A numerical study of a turbulent buoyant helium jet developing in a two-vented cavity is conducted based on a well-resolved numerical simulation. A sufficiently large exterior region is modelled in the computational domain to approach the natural inlet/outlet conditions of the vents, leading to a good agreement between the numerical results and available experimental particle image velocimetry (PIV) measurements. For a release of helium with a flow rate of 5 Nl.min(-1) in an air-filled cavity about 15 cm high, the flow structure and the helium dispersion are analysed illustrating the strong confinement effect enhanced by a cross-flow from the lower vent. By tracking the jet axis deviation and comparing the classic plume description in terms of global fluxes estimated from DNS results to the Morton et al.' theory [1], we quantify the effect of the confinement and the cross-flow on the flow structure. Finally, we highlight a blocking zone at mid-cavity height originated from confinement, where helium accumulates. (C) 2020 Elsevier Ltd. All rights reserved.