Fuel-rich Flames Research Articles

Hybrid fs/ps rotational CARS temperature and oxygen measurements in the product gases of canonical flat flames

A hybrid fs/ps pure-rotational coherent anti-Stokes Raman scattering (CARS) scheme is systematically evaluated over a wide range of flame conditions in the product gases of two canonical flat-flame burners. Near-transform-limited, broadband femtosecond pump and Stokes pulses impulsively prepare a rotational Raman coherence, which is later probed using a high-energy, frequency-narrow picosecond beam generated by the second-harmonic bandwidth compression scheme that has recently been demonstrated for rotational CARS generation in H2/air flat flames. The measured spectra are free of collision effects and nonresonant background and can be obtained on a single-shot basis at 1kHz. The technique is evaluated for temperature/oxygen measurements in near-adiabatic H2/air flames stabilized on the Hencken burner for equivalence ratios of φ=0.20–1.20. Thermometry is demonstrated in hydrocarbon/air products for φ=0.75–3.14 in premixed C2H4/air flat flames on the McKenna burner. Reliable spectral fitting is demonstrated for both shot-averaged and single-laser-shot data using a simple phenomenological model. Measurement accuracy is benchmarked by comparison to adiabatic-equilibrium calculations for the H2/air flames, and by comparison with nanosecond CARS measurements for the C2H4/air flames. Quantitative accuracy comparable to nanosecond rotational CARS measurements is observed, while the observed precision in both the temperature and oxygen data is extraordinarily high, exceeding nanosecond CARS, and on par with the best published thermometric precision by femtosecond vibrational CARS in flames, and rotational femtosecond CARS at low temperature. Threshold levels of signal-to-noise ratio to achieve 1–2% precision in temperature and O2/N2 ratio are identified. The results show that pure-rotational fs/ps CARS is a robust and quantitative tool when applied across a wide range of flame conditions spanning lean H2/air combustion to fuel-rich sooting hydrocarbon flames.

Flame and Emission Characteristics of a Premixed Swirl-stabilised Burner

The flame and emission characteristics of a premixed gaseous flame swirl burner are investigated under various equivalence ratio. The swirl flame is established using propane/air mixture at atmospheric condition. Flame imaging was performed to compare the global flame shape and intensity over a range of equivalence ratios and flow rates. Fuel-rich flame shows increased intensity due to the presence of soot formation. The lean blowout test was performed to determine the operating limit of the burner. Emissions of the propane swirl flame were measured at the exit of the burner outlet. Results show that NOx emissions peak at stoichiometric condition, f =1 as compared to the lean- and rich-burning regions. Carbon monoxide (CO) and unburned hydrocarbons (UHC) emissions were found to be low (< 10 ppm) under premixed, continuous swirl burning conditions for the equivalence ratio range of f = 0.7-1.1.

Investigation of Methyl Pentanoate Flame Structure by Molecular-Beam Mass Spectrometry and Modeling

The structure of four stoichiometric and fuel-rich premixed flames of methyl pentanoate stabilized at low (20 torr) and atmospheric pressures has been studied by molecular-beam mass spectrometry. The data obtained have been compared with results of numerical simulations, performed with implication of two detailed chemical kinetic mechanisms, one of which has been developed by the authors of this work. While both mechanisms have predicted concentration profiles for most of the species quite well, some discrepancies between experimental and modeling data have been observed for carbon monoxide and some intermediate products. Considerable differences in several profiles simulated with different mechanisms have been noted. Analysis of reaction paths in investigated flames has shown most of these differences to be caused by different reactions and kinetics used for isomerization of primary radicals of methyl pentanoate oxidation in these mechanisms

Response of Soot Temperature to Unsteady Inlet Airflow Under Modulated Condition and Naturally Occurring Combustion Dynamics

The response of soot temperature to unsteady inlet airflow is characterized using pyrometry. The unsteady inlet airflow is achieved by either modulating inlet air or naturally occurring unstable flame, running on a jet fuel at fuel-rich conditions. The inlet air is modulated by a siren device running at frequencies between 150 and 250 Hz and up to 60% of modulation level (u'/um) is achieved. Also, the combustor can be run naturally unstable at the same inlet operating condition by changing the combustor length. For pyrometry, the emission from whole flame at 660 nm, 730 nm, and 800 nm is recorded and the three-color pyrometry is used to measure soot temperature. The effect of nonisothermal distribution of soot in flame on the measured temperature is also considered. The level of overall temperature fluctuation under inlet flow modulation (Trms/Tmean) is about an order of magnitude lower than that of flame emission fluctuation (Irms/Imean). Under naturally occurring instability, the measured soot temperature is in phase with the pressure measured in the combustor, indicating that the measured soot temperature can be used as a quantity related to combustion dynamics for fuel-rich sooty flames.

Simultaneous measurement of the concentrations of soot particles and gas species in light hydrocarbon flames using mass spectrometry

Besides gas species concentrations, soot volume fractions are also important data in the study of flames. This work describes the simultaneous measurement of the concentrations of soot and gas species in light hydrocarbon flames by in situ sampling and mass spectrometry (MS).The reaction medium was frozen by sampling into a very low-pressure tube, and the soot selectivity (proportion of carbon atoms in the reactant converted to soot) was determined from the C and H mass balances using the measured concentrations of the gas species and the mass of soot present per unit gas volume. The H/C ratio of the soot was measured by a thermogravimetry–mass spectrometry combination. The soot volume fraction was calculated from the soot selectivity and density of the soot. The soot selectivity measured by this reduced pressure sampling mass spectrometry (RPSMS) method was verified by measurements using the gravimetric sampling technique where the mass of soot collected in a volume of gas was weighed by a high precision balance. For most of the measurements, the uncertainty in the soot volume fraction was ±5%, but this would be larger when the soot volume fractions are less than 1 ppm. For demonstration, the RPSMS method was used to study a methane fuel-rich flame where the soot volume fractions were 1–5 ppm. The simultaneous measurement of concentrations of soot and gas species is useful for the quantitative study of flames.

Effects of high shear on the structure and thickness of turbulent premixed methane/air flames stabilized on a bluff-body burner

The effects of preferential transport and strain on the scalar structure (profiles of major species, elemental ratios) of turbulent premixed bluff-body stabilized flames are examined using line-imaged Raman/Rayleigh/CO-LIF diagnostics combined with crossed-planar Rayleigh imaging to determine the 3D flame orientation. Comparison of the experimental measurements with laminar flame calculations shows strong effects of preferential diffusion on the flame structure and the product state in lean and rich flames. Measurements of the flame orientation show a strong correlation between the flame-front normal angle and the strength of the preferential transport effects. As the flame-front angle decreases (by increasing the reactant velocity or decreasing the distance from the surface), the coupling between the preferential diffusion through the flame brush and the recirculation region is increased, enhancing the preferential transport effects. Spatial profiles and flame thickness measurements are discussed to evaluate how the strain and the turbulence affect the flame. In fuel-lean flames, as the velocity increases, eddies smaller than the flame thickness, but larger than the reaction layer, penetrate the preheat zone, improving the mixing and thickening the flame. Higher velocities are associated with higher levels of strain, which mitigate the thickening effect of the turbulence. Increases of the flame thickness up to 10% were observed. In fuel-rich flames, both the strain and the turbulence contribute to the thickening of the flame, and a flame thickness up to 2.5 times larger than the unstrained laminar thickness is observed. Changes are not limited to the preheat zone, but affect the entire flame structure. Progress variable scalar dissipation rate profiles are also discussed.

Soot and Nanomaterials Formation in Flame

<p>The seventieth anniversary of Professor Jürgen Warnatz is an important event for the scientific community of investigators of chemical kinetics and computation of combustion. His brilliant research in the field of combustion, particularly in the chemical kinetics reactions should be noted. His manuscript «Combustion»<br />with co-authors is a handbook for specialists in the field of combustion and under my supervision was translated to Kazakh. Professor J.Warnatz contributed much to the development of combustion processes modeling and education of scientists from various countries, including Kazakhstan. The general scheme of conversion of hydrocarbon fuels with new experimental data on the formation of fullerenes and graphenes, taking into account the pressure effect is proposed for the fuel-rich flames. It is shown that the formation of fullerenes is important to the corresponding spatial orientation of PAH, possible at low pressures. The formation of hydrophobic soot surface on silicon and nickel substrates during combustion of propane-oxygen flame was studied. It is stated that the hydrophobic properties are due to the presence of soot particles in the form of nanobeads.</p><p> </p>

Linear Stability Analysis of Laminar Premixed Fuel-Rich Double-Spray Flames

This paper considers the stability of a double-spray premixed flame formed when both fuel and oxidizer are initially present in the form of sprays of evaporating liquid droplets. To simplify the inherent complexity that characterizes the analytic solution of multi-phase combustion processes, the analysis is restricted to fuel-rich laminar premixed double-spray flames, and assumes a single-step global chemical reaction mechanism. Steady-state solutions are obtained and the sensitivity of the flame temperature and the flame propagating velocity to the initial liquid fuel and/or oxidizer loads are established. The stability analysis revealed an increased proneness to cellular instability induced by the presence of the two sprays, and for the fuel-rich case considered here the influence of the liquid oxidizer was found to be more pronounced than that of the liquid fuel. Similar effects were noted for the neutral pulsating stability boundaries. The impact of unequal latent heats of vaporization is also investigated and found to be in keeping with the destabilizing influence of heat loss due to droplet evaporation. It should be noted that as far as the authors are aware no experimental evidence is available for (at least) validation of the predictions. However, they do concur in a general and reasonable fashion with independent experimental evidence in the literature of the behavior of single fuel spray laminar premixed flames.

Mapping the Operation of the Miniature Combustion Aerosol Standard (Mini-CAST) Soot Generator

The Jing Ltd. miniature combustion aerosol standard (Mini-CAST) soot generator is a portable, commercially available burner that is widely used for laboratory measurements of soot processes. While many studies have used the Mini-CAST to generate soot with known size, concentration, and organic carbon fraction under a single or few conditions, there has been no systematic study of the burner operation over a wide range of operating conditions. Here, we present a comprehensive characterization of the microphysical, chemical, morphological, and hygroscopic properties of Mini-CAST soot over the full range of oxidation air and mixing N2 flow rates. Very fuel-rich and fuel-lean flame conditions are found to produce organic-dominated soot with mode diameters of 10–60 nm, and the highest particle number concentrations are produced under fuel-rich conditions. The lowest organic fraction and largest diameter soot (70–130 nm) occur under slightly fuel-lean conditions. Moving from fuel-rich to fuel-lean conditions al...

Soot and Nanomaterials Synthesis in the Flame

The general scheme of conversion of hydrocarbon fuels with new experimental data on the formation of fullerenes and graphenes taking into account the pressure effect is proposed for the fuel-rich flames. It is shown that the formation of fullerenes is important to the corresponding spatial orientation of PAH, possible at low pressures. The formation of hydrophobic soot surface on silicon and nickel substrates during combustion of propane-oxygen flame was studied. It is established that the hydrophobic properties are due to the presence of soot particles in the form nanobeads. The photovoltaic properties of solar cells coated by nickel oxide nanoparticles synthesized in counter flow propane-air flame. It is revealed that coated the surface of a silicon solar cell by nickel oxide nanoparticles results in the increase in solar cell efficiency by 3%.

The Impact of Biomass Cofiring on Volatile Flame Length

The physical characteristics and composition of biomass can vary significantly from coal and from one fuel to another, and these differences can impact the structure of the resulting pulverized fuel flame, particularly the volatile flame. The length and location of the volatile flame, which is the zone dominated by the combustion of volatiles, is important not only to flame stability and length but also to the formation of pollutants such as NOx. In this work, the effects of biomass volatile fraction and particle size on the length of the volatile flame were investigated for cofired flames of pulverized coal and wood waste. The volatile flame length was measured experimentally in a 35 kW combustion facility via axial measurements of CO and CO2 made along the centerline. Numerical simulations were employed to aid the interpretation of the impacts of wood waste cofiring on the volatile flame. The length of the volatile flame zone was found to be sensitive to the location of volatile matter release and the amount of volatile matter released within the volatile flame zone. Large biomass particles, with high axial momentum and long heating times, break through the fuel-rich volatile flame zone, releasing volatiles downstream into an oxygen-rich environment. The delayed release of volatiles reduces the amount of volatile fuel in the fuel-rich region, leading to shorter volatile flames, which in turn augments breakthrough of particles before complete release of volatiles. On the basis of CO and CO2 concentration measurements, cofired flames with 20 wt % wood waste and 80 wt % coal were found to have 21% shorter volatile flames compared with coal-only flames. Numerical simulations verified that volatile reactions take place past the end of the volatile flame. To reduce the amount of particles that pass through the volatile flame before complete devolatilization, the wood waste particle size was reduced. The resulting volatile flame was 27% longer for the 20 wt % cofired case compared with coal-only volatile flames. Increased volatile matter content, characteristic of biomass fuels, leads to an increased volatile flame length when all of the volatiles are released in the near-burner region.

Onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a flat-flame heat-flux burner

The onset of cellular instability in adiabatic H2/O2/N2 premixed flames anchored to a heat-flux burner is investigated numerically. Both hydrodynamic instability and diffusional-thermal instability are shown to play an important role in the onset of cellular flames. The burner can effectively suppress cellular instability when the flames are close to the burner, otherwise the burner can suppress the instabilities only at large wavenumbers. Because of differential diffusion, local extinction can occur in lean H2/O2/N2 flames. When the flames develop to take on cellular shapes, the surface length, the overall heat release rate and the mean burning velocity are all increased. For near stoichiometric fuel-rich flames the mean burning velocity can increase by as much as 20%–30%. For lean flames with an equivalence ratio of 0.56, the mean burning velocity can be 2–3 times of the burning velocity of the corresponding planar flame.

Creating Conductive Copper–Silver Bimetallic Nanostructured Coatings Using a High Temperature Reducing Jet Aerosol Reactor

We report production of bimetallic nanostructured copper– silver coatings by in situ deposition and sintering of metal nanoparticles produced as an aerosol. The metal nanoparticles themselves have potential applications in printed electronics, catalysis, antibacterial coatings, and heat transfer fluids. In many applications, nanoparticles are dispersed in an ink, which is then printed or coated onto a substrate and converted into a nanostructured thin film. Direct deposition from the aerosol allows us to produce nanostructured thin films without first dispersing the particles in a solvent. The high temperature reducing jet process allows formation of these metal nanoparticles from low-cost metal salt precursors in the gas phase. In this method, a fuel-rich hydrogen flame provides a low-cost source of energy to drive nanoparticle formation in a reducing environment. The aqueous precursor solution is delivered into the hot combustion product gases within a converging–diverging nozzle. The high-speed gas flow atomizes the precursor and provides exceptionally rapid mixing of the precursor with the hot gases. Here, particles are formed, then immediately quenched and deposited on a glass substrate. The effect of the silver content of the mixed copper–silver films on their electrical conductivity was studied systematically, revealing an abrupt transition from low conductivity to high conductivity between 30 wt.% and 40 wt.% silver. Copyright 2013 American Association for Aerosol Research

R2PI Spectroscopy of Aromatic Molecules Produced in an Ethylene-Rich Flame

Laser spectroscopy, combined with mass spectrometry, was applied to study the spectra of aromatic molecules produced in a premixed ethylene-rich flat flame. These studies produce new gas-phase electronic spectra of polyaromatic compounds, which ultimately will guide the understanding of the chemical processes that lead to polycyclic aromatic hydrocarbon (PAH) growth or PAH formation locking. Resonant two-photon ionization (R2PI) spectra of all species detectable in a specific fuel-rich flame were recorded simultaneously during a single scan of the laser wavelength, within the 220-330 nm range. Comparison with spectra available in the literature allowed us to identify 16 aromatic species. In the PAH forming region of this flame, we found that the main PAHs are accompanied by a great diversity of other species, including in particular various side-chains on aromatic networks. We also show that this technique allows, at least in some cases, to distinguish between different isomers associated with the same mass peak, although the extracted PAHs are only cooled down to room temperature.

Electric Field Control of the Polluting Emissions from a Propane Flame

The present study is performed with the aim to reduce the levels of polluting emissions from fuel combustion that produce acid rains and the greenhouse effect (NOx, CO2). The electric field effects on the processes of heat/mass transfer and propane combustion are studied in order to perform electric control of the levels of polluting emissions from the flame. The results of experimental studies show the direct influence of the electric field's enhanced mass transfer on local variations of the flame composition and fuel combustion. The related variations of the flame temperature, processes of soot formation, carbon capture and deposition along the flame channel flow are studied by varying the field strength and the equivalence ratio of the propane-air mixture. The results show that the electric field effect on soot for- mation, carbon capture and sequestration, for fuel-rich flame flow, can be used to reduce the levels of CO2 emissions from the flame. In addition, the field-enhanced heat/ mass transfer to the channel walls, for fuel-lean conditions, can be used to control the fuel combustion, flame temperature and temperature- sensitive levels of NOx emissions. The most pronounced electric field effects on fuel combustion and composition of the products are observed in the limit of the weak fields (U<1,2 kV, E<105 V m-1).

Prediction of electron and ion concentrations in low-pressure premixed acetylene and ethylene flames

Flame stabilisation and extinction in a number of different flows can be affected by application of electric fields. Electrons and ions are present in flames, and because of charge separation, weak electric fields can also be generated even when there is no externally applied electric field. In this work, a numerical model incorporating ambipolar diffusion and plasma kinetics has been developed to predict gas temperature, species, and ion and electron concentrations in laminar premixed flames without applied electric fields. This goal has been achieved by combining the existing CHEMKIN-based PREMIX code with a recently developed methodology for the solution of electron temperature and transport properties that uses a plasma kinetics model and a Boltzmann equation solver. A chemical reaction set has been compiled from seven sources and includes chemiionisation, ion-molecule, and dissociative–recombination reactions. The numerical results from the modified PREMIX code (such as peak number densities of positive ions) display good agreement with previously published experimental data for fuel-rich, non-sooting, low-pressure acetylene and ethylene flames without applied electric fields.

Heat transfer optimization of an impinging port-array inverse diffusion flame jet

This paper is an experimental study on the heat transfer optimization of an Innovative Inverse Diffusion Flame (IDF) characterized by a central air jet surrounded by an array of fuel jets for impingement heating. It is found that the diameter ratio between air port and fuel port (da/df) exerts significant influence on the impingement heat transfer performance via changing the hydrodynamics and the thermal structure of the IDF. The effects of the overall equivalence ratio (Φ), the burner-to-plate distance (H), and the air jet Reynolds number (Rea), on the local and average heat fluxes, are investigated and compared between IDFs with different da. Under the same air flow rate (Q˙a) and fuel flow rate (Q˙f), it is found that the IDF jet with smaller da produces higher maximum heat flux (qmax′) and average heat flux in the impingement region (q¯), due to the increased flame temperature (Tf) and turbulence level under larger Rea. Fuel-rich flames produce low fuel utilization efficiency and low heat flux and should be avoided in the practical applications. Fuel-lean condition in the range of Φ = 0.9 to Φ = 1.1 is found to be a desirable operation condition for its high heat flux and fuel efficiency. The current study provides valuable information on the heat transfer optimization of this innovative port-array IDF.

Effect of ethanol on the chemistry of formation of precursors of polyaromatic hydrocarbons in a fuel-rich ethylene flame at atmospheric pressure

The effect of the addition of ethanol (EtOH) to the initial combustible mixture on the concentration of various compounds, in particular, those preceding the formation of polyaromatic hydrocarbons in a fuel-rich (equivalence ratio of fuel ϕ = 1.7) flat premixed ethylene/oxygen/argon flame at atmospheric pressure was studied experimentally and by numerical modeling using a detailed mechanism of chemical reactions. Concentrations of various stable and labile species, including reactants, major combustion products, and intermediates in C2H4/O2/Ar and C2H4/EtOH/O2/Ar flames were measured along the height above the burner using molecular beam mass spectrometry. Experimental mole fraction profiles were compared with those calculated using the previously proposed mechanisms of chemical reactions. This mechanism was analyzed to determine the cause of the ethanol effect on the flame concentration of propargyl, the main precursor of polyaromatic hydrocarbons.

Prompt NO formation in flames: The influence of NCN thermochemistry

The influence of the route via the NCN radical on NO formation in flames was examined from a thermochemistry and reaction kinetics perspective. A detailed analysis of available experimental and theoretical thermochemical data combined with an Active Thermochemical Tables analysis suggests a heat of formation of 457.8±2.0kJ/mol for NCN, consistent with carefully executed theoretical work of Harding et al. (2008) [5]. This value is significantly different from other previously reported experimental and theoretical values. A combination of an extensively validated comprehensive hydrocarbon oxidation model extended by the GDFkin3.0_NCN-NOx sub-mechanism reproduced NCN and NO mole fraction profiles in a recently characterized fuel-rich methane flame only when heat of formation values in the range of 445–453kJ/mol are applied. Sensitivity analysis revealed that the sensitivities of contributing steps to NO and NCN formation are strongly dependent on the absolute value of the heat of formation of NCN being used. In all flames under study the applied NCN thermochemistry highly influences simulated NO and NCN mole fractions. The results of this work illustrate the thermochemistry constraints in the context of NCN chemistry which have to be taken into account for improving model predictions of NO concentrations in flames.

Experimental and modeling study on the influences of methanol on premixed fuel-rich n-heptane flames

The structures of two laminar premixed n-heptane/O2/Ar flames (F1.60: Ф=1.60, C/O=0.51, and F1.80: Ф=1.80, C/O=0.57) and one laminar premixed n-heptane/methanol/O2/Ar flame (F1.80M: Ф=1.80, C/O=0.51) are studied at low pressure (4000Pa) by using synchrotron photoionization and molecular-beam sampling-mass spectrometry (PI-MBMS) techniques. Calculations are performed with a modified chemical mechanism, which satisfactorily simulates the tested flames. The results show that as equivalence ratio increases, the maximum flame temperature is reduced and the flame front is shifted away from the burner surface. The post-flame CO concentration in F1.80M is lower than that in F1.80, which is attributed not only to the difference in inlet carbon flux but also to the variation in CO formation pathway. As methanol is added, the peak concentrations of C2–C7 hydrocarbon intermediates are reduced substantially, and the extent of the reduction in the case of constant C/O ratio is smaller than that in the case of constant equivalence ratio. The production of formaldehyde is promoted with the addition of methanol. Reaction flux analysis indicates that the self-recombination of propargyl radical (C3H3) and the cross reaction between C3H3 and allyl radical (a-C3H5) are the dominant pathways leading from small aliphatics to benzene for all the flames.