Effect of the initial temperature and composition of a hot transient jet on the ignition of H2-air-diluent mixtures

An original facility has been built, which consists of a shock tube connected to a combustion chamber via an injector, in order to analyze the ignition of a combustible mixture by a subsonic hot gas jet. Using this technique, it is shown that the stagnation temperature, just prior to ejection, of an H2-Ar mixture heated behind a reflected shock wave between 750 and 3000 K is the main parameter for igniting H2-air-diluent (CO2 or H2O) mixtures. The minimum stagnation temperature to ignite an H2-air mixture is approximately equal to the self-ignition temperature of this mixture. If CO2 or H2O is added to the combustible mixture, the minimum stagnation temperature must be increased. Beyond the critical diluent concentration, the mixture cannot be ignited whatever the initial jet temperature is. The temperature corresponding to the critical composition is called the minimum critical temperature. Under these conditions, the ignition limits by a hot jet constituted of a shock-heated H2-Ar mixture have been determined for H2-air-CO2 and H2-air-H2O mixtures. The conditions for which ignition is no longer observed for an H2-air-H2O mixture are the following: equivalence ratio φ = 0.55, initial temperature and pressure, respectively, of 403 K and 100 kPa, and mol% H2O = 48. The ignition limits are compared with the classical flammability limits. The influence of the composition of the hot gas jet on the ignition limits of H2-air-H2O mixtures has been analyzed and discussed. The present result provide a new assessment, particularly important for nuclear reactor safety