In case of severe accidents in Pressurized Water Reactors, a great amount of hydrogen can be released, the resulting heterogeneous gaseous mixture (hydrogen-air-steam) can be flammable or inert and the pressure effects could alter the confinement of the reactor. Water spray systems have been designed in order to reduce overpressures in the containment, but the presence of water droplets could enhance flame propagation through turbulence or generate flammable mixtures since the steam present in the vessel could condense on the droplets and could not inert the mixture anymore. However beneficial effects would be heat sinks and homogenization of mixtures. On-going work is devoted to the modeling of the interaction between fine water droplets and a hydrogen-air flame. We present in this paper an unsteady Lumped Parameter model in detail with a special focus on hydrogen-air flame propagation in the presence of water droplets. The effects of the initial concentration of droplets, steam and hydrogen concentrations on flame propagation are discussed in the paper and a comparison between this model and our previous steady Lumped-Parameter model highlights the features of the unsteady approach. This physical model can serve as a validation tool for a CFD modeling. The results will be further validated against experimental data.