Modeling CO With Flamelet-Generated Manifolds: Part 1—Flamelet Configuration

In laminar flamelet modeling, a laminar flame in a simple 0D or 1D configuration is calculated a-priori and parameterized by a few scalars such as mixture fraction and reaction-progress or strain-rate. Transport equations, or algebraic expressions, for these parameters are then solved in 3D CFD simulations, avoiding computationally expensive in-situ chemical kinetic calculations. Typical configurations for laminar flamelets include, in 1D, opposed flow configurations with either non-premixed or premixed streams, freely propagating premixed flames, premixed flames impinging on a (heated) wall, and burner stabilized premixed flames. In 0D, plug-flow, perfectly-stirred (PSR) and partially-stirred reactors have been employed to build ‘flamelet-like’ ignition and flame-propagation tables. This paper compares 1D strained steady and unsteady non-premixed flamelets, 1D strained premixed flamelets, and 0D PSR simulations at a stochiometric and a lean equivalence ratio. At stochiometric mixtures, all three flamelet configurations show comparable manifolds (i.e. CO and OH mass fractions, and reaction-progress source term distributions). However, at lean equivalence ratios, the different configurations show substantially different manifolds. It is concluded that a unique flamelet configuration cannot be identified for a general partially-premixed model that ranges from the non-premixed to the perfectly premixed limit. Instead, to accurately model CO emissions, it may be necessary to include both premixed and non-premixed flamelets, with a flame-index (e.g. Yamashita et al., 1996, Proc. Combust. Inst., 26) to select the appropriate burning regime.Copyright © 2012 by ASME