The effect of fuel concentration on intrinsic flame instabilities and flame acceleration in lean H₂-CO-air mixtures

In large scale accident scenarios of nuclear power plants core meltdown and molten-core-concrete-interaction can occur. Large amounts of syngas mainly consisting of hydrogen (H₂) and carbon monoxide (CO) might be formed and mixed with the surrounding air, developing a potentially explosive mixture. The ignition of the flammable mixture might lead to flame acceleration and in worst cases to the destruction of the reactor building, causing the detachment of radioactive substances to the environment. To prevent future accidents, the understanding of flame accelerating conditions of syngas-air mixtures, in specific the understanding of the flame topology, must be improved. To the authors’ knowledge, insufficient data on flame topology investigating ignition and early stages of flame propagation in lean H₂-CO-air mixtures is available. Therefore, in this study experiments are carried out to analyze the ignition and slow deflagration of a 50/50 H₂/CO fuel mixture with fuel-to-air concentrations between 13.0 vol.-% - 29.5 vol.-%. A model calculating the flame acceleration in early stages of flame propagation is suggested. The GraVent explosion channel located at Technical University of Munich is used for the experiments in this study in its configuration of 1300 mm in length and a rectangular cross section of 300 × 60 mm. OH-PLIF measurements are used to determine flame front speed, flame acceleration and flame front length. Wrinkling factors are determined by the division of the wrinkled flame front length caused by intrinsic flame front instabilities and the smooth flame front length to quantify the influence of intrinsic flame front instabilities on early stages of flame acceleration. The optical experimental data contains the ignition of the gas mixtures as well as early stages of flame propagation. In conclusion, the results gathered in this study provide information on the topology of H₂-CO-air flames under lean conditions enhancing the understanding of syngas flame acceleration and extending experimental data provided in former studies with relevance to large scale accident scenarios of nuclear power plants.