Abstract
The transient evolution of counterflow diffusion flames can be described in physical space [i.e. by the model of Im et al. (Combust. Sci. Technol. 158:341–363, 2000)], and in composition space through flamelet equations. Both modeling approaches are employed to study the ignition of diluted hydrogen–air, methane–air and DME–air diffusion flames including detailed transport and chemistry modeling. Using the physical space solution as a reference, this work elucidates the capability of flamelet modeling to predict ignition characteristics in terms of ignition temperature and ignition delay time. Varying pressure and strain rate for the hydrogen–air configurations, the agreement between reference solution and flamelet results is shown to strongly depend on the ignition limits as characterized by Kreutz and Law (Combust. Flame 104:157–175, 1996). In limit 2 and at elevated temperatures, where the ignition kernel formation is governed by chemical reactions and less dependent on mass transport (high Damköhler numbers), the flamelet model yields accurate results. Close to the ignition limits 1 and 3 however, significant deviations can be observed. In these limits, the residence time of radicals during ignition kernel formation is strongly influenced by diffusive transport and Damköhler numbers are low. The analysis of the hydrocarbon flames shows that differences between the physical space model and the flamelet model are smaller. This is attributed to a smaller influence of differential diffusion on the ignition process for methane and DME as compared to hydrogen as fuel. This paper underlines that flamelet models can be used to describe ignition processes under strained conditions, but care should be taken if ignition takes place in certain parameter ranges, i.e. close to the ignition limits or at high strain rates.
Highlights
Counterflow diffusion flames have been essential for the research on fundamental flame physics and for the development of combustion models for both, laminar and turbulent combustion
For three reasons the scalar dissipation rate (SDR) is fixed versus time for both flamelet modeling approaches: Firstly, as will be shown below, the SDR remains almost constant during the forma‐ tion and growth of the ignition kernel until the ignition event
Ignition processes of diluted hydrogen-air, methane-air and dimethyl ether (DME)-air diffusion flames are studied employing a transient counterflow (TCF) model formulated in the physical space, and a transient flamelet (FLT) model formulated in mixture fraction space
Summary
Counterflow diffusion flames have been essential for the research on fundamental flame physics and for the development of combustion models for both, laminar and turbulent combustion. In the clas‐ sical flamelet equations for temperature T and species mass fractions Yk by Peters (1984, 1988) (derived for Lei = 1) This gradient information is contained in the scalar dissipation rate (SDR) = 2D|∇Z|2. Considering the physical space model as a reference, it is the objective of this work to elucidate the capabilities of flamelet modeling for capturing ignition charac‐ teristics of diffusion flames. Thereafter, an unsteady flamelet model incorporating detailed trans‐ port and chemistry is presented Both modeling approaches are verified by demonstrating a close agreement with recently published results from the literature and are utilized to study the ignition of diluted hydrogen-air diffusion flames in detail, vary‐ ing pressure and strain conditions.
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