The design of the fluidising air distributors, or nozzles, is one of the most important aspects influencing operation of fluidised beds at industrial scale. In this work, the study of the hydrodynamics in gas-solid fluidized beds where the primary gas injection is achieved through a nozzle-type gas distributor has been carried out, using an innovative X-ray imaging technique. Qualitative and quantitative results are reported, with particular focus on jets penetration length and their evolution. Results show that the lighter and the finer are the particles, the larger is the jet penetration. Since the experimental data do not match predictions available in literature, a new non-dimensional correlation based on hydrodynamic scaling and Froude number is also proposed. The new correlation takes into account the effects of jet velocity, particle density and particle size. A tentative mechanistic explanation for the departure from purely hydrodynamic scaling is offered. (C) 2017 Elsevier Ltd. All rights reserved.