Simulation of Spherically Expanding Turbulent Premixed Flames

Abstract

Statistically spherical expanding turbulent premixed flames are computed using an unsteady Reynolds-averaged Navier-Stokes (URANS) approach. Mean reaction rate is closed using strained and unstrained flamelet models and an algebraic model. The flamelets are parametrized using the scalar dissipation rate in the strained flamelet model. It is shown that this model is able to capture the measured growth rate of methane-air turbulent flame ball, which is free of thermo-diffusive instability. The spherical flames are observed to accelerate continuously. The flame brush thickness grows in time and the role of turbulent diffusion on this growth seems secondary compared to the convection due to the fluid velocity induced by the chemical reaction. The spherical flames have larger turbulent flame speed, the leading-edge displacement speed s t , compared to the planar flames for a given turbulence and thermochemical condition. The computational results suggest s t =s 0 L $ Re n t with 0.57 n 0.58, where Re t is the turbulence Reynolds number and s 0 L is the unstrained planar laminar flame speed, for both spherical and planar flames.

Large Eddy Simulations of a turbulent premixed swirl flame using an algebraic scalar dissipation rate closure

“…The SDR based closure is a flamelet based methodology of turbulent premixed combustion modelling which was demonstrated to perform well based on both a priori [6,7] and a posteriori [8,[33][34][35] analyses in the past for turbulent premixed flames but its performance in turbulent stratified combustion in a complex swirl flame configuration is assessed for LES for the first time in this analysis. In this paper, the performance of SDR based closure is compared to the predictions of the algebraic FSD closures proposed by Fureby [11] and Muppala et al [12], which were shown to perform better than several other alternative FSD models in a previous a posteriori assessment [13].…”

“…Furthermore, the RANS model for SDR, which has been extended for LES to obtain Eq. (8i), was a posteriori assessed based on actual RANS simulations for several practical burners and laboratory-scale configurations in the past and satisfactory agreement with experimental observations was obtained [33][34][35].…”

A Priori Assessment of Scalar Dissipation Rate Closure for Large Eddy Simulations of Turbulent Premixed Combustion Using a Detailed Chemistry Direct Numerical Simulation Database

“…A number of analyses concentrated on SDR closures for premixed turbulent combustion in the context of RANS (Ahmed and Swaminathan, 2013;Chakraborty et al, 2008a;Dong et al, 2013;Kolla et al, 2009;Swaminathan, 2010a, 2010b;Mantel and Borghi, 1994;Mura and Borghi, 2003;Mura et al, 2008Mura et al, , 2009Sadasivuni et al, 2012) but limited effort has been directed to the closure of SDR for LES (Butz et al, 2015;Dunstan et al, 2013;Gao et al, 2014aGao et al, , 2014bGao et al, , 2015aGao et al, , 2015bGao and Chakraborty, 2016;Langella et al, 2015;Ma et al, 2014a). Recently, Gao et al (2014aGao et al ( , 2014bGao et al ( , 2015a proposed algebraic closures ofÑ c for both static and dynamic model coefficients and assessed their validity by extracting the relevant quantities from explicitly filtered DNS data (i.e., a priori assessment).…”

Unstrained and strained flamelets for LES of premixed combustion

“…A closure similar to that in Equation (7) with corrections for non-unity Lewis numbers has also been tested in DNS [19,49] and used in LES [20,21] studies. It has been reasonably established in past RANS studies [32,47,[50][51][52][53] that the above parameters and their values for Equation (7) are not arbitrary, and various terms in Equation (7) are closely related to certain physical aspects of the scalar dissipation rate transport [47,54]. The terms involving (K c s L /δ th ) and (C 3 − τ C 4 Da ) (u / ) arise due to fluctuating dilatation and strain rate resulting from competing effects of turbulence and heat release, respectively.…”