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All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Experimental measurements of soot formation in fuel-rich homogeneous mixtures using an optical rapid compression machine

Advanced combustion strategies are necessary for the use of more environmentally sustainable fuels than traditional diesel. Alcohol fuels and alcohol/gasoline blends are of particular interest as they are readily available in the marketplace. Heavy-duty engines typically use compression ignited, conventional diesel mixing controlled combustion. Mixing controlled combustion features a non-premixed diffusion flame with a wide range of local equivalence ratios, leading to potentially high rates of soot formation. This work studies the sooting behavior of iso-octane and ethanol as a function of equivalence ratio. Measurements are carried out in a rapid compression machine (RCM) and are reported for pre-ignition conditions of 10–30 bar and temperatures of 650–800 K. Theoretical equilibrium and bulk gas temperatures are calculated for both fuels. These data are used to identify the critical equivalence ratio, the lowest equivalence ratio where soot is detected with a single-pass laser extinction diagnostic. The critical equivalence ratio for iso-octane varies between 1.82 and 1.77 for compressed pressures of 10 and 20 bar, respectively. Ethanol, sometimes considered sootless, had a critical equivalence ratio between 2.37 and 2.12 for compressed pressures of 20 and 30 bar, respectively. When characterizing soot formation by oxygenated equivalence ratio, the critical equivalence ratios for ethanol approach those of iso-octane. This suggests the oxygenated nature of alcohol fuels reduces sooting tendency, but other factors such as fuel molecular structure and morphology may play a role. It was observed that ethanol will form soot at equivalence ratios only slightly higher than iso-octane, which could have implications in mixing controlled combustion. It was seen for both fuels that soot formation is pressure sensitive, with the critical equivalence ratio being inversely proportional to compressed pressure and the rate of soot formation. Future work will investigate the sooting behavior of gasoline/ethanol blends.

02 PETROLEUM↗
DOE OSTI2021

Validation of the HyRAM+ physics models for use with propane

The Hydrogen Risk Assessment Model Plus (HyRAM+) toolkit combines quantitative risk assessment with simulations of unignited dispersion, ignited turbulent diffusion flames, and indoor accumulation with delayed ignition of fuels. HyRAM+ is differentiated from HyRAM in that it includes models and leak data for other alternate fuels. The models of the physical phenomena need to be validated for each of the fuels in the toolkit. This report shows the validation for propane which is being used as a surrogate for autogas, which is a mixture of propane and butane and used in internal combustion engines in vehicles. For flame length comparisons, five previously published experiments from peer reviewed journals were used to validate our models. The validation looked at flame lengths and flame widths with respect to different leak diameters, mass flow rates, and source pressures. Most of the sources included more than one set of experimental data, which were collected using different methods (CCD cameras, IR visualization etc.). In general, HyRAM+ overpredicts the flame lengths by around 65%. For heat and radiation models, we compared the heat flux and radiation data reported from two different sources to the values calculated by HyRAM+. For higher mass flow rates, the HyRAM+ calculated flame length results gave a better estimate of what is found in the experiments (65% error), but a higher error (85%) is observed between the HyRAM+ calculated lengths and the experimental flame lengthsfor lower mass flows. Some differences can be attributed to outdoor environmental effects (i.e. wind speed) and uncertainties in jet flame shapes. The propane flame trajectory is predicted for a high Reynolds number case with Re = 12,500 and a low Reynolds number case where Re = 2,000. The Re=12,500 case which is momentum dominated matches well with the experimental flame trajectory, but the agreement for the bouancy driven low Reynolds number case is not as good. Dispersion modeling for unignited propane was also analyzed. We compared the mole fraction, mixture fraction, mean velocity, concentration half width, and inverse mass concentration over an axial distance from different credible journals to the values calculated by HyRAM+. The results display good agreement but generally, HyRAM+ predicts a wider profile for mole fraction and mixture fraction experiments. Overall, HyRAM+’s results are reasonable for predicting the flame length, heat flux, flame trajectory, and dispersion for propane and can be used in risk analyses

08 HYDROGEN↗
DOE OSTI2020

Experimental and theoretical study of the soot-forming tendencies of furans as potential biofuels

Sustainably-produced biofuels, including furans derived from lignocellulosic biomass, are being studied as potential next-generation fuels. Biofuels have unique structures that are absent in conventional fuels and they potentially emit fewer particulates than conventional fuels because of the presence of oxygen atoms. In this work, we focus on quantification and measurement of sooting tendency. A set of furans was selected and their sooting tendencies were characterized by Yield Sooting Index (YSI), which was obtained by measuring the line-of-sight spectral radiance (LSSR) of a coflow laminar diffusion flame doped with 1000 ppm of each test fuel. The measured YSIs are lower than traditional gasoline fuels and benzenoid aromatics (e.g., YSIbenzene = 100). We also performed numerical simulations with a detailed chemical kinetic mechanism to analyze the flame chemistry of 2-methylfuran (2MF). 4→5 H-atom migration in the furan ring causes 31.0% of the total decomposition and is the dominant fuel consumption pathway leading to soot. DFT calculations via the G4 composite method were carried out to obtain activation energies (Ea) and bond dissociation energies (BDE). The results showed that the 4→5 H-atom migration in the furan ring has the second lowest Ea and subsequently forms a 1,2-diene-substituted ketone derivative as a stable species. The ring-opening products of the 4→5 H-atom migration are propyne + a linear ketene for 2-n-alkylfurans, and 1-butyne + a methyl-substituted ketene for 2-t-butylfuran. These results can explain the trends of the measured YSIs of 2-alkylfurans.

09 BIOMASS FUELS↗
DOE OSTI2022

Editorial: Particulate Matter Emissions From Conventional and Reformulated Fuel Combustion: Advances in Experiments and Simulations

Particulate matter (PM or soot) emissions generated by combustion of conventional and reformulated fuels adversely impact the environment and human health (Bond et al., 2013; Landrigan et al., 2018; World Health Organization (WHO), 2016), generating public awareness and motivating efforts towards the mitigation of their harmful effects. Soot formation is one of the most complex phenomena in combustion, involving interactions between combustion chemistry, fluid mechanics, mass/heat transport, and particle dynamics, spanning different spatial and temporal scales. A fundamental understanding of soot formation process is thus necessary to achieve a strong reduction of PM emissions and design cleaner and more efficient combustion systems. This has motivated a longstanding and ongoing research activity aimed at improving our understanding of the physical and chemical processes involved in soot formation, well reviewed in these recent works (Wang and Chung, 2019; Michelsen et al., 2020; Martin et al., 2022). Despite the wide interest, the transition from gas-phase molecules to incipient soot particles is still elusive and the successive particle growth and oxidation processes are far from being fully understood, especially in conditions relevant to real-world applications. Recent advances in combustion PM emission diagnostic and computational capabilities helped in improving the predictability of fundamental chemical and aerosol models for practical applications, thus tackling some of the above-mentioned challenges. The aim of this research topic is to display the ongoing research efforts in addressing the existing gaps on particulate formation from various fuel sources and in conditions typical of practical combustion applications (e.g. flames, engines, pool fires), through both experimental and numerical approaches.

42 ENGINEERING↗
DOE OSTI2023

Observation of two different cool flame regimes of diethyl ether in a counterflow burner

This short communication reports, for the first time, the existence of two different self-sustaining cool flame regimes of diethyl ether (DEE) in a diffusion counterflow burner: a weaker autoignition-assisted cool flame near the fuel burner and a normal diffusion cool flame near the stagnation plane. Here, the results show that the normal diffusion cool flame extinction limit increases monotonically with the fuel mole fraction, while the autoignition-assisted cool flame approaches a plateau and can exist at a fuel mole fraction below the normal diffusion cool flame. It is shown that both flame regimes are governed by the same low-temperature chain-branching reaction pathway of DEE. By using in situ laser diagnostics, entrainment of unburned fuel stream to oxygen stream at the outer edge of the fuel burner is identified as the governing physical mechanism causing a partially premixed self-sustained hollow cool flame structure. The results reveal that when a fuel with high low-temperature reactivity, two different cool flame regimes can be observed in a counterflow flame experiment. Future studies with high-reactivity fuels in a counterflow burner must ensure to distinguish between the two self-sustaining cool flame regimes. Moreover, the existence of these different cool flame regimes needs to be examined so that they would not trigger an uncontrolled combustion phasing in advanced engines fed with high low-temperature reactivity fuels.

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2019

Numerical investigation of the pressure-dependence of yield sooting indices for n-alkane and aromatic species

The pressure-dependence of the Yield Sooting Index (YSI), as a measure of sooting tendency, is numerically investigated for pressures ranging from 1 atm to 15 atm. Straight-chain alkanes, including ethane, propane, n-hexane, n-heptane, and n-dodecane, and aromatic species, including benzene, toluene, xylene, and ethylbenzene, are selected as test fuels. A detailed chemical mechanism is used for all test fuels considered, and a flamelet-based YSI simulation methodology is employed, where the YSI values are computed based on the Polycyclic Aromatic Hydrocarbon (PAH) dimer production rate at each pressure. It is found that the maximum flame centerline soot volume fraction increases with increasing pressure for all test compounds. However, the pressure dependence of the maximum soot volume fraction is similar for the hydrocarbons within the same class (n-alkane or aromatics). As a consequence, the YSI values computed at pressures up to 15 atm show only small deviations from their values computed and measured at atmospheric pressure. These results suggest that YSI is robust also at elevated pressures, close to those found in practical combustion devices. This study is entirely based on numerical simulations without any experimental measurements available for direct validation. Experimental YSI measurements at high pressures would be required to fully confirm the conclusions reported in this manuscript. Still, our numerical results provide some guidelines for the design of such experiments in the future.

42 ENGINEERING↗
DOE OSTI2019

Experimental study of ethanol oxidation behind reflected shock waves: Ignition delay time and H 2 O laser-absorption measurements

The oxidation of ethanol was investigated in shock tubes by measuring ignition delay times and water time history profiles. A laser absorption technique was used in the latter case. Ignition delay times were investigated for three equivalence ratios, 0.5, 1.0, and 2.0, and for pressures ranging from 1.3 to 53 atm. The fuel concentration and the type of diluent used were also varied. The ignition delay times were in very good agreement with results from the literature for the very few overlapping conditions, and most of the experimental conditions have never been studied before. A high level of accuracy was observed for the predictions of the ignition delay times from detailed kinetics mechanisms. Inhomogeneous ignition events were observed in some specific conditions, validating the notions of thermal diffusivity and flame thickness developed in the literature for the reasons behind such inhomogeneous ignition. The water time-history profiles showed the formation of water in two stages. These profiles were also modeled using well-known detailed kinetic mechanisms from the literature. Severe discrepancies were often observed for laser-absorption measurements, and this was the case with most of the models. A numerical analysis (sensitivity and rate-of-production) was conducted using the two best-performing literature mechanisms (namely, AramcoMech3.0 and CRECK).

42 ENGINEERING↗
DOE OSTI2024

Hexamethyldisiloxane pyrolysis: probing H-atom initiation by femtosecond two-photon LIF

Previous experiments have led to the hypothesis that pyrolysis of silica nanoparticle precursors carried in an inert central gas jet surrounded by a laminar flame is initiated not by thermal decomposition (the measured jet temperatures are too low for pyrolysis), but by reactions with H-atoms diffusing from the surrounding flame into the jet. The current work tests this hypothesis using femtosecond two-photon laser-induced fluorescence (fs-TPLIF) to image H-atom concentrations in the same flame configuration as the previous experiments. Methane flames (ϕ = 0.83, 1.01 and 1.13) were generated using a multi-element diffusion (Hencken) burner equipped with a central tube through which the N2 jet was introduced. The fs-TPLIF line imaging showed significant concentrations of H-atoms in the nitrogen jets at heights above the burner (HAB) close to the methane flame front. The measurements indicate radial diffusion of the H atoms from the flame to the jet is significant. The H-atom concentrations in the jet increased with increasing stoichiometry of the methane flame. Additional experiments were performed with hexamethyldisiloxane entrained in the central jet flow. In each case, a diffusion flame producing silica nanoparticles was established at the boundaries between the central jet and the methane flames. The 2D H-atom images showed a transition from over-ventilated diffusion flames to under-ventilated diffusion flames with increasing equivalence ratio. Furthermore, the fs-TPLIF profiles show a significant production of H-atoms within the jet when the surrounding flame is lean. This knowledge contributes to a deeper understanding of the underlying mechanisms governing these synthesis flame systems and opens avenues for advancing silica nanoparticle synthesis techniques.

Kim, Keunsoo↗
DOE OSTI2023

Experimental measurement of the rapid mixing of fuel and air in a multi-element diffusion (Hencken) burner

Flat flame burners are valuable tools for studying complex combustion phenomena like particle synthesis due to the well-controlled and well-behaved initial and boundary conditions. In this study, the methane/air flame temperature and tracer density profiles of a multi-element diffusion burner equipped with a central tube were characterized using krypton (Kr) x-ray fluorescence at the Advanced Photon Source at Argonne National Laboratory. The experiments were performed using a fuel-to-oxygen equivalence ratio of 0.8 and 3.3% Kr by volume was added to the reactant gases. One- and two-dimensional scans of non-combusting (i.e., cold flow) and combusting conditions, both with and without flow through the central tube were performed. Spatially-resolved Kr number density profiles were obtained at different planar locations and heights above the surface of the burner. The results show rapid mixing occurred near the exit-plane of the burner and increased spatial uniformity was achieved from 0.1 mm to 0.5 mm above the surface of the burner. Further, the data also showed the impact of the central tube flow on the main methane/air flame, including evidence of the central tube flow mixing with the main flow. Flame temperatures were calculated from the fluorescence measurements at various heights above the burner surface and were in good agreement with model predictions for a burner stabilized flame. These high-fidelity measurements provide accurate baseline data that are particularly valuable for understanding the effects of fuel and air mixing on downstream phenomena, such as the formation of condensed-phase products.

42 ENGINEERING↗
DOE OSTI2024

Direct numerical simulations of turbulent premixed cool flames: Global and local flame dynamics analysis

The cool flame dynamics, especially in turbulent flows, is of great interest for both practical application and fundamental research. Here, in this study, a series of direct numerical simulations of turbulent premixed n-C 7 H 16 /O 2 /O 3 /N 2 cool flames are performed, with the focus on the influence of turbulence intensity (u'/S L , where S L is the laminar flame speed) on the flame structure as well as the global and local cool flame dynamics. It is found that the cool flame front is considerably wrinkled by turbulence at high u'/S L , leading to significantly thickened turbulent cool flame brush and largely altered local reactivity compared with the reference laminar flame. However, the turbulent flame structure in the temperature space is found to be insensitive to u'/S L . Besides, with increasing u'/S L , the normalized turbulent cool flame speed (S L /S L ) is monotonically increased, attributed to substantial augmentation on the flame surface area (A T /A L ), while the stretching factor (I 0 ) remains almost constant and is smaller than 1. The underlying mechanisms for such variations are revealed through local flame dynamics analysis. Specifically, the local flame displacement speed S d is found to be strongly negatively correlated with flame curvature; meanwhile, such negative correlation and the probability distribution function (PDF) of flame curvature are barely influenced by u'/S L , leading to a weak dependence of I 0 on u'/S L . In contrast, the PDF of the tangential strain rate is found to span a much wider range and shift to the positive side as u'/S L increases, suggesting that the enhanced tangential strain rate is the main cause for the increase in surface area of the turbulent premixed cool flame. Finally, the influence of equivalence ratio on above findings is found to be insignificant, indicating that although the local reactivity of turbulent premixed cool flames is altered due to the differential diffusion, the resultant flame- stretch interaction is insensitive to the equivalence ratio. This study presents some unique cool flame dynamics that are distinct from hot flames, which can help improve the understanding and modeling of turbulent cool flames.

Cool flames↗
DOE OSTI2019

Exploration of a Reacting Jet-in-Crossflow in a High-Pressure Axial Stage Combustor

The present work investigates a reacting methane jet in a vitiated cross flow at five atmospheres. The experiments are run at temperatures and velocities relevant to real world gas turbines. High speed chemiluminescence is utilized to obtain flame behavior and visualize flame trajectories. Experimental results are compared with a corresponding RANS CFD model. The characteristics of a diffusion methane jet injected into a vitiated crossflow are explored at various momentum flux ratios and test section inlet boundary conditions. Temperature and velocity measurements are made at the inlet of the test section to ensure uniform boundary conditions for the axial stage. The effect of different momentum flux ratios on flame stabilization mechanisms and jet trajectories are presented. Momentum flux ratios are varied from 0.5-107; at lower momentum flux ratios the diffusion flame is less stable and burns downstream in the shear layer between the jet and crossflow. At higher momentum flux ratios, the diffusion flame burns more upstream and exhibits improved stability compared to the lower momentum flux ratio cases.

03 NATURAL GAS↗
DOE OSTI2024

In-situ two-dimensional temperature measurements using x-ray fluorescence spectroscopy in laminar flames with high silica particle concentrations

X-ray fluorescence spectroscopy (XRF) was used to measure temperatures and study mixing, for the first time in silica particle synthesis flames. Hexamethyldisiloxane (HMDSO) and trimethylsilanol (TMSO) were the particle precursors. A multi-element diffusion burner was used to produce a flat methane flame, and the precursors, dilute in inert gas, were injected via a central jet. Krypton was the fluorescent medium at 3.2 % concentration by volume. Scans with Kr in the central flow and not in the main flow were made to assess the mixing effects between the central and main gas flows. When HMDSO or TMSO were added, a secondary diffusion flame formed between the jet and the main methane flame. The results revealed a dramatic change in the centerline temperature profile of the jet gases when HMDSO or TMSO were added. The main methane flame stoichiometry also affected the temperature profiles. The results show HMDSO and TMSO reactions are initiated in a lowtemperature and low-oxygen concentration region of the jet where thermal decomposition is not expected to be significant. Reaction of the particle precursors is therefore attributed to radical transport from the main methane flame. In the current work, particle number densities of up to 270 g/m 3 locally are estimated. Thus, the study also demonstrates the capability of the XRF technique for high spatial fidelity measurements in flames with high concentrations of condensed-phase particles, leveraging the attribute that the XRF signal is generally not impacted by condensed-phase interferences. In conclusion, the observations and data obtained in this study inform likely reaction pathways for this important class of siloxane compounds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Hot-spot induced mild ignition: Numerical simulation and scaling analysis

Mild ignition is a 'non-ideal' autoignition process observed at some conditions in rapid compression machine, shock tube and flow reactor experiments where flames and/or reaction fronts initiate due to non-uniformities within the test mixture. These flames/fronts can consume the entire mixture, or compression-heat the unburned gas, causing bulk ignition at times earlier than in homogeneous environments. This study numerically investigates mild ignition features using computational fluid dynamics techniques under a scenario where a small hot-spot is employed to initiate the process in a spherically-symmetric geometry. Only subsonic regimes are considered. The intent is to develop greater fundamental understandings of competing phenomena, and formulate a robust framework to quantify the perturbative behavior. Syngas blends (CO/H 2 = 80/20) are employed as the fuel, mixed with air at lean fuel loadings of phi = 0.2 and 0.5, with the initial temperatures and pressures covering 900-1120K, and 1.5-15 bar, respectively. Hot-spots ranging from 50 to 2,000 mu m in diameter are utilized with temperatures elevated from the surroundings by 1 to 90%, i.e., T hot /T 0 =1.01-1.90. Scaling arguments are derived to analyze competing processes of: (a) quenching vs. ignition, (b) extinction vs. propagation, and (c) flame consumption/forced autoignition vs. homogeneous ignition. Here, this work provides new insight into non-uniform autoignition phenomena, and discusses opportunities and challenges associated with mitigating it. In particular, scaling arguments indicate that experimental parameters could have potential to either suppress hot-spot initiated flame formation (e.g., via bath gases with high thermal diffusivity), or avert flame compression of the end gas (e.g., via lean/diluted mixtures, or bath gases with high heat capacity).

42 ENGINEERING↗
DOE OSTI2019

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.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

100 kHz OH PLIF of High-Pressure H2/NH3 and H2/CH4 Flames

The reaction zones in a multi-element, multi-stage, micromix injector are investigated using planar laser-induced fluorescence (PLIF) imaging of the hydroxyl (OH) radical at a 100 kHz repetition rate. The combustor is operated at a pressure of ∼10 bar. Multiple fuel blends of hydrogen, methane, and ammonia are considered to study the effect of changing fuel composition on the flame structure. Compact regions of high OH signal are observed in each flame for fuel mixtures with high hydrogen content, indicating rapid reactant consumption in the shear layers. Instantaneous images exhibit wrinkled flame fronts and the formation of unreacted pockets due to highly turbulent burning. The addition of methane is observed to reduce the reactivity and flame speed, resulting in more diffuse regions of OH between each flame. The shape of the reactant core is altered, and the length scale of the flame surface wrinkling is increased. Ammonia addition has a more severe effect on the distribution of OH and the local flame structure than methane addition. A time series of images shows the presence of OH vortical structures and significant turbulent fluctuations in the flames near the wall, suggesting the presence of a recirculation zone that could drive the observed disparity in flame lengths.

Hodge, Alexander J. [Purdue University]↗
DOE OSTI2024

Impact of a Bluff Body on a Pressurized Oxy-fuel Flame

Designing an effective burner is vital for the development of pressurized oxy-fuel combustion technologies. In the present work, turbulent jet diffusion burners are adopted for a pressurized oxy-combustor, with a bluff-body employed to create a recirculation zone, thereby stabilizing the flame in such a combustor. The objective of this numerical study is to perform a systematic analysis of the characteristics of such a pressurized non-premixed flame. Specifically, a 15-bar pressurized oxy-fuel combustor of power 100 kWth is modeled by means of the Ansys FLUENT commercial platform, using the Reynolds-averaged Navier-Stokes (RANS) approach. The present work focuses on identifying the aerothermodynamic features of the pressurized oxy-fuel burner with a disk-shape bluff-body. It is shown that the fuel-to-oxidizer stream momentum ratio has a great impact on the temperature profile of the down-fired, co-axial, pressurized oxy-coal, diffusion flame. A parametric study of the blockage ratio of the burner identifies an optimal range for this pressurized burner.

Li, Lei↗