CH Planar Laser-induced Fluorescence Research Articles

CH3 imaging via photo-fragmentation combined with CH planar laser-induced fluorescence employing C–X (0,0) band excitation and detection

In this paper we demonstrate single-shot imaging of the methyl (CH3) radical using a single laser system. The diagnostic approach combines CH3 photo-fragmentation followed by methylidyne (CH) detection. Specifically, the 5th-harmonic beam of a Nd:YAG laser (λPF = 212.8 nm) facilitates CH3 photo-fragmentation, while the frequency-doubled output of the same laser pumps a dye laser, the output of which is frequency-doubled and tuned to excite CH R-branch transitions within the C–X (0,0) band (λLIF ≈ 311 nm). We detect the resulting fluorescence from the C–X band (primarily the Q and P branches), suppressing laser scattering using a custom long-wave-pass filter. Transitions in the C–X (0,0) band are exceptionally strong, which is an enabling feature for the proposed diagnostic. Indeed, the PLIF excitation pulse energy (at 311 nm) was limited to ∼2 mJ, which is sufficient for high-quality measurements of the CH produced via photo-fragmentation. We illustrate the utility the approach in premixed laminar and turbulent flames (over a range of stoichiometry) and in a methane lifted jet diffusion flame. A noteworthy feature of the proposed diagnostic is that one can detect nascent CH (by blocking the 5th harmonic beam) and hydroxyl (OH) in its A–X (0,0) band by tuning the dye laser to a nearby OH transition. We also propose a simple methodology for acquiring simultaneous OH and CH3 images employing a single camera via the use of structured illumination and demonstrate joint OH + CH3 imaging in the lifted jet diffusion flame.

Flame front visualization in highly turbulent jet flames using CH3 photofragmentation laser-induced fluorescence

Flame front visualization using methyl photofragmentation laser-induced fluorescence (CH3-PF-LIF) was demonstrated in turbulent premixed methane/air flames. A pump–probe method was used to detect CH3, where CH3 was first photolyzed to CH (X2П) fragments using a pump laser (212.8 nm), and the fragments were subsequently excited to CH in the C2Σ+ state by a probe laser at 314.4 nm. By detecting fluorescence from CH (C-X), instantaneous two-dimensional flame front imaging with a high signal-to-noise (SNR) ratio was achieved in fuel-lean and turbulent flames. A laser excitation scan was conducted to ensure that the detection of CH3 did not interfere with the other emissions. Visualization using CH3-PF-LIF was compared with visualization with conventional CH planar laser-induced fluorescence (CH-PLIF). The SNR of CH3-PF-LIF imaging is almost ten times higher than those of CH-PLIF, and flame front visualization using CH3-PF-LIF shows more topological structures than CH-PLIF in fuel-lean and highly turbulent flames.

Temporally resolving premixed turbulent flame structures using self-supervised adversarial reconstruction of CH-PLIF

Understanding the turbulence-flame interaction is crucial to model the low-emission combustors developed for energy and propulsion applications. To this end, a novel frame interpolation (FI) method is proposed to better resolve the spatiotemporal evolution of premixed turbulent flame structures. The framework is completely self-supervised, agnostic to optical flow, and driven by leveraging transferrable feature knowledge at lower speeds and adversarial learning to statistically map the flame dynamics across frames. The method is successfully applied on a 10 kHz CH planar laser-induced fluorescence (PLIF) dataset of highly wrinkled premixed flames with turbulent Reynolds numbers (ReT) of 1100, 1400, and 7900, by down-sampling the image sequence to 5 kHz and restoring the sequence back to 10 kHz via FI. All reconstructions recovered important flame events and displayed excellent resemblance of the corrugated CH-layer geometries to that of the ground truths, with average intersection over union (IoU) and structural similarity index (SSIM) scores of 0.49 and 0.82, which are above the high-similarity baselines of 0.36 and 0.75, respectively. The wrinkling parameters (WP) of the flames also matched the ground truths, wherein R2 was roughly 0.95 for ReT = 1100 and 1400 and 0.85 for ReT = 7900 (lower due to the turbulence-induced uncertainties). The FI is further iteratively repeated to 40 kHz on the ReT = 7900 flames to facilitate pocket analysis by confidently linking their origin of formation, thus, enabling distinction from 3D tunnels, and improving statistical characterization of their consumption speeds. Given that the object features do not exhibit highly turbulent motions with regard to the initial time step, the proposed FI method is shown to be highly accurate and useful to analyzing finite-resolution experimental image sets including, but not restricted to, CH-PLIF.

Pump-probe strategy for instantaneous 2D detection of CH3 in flames using a single laser

Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals (CH3) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing CH3 and the probe laser detecting the photolyzed CH (X2Π) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a CH3 photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of CH3 can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.

CH and NO planar laser-induced fluorescence and Rayleigh-scattering in turbulent flames using a multimode optical parametric oscillator.

An optical parametric oscillator (OPO) is developed and characterized for the simultaneous generation of ultraviolet (UV) and near-UV nanosecond laser pulses for the single-shot Rayleigh scattering and planar laser-induced-fluorescence (PLIF) imaging of methylidyne (CH) and nitric oxide (NO) in turbulent flames. The OPO is pumped by a multichannel, 8-pulse Nd:YAG laser cluster that produces up to 225 mJ/pulse at 355 nm with pulse spacing of 100 µs. The pulsed OPO has a conversion efficiency of 9.6% to the signal wavelength of ∼430nm when pumped by the multimode laser. Second harmonic conversion of the signal, with 3.8% efficiency, is used for the electronic excitation of the A-X (1,0) band of NO at ∼215nm, while the residual signal at 430 nm is used for direct excitation of the A-X (0,0) band of the CH radical and elastic Rayleigh scattering. The section of the OPO signal wavelength for simultaneous CH and NO PLIF imaging is performed with consideration of the pulse energy, interference from the reactant and product species, and the fluorescence signal intensity. The excitation wavelengths of 430.7 nm and 215.35 nm are studied in a laminar, premixed CH4-H2-NH3-air flame. Single-shot CH and NO PLIF and Rayleigh scatter imaging is demonstrated in a turbulent CH4-H2-NH3 diffusion flame using a high-speed intensified CMOS camera. Analysis of the complementary Rayleigh scattering and CH and NO PLIF enables identification and quantification of the high-temperature flame layers, the combustion product zones, and the fuel-jet core. Considerations for extension to simultaneous, 10-kHz-rate acquisition are discussed.

Topological imaging of turbulent premixed, prevaporized liquid fuel jet flames using CH (C-X) band PLIF

New imaging capabilities for topology in prevaporized, liquid-fuel flames are presented with the application of CH-radical planar laser-induced fluorescence (PLIF) using the C2Σ+-X2Π (v′=0, v″=0) band. Imaging of turbulent flame structure for ethanol, n-heptane, n-dodecane, and kerosene (JP-8) fuels has been conducted in jet flames using a piloted McKenna burner with a central jet tube. This work presents the first application of CH PLIF for the study of full flame structure and curvature effects with prevaporized liquid fuels, particularly n-dodecane and kerosene. Liquid fuel structure is also compared to results with methane and ethylene. Results show that good signal-to-noise ratio (SNR) is achieved for all fuels, and the method is feasible in the presence of a dilute droplet field, despite the resonant nature of the technique. Reaction-zone structures clearly depict evidence of nonequidiffusive (Le ≠ 1) effects: strong variation in CH fluorescence is observed in regions of high curvature. Simultaneous excitation of the OH radical is also possible with prevaporized liquid fuels, for combined imaging of the reaction and heat-release zones. In kerosene flames, with aromatic species, fuel fluorescence may also be simultaneously captured, in addition to that from CH and OH, providing information on vapor localization.

Structure and burning velocity of turbulent premixed methane/air jet flames in thin-reaction zone and distributed reaction zone regimes

A series of turbulent premixed methane/air jet flames are studied using simultaneous planar laser diagnostic imaging of OH/CH/temperature and CH/OH/CH2O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH2O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH2O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor.

Simultaneous high speed PIV and CH PLIF using R-branch excitation in the C2Σ+-X2Π (0,0) band

Simultaneous particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) utilizing R-branch transitions in the C-X (0,0) band were performed at a 10-kHz repetition-rate in a turbulent premixed flame. The CH lines at 310.690 nm (from the R-branch of the C-X band) used here have greater efficiency than A-X and B-X transitions, which allows for high-framerate imaging with low laser pulse energy. Most importantly, the simultaneous imaging of both CH PLIF and PIV is enabled by the use of a custom edge filter, which blocks scattering at the laser wavelength (below ∼311 nm) while efficiently transmitting fluorescence at longer wavelengths. The Hi-Pilot Bunsen burner operated with a turbulent Reynolds number of 7900 was used to demonstrate simultaneous PIV and CH PLIF utilizing this filtered detection scheme. Instances where pockets of products were observed well upstream of the mean flame brush are found to be the result of out-of-plane motion of the flame sheet. Such instances can lead to ambiguous results when interpreting the thickness of reaction layers. However, the temporally resolved nature of the present diagnostics facilitate the identification and proper treatment of such situations. The strategy demonstrated here can yield important information in the study of turbulent flames by providing temporally resolved flame dynamics in terms of flame sheet visualization and velocity fields.

Visualization of instantaneous structure and dynamics of large-scale turbulent flames stabilized by a gliding arc discharge

A burner design with integrated electrodes was used to couple a gliding arc (GA) discharge to a high-power and large-scale turbulent flame for flame stabilization. Simultaneous OH and CH2O planar laser-induced fluorescence (PLIF) and CH PLIF measurements were conducted to visualize instantaneous structures of the GA-assisted flame. Six different regions of the GA-assisted flame were resolved by the multi-species PLIF measurements, including the plasma core, the discharge-induced OH region, the post-flame OH region, the flame front, the preheat CH2O region and the fresh gas mixture. Specifically, the OH profile was observed to be ring-shaped around the gliding arc discharge channel. The formaldehyde (CH2O) was found to be widely distributed in the entire measurement volume even at a low equivalence ratio of 0.4, which suggest that long-lived species from the gliding arc discharge have induced low-temperature oxidations of CH4. The CH layer coincides with the interface of the OH and CH2O regions and indicates that the flame front and the discharge channel are spatially separated by a distance of 3–5 mm. These results reveal that the discharge column acts as a movable pilot flame, providing active radicals and thermal energy to sustain the flame. High-speed video photography was also employed to record the dynamics of the GA-assisted flame. This temporally resolved data was used to study the ignition and propagation behaviors of the flame in response to a temporally modulated burst-mode discharge. The results indicate that turbulent flame can be sustained by matching temporal parameters of the high-voltage bursts to the extinction time of flame.

CH PLIF and PIV implementation using C-X (0,0) and intra-vibrational band filtered detection

This study demonstrates advancement in a low-pulse energy methylidyne (CH) planar laser-induced fluorescence (PLIF) method that facilitates its application alongside flows seeded for particle image velocimetry (PIV) or other particle scattering based methods, as well as in high scattering environments. The C-X (0,0) R-branch excitation and filtered detection are carefully selected such that the laser line frequency is heavily attenuated by an edge filter while allowing transmission of most of the (0,0) band fluorescence. There are strong OH A-X (0,0) lines in the vicinity, but they can be avoided or utilized through dye laser tuning. As a demonstration of efficacy, PIV is performed simultaneously with the PLIF imaging. Using the edge filter, particle scattering signal is reduced to sub-fluorescence levels, allowing for flame-front analysis. This achievement enables flame-front tracking at high repetition rates (due to the low-pulse energy required) in combination with a scattering method such as PIV or use in high scattering environments such as enclosed combustors or near burner surfaces.

Multi-species PLIF study of the structures of turbulent premixed methane/air jet flames in the flamelet and thin-reaction zones regimes

Simultaneously planar laser-induced fluorescence (PLIF) measurements of OH, CH, CH2O and toluene are carried out to investigate the structures of turbulent premixed methane/air jet flames in the flamelet regime and the thin-reaction zones regime. A premixed flame jet burner of an inner diameter of 1.5mm is employed. Stoichiometric methane/air mixtures introduced as a jet are ignited and stabilized in a hot co-flow generated by a coaxial porous plug pilot flame surrounding the jet. The Reynolds number for the studied jet ranges from 960 to 11,500 with the characteristic Karlovitz number ranging from 1 to 60. The focus of this study is on the characterization of the structures and turbulent burning velocity of premixed flames in the flamelet and the thin-reaction zones regimes. The preheat zone is analyzed using the CH2O and toluene PLIF fields, whereas the reaction zone is analyzed using the CH and OH PLIF fields. Laser Doppler Anemometer (LDA) measurements are performed to characterize the turbulence field and it is noted that when the Reynolds/Karlovitz number increases a successive thickening of the preheat zone is observed, whereas the reaction zone, characterized by the CH layer maintains nearly the same thickness. The heat release zone, characterized by the combination of the OH and CH2O PLIF fields, is shown to nearly maintain the same thickness under the present experimental conditions. The flame surface wrinkle ratio is shown to be Reynolds number and Karlovitz number independent when the Reynolds number is high enough such that the smallest wrinkle scales reach to the length scales of the thin reaction layers. The global fuel consumption speed of the jet flame is analyzed using the toluene PLIF field and the OH PLIF field. A discrepancy in the two consumption velocities is found as the Karlovitz number increases. This is found to be a result of the broadening of the oxidation zone. These findings provide experimental support to the flamelet and thin-reaction zone regime hypotheses of turbulent premixed combustion.

1027 10kHz CH-OH PLIF・ステレオPIV同時計測による乱流予混合火炎の局所燃焼速度に関する研究

Simultaneous CH-OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) measurements at 10 kHz have been conducted in a methane-air turbulent jet premixed flame. Around the flame tip in the high Reynolds number condition, the fine scale unburned mixture islands are ejected downstream. The enhancement of the turbulent burning velocity is statistically analyzed by the local consumption rate, which is estimated from the OH and CH PLIF images with the assumption of the spherical and pillar shapes of the unburned mixture. The most probable consumption rates with different methods give significantly higher burning velocity than the laminar.

Investigation on rapid consumption of fine scale unburned mixture islands in turbulent flame via 10 kHz simultaneous CH–OH PLIF and SPIV

A system based on high repetition rate simultaneous CH–OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) measurement has been developed. The high-speed simultaneous measurement system has an acquisition rate of 10kHz and its measurement duration time exceeds 1.0s. The developed system is applied to a methane–air turbulent jet premixed flame in order to investigate the flame and flow dynamics of the turbulent combustion field. The obtained simultaneous CH and OH PLIF images well represent the turbulent flame dynamics in shear flow. Under highly turbulent conditions, the number of flame wrinkles increases and the flame front shows a very complex geometry. Since large scale vortical structures develop due to Kelvin–Helmholtz instability, large scale unburned mixture islands are often created and ejected into the downstream. Around the flame tip under the condition of high Reynolds number (U0=20m/s, Reλ=257), the formation of fine scale unburned mixture islands is observed and its contribution to enhance the turbulent burning velocity is investigated by statistically analyzing the local consumption rate. The consumption rate is estimated from the OH and CH PLIF images on the basis of the spherical and pillar assumptions. The most probable consumption rates obtained from all adopted methods are much higher than the laminar burning velocity (0.39m/s). The importance of a high repetition rate CH acquisition as a flame marker is also discussed.

Simultaneous visualization of OH, CH, CH2O and toluene PLIF in a methane jet flame with varying degrees of turbulence

This paper presents simultaneous, single shot planar laser-induced fluorescence (PLIF) imaging of four species: OH, CH, CH2O (formaldehyde) and toluene (C6H5CH3) in methane/air jet flames. The jet flames were stabilized by a flat pilot flame generated using a McKenna type burner. Both flames were operated with a stoichiometric premixed methane/air mixture at room temperature and atmospheric pressure. Several flames, with varying jet flow speeds, were investigated, spanning from laminar (10m/s jet exit velocity) up to highly turbulent flame conditions, with high Karlovitz numbers, (150m/s jet exit velocity). Measuring the four species presented above provides detailed information on jet flame structures including the transition from fuel (indicated by the fuel tracer toluene) via the preheat zone (indicated by CH2O) and the inner layer of the flame front (indicated by CH) to the oxidation layer and the postflame zone (both indicated by OH). Furthermore, the simultaneously recorded PLIF images enable the study of correlations between these key species. Especially, overlapping regions between the species in the flames is of interest. The result indicates that turbulence in the present jet flames affects primarily the mixing in the preheat zone and the wrinkling of the reaction layers. It does however not significantly affect the inner flame front structures, represented by the CH radicals, as the thickness of the CH layer remains fairly constant under the investigated flame conditions.

Characterization of a CH planar laser-induced fluorescence imaging system using a kHz-rate multimode-pumped optical parametric oscillator

The performance characteristics of a new CH planar laser-induced fluorescence (PLIF) imaging system composed of a kHz-rate multimode-pumped optical parametric oscillator (OPO) and high-speed intensified CMOS camera are investigated in laminar and turbulent CH4-H2-air flames. A multi-channel Nd:YAG cluster that produces up to 225 mJ at 355 nm with multiple-pulse spacing of 100 μs (corresponding to 10 kHz) is used to pump an OPO to produce up to 6 mJ at 431 nm for direct excitation of the A-X (0, 0) band of the CH radical. Single-shot signal-to-noise ratios of 82:1 and 7.5:1 are recorded in laminar premixed flames relative to noise in the background and within the flame layer, respectively. The spatial resolution and image quality are sufficient to accurately measure the CH layer thickness of ~0.4 mm while imaging the detailed evolution of turbulent flame structures over a 20 mm span. Background interferences due to polycyclic-aromatic hydrocarbons and Rayleigh scattering are minimized and, along with signal linearity, allow semi-quantitative analysis of CH signals on a shot-to-shot basis. The effects of design features, such as cavity finesse and passive injection seeding, on conversion efficiency, stability, and linewidth of the OPO output are also discussed.

High-speed CH planar laser-induced fluorescence imaging using a multimode-pumped optical parametric oscillator

We report on high-speed CH planar laser-induced fluorescence (PLIF) imaging in turbulent diffusion flames using a multimode-pumped optical parametric oscillator (OPO). The OPO is pumped by the third-harmonic output of a multimode Nd:YAG cluster for direct signal excitation in the A-X (0,0) band of the CH radical. The lasing threshold, conversion efficiency, and linewidth are shown to depend on the number of pump passes in the ring cavity of the OPO. Single-shot CH PLIF images are acquired at 10 kHz with excitation energy up to 6 mJ/pulse at 431.1 nm. Signal-to-noise ratios of ~25-35 are the highest yet reported for high-speed CH PLIF.

Visualization of Turbulent Premixed Flames by Multi-Dimensional/Multi-Variable Laser Diagnostics

Detailed understanding of turbulent combustion phenomena is very important to develop high efficiency combustors which contribute to resolve global environmental issues. Since turbulent combustion involves strong interactions between turbulence and combustion reactions, highly-sophisticated measurement techniques both for fluid motion and flames are required. For fluid velocity, state-in-art dual-plane stereoscopic particle image velocimetry (PIV) is shown and its importance in turbulent combustion researches is discussed. For flame measurements, advanced planar laser induced fluorescence (PLIF) techniques such as simultaneous CH-OH PLIF, double-pulsed CH PLIF and simultaneous dual-plane CH/single-plane OH PLIF are presented and investigations on local flame structure, global/local flame dynamics and three-dimensional flame structures are surveyed. Finally, current state and challenges of multi-dimensional/multi-variables laser diagnostics which consist of the advanced PLIF and color-based dual-plane stereoscopic PIV are presented.

Development of high-repetition rate CH PLIF imaging in turbulent nonpremixed flames

We report on the development of high-repetition rate planar laser-induced fluorescence (PLIF) imaging of the CH radical in turbulent flames. This paper presents what the authors believe to be the first multi-frame, high-speed CH PLIF image sequences captured in a turbulent nonpremixed jet-flame. The high-repetition rate CH PLIF measurements were made using a combination of a custom pulse burst laser system operating at a 10-kHz repetition rate, an in-house optical parametric oscillator (OPO) with frequency mixing for 390-nm laser pulse generation, and a high-framing rate ICCD camera for high-speed image capture. The 1064-nm output of the pulse burst laser is frequency-tripled (355nm) and used to pump the OPO, which can be operated in a narrow bandwidth (300MHz) or broadband (4cm−1) mode. The OPO system produces a burst of laser pulses near 615nm, which can be subsequently mixed with residual 1064-nm output from the pulse burst laser to generate a series of 390-nm pulses separated by less than 100μs. The tunable ultraviolet output around 390nm is then used to excite the B–X (0,0) band allowing high-speed image sequences of the CH radical to be acquired from the B–X(0,1), A–X(1,1), and A–X(0,0) bands near 430nm. In this paper, we present results using the narrow bandwidth mode of the laser. By injection seeding the OPO with a single-frequency diode laser, we can obtain spectrally-narrow output at 390-nm, which allows imaging of CH with only 0.4mJ/pulse of laser energy. Ten-kilohertz-CH PLIF image sequences from a turbulent nonpremixed flame are reported as an example of the potential of this diagnostic system. Although, the signal-to-noise ratio (SNR) is marginal, we discuss future improvements to the system to increase the SNR to levels comparable to that achieved with traditional low-repetition rate systems.

Simultaneous dual-plane CH PLIF, single-plane OH PLIF and dual-plane stereoscopic PIV measurements in methane-air turbulent premixed flames

Simultaneous dual-plane CH planar laser induced fluorescence (PLIF), single-plane OH PLIF and dual-plane stereoscopic particle image velocimetry (PIV) measurement has been developed to investigate three-dimensional structures of methane-air turbulent premixed flame. Dual-plane CH PLIF system consists of two independent conventional CH PLIF measurement systems and laser beams from each laser system are led to parallel optical pass using the difference of polarization, and CH PLIF is conducted in two parallel two-dimensional cross-sections. Dual-plane stereoscopic PIV was established by using the difference of wavelength. The dual-plane CH PLIF and stereoscopic PIV are combined with single-plane OH PLIF. The laser sheet for single-plane OH PLIF is located at the center of two planes for CH PLIF and stereoscopic PIV. The separation between each plane is set to about 170μm. The spatial resolution of PIV is set to 175μm (about three times Kolmogorov length scale)×175μm×210μm and is same as that of general direct numerical simulations (DNSs) of turbulence. This simultaneous measurement can provide three-dimensional flame front, three velocity components and nine velocity derivatives. The measurement is conducted in methane-air swirl-stabilized turbulent premixed flames of Reλ=63.1, 95.0 and 115.0. Various complicated three-dimensional flame structures such as the handgrip and spire structures, which have been shown by previous three-dimensional DNSs, are observed in high Reynolds number turbulent premixed flames. Magnitude of strain rate, which is evaluated from nine velocity derivatives, shows high value in unburnt region except for the region engulfed by burnt gas. These results show that the simultaneous dual-plane CH PLIF, single-plane OH PLIF and dual-plane stereoscopic PIV measurement is useful for investigating the characteristics of turbulent premixed flames.

Measurement of three-dimensional flame structure by combined laser diagnostics

To investigate three-dimensional flame structures of turbulent premixed flame experimentally, dual-plane planar laser induced fluorescence (PLIF) of CH radical has been developed. This dual-plane CH PLIF system consists of two independent conventional CH PLIF measurement systems and laser beam from each laser system are led to parallel optical pass using the difference of polarization, and CH PLIF is conducted in two parallel two-dimensional cross sections. The newly-developed dual-plane CH PLIF is combined with single-plane OH PLIF and stereoscopic particle image velocimetry (PIV) to clarify the relation between flame geometry and turbulence characteristics. The laser sheets for single-plane OH PLIF and stereoscopic PIV measurement are located at the center of two planes for CH PLIF. The separation between these two planes is selected to 500 μm. The measurement was conducted in relatively high Reynolds number methane-air turbulent jet premixed flame. The experimental results show that various three-dimensional flame structures such as the handgrip structure, which has been shown by three-dimensional direct numerical simulations (DNS), are included in high Reynolds number turbulent premixed flame. It was shown that the simultaneous measurement containing newly-developed dual-plane CH PLIF is useful for investigating the three-dimensional flame structures. To analyze the flame structures quantitatively, the flame curvature was estimated by using the CH and OH PLIF images, and the probability density function (pdf) of the curvatures was compared with the results of DNS. It was revealed that the minimum radius of curvature of the flame front coincides with Kolmogorov length. However, the feature of pdf of the flame curvature is slightly different from result of DNS, if the curvature was estimated from experimental results in two-dimensional cross section. On the other hand, the feature of pdf of mean curvature that calculated from triple-plane PLIF results is similar to that obtained from three-dimensional DNS.