Structure and extinction limits of counterflow diffusion flames of hydrogen-nitrogen mixtures in air

Abstract

The paper describes the results of a numerical investigation of the structure and properties of counterflow diffusion flames in three fuel-air mixtures, in which the fuels are respectively pure hydrogen, an equimolar mixture of hydrogen and nitrogen, and a mixture of 21 percent hydrogen and 79 percent nitrogen. Realistic chemistry and detailed formulation of the transport fluxes are employed. The extinction limits of all three flames are examined, and occur at velocity gradients of 13000, 8600 and 1520 s −1 , with maximum temperatures of 1422, 1308 and 1214 K respectively. An additional property of diffusion flames is that there is a minimum oxygen content of the nitrogen-oxygen mixture (or “air”) below which no flame can be established. When the fuel is pure hydrogen, this minimum mole fraction is found numerically to be 0.052. Both the value of the limit and the computed properties of the limit flame agree excellently with experiment, and this provides validation for the reaction mechanism in the diffusion flame context. The properties of all the flames are discussed in relation to their chemical mechanism. The major radical production in all the flames is centred more or less on the stoichiometric position, and near the position of maximum temperature. The dominant heat release is always on the fuel-lean side of the flame, and occurs by way of the HO 2 cycle of reaction in the hydrogen-oxygen system. These heat release rates increase with increasing strain rate or velocity gradient, and this is related with increased concentrations of hydrogen and oxygen in the reaction zone, at the expense of lower reactedness and lower temperatures. Eventually the temperature becomes so low that the radical production mechanism fails and the flame extinguishes.