Study of the Flow Structure of A Thermal Plume Evolving in an Unlimited and in A Semi-Enclosed Environment

The basic objective of this study was to better knowledge the fine flow structure of thermal plume created by a heated disk, evolving in an unlimited and in a semi-enclosed environment. The flow visualization and the study of the temperature fluctuations spectrum show clearly a considerable change of the turbulent structure of the internal flow in comparison with the free plume. This difference is especially caused by the thermosiphon effect. Indeed, this study shows that the thermosiphon affects the flow structure plume and causes the appearance of a supplementary zone, just at the system entrance, that is added to the two classic zones concerning the free plume, mentioned in the previous works. This zone is characterised by two symmetrical rotating rolls created in the vicinity to the hot source. The energy spectrum study permits to obtain certain spectral power laws, which characterize the energy transfer between the structures that are present in the flow. In fact, three qualitatively different regions were identified: first, a production region, secondly, a region with behavior as per n^-3 associated with a buoyancy region and; finally, a dissipation region associated with an n^-7 law. The n^-1 Tchen low is observed for the free plume in the large frequency region. The existence of the n^-1 region is related to an important turbulent production. These spectral regions characterize the energy transfers mechanisms among the length scales of flow investigated here.