Laminar flamelet structure at low and vanishing scalar dissipation rate
The laminar flamelet structures of methane/air, propane/air, and hydrogen/air nonpremixed combustion at low and vanishing scalar dissipation rates are investigated, by numerical calculations of a system of conservation equations in a counterflow diffusion flame configuration, together with a transport equation defining the mixture fraction and scalar dissipation rate. The chemical reaction mechanisms consist of 82 elementary reactions up to C{sub 3} species. In the limit of vanishing scalar dissipation rate, two types of structures are shown to appear. In one structure fuel and oxygen are consumed in a thin layer located near the stoichiometric mixture fraction, Z{sub st}, where the temperature and the major products reach their peaks. This is similar to the so-called Burke-Schumann single layer flame sheet structure. One example is the hydrogen/air diffusion flame. The second structure consists of multilayers. Fuel and oxygen are consumed at different locations. Oxygen is consumed at Z{sub l} (near Z{sub st}), where the temperature and the major products reach their peaks. Fuel is consumed at Z{sub r} (>Z{sub st}). Between Z{sub l} and Z{sub r} some intermediate and radical species are found in high concentrations. Hydrocarbon/air nonpremixed flames are of this type. It is shown that for methane/air diffusion flames, some chemical reactions which are negligible at large scalar dissipation rate near flame quenching conditions, play essential roles for the existence of the multilayer structure. This result is used to successfully explain the high CO emissions in a turbulent methane/air diffusion flame.
- Research Organization:
- Lund Inst. of Tech. (SE)
- OSTI ID:
- 20019014
- Journal Information:
- Combustion and Flame, Vol. 120, Issue 3; Other Information: PBD: Feb 2000; ISSN 0010-2180
- Country of Publication:
- United States
- Language:
- English
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