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The study of the ignition of a combustible mixture induced by means of an unsteady gas jet, at an initial temperature ranging from 700 to 3000 K, requires the construction of an original test facility which consists of a shock tube connected to a combustion chamber via an injector. With this new experimental setup, the ignition conditions of hydrogen-air ( + carbondioxide) mixtures, induced by a hot hydrogen-argon mixture, have been extensively studied, resulting in the determination of the ignition limits of these combustible mixtures, at an initial pressure and temperature of 100 kPa and 403 K respectively.

An original facility has been built, which consists of a shock tube connected to a combustion chambervia 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 results provide a new assessment, particularly important for nuclear reactor safety.

L'etude du comportement d'une bulle de gaz piegee dans un liquide soumise a une onde de compression doit conduire a la maitrise des risques d'explosion des systemes a hydrazine. Peu de travaux ont ete effectues jusqu'ici. Les auteurs ont utilise comme dispositif experimental un tube a choc au bout duquel un petit tube de verre rempli d'eau et contenant la bulle a etudier a ete fixe. Cette etude permet de montrer que les conditions d'inflammation sont liees a la taille initiale de la bulle. L'hypothese d'une compression adiabatique a pu etre validee.

The ignition limits of hydrogen-air mixtures diluted with steam or carbon dioxide have been studied in function of the mixture composition. Ignition was produced by a hot transient jet emerging from the downstream end of a shock tube and entering a combustion chamber. The spurted gas was heated by a reflected shock wave between 750 and 2800 K. The ejection velocity was subsonic. It was noticed that, with an initial jet at 2600 K, the ignition area of H2-Air-CO2 mixtures was larger than that of H2-Air-H2O mixtures. The ignition area was located inside the flammability range of these systems for rich mixtures. On the contrary, ignition limits and flammability limits were almost similar for lean mixtures. The chemical effect of steam on the ignition of these combustible systems are discussed.