A thorough understanding of the flow characteristics of vanadium redox-flow batteries is essential for their improvement. With a combined approach of CFD simulations linked with optical measurements, the distribution in a carbon felt electrode of the vanadium electrolyte as well as of an alternative fluid substitute is compared and in all three dimensions analyzed leading to qualitatively consistent results. Yet, the experimentally observed partially inhomogeneous flow distribution is not predicted by the pseudo-homogeneous porous media CFD model.