Leakage of hydrogen during storage and transportation, coupled with its large flammability range, are vital challenges for its acceptability as a viable alternate fuel carrier. Leaks may happen in storage areas, enclosed/semi-enclosed spaces and transportation lines. In Nuclear Power Plants, hydrogen may get generated from metal-steam reaction during a Loss of Coolant Accident; this released hydrogen may eventually find its way in the closed containment space and pose as a potential hazard, due to possible deflagration or severe detonation. A clear understanding of mixing dynamics and associated transport mechanisms of hydrogen gas in such confined areas, which are already filled with atmospheric air, will lead to efficient mitigation strategies. In this paper, experiments, and supporting numerical calculations, have been conducted to observe the role of various parameters on stratification and mixing dynamics of helium (as a substitute for hydrogen) in a fully closed test section. Injection locations, volumetric Richardson number and amount of helium injected have been varied to see its effects on mixing and flow distribution during injection and post injection phases.. The mechanism of mixing in injection phase changes with the location of injection; highest mixing is achieved when helium injection is from bottom of the test section. Also, as we increase the amount of helium in the test section, buoyancy significantly decreases the mixing time in post injection phase, especially in the bottom injection mode. The results clearly discern the interplay of buoyancy, inertia, and diffusional forces in stratification and mixing mechanisms in closed enclosures. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.