For environmental reason, hydrogen can be considered as a fuel of the future. However, the flammability domain is very large and the necessary ignition energy is only of few millijoules which makes the presence of hydrogen in air potentially dangerous in confined environment. It can also be accidentally generated in nuclear plants. The expanding flame is intrinsically unstable. Numerous studies, for example Lee et al. (1985), Knystautas et al. (1986), showed that obstacles located along the path of expanding flames can cause rapid flame acceleration and deflagration to detonation transition. Generally, if the hydrogen quantity is less than 18 mol% in air, the detonation regime is not observed. Nevertheless, in this case, the flame speed can increase up to 70 or 80 % of the burned gas speed of sound. This flame acceleration has considerable effect on the confinement and can destroy the building.
Flame acceleration has been largely studied for a homogeneous hydrogen distribution in the available containment volume (Yang et al. 1991, Dorofeev et al. 2001, Djebaïli-Chaumeix et al. 2005). Only very few data are available on the behaviour of the H2/air flame in a non-uniform mixture (Whitehouse et al. 1996, Sochet et al. 1997). The present work aims at providing an experimental database on flame acceleration of lean H2/ air based mixtures characterized by hydrogen gradient. The experimental work is completed by computations using TONUS CFD code (Rivière et al. 2004). The H2 gradient establishment is controlled using GC gas sampling analysis and the results are compared to the modelling. Finally, the flame speeds measurements in the non uniform mixtures are compared to those for uniform mixtures.