Simulations of edge-flame propagation in turbulent non-premixed jets

Ignition, flame propagation and stabilisation have been simulated and analysed in a turbulent jet of non-premixed methane and air. The first order Conditional Moment Closure (CMC) turbulent combustion model was fully coupled with a Reynolds-Averaged Navier Stokes (RANS) flow simulation. A CMC model was developed to account for spark ignition. The over-prediction of turbulent flame propagation was attributed to the limitations of the first order reaction rate closure, and of the RANS description of the flow in the presence of thermal expansion around the flame front. A new model for the effects of counter gradient turbulent transport in partially premixed flows was implemented and the modification of the flame front was presented. The coupled CMC-CFD model successfully captures the physics necessary to represent unsteady flame evolution and hence may be used for simulation of ignition in practical combustor designs.  Corresponding author: [email protected] Cambridge University Engineering Department, Trumpington Street, Cambridge CB2 1PZ, UK; http://www.eng.cam.ac.uk/~em257/ Proceedings of the European Combustion Meeting 2007 Introduction Deeper understanding of forced ignition and flame propagation is needed by researchers developing modelling for the design of industrial burners. The ability to model ignition of non-premixed flow is of particular interest to manufacturers of aviation gas turbines who must satisfy certification bodies that their designs may be re-ignited at high altitude. The numerical simulations in this paper were based on the experimental study of spark ignition and flame propagation in a partially premixed turbulent jet by Ahmed and Mastorakos [1]. This allowed comparison with a variety of measurements for the flame evolution in a well characterised flow. The configuration investigated is depicted in Fig. 1, and full details may be found in ref. [1]. The complete transient from spark ignition up to the stabilisation of a lifted flame was captured and the evolution of the mean position of the upstream flame front was reported. Fig. 1 Schematic of burner and igniter [1]. A number of studies have used the steady state turbulent lifted jet flame to examine models for partially premixed turbulent flame propagation [2,3]. The present configuration provides a somewhat more stringent test of the turbulent reacting flow model due to the variety of mixing and turbulence conditions experienced by the flame front during the ignition transient, and also due to the influence of thermal expansion on the fluid dynamics as the flame propagates. The Conditional Moment Closure (CMC) is an advanced turbulent reacting flow model which accounts for the interaction of turbulence with chemical reaction schemes of arbitrary complexity. [4]. Modelled transport equations are solved for the conditional expectations of species and temperature. The primary advantage of solving the conditional moment closure is that due to the small size of conditional fluctuations compared to unconditional fluctuations, the conditional mean reaction rate may be given relatively accurately by a low order closure. In non-premixed flows the mixture fraction [4] is commonly used as the conditioning variable due to its physical significance in such flows and the small conditional fluctuations which may result. The CMC has been applied to the solution of stabilised lifted turbulent jet flames [2,3]. These studies have highlighted the role played by conditional turbulent fluxes in the CMC description of the flame propagation, and a lack of validation for the usual modelling of this quantity. An a priori Direct Numerical Simulation (DNS) study of the CMC treatment of a nonpremixed ignition kernel [5,6] indicated that the usual eddy diffusivity model for the conditional turbulent flux can be inaccurate, and in some circumstances it can give the incorrect sign. Furthermore it was noted that both first and second order closures gave poor predictions inside the flame propagating from the non-premixed spark kernel. It was concluded that a double conditioned closure may be beneficial for some ignition problems. Formulation Simulations have been conducted for mean jet velocities of 12.5ms -1 and 25.5ms -1 with a jet nozzle