Oscillatory Ignition Research Articles

A Most Reactive Mixture Analysis of Ozone- and Temperature-Enhanced n-Heptane Droplet Autoignition and Cool Flame Burning in Microgravity

ABSTRACT Numerical simulations are used to study effects of ozone (O3) and thermal enhancement on autoignition and subsequent burning characteristics of isolated n-heptane droplets in microgravity conditions. Particular focus is given to effects of O3 on low temperature combustion, or cool flames. Levels of ambient O3 mole fraction and ambient temperature are varied from 0 to 0.03 and 400 K to 750 K, respectively. A comparison between two low temperature chemistry enhancement strategies – O3 addition and temperature increase – are investigated on an equivalent energy addition basis to evaluate relative effects on combustion characteristics. Results of this study are analyzed using physical and mixture fraction approaches because of unique insights offered by these frameworks. Ignition and ensuing flame propagation are investigated via a most reactive mixture analysis, evaluated using homogeneous reactor simulations of mixture compositions and temperatures surrounding the droplet. Lastly, a Damköhler number analysis is performed to explore oscillatory ignition dynamics.

Flame dynamics in lean premixed [formula omitted]/air combustion in a mesoscale channel

The dynamics and stabilization of fuel lean premixed CO/H2/air atmospheric pressure flames in meso-scale channels were investigated numerically, using detailed gas phase chemistry and transport. Experiments in a channel flow reactor by means of chemiluminescence detection of the excited OH radical allowed for model validation at steady conditions and identification of the conditions at which unsteady flame dynamics were present. A detailed parametric study of the influence of wall temperature and CO:H2 ratio on the ensuing flame dynamics was performed. The numerical results revealed different flame modes which included oscillatory ignition, random ignition spots, as well as steady weak and V-shaped flames. The wall temperature stability intervals of these modes changed with the CO:H2 ratio. The richest variety was found for molar CO:H2 ratios between 4 and 10, while at lower ratios the random and the weak modes were absent. At higher ratios all the dynamic modes were suppressed. The Computational Singular Perturbation (CSP) method was used to obtain insights into the physicochemical processes responsible for the weak flames, which were found at relatively high inflow velocities compared to previous studies, and V-shaped flames. A kinetic explanation of the phenomena was supported by the CSP analysis.

Thermal Oscillations in the Decomposition of Organic Peroxides: Identification of a Hazard, Utilization, and Suppression

The purpose of this research is to identify and characterize oscillatory thermal instability in organic peroxides that are used in vast quantities in industry and misused by terrorists. The explosive thermal decompositions of lauroyl peroxide, methyl ethyl ketone peroxide, and triacetone triperoxide are investigated computationally, using a continuous stirred tank reactor model and literature values of the kinetic and thermal parameters. Mathematical stability analysis is used to identify and track the oscillatory instability, which may be violent. In the mild oscillatory regime it is shown that, in principle, the oscillatory thermal signal may be used in microcalorimetry to detect and identify explosives. Stabilization of peroxide thermal decomposition via Endex coupling is investigated. It is usually assumed that initiation of explosive thermal decomposition occurs via classical (Semenov) ignition at a turning point or saddle-node bifurcation, but this work shows that oscillatory ignition is also characteristic of thermoreactive liquids and that Semenov theory and purely steady state analyses are inadequate for identifying a thermal hazard in such systems.

Ignition and transient dynamics of sub-limit premixed flames in microchannels

We examine, via two-dimensional numerical simulation of a model system, some unsteady transient ignition scenarios and sustained oscillatory combustion modes that can occur in a single-pass, conductive channel, premixed microburner. These issues are relevant to the problem of ignition, evolution to stable combustion and the operational modes of microcombustors. First, we describe an unsteady ignition sequence that may occur when a single-pass microburner with initially cold walls has its exit walls heated and maintained at a fixed temperature. In particular, we demonstrate that as the heat from the exit walls propagates down the microburner walls, a reaction wave is driven rapidly down the channel towards the inlet via a sequence of oscillatory ignition and quenching transients. This scenario has been observed experimentally during the ignition of a single-pass microburner. Secondly, we show how an initial axial wall temperature gradient can lead to a variety of sustained combustion modes within the channel, including stable stationary flames, regimes of periodic motion involving quenching and re-ignition, regimes of regular oscillatory combustion, and regimes consisting of a combination of re-ignition/quenching events and regular oscillatory motions, all of which have been observed experimentally.

A Numerical and Analytical Study of Thermally Driven Combustion Oscillations in a Perfectly Stirred Reactor

The oscillatory behaviors of methane and hydrogen oxidation in a perfectly stirred reactor (PSR) are examined. The work explores the parameter spaces in which oscillatory combustion and ignition take place using heat transfer coefficient, mean residence time, and reactor wall temperatures as variables. An analytic model was developed using an eigenvalue analysis to determine the nature and stability of these oscillations. Both numerical and analytical studies suggest that combustion oscillations occur at the extinction turning point of the hysteresis curve or the boundary of combustion extinction. These oscillations are found to be driven by the coupling of the heat released from the reaction and the heat dissipation through the reactor wall, and these are unstable, which perhaps explains why they were never or rarely observed experimentally. In the case of hydrogen oxidation, we demonstrate the existence of two additional types of oscillations, namely, hybrid oscillation and oscillatory ignition, both of which occur at or near the turning point of ignition. These oscillations are stable and driven by detailed reaction kinetics. The numerical results for hydrogen oxidation were compared with previous experiments and found to be within 5K of the observed wall temperature where oscillations were observed.

HYPERGOLIC REACTION MECHANISMS OF CATALYTICALLY PROMOTED FUELS WITH ROCKET GRADE HYDROGEN PEROXIDE

The coupling of the physical and chemical processes controlling the ignition delays (τ ign ) of hypergolic propellants complicates the direct analysis of the transient ignition processes. The transport properties of both the propellants and the ambient gas, and the heat of vaporization and reaction affect τ ign and must be considered experimentally. Furthermore, since hypergolic ignition occurs, by definition, on contact, the mechanics of initial contact and subsequent mixing can affect τ ign and must also be considered. Experimental results from a study of the hypergolic ignition of a catalytically promoted fuel with rocket grade hydrogen peroxide (RGHP) are presented. Statistical analysis of the experimental data was conducted to evaluate the relative effects of RGHP concentration, initial ambient pressure, and the ambient gas on τ ign . At RGHP concentrations of less than 92% in water, stable ignition was difficult to achieve. At sufficiently high Lewis number, an oscillatory ignition process is observed. A phenomenological model comprising mixing, RGHP decomposition, fuel heating and vaporization, and auto-ignition is presented and compared to the experimental results.

Reduced Description of Complex Dynamics in Reactive Systems

Detailed chemical kinetics typically involve a large number of chemical species and a wide range of time scales. In calculations of chemically reactive flows, dimension-reduction techniques can be used to reduce the computational burden imposed by the direct use of detailed chemistry. In the reduced description, the reactive system is described in terms of a smaller number of reduced composition variables (e.g., some "major" species) instead of the full set of chemical species. Reactive flows exhibiting complex dynamics are especially challenging for dimension-reduction techniques and therefore provide more rigorous validation for such methods. Following the work of Brad et al. [Proc. Combust. Inst. 2007, 31, 455],1 in this paper, we demonstrate the capability of the Invariant Constrained-equilibrium Edge Pre-Image Curve (ICE-PIC) dimension-reduction method [J. Chem. Phys. 2006, 124, Art. No. 114111]2 through calculations of the oxidation of a CO/H2 mixture in a continuously stirred tank reactor (CSTR) at low pressure. The detailed chemical kinetics employed involves 11 species and 33 reactions. The system exhibits complex dynamics such as oscillatory ignition, oscillatory glow, and mixed mode oscillations. It is demonstrated that with five represented species the reduced description provided by the ICE-PIC method is able to quantitatively reproduce the observed complex dynamics. Moreover, the reduced description accurately predicts the boundaries of slow reaction, oscillatory ignition and the steady ignited state.

The application of chemical reduction methods to a combustion system exhibiting complex dynamics

A number of chemical model reduction techniques have been developed over recent years with a growing range of applications in combustion. The following work demonstrates the application of such reduction techniques for a combustion system describing the oxidation of carbon monoxide + hydrogen in a continuously stirred tank reactor (CSTR) at very low pressure. The system exhibits complex dynamics including oscillatory glow, oscillatory ignition and mixed mode oscillations. It is demonstrated that a range of local reduction methods can be applied to such complex systems, as long as sufficient coverage of the accessed regions of phase space are included in the reduction analysis. The methods include sensitivity analysis, the quasi-steady state approximation (QSSA) and repro-modelling based on the concept of an intrinsic low dimensional manifold (ILDM). The system is qualitatively different from some previous applications of ILDM methods where trajectories tend towards a fixed equilibrium. The underlying dimension of the system is seen to vary throughout selected trajectories with rapid increases occurring over very short time-scales during oscillatory ignition. Nevertheless, a final reduced model of only four variables is developed using fitted orthonormal polynomials describing the system dynamics on a slow manifold. The application serves to demonstrate that the relationship between local reduced model error and global errors can be complex for systems exhibiting complex dynamics, with regions of seemingly small local mapping gradients requiring tighter error control in order to control global errors. This feature may be common in cases where nearby trajectories are seen to diverge within the slow manifold over time.

A Complete Pressure−Temperature Diagram for Air Oxidation of Hydrogen in a Continuous-Flow Stirred Tank Reactor

We report the temperature and pressure dependence of the dynamics at short residence times of a fuel-lean hydrogen−air mixture in an isothermal continuous-flow stirred tank reactor. The reaction between hydrogen and oxygen is modeled by a set of twenty reversible reactions involving nine species over the temperature and pressure ranges 300−1100 K and 10-3−10 bar, respectively. Boundaries of dynamically distinct regions in a complete pressure−temperature diagram are calculated with a path-following algorithm based on pseudo-arclength continuation. We find a region of birhythmicity, thus showing that chain-branching alone can account for the coexistence of stable oscillatory states, and a region containing a periodic-orbit isola, a new feature for this system. Although infinite-period orbits exist just a few degrees below the ignition curve, oscillatory ignition does not occur under the conditions of our simulations.

Oscillations and chaos in CO+O(2) combustion.

The gas-phase reaction between carbon monoxide and oxygen (in the presence of small amounts of hydrogen) shows bistability and oscillatory behavior. Typically, the oscillatory ignition has a period-1 relaxation waveform. The limit cycle is born at a saddle-node loop and terminates via a supercritical Hopf bifurcation. For a mean residence time of 8 s there is a period-doubling to a period-2 solution followed by period-halving to quasisinusoidal period-1 oscillations. At longer residence times, more period-doublings forming a full cascade to chaos with subsequent periodic windows are observed. The chaotic attractor has an underlying single-humped next maximum map.

Oscillatory ignitions and cool flames in the oxidation of butane in a jet-stirred reactor

The oxidation of butane ([C4H10] : [ O2] = 1.13:1.00) has been studied over the temperature and pressure ranges 371 ⩽T/K ⩽ 675, 226 ⩽P/Torr ⩽ 489 in a jet stirred reactor with a residence time of 9.4 s (1 Torr ≈ 133.3 Pa), The gas temperature and pressure were probed and phase diagrams constructed delineating regions of oscillatory ignitions and cool flames, and high- and low -temperature stationary states. On heating at an initial pressure of 400 Torr from 570 K sharp transitions were observed, first to an oscillatory ignition and then to an oscillatory cool flame region, followed by a smooth transition to a high-temperature stationary state via a supercritical Hopf bifurcation. On cooling from this high - temperature stationary state, oscillatory cool flames were observed with a sharp extinction at 542 K, without any entry to the oscillatory ignition region. The latter could be entered, however, by suddenly cooling the system from the oscillatory cool flame region by temporarily substituting nitrogen for oxygen in the gas streams. Complex waveforms, consisting of bursts of oscillatory cool flames interspersed with periods of monotonic cooling, were also observed at lower pressures. A Nd : YAG pumped dye laser was used to probe laser induced fluorescence from form aldehyde in the oscillatory ignition region. Variations in the internal surface of the reactor demonstrated the significance of surface reactions. An outline mechanism, based on detailed numerical simulations, is presented to account for the shape of the ignition profiles and the transition from multiple ignitions to oscillatory cool flames.

The application of resonance enhanced multiphoton ionization to the detection of hydrogen atoms during the oscillatory combustion of hydrogen

The relative concentrations of hydrogen atoms were measured during the oscillatory ignition of hydrogen in a well stirred flow reactor. Comparisons were made with the experimental concentration—time profiles of the hydroxyl radical obtained previously under similar experimental conditions. The predicted concentration profiles obtained from numerical analysis of a thermokinetic model were also compared with the experimental results. Experiments were performed in a 600 cm3Pyrex glass, jet-stirred reactor with the reactants, 2H2+ O2, at a total pressure of 16 Torr (ca. 2132.8 Pa) and at a vessel temperature of 753 K. The mean residence time was 1.2 s. Oscillatory ignition was established at a period of 3 s in which high radical concentrations were attained and in which the temperature rise was almost adiabatic. The concentration-time profile of hydrogen atoms was obtained by a resonance enhanced multiphoton ionization (rempi) which was induced by a laser pulse at energies in the vicinity of 364 nm, with ion collection at a stainless steel probe inserted into the reactor. Supplementary studies were made to characterize the signals and to identify effects of the probe within the reaction volume. A measurement of the relative concentrations of hydrogen atoms was obtained from an integration of the area of the rempi spectrum determined over the laser wavelength range 363.8-364.6 nm. The spectrum was measured at successive times in the oscillatory cycle by imposing a variable delay on the laser firing signal. The results show that, during oscillatory ignition, the maximum concentration of hydrogen atoms was reached and a sharp decay was already well advanced before that of the hydroxyl radicals was attained. The numerical analysis was in very good quantitative accord with this experimental result. The phase difference of the cyclic variation in the H atoms relative to that of OH radicals is a key feature of the kinetic mechanisms which control the oscillatory oxidation of hydrogen.

Free radical and atom activity at a pyrex glass surface as a source of sodium D line emissions during combustion reactions

Hydrogen plus oxygen mixtures are able to undergo oscillatory ignition in a stirred-flow reactor. The temperature approaches that of the adiabatic flame during each oscillation and high free radical and atom concentrations are attained. We have measured hydroxyl radical concentrations of 10{sup {minus}8} mol cm{sup {minus}3}. The oscillatory ignition originates in a kinetic interaction between chain branching and chain termination processes, with supplementary effects on the accompanying heat release. The investigation and interpretation of the oscillatory reaction has been reported in detail elsewhere. A visible light emission also pervades the gaseous volume during ignition that corresponds to the frequencies of the sodium D lines. The emission originates from sodium atoms that are released from the Pyrex glass of the reaction vessel surface. (Similar effects are observed when Pyrex glass is held in a Bunsen burner flame.) In this paper, the authors report experimental results obtained by emission spectroscopy that link the release of the sodium atoms and their associated emission to reactions at the vessel surface involving the atoms or free radicals formed during ignition. Experimental details of the stirred-flow experiments have been described previously. Here the authors summarize only the main features.

Modelling complex oscillations for the H2+ O2 reaction in an open system

The complex waveforms observed in the oscillatory ignition of a stoichiometric H2–O2 mixture in a well stirred, continuous-flow reactor are modelled using a 35-reaction Baldwin–Walker mechanism. The behaviour of this reaction is further studied through a series of reduced mechanisms of 17, 13 and 10 steps, respectively. In each case, complex waveforms require the self-heating of this exothermic reaction to be included, whereas simple oscillations arise even with isothermal models. The methods of bifurcation theory are applied to the smallest of these models and suggest a close connection between parameter ranges for which the simple oscillations in the isothermal scheme change rapidly in period and amplitude with the conditions for complex oscillations in the full, non-isothermal model.

Hydroxyl radical concentrations and reactant temperature profiles during oscillatory ignition of hydrogen: experimental measurements by laser resonance absorption spectroscopy and comparisons with numerical calculations

Laser-induced absorption at wavelengths in the vicinity of 306 nm, at which electronic excitations of the hydroxyl radical occur, has been used to measure absolute concentrations of the hydroxyl radicals and the gas temperature during the course of the oscillatory ignition of hydrogen in a jet-stirred, flow reactor. Experiments were carried out on stoichiometric mixtures of hydrogen and oxygen at a mean residence time of 1.2 s and a total reactant pressure of 16 Torr. Oscillatory ignition occurs at vessel temperatures in excess of 710 K. Supplementary numerical studies were also carried out to compare predicted free radical concentrations and gas temperatures with the experimental measurements. A microcomputer-controlled system was used to trigger the pulsed laser each time an ignition occurred. From the study of very long trains of oscillations (greater than 1000), either the absorption spectrum could be scanned at a constant reaction time with respect to the trigger signal, or, alternatively, the temporal variation of the absorption signal at a fixed wavelength could be studied. The hydroxyl radical concentrations and the gas temperature were deconvoluted from these signals via the relative population densities of different absorption bands of the spectrum. Under the present experimental conditions the highest measured hydroxyl radical concentrations corresponded to 7.3 x 10 15 cm -3 . This is within a factor of two of the calculated concentration. The measured hydroxyl radical concentration not only rose to a maximum within 1.5 ms of the onset of ignition, but also decayed rapidly, falling to below 20% of the maximum absorption signal within ca 10 ms. The gas temperature rose simultaneously with the free radical concentration, to reach a measured maximum in excess of 2000 K, but its decay was very much slower than that of the radicals. The measured temperatures were approximately 500 K lower than those predicted numerically. The shapes of the measured temporal profiles for both concentration and temperature were in excellent agreement with the numerical calculations, and we believe that improvements of experimental technique could lead to closer quantitative accord between the absolute magnitudes.

Period doubling and chaos during the oscillatory ignition of the CO + O2 reaction

Experimental observations of period doubling accompanying the oscillatory ignition mode of carbon monoxide oxidation in a well stirred flow reactor are reported. In the presence of 0.5% added H2 and with a mean residence time of 16 s, we find oscillatory ignition for conditions above the second explosion limit. Further into this ignition region, the oscillatory waveform develops through period-2, period-4 ignitions etc., finally under-going a full ‘subharmonic cascade’ into chemical chaos. We have also observed subsequent periodic windows (period-5 and period-3). Simple oscillations are re-established via an inverse sequence back to period-1 ignitions. Our data capture allows reconstruction of the corresponding attractors and limit cycles and the Fourier power spectra.

Numerical interpretation of oscillatory glow and ignition during carbon monoxide oxidation in a well-stirred flow reactor

Numerical results are presented which give very satisfactory simulations of the oscillatory glow and ignition phenomena that accompany the thermal oxidation of carbon monoxide in a well-stirred flow vessel, when small proportions of hydrogen are present in the reactants. Quantitative comparisons are made with experimental results not only in relation to the amplitudes of oscillations but also the p–Ta regimes in which the different phenomena exist. The origins of oscillatory ignition are traced to the competition between the elementary reactions H + O2→ OH + O and H + O2+ M → HO2+ M, as occurs in the oxidation of hydrogen itself. A change in overall third body efficiencies, that results from changes in composition as reaction proceeds, controls the switch from branching to non-branching reaction. The oscillatory glow originates in a different kinetic competition namely O + H2→ OH + H and O + CO + M → CO2+ M. In this case the switch from a branching to non-branching reaction is governed by the change in ratio of [H2]/[CO] as reaction proceeds. The prediction of the oscillatory phenomenon is sensitive to the magnitude of the rate constant for termination.

Stationary-state and oscillatory combustion of hydrogen in a well-stirred flow reactor

The existence of stationary-state and oscillatory ignition phenomena is established in hydrogen oxidation in a jet-stirred, Pyrex glass reactor (0.55 dm 3). The experimental observations and their numerical interpretation from a detailed kinetic mechanism are reported. Compositions containing 2H 2 + O 2, H 2 + O 2, and H 2 + 2O 2 were investigated at total pressures mainly in the range 10–20 Torr, with brief excursions up to 80 Torr. The vessel temperature was varied between 670 and 790K and mean residence times were controlled in the range 0.7–8.0 s. Reactant temperatures were measured using a very fine thermocouple in the vessel and light output was monitored by photomultiplier. The experimental conditions straddle events at the second limit. Thus criticality and ignition occurs as the vessel temperature is raised under continuous flow of reactants. Ignition is an oscillatory phenomenon at low pressures, and adiabatic temperature changes occur momentarily during it. Increases of vessel temperature cause oscillatory amplitudes to fall and their frequencies to rise. Eventually the oscillations dwindle to a stable stationary state. In certain circumstances there can be an abrupt transition from low frequency to high frequency oscillations. Detailed investigations show that there can be reaction multiplicity and even birhythmicity in the vicinity of these transition points. The dependences of oscillatory reaction on pressure, temperature, and composition are investigated. The elementary kinetic scheme adopted for a numerical integration of the mass and energy conservation equations comprises 8 species in 33 reactions and is designed to embrace subcritical, “slow reaction” and high temperature ignition phenomena. The effects of third body efficiencies are taken into account where these are established quantitatively and enthalpy change is computed at each step. Heat loss rates are computed using an appropriate heat transfer coefficient at the vessel walls. Excellent agreement between the numerical and experimental results is obtained over wide ranges of conditions. Even complex phenomena such as oscillations of mixed frequency and the existence of hysteresis including birhythmicity are predicted numerically. The oscillatory phenomena that occur at the transition to ignition at the second ( p- T a) limit are due to chain branching coupled to product inhibition. Temperature change is an accompaniment and augments the effect, but it does not play an essential role. By contrast, the origins of the additional complexities associated with reaction multiplicity are thermokinetic in kind.

Third-body interactions in the oscillatory oxidation of hydrogen in a well stirred flow reactor

Stationary and oscillatory ignition states have been studied in mixtures of hydrogen and oxygen under flowing conditions in a non-adiabatic, well stirred vessel. The conditions bridge the region between ‘slow reactions’ and ignition at the second (p–Ta) limit. An oscillatory mode of ignition is observed at the boundary; it gives way to a stable state corresponding to complete reactant consumption at higher temperatures within the ignition peninsula. The period of oscillations is governed by the composition of the reaction mixture. The effects on the period of addition of diluents (He, Ar, N2, CO, CO2 and C3F8) and changes in the H2/O2 ratio have been studied by monitoring changes in temperature and light output from the system. The global behaviour of these systems can be modelled successfully using the accepted H2/O2 reaction scheme. Comparison of model predictions and experimental findings leads to values of the third-body efficiencies of He, Ar, N2, CO, CO2 and C3F8 relative to H2 of 0.35, 0.35, 0.45, 0.40, 0.90 and 2.90, respectively, in the reaction H + O2+ M → HO2+ M. With some mixtures hysteresis and birhythmicity are observed in their oscillatory behaviour; these effects too can be modelled successfully.

Isothermal interpretations of oscillatory ignition during hydrogen oxidation in an open system II. Numerical analysis

Numerical methods are used to simulate oscillatory ignition under conditions resembling those associated with the second ignition limit for equimolar hydrogen and oxygen mixtures flowing through a well stirred reactor. The starting point is a scheme comprising 17 elementary reactions coupled to heat release and dissipation. The system of equations is then simplified without loss of the major qualitative features of the predicted time-dependent phenomena, first by eliminating the temperature change, so yielding an isothermal branching system, and then by reducing the kinetic scheme to seven elementary steps as follows: H 2 + O 2 → radicals, OH + H 2 → H 2 O + H, H + O 2 → OH + O, O + H 2 → OH + H, H + O 2 + M → HO 2 + M, HO 2 + H → H 2 O + O, HO 2 → inert. This scheme constitutes the minimum viable kinetic foundation from which isothermal chain-branching oscillations are predicted in H 2 + O 2 under flowing conditions. Its features are analysed and the role of each reaction explained. The change in overall third-body efficiency of M leading to HO 2 formation, as the composition within the reactor varies under flowing conditions, is a key kinetic prerequisite to oscillations. The interaction between H and HO 2 that follows is an essential additional feature. The overall third-body efficiency is highest immediately after ignition, when the concentration of water is highest. The displacement of water by inflowing hydrogen and oxygen reduces the overall efficiency, and hence the effective rate constant of the termolecular step relative to that for the branching reactions. This condition is in complete accord with the analytical prediction of periodicity presented in the previous paper.