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A priori analysis of sub-grid variance of a reactive scalar using DNS data of high Ka flames

Published version
Peer-reviewed

Type

Article

Change log

Authors

Nilsson, T 
Langella, I 
Doan, NAK 
Swaminathan, N 
Yu, R 

Abstract

Direct numerical simulations (DNS) of low and high Karlovitz number (Ka) flames are analysed to investigate the behaviour of the reactive scalar sub-grid scale (SGS) variance in premixed combustion under a wide range of combustion conditions (regimes). An order of magnitude analysis is performed to assess the importance of various terms in the variance evolution equation and the analysis is validated using the DNS results. This analysis sheds light on the relative behaviour among turbulent transport and production, scalar dissipation and chemical processes involved in the evolution of the SGS variance at different Ka. The common expectation is that the variance equation shifts from a reaction-dissipation balance at low Ka to a production–dissipation balance at high Ka with diminishing reaction contribution. However, in large eddy simulation (LES), a high Ka alone does not make the reaction term negligible, as the relative importance of the reaction term has a concurrent increase with filter size. The filter size can be relatively large compared with the Kolmogorov length scale in practical LES of high Ka flames, and as a consequence a reaction–production–dissipation balance may prevail in the variance equation even in a high Ka configuration, and this possibility is quantified using the DNS analysis in this work. This has implications from modelling perspectives, and therefore two commonly used closures in LES for the SGS scalar dissipation rate are investigated a priori to estimate the importance of the above balance in LES modelling. The results are explained to highlight the interplay among turbulence, chemistry and dissipation processes as a function of Ka.

Description

Keywords

scalar variance, scalar dissipation rate, turbulent premixed flame, direct numerical simulation, high Karlovitz number

Journal Title

Combustion Theory and Modelling

Conference Name

Journal ISSN

1364-7830
1741-3559

Volume Title

Publisher

Taylor & Francis
Sponsorship
European Commission Horizon 2020 (H2020) Industrial Leadership (IL) (686332)
The authors at LU acknowledges funding from the Swedish Research Council (Vetenskapsrådet) (VR) and the National Centre for Combustion Science and Technology (CeCOST). IL and NS acknowledge funding from the Clean Sky 2 Joint Undertaking under the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No 686332. NAKD acknowledges the support of the Qualcomm European Research Studentship Fund in Technology.