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At least 757 records · Page 42

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NASA NTRS2018

Flame Holder System

A flame holder system includes a modified torch body and a ceramic flame holder. Catch pin(s) are coupled to and extend radially out from the torch body. The ceramic flame holder has groove(s) formed in its inner wall that correspond in number and positioning to the catch pin(s). Each groove starts at one end of the flame holder and is can be shaped to define at least two 90 degree elbows. Each groove is sized to receive one catch pin therein when the flame holder is fitted over the end of the torch body. The flame holder is then manipulated until the catch pin(s) butt up against the end of the groove(s).

Haskin, Henry H.↗

DOE OSTI2019

Effect of Fuel Reactivity and End- Gas Temperature on Autoignition and Flame Propagation Rate in Primary Reference Fuel Mixtures at Elevated Temperature and Pressure

Knock in spark-ignited (SI) engines is initiated by autoignition in the unburned gases upstream of spark-ignited, propagating, turbulent premixed flames. In this study, knock propensity of SI fuels was quantified via observations of end-gas autoignition (EGAI) in unburned gases upstream of laser-ignited, premixed flames at elevated pressures and temperatures in a rapid compression machine. Stoichiometric primary reference fuel (n-heptane/isooctane) blends of varying reactivity were ignited using an Nd:YAG laser over a range of compressed temperatures and pressures, all in excess of 691 K and 18.8 bar. High-speed pressure measurements and schlieren images indicated the presence of EGAI in the unburned gases upstream of spherical, outwardly propagating flames. The fraction of the total heat release attributed to EGAI was found to vary with fuel reactivity and the time-integrated temperature of the end-gas prior to ignition. Flame propagation rates, measured using schlieren imaging, did not vary strongly with fuel reactivity but were affected by turbulence caused by variation in piston timing. Under conditions of low turbulence, measured flame propagation rates agreed with one-dimensional premixed laminar flame speed computations performed at the same conditions.

Zdanowicz, Andrew↗

DOE OSTI2019

End-Gas Autoignition Fraction and Flame Propagation Rate in Laser-Ignited Primary Reference Fuel Mixtures at Elevated Temperature and Pressure

Engine knock in spark-ignited (SI) engines is initiated by autoignition and detonation in the unburned gases (i.e., end-gas) upstream of the spark-ignited, propagating, turbulent premixed flame. Knock propensity of fuel/air mixtures is quantified by research octane number (RON), motor octane number (MON), or methane number (MN; for gaseous fuels), which are typically measured using single-cylinder, variable compression ratio research engines. In this study, we demonstrate the ability to evaluate knock propensity of SI fuels via observations of end-gas autoignition (EGAI) in unburned gases upstream of laser-ignited, premixed flames at elevated pressures and temperatures in a rapid compression machine. Stoichiometric primary reference fuel (PRF; n-heptane/isooctane) blends of varying reactivity (50 ≤ PRF ≤ 100) were ignited using an Nd:YAG laser at five discrete temperature and pressure conditions, all in excess of 485 K and 16.75 bar. Laser ignition produced outwardly-propagating, laminar premixed flames. High-speed pressure measurements, paired with high-speed Schlieren imaging, clearly indicated the presence of EGAI. The fraction of the total heat release attributed to EGAI (e.g. EGAI fraction) varied inversely with octane number. EGAI fraction and flame propagation rate were both influenced by the magnitude of convection in the unburned gases upstream of the propagating flame caused by variation in piston timing. Experiments were accompanied by three-dimensional computational modeling with detailed chemical kinetics performed using CONVERGETM. The model results reveal low-temperature heat release and hydrogen peroxide formation in the end-gas upstream of the propagating laminar flame. Low-temperature heat release increases the temperature and degree of chain branching in the end-gas and ultimately leads to EGAI.

End-Gas Autoignition, Engine Knock, Rapid Compress↗

DOE OSTI2019

Effect of Fuel Reactivity and End-Gas Temperature on Autoignition and Flame Propagation Rate in Primary Reference Fuel Mixtures at Elevated Temperature and Pressure

End-Gas Autoignition, Engine Knock, Rapid Compress↗

DOE OSTI2022

DNS of Hydrodynamic Instabilities of Laminar H2/O2/N2 Flames at Elevated Pressure

The atypical combustion properties of hydrogen have long provided an interest for hydrogen-enriched combustion, emphasized in recent years by increasingly stringent emission regulations. Significant work has been focused on the small hydrocarbon(C1-C3)/hydrogen blends, but much less is known about the effect of hydrogen on heavier hydrocarbon combustion, of interest for the aviation and automotive industry. In this work, we perform direct numerical simulations (DNS) of spherically expanding laminar n-dodecane/H2 flames in a constant-volume vessel at elevated pressure and a range of hydrogen seeding levels. A low Mach number model is used to represent the gas and flame dynamics allowing for a temporally varying, spatially homogeneous pressure field, while also enabling numerical evolution of the system numerically at time step sizes governed by the advective CFL limit rather than acoustic processes. We use an adaptive mesh refinement (AMR) approach to tackle the large separation of scales associated with high pressure premixed flame surfaces propagating in a laboratory-scale closed domain. We incorporate detailed chemistry and transport models for the reacting flow and focus the analysis on the effects of the pressure rise on the flame propagation characteristics and morphology as the hydrogen content is increased. The presence of fast diffusive H2 and the pressure rise results in dramatic thinning of the flame and the potential onset of hydrodyanmic and theromdiffusive instabilities. We assess the potential role of these instabilities on the ability to accurately measure laminar burning speed experimentally based on the mean flame surface propagation speed.

high pressure↗

DOE OSTI2015

A Study of Strain Rate Effects for Turbulent Premixed Flames with Application to LES of a Gas Turbine Combustor Model

Large-scale strain rate field, a resolved quantity which is easily computable in large-eddy simulations (LES), could have profound effects on the premixed flame properties by altering the turbulent flame speed and inducing local extinction. The role of the resolved strain rate has been investigated in a posterior LES study of GE lean premixed dry low NOx emissions LM6000 gas turbine combustor model. A novel approach which is based on the coupling of the lineareddy model with a one-dimensional counter-flow solver has been applied to obtain the parameterizations of the resolved premixed flame properties in terms of the reactive progress variable, the local strain rate measure, and local Reynolds and Karlovitz numbers. The strain rate effects have been analyzed by comparing LES statistics for several models of the turbulent flame speed, i.e, with and without accounting for the local strain rate effects, with available experimental data. The sensitivity of the simulation results to the inflow velocity conditions as well as the grid resolution have been also studied. Overall, the results suggest the necessity to represent the strain rate effects accurately in order to improve LES modeling of the turbulent flame speed.

33 ADVANCED PROPULSION SYSTEMS↗

DOE OSTI2010

Nonlinear effects of stretch on the flame front propagation

In all experimental configurations, the flames are affected by stretch (curvature and/or strain rate). To obtain the unstretched flame speed, independent of the experimental configuration, the measured flame speed needs to be corrected. Usually, a linear relationship linking the flame speed to stretch is used. However, this linear relation is the result of several assumptions, which may be incorrected. The present study aims at evaluating the error in the laminar burning speed evaluation induced by using the traditional linear methodology. Experiments were performed in a closed vessel at atmospheric pressure for two different mixtures: methane/air and iso-octane/air. The initial temperatures were respectively 300 K and 400 K for methane and iso-octane. Both methodologies (linear and nonlinear) are applied and results in terms of laminar speed and burned gas Markstein length are compared. Methane and iso-octane were chosen because they present opposite evolutions in their Markstein length when the equivalence ratio is increased. The error induced by the linear methodology is evaluated, taking the nonlinear methodology as the reference. It is observed that the use of the linear methodology starts to induce substantial errors after an equivalence ratio of 1.1 for methane/air mixtures and before an equivalence ratio of 1 for iso-octane/air mixtures. One solution to increase the accuracy of the linear methodology for these critical cases consists in reducing the number of points used in the linear methodology by increasing the initial flame radius used. (author)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Data reduction considerations for spherical R-32(CH 2 F 2 )-air flame experiments

The burning velocities of mixtures of refrigerant R-32 (CH 2 F 2 ) with air over a range of equivalence ratios are studied via shadowgraph images of spherically expanding flames (SEFs) in a large, optically accessible spherical chamber at constant pressure. Numerical simulations of the 1-D, unsteady, spherical flames incorporating an optically thin radiation model and detailed kinetics accurately predict experimental results for a range of equivalence ratios. For these low burning velocity flames, the effects of stretch and radiation occur simultaneously and make extraction of the unstretched burning velocity from the experimental data difficult. Different data reduction approaches are shown to have large effects on the burning velocities inferred from the experiments and simulations. A new flame radius tracking approach for the experimental images is shown to provide improved agreement of the burned gas velocity variation with stretch predicted by the simulations and helps to compensate for mild flame distortion due to buoyancy.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

NASA NTRS1999

Soot Aerosol Properties in Laminar Soot-Emitting Microgravity Nonpremixed Flames

The 0-g flame soot measurements reported in previous studies are extended by adding new 0-g data for different fuel flow rates and burner diameters. The new flame conditions allow more conclusive comparisons regarding the effect of characteristic flow residence times on soot field structure, the influence of fuel preheat on fuel pyrolysis rates near the flame centerline, and the premature cessation of soot growth along the soot annulus in 0-g when the fuel is preheated. The paper also reports on the implementation of thermophoretic soot sampling in a specific 0-g flame featuring burner exit velocities typical of buoyant flames and presents quantitative data on the radial variation of soot microstructure at a fixed height above the burner mouth.

FLAME STRUCTURE↗

NASA NTRS2024

Simplified Structure Models for Premixed n-Alkane Cool Flames

The chemical kinetics of cool flames in mixtures of normal alkanes, oxygen, and nitrogen recently have been reduced to obtain simplified asymptotic flame-structure models that can be used to compare theoretical predictions with experimental measurements. A key step that affects flame structures strongly, namely the abstraction of a hydrogen atom from the alkyl radical by an oxygen molecule, had not been taken into account in those models. This deficiency has now been remedied for diffusion flames. The present contribution reports the corresponding remedy for premixed-flame analyses. In the new model, non-zero chemical reaction rates are restricted to narrow regions in the vicinity of the maximum temperature, whereas previously they extended throughout the preheat zone. Comparisons of the new predictions with experimental burning-velocity measurements seem favorable.

premixed cool flames↗

DOE OSTI2010

Direct numerical simulation of the effect of an electric field on flame stability

The role of electric fields in stabilising combustion is a well-known phenomenon. Among the possible mechanisms favouring the anchorage of the flame base, the ion-driven wind acting directly on flow momentum ahead of the flame base could be the leading one. Direct numerical simulation has been used to verify this hypothesis and lead to a better understanding of diffusion flame base anchoring in the presence of an externally applied voltage. In this context, a simplified modelling approach is proposed to describe combustion in the presence of electric body forces. The model reproduces the tendencies of experimental observations found in the literature. The sensitivity of the flame lift-off height to the applied voltage is studied and the modification of the velocity field ahead of the flame base induced by the electric volume forces is highlighted. (author)

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Five kHz thermometry in turbulent spray flames using chirped-probe-pulse femtosecond CARS, part II: Structure of reaction zones

Temperature was measured in turbulent spray flames of ethanol and acetone stabilized on the piloted Sydney Needle Spray Burner (SYNSBURN TM ) using single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS) with a repetition rate of 5 kHz. The burner features air-blast atomization of liquid injected from a needle that can be translated by a length Lr within a co-flowing air stream so that piloted spray flames ranging from dilute to dense can be studied. Part I of these investigations has reported on the CPP-fs-CARS technique and extensive details of data processing methodology. Part II is concerned with the structure of the reaction zones at different spray loadings and for different departures from blow-off. While not performed simultaneously, measurements of the size distribution of liquid fragments are also reported and discussed in conjunction with the measured temperature. Measured probability density functions of temperature show that for flames with the same liquid loading but different recess lengths, Lr, the near-field spray structure that forms upstream of x/D = 10 affects flame structure and stability further downstream. As the spray exiting the burner becomes denser, with a higher proportion of ligaments and ‘irregular’ shaped objects, the entrainment of hot pilot gases into the spray envelope is affected, hence changing the rates of vaporization and subsequent combustion. The reported results will also form a useful platform for validating sub-models of atomization and combustion in turbulent, dilute to dense spray flames.

42 ENGINEERING↗

DOE OSTI2019

Effects of water addition on the combustion of iso -octane investigated in laminar flames, low-temperature reactors, and an HCCI engine

In this work, the effect of H 2 O injection on the combustion process of iso -octane was investigated with the aim to better understand the suitability of water addition as a potential engine control parameter for homogeneous-charge compression ignition (HCCI) combustion. Several experiments were combined including premixed low-pressure flames, a jet-stirred reactor (JSR) and a plug-flow reactor (PFR), both at atmospheric pressure, and a single-cylinder research engine (SCRE) operated with either iso -octane or RON 98 gasoline. The thermal effect of H 2 O addition was determined in laminar premixed iso -octane/O 2 /Ar flames (equivalence ratio $Φ$=1.4, 40 mbar) with H 2 O mole fractions of 0 to 0.22, where water addition reduced the temperature measured by laser-induced fluorescence (LIF) by up to 110 K. Speciation data were obtained from these flames as well as in the JSR ($Φ$=0.65, 933 mbar) and PFR experiments ($Φ$=0.65, 970 mbar) with and without H 2 O addition in the low- to intermediate temperature regime from 700–1100 K. The chemical analysis in these flame and reactor experiments was performed using molecular-beam mass spectrometry (MBMS) employing either electron ionization (EI) in the PFR and premixed flame or single-photon ionization (PI) by tunable vacuum-ultraviolet radiation in the JSR. The effects on species mole fractions were small which is supported by predictions from chemical-kinetic simulations. Quantitative speciation data of the exhaust gas of the SCRE were obtained by using Fourier-transform infrared (FTIR) spectroscopy. A very similar species pool was detected in the laboratory-scale experiments and for the engine operation. It is thus assumed that these results could assist in guiding both the improvement of fundamental chemical-kinetic as well as HCCI engine control models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Adaptive mesh based combustion simulations of direct fuel injection effects in a supersonic cavity flame-holder

Here, we present high-fidelity reacting simulations of a supersonic cavity flame-holder configuration. The focus of this work is on flame stabilization brought about by varying the location of fuel injection in a cavity stabilized supersonic flow of air. Central to our approach is a compressible multi-species reacting flow solver that uses adaptive-mesh-refinement (AMR), enabling the resolution of flame, shock-waves, boundary-layers, and small-scale structures in the computational domain. Our analysis indicates that fuel injection closer to the ramp at the aft end of the cavity allows for greater mixing and lower peak temperatures compared to fuel injection upstream that is closer to the backward facing step of the cavity. This difference is mainly due to greater turbulent fluctuations generated from the shear-layer towards the cavity ramp, thereby enhancing the mixing of fuel and air. A low frequency oscillatory behaviour in heat-release and pressure was also observed for the upstream injection case while a much higher-frequency phenomena was observed in the near-ramp injection case. By identifying the important physical determinants of the combustion processes, this study illustrates a promising pathway to design and optimize direct fuel injection strategies in supersonic cavity flame-holders that can improve flame stability, combustion efficiency, and reduce emissions.

42 ENGINEERING↗

DOE OSTI2021

Computational fluid dynamics modeling and analysis of silica nanoparticle synthesis in a flame spray pyrolysis reactor

Flame Spray Pyrolysis (FSP) is a method for large-scale production of nanoparticles and nanoscale powders employed in a wide range of industrial applications. Particle size and morphology are complex functions of the physicochemical phenomena occurring in the FSP reactor. An extensive study of FSP-related phenomena can be utilized to develop effective strategies for achieving desired particle size/morphology and scaling up the overall yield of an FSP system. In this work, a computational fluid dynamics (CFD) model of an FSP reactor is developed to simulate the coupling of key phenomena involved in the particle synthesis process: liquid spray breakup and evaporation, mixing, combustion, and particle formation/growth of silica nanoparticles. Herein, the particle sizes and their distributions from the CFD simulations are validated against experimental data. Subsequently, the simulations are utilized to investigate the impact of process parameters on the resultant flame dynamics and particle growth. Firstly, the CFD results show that the particle sizes are strongly correlated with the precursor concentration in the solvent. At lower precursor concentrations, the spread of the distribution is relatively insensitive to the value of the concentration. At higher concentrations, the spread is higher as the collision probability between particles is higher. Secondly, increasing the pilot flow rate increases the length of the pilot flames impacting the local ignition location of the spray flame. Lastly, it is shown that the dispersion gas flow rate strongly influences the spray flame shape. This shape can be used for control of particle growth as it helps determine the regions of high temperature and the residence time of the particles in the high temperature region enabling the design and process optimization of the FSP reactor.

42 ENGINEERING↗

DOE OSTI2018

Flame-made TiO2(B)

Highlights: • Flame-made titania can contain up to 27% TiO{sub 2}(B). • TiO{sub 2}(B) is present due to quenching. • The fraction of TiO{sub 2}(B) depends on residence time at high temperature. • Flame-made titania containing TiO{sub 2}(B) photocure epoxy more efficiently than P25. - Abstract: Titania nanoparticles containing both monoclinic (TiO{sub 2}(B) – a promising battery material and catalyst) and anatase crystal phases are synthesized with a low-cost, scalable flame spray pyrolysis (FSP) process, as confirmed by X-ray diffraction, transmission electron microscopy and Raman spectroscopy. The latter reveals TiO{sub 2}(B) contents of up to 27% in the crystalline phase fraction. The amount of TiO{sub 2}(B) increases as the time for nanoparticle growth in the flame decreases through quenching with ambient air. It can be controlled by varying the flow rates of precursor solution and dispersion oxygen into the flame. The FSP-made titania outperforms commercial titania P25 in the photocuring of epoxy, indicating its potential as a photocatalyst.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2024

Effects of radiative heat loss on extinction limits of counterflow premixed ammonia-air flames

The effect of radiative heat loss on counterflow premixed ammonia-air flames has been investigated as a function of equivalence ratio and system pressure. Non-adiabatic counterflow premixed flame simulations incorporating detailed chemistry, transport, and radiative heat transfer in the optically thin limit have been employed to elucidate important underlying physics and extinction characteristics. Similar to methane-air flame systems, non-adiabatic counterflow premixed computations show that the combined effects of positive flame stretch, sub-unity Lewis number, and radiative heat loss lead to the extension of the lean flammability limit for ammonia-air flames, revealing a C-shaped curve near the lean flammability limit that exhibits both radiation-induced and stretch-induced extinction states. Here, the computations compare favorably with the experimentally determined extinction stretch rate values for the stretch-induced extinction states over a wide range of equivalence ratios at different pressures up to 5 atm. A novel feature sensitivity analysis has also been developed to highlight important sensitive reactions for the stretch- and radiation-induced extinction states. Furthermore, the controlling chemistry at the dual-extinction states and at varying pressures are compared and discussed.

10 SYNTHETIC FUELS↗

DOE OSTI2025

Application of soot carbonization kinetics to deduce meaningful soot formation rates in premixed flat flames

Soot formation rates measured in fuel-rich premixed flat flames are frequently used to calibrate or validate chemical kinetic models of soot formation. Unfortunately, these flames feature an extended region of soot precursor particle inception and carbonization that complicates interpretation of soot measurements and leads to a fundamental inconsistency in the nature of the soot material that is modeled versus what is being measured when using non-intrusive, optical techniques. In the work presented here, previously reported data on two canonical sooting ethylene-air premixed flames at 1 atm pressure are interpreted via a new analysis approach that combines soot optical dispersion coefficient measurements with soot carbonization kinetics. This analytical approach explicitly accounts for the production of poorly ordered soot precursor particle mass and its carbonization over time in the flames, providing a clear distinction between the formation rate of precursor particles and their transformation into ordered, solid soot particulate mass. In particular, the results of the analysis show that the precursor particles form much earlier in the flame than the majority of the carbonized soot and their formation rate is two to three times faster than that of ordered soot. The results also show that particle agglomeration begins when the particles are at an intermediate state of carbonization. In conclusion, these results offer a valuable new interpretation of these important datasets and should lead to substantial improvements in the development and calibration of quantitative soot models.

Soot formation↗