A numerical study of cool flame development under microgravity

Fairlie, R.; Griffiths, J. F.; Pearlman, Howard

doi:10.1016/s0082-0784(00)80569-x

Cited by 15 publications

(10 citation statements)

References 18 publications

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“…Further peaks of decreasing amplitude in the temporal trace of T g,max are observed after ignition of the secondary cool flame, until a plateau is eventually reached; these oscillations can be explained again as the consequence of rapid depletion and formation of ketohydroperoxides at rich mixture compositions, which result in subsequent ignitions and spatial propagations of progressively weaker cool flames. The mechanism for cool flame oscillations described here is somewhat similar the one observed in homogeneous mixtures[26], where a phase relationship between temperature and concentration of a peroxydic, molecular branching intermediate is observed.The cool-flame ignition case does not result in hot flame ignition after the cool flame appearance, differently from what observed for the low-temperature ignition case. To better understand this point, we plotted inFigure 6the budget of terms appearing in the gas-phase energy equation at t = 1.2 s. For the sake of the present analysis, five different contributions (including the unsteady term) to the energy balance were identified from Equation 4:…”

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confidence: 76%

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Borghesi

Mastorakos

2016

Flow Turbulence Combust

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Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, twostage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the hightemperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.

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confidence: 76%

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Borghesi

Mastorakos

2016

Flow Turbulence Combust

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“…A one-dimensional five-step, skeletal Gray-Yang model that includes diffusion of species and heat [9] has been extended to two-dimensions to include the effects of natural convection. The model is summa- Table 1 The skeletal Gray-Yang five-step modified mechanism adopted from [9] Five-step modified Gray-Yang reaction mechanism…”

Section: A Two-dimensional Five-step Gray-yang Modelmentioning

confidence: 99%

Cool flames at terrestrial, partial, and near-zero gravity

Foster

Pearlman

2006

Combustion and Flame

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Natural convection plays an important role in all terrestrial, Lunar, and Martian-based, unstirred, static reactor cool flame and low-temperature autoignitions, since the Rayleigh number (Ra) associated with the self-heating of the reaction exceeds the critical Ra (approximately 600) for onset of convection. At near-zero gravity, Ra < 600 can be achieved and the effects of convection suppressed. To systematically vary the Ra without varying the mixture stoichiometry, reactor pressure, or vessel size, cool flames are studied experimentally in a closed, unstirred, static reactor subject to different gravitational accelerations (terrestrial, 1g; Martian, 0.38g; Lunar, 0.16g;and reduced gravity, ∼10 −2 g). Representative results show the evolution of the visible light emission using an equimolar n-butane:oxygen premixture at temperatures ranging from 320 to 350 • C (593-623 K) at subatmospheric pressures. For representative reduced-gravity, spherically propagating cool flames, the flame radius based on the peak light intensity is plotted as a function of time and the flame radius (and speed) is calculated from a polynomial fit to data. A skeletal chemical kinetic Gray-Yang model developed previously for a one-dimensional, reactivediffusive system by Fairlie and co-workers is extended to a two-dimensional axisymmetric, spherical geometry. The coupled species, energy, and momentum equations are solved numerically and the spatio-temporal variations in the temperature profiles are presented. A qualitative comparison is made with the experimental results.

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“…Such experiments are not possible at terrestrial conditions since the pressure and the vessel size cannot be reduced to the extent needed to decrease the Rayleigh number (Ra) below the critical value associated with the onset of buoyant convection (Ra cr % 600) (Yang and Gray, 1969b; Barnard and Harwood, 1974;Fairlie et al, 2000;Pearlman, 2000;Cardoso et al, 2004a,b;Campbell et al, 2006;Foster and Pearlman, 2006). As a result, static reactor cool flame and ignition experiments at terrestrial conditions are inherently multi-dimensional due to buoyant convection.…”

Section: Introductionmentioning

confidence: 99%

“…To model the reactive-diffusive structure of cool flames and ignitions in closed reactors, models of various fidelities have been reported; these models have been based on the two-step Sal'nikov mechanism (Cardoso et al, 2004a,b;Campbell et al, 2006), the Gray-Yang mechanism Gray, 1969a, 1969b;Fairlie et al, 2000), and a reduced propane oxidation mechanism (58 species, 378 reactions) (Fairlie et al, 2004). The Sal'nikov model is the simplest, includes thermal feedback, and has been used successfully to predict the temperature and the species concentration distributions associated with low temperature oxidation and cool flames.…”

Section: Introductionmentioning

confidence: 99%

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The Role of Diffusive Transport on Low and Intermediate Temperature Hydrocarbon Oxidation: Numerical Simulations Using the Wang-Mou Mechanism

Pearlman

Foster

2007

Combustion Science and Technology

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The spatio-temporal temperature and species concentration distributions associated with low and intermediate temperature hydrocarbon oxidation are computed using a global thermokinetic scheme augmented with diffusive transport. The scheme used for the computations was proposed by Wang and Mou and is extended to include diffusion of species and heat. The conservation equations for species and energy are then derived and solved for a one-dimensional and an axisymmetric, spherical domain for temperatures ranging from 540 to 660 K at subatmospheric pressures. The predictions are then used to develop ignition diagrams for different Lewis (Le) numbers. Increasing Le is found to promote oscillatory cool flames and two-stage ignition in the one-dimensional model, while the ratio of the mass diffusivity of the parent fuel to that associated with the autocatalytic chain carrier has a negligible effect on the structure of the ignition diagrams. In the spherical model, oscillatory cool flames and two-stage ignition were also predicted albeit at lower values of the Le.

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A numerical study of cool flame development under microgravity

Cited by 15 publications

References 18 publications

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Cool flames at terrestrial, partial, and near-zero gravity

The Role of Diffusive Transport on Low and Intermediate Temperature Hydrocarbon Oxidation: Numerical Simulations Using the Wang-Mou Mechanism

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