Effect of swirl on pure turbulent thermal plume development

Pure thermal plumes have been experimentally investigated under swirling conditions. Such conditions are applied upon the heated source impinging directly on the plume development. Results clearly underlined the existence of a threshold swirling number. Below the latter, i.e., at small swirling velocities, ascendant flow behaves like a classical static plume while at a high swirling number, the flow is clearly affected even if characteristic power laws are still valid. However, swirling of the source favors transition from laminar to turbulence as well as breakdown of coherent structures all around the plume motion. A direct measurement method was applied in order to assess instantaneous entrainment coefficients, and results clearly show appreciable influence of the swirling motion on the plume development. The driving mechanisms, i.e., contraction and expulsion phenomena, are strongly amplified and as a consequence directly interact on entrainment.