Measurements of ignition probability in turbulent non-premixed counterflow flames

Flow Turbulence Combust

2007

Self Cite

104

The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations. It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface. By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich mixture is engulfed within the hot gas kernel.

Section: Non-local Effects Of Ignition In Inhomogeneous Mixturessupporting

confidence: 92%

Section: Introductionsupporting

confidence: 65%

Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient

Chakraborty

Flow Turbulence Combust

2007

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104

“…The Favre-averaged quantities can be recovered by integrating the conditional averages over mixture fraction space. Recent evidence from experiments with PLIF of acetone in an inert counterflow suggest that the β-function PDF is a very good approximation for the mixture fraction PDF [45].…”

Section: Cmc Modelmentioning

confidence: 99%

“…Geyer et al [9,10] compared Large Eddy Simulations with their opposed jet experiment and found good agreement when using a modified turbulent Schmidt number of 0.4-0. 45. Further LES calculations of flames with a steady flamelet model for the sub-grid reaction rates [17] observed localized extinction events on the centreline, however these extinction events are too sparse to allow for conclusions on the global extinction process.…”

Section: Introductionmentioning

confidence: 99%

Simulations of Turbulent Non-Premixed Counterflow Flames with First-Order Conditional Moment Closure

Kim

Flow Turbulence Combust

2006

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Simulations of turbulent CH 4 -air counterflow flames are presented, obtained in terms of zero and two-dimensional first-order Conditional Moment Closure (CMC) to study the flame structure and extinction limits. The CMC equation with detailed chemistry is solved without the need for operator splitting, while the accompanying flow field is determined using a commercial CFD software employing a Reynolds stress turbulence model and additional transport equations for the turbulent scalar flux and for the mean scalar dissipation rate. Two detailed chemical mechanisms and different conditional scalar dissipation rate models have been examined and small differences were found.The first-order CMC captures the overall structure of the counterflow flame accurately for the unconditional averages. The calculated conditional averages behave as if the scalar dissipation rate were under-predicted, although a comparison with measurement of the conditional scalar dissipation rate is reasonable. The calculated extinction velocity is found to be much higher than the experimental value, but the trend of increasing extinction velocity with air dilution of the fuel stream is captured well. The discrepancies with the data are mostly attributed to the neglect of conditional fluctuations.

“…Spark ignition has also been studied experimentally for non-premixed flames in various geometries (Ahmed et al, 2007a(Ahmed et al, , 2007bAhmed and Mastorakos, 2006 and through Large Eddy Simulation (LES) (Bulat et al, 2013;Jones and Prasad, 2011;Jones et al, 2012;Lacaze et al, 2009;Subramanian et al, 2010;Triantafyllidis et al, 2009). Moreover, low-order ignition models providing lower computational cost have been developed in order to provide easier prediction of the ignition behavior in non-premixed and spray single burners (Eyssartier et al, 2013;Neophytou et al, 2012).…”

Section: Introductionmentioning

confidence: 99%

Numerical Investigation of the Stochastic Behavior of Light-Round in Annular Non-Premixed Combustors

Machover

Combustion Science and Technology

2017

The ignition behavior of a non-premixed multiple-burner annular combustion chamber was investigated numerically, focusing on the stochasticity and average speed of the light-round mechanism ensuring flame propagation from burner to burner that have been observed experimentally. During the propagation sequence, the flame expansion process is tracked by a previously developed stochastic low-order ignition model adapted to full combustor ignition. A stochastic model based on the probability that a flame fragment coming from an ignited burner leads to successful ignition of the next un-ignited one is developed in order to explain and quantify the global ignition behavior of the combustor. The stochastic behavior of the rig, highlighted through the experimentally observed variability of the burner-to-burner propagation times during the ignition sequence, was clarified and quantified. The lean light-round ignition limiting conditions and the mean light-round speed measured experimentally are explained and reasonably accurately predicted, demonstrating the validity of the use of the probabilistic model together with the low-order ignition model for the combustor considered. The results presented in this paper can be used to predict the ignition envelope of annular gas turbines combustors at the design stage.ARTICLE HISTORY