Flamelet budget and regime analysis for non-premixed tubular flames

A recently developed flamelet model for non-premixed combustion accounting for curvature effects is examined. The work focusses on the curvature-induced differential diffusion in flame-tangential direction that is not included in classical flamelet formulations. Non-premixed tubular hydrogen and hydrocarbon counterflow flames are computed in physical and in composition space for various stretch rates. These flames were studied experimentally in previous works (Hu et al., 2007; Hu and Pitz, 2009) and allow a detailed investigation of stretch and curvature effects, while retaining a well-defined one-dimensional flame structure. First, the influence of curvature effects on the flame structure of a non-premixed tubular hydrogen flame is assessed by comparing flamelet solutions with and without curvature to the physical space solution. For the selected flame configuration, curvature-induced differential diffusion leads to a flame temperature deviation of 11% compared to a planar counterflow flame with the same scalar dissipation rate profile. Thereafter, the curvature terms are examined in a budget analysis of the flamelet equations. Finally, hydrogen and hydrocarbon tubular flames are placed in an extended regime diagram that allows non-premixed flame structures to be characterized with respect to curvature effects. The regime diagram contains the classical flamelet regime (Regime I) and the curvature-affected flamelet regime (Regime II) enclosing a novel transition zone, where curvature effects are not dominant, but already influence the flame structure significantly. It is shown that the hydrogen flames (Le(f) = 0. 34) fall into the transition zone while the hydrocarbon flames (Le(f) = 0. 88 -1. 49) reside in the classical flamelet regime with negligible curvature influence. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.