Nitrogen-oxygen Mixtures Research Articles

The influence of carbon monoxide and hydrogen on the structure and extinction of nonpremixed and premixed methane flames

An experimental and computational study is carried to elucidate the influence of carbon monoxide (CO) addition to nonpremixed methane (CH4) flames. A Burke–Schumann (flame-sheet) formulation is applied to laminar counterflow diffusion-flame experiments, reported here, in which CO was added to either methane–nitrogen mixtures or oxygen–nitrogen mixtures at normal atmospheric pressure, with both feed streams at normal room temperature. Experimental conditions were adjusted to fix selected values of the stoichiometric mixture fraction and the adiabatic flame temperature, and the strain rate was increased gradually until extinction occurred. At the selected sets of values, the strain rate at extinction was measured as a function of the CO concentration in the fuel or oxidizer stream. It was found that with increasing amounts of CO in the oxidizer stream, the strain rate at extinction first increased and then decreased. With increasing amounts of CO in the fuel stream, changes in values of the strain rate at extinction was small in comparison to those measured for CO addition to the oxidizer stream. The experimental results are in agreement with predictions obtained employing detailed chemistry. A numerical investigation was carried out to characterize the influence of hydrogen and carbon monoxide on the structure and burning velocities of stoichiometric premixed methane flames. The mass fraction of the reactants in the mixture were so chosen that the adiabatic temperature is constant. The flame speed increases when hydrogen is added to the reactant stream. For CO addition the flame speed first increases and then decreases.

Flame structure and particle-combustion regimes in premixed methane–iron–air suspensions

Flame structures and particle combustion regimes are studied in hybrid fuel mixtures of methane and iron using a modified Bunsen burner with two different oxidizing environments. The first is a stoichiometric methane–air mixture, in which the iron reacts with the hot gaseous combustion products in a kinetically-controlled regime; the second is a lean methane–oxygen–nitrogen mixture, allowing iron to react with the excess oxygen in a diffusion-controlled regime. The particle seeding concentration is monitored using laser attenuation and varies from 0 to 350g/m3. Burning velocities are obtained from flame photographs and condensed phase temperatures are determined from particle emission spectra via polychromatic fitting of the spectra to Planck’s law of blackbody radiation. High-speed imaging is also used to qualitatively characterize the different flame structures. Results are also compared to a methane flame seeded with inert silicon carbide particles instead of iron. It is shown that, independent of the combustion regime, a critical concentration of iron powder is required to form a coupled flame front in the combustion products of the methane flame. Furthermore, after the metal-powder flame formation, a double front structure separated by a dark zone is observed in the kinetic regime, whereas the two flames overlap and form a single Bunsen cone in the case of the diffusion regime.

Physicochemical Investigation of Homogeneous Pulsed Discharge in N2/O2 for Ozone Production

The purpose of this work is to draw attention to the plasma kinetics in a nitrogen-oxygen mixture. The model includes the plasma chemistry module and the circuit module investigating the electrical and the physical characteristics of high-pressure homogenous pulsed discharge. The fundamental chemistry governing ozone generation developed in this work is based on a full set of processes regrouped in 117 reactions involving 19 charged particles, atomic, and molecular species. The results of simulations show the temporal variation of the discharge electrical parameters and the effect of the voltage applied to ozone production. The role played by different reactions and species is also given by analyzing the time evolution of species concentrations, as well as the ozone production and loss terms.

Investigation of positive streamers by double-pulse experiments, effects of repetition rate and gas mixture

Streamer discharges are often operated in a repetitively pulsed mode and are therefore influenced by species left over from the previous discharge, especially free electrons and ions. We have investigated these effects by applying two consecutive positive high voltage pulses of 200–700 ns duration to a point-plane gap in artificial air, pure nitrogen, other nitrogen–oxygen mixtures and pure argon at pressures between 67 and 533 mbar. The pulses had pulse-to-pulse intervals (Δt) between 200 ns and 40 ms. We imaged both discharges with two ICCD cameras and combined this to a compound image. We observe for values of Δt below 0.5–15 µs (at 133 mbar, with Δt depending on gas mixture) that during the second pulse the streamers continue the paths of the first-pulse streamers. We call the maximal time for which this continuation still occurs the continuation time. For N2–O2 mixtures, this time has a maximum at an oxygen concentration of about 0.2%. According to our plasma-chemical modelling this maximum is determined by the electron loss rate which has a minimum around this oxygen concentration. Depending on oxygen concentration the dominant recombining positive ion is , or where dominates around 0.2% O2 and recombines slowest.For increasing values of Δt we observe that after the continuation phase first no new streamers occur at all, then new streamers show up that avoid the entire pre-ionized region. Next we see new thin streamers that follow the edges of the old channels. For larger Δt (10–200 µs) the new streamers start to increase in size and move to the centre of the old channels. Finally, around millisecond timescales the new channels are completely independent of the old channels.Together this points to the combination of two mechanisms: streamers search the proximity of regions with increased electron density, but cannot penetrate regions with very high electron density.

Ignition and combustion of charcoal particles are in the nitrogen-oxygen mixtures at room temperature. Part 1. Experimental studies in the air.

Ignition and combustion of large charcoal particles studied in air at room temperature. It is shows the influence of the ash layer, initial particle diameter and velocity cold air on the dynamics of changes of temperature, density and diameter particle during its combustion.

Laboratory and gas-fired furnace performance tests of epoxy primers for intumescent coatings

Protection of steel structures, using so-called intumescent coatings, is an efficient and space saving way to prolong the time before a building, with load bearing steel constructions, collapses in the event of a fire. In addition to the intumescent coating, application of a primer may be required, either to ensure adhesion of the intumescent coating to the steel or to provide corrosion resistance. It is essential to document the performance of the intumescent coating together with the primer to ensure the overall quality of coating system. In the present work, two epoxy primers were used to investigate the potential failure mechanism of a primer applied prior to an intumescent coating. The analysis was carried out using; (1) gas-fired test furnace, (2) a specially designed electrically heated oven, and (3) thermo gravimetric analysis. When tested below an acrylic intumescent coating, exposed to a gas-fired furnace following the ISO834 fire curve (a so-called cellulosic fire), one of the primers selected performed well and the other poorly. From tests in the electrically heated oven, it was found that both primers were sensitive to the film thickness employed and the presence of oxygen. At oxygen-rich conditions, higher primer thicknesses gave weaker performance. In addition, a color change from red to black was observed in nitrogen, while the color remained red in the oxygen–nitrogen mixture. In summary, the results suggest that an adequate choice of primer, primer thickness, and intumescent coating is essential for a good performance of an intumescent coating system.

High flow nasal cannula (HFNC) with Heliox decreases diaphragmatic injury in a newborn porcine lung injury model

High flow nasal cannula (HFNC) improves ventilation by washing out nasopharyngeal dead space while delivering oxygen. Heliox (helium-oxygen gas mixture), a low-density gas mixture, decreases resistance to airflow, reduces the work of breathing, and facilitates distribution of inspired gas. Excessive lung work and potential injury increases the workload on the immature diaphragm predisposing the muscle to fatigue, and can lead to inflammatory and oxidative stress, thereby contributing to impaired diaphragmatic function. We tested the hypothesis that HFNC with Heliox will decrease the work of breathing thereby unloading the neonatal diaphragm, and potentially reducing diaphragmatic injury. Spontaneously breathing neonatal pigs were randomized to Nitrox (nitrogen-oxygen gas mixture) or Heliox, and studied over 4 hr following oleic acid injury. Gas exchange, pulmonary mechanics indices, and systemic markers of inflammation were measured serially. Diaphragm inflammation biomarkers and histology for muscle injury were assessed at termination. Heliox breathing animals demonstrated decreased respiratory load and work of breathing with lower pressure-rate product, lower labored breathing index, and lower levels of diaphragmatic inflammatory markers, and muscle injury score as compared to Nitrox. These results suggest that HFNC with Heliox is a useful adjunct to attenuate diaphragmatic fatigue in the presence of lung injury by unloading the diaphragm, resulting in a more efficient breathing pattern, and decreased diaphragm injury.

Formation of Nitrates in Aqueous Solutions Treated with Pulsed Corona Discharge: The Impact of Organic Pollutants

Pulsed corona discharge (PCD) in oxygen-nitrogen mixtures results in formation of nitrogen oxides, transformed to aqueous nitrates in contact with water. The experimental research into the impact of formate and oxalate to nitrate formation in aqueous solutions treated with PCD was undertaken. The impact of paracetamol, ibuprofen, indomethacin and their oxidation products to nitrate formation was also analyzed. Pharmaceuticals obstructed nitrate formation, while carboxylic anions and pharmaceuticals’ oxidation products noticeably improved nitrate formation in treated solutions as compared to water. The nitrate formation enhancement is explained by the aqueous ozone decomposition and hydroxyl radical formation known to be improved by carboxylic anions.

Alteration of Striatal Dopamine Levels Under Various Partial Pressure of Oxygen in Pre-convulsive and Convulsive Phases in Freely-Moving rats

The purpose of this study was to investigate the change in the striatal dopamine (DA) level in freely-moving rat exposed to different partial pressure of oxygen (from 1 to 5ATA). Some works have suggested that DA release by the substantia nigra pars compacta (SNc) neurons in the striatum could be disturbed by hyperbaric oxygen (HBO) exposure, altering therefore the basal ganglia activity. Such changes could result in a change in glutamatergic and GABAergic control of the dopaminergic neurons into the SNc. Such alterations could provide more information about the oxygen-induced seizures observed at 5ATA in rat. DA-sensitive electrodes were implanted into the striatum under general anesthesia. After 1week rest, awaked rats were exposed to oxygen-nitrogen mixture at a partial pressure of oxygen of 1, 2, 3, 4 and 5ATA. DA level was monitored continuously (every 3min) by in vivo voltammetry before and during HBO exposure. HBO induced a decrease in DA level in relationship to the increase in partial pressure of oxygen from 1ATA to 4ATA (-15% at 1ATA, -30% at 2ATA, -40% at 3ATA, -45% at 4ATA), without signs of oxygen toxicity. At 5ATA, DA level strongly decreases (-75%) before seizure which occurred after 27min±7 HBO exposure. After the epileptic seizure the decrease in DA level disappeared. These changes and the biphasic effect of HBO were discussed in function of HBO action on neurochemical regulations of the nigro striatal pathway.

Effect of addition of a non-equidiffusional reactant to an equidiffusional diffusion flame

A Burke–Schumann (flame-sheet) formulation is developed for diffusion flames between a fuel and oxidiser with Lewis numbers of unity, subject to addition to the fuel and/or oxidiser stream of a different reactant for which the Lewis number differs from unity. This formulation is applied to laminar counterflow diffusion-flame experiments, reported here, in which hydrogen was added to either methane–nitrogen mixtures or oxygen–nitrogen mixtures at normal atmospheric pressure, with both feed streams at normal room temperature. Experimental conditions were adjusted to fix selected values of the stoichiometric mixture fraction and the adiabatic flame temperature, and the strain rate was increased gradually, maintaining the momentum balance of the two streams, until extinction occurred. At the selected sets of values, the strain rate at extinction was measured as a function of the hydrogen concentration in the fuel or oxidiser stream. The ratio of the fraction of the oxidiser flux that consumes hydrogen to the fraction that consumes fuel was calculated from the new Burke–Schumann formulation, and it was found that, within experimental uncertainty, the ratio of the extinction strain rate with hydrogen addition to that without was the same at any given value of this oxygen flux ratio, irrespective of whether the hydrogen was added on the fuel or oxidiser side. This experimental result was also in close agreement with computational predictions employing detailed chemistry. These results imply that differences in detailed hydrogen concentration profiles within the reaction zone have little or no influence on the chemical kinetics of extinction when the stoichiometric mixture fraction, the adiabatic flame temperature, and the proportion of oxygen that consumes the added fuel are fixed. This same correspondence may be expected to apply for other fuels and additives.

Modelling the effect of water fog on the upper flammability limit of hydrogen–oxygen–nitrogen mixtures

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of controlling flammable hydrogen–air mixtures by modifying the flammability limits to prevent ignition. A simplified theoretical model, based upon the inhibition parameter concept, is developed allowing the effect of nitrogen dilution and very fine water fog upon the upper flammability limit (UFL) of hydrogen–oxygen–nitrogen mixtures to be predicted and compared with corresponding experimental test data. The model predicts similar general trends in behaviour with regard to the effect of water fog upon the UFL to those observed in the experimental tests. The results suggest that the combination of nitrogen dilution and high density water fog can produce a significant reduction in the range of the UFL and total extent of the flammable region.

Extremely low flow tracheal gas insufflation of helium-oxygen mixture improves gas exchange in a rabbit model of piston-type high-frequency oscillatory ventilation

ObjectiveThe purpose of this study was to show the effects of the tracheal gas insufflation (TGI) technique on gas exchange using helium-oxygen mixtures during high-frequency oscillatory ventilation (HFOV). We hypothesized that a helium-oxygen mixture delivered into the trachea using the TGI technique (0.3 L/min) would enhance gas exchange during HFOV.MethodsThree rabbits were prepared and ventilated by HFOV with carrier 70% helium/oxygen or 70% nitrogen/oxygen gas mixture with TGI in a crossover study. Changing the gas mixture from nitrogen70% to helium70% and back was performed three times per animal with constant ventilation parameters.ResultsCompared with the nitrogen-oxygen mixture, the helium-oxygen mixture of TGI reduced PaCO2 by 7.6 mmHg (p < 0.01) and improved PaO2 by 14 mmHg (p < 0.01). Amplitude during TGI was significantly lower with the helium-oxygen mixture than with the nitrogen-oxygen mixture (p < 0.01) and did not significantly affect mean airway pressure.ConclusionsThis study demonstrated that a helium-oxygen mixture delivered into the trachea using the TGI technique would enhance CO2 elimination and improve oxygenation during HFOV.

Effective ionization rate in nitrogen–oxygen mixtures

The effective Townsend ionization coefficient in nitrogen–oxygen mixtures at various pressures is determined. In addition to the commonly accepted difference of the ionization and the attachment coefficients, α and η, respectively, the electron detachment from the negative ions created by the avalanche itself is taken into account. This leads to non-zero effective ionization rate below the threshold field corresponding to α − η = 0.

Combustion and spontaneous extinction of porous carbon particles in nitrogen-oxygen mixtures at room temperature

The effect of the initial temperature of a porous carbon particle on the characteristics of its combustion and spontaneous extinction is analyzed taking into account Stefan flow and heat loss by radiation. It is shown that in the case of forced ignition (increase in the initial temperature of the particle), the diameter and density of the particle after spontaneous extinction remain virtually unchanged. As a result, the extinguished particles have the same diameter but different density. It is shown that the dependence of particle diameter on stationary temperature can be used to determine the maximum combustion temperature and diameter of the particle during its spontaneous extinction. The effect of oxygen concentration on the region of ignition of a porous carbon particle, determined by the initial diameter and temperature of the particle, is analyzed. Analysis of the diffusion-kinetic relations shows that each of the two main heterogeneous reactions of carbon oxidation make similar contributions to the heat and mass transfer of the particle with the surrounding gas.

Modelling the mitigation of hydrogen deflagrations in a vented cylindrical rig with water fog and nitrogen dilution

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of mitigating the overpressure rise, should a hydrogen deflagration in a vented enclosure occur. A numerical CFD gas explosion code (FLACS) has been used to simulate the pressure-time curves and the rate of pressure rise generated following the ignition of different hydrogen–oxygen–nitrogen mixtures in a small scale vented cylindrical explosion rig. This has allowed the potential mitigating effect of nitrogen dilution (reduced oxygen) and very fine water fog, used both alone and in combination, to be explored and permitted their direct comparison with corresponding experimental test data.The numerical simulation results obtained reproduce general trends in behaviour similar to those found in the experimental tests. The initial rate of pressure rise is slowed and the time taken to reach 0.1 barg increased, as the oxygen index is reduced and the number of fogger units operating is raised. However, the combination of nitrogen dilution and water fog is also more effective than predicted in some cases. Thus, the numerical simulation under-predicts the level of mitigation produced at higher fog densities for nitrogen diluted H2–O2–N2 mixtures, where the experimental results suggests that total explosion suppression can be achieved.

Local electron mean energy profile of positive primary streamer discharge with pin-plate electrodes in oxygen—nitrogen mixtures

Local electron mean energy (LEME) has a direct effect on the rates of collisional ionization of molecules and atoms by electrons. Electron-impact ionization plays an important role and is the main process for the production of charged particles in a primary streamer discharge. Detailed research on the LEME profile in a primary streamer discharge is extremely important for a comprehensive understanding of the local physical mechanism of a streamer. In this study, the LEME profile of the primary streamer discharge in oxygen-nitrogen mixtures with a pin-plate gap of 0.5 cm under an impulse voltage is investigated using a fluid model. The fluid model includes the electron mean energy density equation, as well as continuity equations for electrons and ions and Poisson's electric field equation. The study finds that, except in the initial stage of the primary streamer, the LEME in the primary streamer tip tends to increase as the oxygen-nitrogen mole ratio increases and the pressure decreases. When the primary streamer bridges the gap, the LEME in the primary streamer channel is smaller than the first ionization energies of oxygen and nitrogen. The LEME in the primary streamer channel then decreases as the oxygen-nitrogen mole ratio increases and the pressure increases. The LEME in the primary streamer tip is primarily dependent on the reduced electric field with mole ratios of oxygen-nitrogen given in the oxygen-nitrogen mixtures.

The Effect of Ambient Conditions on the Burning Rate of Gel Fuel Droplets

AbstractAn experimental study was conducted to determine the dependence of the burning rate coefficient of gel fuel droplets on the pressure at different ambient oxygennitrogen mixtures. Experiments were conducted using a pressure chamber, in which the droplet was suspended and the combustion process was video‐photographed by a high‐speed digital video camera. The tests were conducted at pressures between 0.1–4 MPa at different ambient oxygennitrogen compositions (air, 40 % O2 – 60 % N2, and 60 % O2 – 40 % N2). The fuel was a compound of 95 % kerosene and 5 % gellant. At sub‐critical pressure conditions, the burning rate coefficient was found to increase with increasing ambient oxygen mass fraction. At supercritical conditions, no dependence of the burning rate coefficient on the ambient mixture was found. The results indicate that the burning rate coefficient depends on the oxygen partial pressure, at least at low pressures.

Experimental determination of dielectric barrier discharge capacitance

The determination of electrical parameters (such as instantaneous power, transferred charge, and gas gap voltage) in dielectric barrier discharge (DBD) reactors relies on estimates of key capacitance values. In the classic large-scale sinusoidal-voltage driven DBD, also known as silent or ozonizer discharge, capacitance values can be determined from charge-voltage (Q-V) plot, also called Lissajous figure. For miniature laboratory reactors driven by fast pulsed voltage waveforms with sub-microsecond rise time, the capacitance of the dielectric barriers cannot be evaluated from a single Q-V plot because of the limited applicability of the classical theory. Theoretical determination can be problematic due to electrode edge effects, especially in the case of asymmetrical electrodes. The lack of reliable capacitance estimates leads to a "capacitance bottleneck" that obstructs the determination of other DBD electrical parameters in fast-pulsed reactors. It is suggested to obtain capacitance of dielectric barriers from a plot of the maximal charge versus maximal voltage amplitude (Q(max) - V(max) plot) in a manner analogous to the classical approach. The method is examined using measurements of current and voltage waveforms of a coaxial DBD reactor in argon at 100 mbar driven by square voltage pulses with a rise time of 20 ns and with different voltage amplitudes up to 10 kV. Additionally, the applicability of the method has been shown for the data reported in literature measured at 1 bar of nitrogen-oxygen gas mixtures and xenon.

Barrier discharges driven by sub-microsecond pulses at atmospheric pressure: Breakdown manipulation by pulse width

Barrier discharges at atmospheric pressure in nitrogen-oxygen mixture powered by high voltage pulses of widths between 10 μs and 200 ns were investigated. The development of the microdischarges on rising and falling slopes was recorded by streak and intensified CCD cameras simultaneously. The breakdown on the falling slope strongly depends on the pulse width. As a result of pulse width variation the starting point of ignition changes and positive and negative streamers occur simultaneously in the falling slope. The observed effect is caused by the electric field rearrangement in the gap due to the different positive ion densities related to their gap crossing times.

Air adsorption and separation on carbon nanotube bundles from molecular dynamics simulations

Using molecular dynamics (MDs) simulations, the adsorption of a nitrogen–oxygen mixture (representing air) on (8, 8), (10, 10), and (12, 12) single-walled carbon nanotube (SWCNT) bundles is investigated as a function of temperature, air loading and diameter of nanotubes at subcritical (T=100K) and supercritical (T=200 and 300K) temperatures of the air. In order to do this, heat of adsorption, diffusion coefficients, activation energy, and radial distribution functions (RDFs) were computed for further analysis of the adsorption process. The simulation of exposing air on nanotube bundles, show that the amount of adsorption, heat of adsorption, and diffusion coefficients are strongly influenced by the applied temperature, i.e., along with the temperature increase, the amount of adsorption would be reduced. Furthermore, the results show that oxygen is selectively adsorbed relative to nitrogen, so carbon nanotubes (CNTs) can be considered as promising gas-filtering systems in addition to their previous gas-storage capability.