Since the pioneering work of Harrison and Eyre (1986), the existence of secondary or external explosion outside explosion vents is recognized and rather systematic. This explosion can be much more powerful (Proust, 2004, 2010) than the internal explosion particularly when the mixture is very reactive. But today, the understanding of the formation of the external cloud and its subsequent combustion remains largely outstanding. Very rapid burning was noticed and significant UVCE pressure effects. In some circumstances, a preexisting flammable cloud encompasses the vented vessel, like in Buncefield for instance. This paper presents experimental work and CFD simulations (OpenFoam) which investigate the aerodynamics of the flow and the flame propagation. The experimental device is composed of a 4 m(3) chamber linked to an unconfined 54 m(3) volume via a square vent. These two volumes are filled with a stiochiometric propane air mixture and ignited by a pyrotechnical match in the 4 m3 chamber. When the vent area is small enough, the vortex bubble formed by the gas ejection is disrupted and a jet is formed entraining a significant portion of the outside atmosphere. The explosion overpressure outside can be 10 time larger as compared to the fully unconfined case (no chamber). (C) 2017 Published by Elsevier Ltd.