Maximum Concentration Of Radicals Research Articles

Perturbations of the flame structure due to a thermocouple. I. Experiment

In thermocouple measurements in flames of gaseous or condensed systems, it is usually assumed that due to the small sizes of thermocouples, the flame perturbations caused by thermocouples are negligible. It is shown, however, that these perturbations can be significant. Temperature measurements in a laminar methane flame at atmospheric pressure revealed a systematic overestimation of measured temperatures compared with the temperature of the unperturbed flame in the temperature gradient region and in the region of the maximum concentration of radicals. This overestimation was measured, and its causes were analyzed. Previously, such effects have not been studied in the literature.

Experimental and numerical study on laminar burning velocities and flame instabilities of hydrogen–air mixtures at elevated pressures and temperatures

Experimental and numerical study on hydrogen–air flames at elevated pressures and temperatures was conducted. Meanwhile, the calculation is extended to initial pressure and temperature up to 8.0 MPa and 950 K, respectively. Laminar burning velocities and Markstein lengths were obtained at the elevated pressures and temperatures. Sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The study shows that laminar burning velocities are increased with the increase of initial temperature, and they decrease with the increase of initial pressure. With the increase of initial pressure, advancement of the onset of cellular instability is presented and Markstein length is decreased, indicating an increase of flame instability with the increase of initial pressure. The study shows insensitivity of flame instability to initial temperature. Laminar burning velocity is depended on the competition between the main chain branching reactions and chain termination reaction. The chain branching reactions are the temperature-sensitive reaction, while the termination reaction is the temperature-insensitive reaction. Through the extraction of the overall reaction orders, it is demonstrated that with increasing pressure, the overall reaction orders give a decreasing trend and then increasing trend. This behavior suggests an analogy to three explosion limits of hydrogen/oxygen mixtures. Numerical study also shows that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) is closely linking to the decrease (or increase) of H, O and OH mole fractions in the flames. Strong correlation is existed between burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames. On the basis of the numerical data, an empirical formula for laminar burning velocity is correlated for the hydrogen–air premixed mixture at elevated pressures and temperatures. The correlated laminar burning velocities are in good agreement with the known experimental results and simulated results with CHEMKIN. The correlation can be used in the calculation of laminar burning velocities at evaluated pressures and temperatures.

Experimental and numerical study on lean premixed methane–hydrogen–air flames at elevated pressures and temperatures

Abstract Experimental and numerical study on the lean methane–hydrogen–air flames at elevated pressures and temperatures was conducted. The unstretched laminar burning velocities and Markstein lengths were obtained over a wide range of hydrogen fractions at elevated pressures and temperatures. The sensitivity analysis and flame structure were also analyzed. The results show good agreement between the computed results and experimental data. The unstretched laminar burning velocities are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. With the increase of initial pressure and hydrogen fraction, Markstein lengths are decreased, indicating the increase of flame instability. Laminar burning velocity is depended on the competition between the main chain branching reaction and chain recombination reaction. The chain branching reaction is a temperature-sensitive reaction, while the recombination reaction is a temperature-insensitive reaction. Numerical study shows that the suppression (or enhancement) of overall chemical reaction with the increase of initial pressure (or temperature) is closely linking to the decrease (or increase) of H, O and OH mole fractions in the flames. Strong correlation is existed between burning velocity and maximum radical concentrations of H and OH radicals in the reaction zone of premixed flames.

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Abstract Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40% , laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.

Effect of heat transfer on the distributions of OH and CH radicals in the boundary layer with ethanol combustion

The reasons for formation of superequilibrium concentrations of radicals are studied by means of joint consideration of experimental data on the distributions of CH and OH molecules formed during diffusion combustion of ethanol and data on heat transfer in the chemical reaction region. The air flow velocity near the stagnation point in experiments with combustion is 0.7 m/sec, and the flow velocity along a flat plate is 10 m/sec (the turbulence levels are 1 and 18%). Mutual locations of specific features in the distributions of the heat-release rate and temperature are analyzed and compared with the distributions of OH and CH radicals. For all turbulence levels and flow velocities considered, the maximum concentration of radicals is reached on the boundaries of the heat-release region, whose locations are determined by molecular transport mechanisms. It is demonstrated that this conclusion is applicable to experimental data on diffusion combustion of a submerged hydrogen jet in air.

Organic radicals and paramagnetic metal complexes in municipal solid waste composts. An EPR and chemical study

The physicochemical properties of composted municipal solid wastes (MSW) were investigated by chemical and spectroscopic methods (EPR, NMR, IR, UV-Vis, ICP). Transformations of the composted organic matter during 150 days were observed. The water content was controlled in the composts; in some cases the composted MSW were nitrogen enriched by urea addition. The maturation of the composts was observed using standard methods. Quantitative EPR measurements were made on isolated humic and fulvic acids; the concentration of semiquinone free radicals and g parameters were determined. The maximum radical concentration (1.2 · 10 17 spins/gram) in isolated humic acids was observed in about the 6th week of the composting process in good quality composts. Moreover, distinct linear changes in g parameters of isolated humic acids (HA) from 2.0031 to 2.0037 were observed during the composting process. These changes were correlated to increasing E 4/E 6 ratio characterizing the intensity of the oxidation processes and formation of oxygen-rich functional groups. The heavy metal (Cu, Fe, Mn, Pb) concentrations were examined in the composts and humic (HA) and fulvic acid (FA) fractions. The increase of copper binding by HA extracted from composts enriched in nitrogen was observed;the coordination sites of the ligand were characterized on the basis of EPR spectra. The various Fe(III) and Mn(II) complexes in the HA and FA fractions detected by EPR were used as additional indicators of oxidation-reduction processes in the composts.

Determination of curing time in visible-light-cured composite resins of different thickness by electron paramagnetic resonance.

The irradiation time of a visible-light-activated composite necessary to achieve full polymerization throughout the material was studied. Curing-time dependence on the thickness of the material was also investigated. To monitor the visible light-activation effect, the free radical concentration was measured as a function of irradiation time. If the composite sample is less than 0.5 mm thick and exposed to light for a time interval recommended by the manufacturer, full radical concentration is indeed created uniformly. This is not the case in thicker samples. Electron paramagnetic resonance (EPR) was used to monitor the concentration of free radicals in the samples. The number of radicals was monitored as a function of irradiation time during which the radicals were generated in samples 0.5, 0.8, 2.0, 3.0 and 5.0 mm thick. An EPR X-band spectro-meter was used to detect the free radical spectra. The number of free radicals per unit mass as a function of irradiation time shows that 60% of the maximum concentration of radicals in a 1 mm sample is reached in 24 s curing time, while in thicker samples it takes hundreds of seconds. On the basis of the experiments, a depth and irradiation time-dependent radical concentration model was developed. This model shows that a 2.0 mm thick sample is cured at the bottom side if irradiated for 60 s. It is proposed that the measure of the degree of polymerization in composite materials should be the polymerization of the bottom layer of the sample which is modelled from the number of free radicals generated in the sample.

Theory of Structures of Hydrogen-Air Diffusion Flames

ABSTRACT The structure of the hydrogen-air counterflow diffusion flame is investigated by Damkohler-n umber asymptotics. Attention is restricted to flowfield strain times smaller than dissociation times but larger than the characteristic chemical time of three-body recombination reactions, thereby placing the system on the upper, vigorously burning branch of the characteristic S-curve of peak temperature as a function of strain time without equilibrium broadening. Under these conditions, it is shown that the reactants can coexist only within a thin reaction zone separating two radical-free equilibrium regions. First, the equations for the counterflow mixing layer in the two outer equilibrium regions are solved in the classical Burke-Schumann limit by the introduction of appropriate conserved scalars that account for variable transport coefficients and variable Schmidt numbers, different for different species. Then, the reaction zone is investigated with scaling that identifies the relevant reduced Damkohler number. While the solutions in the outer zones are peculiar to the counterflow configuration because of their convective-diffusive character, the reactive-diffusive nature of the reaction zone at leading order enables its structure in transformed coordinates to be expressed independently of the geometrical configuration. Matching the solutions from the inner reaction region with those from the outer equilibrium regions yields the first-order asymptotic solution to the problem, which compares favorably with results obtained by numerical integration of the full conservation equations with detailed chemistry and transport descriptions. In particular, the reason that the maximum radical concentration and temperature deficit vary linearly with the one-third power of the strain rate is explained.

Characterization of ubisemiquinone radicals in succinate-ubiquinone reductase

A thenoyl trifluoroacetone-sensitive and antimycin-insensitive ubisemiquinone radical (Qs) is readily detected in purified succinate-cytochrome c reductase. When this reductase is resolved into succinate-Q and ubiquinol-cytochrome c reductases, Qs was not detected in either reductase. The difficulty in detecting such a radical in purified succinate-Q reductase has puzzled investigators for years. A deficiency of Q in the isolated complex is the reason for the failure to detect Qs. Upon addition of exogenous Q, a thenoyl trifluoroacetone-sensitive Q-radical is readily detectable in isolated succinate-Q reductase under a controlled redox potential. Maximum radical concentration is observed when 5 mol of exogenous Q, per mole of flavin, is added. The radical gives an EPR signal with a g-value of 2.005 and a line-width of 12 G. The E m of Qs is 84 mV at pH 7.4, with half-potentials of E1 = 40 mV and E2 = 128 mV. The Qs-radical does not show power saturation, even at 200 mW.

Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.

The effect of heating rate on radical concentration during coal pyrolysis

The effect of heating rate on the change in radical concentration during coal pyrolysis has been studied using high temperature ESR with a rapid scan unit. The maximum radical concentration of Yallourn coal decreased with an increase in the heating rate. The results were compared with those obtained from a theoretical treatment of the kinetics of radical formation. The effect of a catalyst on the change in radical concentration has also been monitored under rapid heating condition. The maximum radical concentration of the coal/ZnCl 2 system showed a significant shift to temperature about 100 K lower than that of coal alone.

Characterization of benzidinediimine: a product of peroxidase metabolism of benzidine.

[U-14C]Benzidine and unlabeled benzidine were used to synthesize benzidinediimine for which n.m.r., i.r. and u.v./visible spectra were obtained. Examination of benzidinediimine in acetate pH 4 buffer by electron spin resonance spectroscopy revealed that a free radical cation of benzidine was produced with maximum radical concentration occurring in 3-4 min. In contrast, u.v./visible spectroscopy revealed the production of a charge-transfer complex with maximum concentration occurring in 6-7 min. Liquid chromatography with electrical and radiochemical detection indicated that benzidine was the final product of benzidinediimine reduction in acetate buffer. The rate of reduction was greater at higher pH's and the time course for production of the free radical and charge-transfer complex did not coincide at any pH examined. Addition of DNA to [14C]benzidinediimine resulted in DNA labeling which was completely inhibited by glutathione. These data suggest that charge-transfer complex formation neither precedes nor coincides with free radical formation. They are consistent with the hypothesis that the free radical cation observed during peroxidatic oxidation of benzidine is derived from one-electron oxidation.

ESR study of polypropylene photostabilisation by sterically hindered piperidine derivatives

ESR spectra of sterically hindered piperidine derivatives embedded in polypropylene are superpositions of signals arising from radicals of different mobility. Analysis of the spectra at different temperatures gives information on the mobility and distribution of radicals in solution and in polymer media. At the temperature of irradiation, radicals are homogeneously distributed also in a polymer matrix. During irradiation the concentration of radicals first increases then, after reaching a maximum, decreases. This intermediate character is the result of simultaneous formation and reaction of nitroxyl radicals with radicals arising from polypropylene photo-oxidation. Maximum radical concentration is reached at higher dosage, and subsequent decrease in concentration is slower when hydroxy-benzophenone is applied together with the piperidine derivative. In case of joint application of the latter with a nickel complex, on the other hand, no radical formation can be detected in the polymer before, or in the course of irradiation.

Highly efficient electron-transfer processes over thermally activated alkaline earth oxides. Monolayer anion radical formation with nitrobenzene-magnesium oxide and high concentrations of carbon monoxide radicals over magnesium oxide, calcium oxide, strontium oxide, barium oxide, and thorium dioxide

Authors have attempted to determine quantitatively whether electron-transfer processes are of major importance on the surface of alkaline earth oxides and what types of sites are involved. Purified, powdered magnesium oxide was heat treated in vacuo overnight at 600/sup 0/C. Total MgO/surface MgO molecules ratio was determined to be 9.4. Sample was treated with nitrobenzene. Color change from white to brown; radical formation was allowed to proceed for about 1 week. Spectra recorded during this period showed that maximum radical concentration was reached in 1 to 2 days at 25/sup 0/C. Spin counts indicated that 20 to 25 surface molecules are necessary for the formation of the anion radical. Electron-transfer process appears to be extremenly important on the surface even in a stoichiometric sense. Essentially, a monolayer of nitrobenzyl radical is formed on the surface of the MgO. Carbon monoxide studies were conducted. Maximum radical production was attained in 30 to 60 days. Evidence showed that the CO radical formation was much more selective process than the nitrobenzyl formation. Ca 15% of the CO adsorbed eventually formed radical species. The authors believe that the radical forming processes are not due to artifacts or to impurities, which could not possibly accountmore » for the number of radical species formed. They have concluded that coordination defects on the MgO surface are responsible for the reactivity. Further studies with other alkaline earth oxides are being conducted.« less

The mechanical degradation of polystyrene

AbstractThe catastrophic failure of a polymeric material is preceded by a number of complex, partially understood events occurring on the molecular level. These events range from the flow of ordered regions to the cleavage of primary bonds in the chain. In recent years, stress‐induced bond cleavage in polymers has received increased attention, many authors nothing the presence of free radicals and/or volatile products released upon fracture; a free‐radical decomposition mechanism involving up to 103 molecules per chain rupture also has been postulated. A special tensile stress–strain and shear apparatus was developed and located inside the ion‐source housing of a time‐of‐flight mass spectrometer to characterize the volatile products released during mechanical degradation of polystyrene. Volatile compounds evolved during stress and fracture of polystyrene were monitored either continuously or by z‐axis modulated oscilloscopic display. The polystyrene was purified by two methods: vacuum outgassing and fractional reprecipitation. Large amounts of styrene evolved from both the as‐received and outgassed samples; however, essentially none was observed from the reprecipitated samples. Previous reports on monomer evolution during mechanical stress of polystyrene may be the result of residual monomer and not mechanical degradation products. The product of the surface density of primary radicals and the chain length for unzipping is less than 3 × 1010 radicals/mm2 indicating a maximum radical concentration of approximately 1010 radicals/mm2.

Effects of crystalline environment on pyrimidine free-radical formation: barbital crystal polymorphs and sodium barbital.

SummaryThe effects of crystalline environment on radiation-induced free radicals in a model system of pyrimidines (barbital I, II, IV, and sodium barbital) were investigated. The characteristics of the stable room-temperature radical were determined by e.s.r. spectroscopy. Single-crystal studies were performed on barbital I and IV. The major free radical in all four species is formed by an ethyl abstraction from C5. The unpaired spin density was 0·4 in all cases. A reorientation of the free radical in the crystal structure was observed in barbital I. An increase in radical yield was observed in sodium barbital (G=0·41) as compared to barbital I and II (G=0·17). Maximum radical concentrations were comparable in sodium barbital and barbital I and slightly lower in barbital II. These radical characteristics, together with the known crystal structures of the model system, indicate the relative importance of intermolecular crystalline environment parameters to solid-state free radical formation.

Kinetics of nitric oxide formation incombustion processes

Recent investigations of nitric oxide formation in combustion are reviewed with the objectiveof obtaining mechanistic information on nitric oxide formation from atmospheric (molecular) nitrogen and from nitrogen-containing compounds in the fuel. Comparison of existing data on formation of nitric oxide from atmospheric nitrogen with results from detailed kinetics calculations shows that, for the most part, the experimental observations are consistent with a three-reaction mechanism for nitric oxide chemistry if radical concentrations and temperature during combustion are correctly evaluated. The principal nitric oxide formation reactions are O+N2”NO+N, (1) N+O2”NO+O; (2) however, N+OH”NO+H, (3) can be of importance in fuel-rich mixtures. Experimental studies of the formation of nitric oxide from nitrogen-containing compounds indicate that the formation rates are rapid, occurring on a time-scale comparable to that of the combustion reactions. A simplified mechanism for conversion of nitrogen-containing compounds to nitric oxide is proposed. In this mechanism, a partial equilibrium approximation is used to relate the maximum nitric oxide concentration in the combustion zone to the maximum radical concentrations. The proposed mechanism adequately models nitric oxide formation in the shock-induced combustion of lean hydrogen-oxygen mixtures containing small amounts of ammonia.