Laser-induced Fluorescence Imaging Research Articles

Effects of ozone on n-heptane low temperature chemistry and premixed cool flames

Abstract Ozone (O3) has been shown to accelerate low and high temperature chemistry and is a promising strategy to effectively enhance and control combustion and ignition processes, such as for engines. There has, however, been a lack of experimental investigation of O3 enhancement effects on low temperature chemistry and premixed cool flames. This study undertook a detailed experimental and numerical assessment of O3 enhancement effects on ignition behavior, propagation speeds, flame temperatures, and CH2O production of n-heptane cool flames as functions of O3 addition. Zero-dimensional simulations were used to gain insights into the effects of O3 on two-stage ignition behavior and gradual suppression of NTC phenomena. Propagation speeds of O3-enhanced cool flames were experimentally determined using a flame lift-off technique, and one-dimensional simulations utilizing several full and reduced mechanisms were performed for comparison. Enhancement by O3 versus enhancement by preheating were experimentally compared and analyzed to evaluate relative effectiveness of these enhancement strategies. Finally, downstream temperature measurements and formaldehyde planar laser-induced fluorescence imaging (CH2O PLIF) were performed at different O3 enhancement levels to further elucidate O3 impacts, and numerical simulations were performed to support experimental insights. Results of this study reveal O3 impacts on low temperature kinetics and resulting impacts on low temperature ignition and cool flames.

Detailed measurements of transient two-stage ignition and combustion processes in high-pressure spray flames using simultaneous high-speed formaldehyde PLIF and schlieren imaging

This study investigates the low- and high-temperature ignition and combustion processes in a high-pressure spray flame of n-dodecane using simultaneous 50-kHz formaldehyde (HCHO) planar laser-induced fluorescence (PLIF) and 100-kHz schlieren imaging. The PLIF measurements were facilitated through the use of a pulse-burst-mode Nd:YAG laser, producing a 355-nm pulse-train with 300 pulses at 70 mJ/pulse, separated by 20-µs, in a 6-ms burst. The high-speed HCHO PLIF signal was imaged using a non-intensified CMOS camera with dynamic background emission correction. The acquisition rate of this HCHO PLIF diagnostic is unique to the research community, and when combined with high-speed schlieren imaging, provides unprecedented opportunity for analysis of the spatiotemporal evolution of fuel jet penetration and low- and high-temperature ignition processes relevant to internal combustion engine conditions. The present experiments are conducted in the Sandia constant-volume preburn vessel equipped with a new Spray A injector. The influences of ambient conditions are examined on the ignition delay times of the two-stage ignition events, HCHO structures, and lift-off length values. Consistent with past studies of traditional Spray A flames, the formation of HCHO is first observed in the jet peripheries where the equivalence ratio (Φ) is expected to be leaner and hotter and then grows in size and in intensity downstream into the jet core where Φ is expected to be richer and colder. The measurements demonstrate that the formation and propagation of HCHO from the leaner to richer region leads to high-temperature ignition events, supporting the identification of a phenomenon coined “cool-flame wave propagation” during the transient ignition process. Subsequent high-temperature ignition is found to consume the previously formed HCHO in the jet head, while the formation of HCHO persists in the fuel-rich zone near the flame base over the entire combustion period.

Fuel placement and atomization inside a gas-turbine fuel injector at realistic operating conditions

The atomization process and fuel placement inside a gas turbine fuel injector are investigated at realistic operating conditions to understand the role of individual flow processes and hardware components. A high pressure, high temperature spray test facility is developed that can simulate a wide range of aero-engine operating conditions. The fuel injector is a swirl cup - dual orifice nozzle combination which is used in the rich dome combustors of various aero-engines. The internal flow field is captured using time-resolved laser induced fluorescence imaging and phase Doppler interferometry measurements. A stage-wise characterization is adopted to track the spray formation as the flow evolves through the swirl cup. The fuel placement at the cup exit is mainly decided by pilot nozzle spray characteristics and the relative momentum exchange between the gas and liquid phases. At the swirl cup exit, two class of droplets are formed: (1) droplets originated directly from the pilot nozzle, which are convected downstream by the primary air swirl, (2) droplets originated from the rim, which are atomized by the shear layer. The relative percentage of these two droplet classes depends on the pilot nozzle operation at low and high power conditions. The effect of primary spray and wall-filming are minimal towards improving the atomization. In both low and high power operating cases, the counter-rotating shear layer interactions are identified as the major mechanism that is improving the atomization process in swirl cup.

Elevated OH production from NPHFD and its effect on ignition

This work explores ignition in flowing mixtures of methane and air at 100 kPa and 295 K initial temperature using a nanosecond-pulsed high-frequency discharge ignition source. Simultaneous OH planar laser-induced fluorescence (PLIF) and schlieren imaging were utilized at a frequency of 50 kHz to examine the time dependent radical generation during and after the plasma discharge. The results indicate that a significant volume of OH was generated in the discharge, and the magnitude of the said volume was a strong function of the pulse repetition frequency (PRF). In addition, it is demonstrated that the intensity of the OH-PLIF signal during and shortly after the discharge is elevated for PRF≥10 kHz, indicating that both the volume and concentration of OH are built up at high PRF. This accumulation of OH radicals in the inter-electrode region is directly correlated with high ignition probability, with higher PRF leading to faster OH accumulation in a step-wise fashion. It is shown that in cases in which the PRF is below a certain threshold (<10 kHz), OH accumulation does not occur, and it is believed that this condition, along with lower discharge temperatures, is directly responsible for reduced ignition probability.

Megahertz-rate OH planar laser-induced fluorescence imaging in a rotating detonation combustor.

Megahertz-rate hydroxyl radical planar laser-induced fluorescence (OH-PLIF) was demonstrated in a hydrogen/air rotating detonation combustor for the first time, to the best of our knowledge. A custom injection-seeded optical parametric oscillator (OPO) pumped by the 355 nm output of a high-energy burst-mode laser produced narrowband pulses near 284 nm for OH excitation. The system generated sequences of more than 150 ultraviolet pulses with 400 µJ/pulse at 1 MHz and 150 µJ/pulse at 2 MHz. The order of magnitude improvement in the repetition rate over prior OH-PLIF measurements and in the number of pulses over previous megahertz burst-mode OPOs enables spatiotemporal analysis of complex detonation combustion dynamics.

Monitoring the Restoration of a Seventeenth-Century Wooden Artwork Using Laser-Induced Fluorescence and Digital Image Analysis.

The present paper is aimed at demonstrating the capabilities of digital image analysis (DIA) to support conservation of painted artwork. Laser-induced fluorescence (LIF) imaging has been usually used in the diagnosis of wall paintings. In this case, LIF is applied to the case study of a painted wooden canopy, and most successful data processing techniques are presented in the discussion of results. The Jesús del Gran Poder canopy, completed by Francisco Ruiz Gijón in 1692, is an oil panel painting on wood. Eight panels of the canopy have been study by LIF. This technique is capable of remotely acquiring hyperspectral images operating in fluorescence mode following ultraviolet laser excitation. LIF spectra combined with principal component analysis, spectral angle mapper, and DIA provide a chemical mapping of the treated wooden surface of the panels. Besides, LIF spectrum is as a fingerprint of the panels that allows stablishing differences between them. LIF imaging analysis has proven to be a very useful tool for mapping retouching work, tracking previous restorations, and detecting chemicals on the wood in order to monitor restorations.

Application of FRAME for Simultaneous LIF and LII Imaging in Sooting Flames Using a Single Camera.

In this article, the application of the FRAME (Frequency Recognition Algorithm for Multiple Exposures) technique is presented for multi-species measurements in symmetric and asymmetric ethylene/air diffusion flames. Laminar Bunsen-type and swirled diffusion flames are investigated to gain a better understanding of sooting combustion. For this purpose, simultaneous imaging is conducted in terms of Laser-Induced Fluorescence (LIF) of Polycyclic Aromatic Hydrocarbons (PAH) and Laser-Induced Incandescence (LII) of soot particles. Subsequently, the approach is utilized for simultaneous imaging of hydroxyl (OH)-LIF and soot-LII. Here, the modulated LIF- and LII-signals are acquired together as a single sub-image—with a single exposure utilizing the full sensor size of a single camera. By employing the frequency-recognition algorithm on the single image, the LIF- and LII-signals are spectrally isolated—generating two individual LIF- and LII-images. The flame luminosity and out-of-focus light such as reflected surrounding laser light are detected as non-modulated signals in the unprocessed image. These unwanted signals are suppressed using the image post-processing, and, therefore, the image contrast of the two resulting images is improved. It is found that PAHs mainly exist in the inner region near the burner and are surrounded by soot. The majority of the OH is distributed on the outer edges of the flame—representing the reaction zone and soot-oxidation region of the flame.

Particle temperature and potassium release during combustion of single pulverized biomass char particles

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O2 concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them.

Experimental study of vorticity-strain interactions in turbulent premixed counterflow flames

The effects of heat release on interactions between vorticity (ω) and strain rate (s) in turbulent premixed CH4/O2/N2 counterflow flames are investigated using simultaneous OH laser-induced fluorescence (LIF) and tomographic particle image velocimetry (TPIV) measurements. A comparison between the flames and a corresponding turbulent non-reacting variable density N2-vs-products counterflow reveals the impact of heat release on vorticity-strain rate alignment statistics. Vorticity and strain rate statistics in the flames and non-reacting flow are conditioned on distance from the local flame front and gas mixing layer interface (GMLI) contours, respectively. The magnitude, alignment, and spatial distribution of the vorticity and principal strain rates (s1,s2,s3) are rather different when heat release is present. Density variations without heat release enhance the ω-s2 alignment while significantly reducing the ω-s3 alignment and modestly reducing the ω-s1 alignment. In contrast, heat release at the flame front further reduces the ω-s1 alignment but increases the ω-s3 alignment and suppresses the preferential ω-s2 alignment. Furthermore, increasing turbulence diminishes the effect of heat release on this preferential alignment. In regions with the largest vorticities, both the reacting and non-reacting counterflows show an increase in the probability of ω-s2 alignment. All counterflow cases have a net positive vortex-stretching contribution to the enstrophy production with a peak production rate at the flame front or GMLI, but the peak values depend on the density variation, heat release, and turbulence level. Elucidation of the complex interplay between these factors contributes to the understanding of the dynamics of turbulence-flame interactions.

Plasma-assisted stabilization of premixed swirl flames by gliding arc discharges

This paper presents key effects of gliding arc discharges on the stability limits of externally perturbed methane/air swirl flames. The low-frequency flow disturbance mimicking the transient conditions in practical combustor was generated by a flow pulsation system with repetition of ∼5 Hz. The synchronized OH planar laser-induced fluorescence and particle imaging velocimetry (OH-PLIF/PIV) measurements were utilized to visualize the instantaneous flow and flame structures. We find that, with gliding arc discharges, the lean blowout limits in perturbed flames are dramatically extended by up to 60%, which is more pronounced than in non-perturbed flames. OH-PLIF/PIV results show that, the stabilization effect can be explained by the expansion of the inner recirculation zone, wherein the opening angle increases from 52° to 62° due to the plasma enhancement. Time-resolved OH-PLIF images illustrate that, in the presence of gliding arc discharges, the flame response to the flow disturbance consists of three stages, including enhancement, extinction, and re-ignition processes. Furthermore, proper orthogonal decomposition (POD) analysis of OH-PLIF images reveals a considerable increase of mean energy contents and suppression of turbulent fluctuations. It suggests that the enhancement of gliding arc discharges is not a simple superposition of the energy deposit, but a strong non-linear coupling between the plasma and the turbulent flame with closed-loop feedbacks.

Using Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry to study the heating of a droplet with different tracers and schemes of attaching it on a holder

The Planar Laser Induced Fluorescence optical technique is used to measure the droplet temperature field, and the Micro Particle Image Velocimetry – the velocities of natural convection in the droplet. We focus on the convection, vortex formation centers, convection velocities, and the influence of convection on the temperature in different droplet sections. The most common tracer particles are used (Al2O3, TiO2, polyamide). Differences in the temperature fields of evaporating droplets with tracers and without them are analyzed. We compare the reliability of measuring droplet temperature using Planar Laser Induced Fluorescence and fast-response thermocouples. The concentrations of tracers are varied to establish the ranges in which the fields of temperature and convection velocities can be reliably measured in rapidly heated and evaporating droplets. We study the influence of typical holder materials on droplet heating rates. Metal rods, wires, and nichrome wires are used. The focus is on illustrating the influence of the holder type and material on the droplet temperature field, its heating and evaporation rates, as well as its lifetime. We consider typical schemes of droplet fixation on holders, as well as the main differences in the fields of temperature and convection velocities in different droplet sections. As a result, we have obtained the concentrations of Al2O3 and TiO2 tracers that make it possible to use both Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry at the same time. Examples are given of particle agglomeration and photobleaching. The use of polyamide particles as tracers is found not feasible for evaluating the convection rates in droplets, because they are displaced from the deep layers of a droplet to the surface. It is established that if a Rhodamine B solution contains Al2O3 and TiO2 tracers, the fluorophore settles on those particles, which may degrade the accuracy of temperature measurement using Planar Laser Induced Fluorescence. The experimental findings are important to develop the fields of research into the conditions and characteristics of heating droplets, films, and liquid jets, using Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry.

Effect of incident laser sheet orientation on the OH-PLIF imaging of detonations

This study aims to investigate the effect of laser sheet orientation on the OH-planar laser-induced fluorescence (OH-PLIF) imaging of a detonation wave and to identify the potential benefit of using a transverse laser orientation, compared to the conventional frontal orientation. In this study, we developed a new laser-induced fluorescence (LIF) model, based on a preexisting one, to include a more fundamental calculation of the absorption lineshape and the effects of the laser orientation on the fluorescence signal. One- and two-dimensional simulations are used to validate this new LIF model for both laser orientations using OH-PLIF images from the literature. The effect of the two laser orientations on detonation imaging is investigated numerically on a two-dimensional OH field. Based on these results, the frontal orientation seems more suitable to collect information near the detonation front, while the transverse laser orientation can give more quantitative information far from the front and only for short optical paths. With the present laser configuration and experimental conditions, both laser orientations present the same limitation with a strong laser energy absorption for long optical paths. While these first results show a qualitative agreement with experimental data, new experimental data are required to quantitatively validate this promising transverse laser orientation and confirm its relevance for OH-PLIF imaging of detonations.

High-fidelity flame-front wrinkling measurements derived from fractal analysis of turbulent premixed flames with large Reynolds numbers

This work assesses the level of wrinkling that premixed flames exhibit when subjected to turbulence levels in excess of ten times that of previous experimental studies of this nature. This assessment was facilitated by performing fractal analysis of isocontours extracted from simultaneously acquired planar Rayleigh scattering (PRS) and planar laser-induced fluorescence (PLIF) images of OH. Images were collected from 21 separate flames with turbulent Karlovitz (KaT,P) and Reynolds numbers (ReT,0) as large as 210 and 99,000, respectively. Values of the inner-cutoff scale (εi) and fractal dimension (FD) were extracted from two separate isocontours associated with each imaging technique. Results indicate that values of εi normalized by the laminar flame thickness (δL,P) follow the scaling: εi/δL,P∼KaT,P−0.22, which holds for results from prior studies but deviates from those derived from theoretical principles. Cases with main-flame equivalence ratios (ϕm) substantially less than that of their pilot flame (ϕp) did not follow the aforementioned scaling; however, adjusting δL,P and SL values such that they corresponded to the average of ϕm and ϕp was observed to reconcile this discrepancy. Yet, cases with ϕm ∼ ϕp did not deviate much from the scaling above. The influence of isocontour choice and targeted scalar on values of εi and FD was assessed and found to be much more significant for the former than the latter. Additionally, εi and FD exhibit a modest increase with axial distance from the flame base, corroborating prior results.

Hydroxyl radical planar laser-induced fluorescence imaging in flames using frequency-tripled femtosecond laser pulses.

Ultra-short optical pulses in the ultraviolet (UV) region are of significant interest for combustion and reacting flow diagnostics, as most important chemical species have electronic resonance transitions in the UV region. Optical parametric amplifiers are typically used for frequency conversion of femtosecond (fs) pulses from near-IR to UV; however, their implementation for practical imaging applications is limited because of the low conversion efficiency and extreme sensitivity to ambient conditions. In this work, we report the implementation of direct-frequency-tripled, fs laser pulses from a tunable amplified laser system for high-resolution imaging of hydroxyl (OH) radical in flames. The fundamental laser output near 850 nm is frequency tripled to obtain approximately 283.3-nm UV radiation. OH planar laser-induced fluorescence (PLIF) imaging at 1 kHz is demonstrated in turbulent flames with image sheet heights in excess of 45 mm and a signal-to-noise ratio better than 25. These results represent over 3× increase in the imaging dimensionality compared to traditional OPA-based systems. Additionally, the third-harmonic generation apparatus is compact, robust, and easy to operate while providing near-Gaussian beam profiles. Simple power scaling suggests another factor of 3 or more increase in sheet height can be achieved for kilohertz-rate practical combustion diagnostics applications.

Effects of Flame Behaviors on Combustion Noise from Lean-Premixed Hydrogen Low-Swirl Flames

This experimental study explored the influence of global/local flame behaviors on direct combustion noise produced by a lean-premixed gaseous low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten kilohertz chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuations, respectively, whereas 2-D particle image velocimetry (2-D PIV) measurements at 4 Hz were applied to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of . Spectral proper orthogonal decomposition of the chemiluminescence and OH-PLIF images revealed that the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, whereas the secondary peak was attributed to periodically generated vortical flame structures near the downstream side of the flame boundary. The 2-D PIV results suggest that vortical flame structures are likely generated by the interaction between the flame and the inner/outer shear layers.

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ϕ, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH3 emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H2 from NH3 decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH3 and N2O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N2O and 4457 ppmv of NH3 at ϕ = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH3 and N2O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N2O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH3-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.

A poly-diagnostic study of bipolar high-power magnetron sputtering: role of electrical parameters

Ion acceleration in a bipolar high-power impulse magnetron sputtering discharge working with Ar gas and using a Ti cathode has been investigated. The multiple discharge diagnostic approaches have been implemented, including the target and substrate current probe measurements, mass spectrometry and laser-induced fluorescence. In particular, the current waveforms were analysed in detail, showing the particularities of the current evolution during the negative (plasma) and positive voltage pulses. It is shown that the substrate current during the positive voltage pulse reveals two peaks, corresponding, as suggested, to two groups of the positive ions repulsed from the cathode by the applied positive potential. These observations correlate well with the mass spectrometry measurements as well as with the two-dimensional ion density mapping performed for several discharge conditions by laser-induced fluorescence imaging technique.

The Leidenfrost transition of water droplets impinging onto a superheated surface

Water droplets impinge on a sapphire wall heated to a temperature ranging from 300∘C to 700∘C. Advanced measurement techniques are used to characterize the thermal processes associated with the drop impact. IR thermography, implemented by coating the impacted surface with an opaque and emissive material in the IR domain, makes it possible to measure the temperature of the solid surface during the impact process. Laser-induced fluorescence imaging is used to characterize the temperature field in the spreading droplet. At the onset of film boiling, the temperature distribution on the solid surface is marked by the formation of a fingering pattern. This latter corresponds to spatial fluctuations in the thickness of the vapor film. When a water droplet hits an overheated wall with a significant impact velocity, the thermal contact is so rapid and intense that the liquid temperature can largely overtake the saturation temperature and reach the spinodal temperature, i.e. the highest temperature at which water can exist in the liquid state. In this situation, experiments show that the dynamic Leidenfrost point is directly linked to the spinodal temperature. A superheating of the liquid by several hundred of ∘C and the subsequent homogeneous nucleation, have to be considered to describe the heat transfer in the film boiling regime.

Heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel

This paper extends the application of laser-induced fluorescence imaging of OH & CH2O (LIF-OH-CH2O) to provide a measure of heat release zones in turbulent, moderately dense spray flames of ethanol and biodiesel fuels. A custom filter is used to remove interference from stimulated Raman scattering arising from the liquid fragments. Mie scattering is measured jointly on a separate camera to co-locate the fluid fragments with respect to the reaction zones. It is found that while the overall spray flame structure is similar to that of a turbulent gaseous diffusion flame, additional complexities arise due to the continuous release of fresh fuel vapor from the evaporating fragments. Structures referred to as burning rings of different sizes are observed, and these are ignited by interfacing with a hot edge before they grow, propagate, and burnout. Statistics on the occurrence of these rings, with and without clouds of liquid fragments within them, are presented. These findings are common to both ethanol and biodiesel, although measurements in the latter were affected by strong fluorescence interference from the excited components in the biodiesel blend.

Effects of Sinusoidal Excitation on Transverse Jet Dynamics, Structure, and Mixing

This experimental study explored the effects of axisymmetric sinusoidal excitation on structural and mixing characteristics in the equidensity jet in crossflow (JICF). Planar laser-induced fluorescence imaging of acetone seeded in the jet fluid was used in this quantification, where the jet Reynolds number was fixed at and crossflow velocity was varied. Jet-to-crossflow momentum-flux ratios ranged from , with a convectively unstable upstream shear layer (USL) in the absence of forcing, to , with a globally unstable USL. Sinusoidal excitation had a relatively significant influence on the convectively unstable JICF at larger values, especially when the USL was locked-in to the forcing frequency, causing the jet cross section to become more symmetric and causing molecular mixing, quantified in both centerplane and cross-sectional planes, to improve. For a globally unstable JICF, only sinusoidal forcing corresponding to clear lock-in conditions, close to the fundamental shear-layer frequency and/or at very high amplitudes, affected jet structure, with somewhat moderate improvements in molecular mixing. Typically, excitation conditions creating enhanced jet penetration and spread did not correlate directly with improved molecular mixing; there was a stronger correlation of improved mixing with creation of a more symmetric jet cross section, in some cases with a counter-rotating vortex pair structure.