Four-step and three-step systematically reduced chemistry for wide-range H₂–air combustion problems

The feasibility of developing multipurpose reduced chemistry that is able to describe, with sufficient accuracy, premixed and non-premixed flames, one-dimensional detonations, high-temperature autoignition, and also low-temperature autoignition is explored. A four-step mechanism with O and OH in steady state is thoroughly tested and is shown to give satisfactory results under all conditions. The possibility of reducing this to a three-step mechanism, to decrease computation times without compromising the range of applicability is then investigated. The originality of this work resides in introducing a single species X, representing either HO₂ for high-temperature ignition or H₂O₂ for low-temperature ignition. An algorithm is defined that covers the entire range without significant degradation of accuracy. Integrations show promising results for different laminar test cases, and applicability to turbulent flows is indicated.This work was supported by the UE Marie Curie ITN MYPLANET, by the Spanish MCINN through Project # CSD2010-00010, by the Comunidad de Madrid through Project # S2009/ENE-1597, and by the US AFOSR Grant # FA9550-12-1-0138

Explicit analytic prediction for hydrogen–oxygen ignition times at temperatures below crossover

This paper addresses homogeneous ignition of hydrogen-oxygen mixtures when the initial conditions of temperature and pressure place the system below the crossover temperature associated with the second explosion limit. A three-step reduced mechanism involving H2, O2, H2O, H2O2 and HO2, derived previously from a skeletal mechanism of eight elementary steps by assuming O, OH and H to follow steady state, is seen to describe accurately the associated thermal explosion. At sufficiently low temperatures, HO2 consumption through HO2 + HO2 → H2O2 + O2 is fast enough to place this intermediate in steady state after a short build-up period, thereby reducing further the chemistry description to the two global steps 2H2 + O2 → 2H2O and 2H2O → H2O2 + H2. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be used in describing the resulting thermal runaway, yielding an explicit expression that predicts with excellent accuracy the ignition time for different conditions of initial temperature, composition, and pressure.This work was supported by the Comunidad de Madrid through project # P2009/ENE-1597. The first two authors also acknowledge support from the EU through the Marie Curie ITN MYPLANET and from the Spanish MCINN through projects # ENE2008-06515 and CSD2010-00011.European Community's Seventh Framework ProgramPublicad

The chemistry involved in the third explosion limit of H₂–O₂ mixtures

The third explosion limit of hydrogen oxidation in closed vessels has always been thought to be the result of the competition between homogeneous gas-phase reactions and diffusion of hydroperoxyl radicals to the walls, where they are destroyed. It has recently been observed that this species actually follows a chemical-kinetic steady state in this regime, with the consequence that its diffusive rate toward the catalytic walls becomes irrelevant. Here we show that the critical explosion conditions are determined instead by the fate of hydrogen peroxide, which emerges as the controlling reactant for the resulting gas-phase chemistry. A simple, accurate analytic expression for the third explosion limit follows from identification of the critical conditions for existence of weakly reactive, diffusion&-reaction solutions, thereby providing the answer to a long-standing problem that in early work was characterized as being hopelessly difficult.This work was supported by the US AFOSR Grant # FA9550-12-1-0138, by the Comunidad de Madrid through Project #P2009/ENE-1597, and by the Spanish MCINN through Project #CSD2010-00011

An explicit reduced mechanism for H2–air combustion

Proceedings of: 13th International Conference on Numerical Combustion, 27-29 April, 2011, Corfu (Greece) in conjunction with the 3rd International Workshop on Model Reduction in Reacting FlowsFor hydrogen–oxygen–inert systems, just as for other fuel–oxidizer mixtures, systematically reduced chemistry has in the past been developed separately for premixed and diffusion flames and for autoignition. In computational work that addresses turbulent combustion or the transition from deflagration to detonation, however, autoignition and flames both may occur, and reduced chemistry may be required because of computer limitations. To fill that need, systematically reduced chemistry is presented here that encompasses autoignition and flames. The description involves three global steps among five reacting species, H2;O2;H2O;H and HO2, being based on approximations to chemical-kinetic steady states for O, OH and H2O2. These steady states apply well under all conditions except during autoignition in lean and stoichiometric mixtures, where they underpredict induction times substantially. To remedy this deficiency, which occurs only when HO2 is not in steady state, an autoignition analysis is employed to derive a correction factor that reduces the value of the reaction rates to produce agreement of calculated ignition delays. Introduction of a criterion for inclusion of this correction factor, based on a test for the HO2 steady state, results in a generally applicable three-step chemical-kinetic description for hydrogen–air combustion that possesses reasonable accuracy for most computational purposes.This work was supported by the UE Marie Curie ITN MYPLANET, by the Spanish MCINN through Project # ENE2008-06515 and by the Comunidad de Madrid through Project # S2009/ENE-1597.European Community's Seventh Framework ProgramPublicad

Hydrogen-air mixing-layer ignition at temperatures below crossover

This paper addresses ignition histories of diffusion flames in unstrained hydrogen-air mixing layers for initial conditions of temperature and pressure that place the system below the crossover temperature associated with the second explosion limit of hydrogen–oxygen mixtures. It is seen that a two-step reduced chemical-kinetic mechanism involving as main species H₂, O₂, H₂O, and H₂O₂, derived previously from a detailed mechanism by assuming all radicals to follow a steady-state approximation, suffices to describe accurately the ignition process. The strong temperature sensitivity of the corresponding overall rates enables activation-energy asymptotics to be employed for the analysis, following the ideas developed for mixing-layer ignition by Liñán and Crespo in 1976 on the basis of one-step Arrhenius model chemistry. When the initial temperatures of both reactants differ by a relative amount that is of the order of or smaller than the ratio of this temperature to the effective activation temperature, the chemical reaction is seen to occur at a significant rate all across the mixing layer. The ignition time is then determined as a thermal runaway in a parabolic problem describing the evolution of the temperature increment and the H₂O₂ concentration, with local accumulation, chemical reaction, and transverse convection and diffusion, all being important. By way of contrast, when the air side is sufficiently hotter than the hydrogen side, as often occurs in applications, ignition occurs in a thin layer close to the air-side boundary, enabling a simplified description to be developed in which the ignition time is determined by analyzing the existence of solutions to a two-point boundary-value problem involving quasi-steady diffusion–reaction ordinary differential equations.This work was supported by the US AFOSR Grant # FA9550-12-1-0138, by the Comunidad de Madrid through Project # P2009/ENE-1597, and by the Spanish MCINN through Project #CSD2010-00011

A four-step reduced mechanism for syngas combustion

A four-step reduced chemical-kinetic mechanism for syngas combustion is proposed for use under conditions of interest for gas-turbine operation. The mechanism builds upon our recently published threestep mechanism for H2-air combustion (Boivin et al., Proc. Comb. Inst. 33, 2010), which was derived from a 12-step skeletal mechanism by assuming O, OH, and H2O2 to be in chemical-kinetic steady state and includes a correction to account for the failure of the O and OH steady states during autoignition. The analysis begins by appropriately extending the number of elementary steps in the skeletal description to enable computation of the CO chemistry for mixtures with appreciable H2 content, giving a total of 16 elementary steps. It is seen that the formyl radical HCO, which appears as the only additional relevant intermediate in the extended chemical description, follows accurately a steady-state approximation, which can be used along with the steady-state approximations for O, OH, and H2O2 to derive the reduced description, involving the three global steps of our previous H2-air mechanism, 3H2+O2 2H2O+2H, 2H+M H2+M, and H2+O2 HO2+H, along with the additional step CO+ H2O CO2+H2. Expressions are given for the rates of the four global reactions in terms of those of the elementary steps of the skeletal mechanism, with concentrations of the different steady-state species also given in explicit form. Comparisons of results of computations of laminar burning velocities and induction times with published experimental data for H2/CO/O2 mixtures with different diluents at atmospheric and elevated pressures and for varying equivalence ratios and initial temperatures indicate that the reduced description can be applied with reasonable accuracy in numerical studies of gas-turbine syngas combustion.This work was supported by the UE Marie Curie ITN MYPLANET, by the Comunidad de Madrid through project # S2009/ENE-1597, and by the Spanish MCINN through projects # ENE2008-06515 and CSD2010-00011.European Community's Seventh Framework ProgramPublicad

Diffusion-Flame Ignition by Shock-Wave Impingement on a Hydrogen-Air Supersonic Mixing Layer

Ignition in a supersonic hydrogen-air mixing layer interacting with an oblique shock wave is investigated analytically under conditions such that the postshock flow is supersonic and the peak postshock temperature before ignition remains below the crossover temperature. The study requires consideration of the flow structure in the postshock ignition kernel found around the point of maximum temperature, which is assumed in this study to lie at an intermediate location across the mixing layer, as occurs in mixing layers subject to significant viscous dissipation. The ignition kernel displays a balance between the rates of chemical reaction and postshock flow expansion, including the acoustic interactions of the chemical heat release with the shock wave leading to increased front curvature. The problem is formulated with account taken of the strong temperature dependence of the chemical heat-release rate characterizing the ignition chemistry in the low-temperature regime analyzed here. It is shown how consideration of a two-step reduced chemical-kinetic mechanism derived in previous work leads to a boundary-value problem that can be solved analytically to determine ignition as a fold bifurcation, with the turning point in the diagram of peak perturbation induced by the chemical reaction as a function of the Damkohler number providing the critical conditions for ignition.This work was supported by the U.S. Air Force Office of Scientific Research grant FA9550-12-1-0138 and by the Spanish Ministerio de Ciencia e Innovación through the Program CONSOLIDER-Ingenio2010 (project CSD2010-00011).Publicad

Flammability conditions for ultra-lean hydrogen premixed combustion based on flame-ball analyses

Proceeding of: 10th International Conference on Clean Energy 2010, 15-17 September 2010, Famagusta, North CyprusIt has been reasoned that the structures of strongly cellular flames in very lean mixtures approach an array of flame balls, each burning as if it were isolated, thereby indicating a connection between the critical conditions required for existence of steady flame balls and those necessary for occurrence of self-sustained premixed combustion. This is the starting assumption of the present study, in which structures of near-limit steady sphericosymmetrical flame balls are investigated with the objective of providing analytic expressions for critical combustion conditions in ultra-lean hydrogen-oxygen mixtures diluted with N2 and water vapor. If attention were restricted to planar premixed flames, then the lean-limit mole fraction of H2 would be found to be roughly ten percent, more than twice the observed flammability limits, thereby emphasizing the relevance of the flame-ball phenomena. Numerical integrations using detailed models for chemistry and radiation show that a onestep chemical-kinetic reduced mechanism based on steady-state assumptions for all chemical intermediates, together with a simple, optically thin approximation for water-vapor radiation, can be used to compute near-limit fuel-lean flame balls with excellent accuracy. The previously developed one-step reaction rate includes a crossover temperature that determines in the first approximation a chemical-kinetic lean limit below which combustion cannot occur, with critical conditions achieved when the diffusion-controlled radiation-free peak temperature, computed with account taken of hydrogen Soret diffusion, is equal to the crossover temperature. First-order corrections are found by activation-energy asymptotics in a solution that involves a near-field radiation-free zone surrounding a spherical flame sheet, together with a far-field radiation-conduction balance for the temperature profile. Different scalings are found depending on whether or not the surrounding atmosphere contains water vapor, leading to different analytic expressions for the critical conditions for flame-ball existence, which give results in very good agreement with those obtained by detailed numerical computations. The one-step chemistry employed in the present work, which involves a non-Arrhenius rate having a cutoff at the crossover temperature, applies with excellent accuracy to the description of lean premixed hydrogen-air combustion, i.e, for f(0:5 at atmospheric pressure, and could be used for instance in the numerical simulation of the propagation of curved or cellularflames in ultra-lean reactive atmospheres, of interest for safety analyses related to the storage, transport, and handling of hydrogen.This work was supported by the Comunidad de Madrid through project #P2009/ENE-1597. The first three authors also acknowledge support from the Spanish MCINN through projects # ENE2008-06515 and CSD2010-00011

The structure of lean hydrogen-air flame balls

An analysis of the structure of flame balls encountered under microgravity conditions, which are stable due to radiant energy losses from H₂O, is carried out for fuel-lean hydrogen-air mixtures. It is seen that, because of radiation losses, in stable flame balls the maximum flame temperature remains close to the crossover temperature, at which the rate of the branching step H + O₂ -> OH + O equals that of the recombination step H + O₂ + M -> HO₂ + M. Under those conditions, all chemical intermediates have very small concentrations and follow the steady-state approximation, while the main species react according to the overall step 2H₂ + O₂-> 2H₂O; so that a one-step chemical-kinetic description, recently derived by asymptotic analysis for near-limit fuel-lean deflagrations, can be used with excellent accuracy to describe the whole branch of stable flame balls. Besides molecular diffusion in a binary-diffusion approximation, Soret diffusion is included, since this exerts a nonnegligible effect to extend the flammability range. When the large value of the activation energy of the overall reaction is taken into account, the leading-order analysis in the reaction-sheet approximation is seen to determine the flame ball radius as that required for radiant heat losses to remove enough of the heat released by chemical reaction at the flame to keep the flame temperature at a value close to crossover. The results are relevant to burning velocities at lean equivalent ratios and may influence fire-safety issues associated with hydrogen utilization.This work was supported by the Spanish MCINN through Project # ENE2008-06515 and by the Comunidad de Madrid through Project #S2009/ENE-159

The constant-velocity highly collimated outflows of the planetary nebula He 2-90

We present high-dispersion echelle spectroscopic observations and a narrow-band [N II] image of the remarkable jet-like features of He 2-90. They are detected in the echelle spectra in the H-alpha and [N II] lines but not in other nebular lines. The [N II]/H-alpha ratio is uniformly high, ~1. The observed kinematics reveals bipolar collimated outflows in the jet-like features and shows that the southeast (northwest) component expands towards (away from) the observer at a remarkably constant line-of-sight velocity, 26.0+-0.5 km/s. The observed expansion velocity and the opening angle of the jet-like features are used to estimate an inclination angle of ~5 degrees with respect to the sky plane and a space expansion velocity of ~290 km/s. The spectrum of the bright central nebula reveals a profusion of Fe lines and extended wings of the H-alpha line, similar to those seen in symbiotic stars and some young planetary nebulae that are presumed to host a mass-exchanging binary system. If this is the case for He 2-90, the constant velocity and direction of the jets require a very stable dynamic system against precession and warping.Comment: 8 pages (emulate ApJ), 5 figure, 1 tabl