Hydrogen-oxygen reactions mechanism

Das, L.M. 1996. Hydrogen-oxygen reaction mechanism and its implication to hydrogen engine combustion. Int J Hydrogen Energy 21 703-715. 

Based on a hydrogen-oxygen reaction mechanism that is extracted from GRI-Mech, compute the flow field and species profiles for the nominal flow conditions. For the purposes of evaluating the gas-phase flow, assume that surface chemistry can be neglected. 

In a branched-chain mechanism, there are elementary reactions which produce more than one chain carrier for each chain carrier reacted. An example of such an elementary reaction is involved in the hydrogen-oxygen reaction ... 

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4],... 

Detailed Reaction Mechanisms In this approach the chemistry is described by a collection of elementary reactions, referred to as a detailed reaction mechanism. Conceptually the mechanism describes the chemical process as it occurs on a molecular level. Table 13.1 shows the reaction mechanism for the hydrogen-oxygen reaction system. This... 

The hydrogen-oxygen reaction 2H2 -f O2 2H2 O involves a chain mechanism in which the atoms H and O and the hydroxyl radical OH act as chain carriers. One of the elementary steps in the mechanism is... 

Thrush [37] reviewed H atom reactions in 1965 but most of the data listed in Table 1 have been measured since that time. On the whole, the rate coefficients recommended by Thrush for the reactions of H atoms with alkanes [37] have been largely substantiated by subsequent studies, principally of Baldwin, Walker and co-workers. They have measured the rate coefficients by studying the effects of the hydrocarbons on the explosion limits of the hydrogen—oxygen reaction at temperatures around 750°K and obtained Arrhenius parameters by combination of the results with lower temperature data [38, 39]. Subsequent studies by the same group of the slow reaction between H2 and Oj in the presence of hydrocarbon additives, based on the measurement of the rates of depletion of the hydrocarbon and formation of water and analyses of the products of the reactions of the alkyl radicals, have confirmed the original mechanisms proposed and substantiated the rate coefficients [15, 40, 41]. 

The mechanism of the hydrogen-oxygen reaction is probably not fully understood even today. Much of the modern work has been done by C. N. Hinschelwood and his coworkers. The steps in the chain reaction are... 

There is little doubt that a reaction such as H-f-Oj = OH+0 follows the second-order law, but this can only be inferred from the fact that the assumption leads to correct results when introduced as part of the theory for a more complex mechanism in the hydrogen-oxygen reaction. 

The shock tube investigations of the hydrogen-oxygen reaction described in section 2.2 have revealed the significant qualitative features of the high-temperature mechanism considered above, viz. exponential growth of intermediate and product concentrations, the induction... 

The primary emphasis in shock tube interferometric studies of the hydrogen-oxygen reaction has been on induction period phenomena. Recently, however, the entire postshock density profiles of a selection of rich, lean and near stoichiometric Ha-Oa-Ar mixtures have been studied by numerical integration of an assumed reaction mechanism. In this manner it was shown that the characteristic features of the profile prior to the end of the density plateau are essentially independent of the recombination kinetics. Thereafter, however, the shape of the profile is largely accounted for by termolecular reactions (e)-(g). Systematic variation of the termolecular rate coefficient values in experimental regimes where recombination is most sensitive to reactions if) or ig)> respectively, has yielded temperature-dependent expressions of the form kf< = AT for kf and kf believed valid over the range 1400-3000 K. The expression of Jacobs et al. was found satisfactory for kf. In all three cases, variation with temperature is small (1-0 m 0-5). Values at 1700 K, kf = 5-9 x 10 (cited above), kf z= 1-9 X 10 , and kf = 3-6 x 10 cm mole sec, are in excellent accord with those listed in Table 2.2. 

Gray and Yang proposed the simple kinetic mechanism discussed here as a basis to explain the multiple limits found in the hydrogen-oxygen reaction. At the present time too few direct measur nents of reactant temperatures have been made in this system to justify this proposal. However, experimental confirmation of some of the qualitative aspects has been found in a similar gas-phase oxidation, that of hydrogen sulphide.- ... 

Carefully study the reaction mechanism for the stearoyl-CoA desaturase in Figure 25.14, and account for all of the electrons flowing through the reactions shown. Also account for all of the hydrogen and oxygen atoms involved in this reaction, and convince yourself that the stoichiometry is correct as shown. 

In absence of oxygen some hydrogen does manage to evolve and polarize the cathode to some extent. However, if oxygen is present, this polarization does not occur as discussed earlier, and results in accelerated corrosion attack. Hydrogen sulfide ionizes in two main stages when dissolved in fluid. The reactions mechanisms are... 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

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