High-speed Planar Laser-induced Fluorescence Research Articles

High spatiotemporal resolution optical measurements of two-stage ignition and combustion in Engine Combustion Network Spray D flames

This study explores flame structures and combustion dynamics in high-pressure n-dodecane fuel sprays, focusing on the formation and consumption of formaldehyde (CH2O) during autoignition and the development of poly-aromatic hydrocarbons (PAH) as soot precursors. These processes are crucial for optimizing combustion efficiency and reducing emissions. However, traditional approaches, which rely on single-shot measurements or ensemble-averaged visualizations, often overlook critical early-stage processes during low-temperature ignition. To overcome these challenges, we employed an innovative high-speed planar laser-induced fluorescence (PLIF) technique at 50 kHz using a pulse-burst Nd:YAG laser system with an excitation wavelength of 355 nm. This approach, applied for the first time to Engine Combustion Network (ECN) Spray D flames, provides unprecedented insights into the combustion processes at varying ambient temperatures and oxygen concentrations. Additionally, simultaneous high-speed schlieren imaging at 100 kHz was used to visualize spray penetration, first-stage ignition, and thermal expansion zones. Our findings reveal that, similar to Spray A flames, CH2O forms in cold, fuel-rich zones well upstream of the combustion zone. However, in Spray D flames, the schlieren signal softening observed in the jet's head does not lead to complete disappearance, and the CH2O signal is absent from the full head of the spray. During the second-stage ignition, CH2O consumption accelerates due to high-temperature reactions, leading to a significant reduction in its signal. Unlike the mushroom-shaped structure seen in Spray A flames, Spray D flames exhibit a quasi-steady PAH phase structure, with lean peripheral mixtures insufficient for soot precursor formation. Notably, reducing ambient oxygen concentration to 13 % while maintaining or increasing temperature prolongs the presence of CH2O, highlighting its influence on ignition dynamics and oxidation processes in dodecane spray flames. This study provides new insights into the combustion mechanisms of high-pressure sprays and offers valuable data for developing next-generation combustion technologies, including models, aimed at improving efficiency and reducing emissions.

Flame development in prechamber assisted engine: High-speed PLIF

Prechamber-assisted combustion reduces emissions and improves engine performance through lean and knock limit enhancement. The spark plug ignition is replaced by multiple, high-temperature, radical-rich jets that entrain and ignite the main chamber charge, enabling the engine to operate at leaner air–fuel mixtures. Current work reports first cycle resolved planar laser-induced fluorescence (PLIF) measurements following the flame development process, starting from the mixture formation inside the prechamber to the post-combustion jets in the main chamber. The mixing and flame development inside the prechamber is visualized at 100 kHz with Acetone PLIF using an inventive engine-mounted optical prechamber (OPC) setup. Following the burning (/burned) prechamber jet interaction with the unburned main chamber, the mixture is imaged using the fuel and flame tracer (FFT) PLIF approach. The innovative 50 kHz FFT-PLIF approach is based on the fluorescence of acetone as fuel (unburned) and combustion-generated SO2 as a flame (burned) tracer with 266 nm laser excitation. The main chamber is fueled with premixed methanol seeded with 6.8 acetone and 2.6% (m/m) di-tert-butyl disulfide (DtBDS) while prechamber is injected with methane at 6 bar using solenoid and check valve assembly. The flow of the main chamber mixture into the prechamber creates a turbulent jet inside the prechamber. The inflow increases as the pressure ratio drops, generating significant recirculation inside the prechamber. Partially oxidized products remain near the top center of the prechamber even as the flame propagates through the throat. Combined flame and fuel images reveal the dynamics of the interaction layer between the prechamber jet and the main chamber fuel–air mixture.

High-speed planar laser-induced fluorescence investigation of nitric oxide generated by hypersonic Mach reflections for computational fluid dynamics validation

In this article, the authors present an experimental investigation of the thermochemical relaxation regions downstream of symmetric Mach reflections generated in Mach 8.5 flows with stagnation enthalpies from 7 to 10 MJ/kg. The experiment was performed in the Hypervelocity Expansion Tunnel at the National Aerothermochemistry and Hypersonics Laboratory of Texas A&M University. The investigation focused on studying the nitric oxide morphology in the vicinity of a strong normal shock wave and shear layers. Experimental planar laser-induced fluorescence measurements at 250 kHz repetition rate with a quasi-simultaneous natural emission photography demonstrate a sufficient rejection of natural emission and reveal intricate flow structures in the shock vicinity. Steady computational fluid dynamics (CFD) flow solutions obtained with the US3D software were used to simulate laser-induced fluorescence signal levels for quantitative comparison against the experimentally acquired data. The investigation accomplished four objectives: complex flowfield visualization in the vicinity of the Mach stem with 4 μs temporal resolution, evaluation of the level of high-temperature chemistry effects introduced to the freestream by the expansion tunnel, both qualitative and quantitative CFD simulation validation, and cross-checking or introduction of fundamental properties of nitric oxide laser-induced fluorescence including absorption cross section for vibrationally excited states of nitric oxide and a collisional quenching cross section of nitric oxide by atomic nitrogen.

High-speed formaldehyde planar laser-induced fluorescence and schlieren to assess influences of injection pressure and oxygen concentration on Spray A flames

The present study investigates the effect of injection pressure and ambient oxygen concentration variations on the Engine Combustion Network Spray A flame. The two-stage ignition and combustion processes for individual injections are characterized using a high-speed planar laser-induced fluorescence (PLIF) and schlieren imaging diagnostic at high pressure (near 6 MPa). For PLIF measurements, we use a pulse-burst-mode Nd:YAG laser capable of producing a 6 ms long 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20 µs. The PLIF imaging at high-acquisition rate offers unique insights into understanding the spatiotemporal distribution of formaldehyde (CH2O) and polycyclic aromatic hydrocarbons (PAH). High-speed line-of-sight schlieren with a pulsed infrared light emitting diode (LED) is applied simultaneously with PLIF to visualize the spray penetration, low- and high-temperature ignition, and turbulent flame dynamics, while in-vessel pressure is used in concert with the optical diagnostics to assess the ignition delay time of the high-temperature ignition events. The PLIF and line-of-sight measurements, documenting the position and timing for formation and consumption of CH2O and the inception and distribution of PAH downstream with changes in ambient oxygen concentration and injection pressure, provide a unique database for detailed evaluation of single-event (e.g. LES) or ensemble-average CFD.

Hybrid algorithm for the detection of turbulent flame fronts

This paper presents a hybrid and unsupervised approach to flame front detection for low signal-to-noise planar laser-induced fluorescence (PLIF) images. The algorithm combines segmentation and edge detection techniques to achieve low-cost and accurate flame front detection in the presence of noise and variability in the flame structure. The method first uses an adaptive contrast enhancement scheme to improve the quality of the image prior to segmentation. The general shape of the flame front is then highlighted using segmentation, while the edge detection method is used to refine the results and highlight the flame front more accurately. The performance of the algorithm is tested on a dataset of high-speed PLIF images and is shown to achieve high accuracy in finely wrinkled turbulent hydrogen-enriched flames with order of magnitude improvements in computation speed. This new algorithm has potential applications in the experimental study of turbulent flames subject to intense wrinkling and low signal-to-noise ratios.Graphic abstract

Flame features and oscillation characteristics in near-blowout swirl-stabilized flames using high-speed OH-PLIF and mode decomposition methods

Flame features and dynamics are important to the explanation and prediction of a lean blowout (LBO) phenomenon. In this paper, recognition of near-LBO flame features and oscillation characterization methods were proposed based on flame spectroscopic images. High-speed planar laser-induced fluorescence measurements of OH were used to capture unique dynamic features such as the local extinction and reignition feature and entrained reactant pockets. The Zernike moment demonstrated a good performance in recognition of stability and near-LBO conditions, though the geometric moment had more advantages to characterize frequency characteristics. Low-frequency oscillations, especially at the obvious self-excited oscillation frequency around 200 Hz, were found when approaching an LBO condition, which can be expected to be used as a novel prediction characteristic parameter of the flameout limit. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were used to conduct dynamic analysis of near-LBO flames. POD modes spectra showed the unique frequency characteristics of stable and near-LBO flames, which were basically in line with those at the heat-release frequency. The primary POD modes demonstrated that the radial vibration mode dominated in a stable flame, while the rotation mode was found to exist in a near-LBO flame. Analysis of modal decomposition showed that flame shedding and agminated entrained reactant pockets were responsible for generating self-excited flame oscillations.

Flame characteristics of a cavity-based scramjet combustor using OH-PLIF and feature extraction

It is vital to analyze the flame characteristics and identify the flame states efficiently in scramjet. Flame combustion and oscillation behaviors in a hydrogen-fueled cavity-stabilized scramjet combustor have been investigated in this study. A 500 Hz high-speed PLIF (planar laser-induced fluorescence) technique with a 20-cm-wide view field is introduced to characterize the combustion flow. The spatial distribution of the flame structure under different fuel injection conditions is studied. The results indicate that the position of the flame peak shifted downstream of the cavity when increasing the injection pressure to a high level. The dynamic characteristics of flames under different flow conditions are analyzed, and the correlation between flame features and hydrogen injection conditions is obtained. These flame features include the vertical range, the flow direction position of the peak flame, the flow direction position of the centroid, the vertical position of the centroid, the flame area, and the flame circumference. For the present cases, the flame of 4.0 Mpa is more unstable than that of 1.5 Mpa under any flow conditions. Moreover, an experiment based on feature extraction results shows that the KNN (K-nearest neighbor) classifier could achieve high accuracy for flame state recognition in this scramjet combustor.

Combustion oscillation characteristics of a supersonic ethylene jet flame using high-speed planar laser-induced fluorescence and dynamic mode decomposition

Flame dynamics and combustion oscillation are complex problems in propulsion systems. In this study, the combustion oscillation characteristics of a supersonic ethylene jet flame in a hot coflow were investigated utilizing a 5 kHz high-speed hydroxyl planar laser-induced fluorescence (OH-PLIF) technique and an advanced postprocessing method, namely, dynamic mode decomposition (DMD). A PLIF system equipped with a large laser sheet was used to collect the dynamic development process of the jet flame. An ethylene jet flow was burned at a hot coflow temperature of 1900 K at different Mach numbers (Mach 0.55–1.6). The dynamic evolution of flame microstructures was clearly obtained. The local extinction events and flame area distribution in the flame were statistically analyzed to characterize the instability of the jet flame. The results indicated that flame instability was enhanced with increasing Mach number, but the jet velocity did not affect the global flame oscillation frequency. Based on DMD analysis, the spatiotemporal three-dimensional oscillation characteristics of the jet flame were quantified. The DMD results indicated that during different time periods, the jet flame is dominated by coherent structures with different frequencies. For supersonic flow, increased jet velocity might lead the dominant mode frequency shift to a higher level.

Investigation of Fuel and Load Flexibility in a Siemens Gas Turbine-600/700/800 Burner Under Atmospheric Pressure Conditions Using High-Speed Hydroxyl-PLIF and Hydroxyl Radical Chemiluminescence Imaging

Abstract Fuel and load flexibility have been increasingly important features of industrial gas turbines in order to meet the demand for increased utilization of renewable fuels and to provide a way to balance the grid fluctuations due to the unsteady supply of wind and solar power. Experimental investigations were performed using a standard third-generation dry low emission (DLE) burner under atmospheric pressure conditions to study the effect of central and pilot fuel addition, load variations, and hydrogen (H2) enrichment in a natural gas (NG) flame. High-speed kHz planar laser-induced fluorescence (PLIF) of OH radicals and imaging of OH chemiluminescence were employed to investigate the flame stabilization, flame turbulence interactions, and flame dynamics. Along with the optical measurements, combustion emissions were also recorded to observe the effect of changing operating conditions on NOX level. The burner is used in Siemens industrial gas turbines SGT-600, SGT-700, and SGT-800 with minor hardware differences and the study thus is a step to characterize fuel and load flexibility for these turbines. Without pilot and central fuel injections in the current burner configuration, the main flame is stabilized creating a central recirculation zone (CRZ). Addition of the pilot fuel strengthens the outer recirculation zone (ORZ) and moves the flame anchoring position slightly downstream, whereas the flame moves upstream without affecting the ORZ when central fuel injection is added. The flame was investigated utilizing H2/NG fuel mixtures where the H2 amount was changed from 0 to 100%. The results show that the characteristics of the flames are clearly affected by the addition of H2 and by the load variations. The flame becomes more compact, the anchoring position moves closer to the burner exit and the OH signal distribution becomes more distinct for H2 addition due to increased reaction rate, diffusivity, and laminar burning velocity. Changing the load from part to base, similar trends were observed in the flame behavior but in this case due to the higher heat release because of increased turbulence intensity.

Scalar Power Spectra and Scalar Structure Function Evolution in the Richtmyer–Meshkov Instability Upon Reshock

Abstract The Richtmyer–Meshkov instability of a twice-shocked gas interface is studied using high-speed planar laser-induced fluorescence in the Wisconsin Shock Tube Laboratory's vertical shock tube. The initial condition is a shear layer with broadband diffuse perturbations at the interface between a helium–acetone mixture and argon. This initial condition is accelerated by a shock of nominal strength M = 1.9, and then accelerated again by the transmitted shock that reflects off the end wall of the tube. Three individual experiments are analyzed, the energy spectrum and the structure functions of the light gas mole fraction field are calculated and compared.

Effects of Water Washing and Deashing on the Combustion Characteristics of the Zhundong Lignite

ABSTRACT To examine the effects of water washing and acid-based deashing on the structure as well as combustion characteristics of Zhundong lignite, this study used high-speed planar laser-induced fluorescence measurements (OH-PLIF) with a specific burner and characterization techniques including XRD, BET, and ICP-AES as the method. The results indicated that both water washing and deashing can remove the ash and the main mineral alkali metals. Compared with RC, the total amount of OH release from WWC and UCC was increased attributing to the addition of combustibles and volatiles. The ignition of WWC and UCC was faster than that of RC, while the burning rate followed the opposite order. Experimental result confirms that the alkali metal and alkaline earth metals play a catalytic role in coal combustion. Meanwhile, the increase of the BET surface area and the reduction of ash are conducive to heat transfer, which is favorable for the ignition of pulverized coal.

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

Continuous 500-Hz OH-PLIF Measurements in a Hydrogen-Fueled Scramjet Combustor

In recent years, significant research attention has focused on scramjet bench-testing using optical measurement techniques to obtain significant experimental data. The work aims to verify the environmental reliability of high-speed planar laser-induced fluorescence (PLIF) technique and satisfy bench-test requirements. A high-speed OH-PLIF system with ~1.8 mJ/pulse at 500 Hz is developed, and conducted tests at a direct-connect supersonic combustion facility. In this paper, OH-PLIF signals and pressure were measured simultaneously, and flame dynamics during pressure buildup were investigated, with a special focus on the ignition and flameout processes. In addition, the flame trajectory of the ignition process is depicted. The results show that the time scale of flame development is consistent with that of pressure stability, whereas the pressure change lags the change in flame intensity. The flame structure is discussed in both scramjet and ramjet modes. An unstable flame structure develops in ramjet mode, and the flame area decreases similarly in both combustion modes. Flame oscillations are captured for three hydrogen equivalence ratios.

High speed PLIF study of the Richtmyer–Meshkov instability upon re-shock

The Richtmyer–Meshkov instability (RMI) of a twice-shocked gas interface is studied using high-speed planar-laser induced fluorescence (PLIF) in the Wisconsin Shock Tube Laboratory’s vertical shock tube. The initial condition (IC) is a shear layer with broadband diffuse perturbations at the interface between a helium-acetone mixture and argon. This IC is accelerated by a shock of nominal strength M=1.8, and then accelerated again by the transmitted shock that reflects off the end wall of the tube. An estimate of the light gas mole fraction is extracted from high-speed imaging using an iterative process that accounts for the nonlinear temperature dependence of the acetone’s fluorescence quantum yield (FQY) and absorption cross-section. A vorticity deposition model for the initial growth rate after re-shock is compared with the Mikaelian model for re-shock. Previously used in literature, the number of generations is shown to naturally arise from a normalisation of the scalar transport equation. A self-similar analysis is then performed using the mole fraction data to explore the evolution of the RMI after reshock and the higher order moments of the light gas mole fraction are compared with a proposed model.

Experimental study of a free-surface circular liquid sheet using planar laser-induced fluorescence

This work presents an experimental study of various spatiotemporal events leading to the primary atomization of a radially expanding circular liquid sheet. A cylindrical liquid (water) jet orthogonally made to impinge on the horizontally placed cone-disc deflector to produce a thin free-surface circular liquid sheet from the peripheral edge of the deflector disc. High-speed planar laser-induced fluorescence (PLIF) at 6 kHz framing rate was carried out to capture the atomizing liquid sheet in the side and front views at various jet Weber numbers ($${\text{We}}_{{{\text{jet}}}}$$) over a range of 994 < $${\text{We}}_{{{\text{jet}}}}$$ < 8029. The side view PLIF comprising of three fields of view to capture the complete atomization process of expanding liquid sheet in a time-resolved manner starting from capturing radial wave features on liquid sheet down to the sheet breakup, ligament formation, and droplet production. The azimuthal wave features were captured in the front view PLIF experiments. The quantitative information of different spatiotemporal features was extracted from experimental data using rigorous image processing methods. Advanced feature extraction and transient analysis methods like proper orthogonal decomposition (POD) was applied to obtain the dominant flow features associated with sheet dynamics. The POD analysis showed that the most dominant phenomenon in the flow field for $${\text{We}}_{{{\text{jet}}}}$$ < 1500 is found to be the oscillation of the liquid sheet, the radial wave propagation in the radial direction for 1500 ≤ $${\text{We}}_{{{\text{jet}}}}$$ < 2000, and sheet breaks up at the outer edge for $${\text{We}}_{{{\text{jet}}}}$$ ≥ 2000. It is shown that the dominancy of each of these phenomena changes with $${\text{We}}_{{{\text{jet}}}}$$. Statistical analysis-based ligament length and thickness ($$L_{{{\text{lig}}}}$$ and $$t_{{{\text{lig}}}}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.6}$$ and droplet diameter ($$D_{32}$$ and $$D_{10}$$) follow $${\text{We}}_{{{\text{jet}}}}^{ - 0.33}$$ power law. Further, the direct correlations obtained showed that the droplet diameter is a stronger function of ligament thickness compared to ligament length. Schematic of optics, laser light and camera arrangements for side view PLIF experiments of a circular liquid sheet. Three FOVs adopted for side view PLIF experiments at different radial locations; RA= 280 mm for A = 100 mm × 100 mm, RB = 360 mm for B = 80 mm × 80 mm, and RC = 420 mm for C = 40 mm × 40 mm Direct imaging of azimuthal waves of liquid sheet using front view PLIF experimental technique

Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy.

Study on the combustion behaviours of two high-volatile coal particle streams with high-speed OH-PLIF

An optical entrained flow reactor, based on a downward flat-flame, was established to explore the devolatilization and volatile combustion behaviours of coal particle streams with surrounding temperatures from 1600 K to 1800 K and oxygen mole fractions in the range of 10%–30%. The coal particle streams with particle sizes of 53–80 μm are two kinds of high-volatile pulverized bituminous coal and lignite. Using High-speed Planar Laser-Induced Fluorescence to diagnose OH (OH-PLIF), the planar sheet-imaging of OH-radicals was obtained to visualize the coal particle flames, and then, the normalized fluorescence signal intensity along the distance downstream from the burner and along the radial distance was obtained to investigate the volatiles combustion process. From the planar sheet-imaging, the ignition of tested high-volatile bituminous coal and lignite is dominated by the combustion of their volatiles. The combustion processes of volatiles can be divided into 4 stages: ignition, accelerating burning, stable burning and feeble burning, to investigate the intrinsic combustion correlation phenomena. The relative standard deviation (RSD) changes of the OH radial profiles can provide a new insight into flame stabilization, wherein the flame stability of the bituminous coal particle stream is higher than that of lignite under the same combustion conditions. The volatiles combustion of lignite is more sensitive to the oxygen concentration and temperature than that of bituminous coal, and the volatile releasing ratio of bituminous coal (65%–81%) was substantially less than that of lignite (92%–100%) by the analysis of the char residues.

Investigation of Flame Evolution in Heavy Oil Boiler Bench Using High-Speed Planar Laser-Induced Fluorescence Imaging

Over recent years, much attention has been paid to the performance evaluation of industrial-type burners. The ignition and stable combustion process are of great significance in assessing the quality of burner. The planar laser-induced fluorescence (PLIF) technique can be applied to heavy oil boilers, extending this technique to engineering applications. Considering the complex environment of the bench test, measures such as temperature control and moisture proofing are made to improve the possibility of detection using PLIF. In this paper, an experimental investigation of flame growth following ignition is reported. A wrinkled structure could be observed from the configuration of the ignition flame; its trajectory will be depicted. The results showed that the wrinkled structure developed downward, i.e., by deviation from the direction of the airflow. The displacement velocity of the flame was used to describe the combustion rate. Good agreement was obtained for the flame shapes of both forced ignition and autoignition. In addition, the center of combustion deviated from the center of boiler, possibly due to some irregularity in the burner’s assembly which was critical to the design of the combustion chamber.

Flame/flow dynamics at the piston surface of an IC engine measured by high-speed PLIF and PTV

Resolving fluid transport at engine surfaces is required to predict transient heat loss, which is becoming increasingly important for the development of high-efficiency internal combustion engines (ICE). The limited number of available investigations have focused on non-reacting flows near engine surfaces, while this work focuses on the near-wall flow-field dynamics in response to a propagating flame front. Flow-field and flame distributions were measured simultaneously at kHz repetition rates using particle tracking velocimetry (PTV) and planar laser induced fluorescence (PLIF) of sulfur dioxide (SO2). Measurements were performed near the piston surface of an optically accessible engine operating at 800 rpm with homogeneous, stoichiometric isooctane-air mixtures. High-speed measurements reveal a strong interdependency between near-wall flow and flame development which also influences subsequent combustion. A conditional analysis is performed to analyze flame/flow dynamics at the piston surface for cycles with ‘weak’ and ‘strong’ flow velocities parallel to the surface. Faster flame propagation associated with higher velocities before ignition demonstrates a stronger flow acceleration ahead of the flame. Flow acceleration associated with an advancing flame front is a transient feature that strongly influences boundary layer development. The distance from the wall to 75% maximum velocity (δ75) is analyzed to compare boundary layer development between fired and motored datasets. Decreases in δ75 are strongly related to flow acceleration produced by an approaching flame front. Measurements reveal strong deviations of the boundary layer flow between fired and motored datasets, emphasizing the need to consider transient flow behavior when modeling boundary layer physics for reacting flows.

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.