Formaldehyde Planar Laser-induced Fluorescence Research Articles

Active and passive prechamber assisted engine combustion: simultaneous 50kHz formaldehyde PLIF and OH* visualization

Prechamber-assisted internal combustion engines allow lean limit extension by replacing the spark plug with multiple, high-temperature, radical-rich jets that entrain and ignite the main chamber charge. This work reports the first investigation of active ultra-lean, active rich, and passive prechamber assisted prechamber engine operation (with a fixed prechamber geometry) through 50 kHz cycle-resolved measurements of formaldehyde planar laser-induced fluorescence (PLIF) and OH* imaging in a heavy-duty optical engine operating at 1200 rpm. The third harmonic of a burst mode laser (355 nm) provided the excitation source for formaldehyde PLIF, and a high-speed CMOS camera collected the fluorescence signal from the side window of the engine. Bottom-view imaging of OH* chemiluminescence was achieved using an intensified high-speed camera. The high-speed image sequence of the prechamber jet formaldehyde layer and OH* captures the main chamber heat release stages and the interaction between the moving piston and the prechamber jet. The prechamber-assisted combustion process in the main chamber could be segmented into three phases: initial jet, jet wall interaction, and Post Jet Combustion. The first two phases differ between active and passive operations owing to the global lambda, while the last phases mostly remain flame propagation driven. Jet wall interaction leading to mixing and vortical transport across the main chamber is pivotal in main chamber heat release rate shaping. The study reports the dynamics and evolution of main chamber combustion in detailed time-resolved setups.

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.

Characterization of the oxymethylene ether fuels flame structure for ECN Spray A and Spray D nozzles

Synthetic fuels will play a major role on the reduction of pollutant emissions and carbon footprint of ICE engines. The use of renewable energy and CO2 for its production maps out a promising route to achieve ICE carbon neutrality. Among these fuels, oxymethylene ethers are also interesting for their potential to drastically reduce soot formation. However, a proper characterization of their properties and behaviour is mandatory to reach full implementation in commercial engines. For this reason, this work presents a detailed characterization of the flame structure of two types of oxymethylene ethers (OMEX and OME1) using high-speed chemiluminescence imaging and Planar Laser-Induced Fluorescence (PLIF). Test were performed in a constant-pressure combustion vessel, with the operating conditions and two different injector nozzles from the Engine Combustion Network (Spray A and Spray D). Regions associated with low-temperature chemical reactions are observed thanks to the formaldehyde PLIF while evolution of the high-temperature reactions has been analysed based on hydroxyl excited state (OH*) chemiluminescence and hydroxyl PLIF. On-resonant and off-resonant measurements were performed for OH PLIF with a dye laser in order to remove other radiation sources not linked to OH fluorescence, whilst 355 nm radiation from the third harmonic of a Nd:YAG laser are used to excite CH2O molecules. On the one hand, the results show that the combustion of OME1 with Spray A is characterized by a flame structure very different to that of a diffusion flame. It is characterized by large cool flame region followed by a short high-temperature zone. On the other hand, the combustion of OMEX with both nozzles and OME1 with Spray D show more similarities with a diffusion flame structure. The stoichiometry of the fuel and the equivalence ratio fields achieved strongly affect the structure and latter evolution of the flames.

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.

Detonation wave driven by aerosolized liquid RP-2 spray

Detonation-based engines such as Rotating Detonating Engines (RDEs) have been of significant interest for aerospace propulsion. However, most detonation-related studies have focused on gaseous reactants with the majority of investigations focusing on liquid water interactions with gaseous detonations and shocks. This study explores the dynamics of detonation waves driven by aerosolized liquid fuel sprays. An unlike-doublet impinging-jet injector is used to atomize RP-2 and water into aerosolized liquid droplet cloud of measured droplet size distribution where the detonation wave interacts with the cloud mixture. Evidence of RP-2 driving the detonation phenomenon is quantified using dynamic pressure measurements and four simultaneous optical diagnostic measurements: high-speed schlieren, CH* chemiluminescence, formaldehyde planar laser-induced fluorescence (PLIF), and particle Mie scatter. The results show formaldehyde and CH* generation along with a substantial increase in pressure and wave speed when the detonation wave interacts with the RP2 mixture cloud. On the contrary, the detonation pressure and wave speed decrease are observed when the detonation wave interacts with the water droplet cloud. The investigation provides supporting information on liquid fuel droplet burning and heat release driving the detonation wave.

Experimental investigation of the two-stage ignition delay and flame structure of pentanol/n-dodecane binary fuel

High thermal efficiency and low emissions are achievable with pentanol direct injection compression ignition, but there are issues of difficulty and unstable combustion at low loads. To solve the above problem, a binary fuel is obtained by blending high-activity n-dodecane with pentanol. In a constant-volume combustion chamber, the natural flame luminosity, schlieren, formaldehyde planar laser-induced fluorescence, and the OH* chemiluminescence methods were used to investigate the combustion characteristics of the two-stage ignition delay, the formaldehyde, and the flame liftoff length. The results show that, compared with n-dodecane, the two-stage ignition delay of pentanol/n-dodecane binary fuel is longer; and the initial time of the formaldehyde signal of pentanol/n-dodecane binary fuel is delayed by about 100 μs? Additionally, under all operating conditions, the initial position of formaldehyde varies concurrently with changes in the flame lift-off position, and the formaldehyde signals of the pentanol/n-dodecane binary fuel and n-dodecane gradually decrease downstream from the flame liftoff position. The pentanol/n-dodecane combustion model was built with experimental data, and it can be helpful in further numerical simulation.

Optical diagnostics of misfire in partially premixed combustion under low load conditions

To clarify the misfire mechanism is important for stabilizing combustion in partially premixed combustion (PPC) under low load. Fuel-tracer planar laser-induced fluorescence (PLIF), formaldehyde PLIF, flame and OH* natural luminosity imaging were utilized to qualify the local equivalence ratio, low-temperature reaction and the high-temperature flame features in an optical engine. Results show that in high direct injection (DI) pressure (1000 bar), due to excessive premixing, the local equivalence ratio in the initial timing of the high temperature heat release (HTHR) is low. Although the auto-ignition flame kernels are formed in high DI pressure, they cannot stably develop, resulting in misfire during the flame development process. In late DI timing (-5 crank angle degree after top dead center, °CA ADTC), since the whole heat release process occurs in the expansion stroke, the in-cylinder temperature and pressure continue decreasing. Although the local equivalence ratio in some regions is high enough, the in-cylinder thermodynamic environment does not support the generation of more auto-ignition flame kernels, thus a small amount of auto-ignition flame kernels can only develop through flame propagation. In short, the misfire of PPC occurs in regions where the equivalence ratio is low or the in-cylinder thermodynamic environment does not further support flame development. Therefore, the trade-off relationship between equivalence ratio and temperature determines the formation of auto-ignition kernels. The local equivalence ratio and temperature distribution near the initial timing of HTHR is the key factor to ensure the subsequent stable combustion. Taking the ambient pressure of 18 bar as an example, the boundary condition where the autoignition kernels are most likely formed or the charge is most likely ignited by the nearby flame kernels is in the range of 0.53–0.62 for equivalence ratio and 740–757 K for temperature. The misfire region most likely appears when the equivalence ratio is lower than 0.49. It can be concluded that the misfire of PPC results from the synergistic effect of local equivalent ratio and temperature. The controlling parameters of injection pressure and injection timing are actually optimizing the suitable combinations of equivalence ratio and temperature to stabilize combustion.

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.

Studies of autoignition-assisted nonpremixed cool flames

Autoignition-assisted nonpremixed cool flames of diethyl ether (DEE) are investigated in both laminar counterflow and turbulent jet flame configurations. First, the ignition and extinction limits of laminar nonpremixed cool flames of diluted DEE are measured and simulated using detailed kinetic models. The laminar flame measurements are used to validate the kinetic models and guide the turbulent flame measurements. The results show that, below a critical mixture condition, for elevated temperature and dilute mixtures, the cool flame extinction limit and the low-temperature ignition limit merge, leading to autoignition-assisted cool flame stabilization without hysteresis. Based on the findings from the laminar flame experiments, autoignition-assisted turbulent lifted cool flames are established using a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner. The lift-off heights of the turbulent cool flames are quantified using formaldehyde planar laser-induced fluorescence. Based on an analogy with autoignition-assisted lifted hot flames, a correlation is proposed such that the autoignition-assisted cool flame lift-off height scales with the product of the flow velocity and the square of the first-stage ignition delay time. Using this scaling, we demonstrate that the kinetic mechanism that most accurately predicts the laminar flame ignition and extinction limits also best predicts the turbulent cool flame lift-off height.

Turbulent nonpremixed cool flames: Experimental measurements, Direct Numerical Simulation, and manifold-based combustion modeling

Turbulence, low-temperature chemistry, and their interactions in the form of turbulent cool flames are critical to understanding and improving advanced engines. Design of such engines requires tractable simulations which in turn necessitate turbulent combustion models that can account for cool flames. While manifold-based turbulent combustion models are an attractive option for hot flames, their applicability to cool flames is not yet fully understood. This is partially due to the lack of turbulent cool flame experiments, which has made model validation difficult. This work addresses these points with a combined experimental and computational investigation of turbulent nonpremixed dimethyl ether cool flames. First, a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner is developed and tested. Turbulent cool flames are studied using the formaldehyde planar laser-induced fluorescence (PLIF), acetone PLIF, and planar Rayleigh scattering techniques. The acetone PLIF signals are converted into mixture fraction values, and measurements of time-averaged temperature are derived from the Rayleigh scattering signals by taking advantage of the similarities between cool flames and unburned mixtures. Second, a Direct Numerical Simulation (DNS) of the CARAT burner is conducted. The flame is shown to be sensitive to thermal boundary conditions such that the stabilization method is dependent on a 10 K change in inlet temperature. Comparisons of first- and second-order statistics of temperature, mixture fraction, and formaldehyde between the DNS and experiment show good agreement. The validity of manifold-based turbulent combustion models in turbulent cool flames is then explored by first studying the effective Lewis numbers in the flame using a local differential diffusion parameter analysis. Turbulent cool flames are shown to have effective unity Lewis number transport even at a much lower Reynolds number than typically required for hot flames. Good agreement is shown for temperature and formaldehyde between DNS conditional means and one-dimensional nonpremixed flame solutions. Additionally, the motion of the cool flame in mixture fraction space is shown to be well described by one-dimensional nonpremixed flame solutions. These points indicate that the fundamental physics of turbulent cool flames are analogous to turbulent hot flames, implying that the same reduced-order manifold-based modeling approaches utilized for turbulent hot flames can be utilized for turbulent cool flames.

Flame Structure and Dynamics in a Premixed Dual-Mode Scramjet Combustor from Fluorescence Imaging

Formaldehyde () planar laser-induced fluorescence (PLIF) imaging was performed in a premixed, ethylene-fueled dual-mode scramjet combustor to study the flame structure and dynamic flame behavior. PLIF is used as a marker to provide information on the structure and position of the lower-temperature portion of the reaction zone. Low-repetition-rate (10 Hz) measurements were acquired at several cross-flow planes along the axial flow direction. Statistical analysis of the topology indicates sparse concentrations of formaldehyde that are intermittently detected by the PLIF measurement. Comparisons between previous large-eddy simulations, laboratory-scale flames, and the current experimental measurements indicate discrepancies between the expected and measured formaldehyde concentrations and structure. High-speed formaldehyde PLIF also was conducted at 50 kHz to further investigate the intermittency of the formaldehyde layers in the flow.

Simultaneous 36 kHz PLIF/chemiluminescence imaging of fuel, CH2O and combustion in a PPC engine

The requirements on high efficiency and low emissions of internal combustion engines (ICEs) raise the research focus on advanced combustion concepts, e.g., premixed-charge compression ignition (PCCI), partially premixed compression ignition (PPCI), reactivity controlled compression ignition (RCCI), partially premixed combustion (PPC), gasoline compression ignition (GCI) etc. In the present study, an optically accessible engine is operated in PPC mode, featuring compression ignition of a diluted, stratified charge of gasoline-like fuel injected directly into the cylinder. A high-speed, high-power burst-mode laser system in combination with a high-speed CMOS camera is employed for diagnostics of the autoignition process which is critical for the combustion phasing and efficiency of the engine. To the authors’ best knowledge, this work demonstrates for the first time the application of the burst-system for simultaneous fuel tracer planar laser induced fluorescence (PLIF) and chemiluminescence imaging in an optical engine, at 36 kHz repetition rate. In addition, high-speed formaldehyde PLIF and chemiluminescence imaging are employed for investigation of autoignition events with a high temporal resolution (5 frames/CAD). The development of autoignition together with fuel or CH2O distribution are simultaneously visualized using a large number of consecutive images. Prior to the onset of combustion the majority of both fuel and CH2O are located in the recirculation zone, where the first autoignition also occurs. The ability to record, in excess of 100 PLIF images, in a single cycle brings unique possibilities to follow the in-cylinder processes without the averaging effects caused by cycle-to-cycle variations.

Optical diagnostics on the reactivity controlled compression ignition (RCCI) with micro direct-injection strategy

Micro direct-injection (DI) strategy is often used to extend the operation range of the reactivity controlled compression ignition (RCCI) to high engine load, but its combustion process has not been well understood. In this study, the ignition and flame development of the micro-DI RCCI strategy were investigated on a light-duty optical engine using formaldehyde planar laser-induced fluorescence (PLIF) and high-speed natural flame luminosity imaging techniques. The premixed fuel was iso-octane and an oxygenated fuel of polyoxymethylene dimethyl ethers (PODE) was employed for DI. The fuel-air equivalence ratio of DI was kept at 0.09 and the premixed equivalence ratio was varied from 0 to 1. RCCI strategies with early and late DI timing at –25° and –5° crank angle after top dead center were studied, respectively. Results indicate that the early micro-DI RCCI features a single-stage high-temperature heat release (HTHR). The combustion in the low-reactivity region shows a combination of flame front propagation and auto-ignition. The late micro-DI RCCI presents a two-stage HTHR. The second-stage HTHR is owing to the combustion in the low-reactivity region that is dominated by flame front propagation when the premixed equivalence ratio approaches 1. For both early and late micro-DI RCCI, the intermediate-temperature heat release (ITHR) of iso-octane, indicated by formaldehyde, takes place in the low-reactivity region before the arrival of the flame front. This is quite different from the flame front propagation in spark-ignition (SI) engine that shows no ITHR in the unburned region. The DI fuel mass is a key factor that affects the combustion in the low-reactivity region. If the DI fuel mass is quite low, there is more possibility of flame front propagation; otherwise, sequential auto-ignition dominates. The emergence of the flame front propagation in micro-DI RCCI strategy reduces its combustion rate and peak pressure rise rate.

DME/Oxygen wall-stabilized premixed cool flame

Wall-stabilized cool flames have been studied through numerical analysis and a series of experiments. One- and two-dimensional numerical simulations were performed to estimate the characteristics of the wall-stabilized cool flames, such as flammability and temperature/species distributions. Based on the computational results, the ignition condition of the cool flame at a fixed wall temperature has been identified with the strain rate between the van't Hoff point and the cool flame extinction point. In experiments with an impinging jet burner and a heated plate, the spontaneous ignition of the cool flame on the heated wall has been successfully established under the conditions predicted by the present numerical simulations. Spatial distributions of the HCHO concentration and flame temperature were measured through formaldehyde Planar Laser-Induced Fluorescence (HCHO-PLIF) and thermocouple measurements, respectively. It is found that the measurement data show a reasonable accordance with the simulation results with reduced low-temperature reactivity.

Optical study of spray-wall impingement impact on early-injection gasoline partially premixed combustion at low engine load

Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray-wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about −30° to −60° ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of −35° (PPC-35) and −60° (PPC-60) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel-lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than −60°), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing.

Investigation of transient ignition processes in a model scramjet pilot cavity using simultaneous 100 kHz formaldehyde planar laser-induced fluorescence and CH* chemiluminescence imaging

Ignition processes in scramjet pilot cavities are highly transient events that are influenced by factors including freestream Mach number and inlet geometry, turbulence intensity, cavity geometry, ignition source, and fueling composition and flow rate. In particular, the location of the flame kernel and associated propagation rate of the flame front throughout the cavity can significantly influence the end state of the ignition process. In this work formaldehyde (CH2O) was used as a flame marker to track ignition progress in a plane throughout the span-wise width of the cavity, while chemiluminescence imaging provided path-integrated flame location along the span-wise and axial directions. Planar laser-induced fluorescence (PLIF) excitation utilized the 355nm frequency-tripled output of an Nd:YAG burst-mode laser operating at 50–100kHz over 10ms with available pulse energy up to 80mJ. Simultaneous CH* chemiluminescence imaging from the top of the cavity was obtained with a high-speed complementary metal-oxide semiconductor camera. A freestream Mach number of 2 with ethylene fuel rates from 55–90 standard liters per minute were examined along with two different ignition sources: a spark discharge and pulse detonator. The resulting formaldehyde PLIF and chemiluminescence images indicate a strong correlation between fueling rate and the delay between the onset of ignition and stable combustion. More importantly, the span-wise propagation rate and structure of the flame front is highly dependent on the fueling rate, burning region, and ignition source.

Coupling of flame geometry and combustion instabilities based on kilohertz formaldehyde PLIF measurements

In the past, flame geometry was found to play an important role in determining when strong pressure fluctuations associated with a combustion instability occur in a gas turbine model combustor. The goals of this study were to use flame surface area as a more accurate way to quantify the flame shape and the heat release rate, in lieu of chemiluminescence; to accurately resolve both the spatial structure and the time history of the heat release fluctuations, and to show that kilohertz formaldehyde planar laser-induced fluorescence provides a new way to achieve these goals. The dual-swirl burner, developed at DLR Stuttgart by Meier et al. was operated using dimethyl ether (DME) to study fluctuations in the flame surface area, flame brush, flame length, and heat release rate. To understand the instability, accurate measurements are needed of the correlation between heat release rate fluctuations and pressure fluctuations. Thus heat release rate must be recorded as a function of time and space. However conventional chemiluminescence offers only a line-of-sight measurement. High-speed CH2O PLIF was applied to study the motion of flame surfaces in response to the pressure oscillations. Results show that the flame surface density fluctuated at the acoustic frequency (typically 320Hz). The frequency of the combustion instability was found to increase as either the gas velocity or the flame speed was increased. These trends are consistent with previous n-τ theories that model the convective time delay. Strong pressure oscillations occur for conditions when the flame is nearly flat; the instability disappears when the flame becomes V-shaped. The explanation is that all points on a flatter flame have the same convective time delay but this is not true for the V-flame geometry.

Simultaneous formaldehyde PLIF and high-speed schlieren imaging for ignition visualization in high-pressure spray flames

We applied simultaneous schlieren and formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) imaging to investigate the low- and high-temperature auto-ignition events in a high-pressure (60bar) spray of n-dodecane. High-speed (150kHz) schlieren imaging allowed visualization of the temporal progression of the fuel vapor penetration as well as the low- and high-temperature ignition events, while formaldehyde fluorescence was induced by a pulsed (7-ns), 355-nm planar laser sheet at a select time during the same injection. Fluorescence from polycyclic aromatic hydrocarbons (PAH) was also observed and was distinguished from formaldehyde PLIF both temporally and spatially. A characteristic feature previously recorded in schlieren images of similar flames, in which refractive index gradients significantly diminish, has been confirmed to be coincident with large formaldehyde fluorescence signal during low-temperature ignition. Low-temperature reactions initiate near the radial periphery of the spray on the injector side of the spray head. Formaldehyde persists on the injector side of the lift-off length and forms rapidly near the injector following the end of injection. The consumption of formaldehyde coincides with the position and timing of high-temperature ignition and low-density zones that are clearly evident in the schlieren imaging. After the end of injection, the formaldehyde that formed on the injector side of the lift-off length is consumed as a high-temperature ignition front propagates back toward the injector tip.

Near-wall formaldehyde planar laser-induced fluorescence measurements during HCCI combustion

The role of thermal boundary layers on homogeneous charge compression ignited engine combustion was directly evaluated by measuring the spatial profile of the formaldehyde concentration near the combustion chamber wall. Formaldehyde forms naturally as an intermediate species of the combustion process. A through-the-wall imaging scheme, in which the laser propagated at an angle through a window and the fluorescence signal was collected through the same window, was employed to avoid the effect of optical vignetting. Intensity profiles acquired in the wall-normal direction from the mean image show that during the first stage of combustion (cool flame) there is a substantial region of lower intensity, and presumably lower formaldehyde concentration, near the wall. This region extends more than 5 mm. After the completion of the first stage of combustion, the formaldehyde is found adjacent to the wall within the measurement limits of the system. During the second stage of combustion, the intensity near the wall and the intensity gradient in the normal direction remain constant, but the intensity is found to decrease preferentially away from the wall at a distance comparable to that observed in the first-stage combustion. Both of these findings are consistent with a thermally affected region near the wall that delays combustion. Individual cycle images, however, show that the true spatial gradients in the flow field are much steeper than in the mean data, and that regions of high temperature fluid can be found in close proximity to the wall. No appreciable thickening of the thermally affected region was seen at low equivalence ratios where there was incomplete combustion.

In-cylinder unburned hydrocarbon visualization during low-temperature compression-ignition engine combustion using formaldehyde PLIF

Formaldehyde (H 2CO) is visualized by planar laser-induced fluorescence (PLIF) in a heavy-duty direct-injection diesel engine to better understand the sources of unburned hydrocarbon (UHC) emissions. H 2CO is used as a tracer for UHC based on chemical kinetic simulations, which show that after the first-stage of ignition, the evolution of H 2CO is very similar to that of UHC in the bulk gases. Modern low-temperature, low-nitrogen oxides (NO x ) combustion conditions are achieved by diluting the intake stream with nitrogen to 12.7% oxygen by volume, simulating exhaust gas recirculation. A range of ignition delays (IDs) is produced by adjusting the intake temperature and/or the fuel injection timing. Spectral measurements of the laser-induced emission after 355-nm excitation show that broadband emission from sources other than H 2CO can be significant, so the PLIF images must be interpreted with care. A frequency-domain covariance analysis is offered as a way to quantify the contribution of H 2CO fluorescence in the images. Longer ID conditions are found to have both higher-UHC emissions and stronger H 2CO fluorescence persisting late in the cycle, especially near the injector. Based on kinetic model predictions of longer H 2CO lifetimes in leaner regions, it is concluded that for long ID conditions, mixtures near the injector after the end of injection are too lean to achieve complete combustion, thus contributing to UHC emissions.