Paper # 070DI-0146 Topic: Diagnostics

8th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute

and hosted by the University of Utah May 19-22, 2013

High-Repetition-Rate Imaging of Atomic Oxygen In Flames

Waruna D. Kulatilaka1, Naibo Jiang1, Sukesh Roy1, and James R. Gord2

1 Spectral Energies, LLC, 5100 Springfield Street, Suite 301, Dayton, Ohio 45431

2 Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio 45433

Abstract: Two-photon laser-induced fluorescence (TPLIF) is a well-recognized approach for detecting atomic oxygen however, measurements to date have been restricted to a single point within a flow, flame, or plasma, due to the inherently high intensity requirements for the excitation laser. Furthermore, the high-intensity ultraviolet (UV) photons used for two-photon excitation process can photochemically produce O atoms by dissociating other oxygen containing species in the medium. In this work, we demonstrate nearly photolytic-interference-free femtosecond (fs) TPLIF line imaging of atomic oxygen in flames. By using amplified Ti:sapphire-based laser systems, the data acquisition rate can be increased to the 1–10 kHz regime, enabling capture of dynamic events in turbulent media. Furthermore, as a result of efficient two-photon excitation and subsequent generation of strong fluorescence signals, the fs-TPLIF scheme is readily extended for 2-D imaging for studying spatiotemporal dynamic in systems such as practical combustion and plasma devices. In O-atom TPLIF, two-photon-allowed absorption of 225.7-nm light, drives the transition from the ground 2p 3P electronic state to the excited 3p 3P state. From this upper level, de-excitation to the 3s 3S state occurs by single-photon emission, allowing fluorescence detection at 844.6 nm. By using high-peak-power but low-energy fs pulses, the photolytic interferences are virtually eliminated while substantially increasing the two-photon excitation efficiency. Similar observations have previously been made in TPLIF detection of atomic hydrogen. The broad bandwidth of nearly Fourier transform– limited (TL) fs pulses contributes to enhanced excitation through the combination of a large number of photon pairs within the spectral bandwidth of each pulse. In TL pulses, all photons of different colors—corresponding to different frequencies—have the same spectral phase, thus collectively contributing to two-photon excitation. Additionally, fs-duration pulses become favorable for TPLIF because the signal scales as the laser irradiance squared, whereas single-photon-induced photodissociation processes scale linearly. As a result, photolytic interferences become virtually negligible at reasonable TPLIF detection levels of nascent O atoms. The measurement techniques described in this work can provide invaluable experimental data of spatially and temporally resolved number density of atomic species such as O, H, and N to validate complex combustion and plasma flow models.

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1. Introduction Spatially and temporally resolved, concentration measurements of highly reactive atomic species such as O, H and N in chemically reacting flows such as flames and plasmas provide a key insight into the physical and chemical nature of such systems. Noninvasive and highly sensitive laser-based spectroscopic techniques such as laser-induced fluorescence (LIF) are widely used for such measurements [1,2]. LIF also offers the ability to readily extend to two-dimensional (2-D) imaging by using planar LIF (PLIF), particularly when only one color laser is used for excitation. LIF-based measurements of key atomic species such as H, O, and N as well as molecular species such as CO require multiphoton excitation, as the frequencies of single-photon electronic transitions fall in the vacuum ultraviolet region, for which the medium becomes optically thick in most practical devices [3]. In such cases, the relatively weaker multiphoton excitation cross sections necessitate the use of high-energy UV pulses for sufficiently intense signal generation. Such high-energy UV photons can simultaneously photodissociate certain other radicals in the medium, generating the species being probed. For example, in two-photon excitation LIF (TPLIF) detection of atomic hydrogen, a substantial amount of additional H can be produced via photodissociation of vibrationally excited water vapor and methyl (CH3) radicals [4], whereas in TPLIF of atomic oxygen, CO radical has shown to be a dominant photolytic precursor [5]. In some earlier studies, improved TPLIF detection has been reported in H and O using picosecond (ps) duration pulses as opposed to the traditionally used nanosecond (ns) duration pulses [3, 5, 6]. It was shown that the high peak power of ultrashort pulses favors the nonlinear excitation while low average power reduces the interfering photolytic processes.

In our earlier work, we have demonstrated that femtosecond(fs) duration pulses are significantly advantages for photolytic-interference-free imaging of atomic hydrogen in flames [7]. In the present study, we extend the femtosecond (fs) TPLIF scheme for interference-free line imaging of atomic oxygen in flames. By using high-peak-power but low-average-power fs pulses, the photolytic interferences are virtually eliminated for a range of laser pulse energies while substantially increasing the two-photon excitation efficiency enabling single-laser shot line imaging. Additionally, by using amplified Ti:sapphire-based laser systems, the measurement bandwidths are increased to the 1–10 kHz regime, enabling the study of the spatiotemporal dynamics of turbulent reacting flows. O is a key intermediate species in number of elemental hydrocarbon combustion reactions [5, 6]. Furthermore, O atoms play a key role in fundamental plasma chemistry and plasma-flame interactions [8]. The excitation dynamics of the TPLIF process are shown in Fig. 1. As shown in Fig. 1(a), two-photon excitation of the 𝑛 = 1 →→ 𝑛 = 3 transition with 226-nm photons is used to populate the 𝑛 = 3 level, and the subsequent fluorescence at 845 nm from the 𝑛 = 3 → 𝑛 = 2  decay is detected. Several other simultaneous processes, including photolytic production of O, photo-ionization, stimulated emission (SE), and collisional quenching, can also complicate the quantitative interpretation of the measured LIF signal. Photodissociation of numerous O-containing flame radicals and molecules can produce substantial quantities of additional O in the medium, as shown in previous studies [5, 6]. In these conditions, fs-duration pulses become favorable for TPLIF because the signal scales as the laser irradiance squared whereas single-photon-induced photodissociation processes scale linearly. Additionally, the broad bandwidth of nearly Fourier transform–limited (TL) fs pulses contributes to enhanced excitation through the combination of a large number of photon pairs as shown in Fig. 1(b). In TL pulses, all photons of different colors—corresponding to different frequencies—have the same spectral phase, thus collectively contributing to two-photon excitation. In our previous work we used 205-nm two-photon excitation for atomic H TPLIF and

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showed that photodissociation of H2O and CH3 can be minimized by using low-power, fs-duration laser pulses [7]. Although beyond the scope of the present study, the temporal resolution provided by the fs excitation can be utilized to define the onset of fluorescence excitation and hence can be critical for time-gated fluorescence detection schemes. We plan to investigate such schemes for quenching-free LIF detection of important intermediate species in combustion environments.

2. Experimental The experimental apparatus consists of a 1-kHz repetition rate, amplified fs Ti:sapphire laser system (Spectra-Physics Solstice) pumping an optical parametric amplifier (OPA) (Coherent OPerA Solo). The peak pulse energy of the pump laser was 2.5 mJ at 800 nm; parametric conversion of the pump beam followed by frequency mixing and then upconversion of the idler beam resulted in up to 15 µJ of UV radiation at 226 nm. The 226-nm beam was collimated to a diameter of approximately 1 mm and transmitted through a Φ=1.2, CH4/O2/N2 Bunsen flame stabilized over a 5-mm diameter underexpanded jet. Two +50-mm focal length f/1.2 camera lenses (Nikon Nikkor AIS) were used in the conjugate configuration to collect the fluorescence signal and focus it back onto the detection system. A 50-mm diameter 840-nm bandpass filter (Semrock FF01-840/12-50) was mounted in between the two camera lenses, which enabled >90% transmission of O-atom fluorescence signal while blocking all other nearby wavelength with optical density (OD) >6. Two camera systems were used for imaging the O-atom signal. For detailed study of the photolytic processes, an intensified CCD camera (Princeton Instruments PI Max II) was used. This camera enabled single laser shot and averaged detection of the O-atom line image. For high-speed detection, particularly for 1 kHz imaging, a CMOS camera (Photron Model SA-X) coupled to a high-speed intensifier (LaVision Model IRO) was used. This camera/intensifier system enabled full frame (1024 x 1024 pixels) imaging at frame rates as high as 12 kHz. Such high-speed imaging system is critical for utilization of the current fs-TPLIF scheme for investigation of turbulent flames where flame dynamics can be captured

Figure 1: (a) Excitation/detection scheme for fs-TPLIF of atomic oxygen. Two-photon excitation at 226 nm is followed by single-photon emission detected at 845 nm. (b) Broad spectral bandwidth of femtosecond duration pulses can effectively contribute for two-photon excitation schemes via matching multiple frequency pairs within the pulse bandwidth.

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3. Results and Discussion Fluorescence line images were recorded in the laminar Bunsen flame, stabilized over a 5-mm diameter jet. A flame luminosity image is shown in the left panel of Figure 2. The steep temperature and species gradient present in this Bunsen flame is ideal for investigated the potential of fs-TPLIF technique for interference-free detection of O atoms in flames. A sample fs-TPLIF line image from the one half of the flame is shown in the right panel of Figure 2. The peak intensity region corresponds to the maximum O atom distribution present at the flame front. The inner cone of the flame contained the cold premixed reactants and thus does not contain any intermediate species such as O atoms.

Fs-TPLIF images were recorded in the above Bunsen flame while varying the laser pulse energy between 0.4 µJ to 8 µJ (i.e., the maximum pulse energy available at the probe region). As seen from Figure 3, when the laser pulse energy was increased above ~ 5 µJ, a change in the peak normalized O-atom LIF profile was observed. This observation is in consistent with previous O-atom LIF imaging using nanosecond (ns) and picosecond (ps) duration laser pulses where photodissociation of CO present in the post flame region of the rich hydrocarbon flame contributing to the additional O-atom LIF signal [5]. However, in the present case, the average signal-to-noise ration (SNR) when the laser pulse energy was ~5 µJ was well above 100. Such and advantage provided by fs-duration pulses is a significant step forward in utilizing fs-TPLIF for interference-free detection of O-atoms in combustion environments. Furthermore, unlike in previous ns or ps LIF experiments where the typical pulse repetition rates are on the order of 10 Hz, the present fs-TPLIF technique can provide data acquisition bandwidth on the order of 1-10 kHz as a result of similar pulse repetition rates available in ultrafast regenerative amplifiers generation these pulses. The photodissociation process can be studied in detail by observing the spectrally resolved flame emission and by observing the dependence of key intermediate species concentration as a function of laser energy. We are in the process of employing such diagnostics and a complete

Figure 2: Flame luminosity image of the premixed CH4/O2/N2 Bunsen flame stabilized over a 5-mm diameter underexpanded jet. Also shown in the right panel is a sample O-atom fs-TPLIF image and a corresponding line profile.

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parametric study of a range of laminar flames from lean to rich using the fs-TPLIF apparatus. Results of such studies will be described in detailed in future publications.

To illustrate the feasibility of performing 2D-PLIF imaging of atomic oxygen using the current scheme, we expanded the laser beam into a small sheet using a f=+50 mm cylindrical lens and passed it over the same plane. In this case, because the laser fluence is much less than the threshold fluence for initiating photolytic breakdowns, the 2D images are essentially free of interferences. A corresponding single-laser-shot O-atom PLIF image is shown in Figure 4. The image quality is somewhat degraded because of the limited laser energy and the non-uniformity of the spatial beam profile. We are currently exploring possibilities of improving our laser system to overcome both of these issue.

Figure 3: Peak-normalized fs-TPLIF line profiles (right half of the flame) of atomic oxygen as a function of excitation beam pulse energy. The peaks appearing on the right side of the profile as the pulse energy is increased above 5 µJ are arising from various photolytic processes such as photodissociation of excess CO in the medium.

Figure 4: A sample O-atom PLIF image recorded using fs-TPLIF scheme in a Φ=1.2, CH4/O2/N2 Bunsen flame stabilized over a 2.5-mm diameter nozzle.

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4. Conclusion We have investigated the utilization of fs duration laser pulses for two-photon-excited LIF detection of atomic oxygen in flames. The high-peak power, but low-average power ultrashort pulses significantly reduce the interfering photolytic processes during TPLIF detection. Furthermore, high repetition rate measurements, in the range of 1-10 kHz is readily achievable by using Titanium:sapphire based femtosecond amplifier systems. Certain photolytic interferences observed in high pulse energies are under investigation. We have also demonstrated the potential of fs-TPLIF scheme for photolytic-interference-free 2D imaging of atomic oxygen in flames. 5. Acknowledgements Funding for this research was provided by the United States Air Force Research Laboratory under Contract No. FA8650-10-C-2008 and by the United States Air Force Office of Scientific Research (Drs. Enrique Parra and Chipping Li, Program Managers). 6. References 1. R. P. Lucht, J. T. Salmon, G. B. King, D. W. Sweeney, and N. M. Laurendeau, "2-Photon-excited

fluorescence measurement of hydrogen-atoms in flames," Opt. Lett. 8, 365-367 (1983). 2. K. Niemi, V. Schulz-von der Gathen, and H. F. Dobele, "Absolute atomic oxygen density measurements

by two-photon absorption laser-induced fluorescence spectroscopy in an RF-excited atmospheric pressure plasma jet," Plasma Sources Sci. Technol. 14, 375-386 (2005).

3. W. D. Kulatilaka, B. D. Patterson, J. H. Frank, and T. B. Settersten, "Comparison of nanosecond and picosecond excitation for interference-free two-photon laser-induced fluorescence detection of atomic hydrogen in flames," Appl. Opt. 47, 4672-4683 (2008).

4. W. D. Kulatilaka, J. H. Frank, B. D. Patterson, and T. B. Settersten, "Analysis of 205-nm photolytic production of atomic hydrogen in methane flames," Appl. Phys. B 97, 227-242 (2009).

5. J. H. Frank, X. L. Chen, B. D. Patterson, and T. B. Settersten, "Comparison of nanosecond and picosecond excitation for two-photon laser-induced fluorescence imaging of atomic oxygen in flames," Appl. Opt. 43, 2588-2597 (2004).

6. J. H. Frank, T. B. Settersten, “ Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation,” Proc. Combust. Inst., 30, 1527-1534 (2005).

7. W. D. Kulatilaka, J. R. Gord, V. R. Katta, and S. Roy, “ Photolytic-interference-free, femtosecond two-photon fluorescence imaging of atomic hydrogen,” Opt. Lett. 37, 3051-3053 (2012).

8. H F Döbele, T Mosbach, K Niemi, and V Schulz-von der Gathen, “Laser-induced fluorescence measurements of absolute atomic densities: concepts and limitations,” Plasma Sources Sci. Technol. 14, S31-S41 (2005).