Merging mechanisms of triple thermal plumes

Abstract

Thermally induced plumes exist on a wide scale and play an important role in engineering and environmental issues. Understanding dynamic characteristics of triple plumes can greatly improve underlying knowledge of the merging mechanisms in multiple thermal plumes. This thesis presents a series of laboratory water tank modelling on triple thermal plumes in a calm environment. Various flow fields are investigated in a starting period and a period of established flows. The aim of the studies is to provide systematic insights into temporal and spatial triple plume dynamics ranging from the characteristics of small scale turbulence to large-scale coherent structures. In a starting period, examination of the instantaneous velocity fields and corresponding vorticity distribution reveals that a typical temporal plume merging process involves successive self-merging, independent development, global merging, and combination stages. Buoyancy distribution, mutual entrainment, and shear stress from vertical plate jointly determine the normalized temporal plumes penetrations at different sources conditions. Faster plumes penetration usually exists in a concentrated buoyancy force or a spare sources layout with less lateral turbulent mixing. Inherent flow instability leads to three global patterns in triple plumes: right-slanting asymmetrical pattern, left-slanting asymmetrical pattern and central-symmetrical pattern. Proper orthogonal decomposition is used to resolve spatial and temporal features of the coherent structures in each flow pattern. A four-scale flow structure is found, which is made up of global flapping motion, large-scale vortex rings, shear layer vortical structure and small-scale turbulent eddies. A moderate source spacing can significantly improve the large-scale flow instability since the comparative vertical buoyant force and lateral enhanced mutual entrainment. Similar to the coherent structures, the turbulent intensity of the u-component is also determined by both buoyant force and lateral entrainment. The turbulent intensity of the w-component, on the contrary, is dominated by vertical buoyancy force. Both the turbulent intensities and skewness are significantly related to the plume intermittency. Temporal auto-correlation series are quantitatively examined for self-merging points, starting axial points, and axial points in global merging region. It finds that regions with obvious regular periodic motion can exhibit higher flow autocorrelation. For near-source points, the flow fluctuations is mainly dominated by small-scale turbulent motions, and consequently exhibit lower autocorrelation. Independent from the heat rate and configuration of the sources, the power spectra of the half-width points and axial points in the end of merging region all exhibit a -5/3 power law in an inertial subrange followed by a -1 power law decay region. The energy accumulations due to the merging effect, buoyant forces preservation in the near-field, and insufficient turbulent mixing are main reasons for a lower decay rate of ‘-1’ following the inertial subrange. This thesis presents a systematic experimental study on the temporal and spatial merging of triple thermal plumes. Further investigations on three-dimensional triple plume structures are planned to further improve the understanding on the complex interaction of triple plumes.