High Repetition Rate Simultaneous CH/OH PLIF in Turbulent Jet Flame

Transcription

1 High Repetition Rate Simultaneous CH/OH PLIF in Turbulent Jet Flame Ayane Johchi 1*, Mamoru Tanahashi 1, Masayasu Shimura 1, Gyung-Min Choi 2, Toshio Miyauchi 1 1: Department of Mechanical and Aerospace Engineering, Tokyo Institute of Technology, Tokyo, Japan 2: School of Mechanical Engineering, College of Engineering, Pusan National University, Busan, Korea * correspondent author: ajohchi@navier.mes.titech.ac.jp Abstract High repetition rate simultaneous planar laser induced fluorescence (PLIF) of CH and OH radicals and stereoscopic particle imaging velocimetry (PIV), both at 10 khz, was developed. The laser system for high repetition rate PLIF consists of an Nd:YAG laser and two dye lasers. The fundamental output of the Nd:YAG laser at 1064 nm is converted to third and second harmonic wavelengths and pump the dye lasers. Laser pulses are emitted with 8 watts for 390 nm and 2 watts for 283 nm at 10 khz. The spatial resolution of the measurements are 60 µm 60 µm for CH and OH PLIF. The simultaneous CH and OH PLIF are combined with high repetition rate stereoscopic PIV. The spatial resolution for stereoscopic PIV is 900 µm 900 µm. The high repetition simultaneous system is applied to methane-air turbulent jet premixed flame of Re λ = 93.4, 173.1, and to show the performance of developed system. The obtained simultaneous CH and OH PLIF images well represent flame dynamics in turbulent jet premixed flame. Simultaneously obtained three-component velocity vectors also represents relation between fluid velocity and flame dynamics. The importance of simultaneous PLIF of CH and OH is also shown in the measurement in high speed and high Reynolds number conditions. The most important advantage of the developed high repetition simultaneous measurement is that these kinds of measurement enable us to do temporal analysis. From the temporal analyses, the injection frequency of the large-scale unburnt mixture island is clarified. The creation of small scale unburnt mixture islands from the large scale ones, which may contribute the enhancement of turbulent burning velocity, is also presented. For general understanding of flame structure, Proper Orthogonal Decomposition (POD) analysis has been applied to the OH PLIF images. It is revealed that dominant structure varied significantly depends on the mean velocity and the measurement location. 1. Introduction Laser diagnostics combining planar laser induced fluorescence (PLIF) of molecules or radicals produced in chemical reactions and particle image velocimetry (PIV) have been applied for the investigations of turbulent flame structures (Kalt et al. 1998; Kothnur et al. 2002; Tanahashi et al. 2005; Boxx et al. 2009a; Sadanandan et al. 2008). Tanahashi et al. (2005) conducted simultaneous measurement of stereoscopic PIV and OH-CH PLIF to investigate complicated flame structure in high Reynolds number turbulent premixed flames. To investigate three-dimensional flame structure, simultaneous dual-plane CH PLIF/single-plane OH PLIF and dual-plane stereoscopic PIV (Shimura et al. 2011) was developed and flame front characteristics such as flame curvature and strain rate at the flame front have been discussed. Although these simultaneous measurements have clarified the instantaneous flame structure, the repetition rate is not enough high to investigate flame dynamics in turbulent combustion field. Multi-shot PILF measurements with short time separation have been used previously (Kaminski et al. 1999; Hult et al. 2005, Filatyev et al. 2007, Tanahashi et al. 2008a). Simultaneous double-pulsed CH PLIF and OH PLIF measurement have been realized to investigate local flame displacement speed in turbulent premixed flame by Tanahashi et al. (2008a). These measurements clarify instantaneous flame dynamics. However, their duration time is not enough high to investigate temporal development of turbulent flame. Detailed information on the flame dynamics is very important to develop highly-accurate turbulent combustion models

2 Especially, to validate large eddy simulation (LES), experimental information with multidimensions and multi-variables on the unsteady characteristics of turbulent flame will give great impacts on the development of sub-grid scale combustion models. Recently, high repetition rate measurements of flame structure and flow field have been developed. Cundy et al. (2009) performed OH PLIF at khz frame rates using frequency-quadrupled Nd:YLF laser. By Nd:YAG-pumped dye laser systems, Juddoo et al. (2011) measured OH fluorescence at a repetition rate of 5 khz. High repetition rate simultaneous measurements of OH PLIF and PIV have been developed and applied to several types of combustors (Böhm et al. 2009; Boxx et al 2009b, 2010; Steinsberg et al. 2011). As is discussed by Carter et al. (1998), OH PLIF is not always enough to investigate flame structure in high Reynolds number turbulence where flame front is highly distorted by turbulence and quite complicated flame structures will be observed (Nada et al. 2004; Shim et al. 2011). Although the layer with high reaction rate can be estimated by distribution of CH radical in hydrocarbon flames, the technique has not been developed enough. Jiang et al. (2011) have constructed high repetition rate CH PLIF system using a combination of a custom burst laser and an optical parametric oscillator (OPO) and applied to non-premixed flame. However, since the laser power in their measurement system is not so high, applicability of their system to turbulent premixed flame is ambiguous. To clarify the interaction between local flame structure and turbulent flow, the simultaneous measurement of CH and OH radicals is important. In this study, high repetition rate simultaneous CH/OH PLIF and stereoscopic PIV is developed to investigate flame dynamics in turbulent combustion. The high repetition rate CH/OH PLIF and stereoscopic PIV is applied to methane-air turbulent jet premixed flame and unsteady phenomenon of flame front are clarified. 2. Experimental method 2.1. High repetition rate CH/OH PLIF The schematic diagram of the experimental setup for the high repetition rate simultaneous CH/OH PLIF and stereoscopic PIV measurement is shown in Fig. 1. For high repetition rate CH PLIF measurement, the Q 1 (7,5) transition of the B 2 Σ -X 2 Π R(0,0) band at nm is excited, and fluorescence from the A-X (1,1), (0,0), and B-X (0,1) bands between 420 and 440 nm is detected. For OH PLIF, the Q 1 (7) transition of the A 2 Σ-X 2 Π(1,0) band at nm is excited and fluorescence from the A-X(1,1), (0,0) and B-X(0,1) bands between 306 and 320 nm is detected. The laser system consists of an Nd:YAG laser (Edgewave, a special version of HD-40-III made for Tokyo Tech) and two dye lasers (Sirah, Credo) which is specialized for high repetition pump with a blend dye of Exalite389 and Exalite398 for CH PLIF and with Rhodamine6G for OH PLIF. Originally, HD series from Edgewave is designed as to generate 532 nm or 355 nm laser beam separately. For this study, the version is made as to generate two laser beams with different wavelength simultaneously at 10 khz repetition rate with extremely high power. The fundamental output of the Nd:YAG laser is converted to third (6 mj/pulse) and second (5 mj/pulse) harmonic wavelengths and pumps the dye lasers. Laser pulses are emitted with about 8 W for 390 nm and 2 W for 283 nm at 10 khz. After two laser beams are lead to the same axis by a dichroic mirror, the beams are expanded by sheet forming optics. The collection devices are located perpendicular to the laser sheet. The fluorescence of CH radical is collected by 100 mm f/2.0 lens (Carl Zeiss, Makro Planar T*2/100 ZF) with the band pass filter (Semrock, FF01-434/17-25) and imaged onto an image intensifier (Hamamatsu photonics, C F). The amplified images are detected by a high speed camera (Photoron, SA-X, at 10 khz). The fluorescence of OH radical is collected by 105 mm f/4.5 UV lens (Nikon, UV-Nikkor) with the band pass filter (Semrock, FF01-320/40-25) and imaged onto an image intensifier (Hamamatsu photonics, C6534). The amplified images are detected by a high speed camera (Photoron, SA5, at 10 khz)

3 Fig. 1 Schematic diagram of the high repetition rate simultaneous CH/OH PLIF and stereoscopic PIV measurement High repetition rate stereoscopic PIV The laser system for high repetition rate stereoscopic PIV consists of two high repetition rate Nd:YAG lasers (Lee Laser, LDP-100MQG). The maximum power of these lasers is 50 W at 10 khz. Laser beams from two lasers become double-pulsed beams through the laser beam combining optics which consist of a half-wave plate (CVI-Laser) and a polarizer (CVI-Laser). The laser sheets expanded by laser sheet forming optics are adjusted to those for CH/OH PLIF. Details of the timeresolved stereoscopic PIV can be found in the previous study (Tanahashi et al. 2008b) except for the high speed cameras to particle images. The high speed cameras (Photoron, SA-5, at 20 khz) are located with about ±20.0º to capture stereoscopic particle images, and Scheimflug condition (Prasad et al. 1995) is applied. Band-pass filters for 532 nm (Shonan Optical Thin Film Laboratories, 532 nm) are installed to the cameras. The particle is SiO 2 and its size is about 1 µm. In this study, a high spatial resolution PIV algorithm developed in our previous studies (Tanahashi et al. 2002; Tanahashi et al. 2008b) is used to calculate the two-dimensional velocity field from successive particle images obtained by each high speed camera. To ensure the accuracy of the PIV measurement, the eliminate scheme of spurious vectors and noise is established by a PIV simulation based on DNS of particle-laden homogeneous isotropic turbulence, and gives high correlation (about 98%) with DNS of turbulence (Tanahashi et al. 2008b). The eliminate scheme is mainly used for cut off of the high wave number noise is introduced by overlap of the interrogation regions. From two-dimensional velocity fields obtained by each high speed camera, three components of fluid velocity are calculated by using a geometrical relation (Arroyo et al. 1991) Timing control for high repetition rate simultaneous CH/OH PLIF and stereoscopic PIV measurement Timing control of the present simultaneous measurement is conducted by three pulse/delay generators (Stanford Research Systems, DG535 and DG645; LabSmith, LC880) and delay system of the image intensifiers and high speed cameras. The gate time of the CH and OH PLIF are - 3 -

4 Table 1. Turbulent characteristics at central position of measurement region. u 0 [m/s] x/d Re D Reλ u rms [mm] l [mm] λ [mm] η [mm] u rms /S L l/δ F case case case case Surrounding(CH4+Air) Main Nozzle(CH4+Air) 10m/s 20m/s Fig. 2 Schemaric of a turbulent jet burner (a) and a CH chemiluminescence image and OH fluorescence image of turbulent jet premixed flame (b). set to about 200 ns. The time separation between first and second frames of PIV are determined by turbulent characteristics (about 9 µs). 3. Experimental apparatus and conditions This simultaneous measurement is applied to methane-air turbulent jet premixed flames shown in Fig. 2(a). This burner has a main jet nozzle and a surrounding nozzle for flame holding. The inner diameter of the main (D) and the surrounding nozzles are 10 mm and 60 mm, respectively. In this study, high repetition rate simultaneous CH/OH PLIF is conducted for three different jet velocity conditions: U 0 = 10, 15 and 20 m/s. Equivalence ratio is fixed to 1.0 for the main flame and 0.86 for the surrounding flame. Figure 2(b) shows direct photo, CH chemiluminescence and OH fluorescence images of the turbulent jet premixed flame. Measurements are conducted at axial distance of x/d = 5, 7, 8 and 10, for each conditions. Here, x is distance from the jet exit. The measurement heights are indicated with gray dotted lines in Fig. 2(b). Table 1 shows experimental conditions and turbulence characteristics of inert flow at the center of jet nozzle. The characteristics were measured by a hotwire constant temperature anemometer with X-probe (Kanomax Japan, Model0250R, tungsten Φ = 5 µm) preliminarily. Re D is Reynolds number based on the nozzle diameter (D) and mean axial velocity at the jet exit, l is the integral length scale, Re l is the Reynolds number based on l and u rms, Re λ is the Reynolds number based on Taylor micro scale λ and u rms, η is Kolmogorov length scale, δ F is a nominal flame thickness and S L is the laminar burning velocity. With increase of inlet velocity, turbulent intensity and Reynolds number at the center of the measurement region increases. These conditions are classified into the corrugated flamelets in the turbulent combustion diagram by Peters (2000). Measurement regions are set to 62.7 mm 52.7 mm for CH PLIF and 55.5 mm 44.6 mm for OH PLIF, respectively. Therefore, spatial resolution of CH and OH PLIF is about 60 µm/pixels. For - 4 -

5 (a) t = 0.0 ms t = 0.2 ms t = 0.4 ms t = 0.6 ms t = 0.8 ms (b) (c) = 4.00 [m/s] Fig.3 Example of sequential CH PLIF images (a), OH PLIF images (b) and fluctuating velocity vector map images (c) at x/d = 5 for U o = 10 m/s. stereoscopic PIV, the camera resolution was determined to be pixels for 13.5 mm 16 mm regions, which means that the spatial resolution of PIV is 900 µm for pixels interrogation region. Note that the velocity vectors evaluated with 50% overlap. In this sense, velocity vectors are obtained every 450 µm. The intensity profiles of laser sheets in the perspective direction at the center of the measurement region are measured by a beam profiler (Ophir, FX-50). The FWHM thicknesses of laser sheets are set to about 400 µm for CH/OH PLIF, about 800 µm and 500 µm for first and second lasers for stereoscopic PIV. 4. Flame dynamics in turbulent jet premixed flame The simultaneous high repetition rate CH/OH PLIF measurements in the different conditions give insights on various dynamics of turbulent premixed flames. Figure 3 shows an example of 5 sequential distributions of CH and OH radicals and fluctuation velocity for the case with U o = 10 m/s at x/d = 5. Vectors and grayscale distribution on the back shows u, v and w, respectively. Mean velocity distribution is obtained from 1,000 samples and subtracted from instantaneous velocity. The largest vector corresponds to 4.0 m/s. Time interval of each images is 200 µs since every two images are illustrated here, corresponding to a 5 khz acquisition rate. Illustrated area is about 14.8 mm 13.0 mm. Simultaneous measurement of PLIF and PIV allows identifying the effect of flow field on flame structure. In addition, high repetition rate acquisition enables us to understand sequential movement. For instance, CH and OH PLIF images show that burnt gas on the bottom at t = 0.2 ms is glowing during the period. The growth is caused not only by flame propagation, but also by convection from left to right side, which is indicated by velocity distribution

6 16th Int Symp on Applications of Laser Techniques to Fluid Mechanics (a) t = 0.0 ms t = 0.1 ms t = 0.2 ms t = 0.3 ms t = 0.4 ms (b) A (c) = 3.00 [m/s] Fig.4 Example of sequential CH PLIF images (a), OH PLIF images (b) and fluctuating velocity vector map images (c) at x/d = 7 for Uo = 15 m/s. (a) t = 0.0 ms t = 0.1 ms t = 0.2 ms t = 0.3 ms t = 0.4 ms C (b) B (c) = 4.00 [m/s] Fig.5 Example of sequential CH PLIF images (a), OH PLIF images (b) and fluctuating velocity vector map images (c) at x/d = 8 for Uo = 20 m/s. -6-

7 10 5 P f [Hz] Fig.6 Spectrum of OH fluorescence intensity. (a) t = 0.0 ms t = 0.1 ms t = 0.2 ms t = 0.3 ms t = 0.4 ms (b) Fig.7 Example of sequential CH PLIF images (a) and OH PLIF images (b) at x/d = 10 for U o = 20 m/s. In Figs. 4 and 5, representative sequential images are shown for the case with U o = 15 m/s at x/d = 7 and U o = 20 m/s at x/d = 8, respectively. Time interval is 100 µs, which corresponds to 10 khz acquisition rate. Even if obtained fluorescence distributions are not sequential, flame structure can be discussed from an independent image. For instance, the flame structure indicated by circle A in Fig. 4 could be considered that burnt gas is engulfed by unburnt mixture or that unburnt mixture is being divided by burnt gas. However, sequential data allows understanding that the unburnt mixture is split by burnt gas since burnt area increases with temporal development. In the period, there is strong flow structure in a direction from merged burnt region toward right side. The flow makes thin unburnt layer split during the period. The importance of simultaneous PLIF of CH radicals and OH radicals is emphasized in the measurement in high speed and high Reynolds number conditions. In the region B in Fig. 5, the distributions of CH radical shows that flame fronts are divided completely in downstream region, although the unburnt gas region which are estimated from OH radical distribution are still in the process of split-off. In the region C, the fluorescence intensity of CH radical is getting high in a large area, whereas the intensity of OH radical does not shows high value, which means that flame - 7 -

8 16th Int Symp on Applications of Laser Techniques to Fluid Mechanics (a) (b) (c) (d) Fig. 8 POD Eigenmodes for (a) U0 = 10 m/s at x/d = 5, (b) U0 = 15 m/s at x/d = 7, (c) U0 = 20 m/s at x/d = 8, (d) U0 = 20 m/s at x/d = 10. front propagates from normal direction of the measurement plane. The region in PIV image also shows strong 3D motion. Another advantage of the high repetition PLIF is temporal analysis of the flame dynamics. At x/d = 8 for Uo = 20 m/s, large scale islands of unburnt mixture is ejected in the downstream. Figure 6 shows a result of spectral analysis of OH fluorescence intensity at x/d = 8. The spectrum has a distinct peak at 1.92 khz. This peak represents the passing frequency of unburnt mixture island, which is shown in Fig. 5. This kind of information on the flame dynamics may contribute to modeling of turbulent premixed flame such as Large Eddy Simulation. In further downstream for U0 = 20 m/s, complicated flame structure is observed. Figure 7 shows sequential CH and OH PLIF images at x/d = 10 for Uo = 20 m/s. In this region, the injected large scale unburnt mixture islands are suddenly split into small ones. These small scale flame balls are consumed very rapidly. This result suggests that the broken small scale unburnt mixture island or small scale flame balls contribute the enhancement of turbulent burning velocity. 5. POD analysis To better characterize the nature of flame structure, proper orthogonal decomposition (POD) analysis (Berkooz et al. 1993) is introduced in this study. The snapshot method (Sirovich 2009) of POD is used for a series of 1,000 OH PLIF images. Figure 8 shows obtained POD eigenmodes. Illustrated area is about 14.8 mm 13.0 mm. For all cases, the mean distribution of OH radical accounts for more than 68 %. For the following modes, flame structure is much different from each -8-

9 condition. In the relatively low Reynolds number case, U o = 10 m/s at x/d = 5, the mode 1 and mode 2 are symmetric each other and represent the flame structure which bend toward one side. Mean structure covers 70% of the intensity and modes 1 and 2 have about 5 % each. 90% of energy can be captured up to mode 8. In the case with U o = 15 m/s at x/d = 7, the lower modes show that unburnt mixture exists. As increasing the Reynolds number, for the case of U o = 20 m/s at x/d = 8, the burnt region is divided into two parts in modes 4 and 5. Further downstream region at x/d = 10, mode 1 and mode 2 show the divided unburnt mixture as expected from OH PLIF images. 6. Summary and conclusions In this study, high repetition rate simultaneous CH/OH PLIF and stereoscopic PIV at 10 khz were developed to investigated flame dynamics in turbulent premixed flame. The developed measurement system is applied for turbulent methane-air jet premixed flame and the performance of the system is validated. The unsteady phenomenon of turbulent jet premixed flame is also discussed from the high repetition experimental results. The developed measurement technique will contribute to investigations of various unsteady phenomena observed in applications of turbulent combustion. Acknowledgment This research is partially granted by the Japan Society for the Promotion of Science (JSPS) through the "Funding Program for Next Generation World-Leading Researchers (NEXT program)" (No. GR038) initiated by the Council for Science and Technology Policy (CSTP).. Reference Arroyo MP, Greated CA (1991) Stereoscopic particle velocimetry. Meas Sci Technol 2: Berkooz G, Holmes P, Lumly JL (1993) The proper orthogonal decomposition in the analysis of turbulent flows. Ann Rev Fluid Mech 25: Böhm B, Heeger C, Gordon RL, Dreizler A, (2011) Flow Turb Combust 86: Boxx I, Heeger C, Gordon R, Böhm B, Aigner M, Dreizler A, Meier W (2009a) Simultaneous three-component PIV/OH-PLIF measurements of a turbulent lifted, C3H8-argon jet diffusion flame at 1.5 khz repetition rate. Proc Combust Inst 32: Boxx I, Stöhr M, Cater C, Meier W (2009b) Sustained multi-khz flamefront and 3-component velocity-field measurements for the study of turbulent flames. Appl Phys B 95: Boxx I, Stöhr M, Cater C, Meier W (2010) Temporally resolved planar measurements of transient phenomena in a partially pre-mixed swirl flame in a gas turbine model combustor. Combust Flame 157: Carter CD, Donbar JM, Driscoll JF (1988) Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent nonpremixed flame. Appl Phys B 66: Cundy ME, Sick V (2009) Hydroxyl radical imaging at khz rates using a frequency-quadrupled Nd:YLF laser. Appl Phys 96: Filatyev SA, Thariyan MP, Lucht RP, Gore JP (2007) Simultaneous stereo particle image velocimetry and double-pulsed planar laser-induced fluorescence of turbulent premixed flames. Combust Flame 150: Hult J, Meier U, Meier W, Harvey A, Kaminiski CF (2005) Experimental analysis of local flame extinction in a turbulent jet diffusion flame by a high repetition 2-D laser techniques and multiscalar measurements. Proc Combust Inst 30: Jiang N, Patton RA, Lempert WA, Sutton JA (2011) Development of hogh-repetition rate CH PLIF - 9 -

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