Time-resolved temperature images of laser-ignition using OH two-line laser-induced fluorescence (LIF) thermometry

Transcription

1 IFRF Combustion Journal Article Number , November 2005 ISSN X Time-resolved temperature images of laser-ignition using OH two-line laser-induced fluorescence (LIF) thermometry Wenhong Qin 1, Ying-Ling Chen 2, and J. W. L. Lewis JDS Uniphase Corp. Shenzhen , China Center for Laser Applications, The University of Tennessee Space Institute, 411 B. H. Goethert Parkway, Tullahoma, Tennessee Corresponding Author: Ying-Ling Chen Center for Laser Applications, The University of Tennessee Space Institute 411 B. H. Goethert Parkway Tullahoma, Tennessee Tel. No.: Fax No.: ychen@utsi.edu

2 IFRF Combustion Journal -1 - Chen et al. ABSTRACT The time-resolved spatial distribution of temperature for the transient laser-induced ignition of ammonia / oxygen mixtures is presented. Planar laser-induced fluorescence (PLIF) of OH was used to measure the temperature distribution by means of two selected transition lines observed using a steady flame to provide the collision quenching correction over the necessary range of temperatures. Information on the spatial and temporal evolution of gas temperature and species concentrations is essential for numerical modeling of laser spark ignition. Keywords: laser-induced breakdown, OH laser-induced fluorescence, time-resolved, planar laser-induced fluorescence, laser ignition

3 IFRF Combustion Journal -2 - Chen et al. INTRODUCTION Laser-induced optical breakdown was reported in 1963 by Maker et al. [1] who found that a spark similar to that of electric breakdown between electrodes was produced in air by a focused Q-switched ruby laser of tens-of megawatts. Subsequent to this work, numerous studies [2-5] of laser breakdown have been performed for gases, liquids and solids, and a variety of applications now exist for laser-induced breakdown and laser-induced breakdown spectroscopy. Similar to electric spark discharge, in a combustible environment, the initial plasma of the laserinduced breakdown leads to ignition. Compared to the sparkplug breakdown, the laser plasmas produced by focused pulsed beams are generally smaller in volume, are typically formed in shorter time, and posses a somewhat different spatial symmetry (and boundary condition). The electrode-less spark from a laser pulse overcomes the sensitivity to flow, heat removal, and contamination of the chemistry and has the obvious and positive implications for the selection of combustor wall material. The potential for ignition within the combustor of stoichiometric ratios of the fuel-oxidizer mixture is a significant improvement. Moreover, using flame detection sensors, a laser-induced ignition source has the capability of multi-point ignition-on-demand. Over the past decade, experimental research in laser ignition has progressed significantly. Investigations were performed under various environmental conditions with single- and multipoint ignition [6-22]. The results of these studies showed the spatial features of the phenomena over the time range of nanoseconds to milliseconds following the onset of breakdown. The laserinduced breakdown plasma [9-12] provides the initial and boundary conditions for the computational fluid dynamic simulation of laser-induced breakdown [13]. As well, laser ignition has made it possible to determine the minimum ignition energy [11, 14-15], breakdown threshold [12, 16], ignition time and flame speed [17] and the formation mechanisms of the shock wave [13,18], the creation of the combustion radicals [18-20], and the resulting combustion and vortexlike gas dynamics of the combustion products [13, 18, 20-22]. It is understandable then that the modeling of laser ignition is complex. The phenomena that occur from spark formation to global combustion include aspects of nonlinear electrodynamics, plasma physics, fluid dynamics and combustion kinetics. Construction of such a model requires numerous approximations. It is

4 IFRF Combustion Journal -3 - Chen et al. therefore essential to have experimental results for the initial and different time sequence events in the ignition process to guide and to check the model. Although hydrocarbon fuels are clearly of interest in laser ignition research, this paper describes the results of an extension of our previous work [18-20] using mixtures of ammonia and oxygen. The reason for choosing this system is to take advantage of the simplicity of chemical kinetics, where as few as ~30 equations can be used compared to hundreds of reactions for the hydrocarbon-oxygen system. As well the ammonia-oxygen system has much more simple spectroscopic features. In our previous work resonant planar laser induced fluorescence (PLIF) of the short-lived NH radical was employed to measure the NH concentration and temperature and to define the flame front [20]. Although NH yields an excellent characterization of the region near the flame front, OH is more broadly distributed in space and is preferred as a molecular probe for the temperature of the ignition event [23]. Figure 1 shows the Sandia Chemkin code [24] prediction of the properties of the onedimensional adiabatic ammonia / oxygen flame with an equivalence ratio of 0.7. Figure 1(a) illustrates the production of the major species mole fractions through the flame reaction zone that is located approximately 0.25 mm above the burner surface. Shown in Figure 1(b) are the mole fractions of the NH and OH free radicals produced in the combustion process. A low concentration of NH molecules appears in the flame-front in a region ~0.3 mm thick. As a result, as a probe species, NH yields good spatial resolution for the flame front determination [20]. In contrast, OH is 20-fold higher in concentration and is distributed throughout the combustion region after it is produced. In addition to these properties, extensive studies have been published on the spontaneous emission properties of OH and also on its fluorescence efficiency, and quenching parameters, with numerous collision partners [25-28]. LIF spectroscopy has been applied to combustion diagnostics for decades, and resonant LIF possesses the sensitivity to detect minor species with concentrations in the parts per million (ppm) or even per billion (ppb) range. Many variations of LIF thermometry exist. The discussions in references 27 and 29 are a good review of various non-intrusive temperature measurement techniques and the advantages of the OH LIF over other techniques are shown. The technique used here is an extension of the two line fluorescence methods reported in refs. 30 and 31. The basis of the technique is the sequential resonant excitation of the lower level population

5 IFRF Combustion Journal -4 - Chen et al. such that, at atmospheric pressure, the mixture is characterized by a thermal equilibrium Boltzmann distribution. The resulting fluorescence is related to the temperature, and is dependent upon the spectral emission observed and the density and composition of the mixture. Since the laser excitations from the individual and different lower levels are sequential in time, the implementation of measurements of transient events is more difficult than for time-independent phenomena. 0.6 O 2 H 2 O 0.05 OH 3000 Mole Fraction NH 3 N 2 Mole Fraction NH ( 10-1 ) Temperature (K) Distance (cm) (a) Distance (cm) (b) Distance (cm) (c) Figure 1 Calculated results for a one-dimensional premixed ammonia / oxygen flame. (a) mole fraction of major species, (b) mole fraction of radical OH and NH, (c) temperature across the flame front. X-axis is distance from the unburned premixed gas to the after-burn products. In the present work, PLIF was used to yield two-dimensional results using laser sheet excitation, two-dimensional and broadband spectral detection, and data acquisition for repetitive laserinduced ignition events. For this PLIF application collisional quenching effects are dominant [23, 32-34]. The two-line variation of PLIF [32] is used in this study and the formulation of the theory is similar to that of Seitzman et al. [34]. The objectives of this paper are to apply the method to temperature-field measurements of laser-ignited ammonia/oxygen mixtures and to present the determination and validation of the necessary systematic and collisional quenching corrections.

6 IFRF Combustion Journal -5 - Chen et al. THE SELECTION OF TRANSITION LINE PAIR The theory of PLIF is well developed and discussed elsewhere [27, 34 and 35]. Here we present only those details relevant to the development used here. The linear LIF emission signal, S, is related to the incident laser beam fluence E, the local total OH number density N t and gas temperature T by the equation [23] Ω ε S = η E VN t (2J" + 1) exp( ) B G Φ( T, J ), (1) 4π kt where η is the optical detection efficiency, Ω is the optical collection solid angle, V is the measurement volume, J is the rotational quantum number of the excited lower state, ε is the energy of the lower state, B is Einstein stimulated absorption coefficient, and G characterizes the spectral overlap of the laser and absorption line profiles. The quantum yield Φ of the excited upper level is dependent upon the excited level of rotational quantum number J, temperature T, and the host environment. For two-line LIF thermometry [32-34], the collected broadband fluorescence signals S 1 and S 2 originate from upper levels 1 and 2, respectively. Levels 1 and 2 are pumped from the lower levels a and b, respectively, and the ratio R of the fluorescence signals is S (2J " 1) 1 a + Ba1 ε a ε b R = = C exp( ), (2) S (2J " + 1) B kt 2 b b2 where the two-line correction, C, includes systematic and quenching corrections and is similar to the calibration constant developed in reference 34: ηλ1e C = [ η E λ2 a1 b2 G ][ G a1 b2 Φ ( T, J Φ ( T, J ') ] ') 2. (3) The rotational temperature could be deduced from Equation 2 if the two selected transitions share a common upper rotational state so that the ratio of quenching, Φ 1 /Φ 2, is one, and therefore, G a1 /G b2 approaches one. Unfortunately, the lower levels that can be excited by allowed electronic transitions to a common level of the A 2 Σ + state of OH do not exhibit adequate sensitivity in T. Specifically, for the energy difference ε of the lower levels, the ratio ε/kt is too small to allow sufficient accuracy in the temperature determination. However, the correction

7 IFRF Combustion Journal -6 - Chen et al. of two-line technique, C, can be calibrated if the temperature at a certain location in the combustion can be measured by a second technique. One simple and practical approach is a quantitative comparison between OH-LIF results and those measured by traditional thermocouples in a stable flame of NH 3 /O 2 [34]. Here, we assume that the quenching environments, including temperature, pressure, and major species concentrations, are similar between ignition and the steady combustion of the same fuel and oxidant. Local thermal equilibrium is assumed for both environments. The selection of the two transitions within OH A 2 Σ + -X 2 Π(1,0) band is based on the following considerations: First, to achieve acceptable accuracy in temperature sensitivity, ε of the 2 lower states must be comparable with kt. Next, both fluorescence signals are required to exhibit good signal-to-noise ratio (SNR) in both ignition and flame conditions. Third, both excitation lines should be spectrally clean to preclude LIF excitation of other species such as NO, H 2 O and N 2 that could result in overlapping lines and systematic errors. As well, a clean off-resonant spectral location near the selected lines is required to determine the background of LIF. This background includes the contribution from Rayleigh and Mie scattering of the pumping laser and radiation from the flame or plasma. In laser ignition, the background signal is normally significant, though stable if the initial conditions and timing are well controlled, compared to the fluorescence signal. Finally, since frequency doubling is required to achieve the desired spectral range of OH excitation, closely spaced pumping wavelengths of the two transitions are favored for experimental simplicity. As a result of the this evaluation, the OH transition line pair R 2 (2,5) and R 2 (9,5) was selected, and Table 1 gives the parameters for these 2 lines. Transitions F J (cm -1 ) λ (nm) A (s -1 ) R 2 (2.5) R 2 (9.5) Table 1 Parameters of the two selected OH A 2 Σ + -X 2 Π(1,0) transitions for two-line PLIF thermometry F J : Lower state term value. λ: Wavelength in air; A: Einstein A coefficient

8 IFRF Combustion Journal -7 - Chen et al. These two transition lines are indicated with red arrows in Figure 2. The upper spectrum in Figure 2 is the experimental data from a LIF excitation scan in a CH 4 /Air flame. The spectrum shows a highly resolved spectral structure by scanning the wavelength of the probe laser. The fluorescence was collected through a broadband filter. The lower spectrum in the Figure is the corresponding OH A 2 Σ + -X 2 Π(1,0) band calculated from 281 nm to nm at a temperature of 2152 K. The calculation is performed using tables of the electronic dipole line strength [36]. The interference of multi-species and multi-line excitation in OH thermometry was examined through the same calculation for all potential species. This procedure is used to ensure that no multiple excitations occur in the thermometry. In the excitation scan experiment, (and particularly in the early stages of laser plasma development), there is always some background signal in the offresonance region from the elastic scattering of the probe beam and the spontaneous radiation of the high temperature species. Although the background can be significant compared to the fluorescence signal, it exhibits no sensitivity to excitation wavelength and can be subtracted by tuning the probe laser to the off-resonant positions. Figure 2 provides a clear selection of the offresonant wavelength position for both lines at the same wavelength of nm. R 2 (2.5) R 2 (9.5) W a v e le n g th (n m ) W a v e le n g th (n m ) Figure 2 OH laser-induced fluorescence excitation spectrum: (Upper) experimental data measured in CH 4 /Air premixed steady flame, (Lower) calculated spectrum

9 IFRF Combustion Journal -8 - Chen et al. QUENCHING CORRECTION AND TEMPERATURE CALIBRATION IN A STABLE FLAME Before the two-dimensional two-line thermometry was applied to the transient ignition process, the technique was calibrated in a steady premixed NH 3 /O 2 /N 2 flame. This flame was generated in a standard McKenna flat-flame burner. A flow of nitrogen caused the flame to lift-of from the burner surface and as such decreased the heat transfer from the flame to the burner surface. An annular sheath-gas flow was used to surround the axis-symmetric core flow to stabilize the core flame and decrease the diffusive and gas-dynamic mixing with the ambient air. Three thermocouples of different bead diameters were used to determine independently the flame temperature. The thermocouple beads were located on the centerline of the burner, 10 mm above the 6-cm diameter burner surface. After correcting for both radiative and conductive heat losses, the flame temperatures obtained from the three thermocouples were consistent within a few percent. The resulting average temperatures and the corresponding calculated temperatures (using Sandia Chemkin code [24]) are listed in Table. 2 for various flame conditions. Flame conditions Calculated T (K) Thermocouple T (K) C correction in Eq. 2 NH 3 /O 2, Φ=0.3 N 2 : 15.8% NH 3 /O 2, Φ=0.35 N 2 : 22.8% NH 3 /O 2, Φ=0.7 N 2 : 36.3% NH 3 /O 2, Φ=1.0 N 2 : 39.8% ± 1% ± 1% ± 1% ± 1% 2.5 Table. 2 Summary of the temperature results measured by thermocouple and computed by Chemkin code [24].

10 IFRF Combustion Journal -9 - Chen et al. In Table 2, the measured temperatures range from about 200K to 350K below the calculated adiabatic flame temperatures for the listed four flame conditions. Using these thermocouple temperatures, we applied OH two-line thermometry in the flames at the same location as the thermocouples and, using these measured temperatures, obtained the values for the ratio C (Equation 2) indicated in Table 2. The correction factor decreases slightly with temperature increase in the range of K. Extrapolation of the quenching correction factors of Table 2 is required to span the hightemperature range of the laser-ignition conditions. The validity of this extrapolation was estimated by computing the variation of the quenching correction over this high-temperature range. This was achieved using from Refs. [25, 26] the quenching rate coefficients for the eight major species and the concentration of collision partners calculated by Chemkin [24]. The OH quenching rates in ammonia/ oxygen flames of three equivalence ratios were calculated across their reaction zones. The result is shown in Figure 3. As we observed from the experiment, the quenching rate of OH in premixed ammonia / oxygen flame is insensitive to the flame temperature above 1800 K for the various mixture conditions, and the extrapolation is justified. 8 Quenching rate (*10 9 sec -1 ) NH 3 /O 2 flam e Eq. Ratio φ =1.0 φ =0.7 φ = Temperature (K) Figure 3 OH quenching rate dependence on NH3 / O2 flame temperature. Calculation uses the quenching parameters of Crosley [25, 26] and chemical reaction code of Chemkin [24]

11 IFRF Combustion Journal Chen et al. To test the two-line thermometry with the higher temperature quenching correction, we performed a PLIF measurement of a steady slit-burner flame of NH 3 / O 2 / N 2. The height and width of this premixed flame were approximately 2 cm and 1cm, respectively. The OH PLIF images were acquired separately in the upper and lower portion of the flame using a probe dye laser of about 1 cm width. Three PLIF images, which correspond to the two selected transition wavelengths and one background image with the off-resonant excitation, were taken for each spatial region. One OH PLIF image of each region is shown in the left of Figure 4. The right image in Figure 4 shows the combined two-dimensional temperature map of this stable slit flame. The highest flame temperature measured in this experiment is about 250 K below the adiabatic flame temperature computed by Chemkin in the same flame conditions, which is consistent with the previous results. Figure 4 Steady premixed NH 3 /O 2 /N 2 flame on slit burner. (left): OH PLIF images of the flame., (right) Temperature map of the flame obtained from the PLIF data.

12 IFRF Combustion Journal Chen et al. TIME-RESOLVED TEMPERATURE MEASUREMENTS IN LASER IGNITION The experimental arrangement of the laser ignition experiment is illustrated in Figure 5. The system is comprised of four major parts: the Nd:YAG laser system that is used to create optical breakdown, the burner and gas flow control system, the dye laser system, which serves as the exciting source of OH PLIF spectroscopy, and the detection system, which includes the CCD camera and the computer. Figure 5 Experiment geometry of the time-resolved 2-dimensional OH temperature measurement, M=Mirror, L=Lens, B.E.=Beam Expander, Pol.=Polarizer, C.L.=Cylindrical Lens As shown in Figure 5, optical breakdown was produced using a 1064 nm Nd:YAG laser beam (Continuum YG680S) with a 6.5 nsec (FWHM) pulse width and ~100 mj pulse energy. The fluctuation of the laser pulse energy was less than 1%. A pair of polarizers was used for control of the beam energy. Following expansion of the beam to a width of ~2 cm, the beam was focused to produce breakdown using an anti-reflection coated 10 cm-focal-length lens. An energy meter recorded the transmitted laser energy following breakdown. The absorbed energy for each laser breakdown was obtained by subtracting the transmitted energy from the incident pulse energy.

13 IFRF Combustion Journal Chen et al. The incident pulse energy in this work was set at 31 mj ±1%, about twice the breakdown threshold. For laser ignition, the YAG laser pulse was focused on the centerline and 10 mm above the surface of a 6-cm diameter, sintered stainless steel McKenna flat-flame burner. Premixed research-grade anhydrous ammonia and oxygen flowed through the burner with a linear speed of 6 cm/sec. An annular sheath-gas flow of argon surrounded the core flow to decrease the diffusive and gas-dynamic mixing with ambient air. An argon flow giving a speed of 4 cm/sec was determined using a computational fluid dynamics code [37] to be optimal for this purpose. The flow rates of all gases were individually monitored by calibrated MKS flow meters. PLIF was accomplished using an excimer-pumped dye laser as the OH probe beam. The pump and dye lasers were a Lambda Physik EMG 150 MSC excimer and FL3002E, respectively, and Coumarin 540A dye was used in dye laser. The scanning dye laser beam was frequency-doubled to achieve the OH resonant excitation. Following prism separation of the fundamental and doubled beams, the 10 nsec UV probe beam was expanded to a diameter of ~15 mm, and then focused with a cylindrical lens (fl.= 200 mm) into the ignition reaction zone above the burner with a monitored energy of 100 µj. The focused laser sheet at the location of ignition kernel had a cross-section of 15 mm in height and 270 µm in thickness. The laser sheet intersected with the ignition kernel precisely at the mid-point of a controlled time-delay gate following the breakdown. The excited OH fluorescence was detected perpendicular to the probe beam by a gated intensified CCD camera (EG&G PARC, 2000 OMA SPEC). For the two-dimensional temperature measurement, a nm pass-band filter was used. The exposure time for each PLIF image was 100 nsec. The time-resolved OH PLIF images in laser-induced breakdown of combusting and noncombusting mixtures are shown in Figure 6(a) and (b), respectively. The grid lines in each PLIF image in Figure 6 specify a 2 mm distance in both horizontal and vertical directions. The falsecolor intensity scale in each PLIF image is scaled to its own maximum. The Nd:YAG laser pulses were incident from the left side of the images, and the burner was located 10 mm below the center of each image. The PLIF signals provide a rough measure of the local concentration of the OH radical, and, as expected from the kinetics results in Figure 1, the OH concentration exhibits a broad spatial distribution. The space-time features of the OH distribution can be

14 IFRF Combustion Journal Chen et al. compared with those previously reported for NH PLIF in both combustible and non-combustible NH 3 /O 2 conditions [20]. Figure 6 OH (A-X) planar laser-induced fluorescence (PLIF) images in (a) laser-induced ammonia / oxygen breakdown at 3, 5, 10, 15, 20, 30, 50 and 100 µsec following the optical breakdown, equivalence ratio of ammonia / oxygen = 0.1 and (b) laser-induced ammonia/oxygen ignition at 5, 10, 15, 20, 30, 50, 100 and 200 µsec following the optical breakdown, equivalence ratio = 1.0 Figure 7 shows the OH rotational temperature in the laser-induced breakdown and laser-induced ignition tests. These results were obtained using accumulated PLIF image data (10 shots in each wavelength and each delay time) and Equation 2. Figures 7 (a) and (b), respectively, show the time-resolved temperature maps of the non-combustible and combustible mixtures following laser-induced breakdown. The grid lines in each image in the figure mark a distance of 1 mm in

15 IFRF Combustion Journal Chen et al. both horizontal and vertical directions. It is seen that the measured temperature spans the range of approximately K. During the early stages of laser breakdown, the spontaneous emission from the decaying plasma kernel produces a significant background noise to OH LIF measurement. The spontaneous radiation rapidly decreases to the signal level of OH LIF at about 2 µsec after laser breakdown. Although the background image can be measured at the offresonant wavelength and subtracted from the OH PLIF image as described before, the uncertainty from the comparable noise leads to significant error in the quantitative analysis of temperature determination. As such this early stage of the laser-induced breakdown is ignored. Figure 7 shows the temperature maps of the earliest achievable times of 3 µsec for the noncombustible case, and a much longer time of 10 µsec for the combustible case. For the noncombustible case, the OH LIF signal level constantly decreases with the decay of plasma kernel after the peak intensity occurred at about 3-4 µsec post breakdown. At µsec post breakdown, the OH LIF signal has dropped an order in magnitude. Figure 7 (a) gives the temperature maps at 3, 5, 10, and 20 µsec post breakdown. For the combustible case at an equivalence ratio of 1.0, the OH LIF signal monotonically increases after the ignition delay time of about 10 µsec post breakdown due to the production of OH molecules through chemical reactions at the reaction zone near the kernel surface. The instability of the spatial distribution of OH LIF signal increases with flame front development. Shown in Figure 7 (b) are the temperature maps at 10, 15, 20, and 30 µsec for ignition.

16 IFRF Combustion Journal Chen et al. Figure 7 Time-resolved temperature maps of (a) laser-induced ammonia / oxygen breakdown (equivalence ratio = 0.1) and (b) laser-induced ammonia / oxygen ignition (equivalence ratio = 1.0) The black solid lines in each T-map indicate locations of 25% (outer contour) and 50% (inner contour) of the maximum intensity of OH LIF in each map. The OH LIF signal level and the location in ignition kernel strongly determine the uncertainty of calculated temperature. The

17 IFRF Combustion Journal Chen et al. source of temperature uncertainty was carefully evaluated by considering factors such as the fluctuation of the deposited energy in the laser plasma, the energy fluctuation of the probe laser, the deviation in mixture condition in the premixed gas flow, and the systematic error of the detection system. On the basis of detector counts and the result of the investigation of repeatability, it is estimated that imprecision in the values of T range from four percent to fifteen percent in the spatial regions of higher and lower OH concentrations, respectively. However, of particular interest is the ability of the two-line PLIF data to provide detailed temperature features for the transient events with a sensitivity of approximately 50K. The description of the spatial structures of laser breakdown/ignition and their temporal development, which is not the emphasis of this paper, is given in Ref. [20]. CONCLUSION Both steady and transient ammonia/oxygen flames were studied using OH PLIF measurements. Thermocouple measurements of the steady flame were used to obtain collisional quenching corrections for the two-line OH temperature PLIF results. The quenching results were extended to higher temperature regions of the flame using ChemKin computations and were applied to the transient laser-induced ignition of ammonia/oxygen mixtures. The temperature maps of laser ignition acquired from the experiment are presented for the first time. Comparisons with previous experimental and computational studies showed that NH and OH PLIF data span the important spatial regions of the laser-induced ignition of these mixtures with adequate sensitivity to characterize the time-dependent phenomena.

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