Nitrogen-oxygen Mixtures Research Articles

Simulation on the effect of oxygen concentration on the positive secondary streamer generated in oxygen-rich nitrogen–oxygen mixtures under atmospheric pressure

In this study, we investigated the effect of various O2 concentrations, from 20% to 90%, in nitrogen–oxygen ( N2/O2 ) mixtures on the characteristics of secondary streamers. As oxygen molecules have different molecular characteristics from nitrogen molecules in terms of ionisation threshold and electron attachment property, streamer discharges generated under various nitrogen–oxygen ratios may exhibit differing characteristics such as electron density, electric field, and radical formation. We focused on the changes in these parameters in secondary streamers using simulations. Simulations were first performed under the same conditions as those in previous experiments to compare the results of the ozone production, discharge current, and discharge emission characteristics. To compare the ozone production characteristics, simulated O radicals–the precursor of ozone–were used in the simulation for simplicity. This comparison showed that, although the absolute values of each parameter were different, the simulation exhibited a similar trend in the case of the experimentally obtained oxygen concentration dependence. After the validity of the simulation was verified to some extent via a comparison with the experiment, the results obtained from the simulation were analysed in detail. The results showed that, although the electric field strength in the secondary streamer did not change much as the oxygen concentration increased, the decrease rate of the electron density was greatly accelerated by the electron attachment reaction of oxygen. As a result, many of the electrons had already dissipate during the development of the primary streamer, and few electrons remained when the secondary streamer was formed. This effect suggests that the ratio of the amount of O radicals produced in the primary streamer to that produced in the secondary streamer changes as the oxygen concentration changes.

Neuroprotective Effects of Krypton Inhalation on Photothrombotic Ischemic Stroke.

This is the first in vivo study to investigate the neuroprotective effects of krypton on focal cerebral ischemia. The aim of the study was to analyze the effect of 2 h of inhalation of a krypton-oxygen mixture (Kr 70%/O2 30%) on the recovery of neurological functions and the degree of brain damage in rats after photoinduced ischemic stroke (PIS) and to investigate the possible mechanisms responsible for this neuroprotection. Experiments were performed on male Wistar rats weighing 250-300 g (n = 32). Animals were randomized into four groups. Two groups (n = 20) underwent photoinduced ischemic stroke, followed by 2 h of inhalation of krypton-oxygen mixture consisting of Kr 70%/O2 30% or a nitrogen-oxygen breathing mixture consisting of N2 70%/O2 30%, followed by neurological examinations on days 3 and 7. The other two groups (n = 12) received only gas mixtures of the same concentration and exposure duration as in those in the PIS groups, then Western blot analysis of the potential molecular mechanisms was performed. The results of the study show that treatment with the krypton-oxygen mixture consisting of Kr 70%/O2 30% improves the neurological status on day 7 of observation, reduces the lesion volume according to the MRI examination and the number of Iba-1- and caspase-3-positive cells in the damaged area, promotes the activation of neoangiogenesis (an increase in the von Willebrand factor), and reduces the penumbra area and the number of NeuN-positive cells in it on day 14 of observation. Inhalation of the krypton-oxygen mixture also significantly increases the levels of phosphorylated AKT kinase (protein kinase B) and glycogen synthase kinase 3b (pGSK3b) and promotes the expression of transcription factor Nrf2, which was accompanied by the lowered expression of transcription factor NFkB (p50). Thus, we showed pronounced neuroprotection induced by krypton inhalation after stroke and identified the signaling pathways that may be responsible for restoring neurological functions and reducing damage.

Three-Dimensional Simulation of a High-Velocity Body Motion in a Tube with Rarefied Gas

Flow around a body moving at a high subsonic velocity in a tube filled with rarefied gas is studied. This aerodynamic problem is considered as applied to the task of designing a high-speed vacuum transport at finite Knudsen numbers. Parameters that are close to target characteristics of such systems are chosen, more precisely, speed of about 1000 km/h, significant transverse size of the body, and nitrogen–oxygen mixture (air) as the filling gas are chosen. The problem was solved in a three-dimensional statement.

Simulation study on influence of oxygen concentrations on atmospheric-pressure streamer in oxygen-rich nitrogen–oxygen mixture

This study aims to investigate the behaviour of positive streamer discharge in various mixtures with different concentrations via two-dimensional simulations. The proposed model considers the effects of photoionisation caused by the photons generated by excited oxygen atoms , which enables the simulation of streamers in oxygen rich conditions. The simulation results indicate that the propagation velocity and diameter of the streamer increase with the oxygen concentration, while the emission intensity decreases. The result conforms to the experimental results of our previous work. Moreover, the simulation results suggest that changes in the ionisation rate caused by variations in the oxygen concentration affect the streamer propagation, while the impact of photoionisation is limited.

Investigation of electric field distribution on dielectric exposed to DC-pulsed He plasma jet with shielding gas

Atmospheric pressure plasma jets (APPJs) produce reactive species and electric fields for biomedical applications. Gas shields control plasma plume-surrounding gas interactions, regulating reactive species generation and electric field strength. However, the surface electric field distribution is still unclear and needs urgent attention. Here, the electric field distribution on the surface exposed to a helium APPJ with shielding gas is investigated using the Pockels technique. This study considers the influence of the type of shielding gas (ambient air, dry air, nitrogen, oxygen, nitrogen-oxygen mixture) and the flow rate (2000–6000 sccm). The results show that the surface electric field develops in three phases: establishment, maintenance, and dissipation. Both flow rate and oxygen content of the shielding gas significantly influence surface discharge behavior and the maximum electric field value. The analysis suggests that the establishment phase of the electric field results from charge transfer by ionization waves to the dielectric, while the maintenance of the electric field depends on pulse duration. During the dissipation phase, the positive surface charge attracts negatively charged species to the surface (electrons and negative ions), which causes charge neutralization at the surface. The oxygen content in the shielding gas impacts the electric field establishment phase, with a low oxygen content leading to lower photo-ionization rates and, consequently, surface discharges with branching. Shielding gas flow rates affect the amount of shielding gas mixed into the helium channel. Mixing less oxygen into the APPJ increases the electric field strength, as the ionization potential is lower than nitrogen. Excessive oxygen mixing traps more free electrons due to electronegativity, causing fewer ionized collisions and more negative ions in APPJ, ultimately lowering the electric field strength. This study shows that shielding gas type and flow rates can adjust surface charging, aiding in optimizing biomedical APPJ.

Surface Acoustic Wave Interaction with a Mixture of Oxygen and Nitrogen

Parameters of surface acoustic waves (SAW) are very sensible to change of physical conditions of a propagation medium. In the classical theory formulation, the waves are guided along the boundary of semi-infinity solid state and free space. A real situation is more complex and a medium commonly consists of two physical components: a solid substrate and a gaseous or liquid environment. In the case of stress-free substrate, the strongest impact on SAW properties have surface electrical and mechanical conditions determined by solids or liquids adhering to the boundary. This impact is utilised for constructing sensors for different gases and vapours e.g. (Jakubik et al., 2007; Hejczyk et al., 2011; Jasek et al., 2012). The influence of gaseous environment on the SAW properties is usually very weak and ignored. However, in certain condition it can be significant enough to be applied to sensor construction. In general, it concerns Rayleigh wave devices where energy leakage phenomenon is perceptible, especially when the gas being detected considerably changes the density of environment. The paper presents the results of experiments with oxygen-nitrogen mixture. Their primary aim was focused on finding the dependence of resonant frequency and attenuation in SAW resonator on parameters and concentrations of the gas in the environment.

Simulation of high-temperature zirconium alloy oxidation in a nitrogen, oxygen and steam mixture

The paper presents a correlation model of high-temperature zirconium alloy oxidation in a mixture of gases (steam, nitrogen and oxygen) for the weight gain calculation of the oxidized sample. The essence of the model consists in using the linear interpolation between the kinetic constants of oxidation in pure gases to describe the oxidation kinetics in gas mixtures. The approach allows one to describe the oxygen weight gain with different contents of gases in steam, nitrogen and oxygen mixtures. The model was validated against experimental data on oxidation in an air-steam mixture in a proportion of 50% to 50% of the volume. For temperatures of 1200 °C and 1400 °C, prior to the 625th second, the deviation between the experimental data and the simulation results is not more than 18%; for 800 °C, prior to the 14th hour, the deviation between the experimental data and the simulation results is not more than 40%. Besides the air-steam mixture, the paper presents the results of the weight gain calculation using the proposed model for oxidation in an oxygen-nitrogen mixture and in a steam-oxygen mixture at 800 °C and 1400 °C.

On the formation of highly oxidized pollutants by autoxidation of terpenes under low-temperature-combustion conditions: the case of limonene and α-pinene

Abstract. The oxidation of monoterpenes under atmospheric conditions has been the subject of numerous studies. They were motivated by the formation of oxidized organic molecules (OOMs), which, due to their low vapor pressure, contribute to the formation of secondary organic aerosols (SOA). Among the different reaction mechanisms proposed for the formation of these oxidized chemical compounds, it appears that the autoxidation mechanism, involving successive events of O2 addition and H migration, common to both low-temperature-combustion and atmospheric conditions, leads to the formation of highly oxidized products (HOPs). However, cool-flame oxidation (∼500–800 K) of terpenes has not received much attention even if it can contribute to atmospheric pollution through biomass burning and wildfires. Under such conditions, terpenes can be oxidized via autoxidation. In the present work, we performed oxidation experiments with limonene–oxygen–nitrogen and α-pinene–oxygen–nitrogen mixtures in a jet-stirred reactor (JSR) at 590 K, a residence time of 2 s, and atmospheric pressure. Oxidation products were analyzed by liquid chromatography, flow injection, and soft-ionization–high resolution mass spectrometry. H–D exchange and 2,4-dinitrophenyl hydrazine derivatization were used to assess the presence of OOH and C=O groups in oxidation products, respectively. We probed the effects of the type of ionization used in mass spectrometry analyses on the detection of oxidation products. Heated electrospray ionization (HESI) and atmospheric-pressure chemical ionization (APCI) in positive and negative modes were used. We built an experimental database consisting of literature data for atmospheric oxidation and presently obtained combustion data for the oxidation of the two selected terpenes. This work showed a surprisingly similar set of oxidation products' chemical formulas, including oligomers, formed under the two rather different conditions, i.e., cool-flame and simulated atmospheric oxidation. Data analysis (in HESI mode) indicated that a subset of chemical formulas is common to all experiments, independently of experimental conditions. Finally, this study indicates that more than 45 % of the detected chemical formulas in this full dataset can be ascribed to an autoxidation reaction.

The Effects of Oxygen Dilution on Two-Stage Autoignition of Isolated N-Dodecane Droplets in Microgravity

ABSTRACT Two-stage autoignition of n-dodecane droplets with varying ambient oxygen concentrations in oxygen-nitrogen mixtures are investigated experimentally under microgravity conditions using high-speed shadowgraphy. The ambient pressure and temperature are held constant at 3 atm and 650 K, respectively, while the droplet initial diameter is fixed approximately at 1.2 mm. During the two-stage autoignition process, first a cool-flame front forms in the leaner regions farther away from the evaporating droplet, and it then propagates toward the fuel-rich region closer to the droplet surface, eventually encompassing the droplet. A hot-flame kernel is then initiated in the wake of the cool flame and very quickly expands, establishing a classical diffusion flame around the droplet. The first and second ignition delay times are measured from shadowgraphic images captured at 3000 frames per second. The first induction time is found to be insensitive to the ambient oxygen concentration, while the second induction time varies approximately as the negative 2 power of the oxygen mole fraction.

Effects of pulse rise time on electron dynamics properties in nitrogen–oxygen mixture under repetitive nanosecond pulses

The effects of pulse rise time on the temporal evolution of electron energy and density under repetitive nanosecond pulses in atmospheric nitrogen with 100 ppm oxygen impurities are investigated in this paper by a two-dimensional particle-in-cell/Monte Carlo collision model. It is found that the peak value of mean electron energy increases with decreasing pulse rise time in the single pulsed discharge. However, in the repetitive pulsed discharge approximated by pre-ionization, the peak value of mean electron energy no longer varies with the pulse rise time, showing a saturation trend with decreasing pulse rise time. Whether or not pre-ionization is present, the time required for the mean electron energy to reach its peak is approximately equal to the pulse rise time. It is worth noting that the presence of pre-ionization enhances the tracking ability of the mean electron energy to the pulse waveform during the pulse rise edge. Although after the peak of the pulse, the mean electron energy terminates the tracking process to pulse waveform due to the formation of high-density avalanches and even streamers, its energy decay rate gradually decreases with the increase in the pre-ionization density. Therefore, when the pulse repetitive frequency is greatly increased or the pre-ionization density is increased by other means, it is possible to achieve the complete control of the mean electron energy by pulse waveform modulation.

PHYSIOLOGICAL AND BIOCHEMICAL PARAMETERS OF NORMOBARIC HYPOXIA STRESS IN HUMANS

The aim of the study is to examine the dynamics of the sympathoadrenal and adrenocortical components of acute normobaric hypoxia stress and their correlation with some physiological and biochemical parameters characterizing the energy metabolism in humans.
 Materials and methods. Fourteen trial subjects were exposed to 40-minute hypoxia while breathing nitrogen-oxygen mixture containing 9 % of oxygen. At 5th, 10th, 20th, and 40th minutes of hypoxia, blood was taken from the cubital vein for further analysis. The authors determined a number of physiological parameters, such as pulse rate; blood pressure; oxygen consumption; rheographic index of cerebral blood flow. We also evaluated epinephrine, norepinephrine, cortisol, lactate and glucose blood levels at the time of sampling. The authors estimated the increase in parameters at control points compared to baseline, as well as the correlation of the increase in various indicators.
 Results. A significant increase in blood epinephrine (max. min 20), norepinephrine (max. min 10) and lactate (max. min 40) was found in trial subjects under hypoxia compared to baseline parameters. A significant increase in oxygen consumption and rheographic index of cerebral blood flow was established under hypoxia (max. min 40). The longer the trial subject was exposed to hypoxia, the more the degree of conjugation in the dynamics of sympathoadrenal response, cerebral blood flow, and energy metabolism (lactate, glucose) was.
 Conclusion. The obtained facts prove the heterochrony of sympathoadrenal and adrenocortical stress components in the dynamics of the human body response to acute normobaric hypoxia. With hypoxia development, the impact of the adrenergic component on mobilizing the energy of glycolytic processes increases.

Features of human capillary blood flow on breathing by heated helio-oxygen mixtures

The purpose of the study: to assess the features of microcirculatory blood flow.Materials and methods. A comparative assessment of microcirculation by laser Doppler flowmetry was carried out in 19 male volunteers aged 20–25 years against the background of breathing heated to 40° C oxygen-nitrogen and helium-oxygen gas mixtures.Main results. Breathing with a heated oxygen-nitrogen mixture consisted in an increase (by approximately 25–35%) of the values of the integral indicator of microcirculation against the background of a decrease in its efficiency. Switching to breathing with a heated helium-oxygen mixture «HeliOx 25/75» in all examined persons also led to an increase in the volumetric velocity of the peripheral blood flow. However, the degree of increase in the indicator (by 35–45% compared with normal breathing conditions) turned out to be significantly more pronounced than it was during breathing with a heated oxygen-nitrogen mixture. In addition, when breathing «HeliOx 25/75», starting from about the 2nd minute of breathing, an increase in the efficiency of microcirculation was observed due to an increase in the «contribution» to the blood flow reaction of its «active» (neuromyogenic) components.

Transport properties of a quasisymmetric binary nitrogen-oxygen mixture in the warm dense regime.

Transport properties of mixtures in the warm dense matter (WDM) regime play an important role in natural astrophysics. However, a physical understanding of ionic transport properties in quasisymmetric liquid mixtures has remained elusive. Here, we present extensive ab initio molecular dynamics (AIMD) simulations on the ionic diffusion and viscosity of a quasisymmetric binary nitrogen-oxygen (N-O) mixture in a wide warm dense regime of 8-120 kK and 4.5-8.0 g/cm^{3}. Diffusion and viscosity of N-O mixtures with different compositions are obtained by using the Green-Kubo formula. Unlike asymmetric mixtures, the change of proportions in N-O mixtures slightly affects the viscosity and diffusion in the strong-coupling region. Furthermore, the AIMD results are used to build and verify a global pseudo-ion in jellium (PIJ) model for ionic transport calculations. The PIJ model succeeds in reproducing the transport properties of N-O mixtures where ionization has occurred, and provides a promising alternative approach to obtaining comparable results to AIMD simulations with relatively small computational costs. Our current results highlight the characteristic features of the quasisymmetric binary mixtures and demonstrate the applicability of the PIJ model in the WDM regime.

«ЧЕРНОМОР»: К 50-ЛЕТИЮ САМОГО ПРОДОЛЖИТЕЛЬНОГО ЭКСПЕРИМЕНТА

The work is devoted to a brief description of the preparation and conduct series of scientific experiments in the underwater laboratory “Chernomor” during the period of 1967–1972. The experiments were carried out at the depths from 8 to 30 meters. The place of the experiments is the Southern Branch of the Institute of Oceanology (Gelendzhik). It was unique experiments in the duration of a person’s stay underwater. The maximum depth for “Chernomor” was identified, the optimal methods of organizing work in underwater inhabited houses and the permissible depths for the use of nitrogen-oxygen breathing mixtures were determined. Specialized medical and physiological research was carried out to develop a methodology for the selection and training of aquanauts for future scientific crews. “Chernomor” was the only underwater house in the world that served on the seabed for five seasons in a row. More than 50 scientists from all over the country worked in it in different crews. The work is a tribute to the memory of all enthusiasts and dreamers: designers and developers, engineers and technicians, divers and scientists, everyone involved in those unforgettable events.

Mathematical modelling of a staged pulverized coal gasification using O2/CO2 mixtures

Conversion of pulverized coal in a two-stage gasifier is studied. When considering carbon capture power plants, mixtures of oxygen with carbon dioxide may be used as a gasification agent. Carbon dioxide is a gasification agent, so characteristics of the gasification process change significantly compared to gasification in oxygen-nitrogen mixtures. The conversion efficiency is determined by the thermophysical factor (change in the heat capacity of the gas mixture and transfer coefficients) and the concentration factor (increase in the concentration of the gaseous reagent). The ratio of primary and secondary fuel consumption determines the leading stage of the process. The influence of process efficiency on oxygen concentration is estimated in the range of 21–30 vol. %.

Electron runaway rate in air

A Monte Carlo code is developed in terms of the null-collision technique capable of simulating the transport of electrons in nitrogen–oxygen mixtures. Cross-sections for the angular scattering of electrons in all types of interactions are incorporated as accurately as possible. To demonstrate the opportunities and reliability of the code, basic transport parameters of electrons in nitrogen for a wide range of the reduced electric field strength E/N are computed and compared to some available theoretical and experimental data. The ‘electron runaway rate’, as a physical quantity, is tentatively defined. The influence of different models ofangular scattering upon the electron runaway in strong electric fields is studied. Dependence on the field strength E of the generation rate in air of electrons with energies above ϵ max in the range from 0.2–4 keV is computed assuming immobile molecules with a concentration equal to Loshmidt’s number cm−3 (STP conditions) in the range of the field strength E from 240–400 kV cm−1.

Numerical analysis on evolution process of multiple rotating detonation waves with ethylene–oxygen–nitrogen mixture

Two-dimensional numerical simulations of a rotating detonation combustor (RDC) fueled by ethylene–oxygen–nitrogen were performed to investigate the evolution process of multiple rotating detonation waves (RDWs), that is, initial chaotic stage, self-adjustment stage, and the final stable stage. The number of stable RDWs corresponding to the initial conditions was in good agreement with that predicted by an empirical model proposed by Daniau et al. from Lavrenty’ev Institute of Hydrodynamics (LIH). From the simulation results, the spontaneous formation of a reverse RDW and the transition of trailing waves to detonation waves contribute to the emergence of the chaotic stage. The self-adjustment stage comprises three distinctive parts, which are the self-adjustment of the propagation direction, the number, and the propagation characteristics of RDWs. Furthermore, the double-wave collision was found to play a significant role in modulating the propagation direction as well as the number of RDWs. The same self-adjusting trend of different instantaneous propagation characteristics (e.g., peak pressure, propagation velocity, and wave front height) of the RDWs was observed, suggesting the self-similarity property of the adjusting mechanism demonstrated in the evolution of rotating detonation dynamics.

Influence of gaseous mixture parameters on the maximum burning temperature and combustion rate of carbon particle

The method is developed to determine the maximum temperature of burning carbon particle and corresponding value of particle diameter by using the maximum temperature as a parameter depending on surrounding gas temperature and oxygen content. This method allows finding a limiting value of oxygen content below which ignition of a carbon particle is not possible in cold oxygen-nitrogen mixture. It is shown that the particle temperature attains a maximum value in the transitional regime of chemical reactions. The rise of the burning temperature with the particle size decrease is caused by relative decrease of radiation heat loss.

On the similarities and differences between the products of oxidation of hydrocarbons under simulated atmospheric conditions and cool flames

Abstract. Atmospheric oxidation chemistry and, more specifically, photooxidation show that the long-term oxidation of organic aerosol (OA) progressively erases the initial signature of the chemical compounds and can lead to a relatively uniform character of oxygenated organic aerosol (OOA). This uniformity character observed after a long reaction time seems to contrast with the great diversity of reaction mechanisms observed in the early stages of oxidation. The numerous studies carried out on the oxidation of terpenes, and more particularly on limonene for its diversity of reaction sites (endo- and oxocyclic), allow this evolution to be studied. We have selected, for their diversity of experimental conditions, nine studies of limonene oxidation at room temperature over long reaction times to be compared to the present data set obtained at elevated temperature and short reaction time in order to investigate the similarities in terms of reaction mechanisms and chemical species formed. Here, the oxidation of limonene–oxygen–nitrogen mixtures was studied using a jet-stirred reactor at elevated temperature and atmospheric pressure. Samples of the reacting mixtures were collected and analyzed by high-resolution mass spectrometry (Orbitrap) after direct injection or after separation by reverse-phase ultra-high-pressure liquid chromatography and soft ionization, i.e., (+/-) HESI and (+/-) APCI. Unexpectedly, because of the diversity of experimental conditions in terms of continuous-flow tank reactor, concentration of reactants, temperature, reaction time, mass spectrometry techniques, and analysis conditions, the results indicate that among the 1138 presently detected molecular formulae, many oxygenates found in earlier studies of limonene oxidation by OH and/or ozone are also produced under the present conditions. Among these molecular formulae, highly oxygenated molecules and oligomers were detected in the present work. The results are discussed in terms of reaction pathways involving the initial formation of peroxy radicals (RO2), isomerization reactions yielding keto-hydroperoxides, and other oxygenated intermediates and products up to C25H32O17, products which could derive from RO2 autoxidation via sequential H shift and O2 addition (C10H14O3,5,7,9,11) and products deriving from the oxidation of alkoxy radicals (produced by RO2 self-reaction or reaction with HO2) through multiple H shifts and O2 additions (C10H14O2,4,6,8,10). The oxidation of RO2, with possible occurrence of the Waddington mechanism and of the Korcek mechanism, involving H shifts is also discussed. The present work demonstrates similitude between the oxidation products and oxidation pathways of limonene under simulated atmospheric conditions and in those encountered during the self-ignition of hydrocarbons at elevated temperatures. These results complement those recently reported by Vereecken and Nozière and confirm for limonene the existence of an oxidative chemistry of the alkylperoxy radical beyond 450 K based on the H shift (Nozière and Vereecken, 2019; Vereecken and Nozière, 2020).

First-order liquid-liquid phase transition in nitrogen-oxygen mixtures

The first-order liquid-liquid phase transition (LLPT) describes the counterintuitive nature between two distinct liquids in a single system. However, the physical understanding of LLPT in simple liquid mixtures has remained elusive. Here, a first-order LLPT for nitrogen-oxygen (N-O) mixtures is investigated via extensive ab initio molecular dynamics simulations. The first-order LLPT is characterized by discontinuities of a short-range order and transport properties, occurring at the transition pressures identified by looking at isotherms. The LLPT in N-O mixtures is dominated primarily by the molecular-to-polymeric transformation. The resulting phase boundaries and further structural analysis show the crucial influence of oxygen on weakening the LLPT of N-O mixtures by promoting chemical reactions with nitrogen and crippling polymerizations. Furthermore, noncovalent interactions of short- to long-range van der Waals have evident impacts on the LLPT boundary. The introduction of the strongly constrained and appropriately normed functional with revised Vydrov--van Voorhis nonlocal correlation (SCAN-$r\mathrm{VV}10$) functional to describe the phase-transition behaviors improves the agreement with experimental data. The results highlight the characteristic features of the complex interplay in binary mixtures and revisit the first-order phase transition of dense nitrogen.