Proceedings of the Australian Combustion Symposium November 6-8, 2013, The University of Western Australia __________________________ * Corresponding author: Phone: (+61) 2 93856196 Email: s.karami@unsw.edu.au - 1 - A numerical study of species transport budgets in a turbulent lifted flame Shahram Karami 1, * , Mohsen Talei 2 , Evatt R. Hawkes 1,2 1 School of Photovoltaic and Renewable Energy The University of New South Wales, NSW 2052, Australia 2 School of Mechanical Engineering The University of New South Wales, NSW 2052, Australia Abstract A turbulent lifted slot-jet flame is studied using direct numerical simulation (DNS). A one-step chemistry model was employed with a mixture-fraction dependent activation energy which can reproduce qualitatively the dependence of laminar burning rate on equivalence ratio that is typical of hydrocarbon fuels. Terms in the transport equation of Favre- averaged product mass fraction were examined to determine their relative contributions and thus help understand the structure of the flame and its mechanism of stabilisation. It was found that the leading edge has features which are expected of a turbulent premixed flame, namely streamwise convection balancing turbulent transport upstream; however transverse mean convection due to entrainment and transverse turbulent transport out of the flame were both significant. Downstream, reaction became a dominant term and the flame has features expected of a non-premixed jet flame. The entrainment flows were found to be significant, and divergence of streamlines around the flame base was found to lead to local mean velocities that were actually in the upstream direction, a feature which may be dynamically important in the flame stabilisation. Keywords: Direct numerical simulation (DNS), partially premixed lifted flame, turbulence. 1. Introduction Many burners feature fuel injection into the combustion chamber at a velocity high enough to preclude rim stabilization. Excessive turbulent stretching at the nozzle exit locally quenches the flame and leads to lift-off and stabilisation of the flame some- where downstream of the nozzle where velocities and flame stretch rates are less severe. One of the earliest stabilisation theories proposed by Vanquickenborne and van Tiggelen [1] is referred to as the premixed theory and is based on premixedness of the reactant prior to the flame base. Upstream of the flame base in lifted flames, mixing occurs due to the presence of turbulence leading to formation of a mixture of fuel and oxidiser with an equivalence ratio within the flammability limits. According to the premixed theory, the mixture will burn with approximately the laminar flame speed and propagate towards the nozzle. Stabilisation will occur when the fresh gas velocity balances the propagation velocity of the flame base. Blow-out will occur when the flame propagation velocity cannot resist the flow velocity[1]. Recent advancements in computational technology have made it possible to model turbulent lifted flames with direct numerical simulation (DNS) using detailed chemistry, however the computational cost limits three- dimensional DNS studies to few cases [2-6]. Single step chemistry is expected to be able to capture the basic behaviour of conventional fuels with significantly reduced computational cost, thus allowing larger do- mains, greater Reynolds numbers, or wider parametric studies. In our previous studies [7, 8], DNS was used to model turbulent lifted flames in hot and cold co-flow oxidisers. The general structure of these flames was analysed using the flame index and conditional variables. However, a more in-depth analysis can be performed using the species transport budgets of mean convection, turbulent transport, molecular diffusion and reaction. The magnitude of these different terms can provide information about the flame structure and the stabilisation mechanism in the vicinity of flame base. This analysis is therefore the aim of this study. 2. Governing Equations The conservation equations of mass, momentum, sensible energy and species mass fraction were solved in a non-dimensional form. These equations are non- dimensionalised using the inlet jet width, H, the speed of sound, a ref , temperature, T in , and thermodynamic properties on the jet centerline at the inlet. For a single- step irreversible reaction of F +rO→ (1+r)P where r is the stoichiometric ratio, the source terms take the following non-dimensional forms:                        and           (1) where Da is the non-dimensionalisation Damköhler number, α and τ=α/(α−1) are the heat-release parameters, T=        is the non-dimensional