τ t / τ ( e ) e o = I Cε Φ. (6) i In the resent study, the hoshorescence signal is imaged at two successive times within the lifetime of the tracer (Fig. ). It is assumed that the first image is detected at the time t = t after laser excitation for a gate eriod δ t to accumulate hoshorescence intensity D. While, the second image is detected at the time eriod t = t + t for the same gate δ t to accumulate hoshorescence intensity D 2. The hoshorescence intensities D and D 2 will be and D Ii f ε Φ δ t / τ t τ ( e ) e / =, (7) D2 Ii f ε Φ δ t / τ ( t + t) / τ ( e ) e =. (8) The intensity ratio of the second image intensity ( D 2 ) to the first image intensity ( D ) will be

5 D D 2 = e t τ. (9) From the Equation (9), it is evident that the intensity ratio of the two successive hoshorescence images is only a funtion of hoshorescence lifetime, not the laser excitation intensity. This indicates that resent method can eliminate the effects of the variation of the incident illumination. The hoshorescence lifetime can be calculated on a ixel-by-ixel basis from Equation (9) according to t τ =, (0) ln( D / D2) resulting in the distribution of the hoshorescence lifetimes in a two-dimentional domain based on two sucessive hoshorescence images. hoshorescence intensity D lifetime D2 τ = t ln( D / D 2) δ t t δ t time Fig.. Calculation of hoshorescence lifetime fromtwo successive hoshorescence images 2.3. Chemical comosition The hoshorescent dye used in the resent study is -BrN Gβ-CD ROH comlex, which has been used extensively as long-lived tracer for MTV measurements. Further information about the chemical structure and hotoluminescence erformances of the - BrN Gβ-CD ROH comlex can be found from Ponce et al. (993), Gendrich et al. (997) and Koochesfahani (999). Since the concentration of the chemical comositions affect the intensity and lifetime of the -BrN Gβ-CD ROH comlex significantly, we used a concentration of M for Gβ-CD, a saturated (aroximately 0 5 M ) solution of - BrN and a concentration of 0.06M for the alcohol (ROH) in the resent study by following the suggestion of Gendrich et al. (997).

6 2.4. Exerimental setu for calibration Figure 2 shows the schematic setu for the calibration rocedure to get the quantitative relationshi between the temerature and the hoshorescence lifetime of the -BrN Gβ- CD ROH comlex. A XeCl excimer Ultraviolent (UV) laser (wavelength λ = 308 nm, energy 50 mj/ulse, 20 ns ulse width) with aroriate otics was used to generate a laser sheet (thickness is about mm) to illuminate the cube-shaed test cell (about 3.0 liters in volume) containing an aqueous solution of -BrN Gβ-CD ROH comlex. Two faces of the cubeshaed aaratus were made of UV-transarent quartz. The cube-shaed aaratus was laced on a heating late. A stirring rod was used to achieve thermal equilibrium in the test cell. A thermocoule was laced in one corner of the aaratus to measure the actual temerature in the test cell. A 2-bit high-resolution ( ixel) gated intensified CCD camera (PCO DiCAM- Pro) with a fast decay hoshor (P46) was used in the resent study to conduct hoshorescence image recording. The laser and the camera were synchronized using a digital delay generator (SRS DDG535), which controlled the timing of the laser sheet illumination and the CCD camera data acquisition. The hoshorescence images catured by the CCD camera were subsequently transferred to a host comuter for analysis. thermocoule mirror laser sheet otics 2 bit gated intensified CCD camera (DICAM-PRO) stirring rod XeCl excimerlaser (308nm) -BrN Gβ-CD ROH aqueous solution heating late host comuter digital delay generator (SRS DDG535) Fig. 2. The schematic setu for calibration rocedure

7 Relative intensity of hoshorescence t=50.0c t=47.5c 0. t=45.0c t=42.5c t=40.0c t=37.5c t=35.0c t=32.5c t=30.0c t=27.5c t=25.0c t=22.5c time delay (ms) Fig. 3. The decay of the hoshorescence intensity at different temerature levels 5 4 data set 4 (solution 3) data set 3 (solution 2) data set 2 (solution ) data set (solution ) lifetime (ms) temerature (c) Fig. 4. The hoshorescence lifetime of -BrN Gβ-CD ROH comlex vs. temerature 2.5. The hoshorescence lifetime of the -BrN Gβ-CD ROH comlex as a function of temerature To acquire temerature data, the aqueous solution of the -BrN Gβ-CD ROH comlex was first heated to a redetermined temerature level. After the thermal equilibrium was established, the hoshorescence intensity decay rofile was measured by changing the time delay of the camera image acquisition after the laser excitation with a fixed gate eriod ( δ t = 500µ s for the resent study). Figure 3 shows the decay curves of the hoshorescence

8 intensity of -BrN Gβ-CD ROH comlex in the temerature range of C. It is found that all the hoshorescence decay curves at different temerature levels are almost singleexonential curves, which is tyical of hoshorescence decay behavior as exected theoretically. Therefore, Equation (0) can be used to calculate the hoshorescence lifetime of the -BrN Gβ-CD ROH comlex. Based on the intensity decay curves, the hoshorescence lifetimes of the -BrN Gβ- CD ROH comlex at different temerature levels were calculated. The results are shown in Fig. 4. It can be seen that the hoshorescence lifetime of the -BrN Gβ-CD ROH comlex varies significantly with increasing temerature, droing from about 4.5 ms to 0.4 ms as the temerature changes from 2 0 C to 50 0 C The effect of incident light intensity A mentioned above, the emission intensity is used directly to infer the temerature in the conventional single-dye LIF technique and the method suggested by Thomson and Maynes (200). Therefore, the satial and temoral variations of the incident light intensity have to be corrected for searately in using these aroaches. In the resent technique, the relationshi between the temerature and the hoshorescence lifetime is used to conduct temerature measurement. This rocedure can eliminate the effects of the variation of the incident illumination, which has been discussed in the revious sections. Figure 5 shows the intensity and intensity ratio curves of the first and second hoshorescence images in the measurement regions at two temerature levels (T= C and T= C). The hoshorescence intensities are measured along the length of the beam illuminating the constant temerature bath used for these calibrations. It can be seen that the intensities of the first and second hoshorescence images change significantly along the laser beam due to the combined effect of non-arallel beam roagation and attenuation effects. However, the intensity ratio of the second image to the first hoshorescence image is almost constant along the laser beams for both temerature levels. This indicates that the value of the hoshorescence intensity ratio changes only with temerature, demonstrating the basis of the unique technique described in this aer hoshorescence intensity (gray level) intensity of second image D 2 ( T=25 0 C ) intensity of first image D ( T=25 0 C ) intensity ratio of sencond image to first image (D 2 /D ) intensity ratio of second image to first image hoshorescence intensity (gray level) intensity of second image D 2 ( T=40 0 C ) intensity of first image D ( T=40 0 C ) intensity ratio of sencond image to first image (D 2 /D ) intensity ratio of second image to first image distance along laser beam (mm) distance along laser beam (mm) a. T= C b. T= C Fig. 5. The intensity and intensity ratio curves of the first and second gated hoshorescence images along the laser beam at two temerature levels 2.7. Simultaneous temerature and velocity measurements Another great advantage of the resent novel technique is that it allows the simultaneous qualification of velocity and temerature fields in fluid flows by using the same hoshorescent dye (-BrN Gβ-CD ROH) and the same otical and equiment setu. As described above, the hoshorescence signal is imaged at two successive times within the lifetime of the tracer. The hoshorescence lifetime, estimated from the intensity ratio of

9 the two successive images by using Equation (0), can be used to get the temerature distribution by using the relationshi between the hoshorescence lifetime and temerature shown in Fig. 4. While, the fluid velocity vectors can be estimated from the dislacements of the tagged regions between two interrogations through a standard MTV image rocessing routine (Gendrich and Koochesfahani, 996 and Koochesfahani, 999). 3. Demonstration exeriment In order to demonstrate the feasibility of the technique described above, we set u a demonstration exeriment to conduct simultaneous temerature and velocity measurements in a ulsed jet flow. Figure 6 shows a schematic setu of the demonstration exeriment, where a cold ulsed jet exhausts into hot ambient fluid. A turkey baster is used in the resent study to generate the cold ulsed jet flow. The diameter of the turkey baster at the exit is about 6.0mm ( D=6.0 mm). The temerature of the -BrN Gβ-CD ROH aqueous solution in the turkey baster is about C, while the temerature of the - BrN Gβ-CD ROH aqueous solution in the container is about C. The cold -BrN Gβ- CD ROH aqueous solution in the turkey baster exhausts into the hot ambient fluid as a ulsed jet by manually squeezing the rubber bulb at the end of the turkey baster. The maximum jet velocity at the exit of the nozzle is about 0.27 m/s. The test section is illuminated with a XeCl excimer UV laser sheet (wavelength λ = 308 nm, energy 50 mj/ulse, 20 ns ulse width). A beam blocker is used to turn the UV laser sheet into multile laser beams to tag the fluid. rubber bulb turkey baster Tagging lines T= C T= C measurement region Fig. 6. The schematic setu of the demonstration exeriment The 2-bit high-resolution ( ixel) intensified CCD camera (PCO DiCAM-Pro) is used to conduct the hoshorescence image recording. The camera is oerated in the dual-frame mode, where two full-frame images of hoshorescence are acquired in quick succession from the same laser excitation ulse. For the resent demonstration exeriment, the first image is catured at the time delay of 500 µs after the laser ulse with the interrogation gate eriod of 500 µs. The second hoshorescence image is catured at the time delay of 3.5 ms after the laser excitation with the same interrogation gate eriod. A

10 tyical air of first and second hoshorscence images is given in Fig. 7. From the comarison of the first and second hoshorescence images, it can be seen that the tagging lines in the first image have deformed into curved lines in the second image. The velocity distribution of the flow field can be obtained from the dislacements of the corresonding lines between the first image and second image through a standard linetagging-tyed MTV image rocessing routine (Koochesfahani, 999). Since the temerature of ambient fluids (T= C) is higher than the core jet flow (T= C), the hoshorescence intensity of the hot ambient fluids decay faster than those of the cold core jet. Therefore, the ortions of the tagged lines in the hot ambient fluid are much dimmer than those in the cold core jet in the second image. The hoshorescence lifetime distributions can be obtained from the hoshorescence intensity ratios of the first and second hoshorescence images based on Equation (0). By using the relationshi between the temerature and hoshorescence lifetime of -BrN Gβ-CD ROH comlex shown in Fig. 4, the temerature distribution can be constructed. Samle simultaneous temerature and velocity data are shown in Fig. 8 in terms of quantitative transverse rofiles along the tagging line A and tagging line B marked in the Fig. 7(a). Along the tagging line A, which is located about 0.4 diameter downstream the nozzle exit, the velocity and temerature distributions were found to have nearly to-hat rofiles, as exected. At the further downstream location of Y/D=3.5 (tagging line B), some hot ambient fluid has been engulfed into the core jet through the enrtainment rocess between the core jet and ambient fluid. The to-hat velocity rofile is rounded u and the velocity at the center of the jet decreases from 0.27 m/s at the nozzle exit to 0.20 m/s. The temerature of the core jet flow also increases to about 27 0 C at this downstream location. There are two temerature eaks in the transverse temerature rofile of the core jet, which is due to the engulfing of hot ambient fluid from the two sides of the core jet flow. The engulfing of the hot ambient fluids to form two temerature eaks in the core jet is visualized clearly in the hoshorescence images shown in Fig. 7, which are the two dark regions in the core jet flow. tagging line A tagging line B a. the first hoshorescence image b. the second hoshorescence image Fig.7. A tyical air of the first and second hoshorscence images

11 temerature velocity U temerature T temerature temerature T velocity U 0.2 temerature ( 0 C) velocity 0. velocity (m/s) temerature ( 0 C) velocity 0. velocity (m/s) X/D a. along tagging line A (Y/D=0.4) b. along tagging line B (Y/D=3.5) Fig. 8. The simultanous velocity and temerature rofiles along tagging line A and B X/D 4. Conclusions A novel method for conducting quantitative temerature field measurements in liquid has been develoed by taking advantage of the hoshorescence lifetime deendence on temerature. -BrN Gβ-CD ROH comlex was used as the long-lived hoshorescent dye in the resent study. The decay rofiles of the hoshorescence intensity of the hoshorescent dye (-BrN Gβ-CD ROH) at different temerature levels were found to follow a single-exonential law as they were exected theoretically. The decay rate of the hoshorescence intensity, in terms of the hoshorescence lifetime, was found to change significantly with the temerature. The methodology described in the resent study reresents an extension of Molecular Tagging Velocimetry (MTV) technique. As in MTV measurements, the hoshorescence signal is imaged at two successive times within the lifetime of the tracer. The hoshorescence lifetime, estimated from the intensity ratio of the two images at each oint, is calibrated against temerature. This rocedure eliminates the effects of the variation of the incident illumination. This methodology also allows the simultaneous qualification of velocity and temerature fields in a fluid flow by using the same hoshorescent dye (- BrN Gβ-CD ROH) and same otical and equiment setu. While the dislacements of the tagged regions between two interrogations rovide the estimate of fluid velocity vectors, the intensity ratio of the two images reresents the fluid temerature distribution. The imlementation and alication of the novel technique are demonstrated here by the simultaneous temerature and velocity measurements in a ulsed cold jet discharging into hot ambient fluid. Acknowledgments This work was suorted by the MRSEC Program of the National Science Foundation, Award Number DMR

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