Hydrogen-oxygen System Research Articles

Chemiluminescence during the high-temperature pyrolysis and oxidation of ammonia

Chemiluminescent emissions from NH2*, NH*, NO*, and OH* during the pyrolysis and oxidation of ammonia (NH3) are quantitatively characterized to gain insight into their reaction mechanisms. Time profiles of light emitted from high-temperature reactions of NH3/Ar and NH3/O2/Ar mixtures have been measured behind reflected shock waves at temperatures of 2300–2600 K and pressures of 1.6–1.9 bar in a high-repetition-rate shock tube. The emission intensities have been calibrated based on the well-characterized OH* chemiluminescence in a hydrogen-oxygen system and converted to photon emission rates for quantitative comparison with kinetic simulations. A kinetic model describing the pyrolysis and oxidation of ammonia and reactions of excited species has been constructed by combining the reaction mechanisms proposed in recent modeling studies. With only modest updates of the thermodynamic functions and the rate constants for the formation and quenching of excited species, the observed chemiluminescence profiles could be reasonably reproduced, with a few exceptions. The rate of production analysis indicates that NH2* is produced by the reaction of NH3 with H as well as thermal excitation of NH2, that the energy transfer reactions from 3N2 to NH and NO are responsible for the formation of NH* and NO*, respectively, and that the formation of OH* is competitively contributed by the reactions of H with N2O, 3N2 with OH, and NH with NO. Remaining discrepancies between the experiment and modeling are noted, and potential directions for further model improvement are discussed.

Molecular Dynamics Simulation of Thermal Nonequilibrium and Chemical Reaction Processes in Hydrogen Combustion.

Using reactive force field (ReaxFF) and molecular dynamics simulation, we investigate the combustion process of hydrogen-oxygen systems in initial thermal nonequilibrium states with different translational and rovibrational temperatures for oxygen. The system studied in this work contains 300 oxygen molecules and 700 hydrogen molecules with a density of 7 times the air density. For this system, the characteristic relaxation times of oxygen and hydrogen vibrational energies are 0.173 and 0.249 ns, respectively. 0.6% of hydrogen undergoes a chemical reaction with oxygen during the thermal nonequilibrium relaxation stage. For the distribution of translational energy and vibrational energy of oxygen in the thermal nonequilibrium state, the maximum mean error of the statistical distribution in the simulation and the Boltzmann distribution at temperature calculated from the average kinetic energy of molecules is about 2.25 × 10-5. At the same time, it was observed in the simulation that many-body interactions play a certain role in the combustion process. Furthermore, we compare the ignition time and temperature rise behavior of different combustion mechanisms and molecular dynamics simulations starting from the thermal equilibrium state. These results will provide meaningful references for the construction of thermal nonequilibrium combustion chemical reaction mechanisms.

Kinetics study on the catalytic recombination of non-premixed hydrogen–oxygen system: Effects of initial temperature, oxygen flow rate, and hydrogen diffusion time

Hydrogen-oxygen catalytic recombination reaction, as a promising technology for hydrogen elimination in the confined environment, can spontaneously eliminate the leaking hydrogen in the environment. However, the kinetics characteristics of this reaction process under different environmental factors have been poorly reported. A series of hydrogen–oxygen catalytic recombination experiments are performed in a little cylindrical container using Platinum-Carbon powder as the catalyst, to investigate the effects of the initial temperature, oxygen flow rate, and hydrogen diffusion time on the temperature characteristics. In addition, the amount of residual hydrogen at the end of the reaction is also taken into account. The results show that the reaction intensity increases with the increase of the initial temperature, and the first temperature peak PT1 is gradually smaller than PT2 and finally disappears. However, the intensity of temperature increase caused by the reaction exotherm does not enhance continuously with the increase of the reaction intensity. With the decrease in oxygen flow rate, the reaction temperature gradually reduces and the temperature peak gradually forms before the end of oxygen inflow. Nevertheless, the direction of temperature transfer is hardly affected by the oxygen flow rate and the heat is still accumulating below the catalyst. The effect of hydrogen diffusion time on the reaction at hydrogen-poor, stoichiometric ratio hydrogen, and hydrogen-rich is diverse and complex, but the temperature–time profile on the catalyst surface has been changed accordingly. In addition, under any conditions, the hydrogen can be completely reacted below the stoichiometric ratio.

Increasing the manoeuvrability of power units of the thermal power plants due to applying the hydrogen-oxygen systems

The article describes a method for providing additional peak power of a power-unit with help of an additional energy circuit involving a peak turbine and a hydrogen–oxygen system for energy storage and generation. This method is described on the example of K-300-240 turbine power unit. The proposed technical solutions result in the nominal, most economical mode of a power-unit operation during a day. Consequently, the specific fuel consumption is decreased, and reliability of equipment operation is increased. We have developed the technology for high-pressure gases generating that is realized in the membrane-less electrolysers. Thus, there is no need in the use of expensive and energy-intensive compressor equipment for the energy storage systems. That means that the cost of power unit modernization is reduced. Taking into account the environmental and economic factors, the use of hydrogen–oxygen systems for energy storage at the power generating enterprises provides a value of the energy return coefficient that is practically the same as for hydro-storage power plant. But capital costs of power unit modernization are significantly lowered and negative impact on the environment is minimal.

Stability of H3O at extreme conditions and implications for the magnetic fields of Uranus and Neptune

The anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune have long challenged conventional views of planetary dynamos. A thin-shell dynamo conjecture captures the observed phenomena but leaves unexplained the fundamental material basis and underlying mechanism. Here we report extensive quantum-mechanical calculations of polymorphism in the hydrogen-oxygen system at the pressures and temperatures of the deep interiors of these ice giant planets (to >600 GPa and 7,000 K). The results reveal the surprising stability of solid and fluid trihydrogen oxide (H3O) at these extreme conditions. Fluid H3O is metallic and calculated to be stable near the cores of Uranus and Neptune. As a convecting fluid, the material could give rise to the magnetic field consistent with the thin-shell dynamo model proposed for these planets. H3O could also be a major component in both solid and superionic forms in other (e.g., nonconvecting) layers. The results thus provide a materials basis for understanding the enigmatic magnetic-field anomalies and other aspects of the interiors of Uranus and Neptune. These findings have direct implications for the internal structure, composition, and dynamos of related exoplanets.

Evaluation of the possibility of ignition of a hydrogen–oxygen mixture by erosive flame of the impulse laser

The paper presents the mathematical model of combustion initiation in a mixture of hydrogen and oxygen gases (oxyhydrogen) by a laser flare created near a metal barrier. For numerical resolution of the ‘hyperbolic’ part of Reynold’s equations system, we give a new nonlinear quasimonotone compact polynomial difference scheme of improved accuracy. Calculations of all the main parameters: the laser flare, the heating of the metal barrier, and the combustion processes in the hydrogen–oxygen system are performed.

Investigation of the chemical and dilution effects of major EGR constituents on the reactivity of PRF by weak flames in a micro flow reactor with a controlled temperature profile

The chemical and dilution effects of the major exhaust gas recirculation constituents N2, CO2 and H2O on the reactivity of iso-octane/air and n-heptane/air mixtures were investigated by separated weak flames in a micro flow reactor with a controlled temperature profile. Experimental results showed that high dilution ratios of 20% and 50% of N2 and CO2 shifted the blue flame (reactions of intermediate species) as well as the hot flame (reactions of the hydrogen/oxygen-system and CO) to higher wall temperatures, indicating a decrease of reactivity. The cool flame at low temperatures was not affected by these dilutions. On the other hand, dilution by H2O showed a promoting effect on the reactivity as it shifted the blue flame and the hot flame to lower wall temperatures. In 1-D steady computations by the modified KUCRS mechanism, good agreement was found with the experiment except for dilution by H2O. For the comparison of flame locations, a new method was employed that calculated the photon emission from excited CH* and CO2*. While the strong effect of H2O on the reactivity was not observed in the simulation, it was attributed to its high enhanced third body efficiency, which is important for three body chain branching and chain termination reactions of the hydrogen–oxygen system. The contribution of each separated weak flame to the total heat release was calculated by the heat contribution index. This index revealed a stronger effect of the three diluents on n-heptane than on iso-octane. This effect was found to be corresponding to an increase of the research octane number by 28 for the addition of 50% H2O to the n-heptane/air mixture. For iso-octane, the corresponding increase of the RON was 12.

Numerical study on laminar flame velocity of hydrogen-air combustion under water spray effects

In the context of hydrogen safety and explosions in hydrogen-oxygen systems, numerical simulations of laminar, premixed, hydrogen/air flames propagating freely into a spray of liquid water are carried out. The effects on the flame velocity of hydrogen/air flames of droplet size, liquid-water volume fraction, and mixture composition are numerically investigated. In particular, an effective reduction of the flame velocity is shown to occur through the influence of water spray.To complement and extend the numerical results and the only scarcely available experimental results, a “Laminar Flame Velocity under Droplet Evaporation Model” (LVDEM) based on an energy balance of the overall spray-flame system is developed and proposed. It is shown that the estimation of laminar flame velocities obtained using the LVDEM model generally agrees well with the experimental and numerical data.

Effects of stoichiometry on premixed flames propagating in narrow channels: symmetry-breaking bifurcations

Recent studies within the diffusive–thermal (constant-density) approximation have shown that, for premixed flames freely propagating in narrow adiabatic channels, the instabilities induced by differential diffusion may result in non-symmetric solutions and/or oscillating and rotating propagation modes. This has been shown in the context of lean mixtures, for which a single species transport equation with a single Lewis number (corresponding to the ratio of thermal to molecular diffusivity of the deficient reactant) can be used to describe the flame propagation problem. In the present work we extend the analysis to mixtures of any equivalence ratio. To this end, we consider a two-reactant model, where the different diffusivities of the two reactants introduce two different Lewis numbers. Steady-state computations and linear stability analysis are carried out for mixtures with large disparity between the Lewis number of the fuel () and the oxidiser (), such as hydrogen–oxygen systems. It is shown that both differential diffusion and preferential diffusion have influence on the stability of the symmetric flame shape. For sufficiently lean and rich mixtures, the flame behaves as dictated by the Lewis number of the deficient reactant, i.e. the flame destabilises toward non-symmetric solutions for large mass flow rates when the mentioned Lewis number is less than one. In near-stoichiometric mixtures the stability of the symmetric flame depends on a weighted average value of and . In particular, the symmetric solution is stable for large mass flow rates because of the difficulty found by the less diffusive reactant to reach the reactive zone of the flame.

Calculation of intermolecular potentials for H2[sbnd]H2 and H2[sbnd]O2 dimers ab initio and prediction of second virial coefficients

The intermolecular interaction potentials of the dimers H2H2 and H2O2 were calculated from quantum mechanics, using coupled-cluster theory CCSD(T) and correlation-consistent basis sets aug-cc-pVmZ (m=2,3); the results were extrapolated to the basis set limit aug-cc-pV23Z. The interaction energies were corrected for the basis set superposition error with the counterpoise scheme. For comparison also Møller–Plesset perturbation theory (at levels 2–4) with the basis sets aug-cc-pVTZ were considered, but the results proved inferior. The quantum mechanical results were used to construct analytical pair potential functions. From these functions the second virial coefficients of hydrogen and the cross virial coefficients of the hydrogen–oxygen system were obtained by integration; in both cases corrections for quantum effects were included. The results agree well with experimental data, if available, or with empirical correlations.

Chemical reaction kinetics in the afterglow of electrical discharges in hydrogen-oxygen systems (H2 + O2, H2O2, H2O) and in the mixture H2 + CO2

Mathematical modeling of chemical reaction kinetics in the afterglow of electrical discharges in gas mixtures 66.7% H2 + 33.3% O2, 3% H2 + 97% O2, and 75% H2 + 25% CO2 and in vapors of H2O2 and H2O has been performed. The model included 50 chemical reactions for hydrogen-oxygen systems and 82 reactions for H2 + CO2 gas mixture. The reactions of heterogeneous loss of active particles were also included in the model. For every system investigated, a minimal set of significant chemical reactions was constructed sufficient for adequate description of experimental results. The concentration of particles not detected in experiment was estimated. The initial gas-phase particles were revealed for the synthesis of peroxy-radical condensates in hydrogen-oxygen systems and formic acid from the mixture H2 + CO2.

Reactive hydrogen-oxygen collisions in combustion and radiolysis

Reactive collisions between neutral molecules play important roles in combustion, radiological, and atmospheric physics. Utilizing the current state of knowledge of hydrogen-oxygen reaction rates, the corresponding reaction cross sections are inferred. For these relatively simple reactions, the time might be ready to undertake ab initio quantum-mechanical calculations of such cross sections. In this case, it might be possible to apportion a reaction rate to reactions leading to various excited states. The reaction rates for a hydrogen-oxygen system in a kinetic time simulation and a detailed steady-state combustive flow-field calculation are used to illustrate the apportionment.

Oxygen/Hydrogen-Planar-Laser-Induced Fluorescence Measurements and Accuracy Investigation in High-Pressure Combustion

DOI: 10.2514/1.39013 Inflow species concentration measurements in reacting flows at high pressures, based on nonintrusive methods, have been acquired so far for isolated conditions in a range of experimental devices and by using a variety of methods. Furthermore, extensive assessments of the uncertainties associated with the measurement techniques are lacking. In general, these methods have been based on calibrations determined from assumptions that were not sufficiently quantified to provide a detailed range of the uncertainties associated with these measurements. This work quantifies the uncertainties associated with OH measurement in an oxygen–hydrogen system produced by a shear, coaxial injector typical of those used in rocket engines. Planar OH distributions are obtained for a range of pressures from 10 to 53 bar, providing instantaneous and averaged distribution in a unified study, using the same experimental setup and maintaining the rest of the parameters the same. The uncertainty of 18 different parameters was evaluated and the overall root mean square error was found as 21.9, 22.8, 22.5, and 22.9% at 10, 27, 37, and 53 bar, respectively.

Detailed and reduced chemistry for hydrogen autoignition

Reaction-rate parameters are given for the detailed chemistry of gas-phase hydrogen combustion, involving 21 reversible elementary steps. It is indicated that, when attention is restricted to specific combustion processes and particular conditions of interest, fewer elementary steps are needed. In particular, for calculating autoignition times over a wide range of pressures for temperatures between about 1000 and 2000 K, five irreversible elementary steps suffice, yielding a remarkable reduction in complexity. It is explained that, from a mathematical viewpoint, in terms of global reaction-kinetic mechanisms, the hydrogen–oxygen system in principle comprises only six overall steps. Rational reduced chemical mechanisms for hydrogen combustion therefore necessarily must have fewer than six overall steps. For autoignition over the range of conditions specified above, ignition times can be determined accurately by considering, in addition to an elementary initiation step and an elementary termination step, at most three overall steps for reaction intermediaries, which reduce to two for very fuel-lean conditions and to one for stoichiometric or fuel-rich conditions. The resulting reductions can simplify computations that need to be performed in risk analyses for hydrogen storage and utilization.

Erratum: “Effect of a gas discharge on the ignition in the hydrogen-oxygen system” [Kinetic and Catalysis 46, 776 (2005)

Erratum: “Effect of a gas discharge on the ignition in the hydrogen-oxygen system” [Kinetic and Catalysis 46, 776 (2005)

Effect of a Gas Discharge on the Ignition in the Hydrogen-Oxygen System

A kinetic model is constructed for ignition initiated by nonequilibrium gas-discharge plasma in the hydrogen-oxygen system. The model takes into account the effect of the electric field on the dissociation of molecules and on the buildup of active radicals, excited species, and charged species (electrons and positively and negatively charged ions). It is demonstrated by mathematical modeling that the induction period depends strongly on the reduced strength of the electric field (electron temperature). Four reduced kinetic networks are considered, and various components and reactions are shown to exert an effect on the nonthermal initiation of ignition in the H2-O2 system by low-temperature plasma.

Inhibition of the Combustion and Detonation of Hydrogen-Air Mixtures behind the Shock Front

The chain avalanche plays the determining role in all regimes of combustion in the hydrogen-oxygen system in the initiating shock wave near atmospheric pressure in a wide range of high initial temperatures. The characteristics of combustion and detonation can be effectively controlled by varying the rates of the competing chain-branching and chain-termination reactions with the use of small amounts of admixtures. The reactivity of the combustible mixture is correlated with the chemical structure of the admixture. This correlation is manifested in the fact that changing a single functional group in the admixture molecule produces a strong effect on the kinetics, macrokinetics, and gas dynamics of the overall process.

On molecular transport effects in real gas laminar diffusion flames at large pressure

Direct numerical simulations are conducted of unsteady, exothermic and one-dimensional laminar diffusion flames at large pressures. The simulations are used to assess the impact of molecular diffusion and real gas effects under high pressure conditions with simplified chemical kinetics. The formulation includes the fully compressible form of the governing equations, real gas effects modeled by the cubic Peng–Robinson equation of state, and a generalized form of the Soret and Dufour mass and heat diffusion vectors derived from nonequilibrium thermodynamics and fluctuation theory. The cross diffusion fluxes are derived for a ternary species system and include the effects of both heat and mass diffusion in the presence of temperature, concentration and pressure gradients (i.e., Soret and Dufour diffusion). The ternary species formulation is applied to a simplified single step reaction elucidating molecular and thermodynamic effects apparent in general combustion. Realistic models for pressure, temperature and species dependent heat capacities, viscosities, thermal conductivities and mass diffusivities are also included. Three different model reactions are simulated both including and neglecting Soret and Dufour cross diffusion. The simulation results show that Soret and Dufour effects are negligible for reactions comprised of species with equal or near equal molecular weights. However, Soret diffusion effects are apparent when species with nonequal molecular weights are involved in the reaction and result in reductions of the peak flame temperature. In addition, it is shown that neglect of cross diffusion leads to deviations in the predicted flame thicknesses, with under predictions for a hydrogen-oxygen system and over predictions for a heavy hydrocarbon reaction. These effects are explained in detail through examinations of the individual heat and mass flux vectors as well as through associated thermodynamic properties. A parametric study addresses the effects of the ambient pressure, the initial “flame Reynolds number,” the Damkohler number and the heat release parameter.

HYDROGEN–OXYGEN INDUCTION TIMES ABOVE CROSSOVER TEMPERATURES

Ignition in hydrogen–oxygen systems above crossover temperatures and under various conditions of pressure and composition is addressed computationally and by asymptotic methods. Different descriptions of the detailed chemistry are evaluated through comparison of computed and measured ignition times, and a balance between accuracy and simplicity is struck in selecting rate parameters to be used in investigating reduced chemistry. Through numerical calculations for isobaric, homogeneous, and adiabatic hydrogen–air mixtures it is shown that the detailed chemistry can be reduced to only six elementary steps for determining induction times over the range of conditions addressed. From these six steps, an analytical expression for ignition time is derived which agrees well with computational results concerning dependence on pressure, temperature, and composition. It is shown that O and OH maintain steady states during ignition for stoichiometric and fuel-rich mixtures, whereas H maintains, a steady state for sufficiently fuel-lean conditions. Simple asymptotic formulas for the induction times are derived for these two limits that demonstrate the limiting effect of O2 under rich conditions and of H2 under lean conditions. These formulas are combined in an approximate way to obtain an expression for the ignition time that can be used for all equivalence ratios.

Computational modeling of the SiH 3+O 2 reaction and silane combustion

Recent theoretical and experimental studies have provided improved estimates of thermochemical and chemical kinetic data for the silicon–hydrogen–oxygen system. In particular, the SiH 3+O 2 reaction has been the subject of considerable interest. Estimates of rate coefficients for specific SiH 3+O 2 channels as a function of temperature and of pressure and branching fraction assignments are made in the current work using Quantum Rice–Ramsperger–Kassel (QRRK) analyses for k( E) and Master Equation (ME) analyses for fall-off. The QRRK/ME analyses were based on potential energy surface data provided by previous ab initio studies. The overall rate coefficient for SiH 3+O 2→ products shows a slight negative temperature dependence, which agrees with previous experimental studies. The results indicate significant pressure and temperature dependence for the product branching fractions and are in good agreement with experimental measurements of the SiH 3+O 2 product channels. In particular, the channel producing H atoms (SiH 3+O 2→cyclic-OSiH 2O+H) is dominant at high temperatures and/or low pressures, and the O-atom channel (SiH 3+O 2→H 3SiO+O) is not significant (>5% branching fraction) below 1000 K at any pressure examined in the QRRK study ( P=0.001–10 atm). A detailed chemical mechanism for silane combustion is presented based on the QRRK estimates and other evaluated rate coefficient and thermochemical parameters. The mechanism is validated at high temperatures by comparison of calculated ignition delay times with shock-tube data and by comparison of calculated OH radical profiles with atmospheric pressure burner data. The mechanism is validated at low temperatures by comparison of ignition times with constant volume explosion data.