CH2O Planar Laser-induced Fluorescence Research Articles

Strategy for instantaneous formaldehyde (CH2O) imaging by planar laser-induced fluorescence (PLIF) in a scramjet with intense flame emissions

Conducting CH2O PLIF imaging in scramjets is challenging due to strong flame emissions (FEs). The FE interference is comparable to the CH2O fluorescence in the wavelength range of the fluorescence. To deal with this challenge, we proposed a CH2O PLIF imaging strategy considering CH2O excitation at 353.06 nm and FE reduction based on capturing two frames with a reasonable time delay. The fluorescence efficiency with the 353.06 nm excitation was shown to be approximately four times that with the excitation of ∼355 nm in the linear regime, where the two wavelengths corresponded to full-width-at-half-maximum line widths of ∼0.16 and ∼1 cm−1, respectively. During the two frames being captured, the FE background remained nearly constant. Then collecting the CH2O fluorescence in the first frame and subtracting the second frame from the first substantially reduced FE interference. The signal-to-background ratio in the raw image was approximately 1–2, but it could be increased to 10 using the proposed strategy. This strategy enables CH2O PLIF imaging to be conducted in scramjets and scramjet-like combustion facilities with intense FE interference.

Experimental investigations on central vortex core in swirl spray flames using high-speed laser diagnostics

Centrally staged swirl combustion can effectively reduce NOx emission. However, the complex combustion field is susceptible to producing large-scale coherent structures, such as precessing vortex core and central vortex core (CVC). This study mainly investigates the effect of CVC on the flow and flame in a centrally staged swirl spray combustor at elevated temperature and pressure using 10 kHz high-speed CH* chemiluminescence (CL), 20 kHz particle image velocimetry, and CH2O planar laser-induced fluorescence (PLIF). For the pilot flame, both CH* CL and CH2O PLIF flame are fork-shaped with three long parts, and the middle parts of flame dynamics indicate CVC structure. For the stratified flame, the CVC structure exists in an extended strip area of strong vorticity near the centerline of the combustor. The analysis of proper orthogonal decomposition modes shows that the motion of CVC is mainly swing, followed by precessing. Simultaneous diagnostics indicates that the entrainment of CVC leads to CH2O transport from the shear layer to the central region of the combustor. In general, the CH2O signal is mainly distributed in two positive velocity regions, the pilot/main jet and around CVC. Taking advantage of the CVC effect on radical transportation is a potential method to improve the mixing of the combustor, such as temperature distribution.

Optimal approaches to formaldehyde planar laser-induced fluorescence

We assessed approaches to i) detecting formaldehyde (CH2O) via planar laser-induced fluorescence (PLIF) in atmospheric-pressure premixed flames, including those with pre-vaporized jet fuels, and ii) improving signal-to-noise ratio (SNR) of CH2O PLIF more generally. We compared excitation wavelengths and schemes – employing the third-harmonic output of a Nd:YAG laser vs. the frequency-doubled output of a dye laser. Specifically, we identified the optimal excitation line accessible via the frequency-doubled dye laser at 353.07 nm, which was ∼4× more efficient than tripled-Nd:YAG excitation, but considering that the tripled-Nd:YAG laser output is generally ∼10× more energetic, it will produce more CH2O-LIF signal. We then determined the optimal detection setup to include a 100-mm focal length (f/1.4) objective lens coupled to a standard achromatic lens, which provided ∼4× greater signal relative to our baseline lens configuration, and subsequently we demonstrated that a Gen III GaAsP intensifier produced remarkably high PLIF SNR (>15). Measurements in premixed turbulent flames using pre-vaporized kerosene fuels showed that interference from fuel-based LIF was largely absent in all cases and that high-quality CH2O PLIF was efficacious.

Simultaneous multiple laser beam intensity profile correction and its application to a vitiated bluff body combustor field.

Simultaneous high-speed stereo-particle image velocimetry, OH planar-laser-induced fluorescence (PLIF), and CH2O PLIF measurements in a vitiated bluff body combustor are considered. An ex situ, simultaneous, time-resolved laser sheet intensity profile correction procedure is introduced. This procedure is easily implemented experimentally and is capable of correcting multiple sheets at the same time. As a proof of concept, the procedure is applied to perform correction of the CH2O PLIF images in vitiated and unvitiated conditions. The challenges associated with CH2O PLIF under these combustor operating conditions are also discussed.

Near blowout dynamics of a premixed, swirl stabilized flame

This paper analyzes the flame dynamics of near LBO (lean blowoff) swirl stabilized flames, using simultaneous OH and CH2O (formaldehyde) PLIF (planar laser induced fluorescence) measurements. Prior studies have shown that recirculation stabilized flames approach blowoff through two distinct stages – “stage 1” characterized by local extinction, where the overall flame and flow field remain largely unchanged, and “stage 2”, characterized by a fundamental change in the flow field, accompanied by violent flame flapping and wake disruption. This paper quantifies extinction spots along the flame edge, and entrained reactants within the combustion product region to analyze these stages in greater detail. Extinction spots were quantified by the overlapping regions of OH and CH2O – numerous such spots were found near blowoff. The entraining of unburnt reactants into the recirculation zone was quantified by detecting low intensity OH pockets that were not surrounded by CH2O. As expected, the flame near blowoff displayed significantly more entrained reactant pockets relative to a stable flame. Unexpectedly however, the volume of these pockets is tiny compared to the products, even on the edge of blowoff. Once they enter the wake, they are short-lived, suggesting that they are diluted and/or quickly react. This was surprising given the non-trivial baseflow and flame position disruption at these conditions, suggesting a striking similarity between the average composition of the wake, to that of a stable flame.

Investigation of Dilution Effect on CH4/Air Premixed Turbulent Flame Using OH and CH2O Planar Laser-Induced Fluorescence

Diluting the combustion mixtures is one of the advanced approaches to reduce the NOx emission of methane/air premixed turbulent flame, especially with high diluents to create a distributed reaction zone and mild combustion, which can lower the temperature of reaction zone and reduce the formation of NOx. The effect of N2/CO2 dilution on the combustion characteristics of methane/air premixed turbulent flame with different dilution ratio and different exit Reynolds number was conducted by OH-PLIF and CH2O-PLIF. Results show that the increase of dilution ratio can sharply reduce the concentration of OH and CH2O, and postpone the burning of fuel. Compared with the ultra-lean combustion, the dilution weakens the combustion more obviously. For different dilution gases, the concentration of OH in the combustion zone varies greatly, while the concentration of CH2O in the unburned zone is less affected by different dilution gas. The CO2 dilution has a more significant effect on OH concentration than N2 with the given dilution ratio, but a similar effect on the concentration of CH2O in the preheat zone of flame. However, dilution does not have much influence on the flame structure with the given turbulent intensity.

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.

Investigation of OH and CH2O distributions at ultra-high repetition rates by planar laser induced fluorescence imaging in highly turbulent jet flames

For better understanding of the transient behavior of the flow/flame interaction in highly turbulent premixed flames, measurement techniques with sufficient temporal resolution are required. Using a burst-mode laser pumped optical parametric oscillator system, the present work demonstrated an ultra-high-speed diagnostic, for the first time, capable of visualizing hydroxyl radicals (OH) and formaldehyde (CH2O) distributions in a highly turbulent flame at repetition rates up to 140 kHz (i.e. with a temporal resolution of 7.1 μs) to access the Kolmogorov time scales of flames in the distributed/broken reaction zone regime (Ka ≥ 100). More than 100 consecutive images were recorded in the present work for OH planar laser induced fluorescence (PLIF) measurement at 100 kHz. Systematic temporal and spatial evolution of the flame structure is clearly resolved over long sequences, e.g. 1.2 ms for OH PLIF and 10 ms for CH2O PLIF respectively. The Kolmogorov time scale for all the cases at x/d = 10 and x/d = 30 are estimated with the minimum Kolmogorov time scale of 7.5 μs, indicating that nearly all time-scales of the flow are resolved with the present experimental setup. The axial velocity of the CH2O structure at its outer layer at different flame height positions, UCH2O, is obtained. The velocity UCH2O is compared with the LDV results, which shows a good agreement with the mean flow velocity. By comparing UCH2O with the local axial velocity, different mechanisms responsible for the large-scale wrinkle structures in the reaction zone and the propagation of CH2O structure along the flame height are discussed. Power spectral density and autocorrelation function based on the CH2O outer layer fluctuation in the radial direction are illustrated. Quantitative analysis of the ultra-high-speed diagnostics data further augments the understanding of turbulent combustion down to Kolmogorov scale experimentally.

The evolution of autoignition kernels in turbulent flames of dimethyl ether

Simultaneous planar laser-induced fluorescence (PLIF) imaging of CH2O and OH was performed at a repetition rate of 10.kHz, jointly with chemiluminescence to explore autoigniting dimethyl ether (DME) flames in a hot vitiated coflow burner. The focus of the study is the imaging of the flame stabilization region and the temporal evolution of ignition kernels upstream of the flame base. Results detail the evolution of kernels throughout their formation, growth and final merging with the flame base. The ignition events were explored for a range of different fuel premixing and dilution ratios over two coflow temperatures which result in different lift-off heights. Images of CH2O and OH over the entire flame length show that not only is the lift-off height much higher at low coflow temperatures, but that the fluctuations are more intense and the region of kernel formation is larger both radially and axially. In these autoignition stabilized flames, increased premixing leads to the lift-off height and location of the maximum kernel formation rate being further downstream. Transient 1-D simulations of hot coflow products opposed against jet fuel mixtures identify that the overlap of CH2O and OH PLIF signals are a reliable marker of heat release in autoignition kernels. Measurements indicate that for the high coflow temperature cases, on average, the heat release of individual kernels is low, despite the high total kernel formation rate. This can be correlated to the slow growth rate and elongated aspect ratio of the kernels. For low coflow temperature cases, kernels are growing faster and have high heat release rates with near unity aspect ratios.

Burst-mode OH/CH2O planar laser-induced fluorescence imaging of the heat release zone in an unsteady flame.

The paper presents simultaneous high-speed (7.5 kHz) planar laser-induced fluorescence (PLIF) of formaldehyde (CH2O) and the hydroxyl-radical (OH) for visualization of the flame structure and heat release zone in a non-premixed unsteady CH4/O2/N2 flame. For this purpose, a dye laser designed for high-speed operation is pumped by the second-harmonic 532 nm output of a Nd:YAG burst-mode laser to produce a tunable, 566 nm beam. After frequency doubling a high-energy kHz-rate narrowband pulse train of approximately 2.2 mJ/pulse at 283 nm is used for excitation of the OH radical. Simultaneously, CH2O is excited by the frequency-tripled output of the same Nd:YAG laser, providing a high-frequency pulse train over 10 ms in duration at high pulse energies (>100 mJ/pulse). The excitation energies enable signal-to-noise ratios (SNRs) of ~10 and ~60 for CH2O and OH PLIF, respectively, using a single high-speed intensified CMOS camera equipped with an image doubler. This allows sufficient SNR for investigation of the temporal evolution of the primary heat release zone and the local flame structure at kHz rates from the spatial overlap of the OH- and CH2O-PLIF signals.

20 kHz CH2O and OH PLIF with stereo PIV.

Planar laser-induced fluorescence (PLIF) of hydroxyl (OH) and formaldehyde (CH2O) radicals was performed alongside stereo particle image velocimetry (PIV) at a 20kHz repetition rate in a highly turbulent Bunsen flame. A dual-pulse burst-mode laser generated envelopes of 532nm pulse pairs for PIV as well as a pair of 355nm pulses, the first of which was used for CH2O PLIF. A diode-pumped solid-state Nd:YAG/dye laser system produced the excitation beam for the OH PLIF. The combined diagnostics produced simultaneous, temporally resolved two-dimensional fields of OH and CH2O and two-dimensional, three-component velocity fields, facilitating the observation of the interaction of fluid dynamics with flame fronts and preheat layers. The high-fidelity data acquired surpass the previous state of the art and demonstrate dual-pulse burst-mode laser technology with the ability to provide pulse pairs at both 532 and 355nm with sufficient energy for scattering and fluorescence measurement at 20kHz.

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.

Heat release rate variations in a globally stoichiometric, stratified iso-octane/air turbulent V-flame

Combustion intensity variations along a globally stoichiometric, stratified iso-octane/air turbulent V-flame were measured in the presence of four different equivalence ratio gradients and compared to a reference fully-premixed case. Instantaneous heat release rate (HRR) images were obtained from the simultaneous acquisition of OH and CH2O Planar Laser Induced Fluorescence (PLIF) images, providing the first direct estimates of local heat release rate in a stratified, iso-octane/air turbulent flame. Detailed numerical simulations were conducted to validate the use of (OH LIF)(CH2O LIF) for measurements of HRR in iso-octane/air flames over a broad range of equivalence ratios. Flame images were analyzed for specific mean equivalence ratio gradient effects for near stoichiometric flame regions within a fixed mean equivalence ratio range of 0.95⩽ϕ⩽1.05. The location of the interrogation window, or region of interest (ROI), was obtained through 3-pentanone tracer PLIF of the reactants and ensured that any observed variation could be attributed specifically to mean equivalence ratio gradient effects, rather than simple mixture strength effects. Results showed small but measurable, decreases in the mean HRR for near stoichiometric flame regions within the ROI as the gradient was varied, and differences of up to 4.4% relative to the reference premixed case were directly attributable to the imposed mean ϕ gradient, separate from potential effects of local mixture strength. By contrast, general stratification effects were also considered for flames of comparable global equivalence ratios but with much broader ranges of mixture strength, and results showed offsetting effects of increased 2D flame length (up to ∼17%) and decreased HRR per flame length (up to ∼22%), resulting in small variations in the overall HRR of ∼9%. The present analysis suggests that gradient effects on local flame properties may differ significantly from general stratification effects in which different ranges of mixture strengths are present, and separating these aspects of partially premixed combustion is critical to the proper interpretation of data, where both may be relevant in different applications.

Narrowband versus broadband excitation for CH2O PLIF imaging in flames using a frequency-tripled Nd:YAG laser

Spectrally-narrow- (~0.003 cm−1) and broadband (>1 cm−1) fluorescence excitation of the \(\tilde{A}^{1} A_{2} - \tilde{X}^{1} A_{1} ,4_{0}^{1}\) electronic transition of formaldehyde (CH2O) in laminar premixed and non-premixed flames is investigated using the third-harmonic output from a tunable, injection-seeded Nd:YAG laser. Spectrally-resolved, CH2O fluorescence excitation spectra are examined over a broad range of conditions including room-temperature vapor cells and lean-to-rich premixed methane/air and dimethyl ether/air flames in order to understand the origin of the fluorescence using both narrowband and broadband excitation strategies. The measured CH2O excitation spectra are nearly identical in all conditions considered which cover a broad range of composition and temperature conditions. These results imply that the predominant emission signature is CH2O and suggest the potential for quantitative in-flame CH2O LIF measurements using room-temperature calibration and existing fluorescence models. A specific emphasis of this study is on CH2O isolation and potential fluorescence interference in the context of single-shot planar laser-induced fluorescence (PLIF) imaging in flames. The PLIF results indicate that for the premixed flames investigated, both narrowband (Nd:YAG laser operating in single mode) and broadband (no injection seeding) excitation yield a reliable marker of the CH2O distribution, with no indication of major interference from additional species. However, frequency-tuned narrowband excitation resulted in a collected fluorescence emission signal that increased by a factor of two as compared to broadband excitation. In the methane-based non-premixed flames, evidence of the excitation of additional species (such as PAH) was noted; however, the impact of this interference is reduced when using narrowband excitation. Similar to the premixed flames, the CH2O fluorescence emission signal increased by approximately a factor of two when using spectrally tuned, narrowband excitation from the third-harmonic output of an injection-seeded Nd:YAG laser. The current results indicate that narrowband excitation of CH2O near 355 nm using the third-harmonic output of an injection-seeded Nd:YAG laser results in increased fluorescence emission signal and hence a reduced effect of interference from additional flame-generated species as compared to conventional broadband excitation using a frequency-tripled Nd:YAG laser.

In situ plasma activated low temperature chemistry and the S-curve transition in DME/oxygen/helium mixture

The effect of non-equilibrium plasma activated low temperature chemistry (PA-LTC) on the ignition and extinction of Dimethyl Ether (DME)/O2/He diffusion flames has been investigated experimentally in a counterflow burner with in situ nanosecond pulsed discharge at 72Torr. A uniform discharge is generated between the burner nozzles by placing porous metal electrodes at the nozzle exits. The ignition and extinction characteristics of DME/O2/He are studied by employing OH and CH2O Planar Laser Induced Fluorescence (PLIF) techniques at constant strain rates and O2 mole fractions on the oxidizer side with varying the DME mole fractions. Contrary to the conventional understanding, strong low temperature reactivity during ignition process is observed for DME with non-equilibrium plasma activation even at 72Torr and flow residence time of a few milliseconds. The OH PLIF shows strong OH signal at and after ignition, whereas extremely low OH signal before ignition. However, the CH2O PLIF experiments demonstrate that, with the increase of DME mole fraction on the fuel side, the CH2O PLIF signal intensity increases significantly before ignition and decreased rapidly after ignition. The low OH number density and high CH2O number density before DME ignition clearly demonstrates the existence of PA-LTC at low pressure. Moreover, at higher O2 mole fraction and discharge repetition frequency, the in situ discharge significantly modifies the characteristics of ignition and extinction, thus creating a new monotonically and fully stretched ignition S-curve without an extinction limit. Compared to our previous study of methane, the existence of strong low temperature reactivity in DME oxidation makes ignitions occur at much lower fuel mole fractions, thus accelerating the transition of ignition curve from conventional S-curve to the fully stretched S-curve. The transition from the conventional S-curve to the new stretched ignition curve at high plasma repetition rate indicates that the plasma could dramatically change the chemical kinetic pathways of DME oxidation by activating the low temperature chemistry even at low pressure. The chemical kinetic model for the plasma–flame interaction has been also developed based on the assumption of constant electric field strength in the bulk plasma region. Both experiments and modeling results reveal that the PA-LTC has a much shorter timescale comparing with that of thermally activated low temperature chemistry owing to the rapid radical production by plasma. The reaction pathways analysis shows that atomic O generated by the discharge is critical to controlling the population of radical pool.

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.

A comparison of turbulent dimethyl ether and methane non-premixed flame structure

We report new results using laser-based imaging techniques (CH2O PLIF and Mie scattering) to understand the structure of turbulent dimethyl ether-based flames in comparison to well-studied methane-based flames. Specifically, the paper introduces a new set of potential target flames using DME fuel. The new DME-based flames are formulated with the specific intention of directly comparing non-intrusive laser-based measurements with the DLR A and B flames, which serve as target cases within the TNF workshop. The new DME-based flames are operated at equivalent Reynolds numbers and have an identical stoichiometric mixture fraction as the DLR flames, thus attempting to isolate kinetic effects and turbulence-chemistry interaction from flowfield and mixing effects. In this paper, we examine the CH2O field using planar laser-induced fluorescence (PLIF) and laser Mie scattering of fuel–seeded oil droplets which yield an approximate marker of the 700–800K isotherms. Results from laminar flame calculations indicate that large amounts of CH2O are formed in DME-based flames as compared to CH4-based flames due to very rapid DME pyrolysis. Additionally, the calculations suggest significant differences in the topology of the CH2O distribution. Consistent with the laminar flame calculations, the measured CH2O PLIF signals from the turbulent DME-based flames are approximately two orders of magnitude higher than the DLR flames. Furthermore, instantaneous, average, and RMS images from the turbulent flames are used to highlight qualitative differences in the CH2O field for the DME-based flames in comparison with the methane-based DLR flames.

High-speed CH2O PLIF imaging in turbulent flames using a pulse-burst laser system

In this manuscript, we demonstrate high-speed (10-kHz-acquisition rate) planar laser-induced fluorescence (PLIF) imaging of formaldehyde (CH2O) in turbulent non-premixed flames. Using the unique pulse-burst laser system (PBLS) at Ohio State University, high-energy laser pulses (∼100 mJ/pulse) at 355 nm with 100 μs pulse separation are generated and used to measure the time-varying CH2O distributions in attached and lifted methane-based turbulent flames. By taking advantage of the tunable, narrow spectral linewidth of the PBLS at 355 nm, the laser output can be frequency-tuned and adjusted to overlap with absorption “peaks” within the tail of the A–X transition of CH2O near 355 nm, thus increasing the acquired signal by as much as a factor of three. The reported signal-to-noise ratio (SNR) exceeds 55, which represents one of the highest SNR reported to date for kilohertz-rate imaging of scalars for comparable spatial resolution. Potential applications and pairings with other diagnostic approaches for high-speed reaction rate and multi-scalar imaging also are discussed.

Spatially resolved heat release rate measurements in turbulent premixed flames

Heat release rate is a fundamental property of great importance for the theoretical and experimental elucidation of unsteady flame behaviors such as combustion noise, combustion instabilities, and pulsed combustion. Investigations of such thermoacoustic interactions require a reliable indicator of heat release rate capable of resolving spatial structures in turbulent flames. Traditionally, heat release rate has been estimated via OH or CH radical chemiluminescence; however, chemiluminescence suffers from being a line-of-sight technique with limited capability for resolving small-scale structures. In this paper, we report spatially resolved two-dimensional measurements of a quantity closely related to heat release rate. The diagnostic technique uses simultaneous OH and CH 2O planar laser-induced fluorescence (PLIF), and the pixel-by-pixel product of the OH and CH 2O PLIF signals has previously been shown to correlate well with local heat release rates. Results from this diagnostic technique, which we refer to as heat release rate imaging (HR imaging), are compared with traditional OH chemiluminescence measurements in several flames. Studies were performed in lean premixed ethylene flames stabilized between opposed jets and with a bluff body. Correlations between bulk strain rates and local heat release rates were obtained and the effects of curvature on heat release rate were investigated. The results show that the heat release rate tends to increase with increasing negative curvature for the flames investigated for which Lewis numbers are greater than unity. This correlation becomes more pronounced as the flame gets closer to global extinction.

Experimental investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations

This paper describes an experimental investigation of acoustically forced lean premixed turbulent bluff-body-stabilised flames in an enclosure short enough so that no coupling of the combustor downstream acoustics occurred for the frequencies studied here, which allows an unambiguous examination of the flame response to inlet velocity fluctuations. Special emphasis was placed on the amplitude dependence of this response. Measurements of the heat release rate were performed with OH ∗ and CH ∗ chemiluminescence, planar laser-induced fluorescence (PLIF) of OH from which the flame surface density (FSD) was computed, and simultaneous CH 2O and OH PLIF imaging from which the local heat release rate (RX) was estimated. The global heat release measured with chemiluminescence and that integrated from the local FSD measurements were in close agreement, while a comparison between FSD and high-resolution RX imaging also showed good agreement. This suggests that estimates of the flame area are sufficient to determine heat release rate for this flow. The heat release response became nonlinear after inlet velocity amplitudes of around 15% of the bulk velocity. This value depended on the forcing frequency and the equivalence ratio. The nonlinearity was found to occur when the shear layers rolled up into vortices. The vortices induced by the inlet velocity fluctuations not only generated flame area when the flame wrapped around them, but also caused cusps and even large-scale flame annihilation events, as observed in time-resolved OH PLIF images. Such events occurred when parts of the flame stabilised on the inner shear layer close to the recirculation zone collapsed on parts of the flame stabilised on the outer recirculation zone, a phenomenon that was made more prominent with increasing forcing amplitude. A further nonlinearity occurred at high amplitudes and at some equivalence ratios, where a significant leakage of energy to higher harmonics was observed, but the origin of this is not yet clarified. The present results suggest that the flame sheet kinematics play a major role in the saturation mechanism of lean premixed flame response, hence extending previous experimental and analytical results from laminar to turbulent flames. Heat release fluctuations due to local fluctuations of strain rate and curvature were less significant, while no localised extinction has been observed even at large forcing amplitudes.