Edge flame structure in a turbulent lifted flame: A direct numerical simulation study

Karami, Shahram; Hawkes, Evatt R.; Talei, Mohsen; Chen, Jacqueline H.

doi:10.1016/j.combustflame.2016.03.006

Cited by 46 publications

(45 citation statements)

References 101 publications

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“…In this study, direct numerical simulations (DNS) of jet premixed flames with a single step chemistry model are carried out using a modified version of the numerical solver S3D [Chen et al, 2009] known as S3D-SC [Karami et al, 2015[Karami et al, , 2016a. This solver features an 8 th order central differencing scheme for spatial derivatives, combined with a 6-stage, 4 th order explicit Runge-Kutta time integrator.…”

Section: Numerical Solvermentioning

confidence: 99%

Sound generation by turbulent premixed flames

et al. 2018

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This paper presents a numerical study of the sound generated by turbulent, premixed flames. Direct numerical simulations (DNS) of two round jet flames with equivalence ratios of 0.7 and 1.0 are first carried out. Single-step chemistry is employed to reduce the computational cost, and care is taken to resolve both the near and far fields and to avoid noise reflections at the outflow boundaries. Several significant features of these two flames are noted. These include the monopolar nature of the sound from both flames, the stoichiometric flame being significantly louder than the lean flame, the observed frequency of peak acoustic spectral amplitude being consistent with prior experimental studies and the importance of so-called ‘flame annihilation’ events as acoustic sources. A simple model that relates these observed annihilation events to the far-field sound is then proposed, demonstrating a surprisingly high degree of correlation with the far-field sound from the DNS. This model is consistent with earlier works that view a premixed turbulent flame as a distribution of acoustic sources, and provides a physical explanation for the well-known monopolar content of the sound radiated by premixed turbulent flames.

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Section: Numerical Solvermentioning

confidence: 99%

Sound generation by turbulent premixed flames

et al. 2018

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“…presents the means and scatter of S d and its components, conditioned upon χ. Similar to tribrachial edge flames at atmospheric conditions[62][63][64][65][66], increasing χ reduces S d . This trend is explained by the increase in the magnitude of the S d C term, as was previously concluded in Ref.[65].…”

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confidence: 91%

Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation

Krisman

Hawkes

Talei

et al. 2016

Combustion and Flame

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With the goal of providing a more detailed fundamental understanding of ignition processes in diesel engines, this study reports analysis of a direct numerical simulation (DNS) database. In the DNS, a pseudo turbulent mixing layer of dimethyl ether (DME) at 400 K and air at 900 K is simulated at a pressure of 40 atmospheres. At these conditions, DME exhibits a two-stage ignition and resides within the negative temperature coefficient (NTC) regime of ignition delay times, similar to diesel fuel. The analysis reveals a complex ignition process with several novel features. Autoignition occurs as a distributed, two-stage event. The high-temperature stage of ignition establishes edge flames that have a hybrid premixed/autoignition flame structure similar to that previously observed for lifted laminar flames at similar thermochemical conditions. A combustion mode analysis based on key radical species illustrates the multi-stage and multi-mode nature of the ignition process and highlights the substantial modelling challenge presented by diesel combustion.

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“…The DNS database of a turbulent lifted slot-jet flame [1,18,19] is used for the presented analysis. For an orientation, a brief description of key details of the DNS dataset is reported here.…”

Section: Configuration and Numerical Methodsmentioning

confidence: 99%

Local extinction and reignition mechanism in a turbulent lifted flame: A direct numerical simulation study

Karami

Talei

Hawkes

et al. 2017

Proceedings of the Combustion Institute

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Local extinction and reignition is studied in a direct numerical simulation (DNS) dataset of a turbulent lifted flame [1]. Extinction holes are identified as regions on the stoichiometric surface which have a product mass fraction less than a critical value. Using this criterion, thirty individual holes are identified and tracked in time. It is observed that large outwardly pushing structures caused compressive strain rates normal to the mixture-fraction iso-surface. These high strain rates caused high dissipation rates and the initiation of the extinction process, leading to the initial creation of holes. Extinction, i.e. hole growth, then occurs in two phases. In the first phase, the edge-propagation velocity is initially negative and the fluid dynamic tangential strain rate on the hole surface is positive, leading to rapid hole growth. Subsequently, in the second phase, local compressive strain rates at the flame edge relax, and the edge-flame propagation velocity switches to positive. However, in this second phase, the hole continues to expand because of positive tangential strain rate on the hole's surface, which dominates over the healing effect of positive edge-flame propagation velocities. When reignition starts, the edge-propagation velocity is mainly affected by the product-mass fraction displacement speed and shows a dependency on curvature and scalar dissipation rate, similar to what is expected in edge-flame propagation. An analysis of thermal diffusion on the unburned portion of the mixture-fraction iso-surface shows that the edge-flame propagation mechanism dominates turbulent engulfment during reignition in this study. AbstractLocal extinction and reignition is studied in a direct numerical simulation (DNS) dataset of a turbulent lifted flame [1]. Extinction holes are identified as regions on the stoichiometric surface which have a product mass fraction less than a critical value.Using this criterion, thirty individual holes are identified and tracked in time. It is observed that large outwardly pushing structures caused compressive strain rates normal to the mixture-fraction iso-surface. These high strain rates caused high dissi-* pation rate, similar to what is expected in edge-flame propagation. An analysis of thermal diffusion on the unburned portion of the mixture-fraction iso-surface shows that the edge-flame propagation mechanism dominates turbulent engulfment during reignition in this study.

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Edge flame structure in a turbulent lifted flame: A direct numerical simulation study

Cited by 46 publications

References 101 publications

Sound generation by turbulent premixed flames

Sound generation by turbulent premixed flames

Characterisation of two-stage ignition in diesel engine-relevant thermochemical conditions using direct numerical simulation

Local extinction and reignition mechanism in a turbulent lifted flame: A direct numerical simulation study

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