Large Eddy Simulation of Combustion in Swirling and Opposed Jet Flows

Abstract

This research concerns the Large Eddy Simulation (LES) of turbulent combustion in both the premixed and the non-premixed regime. Non-premixed hybrid bluff-body/swirl flames are simulated by means of a steady flamelet model (Flamelet-LES) based on detailed chemical kinetics. LES of lean premixed twin flames stabilised on a turbulent opposed jet (TOJ) burner are carried out using an algebraic Flame Surface Density model (FSD-LES) and a newly developed model based on Linear Eddy Mixing (LEM-LES). Isothermal swirling flow at a medium Reynolds and swirl number is simulated first and the LES model is shown to accurately predict the velocity statistics and the complex flow field governed by vortex breakdown and two recirculation zones. The Flamelet- LES model is subsequently used to simulate a low speed swirling methane flame and the capability of the model to predict downstream recirculation, vortex breakdown and central jet precession in the presence of heat release is demonstrated. The simulation of two high speed hydrogen/methane swirl flames with the Flamelet-LES model shows that some quantitative predictions of this challenging test case for combustion simulation can be achieved, while the overall predictions are not satisfactory. The flamelet approach is found sensitive to minor errors in the mixing field which strongly affect the simulation results due to the highly non-linear mixture fraction/density relationship. Non-reacting simulations of turbulent opposed jet flows at moderate Reynolds number are performed and compared to experimental reference data. The inclusion of the flow field inside the nozzles into the computational domain is shown to yield accurate predictions of the velocity statistics between the nozzles. For these predictions the detailed knowledge of the initial jet development region near the turbulence generating plates is vital and provided by PIV measurements inside a glass nozzle. FSD-LES of the twin premixed TOJ flames show that the velocity statistics, both along the burner axis and the stagnation plane, can be predicted to high accuracy. However, the algebraic flame surface density model employed in the present study requires the adjustment of a model parameter and as a result, predictions of the turbulent burning velocity cannot be achieved. The comparison of two different interpretations of the FSD model show that a formulation using an additional diffusion term allows for a better resolution of the premixed flames in LES than the original formulation without diffusion. A complex LEM combustion model is first developed as a Stand-Alone approach to simulate premixed combustion and subsequently coupled to LES. The LEM-LES model requires a number of sub-models to represent the effects of sub-grid stirring, finite-rate chemistry, sub-grid expansion, 3D convection (splicing) and flame propagation which are described in detail. The LEM-LES model is – to the author’s knowledge – the first attempt to simulate premixed flames with finite-rate chemistry in incompressible turbulent flow. Preliminary results from the application of LEM-LES to the premixed twin TOJ flames are reported and show a high sensitivity to the 3D convection model and the requirement to improve the splicing procedure for premixed flames in anisotropic turbulent flow. The difficulty to accurately resolve the turbulent flow field by LES while simultaneously accommodating a premixed flame of finite thickness on the LEM sub-grid is found to be a challenge for the LEM-LES of premixed TOJ flames.