Asymptotic Analysis of the Structure and Extinction of Spherically Symmetrical n-Heptane Diffusion Flames

Card, J.M.; Williams, Forman A.

doi:10.1080/00102209208951847

Cited by 32 publications

(4 citation statements)

References 17 publications

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“…This model relied on thermal feedback to provide autoignition and consequently could not predict negative-temperature-coefficient (NTC) behavior. Despite this shortcoming, the global scheme has been employed in a number of modeling studies of n -heptane combustion. − Other reported global models for n -heptane involving three to six steps ,− are limited to high temperatures and thus not suitable for ignition predictions. The global modeling approach is an attractive option because of its computational efficiency.…”

Section: Introductionmentioning

confidence: 99%

Evaluation of a Semiglobal Approach for Modeling Methane/n-Heptane Dual-Fuel Ignition

et al. 2021

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Because of the relatively high autoignition temperature of natural gas, its use in conventional diesel engines requires a pilot fuel, typically diesel oil, to promote ignition. To conduct computational fluid dynamics (CFD) simulations of the combustion in such engines, there is a need for dual-fuel combustion models that achieve a balance between computational efficiency and accuracy. This study investigates a semiglobal approach for modeling ignition of n-heptane/methane mixtures. In the semiglobal approach, the heptane oxidation is modeled by using a four-step global scheme from Müller, Peters, and Liñán (MPL) (1992), while the methane chemistry is described by a skeletal mechanism with 22 species, derived from a detailed reaction mechanism. The resulting semiglobal model includes 25 species and 138 reactions. Both the global heptane mechanism and the merged semiglobal dual-fuel model are validated against a wide range of ignition delay data from shock tubes as well as through comparison with predictions of the detailed heptane mechanism by Zhang et al. (2016). The MPL model cannot fully capture the ignition delay across the negative temperature coefficient (NTC) region for an n-heptane/air mixture. Despite this shortcoming, the ignition delay at high pressure (38–55 atm) is predicted typically within a factor of 2 compared to experiments. Under dual-fuel conditions with methane as the main fuel, the NTC behavior is less pronounced. Methane ignition is promoted by heat release from the n-heptane oxidation rather than by any direct chemical interaction. Predictions of ignition delays using the combined global/skeletal model are in good agreement with the n-heptane/methane measurements reported by Schuh et al. (2019; 60 atm, 785–1284 K), while at the higher temperatures and lower pressures of the experiments of Liang et al. (2019; 10 atm, 1257–1763 K), predictions are accurate within a factor of 2 for methane contents of 90% and higher. The present results indicate that it is possible with a small combined model to predict the ignition delay for methane/n-heptane mixtures with sufficient accuracy for practical use. The semiglobal model approach can thus be employed in CFD simulations, facilitating development of sustainable dual-fuel engines as well as identification of optimal fuel compositions and conditions.

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Section: Introductionmentioning

confidence: 99%

Evaluation of a Semiglobal Approach for Modeling Methane/n-Heptane Dual-Fuel Ignition

et al. 2021

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“…To estimate the characteristic chemical timescale for reaction processes in the present problem, τ c , one can use diffusion flamelet theory from Card and Williams [45], for example. In this analysis, τ c takes the form τc.=Zst2(1−Zst2)2/χq, where Z st is the stoichiometric mixture fraction and χ q is the scalar dissipation rate given as χq=aqπexp(−2(erfc−1(2Zst))2), where a q , is of the order ≈ 700 s −1 for ethanol [46] and is of the order ≈ 850 s −1 for FT/JP-8 [47].…”

Section: Resultsmentioning

confidence: 99%

“… * Calculated for T ≈ 295 K and P ≈ 101,325 Pa + Calculated for T = ( T adia. + T b,mean )/2 and P ≈ 101,325 Pa − Estimated using δ f ,mean at θ = 90° for unforced, continually-fed droplet combustion ⋆ Estimated using diffusion flamelet theory presented in [45] † For C 10 H 22 ‡ For C 12 H 26 …”

Section: Figurementioning

confidence: 99%

Periodic partial extinction in acoustically coupled fuel droplet combustion

Bennewitz

Valentini

Plascencia

et al. 2018

Combustion and Flame

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This experimental study explored the response of burning liquid fuel droplets to one-dimensional acoustic standing waves created within a closed, atmospheric waveguide. Building upon prior droplet combustion studies quantifying mean and temporal flame response of several alternative fuels to moderate acoustic excitation (Sevilla-Esparza, et al., Combustion and Flame, 161(6):1604–1619, 2014), the present work focused on higher amplitude acoustic forcing observed to create periodic partial extinction and reignition (PPER) of flames enveloping the droplet. Detailed examination of ethanol droplets exposed to a range of acoustic forcing conditions (frequencies and amplitudes in the vicinity of a pressure node) yielded several different combustion regimes: one with sustained oscillatory flames, one with PPER, and then full extinction at very high excitation amplitudes. Phase-locked OH* chemiluminescence imaging and local temporal pressure measurements allowed quantification of the combustion-acoustic coupling through the local Rayleigh index. Similar behavior was observed for JP-8 and liquid synthetic fuel derived via the Fischer-Tropsch process, but with quantitative differences based on different reaction time scales. Estimates of the mean and oscillatory strain rates experienced by the flames during excitation assisted with interpreting specific relationships among acoustic, chemical, and fluid mechanical/straining time scales that can lead to a greater understanding of PPER.

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“…The reduced chemical-kinetic mechanism developed by Bui-Pham and Seshadri (1991) was employed by Card and Williams (1992a) in their rate-ratio asymptotic analysis of the structure and extinction of a spherically symmetrical nonpremixed¯ame that surrounds an n-heptane fuel droplet. The spherically symmetrical¯ame was found to extinguish if the diameter of the fuel droplet is less than a critical value.…”

Section: Introductionmentioning

confidence: 99%