CH Radical Concentrations Research Articles

Experimental Investigation of Inverse Diffusion Flame Stabilization by Nonequilibrium Plasma

A plasma injector is designed to improve the flame stabilization of a typical coaxial injector. Based on the plasma injector experimental system, the effects of an ac dielectric barrier discharge generated plasma on a inverse diffusion flame are investigated. The electrical characteristics, discharge images, plasma species, thermal effect, gas dynamic effect, flame shape, and heat release are analyzed in detail. The results show that species including C, , , and CH are generated in the methane discharge plasma. The concentration of CH radically increases linearly with increasing voltage, while its spatial distribution tends to be uniform. However, the CH radical concentration decreases with an increasing flow rate. For application to plasma-assisted combustion, the thermal effect of the ac dielectric barrier discharge plasma on the C jet can be neglected. The heating is limited to within the injector. The jet angle, which reflects the transverse movement degree of the methane flow, is expanded by the discharge. Because of the gas dynamic effect of the plasma, which changes the pattern of the original jet, the mixing between the fuel and the oxidizer is improved when the discharge is active. The original unstable flame with a single layer is transformed into a stable double-layer flame, and the heat release from the flame is enhanced. However, the stable flame might become unstable if the discharge voltage is too high. The plasma shows better performance with respect to flame stabilization under a high flow rate or a fuel-rich condition. The lowest cost-effectiveness ratio is only 0.7% in the experiment.

NO formation in rich premixed flames of C1–C4 alkanes and alcohols

A comprehensive study of prompt nitric oxide (NO) formation is presented for rich (φ=1.3) premixed flames of alkanes and alcohols. Measurements of the flame burning rate, the temperature, and the CH radical and NO species are performed in atmospheric-pressure laminar premixed stagnation flames. New data, obtained for all isomers of the C4 fuels, butane and butanol, are merged with existing data for C1–C3 fuels obtained by the same methodology. The combined dataset consists of all isomeric combinations of the fuel molecules, and provides a complete description of pollutant trends related to fuel type and branching structure for C1–C4 alkanes and alcohols. These data show alcohols produce less NO than alkanes of equivalent carbon chain length. This behavior is linked to the tendency of alcohols to produce lower concentrations of the CH radical, due to inhibition of the formation of methyl radicals. The results show a linear scaling of prompt-NO formation with the CH radical concentrations, when these are scaled by the characteristic residence time in the flame. This residence time depends on the burning velocity of the fuels, which decreases with increasing fuel branching. These data are useful for thermochemical model development using detailed simulations of the experiments and a comparative diagnostics approach. Further adjustments to all of the thermochemical models are needed to capture the observed trends. This dataset is made available and provides validation and optimization targets for future model revisions.

Radiative Heat Transfer and Fluorescence Measurements in Laminar Prevaporized Canola Methyl Ester/Diesel Blend Flames

Biofuels, such as canola methyl ester (CME), continue to receive considerable attention for their potential use as alternatives to petroleum diesel fuel. The studies on the application of biofuels in internal combustion engines, in general, have shown a considerable reduction in carbon monoxide (CO), soot, and radiative heat emissions, and a small increase in NOx emissions. Radiative heat transfer from flames, which is important in applications such as gas turbines and glass-manufacturing furnaces, has received little attention. The objective of this investigation was to document radiative heat transfer and radical and gas concentration measurements to understand the dominant mechanism of heat transfer in CME/diesel blend flames. In order to isolate the fuel chemical effects on the combustion characteristics of fuels, laminar flames of prevaporized liquid fuels were studied at injector-exit equivalence ratios of 1.2, 2, 3, and 7. Measurements of radiative heat transfer and flame structure including OH and CH radical concentration field were completed. While the peak temperatures in the various blend flames were comparable at the same equivalence ratio, the total flame radiation decreased with the increase in CME concentration in the fuel. Estimates of radiation from gaseous species and soot indicated that about 27–30% of the radiation was from gases, and the rest from soot. The gaseous species contribution to the flame radiation increased slightly with the biofuel content in the blend.

Flame stability and OH and CH radical emissions from mixtures of natural gas with biomass gasification gas

In this work, a study about laminar premixed flames derived from the combustion of natural gas and producer gas (25 and 50% by volume) mixtures in an atmospheric burner was carried out. The producer gas was obtained from steam gasification process of dealcoholised marc of grape in a small-scale drop-tube gasification pilot plant, this solid fuel being an abundant biomass waste in Spain. These mixtures have different advantages such as replacing fossil fuels with renewable fuels, reducing NOx emissions and other advantages associated with the biomass consumption. The main objectives of this work are to delimitate the stability diagram of the different mixtures, to evaluate the potential of these mixtures to reduce NOx emission and to study the interchangeability of the gas mixtures used. The results obtained show that an increase in the producer gas amount (and therefore in the hydrogen content) makes possible a stable burner operation at leaner conditions, thus leading to lower combustion temperature and NOx emissions. OH and CH radical concentrations were measured by chemiluminescence to confirm the potential to reduce NOx emissions. The Wobbe index results show a possible interchangeability between natural gas and natural gas/producer gas mixtures below 37% vol. of producer gas.

Influence of Steam Dilution on Nitrogen Oxide Formation in Premixed Methane/Hydrogen Flames

The premixed combustion of natural gas and hydrogen at atmospheric pressure is investigated over a wide range of equivalence ratios and different degrees of steam and nitrogen dilution of up to 30% in the air. Emission formation, local temperatures, OH radical concentrations, and flame shape and position are measured in gas-fired tests. The measurement results confirm that steam effectively inhibits the formation of emissions, even at constant flame temperature. For both natural gas and hydrogen, and CO emissions below 10 ppm were measured up to near-stoichiometric fuel concentrations at wet conditions. Nitrogen dilution does not lead to reduced emissions. A numerical analysis conducted with a reactor network shows that the influence of steam dilution on radical concentrations and formation depends strongly on the degree of humidity and on the equivalence ratio. The thermal, the , and the NNH pathway are significantly constrained at wet conditions mainly due to reduced O atom concentrations, whereas prompt can be increased due to higher CH radical concentrations for the natural gas flame. Thus, the inhibiting effect of steam dilution on formation is stronger at high flame temperatures.

Influence of steam dilution on the combustion of natural gas and hydrogen in premixed and rich-quench-lean combustors

The combustion of different natural gas–hydrogen mixtures at dry and steam-diluted conditions is investigated for a rich-quench-lean (RQL) and for a premixed combustor. Atmospheric experiments are conducted over a wide range of equivalence ratios and degrees of steam dilution up to a steam-to-air mass ratio of 30%. Emission formation, flame shape and position, and flame stability are measured in combustion tests. An investigation of the influence of steam on the NOx formation is conducted using reactor networks.Steam effectively inhibits the formation of NOx emissions in both combustion systems. For wet conditions, NOx emissions below 10ppm are achieved for operation on natural gas as well as hydrogen up to high equivalence ratios and temperatures. Increasing the hydrogen content leads to increasing NOx emissions for dry conditions, whereas at wet conditions, NOx is lower for hydrogen-rich fuels. The numerical analysis reveals that in both combustion systems, the thermal, the N2O, and the NNH pathways are significantly constrained at wet conditions. The prompt NOx can be increased due to higher CH radical concentrations for the natural gas flame. CO emissions remain low up to moderate degrees of steam dilution. For a steam content of 20%, the RQL combustor shows increasing CO emissions for low equivalence ratios, whereas they remain low for the premixed combustor.Steam dilution is very effective for NOx reduction in natural gas and in hydrogen flames. Additionally, it lowers the reactivity of hydrogen and already a relatively low steam content prevents flashback. The combination of steam injection and hydrogen combustion provides an efficient and clean combustion mode.

Concentration Measurements of CH and OH Radicals in Laminar Biofuel Flames

An investigation to determine the dominant route of NOx formation in biofuel flames and to confirm the relationship of the NOx increase with iodine numbers in fuels has been presented. This is done through the measurement of the concentration of OH and CH radicals, indicators of the formation of NOx through the Zeldovich and Fenimore mechanisms. A laminar partially pre-mixed flame at an initial fuel equivalence ratio (φ) = 7 was used to minimize the effects of fluid mechanics and isolate the effects of fuel chemistry. Three biofuels, with different iodine numbers, were studied: soy methyl ester (SME), canola methyl ester (CME) and methyl stearate (MS). Planar Laser-Induced Fluorescence (PLIF) images of hydroxyl radicals (OH) and CH radicals were captured with a diagnostic system consisting of a pulsed Nd:YAG laser and an Optical Parametric Oscillator (OPO) with frequency doubler option (FDO) using proper wavelengths. It was found that the population of OH radicals was low in the flames of all fuels, but significant CH radical concentrations were detected in all the flames, with the maximum population occurring in the SME fuel flame. The presence of high concentrations of CH measured in the regions of peak NOx indicate that NOx formation is primarily through the Fenimore mechanism, rather than the thermal mechanism, at this fuel-rich condition. Moreover, the fuel with highest iodine number, SME, produced significantly more NOx because of its tendency to facilitate the production of more soot, C, and CH radicals.

NO x emission characteristics of counterflow syngas diffusion flames with airstream dilution

Syngas is produced through a gasification process using variety of fossil fuels, including coal, biomass, organic waste, and refinery residual. Although, its composition may vary significantly, it generally contains CO and H 2 as the dominant fuel components with varying amount of methane and diluents. Due to its wide flexibility in fuel sources and superior pollutants characteristics, the syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. There are, however, gaps in the fundamental understanding of syngas combustion and emissions, as most previous research has focused on flames burning individual fuel components such as H 2 and CH 4, rather than syngas mixtures. This paper reports a numerical investigation on the effects of syngas composition and diluents on the structure and emission characteristics of syngas nonpremixed flames. The counterflow syngas flames are simulated using two representative syngas mixtures, 50%H 2/50%CO and 45%H 2/45%CO/10%CH 4 by volume, and three diluents, N 2, H 2O, and CO 2. The effectiveness of these diluents is characterized in terms of their ability to reduce NO x in syngas flames. Results indicate that syngas nonpremixed flames are characterized by relatively high temperatures and high NO x concentrations and emission indices. The presence of methane in syngas decreases the peak flame temperature, but increases the formation of prompt NO significantly. Consequently, while the total NO formed is predominantly due to the thermal mechanism for the 50%H 2/50%CO mixture, it is due to the prompt mechanism for the 45%H 2/45%CO/10%CH 4 mixture. For both mixtures, CO 2 and H 2O are more effective than N 2 in reducing NO x in syngas flames. H 2O is the most effective diluent on a mass basis, while CO 2 is more effective than N 2. The effectiveness of H 2O is due to its high specific heat that decreases the thermal NO, and its ability to significantly reduce the concentration of CH radicals, which decreases the prompt NO. The presence of methane in syngas reduces the effectiveness of all three diluents.

Experimental and numerical investigation of effect of fuel on ion sensor signal to determine combustion timing in homogeneous charge compression ignition engines

Homogeneous charge compression ignition (HCCI) engines offer promise owing to low emissions and high efficiency. However, the control of the combustion process in HCCI engines, specifically the control of the start of combustion (SOC) or ignition timing, remains a challenge. Piezoelectric pressure transducers are used in research engines for determination of the start of combustion; however, these pressure transducers are too expensive and fragile for applications in commercial engines. Recent work by the authors as well as other investigators has shown the potential of inexpensive ion sensors in HCCI engines fuelled with propane or gasoline. However, the working range of ion sensors is limited in HCCI engines owing, in large part, to the fact that the peak cycle temperature in HCCI combustion is quite low (∼ 1700–1900 K). With the guidance of detailed chemical kinetic modelling it is shown that fuels or additives producing a higher concentration of CH radicals will probably produce higher ion concentrations. Acetylene (HC=CH) is known to produce large concentrations of CH radicals. Hence, various mixtures of propane and acetylene are numerically and experimentally studied. The ion concentration substantially increases with the addition of a small amount of acetylene. This research is an effort to understand the ion generation mechanism in HCCI engines with a view towards improving the ion signal.

Laser-induced fluorescence measurements and modeling of absolute CH concentrations in strained laminar methane/air diffusion flames

We have applied linear laser-induced fluorescence to obtain spatially resolved profiles of CH radicals in laminar methane/air and methane/nitric oxide/air counterflow diffusion flames at atmospheric pressure. Excitation and detection of transitions in the A–X band and calibrating the optical detection efficiency via Rayleigh scattering allowed the determination of absolute radical concentrations. Flames at strain rates from 59 to 269 s −1 were studied to characterize the strain rate dependence of the CH concentration. The work shows that CH concentrations increase with increasing strain rate. Comparisons have been made with predicted CH levels obtained using two different chemical kinetic mechanisms (Lindstedt et al. and GRI-Mech. 3.0). Computed concentrations are shown to be in good agreement with experimental data. It was furthermore found that the addition of up to 600 ppm NO to the fuel did not have a measurable effect on the CH radical concentration. This is also in agreement with predictions from both mechanisms. The current work has shown that measurements of absolute CH radical concentrations are possible in non-premixed flames without the need for spatial temperature or quenching corrections.

Absolute radical concentration measurements in low-pressure H 2/O 2 flames during the combustion of graphite

Absolute CN and CH radical concentrations were determined in situ during the combustion of a graphite substrate in premixed, laminar, low-pressure, H 2/O 2 flames for two different equivalence ratios, ϕ = 1.0 and ϕ = 1.5. For CN measurements, a small amount of NO (1.8%) was added. The concentration of CN was measured by cavity ring-down spectroscopy (CRDS) probing the absorption of the P 1,2 (13) in the B–X (0, 0) band at 388.1 nm, and the concentration of CH was measured by linear unsaturated laser-induced fluorescence (LIF) exciting the fluorescence of the R 1 (4) in the B–X (0, 0) band at 387.4 nm. Temperature measurements were done based on LIF excitation spectra of OH in the A–X (0, 0) band. It was found that the graphite substrate reduces the flame temperature in the vicinity of its surface. The CN concentrations were found to be three times higher for the rich flame than for the stoichiometric flame. CH concentrations were slightly higher for the stoichiometric flame than for the rich flame. The observed CH/CN concentration ratio is substantially lower compared to NO-doped low-pressure CH 4/O 2 flames. The obtained quantitative information can serve as a first calibration point for detailed numerical simulations of the burning solid graphite, which are based on the concept of surface elementary reactions.

Intermediate radical concentrations in hydrogen?natural gas blended fuel jet flames

An experimental and numerical investigation of flame temperature and intermediate radical (OH, O, H, CH) concentrations in hydrogen (H2)–natural gas (NG) hybrid fuel diffusion flames at a burner exit Reynolds number 150 are presented. Radial concentration profiles of temperature and radicals in the near-burner region (8mm above the burner tip) for different mixtures of hydrogen–NG (100% NG, 80–20% NG–H2, 50–50% NG–H2, 20–80% NG–H2, 100% H2 by volume) are reported. Both experimental and computational findings show that as a result of increased fuel burning, the stoichiometric contour moves inward with the increase of hydrogen content in the fuel mixture. The 100% hydrogen flame temperature is close to its equilibrium value, while due to the effect of radiation the mixed fuels have lower temperatures than their equilibrium values. The OH radical concentration profile shows a peak in the stoichiometric contour. Within experimental uncertainties the measured and the computed values of OH radical concentration agree well. The OH concentration increases with the increase of hydrogen concentration in the mixture. The radial profiles of O atom concentration have a peak in the lean side of the fuel oxidizer interface. The photolytic production of O atom affects the measured values. The maximum H atom concentration occurs in the rich side of the stoichiometric contour. The computed peak concentration of H radical for 100% NG flame is comparable with its equilibrium concentration, which indicates the existence of partial equilibrium of H in hydrocarbon flames. The maximum production of CH radical occurs in the rich side of the flame surface. Compared to the experimental results, computation predicts a lower CH concentration because of the use of reaction mechanism without C2 chemistry. The CH radical concentration decreases with the increase of hydrogen content in the mixed fuel.

Intermediate radical concentrations in hydrogen–natural gas blended fuel jet flames

An experimental and numerical investigation of flame temperature and intermediate radical (OH, O, H, CH) concentrations in hydrogen (H 2)–natural gas (NG) hybrid fuel diffusion flames at a burner exit Reynolds number 150 are presented. Radial concentration profiles of temperature and radicals in the near-burner region ( 8 mm above the burner tip) for different mixtures of hydrogen–NG (100% NG, 80–20% NG–H 2, 50–50% NG–H 2, 20–80% NG–H 2, 100% H 2 by volume) are reported. Both experimental and computational findings show that as a result of increased fuel burning, the stoichiometric contour moves inward with the increase of hydrogen content in the fuel mixture. The 100% hydrogen flame temperature is close to its equilibrium value, while due to the effect of radiation the mixed fuels have lower temperatures than their equilibrium values. The OH radical concentration profile shows a peak in the stoichiometric contour. Within experimental uncertainties the measured and the computed values of OH radical concentration agree well. The OH concentration increases with the increase of hydrogen concentration in the mixture. The radial profiles of O atom concentration have a peak in the lean side of the fuel oxidizer interface. The photolytic production of O atom affects the measured values. The maximum H atom concentration occurs in the rich side of the stoichiometric contour. The computed peak concentration of H radical for 100% NG flame is comparable with its equilibrium concentration, which indicates the existence of partial equilibrium of H in hydrocarbon flames. The maximum production of CH radical occurs in the rich side of the flame surface. Compared to the experimental results, computation predicts a lower CH concentration because of the use of reaction mechanism without C 2 chemistry. The CH radical concentration decreases with the increase of hydrogen content in the mixed fuel.

Laser diagnostics of CH in a diamond depositing flame

Abstract Cavity ring down spectroscopy was applied as a laser diagnostic tool to measure CH radical concentrations in a diamond depositing oxyacetylene flame. CH column densities were determined during diamond growth at about 0.4 mm above the deposition substrate. By applying Abel inversion processing absolute concentration profiles are obtained which are compared to diamond growth rate profiles. A correlation is found between the CH density distribution and the local growth rate. From this it can be concluded that the CH radical possibly plays a role in the formation of an annulus of enhanced diamond growth.

Nitric oxide formation and reburn in low-pressure methane flames

Absolute NO concentrations are measured in low-pressure methane-oxygen-nitrogen flames of varying stoichiometry, using laser-induced fluorescence. These measurements are combined with previously measured absolute CH radical concentrations to test the chemical mechanism of prompt NO formation and NO reburn. The GRI-Mech 2.11 chemical mechanism underpredicts prompt NO by 35–60%. Increasing the rate coefficient for the CH+N2 reaction brings the predicted [NO] into agreement with the measurements. Reburn chemistry is studied by seeding NO at concentrations between 0.1% and 3% in rich and lean flames. The model agrees with the observed reduction of NO in a fuel-lean (=0.80) flame and underpredicts NO reburn in a fuel-rich (=1.27) flame. The model predicts significantly more reduction of the peak CH concentration by reburn of seeded NO in the rich flame than is measured.

Mass-spectrometry and actinometry of the low-pressure arc discharge with an evaporating graphite cathode

Emission spectroscopy, optical actinometry and mass spectroscopy non-equilibrium plasma diagnostics have been applied for the determination of parameters of the low-pressure arc discharge with an evaporating graphite cathode in a methane atmosphere. The absolute density of hydrogen atoms and the relative concentration of CH radicals, as well as the degree of dissociation of hydrocarbon (x = 1 - 4) radicals have been determined as functions of the pressure of methane. The optical properties of the hydrocarbon films deposited by the discharge have been investigated. The results of the investigation show that the transmittance of these films is associated closely with the optical emission intensity ratio of the CH (430 nm) and (656.2 nm) spectral lines.

Simultaneous Rayleigh scattering and laser-induced CH fluorescence for reaction zone imaging in high-speed premixed hydrocarbon flames

/air flames where CH concentration is on the order of 1 ppm based on flamelet calculations. The present experimental conditions are also examined and shown to be suitable for quantitative measurements of CH radical based on the two-level model analysis. A linear relationship can be found between the measured CH signal intensity and the calculated CH concentration within a maximum 30% uncertainty range. The FWHM thickness of the CH profile in a stoichiometric laminar methane flame was shown to be less than 0.3 mm, which is the smallest ever achieved. Simultaneous image pairs of flame temperature and concentration of CH radicals from a premixed turbulent Bunsen flame at an exit velocity of 65 m/sec are obtained to demonstrate the system superiority of application on high-speed reacting flows.

Measurement of the rate coefficient of the reaction CH+O2 ? products in the temperature range 2200 to 2600 K

The rate coefficient of the reaction CH+O2 products was determined by measuring CH-radical concentration profiles in shock-heated 100–150 ppm ethane/1000 ppm O2 mixtures in Ar using cw, narrow-linewidth laser absorption at 431.131 nm. Comparing the measured CH concentration profiles to ones calculated using a detailed kinetics model, yielded the following average value for the rate coefficient independent of temperature over the range 2200–2600 K: The experimental conditions were chosen such that the calculated profiles were sensitive mainly to the reactions CH+O2 products and CH3+M CH+H2+M. For the methyl decomposition reaction channel, the following rate-coefficient expression provided the best fit of the measured CH profiles: Additionally, the rate coefficient of the reaction CH2+HCH+ H2 was determined indirectly in the same system: © 1997 John Wiley & Sons, Inc.

CH5 precursor mechanism for diamond growth

Deposition rates of polycrystalline diamond films are investigated as a function of concentration of water vapor in a microwave chemical vapor deposition (CVD) apparatus. For CH4:H2 = 2%:98% gas composition the measured deposition rates are compared with mass spectroscopic data published in the literature and predictions suggested by the widely accepted methyl radical (CH3) diamond growth model. Our results do not confirm the CH3 model, but a good correlation is found between the concentration of CH radicals and the deposition rates of diamond. The main reaction paths leading to diamond deposition from CH are also presented. [Russian Text Ignored]

Two-channel thermal decomposition of CH 3

The two-channel thermal decomposition of CH3 was investigated by time-resolved measurements of both H atom and CH radical concentrations using ARAS and ring dye laser spectroscopy respectively. As a source for methyl radicals, mixtures of a few ppm ethane diluted in argon were shock heated to temperatures between 2100 K and 3800 K and pressures between 1.1 bar and 1.8 bar. From the initial time histories of the measured H atom and CH radical concentration profiles the rate coefficients of the reaction channels.