Direct numerical simulation of lean premixed CH4/air and H2/air flames at high Karlovitz numbers |
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Affiliation: | 1. Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, 1 University Road, Tainan 70101, Taiwan, ROC;2. Department of Chemical Engineering, National Cheng-Kung University, No. 1, University Road, Tainan 70101, Taiwan, ROC |
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Abstract: | Three-dimensional direct numerical simulation with detailed chemical kinetics of lean premixed CH4/air and H2/air flames at high Karlovitz numbers (Ka ~ 1800) is carried out. It is found that the high intensity turbulence along with differential diffusion result in a much more rapid transport of H radicals from the reaction zone to the low temperature unburned mixtures (~500 K) than that in laminar flamelets. The enhanced concentration of H radicals in the low temperature zone drastically increases the reaction rates of exothermic chain terminating reactions (e.g., H + O2+M = HO2 + M in lean H2/air flames), which results in a significantly enhanced heat release rate at low temperatures. This effect is observed in both CH4/air and H2/air flames and locally, the heat release rate in the low temperature zone can exceed the peak heat release rate of a laminar flamelet. The effects of chemical kinetics and transport properties on the H2/air flame are investigated, from which it is concluded that the enhanced heat release rate in the low temperature zone is a convection–diffusion-reaction phenomenon, and to obtain it, detailed chemistry is essential and detailed transport is important. |
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Keywords: | Turbulent premixed combustion Direct numerical simulation High Karlovitz number Detailed chemistry Differential diffusion |
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