CH Planar Laser-induced Fluorescence Images Research Articles

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.

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.

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.

On Flame-Edge Propagation

Contemporary interest exists for understanding how reaction zones stabilize and counter-propagate against incoming reactants. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. Observations of the leading-edge flame behavior with respect to upstream propagation and recession downstream are made with sequential CH-PLIF imaging, and data have been revisited in light of the recent research of McCraw et al. (Flow Turbul Combust 70(1):83–97, 2007). It is found that in cases where a distinct branch of the outer (fuel-lean) edge of the reaction zone is present, the edge of the flame is either witnessed to propagate upstream or locally disappear. In cases where no distinct branch other than the main branch is observed, the flame is witnessed to either remain stationary or drop back downstream. These observations support the notion that structures in the low speed, outer edge of the reaction zone are involved in the upstream phase of the flame propagation.

UPSTREAM ISLANDS OF FLAME IN LIFTED-JET PARTIALLY PREMIXED COMBUSTION

Contemporary interest exists in understanding the roles of leading edge flow deflection, secondary jet instabilities and islands of ignited gases in permitting lifted flames to stabilize. To assess these issues, elements of the leading-edge of a lifted turbulent jet flame have been investigated using laser-imaging techniques. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. In particular, evidence of flame islands, or flame fragments, upstream of the bulk-flame leading edge are reported and discussed. This evidence is presented in the form of sequential CH-PLIF images and well as CH-PLIF/Rayleigh scattering images. Images showing thermal characteristics of the regions surrounding the edge flame are also described.

Investigations of swirl flames in a gas turbine model combustor: I. Flow field, structures, temperature, and species distributions

A gas turbine model combustor for swirling CH 4/air diffusion flames at atmospheric pressure with good optical access for detailed laser measurements is discussed. Three flames with thermal powers between 7.6 and 34.9 kW and overall equivalence ratios between 0.55 and 0.75 were investigated. These behave differently with respect to combustion instabilities: Flame A burned stably, flame B exhibited pronounced thermoacoustic oscillations, and flame C, operated near the lean extinction limit, was subject to sudden liftoff with partial extinction and reanchoring. One aim of the studies was a detailed experimental characterization of flame behavior to better understand the underlying physical and chemical processes leading to instabilities. The second goal of the work was the establishment of a comprehensive database that can be used for validation and improvement of numerical combustion models. The flow field was measured by laser Doppler velocimetry, the flame structures were visualized by planar laser-induced fluorescence (PLIF) of OH and CH radicals, and the major species concentrations, temperature, and mixture fraction were determined by laser Raman scattering. The flow fields of the three flames were quite similar, with high velocities in the region of the injected gases, a pronounced inner recirculation zone, and an outer recirculation zone with low velocities. The flames were not attached to the fuel nozzle and thus were partially premixed before ignition. The near field of the flames was characterized by fast mixing and considerable finite-rate chemistry effects. CH PLIF images revealed that the reaction zones were thin (⩽0.5 mm) and strongly corrugated and that the flame zones were short ( h ⩽ 50 mm ). Despite the similar flow fields of the three flames, the oscillating flame B was flatter and opened more widely than the others. In the current article, the flow field, structures, and mean and rms values of the temperature, mixture fraction, and species concentrations are discussed. Turbulence intensities, mixing, heat release, and reaction progress are addressed. In a second article, the turbulence–chemistry interactions in the three flames are treated.

00/01530 Gasification combined-cycle power generation equipment

A gas turbine model combustor for swirling CH4/air diffusion flames at atmospheric pressure with good optical access for detailed laser measurements is discussed. Three flames with thermal powers between 7.6 and 34.9 kW and overall equivalence ratios between 0.55 and 0.75 were investigated. These behave differently with respect to combustion instabilities: Flame A burned stably, flame B exhibited pronounced thermoacoustic oscillations, and flame C, operated near the lean extinction limit, was subject to sudden liftoff with partial extinction and reanchoring. One aim of the studies was a detailed experimental characterization of flame behavior to better understand the underlying physical and chemical processes leading to instabilities. The second goal of the work was the establishment of a comprehensive database that can be used for validation and improvement of numerical combustion models. The flow field was measured by laser Doppler velocimetry, the flame structures were visualized by planar laser-induced fluorescence (PLIF) of OH and CH radicals, and the major species concentrations, temperature, and mixture fraction were determined by laser Raman scattering. The flow fields of the three flames were quite similar, with high velocities in the region of the injected gases, a pronounced inner recirculation zone, and an outer recirculation zone with low velocities. The flames were not attached to the fuel nozzle and thus were partially premixed before ignition. The near field of the flames was characterized by fast mixing and considerable finite-rate chemistry effects. CH PLIF images revealed that the reaction zones were thin (⩽0.5 mm) and strongly corrugated and that the flame zones were short (h⩽50 mm). Despite the similar flow fields of the three flames, the oscillating flame B was flatter and opened more widely than the others. In the current article, the flow field, structures, and mean and rms values of the temperature, mixture fraction, and species concentrations are discussed. Turbulence intensities, mixing, heat release, and reaction progress are addressed. In a second article, the turbulence–chemistry interactions in the three flames are treated.

Observations on the leading edge in lifted flame stabilization

Observations on the leading edge in lifted flame stabilization

On the Adequacy of Certain Experimental Observables as Measurements of Flame Burning Rate

This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N 2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO ∗ 2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C ∗ 2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C ∗ 2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO ∗ 2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C (1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.

The influence of heterogeneous kinetics and thermal radiation on the oxidation of graphite particles

This work presents detailed chemical kinetic computations and experimental measurements of a premixed stoichiometric N2-diluted methane-air flame in two-dimensional unsteady vortical flow, which are used to investigate the utility of several experimental observables as measurements of local burning and heat release rates. The computed mole fraction of HCO is found to have excellent correlation with flame heat release rate over the whole range of unsteady curvature and strain-rate investigated, for the flame under consideration. HCO planar laser induced fluorescence (PLIF) imaging is discussed and demonstrated in a V-flame experiment. On the other hand, we find the utility of peak dilatation rate as an indicator of heat release rate to be dependent on the unsteady strain-rate and flame curvature environment, and the associated modification in diffusional thermal fluxes within the flame. The integrated dilatation rate is found to be more robust under unsteady strain-rate, but still questionable in regions of high flame curvature. We also study the utility of a particular formulation for CO∗2 chemiluminescence, OH, and CH PLIF imaging, as well as OH∗, C∗2, and CH∗ chemiluminescence, as measurements of flame burning and heat release rates. We generally find these measures to be inferior to HCO. Experimental results suggest that CH, OH∗, C∗2, and CH∗ are not adequate indicators of local extinction; rather they provide signals of subtle shifts of hydrocarbon consumption among different chemical pathways. Moreover, numerical results suggest that both OH mole fraction and an existing CO∗2 chemiluminescence model do not correlate with burning or heat release rate variations in regions of high unsteady flame curvature. The present numerical investigation uses a single flame/vortex condition and a specific 46-step C(1) chemical mechanism. The conclusions reached herein may be generalized with further studies using more detailed mechanisms over ranges of stoichiometry, dilution, and flow time and spatial scales.