Lean Premixed Flames: A Direct Numerical Simulation Study of the Effect of Lewis Number at Large Scale Turbulence

Abstract

The paper presents results from three-dimensional Direct Numerical Simulations of turbulent lean premixed hydrogen, propane and methane flames. The three fuels studied have Lewis numbers which range from small values (hydrogen) through near-unity values (methane) up to values larger than unity (propane). The computations make use of reduced chemical mechanisms with species number ranging from 9 to 28. It has been found that for the present time-explicit numerical method, the CPU-time scales non-linearly – with the power of 1.6 – with the number of chemical species.A new method to produce turbulent vortexes directly in the computational domain is presented. The vortexes are produced through localized forcing in physical space. This allows the full control of the length-scale of the vortexes as well as of their spatial and time distribution. The parallelization of the method requires no programming efforts, i.e. the parallelization follows automatically. The set of vortexes created for this particular investigation is identical for all flames studied: two methane, one hydrogen and one propane flames. Despite the identical turbulent vortex set, the flames of the different fuels react quite differently to it, exhibiting qualitatively different combustion regimes as well as different growth of the flame front area and of the consumption flame speed with time.The computations are carried out on the CRAY XE6 HERMIT supercomputer at the High Performance Computing Center Stuttgart (HLRS). Numerical issues and performance results for the three chemical mechanisms, corresponding to the three fuels used, are presented and discussed.