TED-AJ03-290 HYDROGEN-AIR PRE-MIXED COMBUSTION IN A CONSTANT VOLUME CHAMBER

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In the present paper, effects of the initial pressure on the combustion of the hydrogen-air mixtures in a constant-volume chamber including stability of the flame are considered. In hydrogen-air mixtures, a flame front for lean mixtures is less stable than that for rich mixtures due to large difference of diffusion coefficients between hydrogen and oxygen and to the Lewis number less than unity. For the fuel-lean condition, the flame front becomes unstable for atmospheric pressure, but it is found that the flame becomes stable for sub-atmospheric pressure for the fuel-lean conditions. Because the Lewis number is independent of pressure, this stabilizing effect could not be attributed to the thermo-diffusive effect but to the hydrodynamic effect. Because the phenomenon is transient, whether the flame is stable or not depend on when or where the instability begins. Onset of turbulence might be dependent on viscosity. Then, Reynolds number based on the flame propagation velocity and a radius of the flame should be a key of the problem. Discussions on the onset of the instability will be made in this paper. Near the lean flammability limit, the flame stays for a while in the same position and it dose not propagate. In this experiment, a spark plug is located on the upper of the chamber and the flame propagates downward. Then, the buoyancy effects should stabilize the flame. A ratio of the maximum pressure to the initial pressure is found to decrease to a value less than that is calculated under the chemical equilibrium assumption with decreasing the initial pressure. This is because a flame propagation velocity decreases to make a heat loss larger. Since the flow velocity of the flame front will decrease if the flame propagation velocity decreases and a density of the gas decreases with the initial pressure, the Reynolds number at the same flame radius decreases to make the flame more stable. Nevertheless, instability becomes large because the influence of the thermo-diffusion effect becomes still larger. But, if the mixture becomes leaner to φ=0.2,instability was no longer observed. Therefore, the instability by the thermo-diffusion effect may not be the direct cause that brings the flammability limit for fuel-lean side. From the observed pressure dependence of burning time in the chamber, overall reaction order n of the hydrogen-air mixture is estimated to be about 2.5. NOx concentration in the burned gas is measured by the chemiluminescence method. In case initial pressure is in the range of 60.8 kPa to 101.3 kPa, the concentration becomes maximum at slightly leaner condition rather than the stoichiometric condition, while in case initial pressure is 40.5 kPa, NOx concentration becomes maximum at φ=1.0. For the lower pressure, the concentration levels become lower due to the lower reactivity.[figure]
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TED-AJ03-290 HYDROGEN-AIR PRE-MIXED COMBUSTION IN A CONSTANT VOLUME CHAMBER

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