A criterion to distinguish autoignition and propagation applied to a lifted methane–air jet flame

Abstract

This numerical study deals with the distinction between autoignition and propagation driven reaction zones using an autoignition index (AI). It allows a clear identification of the two burning regimes based on the relative contribution of two reactions for hydroperoxyl (HO 2) chemistry. AI was applied to a lifted methane-air jet in a hot (1350 K) vitiated coflow, namely the Cabra flame configuration. Large Eddy Simulation (LES) were performed using the Dynamic Thickened Flame model (DTF) with a Analytically Reduced Chemistry (ARC) mechanism with 22 transported species, as well as 18 species in Quasi-Steady State (QSS) approximation. A detailed validation of the numerical methods is presented. Comparisons with experimental data are in good agreement for mixture fraction, temperature and species mass fractions for both a fine and a coarse mesh. In a detailed analysis of the flame structure, AI identifies autoignition as dominant over propagation at the flame base. Autoignition pockets are close to the lean most reactive mixture fraction. Lean and rich propagation is recognised to dominate in regions located at higher mixture fractions closer to the centerline with significantly higher heat release rates compared to autoignition.

Autoignition flame dynamics in sequential combustors

“…This is observed when high temperature stratifications propagate through the mixing zone toward the flame. As shown for example in [22] localised autoignition spots initiate diffusion of heat toward richer mixture fractions and eventually become propagating type flames with significantly high heat release rates, which is observed in the instantaneous flow fields (see Fig. 8 at π/2).…”

“…3 that using ARC 22 GRI211, species mass fractions show excellent agreement with the detailed scheme. A more comprehensive validation of ARC 22 GRI211 can be found in [22]. For turbulent combustion modelling the Dynamic Thickened Flame (DTF) model [6] resolving the flame front on the LES grid is applied.…”

“…A cold methane-air jet is injected into hot vitiated co-flow and turbulence chemistry interactions are similar to the ones in a sequential burner. Autoignition is the main stabilisation mechanism, as shown for example by Schulz et al [22] and the hot-gas temperature is the dominant parameter significantly influencing the liftoff height of the flame as seen for instance in [4,13]. The goal of this study is to determine the heat release rate response to temperature fluctuations of a sequential combustor flame with CFD/SI and harmonic single frequency excitation.…”

A numerical study on combustion mode characterization for locally stratified dual-fuel mixtures

“…To characterize the dominant mode of combustion, Sankaran et al [23] proposed a non-dimensional number based on the laminar flame speed and ignition speed to distinguish between normal deflagration and subsonic spontaneous propagation. Introduction of flame index [24] , autoignition index [25] , and chemical explosive mode analysis [26] are instances of other pathways taken by researchers to distinguish between different regimes of propagation.…”

A conceptual model of the flame stabilization mechanisms for a lifted Diesel-type flame based on direct numerical simulation and experiments

“…In the present DNS, chemistry was modelled using an ARC (Analytically Reduced Chemistry) scheme [26][27][28][29][30] adapted for ndodecane / air flames at 3.4 MPa.…”