The flame acceleration plays a major role on the explosion effects. Then, it is of importance to understand the flame acceleration process and to predict explosion effects in open and congested areas for industrial safety reasons. In this aim, small-scale deflagration experiments were performed in cylindrical congested volumes of hydrogen air mixtures varying from 1.77 L to 7.07 L. The influence of the reactivity was studied since the equivalence ratio of hydrogen air mixtures were ranging from 0.5 to 2. The congestion was realized with varying numbers of grid layers and configurations. Experimental results, in term of flame speeds, were compared to results from correlations of the literature. Correlations were also adapted to the small-scale and modified to take into account the volume and the reactivity of the combustible mixtures. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.