The analysis of chemical time scales in a partial oxidation flame

Time scale analysis is a well-established method in combustion science to identify slow and fast processes, but is also required for multi-scale modeling of turbulence–chemistry interaction. Models such as flamelet are based on scale separation arguments, which require a suitable definition of chemical time scales. In this article we study different time scale definitions. Two definitions previously published in the literature as well as two new definitions are investigated in detail. Most time scales require the computation of the eigenvalues of the chemical Jacobian, which is computationally expensive especially for larger chemical reaction mechanisms. One of the new definitions also requires the Jacobian of the chemical source term, whereas the computation of the second newly proposed time scale is computationally straight-forward since neither one is needed. We evaluate the four methods on a simple one-step reacting system and a partial oxidation flame which combines an oxy-fuel type reaction and a fuel-rich reforming post flame zone. Most of the considered time scale definitions provide similar results whereas the computational effort differs significantly.