Abstract
The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.
Highlights
Any combustion process with imperfectly mixed reactants requires mixing at the molecular level of fuel and oxidiser, as well as energy transport from the reacting region to the unburnt reactants, for the flame to develop
These models have not been validated yet against DNS data and the present paper focuses on different implementations of the models that are in practical use
The case is taken from a larger database (Turquand d’Auzay and Chakraborty 2020), which has been used to analyse the effects of the location of the ignitor in terms of the mean mixture composition, flammability factor and mixture fraction gradient on localised forced ignition of a planar turbulent jet
Summary
Any combustion process with imperfectly mixed reactants requires mixing at the molecular level of fuel and oxidiser, as well as energy transport from the reacting region to the unburnt reactants, for the flame to develop. In presumed PDF models, the SDR controls the rate at which the variance of the resolved conserved scalar decays in time; the SDR affects directly the local flame structure in advanced turbulent combustion models such as the conditional moment closure (CMC) (Klimenko and Bilger 1999) as well as flamelet-based approaches (Peters 1984). Several models have been been proposed so far to model the conditional SDR, including the amplitude mapping closure (AMC) (O’Brien and Jiang 1991) and models derived from the PDF transport equations (Girimaji 1992; Kronenburg et al 2000; Devaud et al 2004) These models were originally developed for RANS applications whereas, in LESCMC, much simpler approaches have been pursued (Navarro-Martinez et al 2005; Triantafyllidis and Mastorakos 2010), based on the assumption that conditional moments change slowly in space and using conditional volume averaging. The objectives are: (i) to assess the modelling of Favre-filtered SDR based on linear relaxation and determine the optimal value of the constant CN ; (ii) to evaluate different formulations for the conditionally-filtered SDR
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